Book excerpt: The high overall efficiency of the scramjet engine is potentially maintained over the speed range from Mach 5 to 20. At higher speeds, hydrogen fuel is essential and a fixed geometry appears feasible. Optimization of the intake diffusion can be carried out most easily in terms of the intake process efficiency and velocity ratio. The optimum amount of diffusion is reduced when realistic nozzle losses are included. The development of a hypersonic flight test vehicle is recommended.