An important point to realise in the question of propulsion for aircraft of Mach 2 and upwards is that efficiency of the power plant is no longer a problem. Due to the high pressure developed from the intake the overall pressure ratio of the engine is high, and the aircraft forward speed being high the Froude propulsive efficiency is also favourable. Fig. 1 shows, in fact, that at Mach 2 overall thermal efficiency as high as 35 per cent is achieved which, of course, compares favourably with the best electrical power station generation. Indeed, it is one of the main reasons why economic transport at high Mach numbers is possible. The falling aerodynamic efficiency, as defined by lift/ drag ratio, is offset by this rising propulsive efficiency, so that a somewhat similar overall performance is obtained as compared with subsonic aircraft.