Thrust is the force that moves an aircraft through the air; it is generated by the propulsion system of the aircraft. Different types of engines develop thrust in different ways, although thrust is usually generated through some application of Newton's Third Law (action <-> reaction). A gas is accelerated by the propulsion system, and the reaction to this acceleration produces a force on the engine. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas.
In the early 1900's some of the original ideas concerning ramjet propulsion were first developed in Europe. Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. To maintain the flow through the nozzle, the combustion must occur at a pressure that is higher than the pressure at the nozzle exit. In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid, much like a turbojet engine. In a turbojet engine, the high pressure in the combustor is generated by a piece of machinery called a compressor. But there are no compressors in a ramjet. Therefore, ramjets are lighter and simpler than a turbojet. Ramjets produce thrust only when the vehicle is already moving; ramjets cannot produce thrust when the engine is stationary or static. Until aerodynamic losses become a dominant factor, the higher the speed, the better a ramjet works. But since a ramjet cannot produce static thrust, some other propulsion system must be used to accelerate the vehicle to a speed where the ramjet begins to produce thrust.
Shown above are pictures of an X-15 rocket-powered airplane with a ramjet slung underneath the body and a wind tunnel test of a ramjet inlet used on a missile. In both propulsion systems, a rocket is used to bring the ramjet up to speed before it produces thrust. Because the ramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a rocket, which must carry all of its oxygen. Ramjets are ideally suited for very high speed flight within the atmosphere.
EngineSim is an interactive Java applet which allows you to test the design of ramjet engines. You can learn the fundamentals of ramjet engine propulsion with the EngineSim simulator.
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