Mumbai University > Mechanical Engineering > SEM 6 > Thermal and Fluid Power Engineering

Marks: 5M

Propulsion is a combination of two Latin words namely pro meaning forward and pellere meaning to drive.

Thus, propulsion means to push forward or drive forward an object.

Propulsion system is a machine that produces necessary thrust to drive an object forward.

Jet propulsion devices are used to propel aircrafts, rockets, missiles etc.

The operation of these devices is based on Newton’s second and third laws of motion.

In jet propulsion a high velocity jet is produced by expanding the high pressure and high temperature gases in the nozzle situated at the rear end of the aircraft.

This accelerating mass of the gases produces a force and its reaction. This is called as propulsive force or thrust is used to propel the aircraft in forward direction.

The propulsive devices or the jet engines which make use of jet propulsion are mainly classified as follows:

1. Air stream jet engine(which use atmospheric air)

   • Indirect reaction: E.g. Propeller, turboprop, turbofan
   • Direct reaction: E.g. Turbojet, Turbofan and Athodyd(Ram jet and pulse jet)

2. Self-contained jet engines (which do not use atmospheric air): Rockets.

   Rockets are further subdivided into three groups

   • Chemical (Solid propellant, liquid propellant)
   • Nuclear
   • Electric

Air Stream Jet Engines:

In this case, the air which is necessary for the combustion of fuel is consumed by the engine from the atmosphere.

In case of indirect reaction air stream engines, the internal combustion engines were used and the power generated using IC engines was used to drive a propeller.

In direct reaction devices, the power required to run the compressor was generated using the gas turbine power plants. Since the exhaust of the turbine is at a greater pressure than the atmospheric, it is used to produce a high velocity jet in nozzles. The change in momentum generates a reaction force that drives the aircraft.

Self-Contained Jet Engines:

In these engines, the necessary oxygen required for fuel combustion is contained in the rockets themselves. Thus, it does not need air for forming the jet. The jet produced is called rocket jet and the relevant equipment is called rocket motor.