The 7,200-pound Seiran was an ingenious response to a directive issued by Japanese military leaders for a “secret weapon” that could attack U.S. cities. To overcome the distance, the Seiran worked in conjunction with the huge Japanese I-400-class submarine. Three of the bombers fit into an 11.5-foot-wide hangar tube inside the sub. The Seiran’s wing spars rotated and allowed the wings to fold and tuck against the fuselage. The supersub was to make the journey across the Pacific and surface within flying range of the target. Then a carefully choreographed crew would open the hangar tube, unpack the first of the amphibious bombers, attach its floats, start the liquid-cooled engine, and catapult the manned Seiran from the sub’s deck—all within a period of seven minutes.
When the floatplanes returned from their mission, a crane would lift them out of the water, allowing crews to remove the floats, fold the wings, and stuff them back into the cargo hold for the trip home. Two Seiran-laden submarines were on their way to Micronesia in August 1945 when the Japanese emperor surrendered. By that time, Aichi had manufactured 28 of the airplanes.
From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating float planes from submarines to search for targets. In December 1941, however, 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.
To equip the submarine aircraft carriers, the Imperial Japanese Navy Air Service requested that Aichi design a folding attack aircraft with a range of 1,500 km (810 nmi) and a speed of 555 km/h (300 kn).
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 1,050 kW (1,410 hp) 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 alight next to the submarine, be recovered by crane and then re-used. One 12.7mm Type 2 machine gun was mounted on a flexible mounting for use by the observer.
Aichi M6A Seiran with folded wings for submarine storage (scale model)
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 via a 26 m (85 ft) 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.
Fitting a high-performance combat aircraft into the confines of pressure-tight hull presented Aichi designers with an enormous challenge. To save space, the Japanese designed a 12-cylinder liquid-cooled Atsuta 32 engine based on the German Daimler-Benz DB603. Also, the entire aircraft could be assembled or disassembled in minutes, due not only to foldable wings, but also tailplanes.
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 60 °C (140 °F) 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.
M6A1-K Nanzan (scale model)
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.
The Aichi M6A Seiran was equipped with one 12.7 mm cabin-mounted Type-2 Machine gun and had a bomb compartment that could hold one 1,760-lb torpedo or two bombs of the same weight.
|Type:||two-man crew attack floatplane|
|Design:||Aichi Aircraft Firm|
|Engine(s):||1 x Aichi Atsuta Type-32 1,400-HP inline radial engine|
|Max Speed:||474 kilometers per hour|
|Max Range:||2,000 kilometers|
|Empty Weight:||3,362 kilograms|
|Loaded Weight:||4,250 kilograms|
Reconstructed Aichi M6A Seiran at the National Air and Space Museum