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A ramjet is a form of airbreathing jet engine that requires forward motion of the engine to provide air for combustion. Ramjets work most efficiently at supersonic speeds around Mach 3 (2,300 mph; 3,700 km/h) and can operate up to Mach 6 (4,600 mph; 7,400 km/h).
Ramjets can be particularly appropriate in uses requiring a compact mechanism for high-speed, such as missiles. Weapons designers are investigating ramjet technology for use in artillery shells to increase range; a 120 mm ramjet-assisted mortar shell is thought to be able to travel 35 km (22 mi).[1] They have been used, though not efficiently, as tip jets on the ends of helicopter rotors.[2]