Aside from expedient aircraft conversions, there were a number of more elaborate conversion attempts to improve the performance of the Tokko attacks. As mentioned earlier, the army pioneered these efforts with the bomber conversions for the Banda and Fugaku Squadrons. The next Ki-67 Hiryu conversion was more elaborate, incorporating the secret Sakura-dan (Cherry Blossom) shaped-charge warhead. This warhead was another case of German technical influence, as it was related to the SHL-3500 Beethoven warhead used on the Mistel guided bomb. A hollow-charge warhead was far more effective against armored ships than conventional blast warheads. However, its 5ft 3in diameter required a more substantial modification, with a plywood dorsal bulge added to the aircraft.
A Nakajima Ki-9 Akatombo (Red Dragonfly/“Spruce”) trainer at Kikuchi airbase outside Nagasaki in Japan in 1945, crudely prepared for a Tokko mission with a drum of gasoline in the rear cockpit. It has been painted with a typical emblem of the Tokko force, the cherry blossom, and the inscription on the tale includes the Kana symbol “To” from Tokko, and the Kazekaoru inscription “Rise on a perfumed breeze, fall in a rain of cherry blossoms.” (NARA)
One of the lesser-known Tokko modification efforts was the development of this ventral fairing for modifying light bombers for kamikaze missions. It was designed to carry a 1,760lb (800kg) warhead, and had special fittings for four booster rockets to accelerate the aircraft during the terminal dive. (NARA)
The conversion effort began in December 1944, but was continually delayed – the first five were completed by February 1945. Nicknamed “hunchbacked wizard grannies” by the crews, these Ki-167 Sakura-dan aircraft were deployed with the 62nd Squadron at Kanoya airbase in Kagoshima prefecture along with a small number of the Ki-67-I Kai To-Go conversions. One aircraft took part in an attempted attack on the US fleet around Okinawa on April 17, 1945, in the company of two Kai To-Go, but suffered a premature mid-air detonation of its warhead near Kikaijima, forcing a delay while the cause was investigated. Two other Sakura-dan were expended on a raid near Naha, Okinawa, on May 25 with unknown results, and there were unfulfilled plans to use additional aircraft of this type against Saipan in August 1945.
The IJN placed more of its hopes in new-build Tokko aircraft such as the Ohka, and some Zero fighters were also manufactured specifically as Tokko aircraft under the codename Kembu, these being able to carry a 1,100lb (500kg) bomb instead of the usual 550lb (250kg) bomb. These were issued mainly to elite Tokko units such as the Jinrai squadrons. Some late production Yokosuka D4Y4 Suisei bombers were modified to carry three solid rocket boosters under the rear fuselage to increase the speed of the aircraft during the terminal kamikaze dive.
OHKA LAUNCH FROM MITSUBISHI G4M2E TYPE 1 ISSHIKI ATTACK BOMBER MODEL 24 TEI | |
The Ohka Type 11 was launched from a slightly modified version of the widely used Mitsubishi G4M2e Type 1 Isshiki Attack Bomber, better known to the Allies by its codename “Betty.” The bomb-bay doors were removed, and a simple launch frame was fitted inside. In addition, the bomb-bay floor was modified to accommodate the cockpit canopy of the Ohka and to permit the Ohka pilot to enter and exit the aircraft in flight. This particular aircraft was originally a standard Model 24 Hei variant, converted to Model 24 Tei standards with the addition of a surface-search radar, evident from the antenna on the nose. This illustration shows the G4M2e in the markings of the 3rd Section, 711th Attack Squadron, 721st Navy Aviation Group, which was operating from Konoike airbase in March 1945. It was the 1st and 2nd Squadrons of this unit that conducted the ill-fated attack of March 21, 1944, when all of the bombers were shot down. The three squadrons in this unit can be identified by the tail flash, with the 1st having one flash, the 2nd two flashes, and the 3rd three flashes. |
The MXY7 Ohka was a simple, rocket-powered glide bomb developed at the navy’s Kugisho facility in Yokosuka. (NARA)
The most important of the custom-designed Tokko aircraft was the Ohka (Cherry Blossom) rocket-assisted glide bomb. This project had been proposed in 1943 by a transport pilot, Ensign Mitsuo Ohta. His ideas were ignored until the spring of 1944, when he was sent back to Japan to brief navy officials. His design was refined by Prof Taichiro Ogawa of the University of Tokyo, who drew up preliminary plans. His effort did not attract formal navy support until after the Marianas defeat in June 1944.
