Aichi M6A1 Seiran (Clear Sky Storm): Japan’s Submarine-Based Strike Aircraft
Story and photos by Corey Beitler, first published in “Distelfink Airlines” – An Online Aviation Newsletter
The Aichi M6A1 Seiran was designed to fulfill a requirement for a bomber that could operate exclusively from a submarine. During World War II, the Imperial Japanese Navy developed a fleet of aircraft carrier submarines to bring aircraft within striking distance of the mainland United States and other strategic targets that were thousands of miles from Japan. No Seiran saw combat during World War II, but the aircraft represents an innovative blend of combining submarine and aircraft technology into a weapons system.
Japan had experimented with and successfully operated small reconnaissance aircraft from submarines before the United States entered World War II. In April 1942, the Japanese Navy issued a requirement to build a new series of submarine aircraft carriers designated the I-400 class. Japanese Navy planners envisioned a large fleet of these submarines but the war turning against the Japanese and a shortage of strategic materials meant only three of these submarines were completed, the I-400 through I-402. These submarines were the largest built during World War II. The I-400 class displaced 6,560 tons submerged and cruised at 18.7 knots when surfaced. The ships could carry three aircraft in a special waterproof hangar and had a range of over 43,000 miles.
After commencing the I-400 program, the Japanese Navy contracted with Aichi to develop a specialized aircraft that the submarines would carry as a strike force. Designated the Special Attack Aircraft Prototype M6A1. Aichi’s chief engineer, Toshio Ozaki, faced a daunting challenge when designing the aircraft. The Navy requirements specified that the aircraft had to be able to carry a bomb or torpedo load of 1,200 lb. The M6A1 had to have a top speed of 294 mph with floats or 347 mph without them and a range of 810 nautical miles. The Navy also stipulated that if the M6A1 needed to be assembled prior to launch from the submarine, the assembly and launching of three of the aircraft had to be completed in 30 minutes. The M6A1 was designed to be launched from a cold start, therefore the engine’s oil and the engine itself would be preheated in a separate chamber. The oil as well as hot water, would be pumped through the engine to bring it to normal operating temperature just a few seconds after being airborne. The aircraft and its components would also have to fit inside a specially designed, watertight, cylinder-shaped hangar on the submarine’s main deck. This hangar’s diameter only measured 11 ft 6 in.
To confront the space challenges on the submarine, Ozaki designed the main wing spar of the M6A1 to rotate 90° once the deck crew removed the floats. After rotating the wings, the crew then folded them back to lie flat against the fuselage. About ⅔ of each side of the horizontal stabilizer and the tip of the vertical stabilizer also folded down so the aircraft could fit in the submarine’s hangar. The floats and pylons were removable and stored in compartments on the submarine’s deck. If necessary, the M6A1 Seiran could be launched from the submarine without the floats installed, or the pilot could jettison the floats in flight. The first prototype was completed in October 1943, and flight tests began in November. In February 1944, a second prototype joined the flight test program. In addition to the prototypes, two land-based M6A1-K Nanzan training aircraft were built for pilots to familiarize themselves with the type.
Unfortunately for the Japanese Navy, the war situation deteriorated rapidly in late 1944. Progress was stopped on the development of the M6A1 Seiran after an earthquake damaged Aichi’s production line in December 1944. Boeing B-29 bombing raids further damaged Aichi’s production facilities. In March 1945, further military setbacks caused the Navy to curtail the submarine program, and work on the last two I-400 class submarines was stopped. The I-402 was turned into a submarine fuel tanker, so only the I-400 and I-401 were completed as intended. Using parts on hand, Aichi eventually built 28 Seirans, the production total included the eight prototypes and two trainers used in flight testing. Production variants of the Seiran used the Aichi AE1P Atsuta 31 or 32 V-12 inverted inline liquid-cooled engine that generated 1,400 horsepower and drove a three-bladed propeller. This engine was a license-built copy of the German Daimler-Benz DB601 engine.
Japanese Navy leaders organized the Seirans and their crews into the 631st Air Corps. The I-400 and I-401 and two AM class submarines, the I-13 and I-14, were organized into the 1st Submarine Flotilla. The whole force was placed under the command of Captain Tatsunosuke Ariizumi. During sea trials, the units focused on reducing the time it took to assemble and launch the Seirans. Eventually, the crews could launch three Seirans without floats in less than 15 minutes. The major drawback was that the Seiran could not land safely on the water without floats, so the pilot would have to ditch following the mission, and the aircraft would be lost.
