The development of the gas turbine engine as an aircraft power plant has been so rapid that it is difficult to appreciate that prior to the 1950s very few people had heard of this method of aircraft propulsion. The possibility of using a reaction jet had interested aircraft designers for a long time, but initially the low speeds of early aircraft and the unsuitably of a piston engine for producing the large high velocity airflow necessary for the jet presented many obstacles.
WHAT is JET ENGINE?
A jet engine is a reaction engine that generates thrust by jet propulsion in accordance with Newton’s Laws of Motion and discharge a fast moving jet.The jet engine although appearing so different from the piston engine-propeller combination, applies the same basic principles to effect propulsion.Both propel their aircraft solely by thrusting a large weight of air backwards.
Although today jet propulsion is popularly linked with the gas turbine engine, there are other types of jet propelled engines, such as the ram jet, the pulse jet, the rocket, the turbo/ram jet, and the turbo- rocket.
Generally jet engines are combustion engines but non-combustion forms also exist.
Jet propulsion is a practical application of Sir Isaac Newton’s third law of motion which states that, ’for every force acting on a body there is an opposite and equal reaction’. For aircraft propulsion, the ’body’ is atmospheric air that is caused to accelerate as it passes through the engine. The force required to give this acceleration has an equal effect in the opposite direction acting on the apparatus producing the acceleration. A jet engine produces thrust in a similar way to the engine/propeller combination. Both propel the aircraft by thrusting a large weight of air backwards, one in the form of a large air slipstream at comparatively low speed and the other in the form of a jet of gas at very high speed.
VARIOUS TYPES OF JET ENGINES
THE RAM JET ENGINE
The ram jet engine also known as an athodyd, means aero-dynamic duct.It has no major rotating parts and consists of duct with a divergent entry and a convergent or convergent-divergent exit. When forward motion is imparted to it from an external source, air is forced into the air intake where it loses velocity or kinetic energy and increases its pressure energy as it passes through the diverging duct. The total energy is then increased by the combustion of fuel, and the expanding gases accelerate to atmosphere through the outlet duct. A ram jet is often the power plant for missiles and .target vehicles; but is unsuitable as an aircraft power plant “because it requires forward motion imparting to it before any thrust is produced.
THE PULSE JET ENGINE
The pulse jet engine uses the principle of intermittent combustion and unlike the ram jet it can be run at a static condition. The engine is formed by an aerodynamic duct similar to the ram jet but, due to the higher pressures involved, it is of more robust construction. The duct inlet has a series of inlet ’valves’ that are spring-loaded into the open position. Air drawn through the open valves passes into the combustion chamber and is heated by the burning of fuel injected into the chamber. The resulting expansion causes a rise in pressure, forcing the valves to close, and the expanding gases are then ejected rearwards. A depression created by the exhausting gases allows the valves to open and repeat the cycle.
The pulse jet is unsuitable as an aircraft power plant because it has a high fuel consumption and is unable to equal the performance of the modern gas turbine engine.
THE ROCKET ENGINE
A rocket engine is a jet engine that only uses stored propellant mass for forming its high speed propulsive jet instead of using atmospheric air.
Combustion of that mass or chemically decomposed fuel with oxygen produces its propelling fluid, that enables it to operate outside the earth’s atmosphere. Therefore, it is only suitable for operation over short period.
THE TURBO RAM JET ENGINE
The turbo/ram jet engine made from combination of the turbo-jet engine (which is used for speeds up to Mach 3) with the ram jet engine, which has good performance at high Mach numbers.
The engine is surrounded by a duct that has a variable intake at the front and an afterburning jet pipe with a variable nozzle at the rear. During take- off and acceleration, the engine functions as a conventional turbo-jet with the afterburner lit; at other flight conditions up to Mach 3, the afterburner is inoperative. As the aircraft accelerates through Mach 3, the turbo-jet is shut down and the intake air is diverted from the compressor, by guide vanes, and ducted straight into the afterburning jet pipe, which becomes a ram jet combustion chamber. This engine is suitable for an aircraft requiring high speed and sustained high Mach number cruise conditions where the engine operates in the ram jet mode.
THE TURBO ROCKET ENGINE
The turbo-rocket engine only differs from turbo ram jet engine that it carries its own oxygen to provide combustion otherwise it could be considered as an alternative engine to the turbo/ram jet engine.
The engine has a low pressure compressor driven by a multi-stage turbine; the power to drive the turbine is derived from combustion of kerosene and liquid oxygen in a rocket-type combustion chamber. Since the gas temperature will be in the order of 3,500 deg. C, additional fuel is sprayed into the combustion chamber for cooling purposes before the gas enters the turbine. This fuel-rich mixture (gas) is then diluted with air from the compressor and the surplus fuel burnt in a conventional afterburning system.
It is smaller and lighter than turbo/ram jet but has a higher fuel consumption which makes it more suitable for an space-launcher type aircraft that requires high velocity, high altitude performance and has a flight plan that is entirely accelerative and of short duration .