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INTRODUCTION Jet
Propulsion is the thrust imparting forward motion to an object, as a reaction
to the rearward expulsion of a high-velocity liquid or gaseous stream.A simple
example of jet propulsion is the motion of an inflated balloon when the air is
suddenly discharged. While the opening is held closed, the air pressure within
the balloon is equal in all directions; when the stem is released, the internal
pressure is less at the open end than at the opposite end, causing the balloon
to dart forward. Not the pressure of the escaping air pushing against the outside
atmosphere but the difference between high and low pressures inside the balloon
propels it. An
actual jet engine does not operate quite as simply as a balloon, although the
basic principle is the same. More important than pressure imbalance is the acceleration
due to high velocities of the jet leaving the engine. This is achieved by forces
in the engine that enable the gas to flow backward forming the jet. Newton's second
law shows that these forces are proportional to the rate at which the momentum
of the gas is increased. For a jet engine, this is related to the rate of mass
flow multiplied by the rearward-leaving jet velocity. Newton's third law, which
states that every force must have an equal and opposite reaction, shows that the
rearward force is balanced by a forward reaction, known as thrust. This thrusting
action is similar to the recoil of a gun, which increases as both the mass of
the projectile and its muzzle velocity are increased. High-thrust engines, therefore,
require both large rates of mass flow and high jet-exit velocities, which can
only be achieved by increasing internal engine pressures and by increasing the
volume of the gas by means of combustion. Jet-propulsion
devices are used primarily in high-speed, high-altitude aircraft, in missiles,
and in spacecraft. The source of power is a high-energy fuel that is burned at
intense pressures to produce the large gas volume needed for high jet-exit velocities.
The oxidizer required for the combustion may be the oxygen in the air that is
drawn into the engine and compressed, or the oxidizer may be carried in the vehicle,
so that the engine is independent of a surrounding atmosphere. Engines that depend
on the atmosphere for oxygen include turbojets, turbofans, turboprops, ramjets,
and pulse jets. Non atmospheric engines are usually called rocket engines.
What
is a Turbofan Engine?
A
turbofan engine is the most modern variation of the basic gas turbine engine.
As with other gas turbines, there is a core engine. In the turbofan engine, a
fan in the front and an additional turbine at the rear surrounds the core engine.
The fan and fan turbine are composed of many blades, like the core compressor
and core turbine, and are connected to an additional shaft.
As
with the core compressor and turbine, some of the fan blades turn with the shaft
and some blades remain stationary. The fan shaft passes through the core shaft
for mechanical reasons. This type of arrangement is called a two-spool engine
(one "spool" for the fan, one "spool" for the core.) Some
advanced engines have additional spools for sections of the compressor, which
provides for even higher compressor efficiency. Jet
Engine Thrust
The force produced by a jet engine is expressed in terms of kilograms of thrust.
This is a measure of the mass or weight of air moved by an engine times the acceleration
of the air as it goes through the engine. Technically, if the aircraft were to
stand still and the pressure at the exit plane of the jet engine was the same
as the atmospheric pressure, the formula for the jet engine thrust would be:
Weight of air in kilograms per second * velocity
Thrust = ___________________________________________
9.81 (normal acceleration due to gravity, in meter per second 2)
Imagine
an aircraft standing still, capable of handling 97.522 kilograms of air per second.
Assume the velocity of the exhaust gases to be 1,500 feet per second. The thrust
would then be:
Thrust = 97.522 kg of air per second * 457.2 m / s
9.81 m / s 2
= 9.941 * 457.2
Thrust = 4545.025 kg.
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