The flight plan of the supersonic transport demands efficient aerodynamics and propulsion over a wide range of speed and altitude. As an example, the typical trans-Atlantic flight shown in Fig. 1 demonstrates that apart from cruising efficiently at, say, M = 2·2 for 2.500 n.m. at altitudes from 50,000 ft. to 65,000 ft., the aircraft must also be efficient at subsonic speeds. This is particularly true on the portion of the flight after the normal stage length, i.e. the diversion and hold, since the fuel required here must be carried the whole way and the “ growth factor” on aircraft size is large.