Mitsubishi J8M - Design and Development

Design and Development

The J8M1 was intended to be a licence-built copy of the Messerschmitt Me 163 Komet. Difficulties in shipping an example to Japan meant that the aircraft eventually had to be reverse-engineered from a flight operations manual and other limited documentation. A single prototype was tested before the end of World War II.

The Japanese were quite aware of the results of the strategic bombing of Germany, and knew that the B-29 Superfortress would be bombing Japan and the resultant problems which would arise from trying to combat this. Japanese military attachés had become aware of the Komet during a visit to the Bad Zwischenahn airfield of Erprobungskommando 16, the Luftwaffe evaluation squadron charged with service test of the revolutionary rocket-propelled interceptor. They negotiated the rights to licence-produce the aircraft and its Walter HWK 509A rocket engine. The engine license alone cost the Japanese 20 million Reichsmarks.

The agreement was for Germany to provide the following by spring 1944:

• Complete blueprints of the Me 163B Komet and the HWK 509A engine.
• One complete Komet; two sets of sub-assemblies and components.
• Three complete HWK 509A engines.
• Inform Japan of any improvements and developments of the Komet.
• Allow the Japanese to study the manufacturing processes for both the Komet and the engine.
• Allow the Japanese to study Luftwaffe operational procedures for the Komet.

The broken-down aircraft and engine were sent to Kobe, Japan in early 1944. It is probable that the airframe was on the Japanese submarine RO-501 (ex-U-1224), which left Kiel, Germany on 30 March 1944 and was sunk in the mid-Atlantic on 13 May 1944 by the hunter-killer group based on the escort carrier USS Bogue. Plans and engines were on the Japanese submarine I-29, which left Lorient, France on 16 April 1944 and arrived in Singapore on 14 July 1944, later sunk by the submarine USS Sawfish on 26 July 1944, near the Philippines, after leaving Singapore.

The Japanese decided to attempt to copy the Me 163 using a basic instructional manual on the Komet in the hands of naval mission member Commander Eiichi Iwaya who had travelled to Singapore in the I-29 and flown on to Japan when the submarine docked.

From its inception, the project was a joint Imperial Japanese Army Air Service (JAAF)/Imperial Japanese Navy Air Service (JNAF) venture. The JAAF wanted a new design to be drawn up. The JNAF, on the other hand, felt the design should mimic the German Komet because it had already proven to be a stable aerodynamic body. It was the JNAF which won and issued the 19-shi specification in July 1944 for the design of the rocket-powered defence fighter. The contract went to Mitsubishi Jukogyo KK, which would produce both the JNAF version the J8M1 Shūsui and the JAAF version Ki-200.

The project was headed by Mijiro Takahashi. The JAAF, however decided to undertake their own design to meet the 19-shi specifications, working at their Rikugun Kokugijitsu Kenkyujo (JAAF Aerotechnical Institute) in secret.

At the 1st Naval Air Technical Arsenal in Yokosuka, in association with Mitsubishi and Yokosuka Arsenal, work began to adapt the Walter HWK 509A engine to Japanese manufacturing capabilities and techniques. This was also where efforts were underway to produce a glider version of the J8M to provide handling data. While working on this glider, the MXY8 Akigusa (秋草, "Autumn Grass"), Mitsubishi completed a mock-up of the J8M1 in September 1944.

Both the JAAF and JNAF approved its design and construction and a prototype was built. In December 1944, the MXY8 was completed and, on 8 December 1944, at the Hyakurigahara Airfield, Lieutenant-Commander Toyohiko Inuzuka took the controls of the MXY8. Once in the air, Inuzuka found the MXY8 almost perfectly emulated the handling characteristics of the Komet. Two additional MXY8 gliders were constructed in the naval yard at Yokosuka, one being delivered to the Rikugun Kokugijitsu Kenkyujo (JAAF Aerotechnical Institute) at Tachikawa for evaluation. The JNAF initiated the construction another prototype, production designation Ku-13. This was to use water ballast to simulate the weight of an operational J8M complete with engine and weapons. This variant was to be built by Maeda Aircraft Institute, while the JAAF version was to be constructed by Yokoi Koku KK (Yoki Aircraft Co). The JNAF also proposed a more advanced trainer, designated the MXY9 Shūka (秋火, "Autumn Fire") which would be powered by a 441 lbf (1.96 kN) thrust Tsu-11 ducted-fan engine. The war, however, ended before this model could be built.

Mitsubishi and partners Nissan and Fuji proceeded with development of the airframe and Yokosuka Arsenal was adapting the engine for Japanese production, designated the Ro.2. The Japanese succeeded in producing prototypes that outwardly looked very much similar to the Komet. The J8M1 had a wet weight that was 900 lb (410 kg) lighter, the aircraft having a plywood main spar and wooden vertical tail. The designers had also dispensed with the armoured glass in the cockpit and the aircraft carried less ammunition and slightly less fuel.

The Ki-200 and the J8M1 differed only in minor items, but the most obvious difference was the JAAF's Ki-200 was armed with two 30 mm (1.18 in) Type 5 cannon (with a rate of fire of 450 rounds per minute and a muzzle velocity of 2,350 ft/s (720 m/s), while the J8M1 was armed with two 30 mm (1.18 in) Ho-105 cannon (rate of fire 400 rounds per minute, muzzle velocity 2,460 ft/s (750 m/s). The Ho-105 was the lighter of the two and both offered a higher velocity than the MK 108 cannon of the Me 163 (whose muzzle velocity was 1,705 ft/s (520 m/s). The Toko Ro.2 (KR10) rocket motor did not offer the same thrust rating as the original, and Mitsubishi calculated that the lighter weight of the J8M1 would not offset this. Performance would not be as good as that of the Komet, but was still substantial.

The engine still used the German propellants of T-Stoff oxidizer and C-Stoff fuel (hydrogen peroxide/methanol-hydrazine), known in Japan as Ko and Otsu respectively.

A total of 60 of the training version (Ku-13, Ki-13, MXY-8, MXY-9) were produced by Yokosuka, Yokoi and Maeda. Seven of the operational version (J8M1/Ki-200) were built by Mitsubishi.