The Empire of Japan and the nation of Germany had maintained a relationship since Prussian authorities made diplomatic contact on the Japanese mainland in 1860. However, World War 1 (1914-1918) delayed a forged alliance when Japan found itself against the German Empire and claimed several German territories in the Asia-Pacific region. During the interwar years that followed, the two nations would adopt similar militaristic-driven governments that controlled both political and military policy and this direction brought the two nations into agreement once more. Japan aligned with the Axis powers in World War 2 (1939-1945), joining Germany and Italy as principles in the fight though distances between the powers ultimately segregated combat into two defined fronts - the Pacific and European Campaigns.
During their alignment in World War 2, certain military agreements came to pass and one such resolution became availability of the technology driving the new German Messerschmitt Me 163 "Komet" rocket-powered interceptor. The Japanese government, not blind to the disastrous results of the Allied day/night bombing campaigns on Italy and Germany, understood the arrival of the high-altitude Boeing B-29 Superfortress would bring similar ruin to the Japanese mainland in time. Due to the advanced nature of the B-29, the heavy four-engined bomber could fly out of reach of available Japanese defenses including its interceptor arm. The Me 163 offered some hope - a supremely fast little aircraft outfitted with cannon armament designed exclusively to combat large bomber formations. Its rocket-powered nature ensured quick response times in achieving the required altitude prior to the bomber formation's arrival.
Of course this sort of evolutionary design came at a price for the rocket fuel powering the Walther engine allowed for just 7.5 minutes of flight time, limiting engagement to a few controlled swoops. The speed at which the Me 163 approached often led to pilots overshooting their intended targets at speed. Additionally, the German fuel mixture consisted of a highly volatile combination of "C-Stoff" (a hydrogen peroxide/methanol-hydrazine combination) mixed with an oxidizer in the form of "T-Stoff" - the mixture having a tendency to ignite and explode. Landing was accomplished by gliding onto its belly, a moment in flight which made the Me 163 extremely vulnerable to passing fighter aircraft. Regardless, the aircraft offered up massive performance benefits over traditional propeller-driven fighter types and turbojet technology was still in its infancy - leaving the Japanese Empire with little choice as to how to proceed.
The Japanese Empire secured the production rights to both the Messerschmitt airframe and the Walther HWK 509A series rocket engine that powered it in 1943. Localized production of both would make the Me 163 immediately available to Japanese aviators while at the same time providing a technological advantage of sorts concerning advanced rocket technology for Japanese engineers to evolve on their own. The Me 163 was already being utilized operationally over German territory with limited success and this showed the Japanese that the weapon could be a viable solution for their own defensive war being forced by the Americans in the Pacific. German factories produced some 300 Me 163 Komets before the end of the war and these claimed between 9 and 16 air victories according to conflicting records.
A complete Me 163 airframe, Messerschmitt technical blueprints and three example Walther rocket engines were stowed aboard two separate Japanese Navy submarines - RO-501 and I-29. RO-501 left Germany (Kiel) on March 30th, 1944 while the I-29 departed the French port of Lorient on April 16th, 1944. While en route, RO-501 was targeted, attacked and sunk by American forces on May 13th - its cargo lost to the deep. The I-29 managed to safely navigate to Singapore with its Me 163 blueprints and Walther engine examples intact.
Despite the loss of the RO-501 and its Me 163-related goods, the Japanese understood the German text and drawings enough to attempt to engineer the aircraft from scratch beginning in July of 1944. The storied concern of Mitsubishi was charged with developing the rocket-powered interceptor for use by the Imperial Japanese Army and the Navy - the project to be led by Mijiro Takahashi. The Army version would be designated as the "Ki-200" while the Navy mount was to be known as the J8M "Shusui" (translating to "Autumn Water", "Sword Stroke" and "Sharp Sword").
Mitsubishi produced a powerless glider to handle in-flight data gathering while also working on flyable prototype to help expedite development. A mockup of the latter was completed in September of 1944 and the powerless glider appeared in December. Naturally, the glider was put to the air first with a test pilot in the cockpit while being towed by a lead aircraft. The control system and handling was found to be adequate and development continued. Yokosuka was commissioned for construction of two more gliders for additional testing while work then began on a second flyable prototype. Testing proved the Japanese copy of the Walther engine somewhat underpowered compared to the original performance figures achieved by Germany though this could be offset with utilization of a lighter airframe. With data in hand and test flights proving the design sound, a consortium led by Mitsubishi (to include Nissan and Fuji) ramped up for airframe production while Yokosuka was gearing its lines for the Walther rocket engine as the "Ro.2".
