Aichi M6A Seiran
The Aichi M6A Seiran was a Japanese attack floatplane designed to be carried inside the giant I-400-class submarines and catapulted against distant targets, above all the Panama Canal. To fit the submarine's watertight hangar its wings rotated and folded flat, and its Atsuta V-12 gave it the speed of a land-based bomber. Aichi built 26, but earthquakes, bombing and a shrinking submarine fleet delayed the type, and its one planned operation, against Ulithi Atoll, collapsed as the war ended. No Seiran ever saw combat. The sole survivor is preserved by the National Air and Space Museum.
Why It Mattered
The Seiran was a full-sized bomber designed to operate exclusively from a submarine. Giant submarine aircraft carriers would ferry it across thousands of kilometres of ocean to strike targets such as the Panama Canal and the American mainland. Three of the huge I-400-class boats were completed, each able to carry three Seirans in a watertight hangar. The National Air and Space Museum calls the aircraft and its submarines an ingenious blend of aviation and marine technology. Yet for all the ingenuity, no Seiran ever saw combat.
Design Notes
The Imperial Japanese Navy assigned the design to the Aichi company in May 1942, and Aichi's chief engineer, Toshio Ozaki, led the project. The navy's specification asked for an aircraft that could haul a torpedo or an 800 kg bomb yet fit inside a cylindrical hangar 3.5 metres across. Ozaki's solution was a wing that rotated ninety degrees at its spar and then folded back flat against the fuselage, while the floats came off and were stowed in separate compartments in the submarine. Parts of the tail folded too. Power came from Aichi's Atsuta, a liquid-cooled inverted V-12 developed from the German Daimler-Benz DB 601A. Speed on the surface mattered as much as speed in the air: the navy set a limit of thirty minutes to ready and launch three aircraft, and drilled crews eventually launched three in under fifteen minutes, albeit without floats.
Operational History
Aichi completed the first prototype in October 1943 and flight tests began the following month. A second prototype joined testing in February 1944, and the navy ordered the type into production before the remaining prototypes had even been delivered. Two of the prototypes were land-based trainers with retractable wheeled gear, given the name Nanzan. A major earthquake in December 1944 severely disrupted the production line, and B-29 raids disrupted it further. Aichi eventually completed 26 Seirans, prototypes included, together with the two trainers. By January 1945 the aircraft was ready for operational use. The navy organised the 1st Submarine Flotilla and the 631st Air Corps around I-400 and I-401, the smaller carriers I-13 and I-14, and ten Seirans. The force's first assignment was the Panama Canal: all ten Seirans were to strike the Gatun Locks, six with torpedoes and four with bombs. On 25 June 1945 the orders changed, and the target became the American fleet anchorage at Ulithi Atoll, which reconnaissance aircraft would scout ahead of a one-way attack by six Seirans. The operation collapsed in transit: I-13 was sunk by an American destroyer, and I-400 missed a vital radio message and steamed to the wrong rendezvous. On 16 August 1945 the flotilla learned the war was over and was ordered home to scuttle its aircraft. The I-400's crews holed the floats and pushed their Seirans overboard, while I-401's sailors catapulted theirs into the sea.
Afterlife
One Seiran survived the scuttling: the last airframe built, serial number 28, which Lieutenant Kazuo Akatsuka ferried from Fukuyama to Yokosuka and surrendered to an American occupation contingent. Shipped to the United States, the aircraft was displayed at intervals at Naval Air Station Alameda, California, before the Navy transferred it to the Smithsonian's Garber Facility, where it arrived in November 1962. It then spent twelve years stored outdoors before indoor space became available. Restoration began in June 1989 and finished in February 2000, carried out by museum staff, volunteers and Japanese assistants, with no production drawings left to guide them. The work exposed the conditions of late-war Japanese production: a flap damaged in a bombing raid and patched over with fabric, poor fit of parts, and an English alphabet scratched by a student's hand inside a wing. It is now the only Seiran in the world, displayed at the Steven F. Udvar-Hazy Center in Chantilly, Virginia.