Types of Jet Propulsion
System
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.
1 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)
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.
2 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.
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