Re: How about the aero-dynamic equivalent of supercavitation?
Supercavitation is solving the problem of high friction between the water and the surface of the weapon. While a supersonic aircraft feels the effect of air friction it is nowhere near the limiting factor in its speed nor does it have anything to do with the sonic boom.
A sonic boom comes about because air is getting compressed ahead of the aircraft faster than it can get out of the way. The release of the compression waves behind the A/C is where the sonic boom comes from.
Changing the geometry of trailing structures - engines being the worst offender, the wings less so - helps the air compression waves release more gently.
This could find broad application commercially in several respects - reduction of noise in helicopters (rotor tips are usually supersonic), the windmills noted above, turbine blades, and so on.