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jet engine:see jet propulsionjet propulsion,
propulsion of a body by a force developed in reaction to the ejection of a high-speed jet of gas. Jet Propulsion Engines
The four basic parts of a jet engine are the compressor, turbine, combustion chamber, and propelling nozzles.
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(also reaction engine), an engine that generates the thrust necessary for motion by converting initial energy into the kinetic energy of a jet of a working fluid. The flow of the working fluid through a nozzle on the engine induces a reactive force in the form of a reaction to the jet, which propels the engine and any attached vehicle in a direction opposite to that of the jet flow. Energy in a variety of forms—chemical, nuclear, electric, or solar energy—can be converted into the kinetic energy of the jet flow. A jet engine functions both as an engine and as a propelling device; that is, it propels itself, without an intermediate mechanism.
Three elements are needed for the production of jet thrust: a source of initial energy for conversion into the kinetic energy of the jet flow; a working fluid, which is ejected from the engine as a jet; and an energy-conversion device, the engine itself. The initial energy is stored on board an aircraft or other vehicle equipped with a jet engine in the form of combustible chemical fuel or nuclear fuel. In principle, the initial energy may also be drawn from outside, for example, solar energy. In order to produce a working fluid, a jet engine may use a substance drawn from the environment, such as air or water, a substance stored either in tanks on the vehicle or directly in the engine chamber, or a mixture of substances obtained from the environment and stored on board the vehicle. Modern jet engines usually consume the initial energy in the form of chemical energy. In this case, the working fluid is a mass of hot gases—combustion products of the chemical fuel. In the operation of a jet engine, the chemical energy of a combustible substance is transformed into the thermal energy of the combustion products; the thermal energy of the hot gases is then converted into mechanical energy manifested as translational motion of the jet flow and, consequently, of the vehicle to which the engine is attached. The primary component of any jet engine is the combustion chamber, where the working fluid is produced. The rear section of the chamber, where the working fluid is accelerated to form a jet, is called the nozzle.
Jet engines can be divided into two basic classes— air-breathing jet engines and rocket engines—depending on whether or not the surrounding atmosphere is used in the engine’s operation. All air-breathing jet engines are heat engines, producing a working fluid upon oxidation of a combustible substance by oxygen in the atmosphere. The major component of the working fluid in such engines is air drawn from the atmosphere. Thus, vehicles propelled by air-breathing jet engines carry their energy source (fuel) on board, but they must rely on the surrounding environment for the greater part of their working fluid. By contrast, a rocket-powered vehicle carries all components of its engine’s working fluid on board. Rocket engines are uniquely suitable for operation in space because they need no propelling device dependent on the surrounding environment and they carry all components of the working fluid on board. There are also hybrid rocket engines, which combine features of both basic engine types.
The principle of jet propulsion has long been known. The aeolipile of Hero of Alexandria is considered to be the forerunner of jet engines. Solid-propellant rocket engines first appeared in China as gunpowder rockets in the tenth century AD. These rockets were used for hundreds of years for fireworks, signal flares, and military rockets, first in the Orient and later in Europe. The fundamental theoretical considerations and basic design elements of liquid-propellant rocket engines were first presented by K. E. Tsiolkovskii in his Investigation of Interplanetary Space by Means of Jet Devices (1903). The first Soviet liquid-propellant rocket engines, ORM, ORM-1, and ORM-2, were designed by V. P. Glushko and constructed under his direction at the Gas Dynamics Laboratory (GDL) in the period 1930–31. In 1926, R. Goddard launched a liquid-propellant rocket. The first electrothermal rocket engine was constructed and tested by Glushko at the GDL between 1929 and 1933. Rockets with ramjet engines designed by I. A. Merkulov were tested in the USSR in 1939. The first design for a turbojet engine was proposed by the Russian engineer N. Gerasimov in 1909.
