No matter how dissimilar tanks developed in different countries might be, the evolution of their principal components has followed to a large extent a common course.
The most important of these components has been their armament, which enables tanks to perform their basic functions of destroying or suppressing enemy personnel or weapons. The performance of these functions has generally involved machine guns as anti-personnel weapons and guns capable of destroying other tanks, as well as dug-in machine guns and other weapon emplacements.
The actual armament of the original tanks consisted of weapons designed for other purposes that happened to exist at the time. In the case of the original British tanks, the main armament consisted of 57mm 6-pounder guns that were furnished by the Royal Navy as the army lacked suitable weapons. Subsequently, similar guns were produced specifically for tanks but with barrels shortened from 40 to 23 calibres to make them protrude less from the sides of the tanks. The first French tanks were armed with standard 75mm field guns, which were the mainstay of the French artillery, or short-barrelled guns of the same calibre. In addition to their guns, all tanks also had two to four machine guns. However, once tanks began to be produced, ‘female’ versions of British tanks were armed only with six machine guns to defend tanks against an imagined onrush of enemy infantrymen! In all cases the machine guns were standard rifle calibre infantry weapons.
Machine guns were also the only armament of one version of the Renault FT light tank that came into use towards the end of the First World War, the other version being armed with a 37mm short-barrelled infantry cannon. The example of the machine gun version of the Renault was followed after the war when machine guns became the only armament of most tanks, and in particular of the light two-man tanks that were widely used for several years, from the Vickers Carden Loyd light tank of 1926 to the German PzKpfw I of 1940.
To maximize their machine gun fire power, the larger tanks built during the 1920s and early 1930s were provided with small machine gun turrets in addition to their main gun turret. The extreme example of this was the British A.1 Independent, which had as many as four machine gun turrets clustered about its main turret. The only plausible justification of them was that they provided all-round protection against enemy infantry, which might have implied a survival of the notion that led to the wartime ‘female’ tanks. In any case, although the concept of a five turret tank aroused a great deal of interest at the time, it led to only one other tank of this kind being built, the Soviet T-35. However, other tanks were built with an additional machine gun turret, oddly located at the rear of the hull behind the engine compartment. The first of them was the French 2C heavy tank built immediately after the war, which was followed in the 1920s, and probably inspired in this respect, the German Grosstraktoren and the first Japanese tank.
Multi-turreted tanks attracted little further interest except in Britain, where machine guns were regarded as the principal tank weapon and where the Independent was followed by a series of tanks with two additional machine gun turrets at the front of the hull. They ranged from the A.6 or Sixteen Tonner of 1928 to the experimental A.14 and A.16 heavy cruisers of 1938. The additional machine gun turrets may have increased the volume of suppressive fire that could be delivered over the front of a tank, but they did not in general justify the weight and the complication that they involved. They were therefore abandoned at the beginning of the Second World War, but one additional machine gun turret was still incorporated in the original version of the Crusader cruiser tank ordered in 1939.
Apart from those mounted alongside the main armament and usually called ‘coaxial’, the use of machine guns was confined for several years on many tanks to one mounted in the front of the hull and operated by a gunner sitting alongside the driver. This arrangement was pioneered by the British A7E2 medium tank built in 1929, and became almost universal during the Second World War. Tanks that incorporated it included all the German tanks from PzKpfw III to Tiger II and Soviet T-34s, as well as US M4 Shermans and M3 and M5 Stuart light tanks, and British tanks from Churchill infantry tanks to the Comet cruiser tanks. They also included Italian M 13/40 and Japanese Type 97 medium tanks.
However, even before the Second World War some of the more advanced tank designs, such as that of the British Matilda infantry tank, dispensed with the hull machine gun and the gunner who went with it, which reduced the average crew from five to four men. A general change to this was led by the British Centurion and the Soviet T-44, both of which came into use in 1945, as well as the Soviet IS-2. From then onwards the use of machine guns was confined in almost all newly built tanks to one per tank mounted alongside the main armament, until an additional machine gun was mounted on top of the turret. This began to be practised initially for anti-aircraft defence, particularly during the latter part of the Second World War on US tanks, but was not generally adopted for the very good reason that the tank commander, who had to operate the machine gun, had to expose himself out of the turret to do it and was also drawn away from his principal function of commanding the tank. However, the objection to externally mounted machine guns was party removed later when they could be operated from within tank turrets.
Long before this stage was reached, it was realized that there was a need for more powerful automatic weapons than the rifle calibre machine guns that were the only armament of almost all the early light tanks. The German Army was the first to do so and shortly before the end of the First World War initiated the development of a heavy dual purpose anti-tank and anti-aircraft T.u.F. (‘Tank und Flieger’ or ‘Tank and Aircraft’) machine gun of 13mm calibre, instead of the usual 0.303in. or 7.92mm. The defeat of Germany prevented this gun being produced, but after the war it was followed by the development in the United States of a somewhat similar 0.50in. heavy machine gun for fighter aircraft.1 By 1931 the use of this ‘50 calibre’ machine gun was extended to US light tanks, and it was the most powerful weapon of the US T4 medium tank built in 1935.2 A 0.5in. version of the Vickers machine gun was also developed in Britain towards the end of the First World War for use in fighter aircraft, and in 1929 it was mounted in the experimental A4 E10 version of the A4 light tank. Five years later it became the main armament of British light tanks from the Mark V to Mark VI B, which formed the great majority of British tanks on the outbreak of the Second World War.
The 0.5in. machine guns fired bullets with much the same velocity as that of rifle bullets, but they were heavier and therefore had five or six times as much kinetic energy, or ‘punch’. They were therefore able to penetrate about 20mm of armour at 200 metres, which made them reasonably effective against contemporary light tanks when they were introduced. But by 1940 their performance was no longer adequate, except against very lightly armoured vehicles, and they ceased to be used as the main armament even of light tanks.
The need for larger calibre automatic weapons was foreseen in Germany even before the First World War came to an end. At first they were intended for aircraft, and as they could not be developed in Germany because of the restrictions imposed on it by the Versailles Treaty, their development was undertaken in Switzerland. The resulting 20mm automatic cannon were offered for use not only in aircraft but also as anti-aircraft and anti-tank weapons. One of the companies involved in this was Solothurn, which was taken over by the Rheinmetall company, and the latter subsequently produced a 20mm cannon for the German Army. Other 20mm automatic cannon were also produced in Denmark by Madsen and in Italy by Breda, and they armed a number of light tanks and armoured cars built during the 1930s. But the only noteworthy use of 20mm cannon was that of the Rheinmetall-produced 2cm KwK 30 used as the main armament of the PzKpfw II, which was the most numerous German tank used during the 1940 campaign in France.
When the PzKpfw II began to be produced in 1937, other tanks of its weight of less than 10 tonnes were already being armed with larger calibre cannon, albeit manually loaded, which could perforate thicker armour than the 20mm cannon. The first to be armed with such a medium calibre cannon was the Vickers Light Tank built in 1923, which was followed by the highly influential Vickers Mediums. However, these tanks were most likely armed with 47mm cannon because Vickers happened to have naval cannon of this calibre rather than as the result of an established military requirement.
Development of medium calibre guns specifically for tanks and as anti-tank guns was began in Germany by Rheinmetall in 1924. The 37mm calibre chosen for the guns by Rheinmetall was the same as that of the cannon of the Renault FT, but the gun was 45 instead of 21 calibres long and fired armour-piercing projectiles with a muzzle velocity of 760 instead of 388m/s, as a result of which it could pierce armour more than twice as thick. By 1930 the 37mm Rheinmetall gun was mounted in the secretly built Leichttraktoren and in 1932 it was adopted by the Red Army for the first of its series of BT tanks, the BT 2.3 In the mid-1930s the US Army also acquired a licence to produce the Rheinmetall gun, and it was adopted in 1938 for the M2 medium tank.4 In Germany the 37mm gun was adopted for a light tank that began to be developed in 1934 and evolved into the PzKpfw III.5 In the mid-1930s a 37mm gun with very similar characteristics was also developed in Sweden by Bofors, which then armed some Swedish and Polish tanks.
In view of all this, the 37mm Rheinmetall gun could be regarded as the typical armament of the ‘light/medium’ tanks of the 1930s. However, towards the end of the decade it began to be superseded by similar guns of larger calibre. This process started in 1933 with the Soviet BT-5, which was armed with a 45mm gun, and two years later the T-26 was also armed with a gun of this calibre.6 In 1936 British tanks began to be armed with a 40mm gun 50 calibres long, which could perforate thicker armour than the 37mm Rheinmetall gun, and at about the same time the French Somua S 35 tank was armed with a 47mm gun, which could also perforate somewhat thicker armour, as did the Soviet 45mm gun. The German Army only began to redress the balance in 1941 by arming the PzKpfw III with a 50mm gun 42 calibres long and in 1942 arming it with another 50mm gun 60 calibres long. The latter could perforate thicker armour than all the others, as it fired heavier projectiles and with a higher velocity. In fact, it could perforate 68mm of armour at 500m. Apart from the Italian, Czech and Japanese 47mm guns, the only other medium calibre tank gun was a new British 57mm 6-pounder, which was mounted in 1942 in Churchill infantry tanks and Crusader III cruiser tanks and which could perforate armour 40 per cent thicker than the German 50mm L/60.
However, by 1942 it was beginning to be generally recognized that tanks should be able not only to defeat the armour of the opposing tanks but also deliver effective high explosive fire against enemy anti-tank guns and other targets. To be able to do this, they had to be armed with guns of not less than 75mm.
As already mentioned, the first French tanks were in fact armed with 75mm guns and after the First World War the French Army embarked on the development of a tank armed with a 75mm gun, which was to be its principal tank. By 1930 this led to the Char B. As in the wartime Schneider and St Chamond tanks, the 75mm gun of the Char B was mounted in the hull and in such a way that it could only be aimed in traverse by turning the whole tank and therefore required the driver to act also as a gunner.7 Such an arrangement might have worked in direct assaults on enemy positions, but it was not suited to the mobile manoeuvre warfare in which Char B became involved in 1940.
In contrast, the British Army did not adopt a 75mm gun for any of its tanks until the latter part of the Second World War. As mentioned in Chapter 5, as late as 1937 British General Staff saw no need for tank guns of more than 40mm. Larger calibre weapons were in fact mounted in British tanks but they were either 3.7in. (95mm) or 3in. (76.2mm) howitzers, which replaced the medium calibre guns in some of them. Tanks armed with the howitzers were called Close Support Tanks and their capabilities were confined almost entirely to firing smoke shells to create smoke screens, which were considered essential.8
The German Army avoided such extravagant over specialization when it began secretly to develop medium tanks in the 1920s and adopted a 75mm gun for the Grosstraktoren. The 75mm gun was only 24 calibres long and fired armour-piercing projectiles with a velocity of only 400m/s, but it could still defeat thicker armour than the contemporary 37mm tank guns. What is more, unlike the latter, it also fired very effective high explosive projectiles. It was consequently adopted as the main armament of the PzKpfw IV, which became the most powerful German tank during the first three years of the Second World War. Its nature was not always recognized at the time, and because of its short-barrelled gun it was often wrongly equated with the British Close Support Tanks, although the latter had none of its capabilities.9
The Russians followed the German example when they began to develop their T-28 medium and T-35 heavy tanks in 1932 and armed them with 76mm guns. The prototype of T-28 was still armed with a 45mm gun, which made it comparable in this respect to the British A.6 Sixteen Tonner, but when it went into production it was armed, like the T-35, with a 76mm gun, albeit only 16.5 calibres long. But in 1938 this was superseded by a gun 26 calibres long, which fired projectiles with a higher velocity of 555m/s instead of 381m/s and therefore was capable of defeating thicker armour. Yet another increase in barrel length and projectile velocity took place in 1940 when a 76mm gun 30.5 calibres long was installed in the KV-1 heavy tank and then in the T-34 medium tank. Finally, from 1941 onwards, the KV-1 and T-34 were armed with a 76mm gun 41.5 calibres long that fired projectiles with a velocity of 625m/s.
In contrast to the progressive development of 76mm guns by the Red Army, no successor to the 75mm L/24 gun of the PzKpfw IV was produced in Germany until after the invasion of the Soviet Union in 1941, when German forces came up unexpectedly against the new and, for their time, heavily armoured Soviet tanks. An outcome of this was the 75mm L/43 tank gun, which had a muzzle velocity of 740m/s and proved superior to the Soviet 76mm as well as US 75mm tank guns when it appeared in 1942 as the new armament of the PzKpfw IV. But, good as it was, the 75mm L/43 gun did not represent the peak of the development of 75mm tank guns. That distinction belonged to the 75mm L/70 gun, which was also developed in response to the appearance in 1941 of the new Soviet tanks but was mounted in the Panther medium tank and fired projectiles at 925m/s, which enabled it to perforate 126mm of armour at a range of 1,000m.
A 76mm 17-pounder gun with performance characteristics very similar to those of the 75mm L/70 was developed in Britain at about the same time. But there was no British tank in which to mount it, apart from the rather clumsy Challenger employed on a limited scale in 1944. However, it was found that the 17-pounders could be mounted in M4 Sherman tanks in place of their 75mm guns and most of them were successfully used in that way, the re-armed tanks being called Fireflies. Other tanks used by the British Army in 1944, such as the Cromwells, were still armed with lower performance 75mm guns similar to those of the US Shermans, which were only 37 calibres long and fired projectiles with a velocity of 619m/s.
The muzzle velocity of Panther’s 75mm L/70 gun approached the limit of what could be done with conventional full calibre armour-piercing projectiles before the law of diminishing returns set in, that is before disproportionate amounts of propellant had to be used to further increase projectile velocity. When that limit was reached, the only practical way of increasing the kinetic energy and therefore the armour-piercing capability of conventional projectiles was to make them heavier and therefore of larger calibre. The move to larger calibres had in fact started before the muzzle velocity limit was reached, driven by the increases in the armour of the opposing tanks and aided by the existence of guns suitable for conversion into tank guns. The guns in question were anti-aircraft guns.
The first of these guns was the German 88mm L/56, which had proved very effective against ground targets as well as aircraft from the Spanish Civil War onwards and which was adopted in 1941, just prior to the German invasion of the Soviet Union, for the Tiger I heavy tank. The Red Army followed the German lead and in 1943 armed its KV and T-34 tanks with an 85mm gun 51.5 calibres long, which was also an adaptation of an anti-aircraft gun. Similarly, in 1944 the US Army adopted a modified 90mm 52.5 calibres long anti-aircraft gun for its M26 Pershing tank.
By 1944 the German Army had followed Tiger I with Tiger II, which was armed with a new and more powerful 88mm gun. This gun had a longer 71 calibres barrel and fired 10.2kg armour-piercing projectiles with a velocity of 1,000m/s, which was higher than that of projectiles fired from other contemporary tank guns. However, its projectiles did not have as much kinetic energy as those fired by the Red Army’s counter to the Tigers, which was the IS-2 heavy tank. The latter was armed with a 122mm gun, which was an adaptation of a field artillery piece and fired projectiles at 781m/s, but they weighed 25kg, as a result of which they had a muzzle energy of as much as 10.1 mega joules (MJ), compared with 5.1 MJ of the projectiles fired by Tiger II. But, because the projectiles were of a larger calibre, their energy was spread over a greater area of a target they hit and consequently they penetrated less armour. In fact, at a range of 1,000m the 122mm gun of IS-2 penetrated 147mm of armour, whereas the 88mm L/71 penetrated 190mm.10, 11
A marginally higher level of kinetic energy of 10.2 MJ was attained in 1945 by projectiles fired from the 128mm L/55 gun of the German Jagdtiger heavy tank destroyer. Only 77 were built before the war in Europe ended, but they foreshadowed the calibre and the kinetic energy of tank guns adopted several years later, although using other types of ammunition.12
Some of the new types of ammunition had already appeared during the war. One of them was called Armour Piercing Composite Rigid, or APCR. Its projectiles consisted of a sub-calibre penetrator of tungsten carbide, which is harder and more dense than steel, in a light alloy carrier or sabot. The penetrator absorbed most of the energy imparted to the projectile by the gun and concentrated it on a smaller area of the target, which resulted in it perforating thicker armour than conventional projectiles of the same calibre. APCR projectiles were also lighter than the latter, so they had higher muzzle velocities. But because of their lighter weight they lost velocity more rapidly with distance, which resulted in their armour penetration becoming less than that of conventional armour-piercing projectiles at longer ranges.
