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The principle of jet propulsion was demonstrated by Hero of Alexandria as long ago as the first century AD in the earliest 'steam engine' on record, but the jet engine did not become a practical possibility until 1930 when Frank Whittle, later Sir Frank Whittle, patented the design of his first reaction motor suitable for aircraft propulsion. The early jet engines were designed solely for aircraft propulsion. However, development was rapid and the range of applications has widened to include ships, hovercraft, power stations and industrial installations, all of which benefit from the jet engine's inherent qualities of high power, small size and low weight.

How does a jet engine work

The jet engine or, more correctly, the gas turbine is an internal combustion engine which produces power by the controlled burning of fuel. In both the gas turbine and the motor car engine, air is compressed, fuel is added and the mixture is ignited. The resulting hot gas expands rapidly and is used to produce the power.
In the motor car engine, the burning is intermittent and the expanding gas moves a piston and crank to produce rotary or shaft power which is transmitted to the road wheels.

Layout of the gas turbine

The gas turbine engine is basically a machine designed to accelerate a stream of gas which is used to produce the reactive thrust necessary to propel the aircraft.

The main types of gas turbines

There are four main types of gas turbine engines:

The turbojet and turbofan types are reaction engines which derive power from the reaction to the exhaust stream. The turboprop and turboshaft types operate in a different way. They use the exhaust stream to power an additional turbine which in turn drives a propeller or output shaft.

Variations

Additions

Afterburning
Afterburning or reheat, as it is sometimes termed, is a method of increasing the engine thrust for short periods to improve the aircraft take-off, climb and, for military aircraft, combat performance. Unlike a piston engine, the fuel in a gas turbine burns in an excess of air, so there is still sufficient oxygen present in the exhaust to support further combustion. This makes it possible to inject and burn additional fuel in the jet pipe, downstream of the turbine, to increase the exhaust velocity and consequently increase the engine thrust. Turbofan engines, where the bypass air provides even more oxygen, can achieve thrust increases up to 100 per cent with afterburning.

Reverse thrust
Thrust reversal is a method of mechanically deflecting the exhaust stream of a gas turbine forward externally, to act as a braking force when the aircraft lands. The system is most efficient when used at the relatively high speeds immediately after touch-down.
Reverse thrust is particularly useful in adverse weather conditions, and can be used to augment, or instead of, the aircraft's wheel-braking system.

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