In what would have been one of the most daring raids on a strategic target during World War II, Japanese Navy planners intended to use the 1st Submarine Flotilla in a strike against the Panama Canal. Ten Seirans would strike the Gatun Locks with six torpedoes and four bombs. To plan the attack, pilots studied a large-scale model of the lock system and memorized important features of the canal. The planning for the attack was very similar to how the Japanese Navy planned for the strike against Pearl Harbor in 1941.
During these preparations, the Japanese Navy decided to strike first at Ulithi Atoll in an attack called Operation Hikari. The plan would require the use of six Seirans and four Nakajima C6N1 Saiun reconnaissance aircraft. The smaller I-13 and I-14 submarines would carry two C6N1s each and offload them at Truk Island. The C6N1 pilots would then take off from Truk and scout the American ships at Ulithi Atoll, relaying target information to the Seiran crews. The Seirans would then attack the most important American ships located there, the aircraft and troop transports being primary targets.
Unfortunately, the mission was a failure. The I-13 was spotted by American aircraft and damaged and later sunk by a U.S. destroyer. The I-400 missed a critical radio message and proceeded to the wrong rendezvous point. On August 16, 1945, the 1st Submarine Flotilla received a radio transmission the war was over and to return to Japan. The crews of both the I-400 and I-401 scuttled the Seiran aircraft before returning home, punching holes in their floats and then catapulting or pushing them into the ocean so they would sink. The Seirans were also scuttled to prevent Allied forces from finding out that they had been painted in American markings to carry out the attack on Ulithi Atoll, a violation of the laws of war.
The world’s only remaining example of the Aichi M6A1 Seiran is currently on display at the National Air and Space Museum’s Steven F. Udvar Hazy Center in Chantilly, Virginia. This Seiran was the last example built by Aichi and was not used aboard any of the I-400 class submarines. Imperial Japanese Navy pilot Lt. Kazuo Akatsuka flew this Seiran from Fukuyama to Yokosuka, where he surrendered it to American forces. After examination by American forces, this aircraft, along with other captured Japanese aircraft, was shipped to the United States. It was periodically displayed at the Naval Air Station in Alameda, California until it was donated to the Smithsonian Institution for the National Air and Space Museum. The National Air and Space Museum took possession of the Seiran in 1962 but had no space to display or store the rare aircraft. Sadly, the Seiran remained in outdoor storage for nearly 12 years at the museum’s Paul E. Garber Restoration and Storage Facility in Silver Hill, Maryland, until space became available to move it indoors.
In June 1989, curators began restoration work on the M6A1 Seiran. Restoring the Seiran proved to be one of the most challenging restorations ever completed by the National Air and Space Museum curators. No production drawings of the Seiran survived, and the restoration team had to conduct exhaustive research into how the various aircraft systems were assembled and operated as no manuals were available. In addition, many original components of the aircraft were missing and had to be fabricated to complete the restoration. The restoration team included museum curators, volunteers, and Japanese nationals working in both Japan and the United States.
During the restoration process, the team discovered engineering and design features on the Seiran that were both innovative and absurd. The Seiran also displayed signs of the difficulties faced by the Japanese aviation industry at the end of World War II. The fit and alignment of parts in many places on the aircraft was poor, the interior of the fuel tanks were contaminated with paper, and a metal flap had been hastily patched and covered with fabric. Contrary to Aichi’s claims, museum curators could find no evidence the pilot could jettison the floats in flight. It is unknown if this feature was deleted during the production run or never installed.
After over ten years of work, the National Air and Space Museum restoration team completed work on the Seiran in 2000. After the restoration work was finished, the Seiran was kept in storage at the Paul E. Garber Restoration Facility and could only be viewed during special tours of the facility. In 2003, with the opening of the museum’s Steven F. Udvar-Hazy Center in Chantilly, Virginia, the Seiran was finally put on public display. Today, the Seiran is on display in the World War II Aviation gallery of the museum, along with other rare examples of World War II aircraft from Japan, Germany, Great Britain, and the United States.