Imagine bomber aircraft being launched from Imperial Japanese Navy submarines at dawns early light or in total darkness, striking targets along the West coast of the United States and spreading chaos in their wake! That was what was intended with the design of the Aichi M6A1 Seiran (“Clear Sky Storm“) floatplane light bomber.
The Seiran was aptly named, as it was planned to be launched from giant I-400 Class submarines that were effectively a small aircraft carrier, conduct its mission as a surprise attack, then (if lucky) return to set down on the water (where the submerged submarine was hopefully still on station) and be recovered via a collapsible deck mounted hydraulic crane, before the submarine disappears under the ocean to pop up and strike again. A clever concept indeed!
The long range I-400 submarines had a 115 foot ondeck watertight hangar with a 12 foot diameter that could house three Aichi M6A1 Seiran aircraft with the floats removed, wings folded against the fuselage and tail fin tip and horizontal stabilisers folded down. Once the huge hangar door was opened, a crew of four from the submarine would winch the aircraft out onto a launching dolly, hydraulically rotate the wings into place (with aid from a power source in the hangar) and fold the tail stablisers and fin tip back into place.
The floats were stored in watertight deck compartments until ready for mounting – they were moved up ramps and attached under the wings (the aircraft could also be launched without them but they would obviously have to ditch upon return!). Within 30 to 45 minutes all three aircraft could be setup, prepared for flight and launched with the aid of the dolly along a gradual inclined deck mounted 120 foot long pneumatic catapult (I wonder how they would have gone assembling and launching them in rough seas?).
The assembly and launch of the aircraft from the submarines could be completed day or night. Surfacing and launching in the cold light of day would have made the submarine vulnerable to air or sea attack, so a launch under the cover of darkness would have been more difficult but far more effective (apparently some sections of the aircraft had luminous surfaces to aid the crew in the assembly of the aircraft). Allied air and seaborne radar would have proven a problem day or night though!
Late in the war, with the once mighty capital ships and aircraft carriers of the Imperial Japanese Navy vastly diminished, such a unique blend of aviation and incredibly long ranged submarine design was intended to allow the Japanese to arrive undetected to strike targets far from their homeland and occupied territories. There were even plans to strike the Panama Canal to destroy the locks and slow down the movement of shipping and vital Allied war supplies from the East Coast of the United States to the Pacific Theatre. If successful, the Japanese believed they could close the canal for up to six months.
At the behest of Admiral Isoroku Yamamoto, design plans began in 1941 but with the loss of the Admiral in April 1943, it seems the project slowed down and only 3 of an originally intended 18 I-400 Class submarines (I-400, I-401 and I-402) were completed between December 1944 and July 1945 (a fourth was destroyed in a USAAF bombing raid). At 122 metres (400 feet) in length, they were at the time, the largest submarines to have been built and remained so until the early 1960’s!
Two smaller Type AM (A Modified) submarines (I-13 and I-14) were also fitted with hangars to carry two Aichi M6A1 Seiran aircraft. The Allies had nothing like this innovative concept but given they had so many aircraft carriers there was never really any need to do so.
Following the commencement of the I-400 submarine project, Aichi were directed by the Imperial Japanese Navy to develop an aircraft to fly from the submarines. Aichi chief engineer Toshio Ozaki was up to the design task and the first prototype flight was conducted in 1943 but only 28 Aichi M6A1 Seiran aircraft were built between 1943 and 1945 (8 M6A1 prototypes, 18 M6A1 Seiran production bombers and 2 M6A1-K Nanzan “South Mountain” trainers fitted with a wheeled landing gear). Production was hampered by USAAF bombing raids, material shortages and then the coinciding reduced production of the submarines meant only a small number of aircraft were actually required. They were operated by the Yokosuka Naval Air Technical Arsenal and 631st Kōkūtai.
Captain Tatsunoke Ariizumi was put in command of the 1st Submarine Flotilla and 631st Kōkūtai. These units were allocated submarines I-400, I-401, I-13 and I-14 with 10 aircraft.
Somewhat surprisingly the Japanese were able to keep the submarines and aircraft a secret until the end of the war. Ultimately though, beyond some testing and training, and I-400 and I-401 setting sail on July 23rd, 1945 to target the US invasion fleet at Ulithi Atoll, the war ended and none of the aircraft or submarines ever saw combat (the Japanese pushed their aircraft off the submarines whilst returning to Japan to surrender). Submarines I-13 and I-14 were also to be involved in this attack but I-13 was destroyed by the US Navy on July 16th, 1945.