The first powered flight of a J8M prototype occurred on July 7th, 1945. However, after clearing take-off and beginning a climb, the engine cutout was activated due to half-full fuel tanks, disrupting all power to the rocket. Despite recovering from the ensuring stall, the test pilot managed to glide the aircraft to safe levels but landing was complicated by one of the wings clipping a building near the runway. The strike jarred the aircraft and the volatile fuel stores ignited, severely injuring the test pilot and completely destroying the aircraft. Test pilot Toyohiko Inuzuka would die of his injuries the following day.
The accident expectedly forced a delay to all scheduled J8M flights while the engines were being reworked to help counter the fatal flaw. In August of 1945, Mitsubishi managed a redesigned J8M in the "J8M2" and manufacturing capabilities were almost ready for serial production of the aircraft. However, the Empire of Japan was forced into surrender through several unfolding events - the Americans had showcased their atomic bombs to deadly effect on Hiroshima and Nagasaki, the Allied bombing campaign had severely disrupted local Japanese war-making capacity, Japanese territorial losses in Asia and the Pacific further restricted availability to natural resources and the Japanese Navy was neutered from years of irrecoverable losses at sea - primarily to its carrier fleet at the hands of the United States Navy. Much of what remained were fanatical island defenders cut off from any prospect of hope with no option to surrender honorably.
The Empire of Japan therefore capitulated on August 15th, 1945, officially ending World War 2 as a whole. The surrender would be signed on the deck of the American battleship USS Missouri on September 2nd 1945 and the Japanese defense industry would be dismantled for the foreseeable future. With the collapse of the Japanese Empire, the Mitsubishi J8M project also came to an end. All existing examples were confiscated by the Allies for evaluation and ultimate destruction. At war's end, some 60 training airframes had been completed while 7 of the combat-capable versions were available.
Overall, there were five defined variants in the J8M family line. The J8M1 was the initial aircraft designation armed with 2 x 30mm cannons and this was followed by the J8M2 for the IJN with its 1 x 30mm cannon armament. The J8M3 would have featured an elongated fuselage and wider wing span while being powered by a Tokuro-3 series engine of 4,400lbs thrust. Training versions by Yokosuka included the MXY-8 "Akigusa" based on the original J8M production model and the MXY-9 "Shuka", this being powered by a Tsu-11 thermojet engine - the Tsu-11 relying on a 4-cylinder inverted inline engine coupled to a single-stage compressor with fuel-injection system collectively (when ignited) producing thrust output via rather primitive means.
The Japanese end-product was highly similar to the original German Messerschmitt series complete with a short, stout fuselage housing a single-seat cockpit, well swept-back wing assemblies and a single vertical tail fin. Armament would have been 1 or 2 x 30mm cannons. The rocket engine was set within the fuselage and exhausted through a circular port under the vertical tail fin at rear. The volatile combination of T-Stoff and C-Stoff was retained and known to Japanese engineers as "Ko" and "Otsu" respectively. The pilot sat under a glazed canopy that hinged to the right side while his rear view was obscured by the raised fuselage spine. The Mitsubishi design would take off and land in much the same way as the Me 163 - via a jettisonable two-wheeled dolly for take-off and land on a skid located on the belly of the fuselage.
Overall, the rocket-propelled interceptor proved something of a technological dead-end from a tactical perspective - rocket powered vehicles being later used for weaponry, testing flight envelopes, breaking the sound barrier and ultimately reaching space. The "turbojet" engine had officially arrived and signified the new way of things, quickly supplanting the rocket engine as the next generation propulsion system. The Me 163 itself proved only a marginal success for the Germans and its reach over the skies of Japan may have met a similar fate. By mid-1945, the Pacific War was going to be won by the Allies regardless of the J8M's introduction into service - though the little aircraft provided the Empire of Japan with a fighting chance nonetheless.