Ramjet

Space vehicles and guided-missile systems use this type of engine. A ramjet is basically a turbojet with no moving parts and the most efficient engine. Its operation is limited to speeds at or higher than supersonic speeds allowing a higher tolerance to higher temperatures. So a ramjet vehicle initially requires another engine, typically on an aircraft, until it reaches supersonic speeds. The ramjet develops little thrust below the speed of sound. The engine is designed on the principle that as flight speeds enter the supersonic regime, the compression ratio (raise in pressure) for high thrust decreases. Once high enough speeds are reached, a compressor is no longer needed. The figure below illustrates the design of a ramjet.