The ramjet engine is the simplest engine imaginable. Also known as a flying stovepipe.The ramjet is an airbreathing jet engine that compresses incoming air using the engine's forward motion rather than an axial compressor. Because ramjets can't produce thrust at zero airspeed, they can't move an aeroplane from a standstill.

As a result, a rocket-assisted take-off is required to propel a ramjet-powered vehicle to a speed where it can begin to create thrust. A high-speed object travelling through the air creates a high-pressure zone upstream. As a result, the ramjet makes use of the high pressure in front of the engine to force air into the tube, which is then heated by combusting a portion of it with fuel, typically hydrogen. It is then accelerated to supersonic speeds by passing through a nozzle. This acceleration gives the ramjet forward thrust.

Ramjets work most efficiently at supersonic speeds around Mach 3 (2,300 mph; 3,700 km/h) and can operate up to speeds of Mach 6 (4,600 mph; 7,400 km/h).

Ramjets can be classified according to the type of fuel, liquid or solid, and the booster. In a liquid fuel ramjet (LFRJ), hydrocarbon fuel is normally injected into the combustor ahead of a flame holder, which stabilizes the flame formed by the combustion of the fuel with compressed air from the intake (s). It is necessary to pressurize and provide fuel to the ram combustor, which can be intricate and costly. A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for the efficient operation of the intake system. The first ramjet-powered missiles used external boosters, usually solid-propellant rockets, either in tandem, where the booster is mounted immediately aft of the ramjet, e.g. The Sea Dart, or wraparound where multipleboosters are attached alongside the outside of the ramjet, e.g. The 2K11 Krug.