Ramjet

A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed, thus cannot move an aircraft from a standstill. Ramjets therefore require some other propulsion system to accelerate the vehicle to a speed where the ramjet begins to produce thrust. Ramjets work most efficiently at speeds around Mach 3. This type of engine can operate up to speeds of Mach 6.