In 1939 production of turbojet engines designed by A. M. Liul’ka began at the Kirov Factory in Leningrad; testing of these engines was disrupted by the Great Patriotic War. A turbojet engine was mounted on an airplane and tested for the first time in 1941, using an engine designed by F. Whittle (Great Britain). The theoretical studies of Russian scientists S. S. Nezhdanovskii, I. V. Meshcherskii, and N. E. Zhukovskii and the work of the French scientist R. Esnault-Pelterie and the German scientist H. Oberth were of great importance in the development of jet engines. An important contribution to the development of air-breathingjet engines was the Soviet scientist B. S. Stechkin’s Theory of the Air-breathing Jet Engine (1929).
Jet engines perform a variety of tasks, and their range of application is steadily expanding. They are used most widely on various aircraft. Turbojet or bypass turbojet engines are used on most of the world’s military and civil aircraft, and they can also be used on helicopters. These jet engines are suitable for flight at both subsonic and supersonic speeds, and they may also be used on cruise-type missiles. Supersonic turbojet engines may be used on the first stage of aerospace planes. Ramjet engines are installed on antiaircraft guided missiles, winged missiles, and supersonic interceptor-fighters. Subsonic ramjet engines may be mounted on the tips of helicopter rotor blades. Pulse-jet engines do not generate great thrust and are designed only for subsonic aircraft; they were used to power the V-l flying bombs during World War II.
Rocket engines are usually used on high-speed aircraft. Liquid-propellant rocket engines are used on launch vehicles for spacecraft and as sustainer engines, retrorockets, and vernier engines on spacecraft; they are also used on guided ballistic missiles. Solid-propellant rocket engines are used on military rockets, such as ballistic, antiaircraft, and antitank rockets, and also on launch vehicles and spacecraft. Small solid-propellant engines assist the takeoff of aircraft. Electric and nuclear rocket engines may be used to power spacecraft.
The basic characteristics of a rocket engine include the following: thrust, specific impulse (the ratio of the thrust produced by the engine to the mass of the rocket fuel [working fluid] consumed per second) or its equivalent, specific fuel consumption (the quantity of fuel consumed per second per newton of thrust), and the specific mass of the engine (the mass of the engine in its operating state per unit thrust). For many types of jet engines, overall dimensions and service life are important considerations.
Thrust, the force by which a jet engine acts upon the supporting vehicle, is determined from the formula
P = mWc + Fc (pc - pn)
where m is the mass of working fluid ejected per sec, Wc is the velocity of the working fluid in the nozzle cross-section, Fc is the cross-sectional area of the nozzle exit, pc is the gas pressure in the nozzle cross-section, and pn is the ambient pressure (usually atmospheric pressure). As the formula indicates, thrust is dependent on the ambient pressure. It is highest in a vacuum and lowest in the densest strata of the atmosphere; that is, in the earth’s atmosphere it varies with the altitude above sea level cf a vehicle equipped with a jet engine. The specific impulse of a jet engine is directly proportional to the velocity of the working fluid ejected from the nozzle. The exhaust velocity increases with an increase in the temperature of the ejected working fluid and a decrease in the molecular weight of the fuel; the lower the molecular weight of the fuel, the greater the volume of gases formed during combustion and, therefore, the greater the exhaust velocity.
The thrust generated by existing jet engines varies widely—from fractions of a gram-force for electric engines to hundreds of tons-force for liquid- and solid-propellant rocket engines. Low-thrust jet engines are chiefly used in aircraft stabilizing and guidance systems. In space, where the force of gravity is weak and there is practically no atmosphere to offer resistance that must be overcome, these engines can also be used to provide acceleration. Rocket engines with maximum thrust are needed to launch and propel rockets to great distances and altitudes and especially for placing vehicles in space, that is, for accelerating them to orbital velocities in space. These engines require a very large amount of fuel and usually operate for a very short time in accelerating the rocket to a given velocity. The maximum thrust attained by an air-breathing jet engine is 28 tons-force (1974). Jet engines that use the surrounding atmosphere as the principal component of their working fluid are appreciably more economical. Air-breathingjet engines can operate continuously for many hours and thus are well suited for use in aviation. References and the developmental history and outlook for specific types of jet engines may be found in the individual articles on the engines.
L. A. GILBERG