Thrust is the force which moves any
aircraft through the air. Thrust is generated by the
of the aircraft. Different propulsion systems develop thrust in
different ways, but all thrust is generated through some
application of Newton's third law of
motion. For every action there is an equal and opposite reaction.
In any propulsion system, a working fluid is
accelerated by the system and
the reaction to this acceleration produces a force on the system. A
general derivation of the thrust equation
shows that the amount of thrust generated depends on the
through the engine and the
of the gas.
During and following World War II, there were a number of rocket-
powered aircraft built to explore
high speed flight.
The X-1A, used to
break the "sound barrier", and the X-15 were
rocket-powered airplanes. In a rocket engine , fuel and
a source of oxygen, called an oxidizer, are mixed and exploded
in a combustion chamber. The
produces hot exhaust which is passed through a
to accelerate the flow and
For a rocket, the accelerated gas, or
working fluid, is the hot exhaust produced during combustion.
This is a different working fluid than you find in a
or a propeller-powered aircraft.
Turbine engines and
propellers use air from the atmosphere as the working fluid,
but rockets use the combustion exhaust gases.
In outer space there is no atmosphere so turbines and propellers
can not work there.
This explains why a rocket works in space
but a turbine engine or a propeller does not work.
There are two main categories of rocket engines; liquid rockets and
solid rockets. In a
the propellants, the fuel and the oxidizer,
are stored separately as liquids and are pumped into
the combustion chamber of the nozzle
where burning occurs. In a
the propellants are mixed together
and packed into a solid cylinder. Under normal temperature conditions,
the propellants do not burn; but they will burn when exposed to
a source of heat provided by an igniter.
Once the burning starts,
it proceeds until all the propellant is exhausted.
With a liquid rocket, you can stop the thrust by turning off the flow of
propellants; but with a solid rocket, you have to destroy the casing to stop
the engine. Liquid rockets tend to be heavier and more
complex because of the pumps and storage tanks. The propellants are
loaded into the rocket just before launch.
A solid rocket is much easier to handle and can sit for years
On this slide, we show a picture of an X-15 rocket-powered
airplane at the upper left and a picture of a rocket engine test at
the lower right. For the picture at the right, we only see the
outside of the rocket nozzle, with the hot gas exiting out the
bottom. The X-15 was powered by a liquid rocket engine and carried a single
pilot to a height of more than 60 miles above the earth. The X-15 flew more
than six times the speed of sound nearly 40 years ago. The
speed record for a piloted aircraft is only exceeded
today by the Space Shuttle. The altitude record is only topped by the Space Shuttle
and the recent Space Ship 1, which also used rocket propulsion.
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