JET PROPULSION
A propulsion unit which obtains the oxygen from the surrounding atmosphere for combustion purpose is called jet propulsion. Jet propulsion plant is used for the propulsion of aircraft and other missiles by the reaction of a jet of gases which are discharged out with a high velocity from the rear side of the unit. The mass and velocity of the jet are obtained from the combustion of liquid fuels. At higher altitudes jet propulsion has marked advantage over screw propulsion. Its overall efficiency increases with higher altitudes.
TYPES OF JET PROPULSION SYSTEMS:
A propulsion system which obtains the oxygen from the surrounding atmosphere for combustion of fuel is called jet propulsion.
The jet propulsion employs the gas turbine as its external component. The different types of units (plants) which use gas turbine are classified as
1. Turbo-propeller unit or plant and
2. Turbo-jet unit or plant
Turbo-Propeller Unit:
A turbo-propeller unit obtains its driving or propulsive force from the action of propeller as well as from the reaction of the jet of burnt gases ejecting from its rear. It is also known as ‘turbo-propengine' Here the expansion of gases takes place partly (= 80%) in turbine and partly (= 20%) in nozzle.
Figure shows a turbo-propeller unit. Air enters the compressor (C) where it is compressed to a high pressure. The compressed air is then passed into combustion chamber (C.C) where combustion of fuel (such as aviation kerosene) takes place. The products of combustion are, thereafter, forced into the gas turbine (T). Power is developed by the turbine which drives the propeller (P) and compressor. The exhaust gases form into a jet and leave the unit from its rear end. A propulsine force is obtained due to the reaction of this jet. The speed of the coupling shaft (S) being very high, a set of reduction gear is at times employed to reduce the speed of rotation of the propeller. The overall efficiency of the turbo-propeller is improved by providing a diffuser (D) before the compressor as shown.
Advantages:
• Simplicity in design
• Low specific weight (wt. per KW)
• High power for take-off
• High propulsion efficiency at speeds below 600 km/hr
Turbo-Jet unit:
In this unit total propulsive force is obtained from the reaction of the jet of exhaust gases, which are discharged out from its rear end.
Figure shows a simple turbo-jet plant or jet-engine. This system uses an air compressor and a gas turbine for producing the jet. The plant is an adaptation of the constant pressure type of gas turbine. The exhaust gases from the turbine being further expanded through a nozzle to produce the jet.
Air from atmosphere enters the compressor (C) with a velocity equal to that of the air craft and is compressed to about 3.5 bar in a rotary compressor. Compressed air passes through combustion chamber (CC) which contains a ring of fuel nozzles. Fuel oil (paraffin) is pumped through these nozzles by a fuel pump (P) under pressure. Fuel oil is burnt in the combustion chamber at constant pressure in the presence of high pressure air. This rises the temperature and volume of the mixture of air and burnt gas in the chamber. The mass of air supplied is 50 to 70 times the fuel burnt; this provides sufficient mass for the propulsion jet and also prevents gas temperature from exceeding practical limits. The hot gases from the combustion chamber then enter the gas turbine (T) where they expand on blades producing power. From the turbine the gases are exhausted through the main expansion nozzle (N). Here the remaining pressure energy in the gases is converted into kinetic energy. They are then discharged out in the form of a high velocity propulsion jet from the rear end of the unit. The unit moves ahead in a direction opposite to that of the jet.
Advantages of Turbo-Jet :
• simplicity in construction
• free from vibrations
• speeds as high as 3000 kmph are possible
• uninterrupted and smooth power supply
• low specific weight (weight to power ratio)
• higher rates of climbing minimum over-hauling
Disadvantages :
• low efficiency
• relatively shorter life
• not efficient in performance less than 550 kmph speed
• more noisy
• expensive materials
• low take off thrust.