The basic principle of jet propulsion is that by releasing a jet of high-pressure gas in one direction movement can be produced in the opposite direction. One of the major advantages of this means of locomotion is that very high speeds (faster than the speed of sound) can be reached: accordingly many rockets, guided missiles and aeroplanes are powered by jet propulsion. Jet propulsion also makes flight at very high altitudes possible and is the technology that allows spacecraft to leave the Earth's atmosphere.
The Development of Mechanical Jet Propulsion
Hero of Alexandria was the first to build a jet engine in about 60 BC. Powered by steam escaping from a hollow sphere through two nozzles that pointed in opposite directions, Hero's toy operated in much the same way that jets of water spin a rotating lawn sprinkler. The first people to exploit the principle of the rocket successfully and on a large scale were the Chinese: after they had developed gunpowder, the Chinese used rockets both for warfare and for entertainment from about 1200 AD.
Although few developments in jet propulsion occurred in the next 700 years the growing tensions that led to World War II accelerated the development of jet engines to propel aircraft. German and Italian prototype jets were built in 1939/40 although they proved impractical - a more successful turbojet soon was developed by Frank Whittle, an RAF officer., and used to power the Gloster E. 28/39, which made its first flight in 1941. It was during the 1950s that turbojets and turboprops were first used to power commercial airliners and although some aircraft still use piston engines to turn propellers, jet engines cause far less vibration, resulting in a smoother and safer ride. In addition, while jet engines burn more fuel than piston engines to produce the same amount of thrust, the engine body itself can be smaller and lighter.
Germany was the first country to use jet engines to power missiles, principally the V1 and V2, although the end of the Second World War brought their production to an end at a time when American guided missiles such as the Bomarc were still being developed. Because of its efficiency and quietness, the turbofan came into widerspread use in the 1970s, soon after its invention. Modern research into jet propulsion has been directed at improving the ramjet as a missile engine.
How Jet Propulsion Works
The principle of jet propulsion can be demonstrated with a garden hose connected to a tap. When the nozzle at the end of the hose is closed, the water pushes in all directions against the inside surface of the nozzle. It also pushes back against the water in the hose that is trying to squeeze into the nozzle. When the nozzle is open, some of the water squirts out through the opening. This action upsets the balance of pressure inside the nozzle. It releases the pressure pushing forward just inside the nozzle opening. But the water that is still in the nozzle continues pressing backward and to the sides. If you let go of the nozzle, the unbalanced, pressure will propel it backward. The nozzle will move in the direction opposite that of the jet of water escaping from the nozzle.
The principle of jet propulsion was implicitly described in 1687 by Sir Isaac Newton in his Third Law of Motion, which states that for every action there is an equal and opposite reaction. In the above example, squirting water out of the end of the nozzle is the action while the reaction is the backward movement of the nozzle. Jet propulsion drives an aircraft engine in much the same way: air pressure builds up inside the engine and is released in a powerful stream of jet exhaust. The action of this exhaust escaping from the rear of the engine causes an equal and opposite reaction that pushes the engine forward.
Consider the particle below. It will only hit the engine (solid line), which is moving to the left, if it is also moving to the left. Particles which are moving upwards, downwards or to the right will not interact with the engine.
<--- | o
However, when a particle travelling at speed v to the left hits the engine (below) it will bounce off with a speed v in the opposite direction.
| v | v
<--- | <-- o <--- | o -->
Thus if the mass of the fuel particle is m and that of the engine is M the momentum change which has occurred is
M(δV) = 2mv
where δV is the change in velocity of the engine.
Jet engines and rockets both use the principle of jet propulsion although they use different sources of oxygen for combustion. While jet engines use atmospheric oxygen, meaning that they cannot leave the earth's atmosphere, rockets carry their own supply of oxygen, often in liquid form. Thrust is created by burning fuel in a combustion chamber and allowing the hot gases which are the product of this combustion rush out through a nozzle creating jet thrust.
Air enters the jet engine through an inlet duct and is then compressed until to between 3 and 30 atmospheres. This highly compressed air flows into the combustion chamber and is mixed with a fine spray of jet fuel - usually a liquid petroleum fuel similar to kerosene. The fuel-air mixture is ignited and burns, releasing a large amount of energy in the form of heat - typically the temperature of combustion ranges from about 1800oC to 2000oC. These extreme temperatures could damage the jet engine so the remainder of the compressed air is mixed with the hot combustion gases to cool the walls of the combustion chamber. The exhaust gases, which are now even more highly pressurized, then escape through the exhaust nozzle at an extremely high speed. While a jet-propulsion engine creates thrust by accelerating a small amount of gas to great speeds, the exhaust of the engine contains much unused energy in the form of heat and jet engines accordingly have a low fuel efficiency.