The project was transferred to the Kugisho (Kaigun-Koku-Gijutsu-Sho: Naval Air Technical Arsenal) at Yokosuka, and it formally began on August 16, 1944, codenamed Project Marudai under Cmdr Masao Yamana. The original scheme was to power the glider using a KR-10 liquid-fuel rocket engine being developed at Mitsubishi’s Nagasaki plant on the basis of the German Walther rocket engine used in the Me-163 rocket fighter. This idea was quickly rejected due to the immaturity of the rocket engine, as well as its cost and complexity. Instead, the design switched to the use of available solid rocket boosters. The propulsion system was intended to be only enough to accelerate the aircraft during its final dive, since it would carried into the combat area by a larger bomber. The requirements focused on a simple aircraft made of wood and non-strategic materials and one that was easy to assemble and a tenth the cost of a conventional fighter. The Mitsubishi G4M2e Isshiki (“Betty”) bomber was adapted for the deployment mission due to its widespread availability and adequate size.
The Ohka Model 11 was based around a massive 2,645lb (1,200kg) Tekkou armor-piercing warhead in the nose of the aircraft, as seen here on Okinawa with the nose fairing removed. (NARA)
The final production batches of Ohka aircraft had a metal fairing at the rear of the cockpit canopy, instead of the clear section on the earlier production run. This particular example was part of an Ohka unit being stored at Kawaya near Nagasaki at the end of the war. (NARA)
The first unpowered prototypes of the Ohka were completed in early September 1944, and in parallel the Ohka K-1 training version was also developed without rockets but with a skid landing gear. Static firing tests of the rocket motor began later in September. An unmanned Ohka was dropped over Sagami Bay on October 23, 1944, and the first manned test flight of an Ohka K-1 was conducted on October 31, 1944, using two wing-mounted rockets for propulsion. This flight was successful enough that series production of 45 Ohka K-1 trainers began immediately. The first successful test of a rocket-powered Ohka Type 11 took place on November 19, 1944. As a result, production of the type was authorized to begin under the designation MXY7, with 155 assembled at the Kugisho and 600 more at the Naval Air Arsenal at Kasumigaura although most of the main assemblies came from the Fuji Aircraft Co. near Hiratsuka and Nippon Hikkoki at Tomioka.
The original propulsion configuration for the Ohka was five solid rocket boosters, three in the rear fuselage and one under each wing. The wing-mounts were abandoned after the October 31 test flight and a second static ground test on November 19 when it was realized that they caused steering problems due to uneven burn rates and the resulting asymmetric thrust. This was not a problem on the fuselage rockets, since they were so near the centerline. The first Ohka, such as those deployed to Okinawa, were issued with all five rockets.
An elite Ohka unit, the Jinrai Butai (Divine Thunder Corps), was formed in September 1944 under Capt Motoharu Okamura, more formally designated as the 721st Naval Squadron. The unit was based at Konoike airbase in Ibaraki Prefecture and began training with the arrival of Ohka K-1 trainers. Only experienced pilots were accepted for the unit, and they were given an initial test flight from an altitude of 9,000ft (2,743m) and two more from 16,000ft (4,877m), at which point they were considered combat qualified.
