A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce 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. 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 . 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 efficiency of a ramjet starts to drop as the air temperature in the inlet increases due to compression. As the inlet temperature gets closer to the exhaust temperature, less energy can be extracted in the form of thrust. To produce a usable amount of thrust at yet higher speeds, the ramjet must be modified so that the incoming air is not compressed (and therefore heated) nearly as much. This means that the air flowing through the combustion chamber is still moving very fast (relative to the engine), in fact it will be supersonic—hence the name supersonic-combustion ramjet, or scramjet. History Cyrano de Bergerac L'Autre Monde: ou les États et Empires de la Lune (Comical History of the States and Empires of the Moon) (1657) was the first of three satirical novels written by Cyrano de Bergerac that are considered among the first science fiction stories. Arthur C Clarke credited this book with conceiving of the ramjet, and being the first fictional example of a rocket-powered space flight. René Lorin The ramjet was conceived in 1913 by French inventor René Lorin, who was granted a patent for his device. Attempts to build a prototype failed due to inadequate materials. His patent FR290356 showed a piston internal combustion engine with added 'trumpets' as exhaust nozzles.