‘Never has so much been expected of so few.’
Message chalked onto Upkeep weapon of 617 Squadron Lancaster AJ–B by Warrant Officer Abram Garshowitz, RCAF, killed during the Dams Raid, 16–17 May 1943
While Harris followed Portal’s directive to proceed with preparing for an attack on the dams, he still refused to withdraw one of his Lancaster squadrons from the ongoing Battle of the Ruhr. Harris elected instead to create a whole new unit, to be formed largely with experienced volunteers.
On 15 March Harris directed the new AOC 5 Group, AVM Cochrane, to establish an as yet undesignated squadron (called ‘Squadron X’ until a number could be assigned by the Air Staff) and recommended 24-year-old Acting Wg Cdr Guy Gibson as its commander. Cochrane had been 5 Group commander for only two weeks and did not yet know Gibson. Nevertheless, on 18 March Cochrane interviewed Gibson, who had just completed his second Bomber Command tour only a week before. The interview went well and next day Cochrane met with Gibson and Gp Cpt J. N. H. ‘Charles’ Whitworth, commander of RAF Scampton, to give them the new unit’s initial operational tasking – albeit in the sketchiest of terms.
By this time the command’s wheels were in motion. What was to become the most famous squadron in the RAF was established on paper on 17 March and designated No 617 Squadron a week later. The unit was to be based at Scampton, a pre-war aerodrome four miles (6.4km) north of Lincoln. A Bomber Command base normally had two squadrons assigned, but in order to permit the construction of concrete runways the station had recently been vacated by 49 Sqn. This allowed, at least temporarily, ample space for aircraft parking, barracks, hangars, offices and shopfloors for the new unit. Gibson arrived on 21 March to examine his new unit’s home and plan the physical establishment.
Three or four days later – while Gibson was in Weybridge receiving his first briefing from Wallis on the Upkeep weapon and how it worked and was to be delivered – the first 14 Lancaster crews were posted into the new unit. Five – two of them captained by NCO pilots – transferred ‘across the aerodrome’ from the co-located 57 Sqn. In those days a Bomber Command squadron was organized in two flights (‘A’ and ‘B’), each with ten or so aircraft and crews. As a unit received additional aircrew, the men would be formed into a third (‘C’) flight which would later be used as the basis of a new squadron.
ACTING WING COMMANDER GUY PENROSE GIBSON, VC, DSO, DFC
Guy Penrose Gibson was born in Simla, India, in August 1918. The son of a British official in the Indian Forestry Service, he grew up under the ‘Raj’. The family were used to having numerous servants over whom even young Guy had authority. This experience perhaps accounts for the rather pronounced prejudices he showed as a commander.
The family returned to England when Guy was six. He was educated at St George’s Preparatory School, Felixstowe, and the independent St Edward’s School in Oxford. Gibson joined the RAF almost immediately after leaving school, and after flight training – having graduated with average marks – he was posted to 83 Sqn, flying Handley-Page Hampdens. Once war broke out Gibson soon became known for his enthusiasm for operations and he flew as often as possible during the critical summer of 1940. He completed 39 combat missions, and near the end of that tour was awarded the DFC and promoted to flight-lieutenant.
Despite his arrogance and outspokenness, Gibson was socially astute and politically adept and soon endeared himself to his commanders with his brazen aggressiveness and implacable attitude towards those who were not so ‘gung-ho’. Consequently, when he was posted to Fighter Command’s 29 Sqn the AOC 5 Group, AVM Sir Arthur Harris, assured him he would return to Bomber Command ‘in the fullness of time’.
Flying a radar-equipped Bristol Beaufighter IF nightfighter, Gibson was officially credited with having ‘destroyed three and damaged a fourth enemy aircraft’ during 99 combat sorties. True to his word, when Harris took over as AOC-in-C Bomber Command he ‘rescued’ Gibson from a posting to 51 Operational Training Unit as a nightfighter instructor. Harris promoted Gibson well ahead of his peers, to acting wing commander – on the same day that the RAF had promoted him to permanent squadron leader. Harris also encouraged the new AOC 5 Group, AVM Sir John Slessor, to find a suitable leadership position for the young prodigy.
Given command of 106 Sqn, Gibson ‘led from the front’, logging four missions in the ill-starred Manchester and another 25 in Lancasters. During this period Gibson was credited with improving the serviceability of the chronically problematic Manchester so that his unit was the best of the eight RAF units operating this type. He was the first to fully equip his aircraft with cameras to assess bombing accuracy. Slessor showcased Gibson’s success during a visit by Sir Archibald Sinclair, the Secretary of State for Air, and other VIPs, thus making Gibson well known at the highest levels of the air force.
Gibson was by his own admission biased against NCO pilots, and he also exhibited disdain towards ‘other-ranks’ aircrew, as well as ground crew and other non-operational types (to one NCO ground crewman, Gibson was a ‘bumptious little bastard’). He relished the company of officer-class professionals among the men he flew with – men from the same social class as himself. For him, his young, aggressive peers ‘gladly went to war in bombers, took great pride in their performance and were not deflected from their purpose by the inherent dangers of their trade’. This, in turn, engendered a strong sense of camaraderie and allegiance among those of his inner circle. Collegial respect and mutual admiration created a cadre of exceptional officer-pilots who followed him to do the extraordinary.