APCR ammunition was first provided in 1940 for the 37mm gun of PzKpfw III and then for other German tank guns, but its use was restricted by shortages of tungsten. Towards the end of the war, APCR ammunition was also produced in the United States for the 76mm guns of the Sherman tanks re-armed with them and for the 90mm guns of the M26 Pershing.
Another type of ammunition to make its debut during the Second World War was Armour Piercing Discarding Sabot, or APDS. This began to be developed in France on the eve of the war by the Brandt armament company, and when France was defeated in 1940 the work on it was transferred to Britain, where it was successfully completed at the Fort Halstead research establishment. Like APCR, it consisted of sub-calibre tungsten carbide penetrators in light alloy sabots, but the latter were discarded at the muzzle so that the penetrators flew to their targets by themselves and, as they suffered less aerodynamic drag, their armour penetration decreased far less with range. APDS ammunition was originally produced for the 57mm guns mounted in Churchill tanks as well as 6-pounder anti-tank guns, and then for the 76mm 17-pounder guns mounted in the A.30 Challenger and in the re-armed Sherman tanks of the British Army, all of which took part in the 1944 Normandy campaign. The APDS projectiles of the 17-pounder had a marginally higher muzzle velocity of 1,200m/s than that of the APCR projectiles of the German 88mm L/71 and could penetrate 187mm of armour at 1,000m.
The third of the new types of ammunition to appear during the Second World War was called High Explosive Anti-Tank, or HEAT, and, more precisely, shaped charge ammunition. Unlike the others it did not rely for defeating armour on the kinetic energy of the projectiles, but on the impact of a very high-velocity small diameter copper jet formed by the collapse of a copper-lined conical cavity in the nose of a high explosive projectile. This type of ammunition was first provided in 1941 for the 75mm L/24 gun of the PzKpfw IV, and it could only perforate 80mm of armour because the projectiles were spun like all others by the rifling of the gun, which interfered with the formation of the copper jet. However, the performance of the 75mm shaped charge projectiles was at least superior to that of the standard armour-piercing projectiles fired from the short-barrelled 75mm L/24 gun, and they were used with some success against Soviet tanks. But shaped charge ammunition did not come into its own until several years later, when its projectiles were no longer spun but were stabilized by fins.
In the immediate post-war period, the US Army continued to arm its medium tanks with 90mm guns that fired full calibre armour-piercing projectiles as their primary anti-tank ammunition. But this was complemented by fin-stabilized HEAT ammunition when slipping driving bands were developed and minimized the spinning of the HEAT projectiles by the rifling of the guns. The French Army adopted an ingenious alternative to minimizing the rotation of shaped charges by mounting them on ball bearings within the spinning projectile body. This approach was incorporated in the Obus G or Gessner projectile, which was adopted as the only anti-tank ammunition of the 105mm gun developed in the 1950s for the AMX 30 tank. Obus G was less effective in relation to its calibre than fin-stabilized 105mm HEAT projectiles because of the smaller diameter of its shaped charge, but it could still perforate 360mm of armour, which was considerably more than conventional armour-piercing projectiles did.
The perforation capabilities of shaped charge projectiles led to them being regarded not only by the French but also by the US Army as the most effective type of anti-tank ammunition. The high opinion of shaped charges contributed to the recommendation made in 1957 by the US Army ARCOV (Armament for Future Tanks and Similar Combat Vehicles) committee that future tanks should be armed with guided missiles, which would depend of course on shaped charge warheads for defeating armour. This led to the development of the Shillelagh missile system and its installation in the M60A2 battle tank as well as the M551 Sheridan light tank. The missile was fired from a 152mm gun launcher, which could also fire more conventional ammunition except for high-velocity armour-piercing projectiles as it was only 17.5 calibres long. It was only in 1967 that the 152mm gun launcher was developed, largely in response to German demands in the context of the US-German MT-70 programme, into the XM152 version, which was 30.5 calibres long and could fire high-velocity projectiles. However, the XM152 gun launcher was abandoned in 1971 together with the MBT-70 tank in which it was to be mounted. The same fate befell the short-barrelled version, in spite of the Shillelagh missile being able to perforate as much as 690mm of armour, which was more than enough to defeat any contemporary tank.
A somewhat similar system consisting of the ACRA guided missile and a 142mm gun launcher was developed at about the same time in France. It was mounted in a modified version of the AMX 30 tank but was not developed beyond the prototype stage. Only the Soviet Army continued the development of gun launched guided missiles, which it started in 1962 or 1963 and which resulted in a whole series of missiles – mostly laser beam-riders – that were fired from guns ranging from the 100mm gun of the T-55 to the 125mm gun of the T-90.
In contrast, the British Army did not develop any projectiles or gun launched missiles with shaped charge warheads, largely because of doubts about their lethality. Instead it concentrated for more than a decade after the Second World War on the development of a series of guns firing APDS ammunition. The first of them was the 83.8mm 20-pounder, which was introduced in 1948 on the Centurion tanks. It fired APDS projectiles with a muzzle velocity of 1,465m/s, which was higher than that of any type of tank gun produced until then and which greatly increased their armour penetration. Then, as the thickness of tank armour continued to increase, the demand grew for guns with even greater armour piercing capability, and in response to it the 20-pounder was developed into the 105mm L7 gun, initially by boring out from 83.8 to 105mm! As already indicated in Chapter 9, the armour-piercing capability of the 105mm L7 with its APDS ammunition was such that it became almost the Western world’s standard tank gun. Its performance is illustrated by its ability to perforate a shot line thickness of 240mm of armour inclined at 60° and at a range of 1,830m.
The performance of the 105mm L7 gun was such that it made redundant the Conqueror heavy tank, which the British Army developed during the 1950s, because it approached that of the latter’s 120mm gun. The Conqueror also fired APDS, but instead of the complementary high explosive ammunition that was usual at the time it fired a novel type called High Explosive Squash Head or HESH. The projectiles of this ammunition were filled with a plastic explosive that was squashed on impact against the armour before it exploded, causing spalling of lethal metal scabs from its inner surface. The British Army regarded HESH more highly than shaped charge ammunition and provided no other anti-armour ammunition for light armoured vehicles, such as the Scorpion light tank. HESH was also going to be the only anti-tank ammunition fired by the FV 215 heavy tank, which began to be developed in 1950 and which was going to be armed with a 183mm gun. The tank did not advance beyond a full size wooden mock-up, but the 183mm gun, which was the world’s largest calibre tank gun ever made, was built and successfully test fired from a Centurion chassis.13
Although APDS outperformed the original armour-piercing ammunition, it was itself outperformed in time by Armour Piercing, Fin Stabilized, Discarding Sabot or APFSDS ammunition. This resembled APDS but its penetrators were much longer and of smaller diameter, which meant that their kinetic energy was concentrated on a smaller area of the target and as a result they penetrated more of the armour. However, because their length to diameter ratio was more than about 5, they could not be spin stabilized like other projectiles but had to be stabilized by means of fins. In consequence they were fired from smooth bore guns, although they could also be fired from rifled guns with the aid of slipping driving bands.
The development of fin-stabilized projectiles began in Germany during the Second World War, but it was only taken up in earnest in the 1950s in the United States and in the Soviet Union. In the United States a decision was taken in 1954 to build 90 and 105mm smooth bore guns, but neither was successfully developed. They were followed by a very promising smooth bore 120mm Delta gun, but this was abandoned around 1961 after the US Army decided to arm its tanks with guided missiles instead of high-velocity guns.
The Soviet Army was more persistent. It started to develop smooth bore guns by 1958 and adopted one of 115mm calibre, mounting it in the T-62 tank that went into production in 1962. The APFSDS projectiles fired by the T-62 looked like a scaled down version of the Peenemunde Arrow Projectiles that were being developed in Germany during the Second World War for long range artillery.14 Their penetrators were only of steel, but they were fired with a muzzle velocity of 1,615m/s and were capable of penetrating 240mm of armour at 1,900m, which made them as good in this respect as the contemporary 105mm APDS projectiles.
The next Soviet tank was at first also armed with a smooth bore 115mm gun, but its improved T-64A version, which succeeded it in 1964, was armed with a new 125mm smooth bore gun. Similar guns subsequently armed other Soviet tanks, from the T-72 of 1973 to the T-90 of 1990. During this period, the effectiveness of these guns was gradually increased by the provision of improved APFSDS ammunition with tungsten alloy or depleted uranium penetrators and saddle or spool type sabots similar to those adopted by the Western armies.
In contrast, no smooth bore gun was adopted by the US Army until 1981. But in the meantime it made considerable progress in the development of APFSDS ammunition fired from rifled guns, thanks to the use of slipping driving bands. This began with the APFSDS fired from the 152mm gun launcher of the ill-fated MBT-70 and came to fruition with the M735 APFSDS developed for the US M68 version of the 105mm L7 gun, which put new life into that widely used gun and led to its adoption for the US M1 tank.
While Germany was still participating in the MBT-70 programme, work began there on a fall-back solution, and this led in 1971 to the building of experimental tanks armed with newly developed 105 and 120mm smooth bore guns. The tank with the 120mm gun was eventually adopted as the Leopard 2, which was accepted for production in 1977 and the first of which was handed over to the German Army two years later. After it was adopted for the Leopard 2, the 120mm L/44 gun developed by Rheinmetall was also adopted for the US M1A1 and then for the Italian Ariete, the Israeli Merkava, the Japanese Type 90 and the South Korean K1A1, spreading its use around the world. With the subsequent acquisition by several other countries of the Leopard 2 from German and other surplus stocks, the use of the 120mm L/44 gun spread even further and it became the standard tank gun of the Western World, just as the 105mm L7 had been. The British Ministry of Defence continued for a time to champion the use of rifled guns and APDS, but in the end it had to accept the superiority of APFSDS and procured it in the mid-1980s. However, it failed to replace the rifled gun of the Challenger by a smooth bore gun, relying on slipping driving bands for firing fin-stabilized APFSDS.
The effectiveness of the 120mm L/44 gun and of its clones has been increased over the years by the development of APFSDS ammunition with progressively longer penetrators able to penetrate more armour because penetrations at their impact velocities was roughly equal to the length of the penetrator.15 In fact, the length to diameter ratio of the penetrators increased over the years from about 10:1 to 32:1. The armour-piercing capabilities of the Rheinmetall gun were also raised by increasing the length of its barrel from 44 to 55 calibres, which was accompanied by an increase in the muzzle velocity from 1,650 to 1,750m/s and in the muzzle energy of the APFSDS projectiles from 9.8 to 12.5 MJ.
The progressive increases in the calibre of tank guns to 120 or 125mm raised the weight of their ammunition to a level at which it was difficult to manhandle. For example, a round of the traditional armour-piercing ammunition fired by the 120mm gun of the US M103 heavy tank weighed 48.8kg, which called for two loaders to handle it, increasing the crew of the turret from three to four men and consequently increasing the size of the turret. The Red Army avoided a similar problem when it armed the IS-2 with a 122mm gun by the use of ammunition with separate projectiles and propellant charges. This halved the load that had to be handled, but reduced the speed with which the gun could be re-loaded and therefore the rate of fire. The British Army also opted for separated ammunition when it developed the Conqueror heavy tank and then the Chieftain and Challenger, in which the handling of the ammunition was made easier by the lighter weight of the APDS or APFSDS projectiles and the propellant being in bags instead of the traditional brass cases. In Germany Rheinmetall followed the lead established in the United States by the 120mm Delta gun and developed one-piece ammunition with combustible cartridge cases instead of the traditional, heavy brass cases. This reduced the weight of a 120mm APFSDS round to a manageable 18 or 19kg and that of the complementary HEAT/Multi-purpose ammunition to 24kg.
The alternative to the manual loading of tank guns has been the use of powered automatic loading systems. This eliminated the restriction on the weight of the ammunition imposed by human strength, but in most cases has been adopted mainly to facilitate firing on the move and to reduce the size of tanks by eliminating the human loader.
Development of automatic loading systems began soon after the Second World War when the AMX 50 heavy tank was built in France with one located in the turret bustle of its oscillating turret, which lent itself particularly well to it. The AMX 50 was not developed beyond prototypes, but a bustle-mounted automatic loading system was installed in the oscillating turret of the AMX 13 light tank that was produced and widely used from 1950 onwards. AMX 13 also inspired the construction in the United States of experimental tanks with automatic loading systems, and a bustle-mounted automatic loading system was also incorporated in the US-German MBT-70. However, except for the AMX 13 and the Swedish S-tank, automatic loading systems did not begin to come into use until they were incorporated in Soviet tanks, starting in the 1960s with the T-64 and following with the T-72, T-80 and T-90 and similar Chinese tanks. All their automatic loading systems have been of the carousel type mounted under the turret. In contrast, when automatic loading systems were eventually adopted elsewhere, they were all mounted in the turret bustles. The first of them was mounted in the Japanese Type 90, which was followed by the French Leclerc and then by the South Korean K-2 and the Japanese Type 10.
A general use of automatic loading systems would have followed if the studies begun in 1982 had led to the adoption of 140mm guns as successors of the 120mm guns because their ammunition was far too large and too heavy to handle, a typical round weighing 38kg and being 1.5m long overall.16 Guns of 140mm were the subject of an agreement reached in 1988 between Britain, France, Germany and the United States about an interoperable tank gun, or Future Tank Main Armament (FTMA), and prototypes of such a gun were built and fired by 1992. The muzzle energy of their APFSDS projectile reached 23 MJ, or almost twice the muzzle energy of the most powerful of the projectiles of the 120mm guns, but as the Soviet threat receded the development of the FTMA was abandoned. What its performance might have been is indicated by a 140mm smooth bore gun that was built in Switzerland and that in 1999 perforated 1,000mm of armour firing APFSDS projectiles with a 900mm long penetrator.17
The problem of handling the increasingly heavy and large charges of solid propellant that accompanied the increases in the calibre of tank guns was thought at one time to be avoidable by the use of liquid propellants that could be piped directly from their containers into the gun. At the same time, the greater density of the liquid propellants and the relative freedom of the shape and location of their containers appeared to offer significant reductions in the internal volume of tanks and therefore in their size.
The development of liquid propellant, or LP, guns started in the late 1940s in the United States, following the lead taken in Germany during the Second World War in the application of liquid propellants to rocket propulsion. It led to an experimental 90mm LP gun that was tested in 1951, but the results obtained with it and other LP guns of up to 120mm calibre showed that their interior ballistics were inconsistent and that liquid propulsion offered no advantage in performance over solid propellants. Moreover, the propellants used in the early LP guns were highly corrosive and toxic and therefore required special handling. It is not surprising therefore that after another catastrophic explosion in the mid-1960s work on LP guns in the United States came to an end.
