The Aichi M6A Seiran

The Aichi M6A Seiran was one of the most unusual aircraft of World War II it was capable of both dive bombing and launching torpedoes against an enemy target.  Additionally this aircraft was specifically build to be launched by the massive I-400 class submarines of the Japanese Imperial Navy.

Japanese Military Desires

Ever since the late 1920s, the Japanese empire has been looking for ways to improve its naval power. One of the military doctrines they believed would achieve this is the development of submarine aircraft-carriers with purpose built aircraft.
The basic idea behind this was simple:
Submarines would be able to approach the enemy coastline undetected and then lunch their aircraft to conduct bombing runs on the unsuspecting foe. This way, they could greatly increase the operational range of their bombers.
While this idea has been present before, with the start of the Second World War it has gained much traction with the Japanese military top. It came as no surprise that the Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto, proposed construction of a large fleet of the newly designed Sentoku submarine aircraft carrier. The main idea was that these submarines would approach the American coast, and subsequently launch their bombers to bomb American military targets and cities.
The construction of these special submarines posed problems of its own, but special design solutions also had to be taken in the construction of the new bomber aircraft that would be on board.
This task fell to the Japanese aircraft manufacturer Aichi, who already had several designs created for the needs of the Japanese navy. While they already had fairly lightweight, small bomber design in the D4Y1 Suisei, but after some deliberation, this design was deemed unsuitable for the new navy needs.

The Transforming Airplane – a novel piece of engineering

M6A1The main problem the Aichi faced was a simple question: How to fit several of these new bombers in the very confined space of a submarine?
To combat this issue, Aichi designed a two seat, single engine monoplane, powered by 1,050 kW (1,410 hp) Aichi AE1P Atsuta 30 engine. The most revolutionary, and important, facet of this new plane was that it had foldable wings and flaps, which allowed it to be stowed in a narrow cylindrical hangar that ran along the length of the Sentoku submarine. The wings would rotate 90 degrees and fold back, while the flaps would fold up, using special hydraulics system designed for this plane.
The new plane was designated M6A1 Seiran, with the name inspired by an 18th century woodblock print titled “Awazu no seiran”. It had a top speed of 474 km/h (295 mph) at an altitude of 5,200 m (17,060 ft). The maximum range was 1,190 km (739 mi), at the cruise speed of 296 km/h (184 mph). Due to its small size, the Seiran was able to carry only a single 850 kg (1,870 lb) torpedo or the same weight in bombs, and it had a single 13 mm (0.51 in) machine gun, that was used by the spotter, who was sitting in the second seat.
The initial design didn’t account for any sort of landing gear – the plane was supposed to be discarded with the pilot either ejecting, or doing a kamikaze run. As this was seen as a very uneconomical solution, the design was revised to include detachable floats, which in effect converted it to a seaplane bomber. These floats could be jettisoned during the flight in order to increase the plane’s performance, or they could be left out altogether in case of a kamikaze run. (Not attaching the floats also significantly decreased the plane’s takeoff preparation time.)
The Seiran was launched from the 28m long compressed-air catapult found on the front deck of the Sentoku submarine. The three planes that were stored in a single submarine could be prepared and launched in 45 minutes (or in just 15 minutes if the floats weren’t attached). One of the more novel features was the ability for the plane’s engine to utilize pre-heated engine oil from the I-400 submarine as a way of reducing the startup / warm up time needed once the submarine surfaced. On the return from the mission, the pilot would land the seaplane on water near the submerged sub, which would resurface and use its onboard crane to hoist the plane back into the hangar.

Operational History

The first Seiran prototypes were finished and tested in the fall of 1943. After additional tests they were green-lighted for production early in 1944, and 28 of these curious aircraft have been built by the end of the war.

Although several missions have been planned for these bombers, they haven’t seen any real action during the war. The first mission planned for the Seiran squadron would include the only 2 Sentoku submarines built at the time, with the full contingent of 6 bombers that were to make a surprise air strike on the locks of the Panama Canal dam. This would have severely hampered the US war effort, as it would cut one of its main supply lines to for the US forces in the pacific war theater.
Besides planned surprise attacks, the Japanese bombers were to be used in the bombing of major American towns and cities, such as New York, in order to demoralize the American people.
As the Americans made a push in their offensive, and the Japanese forces grew desperate, these plans were scratched, and the bombers were to target the American base at Ulithi Atoll. This base was a staging ground for American attack on the Japanese Home islands, with the whole American fleet located there.
The submarines were, however, too late. On their way back to Japan they received the radio message with the news of the Japan’s surrender and instructions to dispose of the Seiran aircraft. The reason for this was because all the Seiran bombers were disguised as American planes, which is a violation of the rules of war. The planes were either catapulted with their wings folded or pushed overboard to prevent their capture.

The Museum Piece

The submarines were promptly seized by the Americans and only one Aichi M6A Seiran was recovered. This plane has been left to rust until the summer of 1989 when a team of experts, consisting of various engineers, volunteers and several Japanese nationals, tried to restore it.

Aichi_M6A1The restoration work took quite some time, as there were no production drawings and schematics that survived, which meant that the team had to do extensive research and reverse engineer the missing and damaged components. The whole process took over 10 years, and was finished in February of 2000.
The only surviving Serian served as a testament to the increasing difficulties that the Japan industry faced near the end of the war, with numerous factories and equipment damaged or destroyed. The lack of proper resources and skilled workers really showed.

This aircraft was of very poor quality, with the damage from a bombing raid patched up by absolutely inadequate fabric patches. The fit and alignment of parts was very poor in some places, and it wouldn’t have passed the Japan’s navy quality control in the beginning of the war. There were graffiti all over the airframe, and someone, probably a Japanese high schooled student that worked in a factory, etched in a complete English alphabet on one wing panel.
One thing that was surprising is that the restoration team hasn’t found any evidence that the floats could be jettisoned mid-flight, contrary to the claims made by Aichi.
While this plane hasn’t seen any real action, it still remains a curious piece of engineering that showed off the creativity of the WWII Japanese engineers. This little bomber is now resting comfortably at the Smithsonian near Dulles International Airport where it serves as a slightly uncomfortable reminder of what could have been.

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