A 617 Sqn Lancaster III dropping an Upkeep weapon during practice at Reculver. (IWM FLM 2365)
Commanding ‘C’ Flight of 57 Sqn was Sqn Ldr Henry Young, DFC and Bar, the 27-year-old son of a London solicitor and an American socialite from southern California. Young had flown 51 operational missions in Western Europe, North Africa and the Mediterranean, with 102 Sqn on Armstrong-Whitworth Whitleys and with 104 Sqn on Wellingtons. On eight of these missions he commanded a ten-aircraft detachment on Malta. During his first tour Young had been forced to ditch twice, on both occasions saving his crew. The fact that he had spent more than 30 hours in the aeroplanes’ tiny life-rafts, as well as his fame as an oarsman in Oxford University’s 1938 championship rowing team, inevitably led to his acquiring the nickname ‘Dinghy’.
Posted at the same time was Gibson’s ‘B’-Flight commander, 21-year-old Sqn Ldr Henry Maudslay, DFC. A 1940 graduate of Eton College, he was also an oarsman, having been named Captain of Boats in his final year. An experienced instructor, Maudslay had flown 29 operations in Handley-Page Hampdens (44 Sqn) and 16 on Lancasters (1654 Heavy Conversion Unit and 50 Sqn) before arriving at Scampton. Also from 50 Sqn was the talented but relatively inexperienced (26 operations) 22-year-old Pilot Officer Leslie (‘Les’) Knight, RAAF, and his crew, all of whom volunteered for what sounded like an interesting assignment.
Three crews came from 97 Sqn.
Flt Lt David Maltby, DFC, a reserved but determined 23-year-old graduate of Marlborough College, had flown five operations with 106 Sqn (Hampdens) and 21 with 97 Sqn (Hampdens, Manchesters and Lancasters) before commanding a section in 1485 Target Towing and Gunnery Flight at Scampton. Returning to 97 Sqn on 17 March, Maltby was given a new and inexperienced crew, all fresh from training.
The 6ft 3in. (1.9m), 24-year-old red-headed Flt Lt Joseph ‘Big Joe’ McCarthy, DFC, was an American from New York City. Having joined the RCAF in May 1941, he had just finished his first operational tour. He and his crew had flown their 27th successful sortie on Lancasters – a raid on St-Nazaire – on 22–23 March.
Relative newcomer Flt Lt John Leslie (‘Les’) Munro, RNZAF, flew his first operational sortie on 2–3 January 1943. He was an aggressive pilot, and he and his crew completed another 19 missions in the next 11 weeks. Returning from the St-Nazaire raid, they responded to a 5 Group circular ‘calling for volunteers from those crews nearing the end of their first operational tour…to form a new squadron to undertake a special mission’.
Towards the end of the month another seven full crews, plus a number of additional personnel to plug gaps in other crews, were posted to the squadron. These included three pilots Gibson knew and personally requested.
Gibson’s favourite was 22-year-old Flt Lt John ‘Hoppy’ Hopgood, DFC and Bar. Hopgood was a Cranwell graduate, a veteran bomber pilot and an experienced Lancaster instructor. Hopgood had flown ten Hampden missions with 50 Sqn as a bomb-aimer/navigator and another 35 as a Manchester/Lancaster captain and instructor with 106 Sqn. While with 106 Sqn Hopgood had taught Gibson to fly the Lancaster four-engined heavy bomber. Hopgood made a strong impression on the commander with his ‘press-on spirit’.
Also from 106 Sqn was 21-year-old Flt Lt David Shannon, DFC, RAAF. He had flown five missions as Gibson’s second pilot, developing a tight bond with his captain. After completing another 31 operations, Shannon had just obtained a posting to 83 Sqn to join the neophyte Pathfinder Force when Gibson recruited him for 617 Sqn. Only Shannon’s navigator, Flying Officer (Fg Off) Danny Walker, DFC, RCAF, followed his captain to Scampton.
Finally, there was Flt Lt Harold ‘Mick’ Martin, DFC, a 23-year-old Australian in the RAF. He was a veteran of 36 combat operations in two bomber tours, with 455 Sqn RAAF (Hampdens) and 50 Sqn RAF (Manchesters/Lancasters). Gibson had met Martin at a Buckingham Palace medals ceremony where they had discussed the tactical advantages of ultra-low flying. From 1654 Heavy Conversion Unit Martin and his well-experienced crew headed to Scampton, with his bomb-aimer, Flt Lt Bob Hay, becoming 617 Sqn’s bombing leader.
In addition to these, Flight-Sergeant (F/Sgt) Kenneth Brown, RCAF, captained one of three additional all-NCO crews also transferred to bring the new unit to its full complement of flying personnel. Just after the briefing for Brown’s seventh mission with 44 Sqn – to Berlin on 27–28 March – his commander, Wg Cdr John Nettleton, VC, notified him that upon return he and his crew were ‘transferred to a new squadron … and I can do nothing about it’. Saying farewell the next day, Nettleton added encouragingly: ‘Brown, you’re going to be the backbone of this new squadron.’ Brown’s wireless operator, Sgt Harry Hewstone, commented upon arriving at Scampton and seeing who were already there: ‘Skip, if we’re the backbone of this squadron, we must be damn close to the ass end!’
THE DANN BOMBSIGHT
One of the most critical parameters for delivering Upkeep was to release it at precisely the right distance from the target. It needed to “bounce” three times, each contact with the water’s surface allowing the backspin of the weapon to retard its forward velocity. If released too late (inside 425yd/388.6m) when it hit the dam wall the impact forces could destroy the device or cause it to “bounce off” the vertical surface instead of “rolling down” the surface – one key to effective destructive force was physical adherence to the dam wall. If released too early (outside 475yd/434.3m) the device tended to begin veering to the left which then made accurately hitting the centre of the dam – another requirement for effective use of its destructive force – problematic.