A similar fate overtook the work on LP guns in Britain. Started in 1952, it led a year later to the construction of an experimental gun based on the contemporary 83.8mm tank gun. This gun used red fuming nitric acid, which was highly corrosive, as part of its propellant mix, and which alone should have dispelled contemporary ideas about LP guns arming future tanks. Nevertheless, further work was carried out using 76mm LP guns and in a fit of remarkable optimism the use of an LP gun was included in the studies that led to the design of the Chieftain tank. But, as in the United States, LP guns failed to come up to expectations and work on them in Britain was terminated in 1957.18
There was little further interest in LP guns until the early 1970s, when it revived in the United States as the result of the development by the US Navy of a new liquid monopropellant for torpedoes, which in addition to a relatively high density exhibited low toxicity, low flammability and low susceptibility to detonation. An attempt was made during the mid-1970s to exploit the properties of this new monopropellant in a bulk-loaded high-velocity 75mm LP gun, which formed part of the US High Mobility Agility Program. But this initiative again ran up against the inconsistent ballistics inherent in bulk-loaded LP guns and ended with another catastrophic explosion. Attention then turned to the use of the new monopropellant in LP guns with regenerative injection during the combustion cycle, on which General Electric began to work in the United States in 1973.19 By 1977 this work had progressed to the stage of an experimental 105mm LP gun, and ten years later studies began of the use of LP guns with regenerative injection in tanks. However, these studies were abandoned in 1991 because the US Army came to the conclusion that liquid propellants might be more appropriate to artillery guns than to tanks. In consequence an order was issued for the development of a 155mm LP howitzer, which was duly built and test fired. However, interior ballistics were again a problem and work on LP guns in the United States was terminated in 1996.
The revival of work in the 1970s in the United States on LP guns resulted in renewed interest in them in Britain in 1981. This led in 1987 to the setting up of a research programme aimed at exploring the possible use of LP guns in tanks. However, before the programme got very far interest in Britain in LP guns switched from tanks to artillery and in 1995 all the work on them was abandoned.
Well before the work on LP guns in the United States and in Britain came to an end, it was overshadowed by the emergence of another potential alternative to solid propellant guns, namely electromagnetic or EM guns. These offered the possibility of much higher projectile velocities, which made them particularly attractive as tank armament because this implied greater armour penetration for a given calibre of gun as well as greater probability of hitting moving targets.
EM guns had been proposed before this, but their development only began after some physics experiments carried out around 1970 at the Australian National University where 3g pellets were accelerated to about 6,000m/s, or almost four times the muzzle velocity of contemporary tank guns. In 1978 this prompted a group of US Army physicists to propose an EM launcher research programme, which was accepted and led to work at several US facilities.20 One of them was the Westinghouse Research and Development Center, which in 1983 used a laboratory EM launcher to accelerate a 317gm projectile to 4,200m/s, while five years later Maxwell Laboratories in California used a capacitor-powered 90mm EM launcher to accelerate a 1.08kg projectile to 3,400m/s. This meant that the energy imparted to the projectile by the 90mm EM laboratory launcher had reached 6.2 MJ, and this brought it to the level of the muzzle energy of tank gun projectiles.
The progress made with laboratory EM guns led in 1987 to a design study carried out by FMC Corporation for the US Defense Advanced Research Projects Agency (DARPA) of a tank armed with a 15 MJ EM gun, which concluded that a prototype of such a tank was ‘achievable by 1991’.21 A similar conclusion was reached a year later by another DARPA-sponsored study, which recommended a ‘tank destroyer’ armed with a 11 MJ EM gun and expected its components to be demonstrated by 1992. A contemporary NATO study also opined that tanks armed with EM guns would begin to be produced and come into service in 2000.
Similar optimism existed in Britain, where the Royal Armament Research and Development Establishment (RARDE) proposed in 1987 the construction of a technology demonstrator consisting of an EM gun mounted on a Chieftain tank. What actually happened was the construction at the University of Texas of a self-contained 90mm EM laboratory gun mounted on a skid so that it could be taken to a range for firing trials. In 1993 another 90mm EM laboratory launcher was installed at the UK-US electromagnetic launch facility built at Kirkcudbright in Scotland. Tests carried out with the 90mm EM laboratory guns established that realistic APFSDS projectiles could be launched at up to 2,340m/s.22 But the ‘skid gun’ proved to weigh 25 tonnes, which showed that EM guns were far too heavy as well as being far too large to be mounted in tanks.
However, such evidence did not deter US and UK military planners from considering arming future tanks with EM guns in the late 1990s. In the case of the United States, the tank in question was the Future Combat System, or FCS, which was to come into service in 2012; in the case of the UK, it was the Mobile Direct Fire Equipment Requirement, or MODIFIER, which was to be introduced in 2020. But before these ideas were shown to be unrealistic, the development of FCS and of MODIFIER was abandoned in favour of lighter armoured vehicles, which were heralded by the transformation policy adopted by the US Army in 1999 and which were even less capable of accommodating EM guns. Research work on EM guns continued, but interest in their possible application shifted to warships, in which weight and space were far less restricted, and the prospects of their use in tanks remained remote.
In contrast, the prospects of arming tanks with another type of electric gun, the electro thermal-chemical or ETC gun, were brighter from the start because only part of the energy it used to launch projectiles was electrical, the rest coming from the chemical reaction of a solid or liquid propellant. In consequence, the ETC guns did not require electrical equipment as large and as heavy as the EM guns.
Development of ETC guns was pioneered by GT Devices, a small US company that started firing 20mm ETC guns in 1985 and was subsequently taken over by General Dynamics Land Systems (GDLS). In 1985 FMC Corporation also started work on what it called Combustion Augmented Plasma Guns, in which originally most of the projectile propulsion energy was expected to be electrical but which were in effect ETC guns. The early work on ETC guns was so promising that by the end of 1989 a competitive trial was arranged between 120mm tank guns converted by GDLS and FMC into ETC guns, which were intended to demonstrate that an ETC gun could arm the next version of the US M1 tank. The trial was clearly premature, and proved so disappointing that it led to an equally rash view that ETC guns were less promising than EM guns. This view was reached, among others, by the US Army Science Board, which recommended in 1990 that development funds be diverted from ETC to EM guns.23 Similar views were held in Britain, where RARDE had already shown little enthusiasm for ETC guns.
However, the US Army continued to support research into ETC guns and ordered a 9 MJ 120mm ETC laboratory gun from FMC, which was installed in 1991 and from which projectiles were fired at up to 2,500m/s. Work on ETC guns was also pursued in Germany, where it started in 1987, and resulted in the construction by Rheinmetall of a 105mm ETC gun thatby 1995 fired projectiles at up to 2,400m/s. This was followed by the design of a 120mm ETC gun that began to be used for firing trials in 1999, and by collaboration with France, where another 120mm ETC gun was built by GIAT and started firing trials in 2003.
Since 1986, work on ETC guns has also been pursued in Israel at the Soreq Nuclear Research Centre, which pioneered the use of solid propellants as the source of the chemical part of the projectile propulsion energy instead of the liquid or slurry propellants used originally by FMC and GDLS. Soreq’s lead was followed by others, and since the early 1990s the development of ETC guns has concentrated on the solid propellant form of them, becoming focused during the 1990s on guns of 120mm calibre.
The object of the development of the solid propellant 120mm ETC guns that was pursued in the United States, Germany and elsewhere became that of making them a potential alternative to 140mm solid propellant guns that were being developed for the defeat of future enemy tanks. In the course of this development, the use of a 120mm ETC gun was considered in the early stages of the US Future Combat Systems programme and in 2004 United Defense LP (originally FMC and now BAE Systems) successfully fired a 120mm ETC gun from a light tank developed from a much modified M8 Armored Gun System. An ETC gun was also included in the plans for a new family of armoured vehicles that were drawn up in Germany in the late 1990s, and by 2002 Rheinmetall demonstrated a 120mm ETC capable of generating 30 per cent more muzzle energy than the 120mm solid propellant gun on which it was based.24
However, even though 120mm ETC guns were considered capable of firing projectiles with a muzzle energy of 15 MJ, their performance still fell short of that of 140mm solid propellant guns, which could fire projectiles with an energy of 18 to 23 MJ and at the same time enjoyed the advantages of being based on well proven technology.
Over the years tanks have faced a number of weapons that have posed an increasingly severe threat to them and have consequently called for progressive increases in the thickness of their armour and for the development of other forms of protection.
To start with, the armour of tanks was very modest, the maximum thickness of that of the original British tanks being only 12mm.1 This was sufficient to resist ordinary rifle bullets but not thick enough to provide protection against steel-cored ammunition fired by machine guns. The armour was of naval origin, as there was virtually no other at the time than that produced for navies.2 It was a nickel chrome steel alloy, plates of which were heat treated to provide them with a high degree of hardness for the defeat of bullets.3 From the latter part of the First World War to the early 1930s, tank armour was generally face hardened by carburizing to make it better able to resist penetration. Such armour was too hard to be machined or drilled after heat treatment so that any machining had to be done before the latter, and it could only be assembled by being bolted or riveted on to angle-iron frames, which became a feature of the early tanks.
Although it had its ballistic advantages, face hardened armour was difficult to produce and its use was abandoned in the 1930s in favour of homogeneous machineable quality armour. However, this did not bring to an end the assembly of armour by riveting or bolting, which continued to be used well into the Second World War, particularly in Britain, Italy and Japan. Elsewhere homogeneous armour was by then already assembled directly by electric arc welding. The change from riveting to welding began in 1934 or 1935 when modified versions of the Soviet BT and T-26 tanks started to be produced using welding technology acquired from Germany, where welding was also used as soon as tanks began to be produced in 1934.4 Other countries followed six or seven years later.
By the mid-1930s an alternative had been developed to the fabrication of tank turrets and hulls from armour plates, which involved casting them. The use of castings was pioneered in France, where cast turrets were already produced during the First World War for some of the Renault FT light tanks and where one-man cast turrets were adopted during the 1930s for most light and all medium and heavy tanks. Larger three-man cast turrets were subsequently adopted for British, Soviet and US tanks, starting in 1939 with the British Matilda infantry tank. Castings were also used for the production of parts of hulls, starting in the 1930s with the French R 35 light tank. During the Second World War the whole of the upper part of the M4A1 version of the US Sherman medium tank was cast in one piece, and in the 1950s the entire hulls of the US M48 and M103 tanks were cast, as were the hulls of the Swiss Pz.61 and Pz.68 tanks. In general, the ballistic properties of cast armour were slightly lower than those of rolled plates, but casting lent itself better to the production of complex shapes, as a result of which most turrets came to be cast.
The beginning of the use of castings coincided with and contributed to a general increase in the thickness of tank armour. Although some of the armour of the Renault FT was already 22mm thick and that of the multi-turreted Independent was 25mm thick, the armour of most other tanks was for many years thinner.5 In fact, the armour of the influential Vickers Medium Mark I was only 6mm thick, and the maximum thickness of armour of most other tanks was 14 or 15mm, which included the original versions of the British cruiser tanks and of the German PzKpfw III and IV. But in the early stages of the Second World War, the maximum thickness rose to at least 30mm in the case of the more mobile tanks and 75 or 78mm in the case of the British Matilda and the Soviet KV-1 heavy tank. It then continued to increase further, reaching by the end of the war 100mm in the case of the German Panther and 120 and 180mm respectively in the case of the Soviet IS-2 and German Tiger II heavy tanks.
Some thicker armour was incorporated in tanks designed after the Second World War, bringing it up to a maximum of 200mm. However, such armour was confined to the front of tank turrets. When inclined at 60º or more from the vertical it had a horizontal shot line thickness of about 400mm, which implied an areal density of more than 3 tonnes per square metre of the area of the tank normal to the direction of attack. Significant increases in the thickness of armour were not practicable because of the consequent increases in the weight of tanks and hence a reduction in their mobility.
Moreover, increasing the thickness of homogeneous steel armour became less profitable as a result of the development of shaped charge weapons, against which it was less effective than against the armour-piercing projectiles of high-velocity guns.
This was brought out particularly clearly by the Panzerfaust anti-tank grenades with shaped charges that were used by the German infantry in the closing stages of the Second World War and that could penetrate up to 200mm of steel armour. The threat to tanks of shaped charge weapons was maintained after the war by rocket propelled anti-tank grenade launchers, like the US 3.5in. M20 ‘bazooka’, which could penetrate 280mm of armour. But it did not emerge in full until the appearance of anti-tank guided missiles, which began to be developed in Germany towards the end of the war.6 Their development was continued after the war in France and in the first instance resulted in the SS-10 guided missile, which had a warhead with a diameter of 165mm and could penetrate 400mm of armour. The SS-10 came into service with the French Army in 1953, but it was first used in action by the Israeli forces during the 1956 Sinai campaign.
The penetration capability of the SS-10’s successor, the SS-11 that was adopted by several countries, rose to 600mm, which was clearly more than any practicable thickness of steel armour. There was a need therefore to develop alternative ways of protecting tanks against shaped charge weapons. The search for the alternatives began in 1952 in the United States, where it was found that glass could be twice as effective in relation to its weight as steel armour in resisting the penetration of shaped charge jets. This led to the development of ‘siliceous armour’, which consisted of fused silica glass encased in steel that was successfully trialed as part of the US T95 tank programme. In 1958 it was subsequently proposed to incorporate it in the M60 tank, which was then being developed, but it was not adopted.7
A somewhat similar solution to the problem was pursued in the Soviet Union when the T-64 tank began to be developed in 1962, which was provided with frontal hull armour consisting of two thick layers of a glass fibre composite sandwiched between steel plates. A similar type of composite armour with a high glass content was subsequently adopted in the T-72 and other Soviet tanks.8
On the other hand, siliceous armour was no longer considered in the United States when the M1 tank began to be developed in 1972. Instead, what was initially considered in its design were arrays of spaced plates of steel and aluminium, which were expected to defeat shaped charge jets by eroding them in stages instead of defeating them by the properties of the armour materials. As it happens, arrays of metallic plates were not adopted for the US M1, but were retrofitted to Soviet T-55 tanks.9
As it proceeded with the development of the M1 tank, the US Army became aware of and decided to adopt a new type of armour developed in Britain called Chobham armour, as already described in Chapter 9.10 Chobham armour was developed at the Fighting Vehicles Research and Development Establishment of the British Ministry of Defence by G. N. Harvey and J. P. Downey from the basis of a research programme initiated in 1963, and was successfully incorporated for the first time in a Chieftain-based experimental tank designated FV 4211, which was built in 1971. It proved to be more than twice as effective against shaped charges as steel armour in relation to its weight, and when its existence became known it did much to restore the faith in tanks, which had been shaken by the grossly exaggerated claims about the vulnerability of tanks to anti-tank guided missiles that arose out of the 1973 Arab-Israeli War. The nature of Chobham armour has been kept secret by the British Ministry of Defence, although it has been succeeded by another type of armour called Dorchester, and in spite of it being obviously some form of spaced armour incorporating non-metallic materials as well as steel.
However, there is no secret about armour developed against shaped charges, which consists of an array of spaced sandwiches of steel plates with a rubber interlayer. When a sandwich is struck obliquely by a shaped charge jet the rubber expands, causing the plates to bulge and to move apart, interfering thereby with the jet, and if there are enough of the sandwiches arranged behind each other, ultimately breaking it up. Because of the way in which the sandwich plates deform, this type of armour is often referred to as ‘bulging armour’, and was described as early as 1973 in a patent applied for by M. Held.11 It has been incorporated subsequently in tanks such as the Soviet T-72M, which began to be produced around 1980 and which contained an array of 20 spaced steel and rubber sandwiches in each of two cavities in the front of its cast turret.12
Some of the armours devised for protection against shaped charges incorporate layers of ceramics, such as aluminium oxide and silicon carbide. Ceramics first came into use as armour materials in the late 1960s in panels made to protect US helicopter pilots against bullets during the war in Vietnam. By the early 1970s ceramics were also recognized as being twice as effective in relation to their weight as steel against shaped charge jets.13 In consequence, they have been incorporated since then in a number of armour systems to erode the jets or the long-rod penetrators of APFSDS projectiles and to absorb their kinetic energy.
