On this slide, we show a schematic of a solid rocket engine. Solid rocket
engines are used on airtoair and airtoground missiles, on
model rockets,
and as boosters for satellite launchers.
In a solid rocket,
the fuel and oxidizer are mixed together into a solid propellant
which is packed into a solid cylinder.
A hole through the cylinder serves as a
combustion chamber.
When the mixture is ignited,
combustion
takes place on the surface of the
propellant. A flame front is generated
which burns into the mixture.
The combustion produces
great amounts of exhaust gas at high
temperature
and
pressure.
The amount of exhaust gas that is produced depends on the
area of the flame front and
engine designers use a variety of hole shapes
to control the change in thrust for a particular engine.
The hot exhaust gas is passed through a
nozzle
which accelerates the flow.
Thrust
is then produced according to Newton's
third law
of motion.
The amount of thrust
produced by the rocket depends
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
mixed into the propellant, solid 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 the oxygen
for combustion and as the working fluid in the generation of thrust.
The
thrust equation
given 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 rocket engines.
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