Therefore accurate determination of range from the dam wall was a critical parameter – one that was met with a rather simple solution.
This was provided by Wg Cdr Charles L. Dann, the Supervisor of Aeronautics at A&AEE Boscombe Down. Since distance between the Möhne’s sluice towers was known, as was optimum release distance, it became a rather simple trigonometry problem. The solution was a small wooden hand-held Y-shaped angular sight.
The ‘Dann bombsight’ was held by a thick dowel mounted beneath its junction, a peephole was mounted in the base of the stem and a small white dowel projected from the end of each arm, angled apart 29.5 degrees. As the Lancaster approached the target the sluice towers would begin to spread and when they touched the dowels, the weapon was released.
Just as the first crews arrived at Scampton, so did ten Lancaster B Mark Is and IIIs, one each from ten of 5 Group’s 15 squadrons operating the type. By the morning of 27 March the full complement of ground crew – 382 men, all from within 5 Group – had arrived, and following aircraft inspections that afternoon the unit was pronounced ready to fly. First off was Flt Lt Bill Astell, who flew his former 50 Sqn Lancaster AJ–B (W4940) at low level for two hours around Leicester, Rugby and Birmingham and ‘photographed nine reservoirs’ to ‘work out cross-country routes for the OTUs’.
That same day a memorandum arrived from 5 Group’s SASO, Gp Cpt Harold V. Satterly. It established the training objectives for the new squadron, which were to be met by 10 May. These included accurate navigation under moonlight conditions at ‘a height which will best afford security against fighter attack’; ‘final approach to the target at 100 feet at [approximately] 240 mph ... it will be convenient to practise this over water’; and bomb release to be carried out visually at an estimated range gauged either by landmark on a nearby shore, or by timing a run from a given landmark, to provide an accuracy of about 40yd.
Sqn Ldr Young, Gibson’s second-in-command, was responsible for designing the training programme to meet these objectives. Training began on 31 March, with navigation exercises using ten standard three-hour-plus cross-country routes. Initially, for familiarisation purposes, these routes were flown singly in daylight at 700ft (213m). Then they were flown at 200ft (61m) for three days, before ‘stepping down’ to 150ft (46m). The routes included dead-reckoning legs – involving calculation of heading, airspeed and time – over the North Sea, and flights across the countryside to various English, Scottish or Welsh lakes that required map-reading. As individual proficiency increased the routes were flown in pairs, and finally in three-aircraft formations at night.
To simulate moonlight conditions a system known as ‘two-stage blue’ was adopted from the US Army Air Forces. This involved placing blue celluloid panels over the windscreen, cockpit side-windows and gunners’ glazed areas, and aircrew wearing amber-lensed flying goggles. Five squadron bombers used this training modification, the first arriving on 11 April.
Not all the crews met the exacting standards required by the mission and the squadron commander. By mid-April Gibson had sent Young’s two NCO crews back to 57 Sqn because they or one of their members ‘did not come up to the standard necessary for this squadron’. These crews were replaced by a single crew only: that of Plt Off William Divall, who had joined 57 Sqn less than two months before. All this reduced 617 Sqn’s strength from 22 crews to 21.
Even harder to achieve than successful low-level navigation by moonlight was the risky requirement to hold an exact altitude close to the water in the dark. Gibson became personally acquainted with this on 28 March when he, Hopgood and Young flew mock attacks on Derwent Reservoir after dusk and almost came to grief. After Gibson recounted this harrowing experience to Satterly, the Group SASO contacted MAP’s director of scientific research (DSR) to ask about a solution. The new DSR, the later Sir Benjamin Lockspeiser, recalled a 1941 Coastal Command experiment with Lockheed Hudson patrol bombers aimed at helping with depth-charging U-boats at night in shallow, inshore waters. This method used triangulating spotlights mounted on the aircraft. Lockspeiser found the experiment had been deemed a failure because inshore waters were too choppy, but he became convinced that the technique would be more effective over smooth-surfaced lakes. A series of tests was undertaken by RAE Farnborough, and Lockspeiser soon came up with what was officially called the ‘spotlight altimeter calibrator’.
The device comprised two Aldis lamps, one mounted in the forward camera aperture under the nose and the other in the disused ventral gun position aft of the bomb bay. The forward lamp or spotlight (shrouded in a length of ‘stovepipe’ to shield the light from enemy gunners) was pointed downwards, perpendicular to the longitudinal axis of the aircraft and starboard 30º, and shone a 10º-wide beam of light downwards forward of the starboard propellers. The aft spotlight was angled forward about 20º from the perpendicular and outwards 40º so that the two beams crossed (but did not intersect) at the prescribed altitude under the bomber’s belly. The aircraft would be at the correct altitude at the point when, viewed from the starboard cockpit blister, the two pools of light touched to form a ‘figure eight’ on the surface below.
It would be the navigator’s job to watch the pools of light get closer as the aircraft descended over the lake and call ‘steady’ when they touched. To help the pilot maintain that height a second altimeter was mounted atop the Lancaster’s glare shield, within the pilot’s field of vision when looking at the target through the windscreen. After the pilot set the barometric pressure to get the indicator needle to point to 60ft (18.3m), he would simply hold the needle steady in order to maintain the prescribed height.