Ceramics have also been used to enhance the protection of light tanks and other light armoured vehicles against rifle and heavy machine gun bullets. In this case, their function has been to shatter the bullets by virtue of their greater hardness, and they have been used in the form of relatively thin tiles assembled into panels mounted on the outside of the basic metallic armour of the vehicles. Early examples of this were the Canadian M113 and the Swedish Pbv 302 armoured carriers that were deployed in support of the peace-keeping operations in Bosnia in the mid-1990s.
The ballistic protection of some light armoured vehicles has also been increased by the addition of a type of armour originally introduced in 1943 on German tanks and assault guns to increase the protection of their sides against Russian 14.5mm anti-tank rifles. It consisted of thin steel plates mounted some distance in front of the vehicles’ armour, which offered little resistance to the attacking bullets but tipped them so that as they struck the armour yawed and therefore hit less effectively. The use of this type of ‘tipping’ armour was revived in 1970 when it was adopted in the United States for a derivative of the M113 armoured carrier called the Armored Infantry Fighting Vehicle that was produced for the Dutch, Belgian and Eqyptian armies and was also produced in Turkey, as well as South Korea.14
The spaced-off tipping type of armour was developed further in Israel by the Rafael organization, who replaced the thin homogeneous steel plates by high hardness steel plates perforated by holes somewhat smaller than the diameter of the attacking bullets, which reduced their weight to one half of that of the equivalent solid plates and increased their ability to tip the attacking bullets. Called TOGA, the perforated plate armour was introduced on Israeli operated M113 carriers around 1985 and has been used since on other armoured vehicles, including some light tanks.
However, from the 1980s onwards the most common method of increasing the ballistic protection of light armoured vehicles has been to bolt on plates of high-hardness steel on to their steel or aluminium hulls, or of titanium on to aluminium hulls. An example of this has been the M2A2 version of the US Bradley Infantry Fighting Vehicle, which around 1986 had its original tipping armour consisting of two spaced 6mm steel plates replaced by a single 32mm thick appliqué armour plate.15
A very different type of armour appeared on Israeli M60 and Centurion tanks during the 1982 Israeli invasion of the Lebanon. This was explosive reactive armour, or ERA, which was devised by M. Held from the basis of the studies he carried out in 1969 in Israel on behalf of the Messerschmitt-Bolkow-Blohm missile company on the effects of shaped charge hits on the tanks disabled two years earlier during the Six Day Arab-Israeli War. Held patented his ideas in 1970 and they were subsequently put into effect in Israel by the Rafael organization in the form of the Blazer explosive reactive armour.16
In essence, ERA consists of sandwiches of two steel plates with an explosive interlayer, which is set off when a sandwich is penetrated by a shaped charge jet and which, when the plates are at an angle to the jet, drives the plates apart into its path, disturbing or disrupting it. Originally the plates were only 2 or 3mm thick, but when the sandwiches incorporating them were at an angle to the jet, as they had to be, they could still reduce its penetration of armour by as much as 70 per cent.
The appearance of ERA on Israeli tanks was followed by its large scale installation on Soviet tanks, starting in 1983 with T-64BV, as already described in Chapter 9. Having decided to use ERA, the Soviet Army took the lead in developing a heavy version of it with sandwich plates of 15mm or greater thickness, which were effective not only against shaped charge jets but also against the long-rod penetrators of APFSDS projectiles. The Soviet Army also took the lead in the development of tandem ERA consisting of pairs of sandwiches separated by an air gap, which was considerably more effective than the original type of ERA against single shaped charges. Tandem ERA could also defeat warheads with tandem shaped charges that incorporated a precursor charge designed to clear any single ERA sandwich out of the way of the main charge. An example of such tandem ERA described in a Russian journal incorporated an outer light ERA sandwich followed by a layer of a damping material and a sandwich of heavy ERA.17 This, together with a tank’s steel armour, was claimed to be capable of defeating the tandem warhead of the US AGM-114F Hellfire guided missile, which has a diameter of 178mm and is thought to be capable of penetrating up to about 1,500mm of armour.
What emerged out of all the development of armour was a trend towards the use of multi-layered protection systems combining several different types of armour. Thus the outer layer of armour might consist of very steeply sloped thin high-hardness steel, which would fracture penetrators striking it or at least throw them to some degree off their trajectory. Examples of this are the sharply pointed noses of the turrets of several tanks modified during the 1990s, including the German Leopard 2A5 and the Chinese Type 99. The nose armour might be followed by tandem ERA to break up long-rod penetrators or disrupt shaped charge jets, and then by the tank’s main armour, which could incorporate ceramics and which would absorb the kinetic energy of penetrator fragments or of jet particles. The effectiveness of some tanks’ frontal armour that has been developed has been estimated to be equivalent to as much as 900mm of steel armour against kinetic energy projectiles and to well over 1,000mm of armour against shaped charges.
After its successful introduction on tanks, the use of ERA was extended to lighter armoured vehicles. This initially created problems because lighter vehicles did not, unlike tanks, have armour thick enough to absorb the front part of a shaped charge jet, which inevitably passes through an ERA sandwich before it is set off, and because the flying rear plate of a sandwich could damage thin armour. To overcome these problems, Rafael developed a hybrid ERA by backing an explosive sandwich with an elastomer and another steel plate.18 This reduced the impact of the ERA on the host vehicle and provided additional resistance to bullets.
The use of ERA on armoured vehicles other than tanks was already being considered in the 1980s but it was not generally implemented until the following decade, partly because there was no urgent requirement for it and partly because of concern about the collateral damage that it could cause. Thus when the second generation of the US M2 Bradley infantry fighting vehicle was being developed in the 1980s, only a part of the fleet was fitted for, but not with, ERA. However, after the US forces invaded Iraq in 2003 hybrid ERA became standard on the Bradleys and it was also fitted to some of the Israeli M113 carriers. Subsequently the British Ministry of Defence was persuaded to have it fitted also to the Bulldog, the modernized version of the FV 432 armoured carrier, and the Warrior infantry fighting vehicle.
Hybrid ERA provided a badly needed response to the extensive use in Iraq of RPG-7 rocket propelled anti-tank grenades by the fedayeen or militants. The situation that had arisen in Iraq in 2003 also revived the use by the US Army of another form of protection against RPG-7 grenades, which was simpler and cheaper than ERA but which was only partially effective against them. It consisted of horizontal steel slats set apart at less than the diameter of the RPG-7 grenades so that one side or the other of a grenade’s nose would hit a slat as it flew between the slats and would be crushed, thereby short-circuiting its fuse and preventing detonation of the grenade. However, some grenades are bound to hit the edges of the slats with their nose impact fuse and thus to detonate. The probability of this happening is such that slat armour is only effective at most against about 60 per cent of the hits.
A form of slat or the very similar bar armour was first used in the 1960s by the US Navy on the gun boats that it operated in the Mekong delta during the Vietnam War.19 It was also used by the Soviet Army in Afghanistan in the 1980s and in Chechnya in 1995 on T-62 tanks, and it was also fitted to the turrets of some Chinese-built Type 69 tanks used by the Iraqi Army in 1991 during the First Gulf War. The US Army developed bar armour for its M113 carriers as early as 1966 but did not start using it until 2003, immediately after the invasion of Iraq, when it came up against the widespread use of RPG-7 by the Iraqi fedayeen.20 Slat armour then began to be used widely not only by the US Army but also by others, including the British Army. Nevertheless, in 2005 the British Ministry of Defence still considered slat armour as something new and regarded a contemporary article about it as revealing secrets.21
Slat armour originally fitted by the US Army to its Stryker eight-wheeled armoured carriers weighed 2,231kg, or about as much as a suite of hybrid ERA, which constituted an undesirable increase in their weight. It was consequently followed by the development of several lighter alternatives, including L-Rod armour developed by BAE Systems in which steel slats were replaced by bars of high strength aluminium and which had half the weight of the original type. An even lighter version was developed in Switzerland by RUAG using a diamond-patterned mesh of very high strength steel wire, and still lower weights have been achieved with fibre net systems, such as RPGNets developed in the United States or Tarian developed in Britain, which squash the noses of the grenades that become enmeshed in them.
The quest for ballistic protection that would be more effective in relation to its weight than steel led several years earlier to the use of aluminium armour. This began to be developed in the United States around 1956 and three years later the US Army ordered the production of the M113 armoured carrier, which became the first aluminium armoured vehicle to be produced in quantity and subsequently the most numerous tracked armoured vehicle to be built outside the Soviet Union. Britain, France, Italy and South Korea followed the example of the United States and produced aluminium armoured infantry fighting vehicles, like the US M2 Bradley, of up to 20 and eventually 30 tonnes. On the other hand, Germany, Sweden and Singapore built similar vehicles of steel armour. In spite of the lower density of aluminium armour, there has been little to choose between vehicles with the two kinds of armour so far as their weight is concerned, but those of aluminium armour have been somewhat easier to manufacture and are structurally stiffer because their walls have to be thicker for a similar level of ballistic protection.
The structural stiffness of aluminium armour hulls makes them particularly attractive where most of the ballistic protection comes from other materials, such as high-hardness steel or ceramic tiles, which are structurally parasitic. This was also the case with the Chobham armour of FV 4211, which was designed with a hull of aluminium armour, relying on the Chobham armour packs for most of the ballistic performance. But the combination of Chobham armour with aluminium armour was not considered entirely satisfactory and it was adopted for the hull of only one other tank, the 43-tonne Vickers Valiant designed for export by Vickers Defence Systems in 1977 but not developed beyond the prototype stage.22 Some light tanks, such as the US M551 Sheridan and the British Alvis Scorpion, have also had hulls of aluminium armour, but the levels of protection they were expected to provide were very much lower than that of FV 4211 and Vickers Valiant.
Interest in the possible alternatives to steel extended at one time beyond aluminium armour even to composite materials made of resin bonded glass fibres. The latter began to be considered by the US Army Materials Technology Laboratory in 1976 and attracted the interest of the US Marine Corps, which in 1983 ordered two M113-type armoured carriers to be made with composite hulls. When one of them was tested, it was adjudged to be superior to the standard aluminium hulled carriers, which encouraged the US Army to order a composite armour analogue of the larger 22-tonne aluminium armoured Bradley infantry fighting vehicle. This was completed by FMC Corporation in 1989, when the writer was able to examine its construction.23
The hull of what became known as the Composite Infantry Fighting Vehicle or CIFV was made of high strength aerospace quality S-2 glass fibres bonded by a thermosetting polyester resin. The laminate that made up its walls contained as much as 68 per cent of glass by weight and was superior ballistically to the aluminium armour of the M113 carriers. CIFV was fitted with the standard turret as well as the engine, transmission and suspension of the Bradley and successfully completed a 6,000 mile automotive test programme, which encouraged further work in the United States on composite hulled armoured vehicles.
One sequel to it was the construction in 1993 of a Heavy Composite Hull, or HCH, which resembled that of the US M1 tank. It was intended to be part of a 45-tonne composite hulled, US tank, but the latter was never built. However, another and more realistic project launched by the US Army in 1993 led to the construction of the Composite Armored Vehicle Advanced Technology Demonstrator or CAV-ATD, a 20-tonne vehicle that might have served as a model for an armoured reconnaissance vehicle but that had no direct follow-up after it was rolled out in 1997.
The incentive to develop composite vehicles was the hope that they would be significantly lighter than conventional vehicles with metallic hulls, and savings in weight of up to 33 per cent were claimed. But, even if this were true, it only applied to hulls, which in general account for only one third of the total weight of an armoured vehicle. In consequence, the overall saving in weight would be only of the order of 10 per cent, and this would hardly justify the adoption of composite armoured vehicles, bearing in mind the problems associated with their production and their considerably higher cost.
Nevertheless, interest in composite armoured vehicles extended beyond the United States. In fact, a study of a composite hull for the Scorpion light tank was carried out in Britain for the Fighting Vehicles Research and Development Establishment as early as the 1960s.24 Nothing came of it, but in 1993 the Defence Research Agency, which succeeded FVRDE, embarked on the development of a composite hulled vehicle of about 22 tonnes to demonstrate the possibility of basing a future reconnaissance vehicle on it. It was called the Advanced Composite Armoured Vehicle Platform or ACAVP, and was completed in 2000, after which it successfully passed extensive automotive trials but, like the US CAV-ATD, it had no successor.
The only composite armoured vehicle to go into production and service has been the CAV 100, which consists of a resin bonded glass fibre body mounted on the chassis of the 3.5-tonne 4x4 Land Rover light truck. More than one thousand CAV 100s were built by Courtaulds Aerospace from 1992 onwards, mainly for use by the British Army in Northern Ireland where it acquired the name ‘Snatch’ because of its use in grabbing rioters. Its composite body provided some protection against small arms, but it proved entirely inadequate, with fatal consequences, when the British Army mistakenly deployed it in the mid-2000s in Iraq and then in Afghanistan, where it was exposed to improvised mines and anti-tank grenades.
The only other large scale and far more effective use of glass fibre composites has been as the intermediate component of the glacis armour of Soviet tanks from the T-64 onwards, which has been mentioned previously. Because of their high glass content, glass fibre composites made a very effective contribution in this case to the frontal protection of tanks against shaped charge weapons.
An entirely different form of protection, particularly against weapons with shaped charge warheads, came to be represented by active protection systems. There are several different types of them, but they all consist of three basic components. One of them is a threat detection system, usually based on millimetre wave radar. Another component is a ‘kill’ system consisting of counter-missiles with blast or fragmentation warheads or of focused blast modules. The third component is a computer-based control system that processes information about the threat and activates the countermeasures.
An active protection system called a Dash-Dot Device, which incorporated radar for threat detection and linear shaped charges as countermeasures, was proposed as early as 1955 in the United States at the Picatinny Arsenal.25 However, actual development of active protection systems did not become evident until the 1980s.26 In fact, in 1983 after six years of development the Soviet Army completed the installation of the Drozd active protection system on a T-55AD tank.27 This pioneer Soviet system consisted of a radar module and a cluster of four launchers of 107mm rockets with fragmentation warheads on each side of a tank’s turret, which formed the countermeasures. Between them they covered a frontal arc of 80º, which would have been sufficient for protection during frontal attacks in open terrain. As it is, some tanks fitted with the Drozd system were employed towards the end of the 1979–89 Soviet occupation of Afghanistan, where according to the system’s developers they defeated 80 per cent of anti-tank grenade attacks.
Elsewhere, during the 1970s and 1980s, attention was focused on simpler ‘soft kill’ protection systems, which were not designed to damage or destroy threat missiles but merely to make them miss their targets. The basic component of such systems were infrared jammers, which interfered with the guidance of anti-tank missiles with semi-automatic command-to-line-of-sight or SACLOS guidance that were perceived at the time to be a major threat to tanks. A ‘soft kill’ defence system based on infrared jammers was deployed on French AMX 30 B2 tanks during the 1991 Gulf War, and at about the same time another such system called Shtora appeared on Russian tanks. The latter also incorporated a laser warning receiver that could trigger smoke grenade launchers to produce smoke screens that would blind laser designated missiles with semi-active guidance.
Further development of the ‘soft kill’ systems exemplified by the MUSS system produced in Germany involved the addition of a missile warning receiver capable of detecting the ultra-violet emissions of missiles’ rocket plumes and consequently of alerting the infrared jammers, which would otherwise have to be switched on continuously when missile attacks were expected and thereby could reveal the tank’s position.
