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. The types of jet engines are explain below.
This is also called as turbo-prop engine. It differs from the turbo-jet in that, it uses a propeller to increase the mass flow of air. In this type, the turbine drives the compressor and propeller.
Air from atmosphere is taken into the compressor through diffuser. In the diffuser, the air is slowed down and compressed. The compressed air from the compressor enters the combustion chamber where the fuel is sprayed. Fuel-air mixture is burnt and thus heat is supplied at constant pressure. The combustion products are then expanded partially in the turbine. The combustion products are then expanded partially in the turbine. The exhaust gases from the turbine are then expanded in the exit nozzle. In the nozzle pressure energy is converted into kinetic energy. Thus the gases emerge from the unit with very high velocity. This increased velocity produces a reaction or thrust in the opposite direction. The total thrust produced in the sum of the thrusts produced by the nozzle and the propeller. This total thrust propels the air craft.
The angular velocity of the shaft is very high. The propeller cannot run at a higher angular velocity. Therefore, a reduction gear box is provided before the power is transmitted to the propeller. In some cases, two turbines are used to drive the compressor and propeller separately.
Advantages of turbo-prop engines
· Low specific weight and less frontal area.
· Simple in construction.
· Lesser vibration and noise. Easy maintenance.
· High power for take-off and climb of the propeller.
· High propulsive efficiency at speeds below 800 kmhr.
A ram-jet is the simplest of all propulsive devices. Compressor and turbine are not required for ram-jet units. It consists of three main components namely diffuser system, combustion chamber and exhaust nozzle.
The diffuser consists of two parts namely supersonic diffuser and subsonic diffuser. The atmospheric air enters the unit with supersonic velocity and slowed down to the sonic velocity in the supersonic diffuser. This produces shock waves and increase in pressure. Then the air passes through the subsonic diffuser and its velocity further reduces to subsonic value. Due to this, the pressure of air increases to ignition pressure. Thus, the compression of air to the ignition pressure takes place only by ram effect in the diffuser. Then the high pressure air enters the combustion chamber. Fuel is injected in the combustion chamber and the air-fuel mixture is burnt. The hot products of combustion are then expanded in the exit nozzle. In the nozzle pressure energy is converted into kinetic energy. This increased velocity of exhaust gases produces an opposite thrust. The aircraft is propelled by this thrust. The ram-jet operates at very high speed.
· No moving parts.
· Light wright when compared with turbo-jet engine.
· More thrust than a turbo-jet
· Unlimited flight speed.