On 4 April Maudslay flew one of the squadron’s loaned Lancasters, AJ–Z (W4926), to RAE Farnborough, where the spotlights were mounted. Four days later, after testing the system over the sea, Maudslay returned to Scampton and demonstrated the system by flying along the runway at low level in the dark. The squadron’s electricians quickly mounted the lamps on three other bombers. After flying down the runway to have their lights calibrated and heights confirmed by theodolite, over the next few nights the crews began training with this simple, reliable way to safely maintain low altitude over water.
Lancaster ‘P-for-Popsie’ (ED909) taxying out on a Chastise practice sortie. This aircraft was the first Type 464 Provisioning Lancaster to arrive at 617 Sqn and was flown by Flt Lt ‘Mick’ Martin and his crew. (IWM MH 6540)
By 26 April Wallis had worked out the final Upkeep delivery parameters – from 450yd (± 25yd, or 411.5m ± 22.9m) range and 60ft (18.3m) height at 220mph (354km/h) – and informed Gibson accordingly. Over the following week the squadron concentrated on weapons delivery practice, dropping 10lb or 20lb (4.5/9.1kg) tetrachloride ‘smoke bombs’ on Wainfleet Range, on the Wash on England’s east coast. A total of 168 practice attacks were made using the newly developed Dann bombsight and aiming between two 30ft x 20ft (9.1m x 6.1m) white ‘cricket boards’ standing some 700ft (213m) apart; 284 bombs were dropped with an average error of 39ft (11.9m).
Having met 5 Group’s training standards, from 4 May the squadron graduated to ‘large force employment’ (LFE) exercises: operating ten-aircraft ‘packages’ nightly and simulating attacks from 60ft (18.3m) mainly against the dam at Eyebrook Reservoir, two miles (3.2km) south of Uppingham in the English east midlands. To facilitate these simulations, two pairs of 20ft x 12ft (6.1m x 3.7m) canvas targets, connected by a camouflage ‘scrim’, were mounted on the dam’s parapet, positioned apart so as to represent the two sluice towers of the Möhne Dam.
The first two Type 464 Provisioning Lancasters arrived at Scampton on 21 April. The most noticeable modification, to the bomb bay, made the great Avro heavy bomber look ‘disembowelled’, and prompted some squadron members to refer to them irreverently as ‘abortions’.
Another obvious modification was the deletion of the mid-upper turret. The aircraft would be ingressing beneath the Luftwaffe’s radar coverage, so interception by night fighters would be avoided. This allowed the upper turret to be sacrificed so as to compensate somewhat for the increased drag caused by the gaping bomb bay.
A Lancaster crew normally consisted of seven men (an eighth man, a ‘second pilot’, might be temporarily included as a way of increasing his experience before he became a bomber captain). The bomb-aimer would man the forward turret until the initial point on the bomb run was approached, at which time he would descend to crouch over the bombsight and determine the release of ordnance. However, delivery of Upkeep involved a low-level frontal attack, which meant the front-turret guns were needed to suppress enemy anti-aircraft fire at the same time as the bomb-aimer was busy peering at the target through his Dann bombsight. Consequently, for this mission the mid-upper turret gunner was moved to the front turret. A further small modification – installing a set of stirrups beneath the turret – was needed to prevent the gunner’s feet bumping the bomb-aimer while he was concentrating on navigation or bomb-aiming.
The delivery vehicle: Avro Lancaster B Mk III ED 825/G, showing modifications to facilitate carrying the Upkeep anti-dam weapon. The B.III was virtually identical to the original B.I but was powered by the 1649-cubic-inch (27-litre) V-12 supercharged Merlin 28, licence-built by the USA’s Packard Motor Car Company. This engine produced the same 1,390hp as the Rolls Royce original. (IWM Photo ATP 11384B)
Avro Lancaster B.III Type 474 Provisioning
| Serial number | Sent to | Date | 617 Sqn code | Fate and remarks |
| ED765 | RAE Farnborough | 8 April | – | Lost in training accident, 5 August 1943 |
| RAF Scampton | July | AJ–M | ||
| ED817 | RAF Manston | 20 April | – | Not flown on Dams Raid. SOC** Lossiemouth, early 1947 |
| RAF Scampton | 16 May | AJ–C* | ||
| ED825 | A&AEE Boscombe Down | 22 April | – | Flown on Dams Raid as AJ–T Shot down 10–11 December 1943 |
| RAF Scampton | 16 May | AJ–T | ||
| ED864 | RAF Scampton | 22 April | AJ–B | Lost on Dams Raid |
| ED865 | RAF Scampton | 22 April | AJ–S | Shot down on Dams Raid |
| ED886 | RAF Scampton | 21 April | AJ–O | Shot down 10–11 December 1943 |
| ED887 | RAF Scampton | 22 April | AJ–A | Shot down on Dams Raid |
| ED906 | RAF Scampton | 23 April | AJ–J | SOC** Scampton, July 1947 |
| ED909 | RAF Scampton | 21 April | AJ–P | As above |
| ED910 | RAF Scampton | 25 April | AJ–C | Shot down on Dams Raid |
| ED912 | RAF Scampton | 1May | AJ–N | SOC** Lossiemouth, Sept 1946 |
| ED915 | RAF Scampton | 25 April | AJ–Q | Ground abort on Dams Raid. SOC** Lossiemouth, 8 October 1946 |
| ED918 | RAF Scampton | 28 April | AJ–F | Lost in training, 20 January 1944 |
| ED921 | RAF Scampton | 27 April | AJ–W | SOC** Lossiemouth, early 1947 |
| ED924 | RAF Scampton | 29 April | AJ–Y | As above |
| ED925 | RAF Scampton | 28 April | AJ–M | Shot down on Dams Raid |
| ED927 | RAF Scampton | 1May | AJ–E | Lost on Dams Raid |
| ED929 | RAF Scampton | 30 April | AJ–L | SOC** Lossiemouth, early 1947 |
| ED932 | RAF Scampton | 30 April | AJ–G | SOC** Scampton, July 1947 |
| ED933 | RAF Scampton | 2 May | AJ–H | Damaged 12 May; not on raid |
| ED934 | RAF Scampton | 1May | AJ–K | Shot down on Dams Raid |
| ED936 | RAF Scampton | 2 May | AJ–X | Lost on landing, 28 July 1944 |
| ED937 | RAF Scampton | 4 May | AJ–Z | Shot down on Dams Raid |
* ED817 arrived at Scampton during the late afternoon of 16 May, too late to be loaded with Upkeep as a spare forthe Dams Raid. The aircraft was coded AJ–C after the raid (the original AJ–C having failed to return).