Although ‘soft kill’ active protection systems can prevent some anti-tank missiles from hitting their targets, they are ineffective against others, and in particular against unguided anti-tank rockets, which became the principal threat to tanks by the time Russian forces moved into Chechnya in 1995 and US forces moved into Iraq in 2003, when the scene of operations shifted to urban environments. In consequence the focus of attention began to turn from soft to hard kill active protection systems, which were potentially capable of defeating a much wider range of threats.
An early object of the renewed interest in hard kill active protection systems was the Russian Arena system, which appeared in 1993.28 In addition to radar, Arena was based on the use of fragmentation cassettes launched from a collar-like mounting around the turret of a tank as its kill mechanism so that, unlike Drozd, it provided almost all-round protection and it produced far less risk of collateral damage. However, although it aroused a great deal of interest when it appeared on a T-80 tank, it did not advance beyond experimental installations.
It was only 27 years after the appearance of the Russian Drozd that another hard kill active protection system came into use. This was Trophy, which began to be developed in Israel by Rafael around 1995 and which fired at the threat missiles a beam of small explosively formed penetrators from one of two automatically reloadable launchers mounted at the sides of a tank’s turret. The development of Trophy was accelerated by the 2006 war in the Lebanon, where Israeli forces came up against the powerful Russian-made Kornet (9M133) anti-tank guided missiles acquired by Hezbollah through Syria. In consequence, 100 Trophy systems were ordered in 2007 for installation on Merkava Mark 4s, and a battalion of them was subsequently deployed along the frontier with Gaza, where in March 2011 for the first time Trophy automatically destroyed an anti-tank rocket fired at a Merkava by Palestinian militants.
Several other hard kill systems have been developed since the 1990s, including AWISS developed in Germany by EADS, Iron Fist developed by the Israel military industries and LEDS 150 developed in South Africa by Saab Avitronics. Although they differ from each other in several respects, all these systems have been designed to defeat attacking missiles at some distance from the defended vehicle by launching counter-missiles with fragmentation or blast warheads at them from rapidly traversable two to six tube launchers, which ensured all-round protection.
Hard kill active protection systems have also been developed that do not launch counter-missiles but fire directly at the attacking missiles from the defended vehicles. The Israeli Trophy belongs to this category of active protection systems, but most of them incorporate counter-measures that are distributed around a vehicle and defeat threats close to it by blast. This minimizes the risk of collateral damage, but because of the very short distance at which the threat is attacked requires the system to have a very short reaction time. The principal example of this kind of system is AMAP developed in Germany by IBD Deisenroth Engineering; others include the Iron Curtain developed in the United States by Artis and Zaslon developed in the Ukraine.
In addition to the threat posed by various missiles as well as other anti-tank weapons, tanks have also had to be protected against anti-tank mines. The latter emerged as a threat almost as soon as tanks came into use during the First World War, when in 1918 the German Army began to use mines improvised from artillery shells.29 However, there was little interest in anti-tank mines for some time after the First World War and there was no significant use of them again until the Spanish Civil War of the 1930s. They were also employed by the Finnish Army during the 1939–40 war between Finland and the Soviet Union, but it was only in 1942 that they began to be used extensively by the German Army in North Africa and by the German and Soviet armies in Russia.
The use of mines resulted in as much as 18 per cent of the Allied tank casualties in North Africa and 23 per cent of the casualties in Western Europe in 1944–45. However, much of the damage was confined to the running gear of tanks and was repairable, particularly when tanks had externally mounted suspension units. Moreover, mines were laid to create minefields to restrict the freedom of manoeuvre of armoured formations rather than to destroy tanks. In consequence, considerable effort was devoted during the latter part of the Second World War and for some time afterwards to the development of devices such as flail tanks for the clearing of paths through minefields instead of improving the mine resistance of individual tanks.
The situation changed in the second half of the 20th century when mines became the principal weapons of the insurgents, terrorists and others involved in the various asymmetric wars of that period. The change was brought out by the war in Vietnam, in which as many as 69 per cent of the US armoured vehicle casualties were caused by mines. However, in contrast to the Second World War where the armoured vehicles concerned were mainly tanks, in Vietnam most of the vehicles were lighter and less robust. Moreover, the Vietnamese forces were short of anti-tank weapons other than mines.
The war in Vietnam had little impact on the design of tanks, although it led to the installation of additional steel belly plates in some of the lighter vehicles, such as the US M551 Sheridan light tank.30 The 1979–89 war in Afghanistan in which a number of Soviet tanks were destroyed by mines laid by the mujahedin produced greater repercussions, at least so far as Soviet tanks were concerned. In particular, it led to a number of modifications to them that were later widely adopted elsewhere. Thus to reduce the risk of the driver’s seat being hit by a belly plate bulging under the impact of a mine explosion, T-62 tanks were fitted with an additional outer spaced-off belly plate under the front part of the hull, although this seriously reduced the ground clearance. Then in T-72 and other tanks the risk of the bulging belly plate hitting the driver’s seat was reduced without affecting the ground clearance by suspending the seat from the roof of the hull instead of keeping it fixed as usual to the floor, which disconnected them and lifted the seat well off the floor and the belly plate.
Like the war in Afghanistan, the 1964–79 war in Rhodesia (now Zimbabwe) also involved extensive use of mines but not of tanks.31 However, it led to the development of a new category of mine resistant wheeled armoured vehicles that were developed further with great success in South Africa.32 They included the 4x4 Buffel, 3,500 of which were built and which reduced dramatically the number of casualties caused by terrorist mines, and its successor, the Casspir. Like the Buffel, the 4x4 Casspir had a hull with a blast deflecting V-bottom and, in spite of its relatively light weight of 11 tonnes, was claimed to be able to survive the explosion of three stacked anti-tank mines, or 21kg, of TNT under one of its wheels or of 14kg of TNT under its hull. Since it was first built in 1981, about 2,500 Casspirs have been produced and they were used as armoured personnel carriers in counter-insurgency operations in South West Africa (now Namibia) and elsewhere, with casualties occurring in them due to mine explosions only when they encountered a penetrator mine.
A few South African Mamba mine resistant vehicles derived from the Casspir were procured by the British Army in 1995 for the contemporary peace-keeping operations in Bosnia that came up against widespread use of mines, including Yugoslav TMRP-6 penetrator mines. At about the same time the Krauss-Maffei company began to develop in Germany the 4x4 Dingo mine resistant vehicle, which was to be produced later in quantity.33 However, mines were still not a major concern to US and other NATO forces, and the design of their tanks that dated from the Cold War was focused on protection against horizontal attack by tank guns and anti-tank weapons and not against mines. US and British forces were therefore unprepared for the extensive use of improvised mines by the Iraqi insurgents that followed the invasion and occupation of Iraq in 2003.34
Prior to these events, the usual threat to tanks was considered to consist of industrially produced blast mines with contact fuses that exploded when a tank’s track ran over them,, or less frequentls with tilt rod or magnetic influence fuses that would set off mines not only under tracks but also, and more dangerously, under the bellies of tanks. Worldwide studies carried out in the United States and Germany established that the most common of the industrially produced anti-tank mines contained 7 to 8kg of explosive and the highest level of mine threat specified by NATO was the explosion, of 10kg of TNT under the hull of a vehicle.35
However, many of the blast mines improvised by the Iraqi insurgents weighed more that this. In fact, one of them that wrecked a US M1A2 tank in October 2003 is believed to have contained more than 100kg of explosive. A year earlier, an Israeli Merkava Mark 3 was similarly wrecked on the border of Gaza by a mine containing almost 100kg of explosive detonated by remote control by Palestinian militants, as already mentioned in Chapter 10. Evidently even well-armoured tanks cannot withstand such large mines, but their resistance can be improved, as has been shown by Merkava Mark 4, which has been provided, among others, with a thick additional belly plate of special steel and one of which even survived the explosion of a 150kg mine laid by the Hezbollah during the 2006 war in the Lebanon with the loss of only one crew member.36 What is more, very heavy mines are not easy to plant and although many mines laid by the insurgents have weighed more than 10 kg they have not, in general, weighed much more than about 20kg, which is about as much as an insurgent could carry any distance.
In addition to improvised blast mines, tanks and other armoured vehicles need to have their protection improved against the use of improvised penetrator mines, which was foreshadowed by the appearance of such mines in Southern Africa and Bosnia. Penetrator mines consist of explosive charges with shallow copper-lined cavities that resemble shaped charges but instead of copper jets shoot copper slugs with velocities of up to 2,000m/s, which may be compared to kinetic energy projectiles. Their armour-piercing capability is less than that of the shaped charges of the same size, but it does not fall off as rapidly with distance as that of the latter, which makes them particularly effective as remotely controlled off route mines, and in this role they were used extensively by the Iraqi insurgents.
Mobility is commonly described as one of the major attributes of tanks, but in relation to them it has at least three different connotations.
One of them is strategic mobility, which implies the ability of tanks to be moved over considerable distances by ship, by rail or by road transport to the zone of operations. Such movement has become increasingly difficult as the weight of tanks has increased, and so the latter has had an adverse effect on the strategic mobility of tanks. Strategic mobility has also been hampered by the dimensions of tanks and in particular by their width, which beyond a certain limit can prevent them from being transported by rail without special arrangements. Thus, to avoid this, tanks designed in Britain before the Second World War were less than 2.67m wide to keep them within loading gauge of British railways. On the other hand, the broader gauge of Russian railways allowed Soviet tanks to be 3.32m wide for unrestrained rail movement, which provided greater latitude in their design.
Width restrictions also apply to movement by air, but what has been far more important in this context is the weight of tanks, which has prevented their strategic deployment by the available aircraft. After General Shinseki launched his plan in 1999 to transform the US Army into a strategically more mobile force, it was expected that the Future Combat Systems’ vehicles that were to take the place of tanks would be light enough to be deployed by Lockheed C-130 Hercules aircraft. This meant that they would weigh not more than about 17.5 tonnes. But within a few years of the inception of the FCS programme the realities of combat operations in Iraq led to the inevitable conclusion that, to achieve an adequate level of survivability, the FCS vehicles would have to be better armoured and consequently would weigh well over 20 tonnes, as already mentioned in Chapter 9. They could not, therefore, be transported in C-130 aircraft, which were the only ones available in quantity.
Much heavier tanks have been flown, of course, in aircraft but only in small numbers. An example of this is the 60-odd tonnes Leopard 2 tanks of the Canadian and Danish forces, a few of which were transported, one by one, in 2009 to Afghanistan in Russian-built Antonov 124 aircraft.
Another aspect of the mobility of tanks is their ability to move under their own power, on and off the roads, in the zone of operations but out of contact with the enemy. This, known as the operational mobility of tanks, is related to a large extent to the power of tanks’ engines in relation to their weight, which governs the average speed with which they can move from one area to another. However, the average speed over longer distances also depends to some extent on how often tanks have to stop for refuelling and for maintenance.
Whatever their other characteristics, the operational mobility of tanks has been inferior to that of the corresponding wheeled armoured vehicles whenever operations take place mainly along roads or over relatively dry, hard ground. This has led to attempts to develop ‘wheeled tanks’, but the resulting vehicles have been generally inferior to tanks, mainly because they have had to be lighter for comparable performance off the roads, particularly over soft wet ground, and therefore have been less well protected.
In addition to being an important component of operational mobility, speed is also an important ingredient of the tactical or battlefield mobility of tanks, which is their ability to operate in imminent or actual contact with the enemy. Under such circumstances, tanks need to minimize their exposure to enemy weapons and therefore to move rapidly over different types of terrain, which requires them to exert a sufficiently low ground pressure in the case of soft soils and to have a resilient suspension in the case of hard, rough ground.
Tanks that are fast and agile can also outmanoeuvre enemy forces. All this leads to a strong case for providing tanks with the highest possible power-to-weight ratio. However, in practice the maximum has been of the order of 25 to 30hp per tonne. That of some experimental vehicles has been higher than this but, whatever benefits it offered, it did not justify the cost of achieving it.
Moreover, the automotive characteristics of tanks are not the only constituent of the tactical mobility of tanks. An important contribution is provided by armour protection, which allows tanks to disregard the threat of some weapons, such as small arms, and therefore to move about more freely. In this respect tanks differ significantly from unarmoured weapon platforms, which may have greater operational mobility but have inferior tactical mobility because they can be immobilized by the fire of machine guns and other light weapons that populate the battlefield. Unfortunately, these facts have been frequently disregarded by the mounting of troops in unarmoured vehicles, such as Humvees, Land Rovers and other light trucks.
On the other hand, because of its impact on the weight of tanks, the provision of armour protection has been in conflict with the achievement of a high level of automotive performance. Striking a balance between the two has been difficult to achieve, and in many cases has led to military requirements being deliberately biased in favour of one or the other. For example, greater importance was attached to protection than to mobility in the case of the French light infantry tanks of the 1930s, while the reverse applied to the British cruiser tanks of the same period. Since then several attempts have been made to develop tanks that were significantly more mobile than their predecessors, but in general they have been overtaken by demands for more armour protection, which adversely affected their automotive mobility.
The starting point of the development of the mobility of tanks was the construction in 1916 of the British Mark I heavy tank, which was powered by the only suitable engine available for it at the time, namely a six-cylinder water-cooled petrol engine of 105hp originally produced by the Daimler company for a large-wheeled tractor. This engine provided the Mark I with a power-to-weight ratio of only 3.7hp per tonne and a maximum speed of 3.7mph on hard level ground.
It was soon realized that tanks needed more powerful engines and to meet their need a special six-cylinder 150hp engine was designed by H. R. Ricardo and produced for the Mark V and other British tanks.1 It proved generally satisfactory but it was not powerful enough for the last and heaviest of the rhomboidal tanks, the Anglo-American Mark VIII, which weighed 37 tonnes compared with 29 or 28 of the Mark V. The problem was solved by the adoption of the V-12 Liberty aero engine, which began to be produced at the time in the United States and which developed 300hp. As a result the maximum road speed of the Mark VIII went up to 7mph, which made it about as fast as any tank produced by the end of the First World War.
The adoption of the Liberty engine for the Mark VIII pioneered the use in tanks of aircraft engines and at the same time of engines with a V-12 cylinder configuration, which was to be a feature of the larger tank engines, even when engines of this kind were no longer produced for aircraft. For several years the Liberty engine was also the most powerful engine available for use in tanks and as such provided the basis for the record speed of 42.5mph attained in 1928 in the United States by J. W. Christie with one of his experimental tanks.2 Christie’s example was followed by the Red Army, which adopted the Liberty engine, uprated to 400hp, to power the early models of the BT series of fast tanks and then had it produced in the Soviet Union as the M5 tank engine. One consequence was that the BT-2 tank had a power-to-weight ratio of as much as 35hp per tonne and a maximum speed on tracks of 32.5mph. Nevertheless, an even more powerful engine, the M17, with a capacity of 45.8 instead of the 27 litres of the Liberty engine, was installed in the BT-7, although throttled down to 400hp. However, the same engine was rated at 500hp when it powered the T-28 medium and T-35 heavy tanks and at 680hp when it powered some of the contemporary Soviet aircraft. The M17 was actually a licence-built copy of the German BMW VI, a V-12 water-cooled aircraft engine, a six-cylinder BMW IV forerunner of which powered some of the secretly built German Grosstraktoren.
