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The
Control of the thrust of an rocket engine is very important to launching
payloads (cargoes) into orbit. Too much thrust or thrust at the wrong time
can cause a satellite to be placed in the wrong orbit or set too far out
into space to be useful. Too little thrust can cause the satellite to fall
back to Earth.
Liquid-propellant rocket engines
control the thrust by varying the amount of propellant that enters the
combustion chamber. A computer in the rocket's guidance system determines
the amount of thrust that is needed and controls the propellant flow rate.
On more complicated flights, such as going to the Moon, the rocket engines
must be started and stopped several times. Liquid rocket engines do this
by simply starting or stopping the flow of propellants into the combustion
chamber.
Solid-propellant rockets are not
as easy to control as liquid rockets. Once started, the propellants burn
until they are gone. They are very difficult to stop or slow down part
way into the burn. Sometimes fire extinguishers are built into the engine
to stop the rocket in flight. But using them is a tricky procedure and
doesn't always work. Some solid-fuel engines have hatches on their sides
that can be cut loose by remote control to release the chamber pressure
and terminate thrust.
The burn rate of solid propellants
is carefully planned in advance. The hollow core running the length of
the propellants can be made into a star shape. At first, there is a very
large surface available for burning, but as the points of the star burn
away, the surface area is reduced. For a time, less of the propellant burns,
and this reduces thrust. The Space Shuttle uses this technique to reduce
vibrations early in its flight into orbit.
NOTE: Although most rockets
used by governments and research organizations are very reliable, there
is still great danger associated with the building and firing of rocket
engines. Individuals interested in rocketry should never attempt to build
their own engines. Even the simplest-looking rocket engines are very complex.
Case-wall bursting strength, propellant packing density, nozzle design,
and propellant chemistry are all design problems beyond the scope of most
amateurs. Many home-built rocket engines have exploded in the faces of
their builders with tragic consequences.
Next
page > • Part
3: Rocket Stability and Control Systems
• Part
4: Rocket Mass
Information and Images Provided by
NASA
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