The Aichi M6A Seiran is the only aircraft to have been designed as a submarine-based attack aircraft and to have entered service, although its only military operation was ended prematurely by the end of the Second World War.
The idea of building large submarines capable of carrying attack aircraft across the Pacific first emerged late in 1941, and originated with Admiral Isoroku Yamamoto. The design of the I-400 class submarines was completed by March 1942, and in June 1942 it was decided to build 18 of these submarines, each of which would be able to carry three aircraft to any point on the planet. Yamamoto had planned to use these submarines to attack the east coast of the United States, but he was killed in April 1943, only three months after the first submarine – I-400 – was laid down at Kure. Work would eventually begin on five of the submarines, of which three would be completed, but only the first two would ever go to sea. Three M6A Seiran attack aircraft were to be carried in a long watertight cylindrical hanger built into the submarine’s superstructure, with the wings folded back against the fuselage and the floats detached.
Aichi were ordered to produce a prototype for the special attack aircraft early in 1942. The new aircraft had to fit within the small submarine hanger, have detachable floats and folding wings, and be capable of being launched by catapult. The floats would be used on training missions, but would be detached on operations. The surviving aircraft would ditch close to their submarines, and the crew would swim back to safety.
Aichi’s first response was to attempt to modify the D4Y1 Suisei (‘Judy’), which they were then producing in large numbers, but this effort failed, and a completely new aircraft had to be developed. The new aircraft looked similar to the Judy, but had a rather more simple structure, to compensate for the limited space on the submarine. It used the Aichi Atsuta Type 32 engine, a licence-built version of the Daimler Benz DB-601A that was never entirely reliable. It would carry a crew of two – pilot and radio operator/ observer/ rear gunner. The folding wings were hydraulically operated, and eventually four trained men could get the aircraft ready for flight in seven minutes.
The M6A prototype made its first flight on 8 December 1943. A number of problems with the design were discovered, and eventually eight prototypes were completed before the aircraft was considered satisfactory. Eighteen production M6A1 aircraft were built between October 1944 and July 1945, only enough to equip a third of the submarines originally planned.
Two M6A1-K training aircraft were produced, with retractable landing gear in place of the floats. These aircraft were given the popular name Nanzan meaning either “Southern Mountain” (the intended meaning) or “Difficult Delivery”.
By 1943 the target for the M6A had changed from American cities to the lock gates at the Atlantic end of the Panama Canal. Two I-400s and two standard fleet submarines (I-13 and I-14) would carry ten aircraft (three each on the I-400s and two each on the other submarines) to a position to the south west of the canal. The aircraft would then cut across the top of South American and dive bomb the lock gates. Work on this plan reached a very advanced stage, and the mission was timetabled for the autumn of 1945, but just before it was due to take place the operation was cancelled in favour of an attack on US Task Force 38, at Ulithi Atoll. This time the submarines left port. I-400 actually reached the rendezvous point off Ulithi Atoll on 16 August and was preparing to launch its M6A1s on the following morning, but at 21.00hrs that evening the Japanese Sixth Fleet formally confirmed the news of the Japanese surrender. The two I-400 class submarines made their way most of the way back to Japan before surrendering to the Americans, having never launched their M6A1s in anger.
Engine: Atsuta 32 in-line (licence-built Daimler-Benz DB 601)
Power: 1,400hp at take-off, 1,340hp at 5,580ft, 1,290hp at 16,405ft
Crew: two in tandem cockpit
Wing span: 40ft 2.75ft
Length: 38ft 2.75ft
Height: 15ft 0.
Weights: 7,277lb empty, 8,907lb loaded, 9,800lb maximum
Max Speed: 295mph at 17,060ft
Cruising Speed: 184mph at 9,845ft
Service Ceiling: 32,480ft
Range: 712-739 miles
Armament: One flexible rear firing 13mm machine gun
Bomb-load: Two 500lb bombs or one 1,764lb or 1,874lb bomb