The Ohka K1 was the basic training version of the Ohka and was fitted with a skid under the nose for the return landing. This hanger at Yokosuka is filled with the trainers. (NARA)
The first 50 Ohka Type 11 aircraft were delivered to the aircraft carrier Shinano in Yokosuka in November 1944 for deployment to the Philippines, but the carrier was sunk on November 28 by the submarine USS Archerfish. Plans to deploy the Jinrai Butai to the Philippines in December were frustrated when the carrier Unryu that was supposed to deliver them was sunk by the submarine USS Redfish on December 19. A second Ohka unit, the 722nd Tatsumaki (Tornado Corps) Naval Squadron, was raised for new versions of the Ohka, but ended up reinforcing the existing Jinrai Butai when the new Ohka types failed to reach production. Plans to use the Ohka in the Iwo Jima campaign in February 1945 were frustrated when a pre-emptive US air attack against the Konoike base destroyed all 24 modified Betty bombers. The Ohka were first identified at Konioke by US intelligence on March 8, 1945, and given the codename “Viper.”
MXY7 OHKA TYPE 11 |
The Ohka K2 trainer was the only version of the Model 43 to enter production and can be distinguished both by its twin canopy configuration but also by the extended wings. This particular example was found at the Kugisho facility in Yokosuka at the end of the war. (NARA)
The first combat sortie by the 721st Naval Squadron took place on March 21 from Kanoya airbase on Kyushu, carried by 16 G4M2 bombers of the 711th Attack Squadron and under the command of LtCdr Goro Nonaka. The Ohka force included two more bombers with conventional bombloads, and it was escorted by about 30 A6M5 fighters. Their target was US Task Force 58 operating off Kyushu, but when they were about 60 miles from their targets, they were intercepted by 24 US Navy fighters – F4U Corsairs of VF-17 and F6F Hellcat fighters of VBF-17 from USS Hornet. In just a few minutes every single bomber was shot down, along with most of the escort force.
The vulnerability of the Ohka/Isshiki combination to enemy fighters was well appreciated before the ill-fated March 21 mission, and the Kugisho had warned navy commanders that Ohka was only viable in the absence of enemy fighters or with heavy escort. Two steps were taken to address this problem. To begin with, tactics were changed so that Ohka/Isshiki combinations were dispatched singly or in very small groups to avoid detection and intercept by US Navy air patrols. In addition, work was already underway on other versions of the Ohka, the Model 21 and Model 22.
The Ohka Model 22 did not enter service due to problems with its engine. This example was restored at the Udvar-Hazy Center at the US National Air & Space Museum. (Author)
The Ohka Model 22 was fitted with a Hitachi Tsu-11, a primitive type of jet engine that used a conventional Hatsukaze 150hp (112kW) gasoline-powered engine, as seen here, to drive a single-stage axial compressor behind it, which is missing in this view. (NARA)
The Model 21 was a smaller version of the basic Model 11, with the warhead reduced to 1,320lb (600kg) and the wingspan shortened so that it could be carried by the faster P1Y1 Ginga bomber. This type was never produced. Instead, design shifted to the jet-powered Ohka Model 22, which was fitted with a Hitachi Tsu-11 jet engine, based on the Campini powerplant that first flew in Italy in 1941. The novel engine would allow the Ohka to be released from the Ginga bomber at a greater distance from the target, as much as 80 miles (130 km), increasing the survivability of both the Ohka and parent aircraft. Development of the Model 22 began on February 15, 1945, but was delayed due to the immaturity of the Tsu-11. In spite of its unproven performance, limited production of 50 aircraft began at Yokosuka in the summer of 1945, even before a successful test flight. About 20 Tsu-11 engines were produced before the war ended, so few aircraft were completed. The first drop test was conducted on June 26, 1945, but ended in tragedy when the wing-mounted rocket boosters accidentally ignited on launch, precipitating a collision with the Ginga and a subsequent crash. The wing rockets were deleted, but another flight test on August 12 was abruptly terminated when the jet engine prematurely ignited before separation.
In the meantime, other propulsion options were studied. Besides the Tsu-11, the more advanced Ne-20 axial-flow turbojet was being developed for the Kikka jet fighter and was a smaller version of the BMW-003 used by the German Me-262 fighter. The Ohka Model 33 with this engine was planned for launch from the G8N1 Renzan (“Rita”) heavy bombers and the Ohka Model 33 Ko version from submarine catapults. Design of this variant, however, was never completed as it was decided to proceed instead to a more radical solution, the Model 43.
