Until recently, aircraft cooling requirements were met by direct heat transfer to free stream. Flight at high subsonic speeds brought forth a need for crew compartment refrigeration. Supersonic flight may result in an extension of this technique to a wider field, particularly the cooling of equipment and temperature-vulnerable functional materials.

This state of affairs has arisen from two major factors. Firstly, the hot boundary layer which surrounds the aircraft and would ultimately soak it to maximum boundary kinetic temperature. Secondly, ducted air either for the engine, equipment cooling, or crew conditioning, is received at nearly stagnation temperature. This temperature increases rapidly with Mach number and reaches its first unacceptable level, i.e. for crew cooling, at low altitudes in the transonic range.