In the meantime, the British Army opted for tanks powered by engines built again specially for them. The first was a V-8 of 90hp, which was produced by the Armstrong Siddeley company soon after it started building other air-cooled engines for aircraft and which was installed in the Vickers Medium Mark I in 1923. A V-12 air-cooled engine of 370hp was subsequently adopted for the A.1 Independent heavy tank, and V-8 air-cooled engines of 180hp were then installed in the experimental Sixteen Tonners and the Mark III medium tanks of the 1928–34 period. A four-cylinder air-cooled engine of 87hp was also produced by Armstrong Siddeley for the widely used Vickers Armstrongs Six Ton Tank and was copied for the Soviet T-26 tank.
Air-cooled engines were considered to offer several advantages compared with water-cooled engines, including the absence of leaks and the elimination of the risks of the coolant boiling or freezing.3 In consequence they were adopted not only for British but also for American and Japanese tanks.
The use of air-cooled engines in US tanks started with the experimental installation between 1929 and 1931 of six-cylinder Franklin engines in seven US copies of the Renault FT. The results obtained were considered encouraging but the US Army had no money at the time to develop air-cooled, or indeed other, engines specially for tanks. In consequence, it turned to the only air-cooled engines of sufficient power that were available at the time and could be used in tanks, which were radial aircraft engines. The configuration of these engines was far from ideal from the tank point of view, mainly because of their height, but the US Army had no choice and, in spite of their adverse effect on the silhouette of tanks, used them from the early 1930s until the end of the Second World War.
The first model to be powered by an aircraft radial air-cooled engine was an experimental light tank built in 1931 for the US cavalry, which had to resort to the subterfuge of calling it a ‘combat car’ because a Congressional edict made the development of tanks a prerogative of the infantry. The engine installed in it was a seven-cylinder Continental of 156hp, but subsequently the more powerful 250hp Continental R-670 engine was used, from the M1 combat car of 1934 to the M3A3 light tank of 1943. Following their use in light tanks, air-cooled radial engines were also adopted for medium tanks, starting with the M2 of 1939 that was powered by a nine-cylinder Wright engine of 350hp. The same engine but developing 400hp also powered the early versions of the M3 and M4 Sherman medium tanks produced during the Second World War. But there were not enough of them as the production of tanks increased, and to make up for the shortage some of the medium tanks were powered by adaptations of General Motors truck diesels and even of Chrysler car engines. As they were not powerful enough individually, the General Motors diesels were used in twin engine installations that took up more room and required more maintenance but were, nevertheless, used successfully in M3A3, M3A5 and M4A2 medium tanks. Undeterred by the complexity of combining more engines, the US Army adopted as many as five six-cylinder Chrysler car engines assembled in a star configuration to power its M3A4 and M4A4 medium tanks, but was glad to be able to pass most of them to allied armies.
The twin General Motors diesels and the multi-bank Chrysler engines were followed by a V-8 version of a V-12 water-cooled engine designed originally by the Ford Motor Company for aircraft, which became available for tanks and was adopted in 1943 for the M4A3 medium tank. The latter became the most popular model of the M4 tank family and one that continued to be used for two decades after the war. The engine provided the final M4A3E8 model of the series with a power-to-weight ratio of 15.3hp per tonne, which was as high as that of any of the M3 and M4 medium tanks and compensated for the increases in weight that took place during the course of their development and made them weigh in the end 33.65 tonnes.
The 500hp Ford GAA engine also powered the M26 Pershing medium tank which was built towards the end of the Second World War and which was in some respects the forerunner of the post-war US tanks. However, when the US Army embarked in 1943 on the development of engines specifically for tanks it decided to return to air cooling. The new engines were not, of course, radial, but their development and production were entrusted to the same company as that which had earlier produced most of the air-cooled engines for tanks, namely the Continental Motors Corporation. The most important of the new engines was AV-1790, a V-12 with a displacement of 29.36 litres that developed 810hp and that began to be produced in 1949 for the M46 medium tank.
Several years earlier the British Army came up against the same problem as that which made the US Army power its tanks by modified aircraft engines, namely a lack of money for the procurement of special tank engines. This made it abandon the use of the air-cooled Armstrong Siddeley engines. In anticipation of this, the Royal Ordnance Factory at Woolwich started to develop in 1928 the A.7 medium tank as an alternative to the A.6 Sixteen Tonner and powered the third prototype of it, built in 1934, by two six-cylinder water-cooled AEC bus diesels that had a combined output of 280hp. The combination of the two engines proved successful and met the need for more power than could be provided by single available engines. In consequence it was adopted in 1937 for the A.12 Matilda infantry tank, which the British Army used successfully in the early stages of the Second World War.
Another consequence of the lack in Britain of adequately powerful engines was a revival in 1937 by the Nuffield organization of the production of the Liberty engine of the First World War. The Nuffield Liberty engine, which developed 340hp, provided some of the early cruiser tanks with 23 or 24hp per tonne, and this together with their Christie-type suspensions enabled them to move at up to 30mph. However, the engine proved troublesome, particularly in the heavier Crusader cruiser tanks, although it still provided them with 17 to 18hp per tonne. There was by then an alternative in the form of the horizontally opposed 12-cylinder engine that was specially designed for the Crusader’s contemporary, the Covenanter cruiser tank. The engine was designed by the Meadows company, which had produced engines for almost all the British light tanks since the 1920s, but its output of 280hp was lower than that of the Nuffield Liberty engine and its cooling system was unsatisfactory. This and their other shortcomings resulted in all the Covenanters being considered unfit for use in battle.
No sufficiently powerful and reliable engine was produced for British tanks until a decision was taken to use a derated unsupercharged version of the V-12 Rolls-Royce Merlin engine that successfully powered the Hurricane and Spitfire fighters of the Royal Air Force as well as some of its bombers. The use of this engine, called Meteor, was proposed in 1941 and was put into effect a year later in a new cruiser tank called Centaur, which was originally powered by a Nuffield Liberty engine but which was renamed Cromwell when powered by the Rolls-Royce engine. Although it had the same displacement of 27 litres as the Nuffield Liberty engine, the Meteor produced 600hp, which provided the 27.5-tonne Cromwell with 21.8hp per tonne and enabled it to reach a speed of 38mph, in spite of being twice as heavy as the original cruiser tanks.
Having successfully powered the Cromwell, the Meteor also powered its successors, the 33-tonne Comet and then the Centurion, which came to weigh 51.8 tonnes. It was also developed in the early 1950s to produce 810hp in the 65-tonne Conqueror heavy tank after being fitted with petrol injection.
Unlike their British and US counterparts, German tanks used during the Second World War were not powered by adaptations of aircraft or commercial automotive engines but by engines specially designed for them. Moreover, except for the original PzKpfw I Model A, their engines were of one make, being produced by the Maybach company. They were all water-cooled petrol engines with six-cylinders in line in the case of light tank engines and with a V-12 configuration in the case of engines that powered what were the principal German tanks until the middle of the Second World War, that is PzKpfw III and IV.
Although it had not made engines for aircraft since the First World War, the Maybach company built engines for airships until the early 1930s, and the V-12 engines that it produced for the PzKpfw III and IV were comparable to contemporary aircraft engines. The HL 108 TR engines that originally powered both tanks had a capacity of 10.8 litres and produced 230hp, which resulted in a power-to-weight ratio of 15.5 and 12.6hp per tonne respectively. The capacity and the output of the engines were increased to compensate for the increases in the weight of the two tanks when they were fitted with more powerful guns and thicker armour, but in spite of this the power-to-weight ratio went down to 11.5hp per tonne. However, this did not prevent them playing a very effective role in mobile operations.
A more powerful engine was clearly needed for the 57-tonne Tiger heavy tank, and another V-12 Maybach engine, the HL 210, was produced for it. It developed 650hp, but this was considered insufficient, and so after the production of the first 250 tanks the engine’s cylinders were bored out to increase its capacity from 21.33 to 23.88 litres and its output to 700hp. The modified HL 230 engine was also adopted for the Panther medium tank, which in its original form weighed 43 tonnes and therefore had a power-to-weight ratio of 16.3hp per tonne. This was higher that that of any German tank except for the light PzKpfw II. Nevertheless, a more powerful version of the HL 230 was being developed for the Panther as well as the Tiger II heavy tank, which needed it as it weighed 68 tonnes. The resulting HL 234 engine was for the first time provided with petrol injection instead of using a carburettor, and this increased its output from 700 to 900hp.
Development of the HL 234 engine did not advance beyond its installation in a Tiger II test bed because of the defeat of Germany in 1945 and its occupation by the Allied armies. However, the Maybach company was located in the zone controlled by the French Army, which very sensibly allowed some of its development work to continue, as well as adopting the HL 230 engine for its ARL 44 chars de transition. The outcome of this was another engine with petrol injection, the HL 295, which had a capacity of 29.5 litres and which developed 1,000hp. But, for all its advanced characteristics, only about ten engines of HL 295 type were built, being used in the AMX 50 family of heavy tanks that the French Army was developing in the early 1950s.4
Maybach’s pioneering use of petrol injection was followed by its use in an uprated version of the Meteor engine developed for the British Conqueror heavy tank and in the AVI-1790 engine adopted around 1954 for the US medium tanks. These engines represented the ultimate form of tank petrol engines. No others were built for medium or heavy tanks and eventually even for light tanks, their place being taken by diesel engines that offered lower fuel consumption and less risk of catching fire.
Interest in the use of diesel engines to power tanks arose soon after the first steps were taken to develop them for airships and aircraft. This attracted the attention of some officers in the British War Office, primarily because of the advantage they offered of longer operating range. In consequence, in 1926 the Ricardo research organization was asked to design a four-cylinder sleeve-valve diesel engine of 90hp, which meant that it was to be of the same power as the air-cooled petrol engines used at the time in the Vickers medium tanks. It was successfully tested a year later in one of them and was followed by the construction of at least four more similar engines and of a larger six-cylinder engine of 180hp, which was tested in 1933 in one of the A.6 Sixteen Tonner tanks.5 However, the Ricardo diesels were not developed further for lack of money, and the use of diesels in British tanks was reduced to the adaptation of bus engines in the A7E3 experimental medium tank and then in the A.12 Matilda infantry tank, as mentioned earlier in this chapter. After this no British-built tank was diesel powered until well after the Second World War, except for the Valentine infantry tanks designed by Vickers Armstrongs, all but the first of which used commercial AEC or General Motors diesels.
In the meantime, the British lead in the development of diesel engines for tanks was followed in several other countries. They included Japan, where development began in 1932 of an air-cooled diesel for the Type 89B tank and where subsequently all other tanks were diesel powered. They also included Poland, where the 7TP derivative of the Vickers Six Ton Tank began to be produced in 1935 with the Swiss Saurer diesel, and Switzerland where a Saurer diesel was also installed in the LTH light tanks imported from Czechoslovakia. By 1938 the French Army also ordered 100 FCM 36 light tanks powered by Berliet-Ricardo diesels, although all its other tanks were powered by petrol engines. In 1936 the US Army tested a Guiberson nine-cylinder radial diesel in an M1 light tank, and this led to its use in M1A1 and M3 light tanks produced during the early part of the Second World War.
All this was overshadowed by the development of a diesel engine for tanks in the Soviet Union. It began in 1931 and was originally intended to produce a V-12 water-cooled engine to power aircraft as well as tanks, as the M17 petrol engine had done. The idea of using it in aircraft was gradually abandoned, but it retained to its advantage the characteristics of an aero engine and in particular light weight. Its characteristics also led to claims that it was a copy of contemporary French or Italian aircraft engines, but in spite of some similarities there has been no convincing evidence of this.
With what was either remarkable foresight or merely a continuation of the power levels already attained by the engines of the T-28 and T-35 tanks, the diesel engine was specified to produce 500hp, which met the needs of Soviet tanks for many years. However, before its definitive V-2 form was reached in 1937, it had to be re-designed, which involved, among other things, an increase in its capacity to 38.8 litres.
While it was still being developed, the V-2 engine was installed in the last tank of the BT series, the BT-7M, and in 1939 it was put into production for the T-34 medium tank rated at 500hp and for the KV heavy tank rated at 600hp. It then powered all the medium and heavy tanks and the assault guns based on them that were produced for the Red Army during the Second World War. Towards the end of that conflict it was modified so that it could be mounted transversely in the T-44 medium tank, which made it take up less of the hull length than the conventional longitudinal engine installations. Surprisingly, this had not been done before, except for the Italian Fiat 3000 and the L.3 tankettes. But, in view of the advantages of it, all the subsequent versions of the V-2 engine were mounted transversely, starting with the V-54 of the T-54 tanks. The output of the engine was increased to 580hp of the V-55 version, which powered the early models of the T-55 and T-60 tanks and finally produced 620hp. A further increase in power to 780hp was achieved in the V-46 version, which was fitted with a mechanically driven supercharger and powered the early models of the T-72 tank, while its later models were powered by the V-84 engine developing 840hp. The output of the engine was increased further to 1,000hp in the V-92S2 version, which was fitted with a single turbo-charger and was installed in the T-90 tanks, and still further to 1,200hp in the V-99 version, which was fitted with two turbo-chargers. All this more than made up for the increases in the weight of Soviet and then Russian medium tanks that took place over the years, as it resulted in a power-to-weight ratio of 25.8hp per tonne of T-90S compared with 18.9hp per tonne of the original T-34.
Thus, by a judicious initial choice of a sound conventional design and its progressive development, the Red Army and its successors were able, to a large extent, to meet most of their needs with a single type of tank engine over a period of 70-odd years and gained thereby considerable economic and operational advantages. Admittedly, in the middle of that period the Soviet Army threw away the advantages of a single type of medium tank engine by developing two others, but in the end the army reverted to the wiser policy of using a single type of engine.
By comparison, other armies dissipated their resources by successively developing different types of engines. One reason for this was changes in the policy concerning the availability of fuels. In particular, the US National Petroleum Board decided during the Second World War that military vehicles should use spark ignition petrol engines because petrol was considered to be more readily available than diesel fuel. Similar views were held after the war within NATO. As a result, engines developed for tanks towards the end of the war and in its aftermath were all petrol engines, and production of petrol-engined tanks, such as the US M48 and British Centurion, did not cease until 1959.
However, in 1957 NATO adopted a policy that tanks should be powered by what were called ‘multi-fuel engines’. This in practice meant diesel engines, as the latter could be adapted to run on a range of fuels, including petrol as well as diesel oil. The change in attitude began to manifest itself in 1954 when work began in the United States on converting the standard AVI-1790 tank petrol engine into a turbo-charged diesel. The resulting AVDS-1790 diesel produced 750hp compared with 810hp of its petrolfuelled forerunner, but when installed in the M60 tank it increased its range on roads to approximately 300 miles, compared with 160 miles of the similar but petrol-engined M48A2.6 Similar improvements in the operating range were achieved by all the other diesel-engined tanks designed during the 1950s, such as the German Leopard 1, French AMX 30 and Swiss Pz.61.
The British Chieftain tank, which was designed at about the same time, was also diesel powered, but its Leyland L.60 engine was not of a well proven four-stroke type like the others but of the opposed-piston two-stroke type, which was adopted because of its perceived ability to operate on a wide range of fuels.7 In fact, the more conventional engines proved equally capable of using different fuels while the opposed-piston engine had peculiar development problems that took time to resolve, particularly by a company that had no previous experience of its type, and which delayed the attainment of the specified output of 700hp.