Although powered by the Ne-20 like the Model 33, the Model 43 had extended wings to avoid the need for a parent bomber altogether. The Model 43A was intended for launch from a submarine or aircraft carrier, while the Model 43B was designed for launch from land-based coastal launchers. In parallel with the Model 43, the Ohka K-2 trainer was designed and ten were built in Yokosuka in May–June 1945 for preliminary training.
The Ohka Model 43 was delayed because of shortages of its NE-20 turbojet engine which was also used by the Kikka jet fighter. This wooden mock-up was found after the war in the Yokosuka workshops with an engine-compartment mock-up on the ground. (NARA)
The Ohka Model 43B was intended to be launched from special coastal bases. The Ohka was stored in a protective cave, and then rolled out to the launch rail on its special launch cart. The cart was propelled along the launch-rail by two rocket boosters. (Author)
Design of the Ohka Model 43 was completed on April 26, 1945, and a wooden mock-up finished in May 1945; production tooling work started at Yokosuka and at Aichi Aircraft, but was disrupted by B-29 bomber attacks. Construction of the first Ohka Type 43 launch base began in July 1945 on the Miura peninsula, and there were plans to construct 41 launchers and 245 underground shelters at seven sites by mid October 1945. A catapult launch site was in operation at Takeyama near Yokosuka for training Ohka 43 pilots using the Ohka K-2 glider and the first test of a catapult using a K-2 trainer took place on June 27. A wooden mock-up of the Model 43B was tested on a catapult at the Mt Hiei site in August 1944. Yet the war ended before Ohka Model 43 production began. Another derivative of the Model 43 was also planned, the Model 53, which was intended to be towed into the air like a sailplane by another aircraft before igniting the engine and proceeding to the target. This variant never proceeded beyond the preliminary design stage.
Since none of the advanced Ohka types reached production, missions continued with the Ohka/Isshiki combination on March 21, 1945, during the campaign around Okinawa. A sortie by three bombers on April 1 was ineffective, and the same day, US Marines first discovered the secret Ohka from a cache of 15 near Kadena airbase on the island. It was dubbed the “Baka” (Fool) after its discovery on April Fool’s Day.
On April 12, the Ohka’s luck changed. A sortie by eight Ohka/Isshiki combinations saw the picket destroyer USS Mannert Abele struck by an A6M5 Zero and then by an Ohka piloted by Lt Saburo Doi, which broke the ship in half, making the destroyer the first US ship sunk by an Ohka. The destroyer USS Stanley was hit and damaged during the same attack. The Ohka raids continued sporadically, with mixed results. Sorties by six bombers on April 16 were ineffective, but a transport and a destroyer were hit and damaged on May 4 and May 11. A final attack on June 22 by six bombers was wiped out by US fighters before reaching its target.
The Ohka Model 22 was designed to be launched by the smaller but faster P1Y1 Ginga bomber, as shown in this illustration. (Author)
Japanese jet and rocket propulsion was heavily dependent on German technology. These are three of the engines intended for various kamikaze aircraft, starting with the Maru Ka-10 copy of the Argus As 0014 pulse-jet on the left (intended for the Baika), the Mitsubishi KR-10 rocket engine copied from the Walter HWK 109-509 (for the Shusui) fighter, and the NE-20 based on the BMW-003 (for the Kikka fighter and the Ohka Model 43). (NARA)
In total, 74 Ohka/Isshiki sorties were conducted during the Okinawa campaign, and 56 Ohka were either released or shot down while still attached to their parent bomber. These attacks sank the destroyer Mannert Abele and damaged at least two other destroyers. Aside from the vulnerability of the Ohka/Isshiki combination, the Ohka was very difficult to steer in its terminal dive, and there are numerous accounts of Ohka missing their targets. Following its combat use in the spring of 1945, there were about 230 Ohka Type 11 still in inventory in July, these being hoarded for the final defense of Japan.
