Jet propulsion is based on newton’s second law and third law of motion. Newton’s second law states that the rate of change of momentum in any direction is proportional to the force acting in that direction. Newton’s third law of motion states that for every action there is an equal and opposite reaction. Turbo-jet engine a type of jet engine is explained below.
The Turbo-jet engine is a modified form of gas turbine engine. A Turbo-jet consists of a diffuser, rotary compressor combustion chamber, turbine and nozzle.
Air from the surrounding atmosphere is drawn by the compressor through the diffuser slows down the entering the speed of the air and compresses it. This is known as ram compression. Inside the rotary compressor the air is further compressed to a pressure of 4 atm. This pressurized air then passes into the combustion chamber. Inside the combustion chamber the fuel is sprayed over the pressurized air. The air fuel mixture is burnt. Thus heat is applied at a constant pressure.
The temperature of the air increases rapidly. The amount of air supplied is about 60 times the amount of fuel burnt. The excess air produces sufficient mass for the propulsion of the jet and at the same time prevents the turbine blades from high temperature gases.
The products of combustion process enter into the turbine and expand partially. The power generated by the turbine is only sufficient to drive the compressor, fuel pump and other auxiliaries. The hot gases from the turbine are at a pressure which is above the atmospheric pressure. These gases then enter into the nozzle where it is expanded which converts pressure energy into kinetic energy. Thus the gases come out from the unit with a very high velocity. Due to the increased velocity of gases coming out of the unit, a reaction or thrust is produced in the opposite direction. This thrust propells the air craft. For initial starting, some starting device is provided. This method of propulsion is best suitable for air crafts which travels at or about 800 kmhr.
Advantages of turbo-jet
· Simple construction.
· Less moving parts. Less wear the tear. Hence maintenance cost is less.
· Thrust is directly applied. Hence no loss of power in transmission.
· Unit runs smoothly without vibration since continuous thrust is produced by continuous combustion of fuel.
· Lighter construction of the unit since low working pressure.
· It can operate at higher speeds than turbo-propeller air crafts since the speed of turbo-jet is not limited by the propeller.
· Low grade fuels can be used. This reduces fuel cost.
Disadvantages of turbo-jet:
· A poor starting characteristic since less power is produced during take-off.
· Costly materials are used.
· Life of the unit is shorter than reciprocating engine.
· Produces more noise than reciprocating internal combustion engines.
· High fuel consumption.