** SOC: struck off charge and scrapped at locality stated.
In fact, the bomb-aimer’s workload was greatly increased due to the fact that ingress and egress as well as the actual attack would be at ultra-low level. At such heights the navigator’s view of the ground immediately below and to each side was considerably restricted. Consequently the bomb-aimer, who had a more panoramic view from the nose, was tasked with helping the pilot keep the aircraft on the desired ground track. To facilitate this each bomb-aimer prepared a route map of his own. Some made ‘strip maps’ mounted on rollers to dispense with having to constantly unfold and refold large charts in the cramped nose compartment. To increase visibility from the nose the new Lancasters featured an enlarged, near-hemispherical Perspex nose bubble in place of the conventional saucer-shaped dome (which was in fact little more than a porthole).
The final aircraft modification was in the radiotelephone (R/T) system. The Dams Raid would be the first to employ the ‘master-bomber’ concept, whereby a single airborne commander would direct the individual attacks and reorganise the strike force as needed to ensure the primary targets were destroyed first. However, it soon became clear during the low-altitude formation and LFE practices at Eyebrook that the Lancaster’s standard TR1154/55 wireless sets would not suffice. Again, RAE was asked to help. Its radio department decided to replace the existing sets with the more reliable TR1143 very-high-frequency (VHF) radios normally used in RAF fighters. Flt Lt Bernard Bone was sent to Scampton with a 35-man team from 26 (Signals) Group to install them, completing the work by 9 May.
On 25 March, in a cunning attempt to mask the true nature of Upkeep and its intended targets, the Air Ministry’s director of intelligence (security) began a disinformation campaign so effective that it has subsequently deceived even most historians. The ‘cover story’ maintained that ‘the weapon is a special type of mine and the [spherical] wooden casing is designed for use in localities where it will be handled by native labour [implying employment outside of Europe]. The spinning device is in connection with the fusing which is effected by centrifugal force. The uses of the weapon are in the main anti-submarine … Units armed with the weapon are to be known as Special Mining Squadrons.’
When the first of 96 Upkeep weapons – 40 concrete-filled ‘training rounds’ and 56 ‘live rounds’ – began arriving at Scampton at the start of May, they were referred to as ‘mines’. This usage has continued ever since. In the meantime, of course, Wallis had forsaken the spherical wooden casing. The weapon was in fact a 9,250lb (4,196kg) cylindrical depth charge.
Because the Upkeep weapon was no longer spherical, it would have to be released in a wings-level (zero-bank) attitude. If released within Wallis’ precisely prescribed parameters of height, range, speed, rotation and aircraft attitude, Upkeep would skip three times, the backspin providing sufficient retardation so that the weapon would contact the target with an impact ‘soft’ enough to prevent disabling damage to the weapon but with enough residual rotational moment to cause it to ‘roll down’ the dam wall at 10–20fps (3–6m/sec). At a depth of 30ft (9.1m), three standard Royal Navy Mk XIV hydrostatic pistols – water-pressure sensor triggers – would detonate the 6,600lb (2,994kg) of Torpex underwater (torpedo) explosive.
In case the weapon did not sink to the required depth – or somehow became deposited on dry land – a 90-second timer provided a ‘self-destruct’ fuse.
Squadron training with Upkeep began on 11 May, when Gibson led Hopgood and Martin to Manston to load training rounds and then flew to Reculver to practise deliveries, aiming at two screens erected onshore – near the abbey – to simulate the Möhne’s sluice towers. The following day Shannon’s, Knight’s and Munro’s crews flew the same exercises, with Maudslay’s and other crews doing so next day. All participants were well aware that release at extremely low altitude was imperative: the angle of contact with the water had to be shallow enough (less than 7º angle of incidence) to cause the device to ‘skip’ along the surface. Some pilots were a bit over-zealous: two Lancasters were damaged by a huge spray of water and shingle engulfing the tail. Maudslay’s aircraft, AJ–X (ED933), was damaged to such an extent that it had to be withdrawn for repair. He was assigned AJ–Z as a replacement, reducing the Chastise strike force to 19 bombers.
On 14 May Gibson and Young organised a final, four-hour, near full-scale ‘dress rehearsal’. By this time Gibson had decided which crews would attack which dams and the practice was organised accordingly. Nine crews in three triple-aircraft formations navigated first to Eyebrook (‘Target A’) to simulate attacking the Möhne, then to Abberton Reservoir just south of Colchester (‘Target B’) to simulate attacking the Eder. Gibson coordinated individual attacks using the new VHF radios. Six other crews followed another route and simulated attacks against the Derwent Dam, near Sheffield (‘Target C’). Finally the remaining crews – the ‘mobile reserve’ – practised at Wainfleet Range.
