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The Submersible Aircraft program is designed to explore the possibility of developing a single platform that is capable of both flying through the air and submerging below the water. The difficulty with developing such a platform arises from the diametrically opposed requirements that exist for an airplane and a submarine. While the primary goal for airplane designers is to try and minimize weight, a submarine must be extremely heavy in order to submerge underwater.
In addition, the flow conditions for a submarine and an airplane are different, due to the order of magnitude difference in the densities of air and water. Platform velocities vary considerably as well and there are also significant differences in the design constraints that arise from the two radically different loading modes that need to be supported by the structural systems. An airplane's structure acts as a pressure vessel and as such the required skin thickness is relatively narrow, while a submarine is required to withstand enormous crushing loads and consequently the required skin thickness is an order of magnitude larger. Structural systems represent one third of the total platform weight for both submersible and aircraft platforms. However, the weight of a submarine’s structure precludes the possibility of flight. The geometry requirements for lifting surfaces capable of operating in air and water have different design drivers.
