Flying submarine

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.

[1] In 1934, a Soviet engineering student, Boris Ushakov, proposed a design for a submersible aircraft that would scout for ships and then submerge itself in order to ambush them.

The project – called the Convair Submersible Seaplane – reached the stage of detailed design and models, but was then cancelled by Congress.

However, one problem identified with this proposal was that the batteries required to achieve DARPA's specifications would make the vehicle too heavy to fly.

A suggested solution was using a ten-metre tall snorkel to supply air to a more conventional petrol turbine engine, although this would limit how far the craft could dive.

However, one issue is that because jet engines run at several hundred degrees when in air, they could not immediately transition underwater, as being exposed to seawater would subject them to extreme temperature change which would damage them, requiring the aircraft wait for several hours on the surface to cool its engines to submerge, thus any such configuration would require a novel cooling system in order to make a faster transition.