A different attempt to improve on conventional diesel engines was made in the United States. It involved the use of variable compression ratio pistons devised by the British Internal Combustion Engine Research Association that offered the prospect of much higher specific output. They were first used in the AVDS -1100 diesel that was being developed for the US T95 tank and enabled its output to be raised from 550 to 700hp and eventually in the AVCR-1100 form to as much as 1,475hp. At that power level it was adopted for the US version of MBT-70, but its capacity was increased from 18.3 to 22.3 litres and it was designated AVCR-1360. When MBT-70 was abandoned AVCR-1360 was adopted by General Motors for their entry into the competition for the US M1 tank, but it proved difficult to achieve consistently good combustion with it, which manifested itself in clouds of black exhaust smoke, and its specific fuel consumption was not as good as that of other diesels. This and the other characteristics of the AVCR-1360 engine handicapped the General Motors prototype of the M1 tank, and when it failed to win the competition in 1976 interest in the variable compression ratio type of engine vanished.
Eight years later, when the US Army showed renewed interest in diesels, it funded the development of another unconventional tank engine as part of the competitive Advanced Integrated Propulsion System, or AIPS, programme. What emerged out of it was another departure from standard diesel engine practice, the Cummins XAV-28, a 27.56 litre V-12 with a high temperature lubricant acting also as the coolant. The engine was to produce 1,450hp but failed to come up to expectations, and in mid-1990 Cummins terminated its involvement with it.
Another departure from established diesel engine practice, which was also aimed at a high specific output, was adopted in France in the 1970s. It involved the use of the Hyperbar high pressure turbo-charging system in which the turbo-charger was driven not only by the exhaust gases but also by additional energy supplied by a gas turbine type combustion chamber. When applied to the V8X-1500 engine, this approach raised its output to 1,500hp in spite of its displacement being only 16.47 litres. It also resulted in a much more rapid engine response that led to high vehicle acceleration, but it complicated the engine installation, made it expensive to produce and resulted in a relatively high specific fuel consumption. Moreover, although its displacement was much smaller than that of conventional diesels of the same power, the space occupied by its whole system within a tank hull was not very different from that of the best of them. In consequence, its use was confined to the Leclerc tanks produced for the French Army. Other Leclerc tanks, produced for the United Arab Emirates, were powered by more conventional MTU diesels.
The most radical departure from the prevailing diesel engine practice was contemplated by the British Army, which in the 1960s funded the development of a rotary diesel by the Motor Car Division of Rolls-Royce. Its development was prompted by the excitement created in the motoring world by the appearance in Germany in 1958 of the Wankel rotary car engine.8 In the unique two-stage twin rotor form devised by Rolls-Royce, the rotary diesel was expected to be lighter than conventional, piston-type diesels and more efficient than automotive gas turbines. However, it turned out to suffer from a number of problems inherent in its configuration and would have required, at best, considerable further development.9 In consequence, the British Army abandoned supporting its development in 1974 and finally opted for a conventional water-cooled four-stroke V-12 diesel that was part of a family of engines developed, initially on its own initiative, by the Diesel Engine Division of Rolls-Royce.
The most consistent and successful development of diesel engines for tanks was that pursued in Germany where, over a period of 60 years, all the engines have been of the same conventional four-troke water-cooled type with a 90° V cylinder configuration. They have been progressively improved, mechanically and thermodynamically, resulting in three generations of them. Their development was preceded by that of the MB 507 diesel of 850hp, which Mercedes Benz proposed as early as 1942 as an alternative to the Maybach petrol engine of the Panther tank but which was not adopted.10 It was only ten years later that Mercedes Benz were able to resume the development of diesel engines for tanks. This led to the first of a new generation of Mercedes Benz diesels, the MB 837, a 630hp V-8 that was adopted for the Swiss Pz.61 tank. The same engine then powered the prototype of the German Leopard 1 tank but was quickly succeeded by its more powerful MB 838 development, a V-10 of 830hp that provided Leopard 1 with a power-to-weight ratio of about 20hp per tonne and made it the most agile tank of the period.
The second generation of Mercedes Benz diesels began to be developed in 1965, initially to provide the US-German MBT-70 with 30hp per tonne. This requirement was met by MB 873 which was designed on much the same lines as MB 838 but was more compact and had two turbo-chargers instead of the two mechanically driven superchargers. When the MBT-70 programme collapsed, MB 873 was developed further for Leopard 2, retaining the 1,500hp rating but having its capacity increased from 39.8 to 47.6 litres in order to increase its torque and consequently the acceleration of the tank.11 In the meantime, the production of Mercedes Benz tank diesels was taken over by Motoren und Turbinen Union, or MTU, which incorporated the high performance diesel divisions of Mercedes Benz and Maybach.
A year after coming into existence in 1969, MTU embarked on its own initiative on the development of the third generation of tank diesels in anticipation of a demand for more compact engines.12 The outcome of this was the MT 883 engine, which appeared in 1979. It was another V-12, although with a smaller capacity of 25.1 litres than the MB 873, but nevertheless with a maximum output of 1,500hp. Moreover, when it was mounted transversely in a tank, following the example of Russian tanks, the Euro Power Pack based on it had a total volume of 4.5m3 compared with 7m3 of the power pack of Leopard 2 with its longitudinally mounted MB 873 engine. It also took up 1m less of hull length.
As a result of its characteristics, the MT 883 was mounted in the export versions of the French Leclerc, the US M1 and the British Challenger 2 as a superior alternative to their standard engines. It has also been adopted as the best available engine for newly designed tanks, such as the Israeli Merkava Mark 4, the South Korean K-2 and the Turkish Altay.
With the possible exception of the Mitsubishi engines of the Japanese Type 74 and Type 90 tanks, the only diesels that have successfully departed from the prevailing four-stroke type appear to have been the two-stroke engines designed in the Ukraine by the Kharkov Engine Building Design Bureau. These water-cooled turbo-charged engines with horizontally opposed pistons have been only 581mm high and are unique in being connected on either side to a transmission gearbox, which when they are mounted transversely results in exceptionally compact power packs. They are also unique in dispensing with cooling fans, using instead exhaust driven ejectors to suck cooling air through the radiators.
Engines of this kind were originally installed in the Soviet T-64 tanks in the form of the five-cylinder 5TDF engine of 700hp, which was followed in the final models of the T-64 series by the six-cylinder 6TD engine of 1,000hp. The 6TD was also mounted in some of the Soviet T-80U instead of their gas turbines because of the high fuel consumption of the latter, the re-engined tanks being designated T-80UD. After the collapse of the Soviet Union the T-80UD was developed in the Ukraine into the T-84, which was powered by 1,200hp 6TD-2. At about the same time 320 T-80UDs were sold to Pakistan, and subsequently the 6TD-2 engine was adopted for the Al Khalid tank and for the very similar MBT 2000 marketed by the Chinese North Industries Corporation, which sold 44 to Bangladesh. The 6TD engine has also been adopted for the 200 T-72 tanks that are to be modernized in the Ukraine for Ethiopia.
By the time diesels became generally accepted as tank engines, a potential alternative emerged to them in the form of automotive gas turbines. Study of their application to tanks began in 1944 in Germany, which already had established a lead in the development of gas turbine powered aircraft, including building the world’s first, a Heinkel He 178, which flew in 1939. German work on gas turbines for tanks had not advanced beyond preliminary designs of a 1,000hp engine when it was brought to an end by the defeat of Germany in the Second World War.13 However, it was taken up in Britain, where a contract was awarded within seven months of the end of the Second World War to the Parsons company for the design study of a 1,000hp gas turbine for tanks. This was followed by the construction of an engine of 655hp, which was installed in 1954 in a Conqueror heavy tank chassis, and then of a second engine, rated at 910hp. However, neither advanced beyond trials because their fuel consumption was unacceptably high, as might have been expected.14 In retrospect it is somewhat difficult to understand why the development of gas turbines for tanks was taken up so readily in Britain, except for the contemporary euphoria engendered by the world lead in the development of gas turbines for aircraft that Britain enjoyed for a time after the war.
The exploratory work in Germany on gas turbines for tanks might also have led to them being considered in 1949 in the Soviet Union, as already mentioned in Chapter 9. But little of consequence happened there until 1963, when experiments began with a helicopter gas turbine installed in a tank chassis. This was followed in 1967 by a decision to develop a 1,000hp gas turbine, which was accepted in 1976 for use in the T-80 tank in spite of its high production cost and high fuel consumption. The T-80 continued to be produced until the collapse of the Soviet Union, after which only a few more were built. It was then offered for export, but only a small number was procured by Cyprus and South Korea, and by mid-1990 the Russian Army decided to abandon it and to concentrate on further development of the diesel-powered T-72, the T-90.
Development of gas turbines for tanks began in the United States, as in the Soviet Union, with tests of an engine built for other purposes. This took place in 1961 when a Solar Saturn gas turbine was mounted in one of the T95 medium tanks that was then being developed.15 Shortly afterwards the US Army funded the competitive development of a 600hp gas turbine by the Solar Aircraft and Ford Motor Companies. But the engines built by them failed to establish an overall advantage over diesels and were abandoned without being tested in a tank. In spite of this, the US Army placed another contract in 1965, this time with the Lycoming Division of Avco Corporation, for what came to be known as the Army Ground Turbine of 1,500hp, or AGT-1500. It began to be tested in 1967 and was originally considered for MBT-70, but after the demise of the latter it was adopted in 1973 by Chrysler Defense in its XM1 prototype, which was accepted by the US Army to become its M1 tank in 1976 – the same year as that in which the Soviet Army accepted the GTD-1000T gas turbine for its T-80 tank!
When AGT-1500 began to be tested, it was claimed that its minimum specific fuel consumption was as low as that of diesel engines. But when the M1 powered by it came to be used, its overall fuel consumption proved to be twice that of diesel-powered tanks. This exacerbated the problem of supplying US tanks powered by it with fuel, which was brought out by the large quantities of it that had to be delivered to M1 tank units in Kuwait in 1990 and Iraq in 2003. AGT-1500 was also relatively expensive, which handicapped the Chrysler designers of the M1 tank who, like their General Motors competitors, had to work within an overall cost target for the tank of $500,000 in 1972, and consequently had to keep down the cost of other components.
During the 1980s, an attempt was made to show that gas turbines could be as fuel efficient as diesels using a Garrett GT-601 engine originally designed for commercial truck operation that was tested in several tanks, including the US M48, British Chieftain and French AMX 30. Their overall fuel consumption was estimated to be only 10 per cent higher than that of their diesel-powered counterparts. However, because of its more robust design and bulky heat recuperator, the GT-601 was twice as large and heavy in relation to its power as the AGT-1500 and did not enjoy any advantage over diesels in terms of weight and volume.
Undeterred, the US Army funded the development of yet another gas turbine as part of its Advanced Integrated Propulsion System programme, which included the award of a contract in 1984 to General Electric and Textron Lycoming for the LV 100, a 1,360hp gas turbine. Two were built by 1991 and one of them was installed in a tank test bed with an electric transmission built as part of the Armored Systems Modernization Program. The latter was abandoned around 1994 when international tension abated, but interest in gas turbines continued and in 2000 General Electric and Honeywell were awarded a contract for the LV 100-S engine, which was intended for the Crusader 155mm self-propelled howitzer and as a replacement of the AGT-1500 in the M1 tanks. However, development of the Crusader was terminated in 2002, having been overtaken by the US Army’s transformation programme, and so was that of the LV 100-5 gas turbine.
Although they were adopted for only three tanks – the US M1, the Soviet T-80 and the Swedish S-tank – the use of gas turbines constituted a significant divergence from the well-established automotive engineering practice. But it did not represent the most radical departure. This would have been the use of a nuclear reactor to power a tank, which was proposed, in all seriousness, at a conference held in 1955 at the US Army Ordnance Tank Automotive Command.16 It was estimated that the proposed nuclear powered tank would weigh 50 tonnes, or about as much as a conventional contemporary tank, but this appears to have grossly underestimated the weight of the shielding that would have been required to protect the crew from radiation.17
Whatever their engines, tanks, like other vehicles, need transmissions to vary the engines’ torque. In most cases, this requirement has been met by providing tanks with multi-speed gearboxes that have in general terms followed contemporary automotive engineering practice. Thus, over the years, tank transmissions have advanced from incorporating sliding gears to automatically controlled epicyclic or planetary gear trains, augmented since the Second World War by hydrokinetic torque converters.
Tanks also require a system of altering the relative speed of their tracks by which they are steered. The earliest method of achieving this appears to have been incorporated in 1904 in the United States in a Holt half-track steam traction engine.18 It amounted to disengaging the drive to one track and then applying a brake to it, which made the vehicle swing around it. Such ‘clutch-and-brake’ steering was used in the first French tanks built in 1916 and as one stage of the steering in the first British tanks up to the Mark IV of 1917. It was subsequently used by most light tanks built during the 1920s and 1930s and by heavy tanks such as the A.1 Independent and even the Soviet T-35. It proved adequate for steering the former but was not suitable for the latter. In consequence, as their weight grew, no British tanks were produced with it after the Valentine.
Another steering system somewhat similar to but more gradual than the ‘clutch-and-brake’ steering that has been used successfully in heavy as well as light tanks has been based on inserting a multi-speed gearbox, usually of the epicyclic type, in the drive of each track: changing gear in one or the other of them produced the desired difference in the speed of the tracks. The first geared steering system of this kind was designed in 1918 for the Anglo-American Mark VIII heavy tank, and other experimental geared steering systems were tried in a number of British tanks during the 1930s. But none was adopted until they were incorporated in the Covenanter and Crusader cruiser tanks designed at the beginning of the Second World War, in which they proved eminently successful. In the meantime a geared steering system was designed in 1925 for the first Japanese tank and similar systems were subsequently adopted in all tanks made in Japan. A geared steering system was also adopted in Czechoslovakia for the LTH light tank that was used extensively by the German forces as PzKpfw 38(t), and proved very successful mechanically. A geared steering system was also produced for the German Panther medium tank, but it differed from all the others in being more elaborate.19
Soviet tanks, including the T-34-85, continued to rely on clutch-and-brake steering well into the Second World War, in spite of it being one of their weak points. However, in 1943 a geared steering system with two-speed epicyclic gearboxes was developed for the KV-13 experimental heavy tank that led to the IS or Stalin tanks, and they became the first Soviet tanks to go into service with such a system.20 After the war a similar system was used on a large scale in T-54, T-55 and T-62 tanks, and was then succeeded by a more elaborate version that incorporated epicyclic gearboxes with as many as seven speeds. This provided several turning radii under power and therefore more gradual control of tanks’ manoeuvres. Such a system was first installed in the T-64 and was then adopted for the T-72 and T-90 as well as Ukrainian tanks and the Chinese Type 98.
From the beginning there was also an alternative to geared steering in the form of differential steering. The simplest and earliest embodiment of it consisted of an ordinary truck differential interposed in the drive of the tracks and fitted with a brake on each of the half-shafts coming out of it. Steering based on it was used in the first successful fully tracked tractor built by Richard Hornsby in 1905 and ten years later was incorporated in the first British tanks, although they were generally steered by clutch-and-brake methods. Braked differential steering was evidently simple, but it is also very inefficient, and its use after the First World War was confined to very light vehicles such as the Carden Loyd tankettes of the 1920s and the Bren Gun Carriers that were produced on a large scale during the Second World War.
The inefficiency of the braked differential steering is avoided in the closely related controlled differential steering systems, which contain supplementary gears that allow the speed of the half-shafts to be reduced instead of bringing them to rest. However, controlled differential systems provide only one minimum radius of turn, and this has to be a compromise between a large radius of turn required at high speeds and tight turns at low speeds. Nevertheless, it has been widely used since it was developed in the United States during the First World War by the Cleveland Tractor Company, after whose trademark it is sometimes called a Cletrac Differential. It was used in almost all French light tanks built since the mid-1920s until 1940 and in the secretly built German Grosstraktoren, but not in later German tanks. It was also used in all US light and medium tanks from 1932 until the end of the Second World War. Since then it has been used in the French AMX 13 light tanks and a number of armoured personnel carriers, but in only one more medium tank, the Japanese Type 61.