Japan also acquired engineering information on the German Fieseler Fi-103 (V-1) “Buzz Bomb” cruise missile, and had begun to manufacture its Argus pulse-jet engine as the Maru Ka-10. Such pulse-jets were much easier to manufacture than axial-flow turbojets such as the Ne-20, and Japan attempted to acquire engineering drawings of the Fi-103 for production of a manned suicide version. Only a portion of the documentation arrived, so the IJN sponsored the development of the similar Baika (Plum Blossom), starting in the summer of 1945. Both the Yokosuka arsenal and Kawanishi offered designs, but neither had progressed beyond preliminary sketches before the war ended.
The Nakajima Kikka (Orange Blossom) was patterned after the German Me-262, but was somewhat smaller. There was controversy over its intended use, with some navy commanders urging its application as a fast bomber, even though it was nominally part of the Tokko aircraft program. It flew only once prior to war’s end, due to delays with the associated Ne-20 turbojet engine effort. (NARA)
Planned Tokko Aircraft Production in 1945 | |||||||||
May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Total | |
Ohka Type 22 | 6 | 49 | 30 | 50 | 60 | 60 | 60 | 315 | |
Ohka Type 43 | 2 | 10 | 22 | 38 | 65 | 80 | 217 | ||
Ki-115/Toka | 110 | 210 | 220 | 277 | 325 | 430* | 500* | 500* | 2572 |
Total | 116 | 259 | 252 | 337 | 407 | 528 | 625 | 580 | 3104 |
*Still under discussion |
The declining inventories of suitable suicide aircraft convinced the IJA to initiate its own program for a dedicated Tokko aircraft. In contrast to the navy’s high-tech jet and rocket approach, the IJA favored low-tech solutions. In January 1945, the IJA started the Ki-115 Tsurugi (Sword) Experimental Army Special Attack Aircraft, which was a joint venture by the Mitaka Research Institute and the Nakajima plant at Ota. The aircraft was of very conventional design, and powered by a Nakajima Ha-115-II 1,130hp (843kW) radial engine. To simplify production, the aircraft used a basic, fixed landing gear and the tray for the bomb under the aircraft lacked any means to jettison the ordnance in the event that the mission had to be aborted.
The first prototype was completed on March 5, 1945, and began flight trials. The initial design proved to be too crude. The simple undercarriage made it difficult to handle the aircraft on the ground, visibility from the cockpit was poor, the wings were too small for adequate lift, and the lack of flaps aggravated its shortcomings during take-off. As a result, a re-design was ordered prior to series production. The first prototype of the revised Ki-115 Ko design was ready in June 1945, and it was rushed into production at Nakajima’s plants at Iwate and Ota.
The IJA began a training program for Ki-115 pilots in late June 1945, but the program was abruptly cancelled after many pilots were killed flying the primitive aircraft. In spite of its shortcomings, the IJN saw the merits of the low-tech concept and approached Nakajima about building a similar aircraft designated as the Toka (Wisteria) for its own kamikaze units. The IJN expected to build thousands of Toka in 1945, more than the rest of its high-tech kamikaze aircraft combined. Its aircraft differed in the type of powerplant and the ability to drop the bomb in case of mission abort. In the event, no navy Ki-115s were ever built and none of the 104 Ki-115 Ko manufactured for the army were actually issued to combat units, due to the type’s flying deficiencies. It is worth noting that a number of other cheap Tokko aircraft were under development at war’s end, such as the elementary Ta Go (Bamboo Spear), which was of wooden construction and even simpler than the Tsuragi. However, most of these designs never left the drawing board or existed only in prototype form at the end of the war.
The Nakajima Ki-115 Tsuragi was the IJA’s low-tech alternative to the Ohka. The design proved to be too difficult for inexperienced pilots to fly, and the program came to a standstill prior to the end of the war. (NARA)