In the summer of 1944 the resolute Japanese Naval officers, who were faced with the overwhelming material strength of the Allied Forces progressing inexorably towards Japan, had little hope of forestalling defeat by conventional tactics, and many of them began to advocate the use of drastic new combat methods. One of these officers was Ensign Mitsuo Ohta, a transport pilot serving with the 405th Kokutai, who conceived the idea of a rocket-propelled suicide aircraft.
With the help of personnel from the Aeronautical Research Institute of the University of Tokyo, Ensign Ohta proceeded to draft preliminary plans for his proposed aircraft, and in August 1944 submitted his drawings to the Dai-Ichi Kaigun Koku Gijitsusho (First Naval Air Technical Arsenal). Ensign Ohta's proposal was favorably received by the Navy, who decided to proceed with the project and assigned the preparation of detailed drawings to a team of engineers led by Masao Yamana, Tadanao Mitsugi and Rokuro Hattori, the type being designated MXY7. The Arsenal, during WW II known by the acronym Kugisho, was based at Yokosuka, hence the type is often incorrectly designated Yokosuka MXY7.
The MXY7 was primarily designed as an anti-invasion or coastal defense weapon to be launched from a parent aircraft. Following an initial glide after release from the mother plane, the MXY7 was to accelerate towards its target on the power of three solid-propellant rockets mounted in the tail, being fired either singly or in unison. The tiny aircraft was built of wood and non-critical metal alloys and great care was taken in its planning to enable it to be mass produced by unskilled labor. As the aircraft was to be flown on its one-way mission by pilots with only limited flying experience, instruments were kept to a minimum and good maneuverability was demanded to achieve reasonable accuracy.
The design and construction of unpowered prototypes was completed in a matter of weeks, and by the end of September 1944 ten MXY7s had been completed. Named Navy Suicide Attacker Ohka (Cherry Blossom) Model 11, the initial version was to carry a 2,646 lb (1,200 kg) warhead in the nose and be transported in the bomb-bay of a specially modified Navy Type 1 Attack Bomber Model 24J (G4M2e). A battery of three Type 4 Mark 1 Model 20 rockets, providing a combined thrust of 1,764 lb (800 kg) for 8 to 10 sec, was fitted. Initial flight trials without power began at Sagami in October 1944, and the first powered flight was made at Kashima during the following month. These trials proved successful and performance measured during an unmanned flight at Kashima in January 1945 indicated that at 11,485 ft (3,500 m) the Ohka Model 11 could reach a top speed of 288 mph (463 kmh) without power and 403 mph (649 kmh) with full thrust.
Without waiting for the results of the trials, the Navy placed the aircraft in full production, and a total of 755 Ohka Model 11s were built between September 1944 and March 1945. Of these 155 were built by the Kugisho at Yokosuka, and 600 by the Dai-Ichi Kaigun Kokusho at Kasumigaura, with Nippon Hikoki K.K. at Yokohama and Fuji Hikoki K.K. at Kanegawa acting as subcontractors for wings and tail units. On March 21, 1945, the Ohka Model 11 was taken into battle by the 721st Kokutai, but the sixteen G4M2e parent aircraft were intercepted and forced to release their weapons short of the target. The first success was achieved on April 1 when Ohkas damaged the battleship West Virginia and three transport vessels, while the first Allied ship sunk by Ohka aircraft was the destroyer Mannert L. Abele, lost off Okinawa on April 12. The slow and cumbersome parent aircraft proved to be extremely vulnerable when approaching to within a few miles of well-defended targets, and production of the Ohka Model 11 ceased in March 1945.
Forty-five examples of the Ohka K-1, an unpowered version with water ballast replacing the warhead and the powerplant, were built by the Kugisho to provide pilots with limited handling experience of their weapon. The water ballast was released during the terminal phase to reduce the landing speed to 138 mph (222 kmh), the aircraft landing on retractable skids.
The Ohka Model 22 was planned as an improved version of this weapon intended to be carried by the faster Navy Bomber Ginga (P1Y1). Because of the limited clearance provided by the parent aircraft, the wing span of the Ohka Model 22 was smaller than that of the Model 11 and the warhead limited to 1,323 lb (600 kg). To increase the Ohka's range so that the parent aircraft could release it at greater distance from the intended target, the Model 22 was powered by a Tsu-11, a Campini-type jet engine with a 100 hp Hitachi four-cylinder in-line engine as a gas generator. Fifty Ohka Model 22s were delivered by the Kugisho with follow-on production assigned to Aichi Kokuki K.K. and with Murakami Hikoki K.K., Miguro Hikoki K.K. and Fuji Hikoki K.K. acting as subcontractors. Aichi's inability to start production led to the decision to concentrate production of the Okha Model 22 in underground factories managed by the Dai-Ichi Kaigun Kokusho, but the war ended before these factories were completed. One test flight was made in July 1945, but auxiliary rockets installed under the wings went off accidentally just after release causing a stall from which the pilot could not recover.
A two-seat training version, designated Ohka Model 43 K-1 KAI Wakazakura (Young Cherry), was built in limited numbers and had the warhead replaced by a second cockpit and the addition of flaps and retractable skids for landing. A single Type 4 Mark 1 Model 20 rocket was mounted in the tail to obtain limited power-handling experience. Other developments of the Ohka series included a single Ohka Model 11 experimentally fitted with wings made by Nakajima out of thin steel; the Ohka Model 21 combining the powerplant of the Model 11 with the air-frame of the Model 22; and the Model 53 which, powered by a Ne-20 turbojet, was to be towed aloft by another aircraft and released over the target.
A total of 852 Ohkas were built by the following major contractors with the co-operation of a series of sub-contractors. Kugisho at Yokosuka produced 155 Ohka Model 11; 50 Ohka Model 22; 45 Ohka Model K-1 and 2 Ohka Model 43 K-I KAI, while Dai-Ichi Kaigun Kokusho at Kasumigaura produced 600 Ohka Model 11.
Description: Single-seat suicide aircraft (Model 11. 21, 22, and 53). Single-seat training glider (Ohka K-1), or two-seat powered-glider (Ohka Model 43 K-1 KAI). Mixed construction.
Powerplant Ohka Model 11 and Model 21: Three Type 4 Mark 1 Model 20 solid-propellant rockets, total thrust 1,746 lb (800 kg). Ohka Model 22: One 551 lb (200 kg) thrust Tsu-11 turbojet. Ohka Model 53: One 1,047 lb (475 kg) thrust Ne-20 axial-flow turbojet. Ohka Model 43 K-1 KAI: One 573 lb (260 kg) thrust Type 4 Mark 1 Model 20 solid-propellant rocket.
Armament (warhead in the nose) Ohka 11: 2,646 lb (1,200 kg). Ohka 22: 1,323 lb (600 kg).