The four basic parts of a jet engine are the compressor, turbine, combustion chamber, and propelling nozzles. Air is compressed, then led through chambers where its volume is increased by the heat of fuel combustion. On emergence it spins the compression rotors, which in turn act on the incoming air.
In the cumbustion chamber of a jet propulsion engine the combustion of a fuel mixture generates expanding gases, which escape through an orifice to form the jet. Newton's third law of motion requires that the force that causes the high-speed motion of the jet of gas have a reaction force that is equal in magnitude and oppositely directed to push on the jet propulsion engine. Hence the term "reaction motor" is often applied to jet-propulsion engines.
The thermal jet engine operates with a continuous blast, but intermittent duct jet propulsion proceeds by a series of pulses, or intermittent explosions. The ramjet, or continuous duct, engine relies on its own forward motion to compress the air that enters it. Although highly efficient, it is designed to operate only after high speed has been attained through the use of some other power source, typically a rocket. The scramjet, or supersonic-combustion ramjet, engine is designed to operate at hypersonic speed (above Mach 5), using hydrogen for fuel; in theory, a scramjet-propelled craft could achieve orbital speed, with an efficiency three times that of liquid- or solid-fuel rockets. In addition, without the need to carry oxygen, an air-breathing, scramjet-powered vehicle can carry a greater payload than a rocket-powered one.
There are various thrust-augmentation methods that can be used to increase the effective driving force of jet engines: the afterburner, water-injection, and air bleed-off methods. An afterburner uses the exhaust gases from the engine for additional combustion, with resulting higher compression; however, it consumes large amounts of fuel. Injection of water into the air-compressor inlet also increases the thrust, but can be used only at take-off because of the high water consumption. Air bleed-off, sometimes called the fan augmentation method, also makes more efficient use of air otherwise wasted.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.