Targets were selected by a new ad hoc committee that met for the first time on 18 March. The meeting was convened by ACM Portal and chaired by his ACAS (Operations), AVM Bottomley, who three years before had authored Portal’s request to the Air Ministry to develop the means to breach the Möhne. Bottomley had ostensibly been commissioned to oversee the development and employment of Upkeep and its unsuccessful ‘little brother’, Highball, which was to have been used by Coastal Command Mosquitoes against Axis capital ships. Bottomley took a generous view of Chastise’s charter because of his own – and his chief’s – personal interest in the project.
The committee determined that ‘the two most important dams vulnerable to attack in Germany are the Möhne and the Eder’. Breaching the Eder would not affect the Ruhr industrial district, but being similar to the Möhne in construction, it was potentially vulnerable to Upkeep. The committee ruled out the Sorpe – an earthen berm-type dam that held back the second-largest reservoir affecting the Ruhr – as ‘unsuitable for attack, for tactical and technical reasons’.
When an independent Ministry of Economic Warfare (MEW) study recommending destruction of the Möhne and Sorpe – and not the Eder – came to Bottomley’s attention, he consulted with Col C. G. Vickers, VC, and MEW analyst O. L. Lawrence. Lawrence opined that destroying both the Möhne and the Sorpe ‘would be worth much more than twice the destruction of one’. However, the Eder Dam, the water from which flowed through Kassel into the Weser-und-Mittelland-Kanal, was of ‘no major economic importance’. Bottomley was convinced and, assured that Upkeep ‘had very good prospects’ of breaching the Sorpe also, he recommended to Portal that simultaneous attacks should be made against the Möhne and Sorpe, with the Eder to be attacked if circumstances allowed.
The first primary target: the Möhne Dam, as seen in an RAF vertical overhead aerial reconnaissance photo. At this stage in weapon development and mission planning, pictures like these were about as much hard intelligence about the target that the RAF possessed. Note the double row of anti-torpedo nets separated by timber ‘spreaders’ positioned well in front of the dam structure. North is at top of the photo. (IWM Photo C 3717)
Consequently, on 29 March, when Gibson travelled to 5 Group HQ – St Vincents, an imposing 78-year-old building set among trees at the edge of Grantham in Lincolnshire – AVM Cochrane informed Gibson that the primary targets of 617 Sqn’s first operation were the Möhne and Sorpe dams. Next day Gibson journeyed again to Weybridge where Wallis informed him how Upkeep was expected to work against the Möhne. However, Wallis was somewhat reserved regarding the prospects for breaching the Sorpe.
The Möhne-Talsperre, located some 30 miles (48km) east of the location of the main Ruhr industries, was the largest dam in Europe. Completed in 1913, it was 2,549ft (777m) across and 120ft (36.6m) tall, from bedrock to crest. It held back 140 million tons of water reportedly needed for ‘drinking purposes and industrial supplies’. There were also 13 hydro-electric power stations downstream. These target considerations paled, however, next to the potential amount of destruction resulting from the deluge inundating the valley – sweeping away structures, flooding factories and washing away barges. It was calculated that the Möhnesee would empty in ten hours, which would ‘cause a disaster of the first magnitude even in the lower reaches of the Ruhr’.
Just six miles (9.7km) south-southwest of the Möhne was the Sorpe-Talsperre, comprising an earthen berm 2,100ft (640m) long that rose 200ft (61m) above the streambed. On both sides a gently sloping embankment stabilised a watertight central concrete core. At 30ft depth on the lakeward side the bank the berm, consisting of granulated rock and hard-packed earth, was 67.5ft (20.6m) thick; this would absorb much of Upkeep’s explosive force. Nevertheless, Wallis initially calculated that the pressure wave resulting from a detonation would displace the central core by 16–20 inches (40–51cm) – enough to crack the watertight seal. It was hoped that a serious leak would cause sufficient erosion of the gravel and earth on the far side of the dam to cause the eventual collapse of the central core due to lack of support on that side.
ACM Harris, never an optimist about this operation, thought the Sorpe attack had ‘a poor chance for success’. In fact, even Wallis lost confidence in the approach here. He requested a preliminary, conventional bombing attack against the Sorpe’s far side so as to weaken it sufficiently for Upkeep to work. However, this was viewed, rightly, as alerting the enemy to the real attack and Wallis’ request was denied. Disappointed, Wallis commented during his portion of the mission briefing that ‘it would need at least six mines to crack’ the Sorpe.
Despite these drawbacks, the Sorpe was still considered a worthy target because it held back another 70 million tons of water. If the dam was breached the deluge would contribute greatly to the desired impact on the Ruhr basin. Therefore, the diversionary force would attack the Sorpe using a weapon actually designed for a different type of target, and hope for the best.
Meanwhile, because there probably would be Upkeep weapons remaining after breaching the Möhne, Gibson wanted the Eder included as a primary target. Rather than use these weapons against the Sorpe, he wanted to take on the Eder since the attack profile was the same as for the Möhne and the chances of success were much greater.
