In April 1944 a Japanese submarine left Lorient carrying the plans for the most advanced interceptor in the world. It reached Singapore on 14 July. Twelve days later, on its way home, USS Sawfish sank it off the Philippines. A second boat carrying the actual aircraft — almost certainly RO-501, the former U-1224 — had already been sunk in the mid-Atlantic in May. A third attempt, aboard U-864, went down off Bergen in February 1945.

What survived was a naval officer. Commander Eiichi Iwaya had travelled aboard I-29 as far as Singapore and then flown on to Japan, carrying a basic instruction manual for the Messerschmitt Me 163 Komet. From that manual, and very little else, Mitsubishi built a rocket fighter.

Why Japan wanted a rocket

By late 1943 Japanese officers had watched the strategic bombing of Germany and drawn the obvious conclusion about their own cities. The B-29 would come, and it would come high. Neither the A6M Zero nor the newer J2M Raiden could reach and fight effectively above 30,000 feet.

The Me 163 could. Attachés who visited Erprobungskommando 16 at Bad Zwischenahn came away convinced. In late 1943 Japan negotiated licences for both the aircraft and its Walter HWK 509A rocket engine. The engine licence alone cost 20 million Reichsmarks. Germany was to supply complete blueprints, one finished Me 163, two sets of sub-assemblies and three engines by spring 1944.

Almost none of it arrived.

Building an aeroplane from a handbook

With the hardware at the bottom of three oceans, the decision was taken to reverse-engineer the Komet from Iwaya’s manual. In an unusually cooperative arrangement, the Army took the rocket engine and the Navy took the airframe. The two services disagreed at once: the Army wanted a clean-sheet design, the Navy wanted an accurate reproduction of a shape that was already known to fly. The Navy won, and the 19-shi specification was issued in July 1944. Mitsubishi got the contract, with Mijiro Takahashi leading the design, working alongside the First Naval Air Technical Arsenal and with Nissan and Fuji as manufacturing partners.

The result looked remarkably like the original but was not a straight copy. The J8M1 came out roughly 400 kg lighter, with a plywood main spar and a wooden vertical tail, no armoured cockpit glass, less ammunition and slightly less fuel. The Toko Ro.2 rocket did not match the German engine’s thrust, and Mitsubishi calculated that the weight saving would not fully compensate. Performance would be short of the Me 163, but not by much.

The glider that proved it

Before anyone lit a rocket, they built a glider. The MXY8 Akigusa — “autumn grass” — was completed in December 1944, and on 8 December at Hyakurigahara Lieutenant Commander Toyohiko Inuzuka towed aloft and let go. He reported that the Akigusa handled almost exactly like the Me 163. Which meant Takahashi’s team had reproduced a German aerodynamic shape from a handbook and got it right.

Around sixty trainers of the Ku-13 / MXY8 family were produced. Pilot conversion courses began. The 312th Naval Air Group was chosen to fly the first operational J8M1. Mitsubishi, Fuji and Nissan tooled up for quantity production.

7 July 1945

The first powered aircraft was ready in June 1945 and flew on 7 July, with Inuzuka again at the controls. The rocket start worked. He jettisoned the take-off dolly, accelerated and pitched into a 45-degree climb. At about 400 metres the engine cut.

The cause was a design flaw in the fuel injection system. With the tanks only half filled for a first flight, the steep climb let the propellant shift, an air lock formed in the fuel line, and an automatic cut-out did what it was designed to do. Inuzuka got the aircraft turned round and gliding back, but came in fast, clipped a small building at the edge of the airfield and the J8M burst into flames. He died the next day.

All flights were grounded. Days later the fuel mixture exploded during a ground test. Testing was to resume in late August with modified fuel pumps on the sixth and seventh prototypes. Japan surrendered on 15 August.

What the plan actually was

It is worth being clear about what the Ki-200 was meant to do. With around five and a half minutes of powered endurance, the envisaged mission was one or two firing passes at a B-29 formation, then a ramming attack with whatever energy remained. Any propellant still aboard would probably detonate on impact. A special attack unit, the Hagakure-Tai, was being planned along the lines of the Luftwaffe’s Sonderkommando Elbe when the war ended.

Ciò che sopravvive

Two aircraft were shipped to the United States aboard USS Barnes in November 1945. One went to the Navy at Glenview and was eventually scrapped; the other went to the Army at Wright Field and was acquired in 1948 by what became the Planes of Fame Air Museum — the first aircraft that museum ever owned. It is prototype number three, serial 403, and it is on static display at Chino, California.

The second survivor turned up in the 1960s: a nearly complete but badly damaged fuselage found in a cave in Japan. It was displayed at a JASDF base near Gifu until 1999, when Mitsubishi restored it for the company’s Komaki plant museum.

Sources: Wikipedia (Mitsubishi J8M); Bob van der Linden, “Japanese Wonder Weapons”, Smithsonian National Air and Space Museum, 11 August 2020; Planes of Fame Air Museum collection notes; Walter E. Grunden, “Secret Weapons and World War II” (2005); William Green, “Rocket Fighter” (1971).