Ramjet

A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to take in air for combustion that produces jet thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an assisted take-off like a rocket assist to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around and can operate up to speeds of . Ramjets can be particularly useful in applications requiring a small and simple mechanism for high-speed use, such as missiles. The US, Canada, and UK had widespread ramjet powered missile defenses during the 1960s onward, such as the CIM-10 Bomarc and Bloodhound. At present weapon designers are investigating the 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 . They have also been used successfully, though not efficiently, as tip jets on the ends of helicopter rotors. Ramjets differ from pulsejets, which use an intermittent combustion; ramjets employ a continuous combustion process. As speed increases, the thermal tfficiency of a ramjet decreases as the air temperature in the inlet increases. This is because as the Ramjet intake structure must accelerate the air entering it to subsonic speed relative to the engine internals. As the speed increases, doing so requires more work to be done on the incoming air to accelerate it, causing its density and temperature to significantly increase. As the difference between the engine inlet temperature and the exhaust gas temperature decreases, less useful energy can be extracted in the form of thrust, relative to the induced drag of the inlet. Meanwhile, if altitude and ambient air conditions are assumed to be the same, then higher speeds produce greater mass flow rate through the engine, therefore fuel flow must be increased to maintain sufficient exhaust temperature to offset ram drag incurred thereby.