Much more sophisticated double differential steering systems began to be developed in France as early as 1921. In them one differential was driven through the gearbox while another was driven directly by the engine, and their outputs were then combined, which resulted in a different minimum radius of turn for each gear in the gearbox – the lower the gear the smaller the radius, as is generally required. They also offered the possibility of making the drive from the engine through a hydrostatic pump and motor and thereby achieving infinitely variable control of steering. This was exploited in the design of the French Char B to make it possible for its driver to aim the tank’s hull-mounted 75mm gun by turning the tank while at the same time driving it.
A double differential system with a simpler direct mechanical steering drive was adopted ten years later for the French S-35 Somua medium tank, and during the Second World War a more refined version of the double differential system was produced in Germany for the Tiger tank. At about the same time a triple differential system, functionally very similar to the double differential, was developed in Britain for the Churchill infantry tank and was subsequently adopted for the Cromwell cruiser tank. It then continued to be used in the Comet, Centurion, Conqueror and Chieftain tanks, although the TN 12 transmission of the last was very different from those of the earlier tanks in having epicyclic gear trains instead of crash gears. But transmissions with triple diffential systems did not lend themselves to the use of progressive hydrostatic steering controls and were therefore succeeded in the Challenger by one with a double differential system that did.
A general use of transmissions with double differential steering systems and hydrostatic steering drives began in the 1950s with the Swiss Pz.61, which was followed by the German Leopard 2 with a Renk transmission, the US M1 with an Allison transmission and the French Leclerc with a SESM transmission. However, other contemporary tanks have used double differential steering systems with mechanical steering drives, including the Italian C-1 Ariete and the South Korean K-1.
An entirely different approach to the problem of engine torque multiplication and steering existed from the start in the form of electric transmissions. The simplest of them consisted of a DC generator coupled to a tank’s engine and a DC motor to drive each track. Such a system was first adopted in 1916 for the French St Chamond tank and had the advantage that it could be put together readily from existing electrical motors and generators. It also made the control of track speeds and hence steering easy. However, it was relatively heavy and inefficient. In consequence, its use between the two world wars was confined to the ten French 2C heavy tanks.
There was relatively little interest in electric transmissions during the Second World War, their use being confined at first to the two prototypes of the British TOG heavy tanks built between 1940 and 1941, which weighed 63.5 and 80 tonnes respectively and were in effect unsuccessful throw-backs to the First World War. A much more successful electric transmission was developed in the United States between 1943 and 1944 for the T23 medium tank, but although 252 of the latter were built none went into service. The only armoured vehicle with an electric transmission to be used during the Second World War was the Ferdinand heavy 65-tonne 88mm self-propelled anti-tank gun. This was based on the unsuccessful prototypes of medium and heavy tanks designed by F. Porshe between 1940 and 1942, and 90 were built and used by the German Army during the latter part of the war. The only other armoured vehicles built by then with an electric transmission were the two prototypes of the 182-tonne Maus heavy tank, which were built between 1943 and 1944.
No other armoured vehicle was fitted with an electric transmission until the 1960s, when the Atelier de Constructions Electriques de Charleroi, or ACEC, installed one in Belgium in a US-built M24 light tank and later in its Cobra armoured carrier. At about the same time FMC Corporation installed another electric transmission in one of the US M113 armoured carriers that it was producing. The ACEC transmission represented a significant advance as it used an alternator with a rectifier instead of a DC generator, while FMC not only did this but also used induction motors, which were not only lighter than the DC motors but were brushless.21
This was followed in the 1980s by a general upsurge of interest in electric transmissions, which led during the following decade to the construction of several experimental armoured vehicles fitted with them in the United States, Germany and France. Their transmissions took advantage of the contemporary development of rare earth permanent magnet alternators and motors, which made them more compact. However, in terms of the total weight and cost as well as the cooling requirements of their power electronics they were not competitive with hydro-mechanical transmissions. This was demonstrated by the most advanced and powerful of them developed as a possible tank transmission and installed in 1994 in the 50-tonne Automotive Test Rig, which was an offshoot of the US Army’s abortive Armored Systems Modernization Program.22
However, interest in electric transmissions persisted, encouraged by the emergence in the 1980s of the concept of the ‘all-electric tank’, which was envisaged to combine an electric transmission with an electromagnetic gun system and electric armour. The combination failed to materialize, but the use of electric transmissions was taken further to form part of hybrid drive systems, in which they would operate alongside battery power packs storing electrical energy that could be drawn upon to meet peak power requirements and thereby make possible the use of smaller engines sufficient for most of the time or enable ‘silent running’ for short distances on battery power.
At first hybrid electric drives, or HEDs, were severely handicapped by the bulk and weight of their energy storage, which was based on conventional lead-acid batteries, but this was largely overcome by the development of lithium ion and other batteries with higher energy densities.
Except for the added complication of the hybrid drives, the electric transmissions referred to have been of the classic two line kind, with two parallel circuits carrying current from an engine-driven generator to two separate motors, each driving one track. This means that the only connection between the engine and the motor attached to it and the track driving motors consisted of cables, which made for greater flexibility in the relative positioning of them within vehicle hulls and has been a major advantage of electric transmission in the case of some types of armoured vehicles. But there is also a problem with the two line systems when it comes to steering, which to be efficient requires the transfer of the power regenerated at one track to the other track. The regenerated power can be considerably greater than that required for vehicle propulsion, and to cope with it the motors and generators have to be correspondingly large. However, their size can be kept down by mechanically and more efficiently transferring the regenerated power by a cross-shaft connecting the track final drives. The resulting electromechanical transmissions, or EMTs, which can have a single propulsion motor and a single steering motor, retain most of the advantages of the two line systems but are more complex and less adaptable to vehicles not specifically designed to use them.
The advantages of EMTs began to attract attention in the 1980s, but the first was only demonstrated in 2005 in Sweden, where it was designed by Hagglunds for the tracked variant of the SEP multi-role armoured vehicle. Another EMT designated E-X-Drive developed in Britain by QinetiQ might have been used in the manned combat vehicles of the US Future Combat Systems programme, but this was not adopted.23
Whatever their type, transmissions determine how effectively tanks’ engine power is used to maximize their automotive performance, including speed. But the latter can be severely restricted by the vibrations set up by the rough ground when tanks move over it. The severity of the vibrations can be reduced by the resilience of the suspensions on which tanks’ road wheels are mounted and which therefore governs the maximum speed of tanks in some circumstances.
As it happens, the first British tanks ran on rollers mounted rigidly in their hulls, which was only acceptable at the low speeds for which the early tanks were designed. But the first French tank, the Schneider, already had rollers mounted on sub-frames sprung by coil springs. During the 1920s and 1930s most tanks had suspensions based on pairs of rollers or road wheels mounted in tandem on balance beams sprung by leaf or coil springs. Suspensions of this kind worked mainly by ‘walking beam’ action, which was reasonably effective at low speeds but did not respond quickly enough to ground irregularities at higher speeds.
Such slow speed suspensions were therefore abandoned during the Second World War, and since then almost all tanks have had their road wheels sprung independently. This was pioneered in the United States by J. W. Christie, who first demonstrated a vehicle with road wheels independently sprung by long coil springs in 1928 and obtained with it a considerable increase in speed. His example was followed on a large scale a few years later by the Soviet BT tanks and then by the British cruiser tanks and Soviet T-34 tanks.
Independent coil spring suspensions were eventually abandoned in favour of suspensions based on torsion bars, which could absorb more energy in relation to their weight and did not take up any hull width. Torsion bar suspensions were first incorporated in 1938 in some of the German PzKpfw II light tanks, and their use became widespread by the end of the Second World War, by which time they had been adopted in the German Tiger and Panther as well as PzKpfw III tanks and Soviet KV and IS heavy tanks. Torsion bar suspensions were then adopted for almost all tanks built after the war, but in the 1960s they began to be superseded in turn by independent suspensions with hydropneumatic spring units, which offered superior, progressive spring action. If linked by suitable controls, they can also be used to vary the pitch of the hulls and their height off the ground. Such adjustable hydropneumatic suspensions were originally adopted in the 1960s for the Swedish S-tank and the Japanese Type 74, while simpler non-adjustable suspensions were adopted for the British Challenger and other tanks.24
While the speed with which tanks can move over rough ground is related to the resilience of their suspensions as well as engine power, their ability to move over soft muddy ground depends to a large extent on how their tracks spread their weight over it, or in other words on their ground pressure. The latter is commonly considered in terms of the nominal ground pressure, or NGP, which is the weight of a tank divided by the area of its tracks in contact with the ground. NGP does not represent the actual pressure exerted by tanks on the ground, but it was a reasonable approximation to it in the case of the early British tanks, which ran on a number of small rigidly mounted rollers and tracks with flat-plate links. In any case, it was accepted as an important characteristic of tanks and was quoted as such as early as 1917.25 Moreover, a low NGP value became the object of some of the earliest tank designs.26 There was no knowledge at first of what it should be, and in the circumstances it was assumed that it should be similar to the pressure exerted by a soldier’s boots, so that tanks could move over the same ground as infantrymen.27 This led to the view in the 1920s that NGP should be about 50kN/m2 and the most numerous tank of the period, the Renault FT, had in fact an NGP of 58kN/m2.
However, little attention appears to have been given for some time in military requirements to ground pressure, and the NGP of some tanks was allowed to rise in the 1930s to more than 100kN/m2, although this could have been avoided by the use of wider tracks. It was only during the Second World War, in particular as a result of the difficult terrain conditions encountered on the Russian Front, that the importance of ground pressure was generally recognized.
Although NGP is only an approximation to the pressure exerted by tanks on the ground, it has been a reasonable indicator of their relative ability to move over soft ground. But this is true only if their running gear is similar and in particular if they have the same number and size of road wheels and similar tracks. Otherwise, NGP fails to provide a correct indication of the soft soil capabilities because it does not take into account the fact that tanks’ tracks are flexible and that ground pressure varies consequently along their length, with peaks occurring under the centres of the road wheels. It is the peak values and not the average of the ground pressure that govern sinkage and therefore soft ground performance. The importance of the maximums of the pressure under the tracks was recognized in Britain in the 1970s, and an empirical equation for calculating their average value, or Mean Maxim Pressure (MMP), was devised by D. Rowland working at the Fighting Vehicles Research and Development Establishment.28 MMP based on this equation has been used since in British armoured vehicle specifications, providing them with a superior alternative to NGP. It has also helped to explain a number of apparent anomalies resulting from the use of NGP. One of them involved the British Matilda infantry tanks, which had a higher NGP of 112kN/m2 than any other tank of the Second World War and yet operated successfully in many parts of the world, which could be explained by their MMP being lower than that of many other tanks.29
Although it provided a far better indication of the relative soft soil capabilities of tanks than NGP, MMP applied to only one particular and not especially difficult type of soil. To assess the performance of tanks over different types of soil, there had to be some measure of the soil properties. This requirement has been commonly met by measuring the resistance of soils to penetration by a simple instrument called a cone penetrometer, which could be described as a scientific descendant of the walking sticks used by British tank commanders during the First World War to probe the ground in order to decide whether tanks could move over it or not.
The cone penetrometer began to be used for military purposes towards the end of the Second World War by the US Army Corps of Engineers, and in spite of its shortcomings it has remained the only instrument widely used for assessing the trafficability of soils, that is their ability to support the movement of vehicles over them. It has also been used to determine the inverse, that is the cone index or the measure of the resistance to soil to penetration, of the weakest soil that a particular vehicle can traverse. This, called the Vehicle Cone Index or VCI, has been determined by experiment and has also been correlated with what has been called the vehicle’s Mobility Index, but the latter consists of a questionable collection of vehicle parameters and arbitrary factors. Nevertheless, the Mobility Index has been incorporated in the NATO Reference Mobility Model or NRMM that has been used to predict the capabilities of vehicles.30
A more rational method of determining the cone index of the weakest soil that a vehicle can traverse was developed in the 1980s by E. B. Maclaurin working at the Defence Evaluation and Research Agency of the British Ministry of Defence. It is based on traction tests and provides the cone index of the soil on which a particular vehicle can no longer generate any traction. This has come to be called the Vehicle Limiting Cone Index or VLCI, and can be predicted from a relationship established between vehicles’ principal parameters and the results of the traction tests.31
Useful as it might be, the use of cone penetrometers gives little insight into the physical phenomena involved in the operation of vehicles on soft deformable soils. Two aspects of it have been identified, one of them being the compaction of the soil and the consequent formation of ruts causing resistance to motion, which was recognized in Germany as early as 1913 by R. Bernstein.32 The other aspect of it, which was identified in Britain in the 1940s by E. W. E. Micklethwaite, is the generation of thrust, or tractive effort, which is related to the shear strength of soils.33 This was followed in the 1950s by M. G. Bekker, who proposed a semi-empirical method of predicting the performance of vehicles using the compression and shear strengths of soil measured simultaneously by a device named Bevameter after him. Bekker’s approach has only been followed to a limited extent and has as yet had little application to armoured vehicles.34 However, versions of the Bevameter have been developed for the characterization of lunar soil.
In addition to leading the work on soil-vehicle mechanics, Bekker was also responsible for some of the upsurge of interest in the United States during the 1950s in articulated vehicles.35 The idea of articulated tracked armoured vehicles was not new, as indicated in Chapter 2, but no successful prototype of one was built until the 1980s in spite of the potential advantages of its kind. The principal advantage is that the total length on the ground of the tracks of articulated vehicles can be significantly greater than that of conventional tracked vehicles because the latter are restricted to about twice the distance between track centres, for otherwise they are unsteerable. In consequence the area of their tracks in contact with the ground is greater and their ground pressure is correspondingly lower. The steering of vehicles by turning the two halves of them relative to each other also imposes lower stresses on the ground than the skid steering of conventional tracked vehicles and thereby reduces the risk of stalling. Articulated vehicles are also better able to negotiate vertical obstacles.
As a result, the performance of articulated vehicles is superior to that of conventional tracked vehicles when the terrain is difficult and in particular when it is very muddy, marshy or covered with deep snow. On the other hand they are more complicated and expensive to produce, less manoeuvrable and difficult to shape well ballistically. Nevertheless, Bekker advocated their development prompted by his studies of vehicle operation off roads, starting in Canada in the 1940s and continuing in the 1950s in the United States, where he became chief of the Land Locomotion Laboratory set up at the time at the US Army Ordnance Tank Automotive Command.36 He was apparently listened to, for when the writer visited the Command in 1961 he found it full of scale models of various articulated vehicles that were a reflection of the number of design studies of them. However, no articulated tracked armoured vehicle was built. All that happened so far as armoured vehicles were concerned was the design of an eight-wheeled articulated armoured vehicle unsuccessfully produced by the Tank Automotive Command to meet a contemporary US Army requirement for an Armored Reconnaissance/Airborne Assault Vehicle, which eventually led to the M551 Sheridan light tank. The design was taken up by the Lockheed company, where it was developed into the XM 808 8x8 ‘Twister’, but the development of this eight wheeled articulated vehicle did not advance beyond the three prototypes that had been built by 1970.37
No prototype of an armoured articulated tracked vehicle was built until the Swedish UDES XX-20 was completed in 1982. As described in Chapter 10, the performance of this prototype was superior in several respects to that of conventional tracked vehicles, but its gun system was difficult to integrate with its two-part chassis and its development was terminated in 1984. The only other articulated tracked armoured vehicles to be built since have been the lightly armoured versions of the Swedish Hagglunds Bv 206 articulated carriers and, more recently, the Warthog versions of the Bionix articulated carrier built in Singapore for the British Army.