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A flying submarine or submersible aircraft is a combination of a seaplane and a submarine. It is supposed to be able both to fly and to travel under water. Starting from the surface of water is also intended.
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Since the requirements for designing a submarine are practically opposed to those of an airplane, the performance expected from such a construction is usually rather moderate.
Soviet Union
The Soviet Union started building a naval fleet in the mid 20th century, including battleships, aircraft carriers and other ships. Encouraged by this, the leadership became more open to respective proposals.
The Soviet Union tried to develop a flying submarine during World War II, whose production never exceeded design phase. At the Naval Engineering Institute, a flying submarine project was headed by engineer Boris Ushakov.
During his studies at the Dzerzhinsky naval engineers' academy in Saint Petersburg, which he finished in 1937, Boris Ushakov presented a draft for a project, which could unite the capabilities of an aircraft with those of a submarine. To be precise, this draft showed a seaplane able to dive.
In the following years, the project was revised and presented in multiple variations, enabling him to test the capacity and durability of particular components. In April 1936 the reviewer determined that Ushakov's was interesting and to be implemented. In July 1936 the draft project “Flying submarine” was checked and evaluated positively by the scientific research committee of the forces. The committee recommended to continue the project, to test its realisability using calculations and lab tests.
In 1937 the project was included into plan “W” (russ. “В”) of the committee. However, the project was cancelled after a reevaluation. Ushakov, by the time a “military technician 1st class” in department “W” of the committee continued the project in his spare time.
In 1938 the draft and the fundamental tactical-technical elements of Ushakov's flying submarine were evaluated once more by the 2nd department of the committee.
The design was supposed to be built as an all-metal construction and operate at 100 knots in the air and 3 knots under water. The engines were supposed to be sealed shut by metal plates while under water. The flying submarine was supposed to house six hermetically sealed chambers in its hull and wings: Three chambers to hermetically seal the three aircraft engines (AM-34; 1000 HP; while starting 1200 HP, utilising the turbo compressor), as well as the hermetically sealed pilot’s chamber, a chamber for the accumulator and another one for the electric engine. All the other empty spaces of the aircraft were supposed to be filled with water while submerging. The time for preparation and actual submerging were projected at 1.5 minutes. The hull was to be constructed as a riveted duralumin cylinder, measuring 1.4 m across and 6 mm thick. The cockpit was supposed to be flooded while submerging, after the flying instruments were lowered into a waterproof shaft. The crew was supposed to enter the aft command centre and control the submarine from there.
The outer skin of the wings and the empennage was to be made of steel, the floats of duralumin. These parts were supposed to be filled with water while submerging, so they did not have to be designed for the water pressure in diving depth. The water was supposed to enter by itself through opened valves.
The containers for fuel and lube were to be placed in rubber reservoirs in the flying submarine's hull.
As a corrosion prevention the flying submarine was supposed to be treated with special paints and varnishes.
Two special mounts for 18 inch torpedoes were supposed to be placed under the hull. The payload was supposed to be 44.5% of the take-off mass. This is a typical value for heavy aircraft of the time.
For surfacing, the excess water was supposed to be pumped out of the empty spaces. The electric engine was designed as an underwater drive.
The flying submarine was supposed to be used on the open sea for torpedo attacks on opposing ships. It was supposed to track these ships and await their arrival at the projected course while submerged. In case the opposing ship would not pass closely enough, an other trial was supposed to be possible. Therefor the flying aircraft would have had to surface after the opposing ship was out of sight, track them again and await them again at the correct position.
An other projected use was entering naval areas and naval bases, which were blocked by mine belts. The flying submarine was supposed to pass the mines while in the air and then land inside an opposing restricted area, bay or naval base at night, submerge and act as a submarine. During the day, it would then have been able to gather intelligence on opposing forces, determine navigable water, or use an opportunity to attack.
Especially the combined use of multiple flying submarines offered great opportunities. In theory, three flying submarines would have been able to form a 15 km long cordon sanitaire, able to block the projected way of opposing ships.
The project was expected to counter a disadvantage of submarines, their bad manoeuvrability. Expectations were especially high for the ability to repeat attack manoeuvres using a short flight to a different attack location, if opposing ships would not pass closely enough. This was seen as one of the flying submarine’s main advantages.
In 1939 the project was temporarily suspended and classified. In 1943, on the orders of NKVD chief Lavrentiy Beria, the project was resumed. In 1947 the first test of the flying submarine was performed. In 1953, the project was closed by order of Communist Party First Secretary Nikita Khrushchev. The design never "got off the ground".
United Kingdom
As early as 1920, the British trade journal, ″Flying,″ reported conversations between the First Sea Lord and other military leaders and one of the principal aircraft manufacturers concerning a flying submarine (or submersible seaplane). The all-metal craft, its hypothetical design illustrated in the article, was to be a twin-propeller airplane with retractable wings and a hermetically sealed fuselage. There was, however, apparently no further development of the project.
USA
In 1961 Donald Reid designed and built a single-seat craft (32.83 ft length) capable of flight and underwater movement, the Reid Flying Submarine 1 (RFS-1). A 65 hp (48 kW) engine mounted on a pylon provided propulsion for flight; a 1 hp electric motor in the tail provided underwater propulsion. The pilot used an aqualung for breathing underwater. The first full-cycle flight [underwater at 6.5 feet (2 m) depth, airborne at 33 ft (10 m) altitude] was demonstrated on 9 June 1964. Reid, his craft, and his son (the test pilot) appeared on the U.S. game show "I've Got A Secret" on March 15, 1965.
In 2008, the American's Defense Advanced Research Projects Agency announced that it was preparing to issue contracts for a submersible aircraft.
Flying submarines in fiction
A flying submarine was a feature in: