JET PROPULSION ENGINES

Theory of jet propulsion

Jet propulsion is based on Newton’s second and third low of motion. Newton’s second law states that ‘the range of momentum in any direction is proportional to the force acting in that direction’. Newton’s third low states that for every action there is an equal and opposite reaction.

In propulsion momentum is imparted to a mass of fluid in such a manner that the reaction of the imparted momentum furnishes a propulsive force. The jet aircraft draws in air and expels it to the rear at a markedly increased velocity; the rocket greatly changes the velocity of its fuel which it ejects rearward in the form of products of combustion. In each case the action of accelerating the mass of fluid in a given direction created a reaction in the opposite direction in the form of a propulsive force. The magnitude of this propulsive force is defined as thrust.

Types

The jet propulsion engines are classified basically as to their method of operation. The two main categories of jet propulsion engines are the atmospheric jet engines and the rockets. The atmospheric jet engines require oxygen from the atmospheric air for the combustion  of fuel. As a result, their performance depends to a great degree on the forward speed of the engine and upon the atmospheric pressure and temperature.

The rocket engine differs from the atmospheric jet engines in that the entire mass of jet is generated from the propellants carried within the engine, i.e., the rocket engine carries its own oxidant for the combustion of the fuel and is therefore, independent of the atmospheric air. The performance of this type of power plant is independent of the forward speed and affected to a maximum of about 10% by changes in altitude.

Air Breathing Engines

Air breathing engines can further be classified as follows:

1.       Reciprocating engines (Air screw)

2.       Gas Turbine engines (i) Turbojet  (ii) Turbojet with after burner (also known as turbo ramjet, turbojet with tail pipe burning and turbojet with reheater) (iii) Turboprop (also known as propjet).

3.       Athodyds (Aero Thermodynamics Ducts)

(i) steady combustion system, continuous air flow – Ramjet (also known as Lorin tube)(ii) Intermittent combustion system, intermittent air flow – Pulse jet (also known as aero pulse, resojet, Schmidt tube and intermittent jet).

The reciprocating engine develops its thrust by accelerating the air with the help of a propeller driven by it, the exhaust of engine imparting almost negligible amount of thrust to that developed by the propeller.

The turbojet, turbojet with afterburner and turboprop are modified simple open cycle gas turbine engines. The turbojet engine consists of an open cycle gas turbine engine (compressor, combustion chamber and turbine) with an entrance air diffuser added in front of the compressor and an exit nozzle added aft of the turbine. The turbojet with afterburner is a turbojet engine with a reheater added to the engine so the extended tail pipe acts as a combustion chamber. The turboprop is a turbojet engine with extra turbine stages, a reduction gear train and a propeller added to the engine. Approximately 80 to 905 of the thrust of the turboprop is produced by acceleration of the air outside the engine by the propeller and about 10 to 20% of the thrust is produced by the jet exit of the exhaust gases. The ramjet and the pulsejet are athodyds, i.e., a straight duct type of jet engine without compressor and turbine wheels.

Rocket Engines

The necessary energy and momentum which must be imparted to a propellant as it is expelled from the engine to produce a thrust can be given in many ways. Chemical, nuclear or solar energy can be used and the momentum can be imparted by electrostatic or electromagnetic force.

Chemical rockets depend up on the burning of the propellant inside the combustion chamber and expanding it through a nozzle to obtain thrust. The propellant may be solid, liquid, gas or hybrid.

The vast store of atomic energy is utilized incase of nuclear propulsion. Radioactive decay or Fission or Fusion can be used to increase the energy of the propellant.

In electrical rockets electrical energy from a separate energy source is used and the propellant is accelerated by expanding in a nozzle or by electrostatic or electromagnetic forces.

In solar rockets solar energy is used to propel spacecraft.

Differentiate between air breathing engines and rocket engines?

AIR BREATHING ENGINES	ROCKET ENGINES
Combustion takes place by using atmospheric air.	Combustion takes place by using its own oxygen supply.
These engines cannot be used at very high altitudes due to deficiency of air.	Rocket engines can be used at very high altitudes. That‘s why it is used in space crafts and for satellite launching
It do not require any oxygen storage system	Requires oxygen storage system.
Design is not much complex.	Design is very complex.
Air breathing engines can be either reciprocating or rotary type.	Rocket engines are of rotary type.
Air breathing engines can be used for domestic applications.	Rocket engines are used for scientific researches  mainly in outer space
Air breathing engines are not much bulkier.	Rocket engines are really bulkier.

AIR SCREW

In an airscrew the source of power is a reciprocating internal combustion engine which drives a propeller connected to it. The propeller displaces rewards a large mass of air, accelerating it in the process (Figure 1). Due to this acceleration of the fluid a propulsive force is produced which drives the aircraft.

Nearly all the earlier aircrafts used reciprocating engines as the source of energy to drive the propeller. The use of reciprocating engines is continuously on the decline because its development has reached a stage of near saturation. Present day aircrafts demand high flight speeds, long distance travels and high load carrying capacities. A power output more than 5000 hp. is difficult to obtain without modifications in the present reciprocating engine plant. The output can be increased by increasing the cylinder size, installing large number of cylinders or by running the engine at high speeds. Unfortunately all those methods of raising the output of the engine increase the engine size, frontal area of the aircraft, complexity and cost of the plant. The drag of the plant will also increase to critical values with increase in engine size.

The speed of airscrew is limited to a range of about 700 km/h. The propeller loses its effectiveness at higher speeds due to separation of flow and shock waves as the air velocity approaches the sonic velocity. At lower speeds the propulsive efficiency of the propeller is about 95%.

For small aircrafts flying at velocities less than about 500 to 650 km/h reciprocating engine has an enviable position due to its excellent fuel economy and good take-off characteristics. However due to comparatively large drop in power with altitude operation and the need of using high octane fuels, along with the difficult cooling and lubrication problems, high weight/power ratio, and larger frontal area of such engines these are being replaced by turbojets in higher speed ranges.

References

1. Compressible fluid flow  - A. H. Shapiro
2. Fundamentals of compressible flow with aircraft and rocket propulsion   - S. M. Yahya