The Eder-Talsperre was indeed very similar to the Möhne. Built at the same time, it was located 46 miles (74km) east-southeast of the Möhne and produced the largest reservoir in Germany. Spanning a deep, narrow valley, it was 145ft (44m) high and 1,290ft (393m) across. The attack profile was greatly complicated by the surrounding high terrain. The dam held back 200 million tons of water, and breaching it would provide a dramatic – if militarily inconsequential – flood downstream, through Kassel.
The second primary target: Eder Dam, as seen in an RAF vertical aerial reconnaissance photo used for mission planning, preparation and briefing. While RAF planners considered the Sorpe the next most important target to strike, 5 Group OpOrd specified the Eder as the second target, assigning the Sorpe to the diversionary force. (IWM Photo C 3718)
Planning Operation Chastise was the personal responsibility of 5 Group’s SASO, Gp Capt Satterly, who initially outlined the concept of operations – along with the squadron’s training objectives – in ‘most secret’ orders to Gibson on 27 March.
As Upkeep was perfected, aircrews trained and the concept of how the weapon was to be employed was refined, the draft operations plan was outlined in a briefing to AVM Bottomley’s committee on 5 May – a meeting also attended by Wallis. Wallis stressed the point that, for maximum effect, the water level of the Möhnesee needed to be within 5ft (1.7m) of the dam’s crest. Disturbed that water was already being drawn off, Wallis urged that Chastise be implemented at the earliest opportunity. Cochrane agreed, wanting to strike as soon as possible after 14–15 May – in clear weather and under the full moon.
Five days later, Satterly completed a handwritten draft operation order (OpOrd) and sent it to Whitworth and Gibson for comment. Two days later, capitalising on the lessons of six weeks of intense training, practice and trials, Gibson responded with numerous recommendations. Satterly incorporated many of them, though not all.
The plan called for 20 bombers to make a succession of individual attacks on the five Ruhr dams, plus the Eder and Diemel. To do this, 617 Sqn would be split into three waves: a main force of nine bombers to attack the primary targets, namely the Möhne (Target ‘X’) and Eder (Target ‘Y’); a diversionary force of five aircraft to attack the secondary targt, the Sorpe (Target ‘Z’); and a reserve force of six aircraft to attack the first pair again or strike the tertiary targets (‘D’– ‘G’, formally called ‘targets of last resort’), depending on the results achieved by the earlier waves.
Since the German nightfighters (Nachtjägder) were most successful at full moon, Bomber Command had no significant concurrent operations planned. The only operations were the following: eight Mosquitoes were to make nuisance raids against four major German cities while others conducted intruder operations over six Luftwaffe bases in the Netherlands; and 54 Lancasters, Stirlings and Wellingtons would drop mines in the German Bight while another ten dropped leaflets in France.
All this meant 617 Sqn would have to provide its own diversionary effort. It was planned for the main and diversionary forces to cross the enemy coast simultaneously at widely separated points – the main force over the Scheldt Estuary, the diversionary force over the Dutch Frisian Islands.
The plan was that the two forces would arrive at their targets at midnight (midnight being the assigned time over target or ‘TOT’). The diversionary force would have to depart Scampton first due to the longer distance to the coast and to the force’s target. The main force would be under Gibson’s direct control; he would coordinate individual attacks via VHF. Taking off 2½ hours later than the main force, the reserve force would be under HQ 5 Group control; each bomber would be directed, via Morse code sent via long-range wireless/telegraphy (W/T), to attack specifically assigned targets.
At 0900 on Saturday, 15 May, AVM Bottomley sent the ‘warning order’ to 5 Group directing: ‘Operation Chastise. Immediate attack of targets “X”, “Y”, “Z” approved. Execute at first suitable opportunity.’ Cochrane informed Whitworth that the attack would be launched the next evening.
The Type 464 Lancasters were stood down (except for air-test sorties) to allow last-minute maintenance. Wallis arrived at Scampton to monitor the loading and arming of weapons. The laborious process of loading involved a ten-ton crane, six modified bomb trolleys and two mobile gantries, which limited the loading rate to about one Upkeep an hour. Meanwhile, Gibson travelled to Grantham to confer with Cochrane on the final details of the mission and publish the OpOrd for the squadron. At this point the Hennesee-Talsperre was deleted from the target list due to its proximity to Meschede, a small industrial town thought to be well defended; instead, Diemel was made Target ‘F’.
At 1800 these three key individuals – Wallis, Gibson and Cochrane – plus Young, Maudslay, Hopgood (Gibson’s deputy mission commander for ingress and for the Möhne attacks), and Hay, gathered at Whitworth’s quarters for the initial mission briefing. Hopgood suggested the routing around the Ruhr be adjusted to the north so as to avoid Hüls, where a rubber factory was also thought to be well defended.
The target realignment was incorporated into Satterly’s product, which became ‘No. 5 Group Operation Order No. B.976’ – with appendices for ‘Routes and Timings’, ‘Signals Procedure for Target Diversions’ and ‘Light and Moon Tables’ – and was promulgated at 1000 the next morning.
The scale model of the Möhne Dam and its surrounding area, photographed from an oblique angle as if from the north. The model, used in the briefing for the mission, may still be seen at the Imperial War Museum, London. (IWM Photo MH 842)
‘The longest briefing I ever attended’
Shortly after noon on Sunday, 16 May, crew members began to be briefed on their specific duties for the mission. Gibson and Wallis met with the pilots and navigators while 5 Group chief signals officer Wg Cdr Wally Dunn met with the wireless operators.