An engine's thrust is measured in a specially designed chamber which can create the different conditions that the engine may encounter - providing a flow of air similar to that occurring in flight at different speeds and altitudes. The thrust produced by an aircraft engine is usually expressed in newtons: each of the four jet engines used on Boeing 747s produces 230,000 newtons of thrust.
Types of Jet Engines
There are four major types of jet engines:
The primary difference between these species is the portion of their total thrust that they produce directly by jet propulsion, although the different types of engine also differ in the way they compress the air that enters the intake ducts.Turboprops and turbofans, as their names imply, generate most of their thrust by turning propellers or propeller-like fans rather than by pure jet propulsion.
The turbojet was the type of engine originally designed by Frank Whittle and used to power aeroplanes during the Second World War - all later jet engines are variations onm this basic design. An inlet duct scoops air into the turbojet and carries the air to the compressor. The job of the inlet duct becomes more complicated in jet fighters and other aircraft that fly faster than the speed of sound since supersonic flight causes shock waves to propagate in the air as it rushes through the inlet duct. These turbulent shock waves may drastically limit the flow of air to the compressor, although a turbojet can reduce the blockage caused by such shock waves by adjusting the shape of the inside of the inlet duct. Nowadays, turbojets are primarily used to power military aircraft
The compressor, a turbine in this case, raises the pressure of the air in the engine and can be either an axial-flow or a centrifugal flow compressor. An axial-flow compressor consists of several wheels with blades attached to them, which are arranged one behind another along a shaft that runs through their centres and turns them at high speeds. Between each pair of wheels is a set of stationary blades, and the air flows through the compressor parallel to the shaft. Each row of blades squeezes the air, increasing its pressure. Some axial-flow compressors can raise the air pressure to 30 atmospheres.
A centrifugal-flow compressor squeezes the air by taking it in near the centre of a rapidly spinning wheel and throwing the air out toward the rim. The wheels of centrifugal-flow compressors cannot be arranged in rows like those of axial-flow compressors. For this reason, centrifugal-flow compressors can only raise the air pressure to about six times that of the outside air. After the air leaves the turbojet compressor, it enters the combustion chamber where the gas behaves as described above.
Some turbojets are equipped with devices called afterburners which are used to greatly increase the thrust of the engine for short periods of time. The afterburner is located between the turbine and the exhaust nozzle in a turbojet where the exiting gases rich in oxygen. In the afterburner, additional jet fuel is mixed with these gases and burned, greatly raising the temperature. The energised gases then accelerate through the nozzle, reaching extremely high speeds and generating a great deal of thrust coupled with a correspondingly high fuel usage. For this reason, afterburners are used only for short periods of time, such as during emergency manoeuvres, rapid take-offs, or steep climbs.
The turboprop is basically a turbojet that uses nearly all its power to turn a propeller. The arrangement of the engine is very similar to that of a turbojet although in this case there is a second turbine just to the rear of the turbine that turns the compressor. Combustion gases spin this 'power turbine' and the spinning motion is transferred by a shaft and a gearbox to the propeller. The gases then exit through the exhaust nozzle, providing a small amount of jet thrust.
The turboprop is smooth running, reliable, and economical, but it is limited to subsonic flight speeds. Turboprops are much smaller and lighter in weight than piston engines of an equivalent power. Large transport planes, small passenger planes and helicopter rotors often use turboprop engines.
A turbofan is essentially a turbojet that uses part of its power to turn a large, propeller-like fan which is enclosed in a pod-like cover at the front of the engine. This fan pushes a large volume of air back toward the engine, part of which enters the engine, where it is compressed, mixed with fuel, burned, and released to generate jet thrust. Most of the air accelerated by the fan, however, goes around the engine and is forced back along the outside of the engine creating thrust and simultaneously providing a cooling effect.
Turbojets are thus capable of combining the efficiency and low noise level of a propeller driven aircraft with the high power of a jet engine. For these reasons, the turbofan is the most common jet engine, used by all large airliners and most military jets.
The ramjet is basically a turbojet without a compressor or a turbine: the simplest type of jet engine. Air entering the ramjet is slowed down in the inlet duct and compressed by the pressure of the moving air behind it. This compressed air enters the engine and provides jet propulsion in the usual way. Due to its simplicity, the ramjet engine is sometimes known as "the flying stovepipe".
However, a major disadvantage of the ramjet is that it cannot function at subsonic flight speeds since it relies on its crusing speed to provide air compression. As a result, aeroplanes which use ramjets must also have a subsidiary power source, usually a turbojet. Although ramjets are not used to power commercial aeroplanes, they do power the Firebrand - a remote-controlled USAF target aeroplane used to imitate anti-shipping missiles.
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