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.
Ramjets can be particularly useful in applications requiring a small and simple mechanism for high-speed use, such as missiles. Weapon designers are looking to use ramjet technology in artillery shells to give added range; a 120-mm mortar shell, if assisted by a ramjet, is thought to be able to attain a range of 22 mi (35 km). They have also been used successfully, though not efficiently, as tip jets on the end of helicopter rotors.
Ramjets are frequently confused with pulsejets, which use an intermittent combustion, but ramjets employ a continuous combustion process. They are also confused with scramjets, a similar, but less efficient system.
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