On this slide, we show a schematic of a liquid rocket engine. Liquid rocket
engines are used on the
Space Shuttle
to place humans in orbit, on many unmanned
missiles to place satellites in orbit, and
on several high speed
research aircraft following World War II.
In a
liquid rocket,
stored fuel and stored oxidizer
are pumped into a combustion
chamber where they are mixed and burned.
The combustion produces great amounts of exhaust gas at high
temperature
and
pressure.
The hot exhaust is passed through a
nozzle
which accelerates the flow.
Thrust
is produced according to Newton's
third law
of motion.
The amount of thrust produced by the rocket depends
on the mass flow rate through the engine, the exit
velocity of the exhaust, and the pressure at the nozzle
exit. All of these variables depend
on the
design of the nozzle.
The smallest crosssectional area of the nozzle is called the
throat of the nozzle. The hot exhaust flow is
choked
at the throat, which means that the
Mach number
is equal to 1.0 in the throat and the
mass flow rate
m dot
is determined by the throat area.
The area ratio from the throat
to the exit Ae sets the
exit velocity
Ve
and the exit pressure pe.
You can explore the design and operation of a rocket nozzle with
our interactive
nozzle simulator
program which runs on your browser.
The exit pressure is
only equal to free stream pressure at some design condition.
We must, therefore, use the longer version of the generalized
thrust equation
to describe the thrust of the system.
If the free stream pressure is given by p0,
the thrust F equation becomes:
F = m dot * Ve + (pe  p0) * Ae
Notice that there is no free
stream mass times free stream velocity term
in the thrust equation
because no external air is brought on board. Since the oxidizer is
carried on board the rocket, rockets can generate thrust in a vacuum
where there is no other source of oxygen. That's why a rocket will
work in space, where there is no surrounding air,
and a gas
turbine or propeller will not work.
Turbine engines and propellers rely on the atmosphere to provide
air as the working fluid for propulsion and oxygen in the air
as oxidizer for combustion.
The
thrust equation
shown above works for both liquid and
solid rocket engines.
There is also an efficiency parameter called the
specific impulse
which works for both types of rockets and greatly simplifies
the performance analysis for rockets.
The details of how to mix and burn the fuel and oxidizer,
without blowing out the flame, are very
complex. It DOES take a rocket scientist to figure it out!
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