The is a submarine-launched attack floatplane designed for the Imperial Japanese Navy during World War II. It was intended to operate from I-400 class submarines whose original mission was to conduct aerial attacks against the United States.
Design and development
From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets. In December 1941, Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto, proposed constructing a large fleet of submarine aircraft carriers (also designated STo or sen-toku — special submarine) whose purpose was to mount aerial attacks against American coastal cities. The submarines would surface to launch their aircraft by catapult, submerge to avoid detection, then surface again to retrieve the aircrews who would ditch their planes nearby. By June 1942, the plan was to build a fleet of eighteen such submarines. This was later cut to nine, then five and finally just three as Japan's wartime fortunes declined.
To equip the submarine aircraft carriers, the Imperial Japanese Navy Air Service requested that Aichi design a folding attack aircraft with a range of and a speed of . Aichi was already manufacturing under license, the D4Y1 Suisei (Judy), a relatively small single-engined carrier dive bomber with exceptionally clean lines and high performance. Detailed engineering studies commenced in an effort to modify the Suisei for use aboard the I-400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated.
Aichi's final design, designated AM-24 by Aichi and given the military designation M6A1, was a two-seat, low-winged monoplane powered by a Aichi AE1P Atsuta 30 engine (a licence-built copy of the Daimler-Benz DB 601 liquid-cooled V12 engine). The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility. If conditions permitted, these would allow the aircraft to land next to the submarine, be recovered by crane and then re-used. The floats could be jettisoned in flight to increase performance or left off altogether for one-way missions. The Seiran's wings rotated 90 degrees and folded hydraulically against the aircraft's fuselage (with the tail also folding down) to allow for storage within the submarine's diameter cylindrical hangar. Armament was a single torpedo or an equivalent weight in bombs. One Type 2 machine gun was mounted on a flexible mounting for use by the observer.
As finalized, each I-400 class submarine had an enlarged watertight hangar capable of accommodating up to three M6A1s. The Seirans were to be launched from a compressed-air catapult mounted on the forward deck. A well-trained crew of four men could roll a Seiran out of its hangar on a collapsible catapult carriage, attach the plane's pontoons and have it readied for flight in approximately 7 minutes.
In order to shorten the launching process and eliminate the need for time-consuming engine warm-ups, the Seirans were to be catapulted from a cold start. This necessitated heating the engine oil for each plane to approximately in a separate chamber and pumping it, as well as hot water, through the engine just prior to launch while the planes were still in the hangar. In this way, the aircraft's engine would be at or near normal operating temperature immediately upon getting airborne. The idea was borrowed from the Germans who planned on using a similar launch method for the aircraft of their unfinished carrier Graf Zeppelin.
The first of eight prototype Seirans was completed in October 1943, commencing flight testing in November that year. A problem with overbalance of the auxiliary wings was eventually solved by raising the height of the tail fin. Further testing was sufficiently successful for production to start in early 1944. In order to aid pilot conversion to the Seiran, two examples of a land based trainer version fitted with a retractable undercarriage were built. These were given the designation M6A1-K Nanzan ("Southern Mountain"). Besides the difference in landing gear, the vertical stabilizer's top portion, which was foldable on the Seiran, was removed.