It seems like folks don't understand the reference to compression in airplanes, so let me share some examples. Airplanes are supported by their wings, which de

It seems like folks don't understand the reference to compression in airplanes, so let me share some examples. Airplanes are supported by their wings, which deflect air downward and are in turn deflected upward. The wingtips flex upward, which puts the bottom surface of the wing under tension and the top surface under compression. This is true of all wing materials, but the point is that carbon fiber is used for wings and works fine as long as it is specced and constructed to handle the load and cycles. The fuselage is supported during flight by the wings at the wing root, so the nose and tail of the aircraft want to droop down; this puts the bottom of the hull under compression and the top under tension. Again--true of all aircraft hulls but carbon fiber works fine in hulls too--again, as long as it is engineered correctly. Finally composites (of which carbon fiber is only one type) are used extensively in marine applications. Fiberglass, aramid, carbon, and other fiber-based composites are commonly used in boat hulls, where they experience static and dynamic compressive loads. A jet ski smacking into the face of a wave at 50 knots puts a lot more than 1G of compressive force on its composite hull. There's really no physical reason that a composite hull could not work in a submersible. However there are a lot of reasons folks don't usually use composites for that purpose--primarily that the cost-benefits of composites disappear as compressive forces go up. A composite hull that performs as well as a metal hull, at the pressures we're talking about, is possible but would be more expensive and have a shorter lifespan than the metal hull. This is why huge ship hulls are still built out of steel, even though almost all smaller boats have composite hulls these days. It's cheaper. Coming back around to the Titan, the concern with the carbon fiber hull was not that it's physically impossible to do right, but rather suspicion that it only looked like a good idea because they were cutting corners in the engineering (because if they weren't cutting corners, it would not have looked economical).