jet propulsion

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jet propulsion

jet 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. 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.

Development of the Reaction Engine

The first reaction engine, the aeolipile (a ball that rotated as a reaction to escaping steam), was constructed by the inventor Heron (or Hero) of Alexandria. Developments through the centuries have resulted in two general types of reaction machines, the true rocket and the airstream engine, commonly known as the jet engine. Unlike a jet engine, a rocket engine carries with it chemicals that enable it to burn its fuel without drawing air from an outside source. Thus a rocket can operate in outer space, where there is no atmosphere. Fritz von Opel, a German automobile manufacturer, made the first flight entirely by rocket power in 1939. The American R. H. Goddard did much of the important pioneer work in modern rocket development.

The second category of reaction motor, the jet engine, is a development of the late 18th-century gas turbine engines, which directed combustion gases against the blades of a turbine wheel. Not until 1908 was it suggested that an aircraft could be driven by jet propulsion. René Lorin, a French engineer, proposed using a reciprocating engine to compress air, mix it with fuel, and thus propel the aircraft by the pulses of hot gas produced by combustion of the mixture. Henri Coanda, a Romanian engineer, experimented with a reaction-powered aircraft in 1910, and observed the phenomenon now known as the Coanda effect. In 1939 the English engineer Frank Whittle developed a jet engine that powered a full-sized aircraft, and a year later Secundo Campini in Italy flew for 10 min using a thermal jet engine.

Jet-propelled aircraft have replaced propeller-driven types in all but short-range commercial applications; turboprop planes, in which a propeller is turned by a turbine engine, are used for short-range flights. The SR-71 Blackbird, a U.S. jet spyplane, holds the current speed record of 2,193.17 mph (3,529.56 kph) for a piloted air-breathing airplane, but NASA's experimental scramjet-powered pilotless X-43A bested this, almost reaching Mach 7 (about 5,300 mph/8,500 kph) and later Mach 10 (about 7,600 mph/12,200 kph) in brief test flights in 2004. The experimental X-51A, also pilotless, reached Mach 5 (3,800 mph/6,100 kph) in a 2010 test flight. The Australian-led HyShot Flight Program successfully tested a British-designed scramjet engine in 2006.

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jet propulsion

[¦jet prə¦pəl·shən]
(aerospace engineering)
The propulsion of a rocket or other craft by means of a jet engine.
Propulsion by means of a jet of fluid.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Jet propulsion

Propulsion of a body by means of force resulting from discharge of a fluid jet. This fluid jet issues from a nozzle and produces a reaction (Newton's third law) to the force exerted against the working fluid in giving it momentum in the jet stream. Turbojets, ramjets, and rockets are the most widely used jet-propulsion engines. See Ramjet, Turbojet

In each of these propulsion engines a jet nozzle converts potential energy of the working fluid into kinetic energy. Hot high-pressure gas escapes through the nozzle, expanding in volume as it drops in pressure and temperature, thus gaining rearward velocity and momentum. This process is governed by the laws of conservation of mass, energy, and momentum and by the pressure-volume-temperature relationships of the gas-state equation. See Nozzle

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

jet propulsion

jet propulsion
In the above case, velocity of exhaust gases (Vje) is much greater than those of gases at the inlet (Va). The accelerated gases push the aircraft forward by reaction in accordance with Newton's third law of motion.
A form of propulsion where a mass of air is accelerated through a large change in velocity, and the body is propelled forward as a result of the action-reaction phenomenon, or Newton's third law of motion. The propulsion unit obtains its oxygen from the air for imparting a higher velocity to the incoming gases by combusting fuel. The thrust is obtained by expelling a mass air flow at a higher velocity than its incoming velocity.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

jet propulsion

1. propulsion by means of a jet of fluid
2. propulsion by means of a gas turbine, esp when the exhaust gases provide the propulsive thrust
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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