This was fortunate as there were also some extreme plans in place that were eventually halted, including Operation Cherry Blossoms at Night, which would have involved Seiran aircraft either bombing or conducting Kamikaze missions against San Diego in California with biological weapons onboard to spread the plague! The even more extreme atomic bombs dropped on Nagasaki and Hiroshima ended any such missions.
It would have been extremely unlikely but you can imagine the chaos that would have been unfurled upon a city like San Diego if all 18 I-400 submarines had been built and suddenly popped up off the west coast to launch 54 Aichi M6A1 Seiran aircraft armed with biological bombs! “Clear Sky Storm” indeed!
Although all three I-400 Class submarines were captured by the Americans in 1945, apparently the Soviets wanted to inspect them, so the US Navy promptly blew up and sunk I-402 at sea (late in the war it had actually been converted to a fuel tanker by the Japanese and never carried any aircraft), then sailed I-400 and I-401 to Hawaii for a thorough inspection. The Soviets must have still been rattling their sabre though, so in June 1946 the Japanese submarines were torpedoed and sunk at sea by the Americans rather than let the Soviets carry out their own inspection and gain knowledge of the Japanese technology! The surviving Type AM submarine I-14 was also scuttled off Hawaii in 1946.
A rather sleek looking machine, the nose of the Aichi M6A1 Seiran has a distinctive German look to it for very good reason. They were fitted with either a Aichi AE1P Atsuta 31 or Atsuta 32 1,400 hp engine which were licence built Japanese variants of the Daimler-Benz DB 601A 12 cylinder inverted liquid cooled inline engine. Being liquid cooled, the submarine crew could heat the coolant and lubricating oil within the hangar, then pump them into the engine, reducing the warming time to enable a prompt take-off. This was important for the protection of the submarine, which had to limit its time on the surface.
The Seiran had a maximum speed of 475 km/h (295 mph) but like all similar aircraft, performance and maneuverability was severely hampered by those large twin floats. In earlier versions it was intended the aircraft could be launched without them (to carry a higher payload), or possibly jettison them in flight, then return to the submarine and ditch in the water nearby but they could not be jettisoned on later production models. Without the floats the top speed was a respectable 560 km/h (348 mph).
The Seiran could carry a single torpedo or 850 kg (1,874 lb) bomb without the floats attached or a single 250 kg (551 lb) with them attached. This was not intended to be a dog fighter, so defensive armament consisted of just one 13mm Type 2 machine gun in the navigators rear position – now that would have been a stressful job, not only navigating to the target but also back to wherever the submarine was located out in the ocean!
Just one Aichi M6A1 Seiran (Serial Number 28, the last built) survives today and is on display in the fantastic Smithsonian National Air & Space Museum, Steven F. Udvar-Hazy Centre in Chantilly, Virginia. This one was surrendered by Imperial Japanese Navy Lt. Kazuo Akatsuka, who flew it from Fukuyama to Yokosuka and handed it over to American forces who shipped it to the United States as part of 145 captured Japanese aircraft taken for evaluation and research.
This Seiran was stored at Naval Air Station Alameda in California for many years and occasionally trotted out for display until 1962 when it was transferred to the Smithsonian Paul E. Garber storage facility in Silver Hill, Maryland. It sat outdoors for 12 years before indoor storage space became available (the floats had suffered plenty of corrosion during that time).
A team of Smithsonian staff and volunteers with help from experts in Japan set about to restore the aircraft in 1989. With no surviving production plans and just a sketchy blueprint it was always going to be a big task and much research had to be carried out to reproduce missing parts including instruments, most of the propeller spinner and a navigators table. Other original parts were purchased or donated.
The Aichi AE1P Atsuta 32 engine was in good condition but much of the linen that covered the control surfaces was missing and the airframe had corrosion from the lengthy period it sat outdoors. Paint was matched to samples of original paint that had been protected in different areas of the aircraft, which enabled the restoration team to bring it back to factory like condition.
During the restoration process the Smithsonian team discovered evidence of the poor quality of Japanese workmanship late in the war due to Allied bombing disruption to supplies and construction facilities, plus a lack of skilled labour. Apparently construction was often completed by high school students!
The state of the airframe set its own challenges for the team and it was not until 2000 that the restoration was complete. The effort was well worth it though and today the Seiran can be seen in all its glory sitting alongside many other Axis treasures and rare survivors of the war within the hallowed halls of the Smithsonian.
References:
Air & Space Magazine – All and Nothing
HistoryNet – Japan’s Panama Canal Buster
Smithsonian National Air & Space Museum – Aichi M6A1 Seiran (Clear Sky Storm)
Aces Flying High - The Survivors 