Two of the 21 crews were not included: Plt Off Bill Divall had a knee injury and Flt Lt Harry Wilson was ill. Thus, there were 19 crews for 19 aircraft. The third prototype aircraft, ED825, was flown in from Boscombe Down as a spare, loaded with an Upkeep and coded AJ–T, but there was not time to mount the TR1143 radio or the spotlight altimeter. The second prototype, ED817, was also flown to Scampton but arrived too late to be loaded.
At this point the targets were finally revealed – to the relief of those who feared they would be up against the mighty Tirpitz – and after refreshments at 1400 the bomb-aimers and gunners joined the pilots and navigators in viewing large tabletop models of the Möhne and Sorpe dams. Including the Eder as a primary target had been an afterthought, so unfortunately RAF Intelligence was unable to build a tabletop model of this dam in time for the mission briefing. This meant three crews would be attacking a dam with little advanced appreciation of the difficulties involved.
During the discussion the ingress routing change was finally agreed upon and implemented. This verbal modification to the OpOrd resulted in some confusion and exposure to yet another (unknown) concentration of flak during the operation; it also later created uncertainty among many historians.
At 1800 all aircrew gathered in the large meeting room at Scampton’s Junior Ranks’ Mess for the final briefing. Gibson began by announcing that the squadron was ‘to attack the great dams of Germany’, then introduced Wallis, who explained how Upkeep worked. After that, Cochrane added information on security concerns and provided some motivational remarks.
Next, Gibson briefed the men on the operational details. The main strike force would comprise Gibson’s favourites: on his wing would be Hopgood and Martin, with Young and Maudslay leading the next two sections. The three triple-aircraft formations would take off ten minutes apart and head southeast, cross the North Sea on a line intersecting Southwold and the Scheldt Estuary, then turn east to fly between Schouwen and the Walcheren Islands, which were well defended by Marine-Flakabteilung 810 with four heavy and three light flak batteries. Similarly, the formations would fly between the Luftwaffe Nachtjagd bases of Gilze-Rijen and Eindhoven in the Netherlands. Circumventing the heavily defended Ruhr to the north, Gibson would lead the attackers to the Möhne-Talsperre, the first primary target.
A historically supported military maxim is ‘you fight like you train’, and because the squadron’s LFE exercises had used ‘A–B–C to reference the three main targets, Gibson deviated from the ‘X–Y–Z’ designations directed by Satterly’s OpOrd. Once Target ‘A’ was breached, Gibson would lead his remaining bombers to Target ‘B’, the Eder. If any weapons were left over after these attacks, the remaining armed Lancasters would attack the Sorpe Dam (Target ‘C’).
Oblique view of the briefing scale model of the Eder Dam and its surrounding area. The initial point for the attack was the hilltop Waldeck Castle at the end of the road complex along the right edge of the photograph and the model. As the picture makes clear, the attack profile was very tight indeed. Unfortunately this model was not actually completed in time for the briefing. (IWM Photo MH 27710)
Operation Chastise codewords, numbers and letters and their meanings
| Codeword | Meaning | |||||
| Cooler | 617 Sqn aircraft callsign for Operation Chastise | |||||
| Pranger | Attack Möhne Dam | |||||
| Nigger | Möhne breached; divert to Eder | |||||
| Dinghy | Eder breached; divert to Sorpe | |||||
| Tulip | Cooler 2 take over at Möhne | |||||
| Cooler 4 take over at Eder | ||||||
| Gilbert | Attack last-resort targets as detailed | |||||
| Mason | All aircraft return to base (RTB) | |||||
| Goner | Upkeep released and… | with (results noted) | in (target) | |||
| 1 | failed to explode | 8 | no apparent breach | A | Möhne | |
| 2 | overshot dam | 9 | small breach | B | Eder | |
| 3 | exploded 100+yd from dam | 10 | large breach | C | Sorpe | |
| 4 | exploded 100yd from dam | D | Lister | |||
| 5 | exploded 50yd from dam | E | Ennepe | |||
| 6 | exploded 5yd from dam | F | Diemel | |||
| 7 | exploded in contact with dam | |||||
| Example: ‘Goner 710A’ indicates ‘Upkeep released and exploded in contact with dam with large breach in Möhne.’ | ||||||
The diversionary force would be flown by ‘crews who did not reach the highest standards of accuracy in practice’. Gibson and Cochrane felt that ‘the attack method against this dam [the Sorpe] was simpler’ and thus could be flown by the less proficient crews.
The remaining five crews – including all of Gibson’s least favoured, the NCO pilots – would not depart Scampton until after midnight, thus allowing time for reports from the first two groups to provide specific information on which target to attack. This group would fly the same route as the main strike force and, without any directions otherwise, attempt further attacks on targets ‘A’ and ‘B’. However, they also had to be ready to navigate to and deliver their Upkeep weapons against any of the three tertiary targets, if necessary. These targets were the Lister (‘D’), the Ennepe (‘E’) and the Diemel (‘F’).
To enable ‘real-time’ target assignment changes – directing individual aircraft to attack different targets based on the initial results – specific codewords would be used to indicate when the Upkeep was released (‘Goner’) and to indicate a results assessment (two to three digits) at a specific target (letters ‘A’ to ‘F’). Based on this information, HQ 5 Group would assign targets to the reserve crews using callsigns and codewords to designate a specific target to each aircraft.
While the level of flexibility planned is admirable, saddling the youngest, least-proficient crews with the need to memorise attack data for five possible alternative targets – and remember it for hours afterwards – did have a deleterious effect on mission performance. In the event, most of the reserve force was directed to attack the only target not discussed in its portion of the OpOrd: the Sorpe.