JET ENGINE

any of a class of internal-combustion engines that propel aircraft by means of the rearward discharge of a jet of fluid, usually hot exhaust gases generated by burning fuel with air drawn in from the atmosphere. The only practical means of propulsion so far devised is to take advantage of Newton's third law of motion. In its simplest form, this law states that action and reaction are equal and opposite. It is observed in nature that forces (e.g., pushes or pulls) never occur singly but always in equal or opposite pairs. Thus the existence of a force on a body requires the coexistence of an equal and opposite force. If a pair of forces acts between two separate bodies (and provided that no other forces act), the bodies will be impelled to separate or come together depending on whether the force pair is repulsive (a push) or attractive (a pull). The combination of Newton's second and third laws shows that the resulting motion of the two bodies can be expressed by the statement that the change in momentum of the two bodies is equal and opposite and along the line of action of the force pair. Momentum is the product of mass and velocity, and, because (at aircraft velocities) the mass of any body remains constant, a force pair acting between two bodies produces a change in the velocity of each. It is clear that in order to propel a body it is necessary to find something to push against. All aircraft-propulsion devices push against the air itself. If the air is used as the propelling medium, it must experience a change in momentum i.e., it has to be accelerated toward the rear of the aircraft and discharged rearward with enough velocity that the reaction produces an appreciable thrust in the opposite direction. From 1903 to the late 1930s the reciprocating internal-combustion engine with a propeller was the sole means used to propel aircraft. The first jet-powered airplane was finally introduced in 1939 in Germany. The jet engine, consisting of a gas-turbine system, significantly simplified the propulsion process and thereby opened the way to substantial increases in aircraft speed, size, and operating altitudes. Over the years, several types of jet engines have been developed. The principal varieties are (1) turbojets, (2) turbofans, (3) turboprops, (4) turboshafts, and (5) ramjets. any of a class of internal-combustion engines that propel aircraft by means of the rearward discharge of a jet of fluid, usually hot exhaust gases generated by burning fuel with air drawn in from the atmosphere. Additional reading Texts on jet engines include two nontechnical works, Rolls-Royce Ltd., The Jet Engine, 4th ed. (1986), with a discussion of basic concepts and a systematic analysis of jet engine components; and Irwin E. Treager, Aircraft Gas Turbine Engine Technology, 2nd ed. (1979), with a section on the history of the jet engine. More technical treatments are found in Jack L. Kerrebrock, Aircraft Engines and Gas Turbines (1977), which deals primarily with the thermodynamic and aerodynamic operation of major engine components; and Gordon C. Oates, Aerothermodynamics of Gas Turbine and Rocket Propulsion, rev. and enlarged ed. (1988). Fredric F. Ehrich