Awatef Hamed, Professor of Aerospace Engineering and Engineering Mechanics at the University of Cincinnati, provides this explanation:
Jet propulsion revolutionized the science of flight by dramatically increasing possible speeds and altitudes, hence enabling space exploration. The term jet propulsion refers to the action produced by a reactor to the ejection of matter. For example, when the matter in a typical rocket (like gunpowder in fireworks) is ignited, the resulting chemical reaction produces heat and gases, which escape from the rocket and cause it to move forward. The oxygen necessary for combustion is carried (in tanks or in a combined form) in the rocket itself so that the rocket thrust is independent of the atmosphere. Other jet propulsion devices depend on the air inducted into the engine to supply the necessary oxygen. After heat is released by the combustion, the hot gases are accelerated through the engine so that the exit velocity is greater than the airstream velocity at the entrance.
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In both self-contained rocket motors and air-breathing jet propulsion power units, the thrust that can be generated is proportional to the mass of material ejected from the unit in a given time, as well as the increase in the mass velocity with respect to the unit. Therefore, the same forward-thrust force can be produced in two ways: by ejecting rearward either a large mass of material at a low velocity during a given time period (as in turbofan engines) or by ejecting a smaller mass of material at higher velocity (as in turbojet and ramjet engines). The two sources of mass are the propellant, or fuel, and the oxidizer, or air.
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Fuels contain a large quantity of potential energy, which is rapidly released during combustion. A portion of this heat energy is converted into useful work, moving the vehicle through the atmosphere or into space. Another portion, however, in the form of the jet's kinetic energy, is lost and dissipates into the atmosphere. The very fuel-efficient turbofan engines used in modern commercial aircraft engines attempt to minimize the latter portion. To do so, they impart a moderate increase in the mass velocity to the combustion products for the large mass of air drawn through the engine in a given time. But turbojet and ramjet engines, which meet more demanding supersonic flight requirements, are less fuel-efficient.
Various types of jet engines have been developed to deliver the required thrust and engine performance for a wide range of flight speeds and altitudes. Air-breathing turbojet, turbofan and ramjet engines operate according to similar principles in the sense that they raise the inducted air pressure before combustion and expand the high-energy gases before they leave, in a nozzle or exhaust system. In turbojet engines, the inducted air goes through a compressor to increase its pressure before entering the combustor, then through a turbine before accelerating in the exhaust nozzle. The ramjet engine, though, has no moving parts; it produces a ram pressure rise from decelerating the high-speed inducted air in the inlet diffuser. The ramjet engine can only operate at high supersonic velocities and therefore requires another launching device, such as a rocket or turbojet engine, to accelerate it to the required speed.