Flying model rockets is a relatively safe and inexpensive way for students
to learn the basics of forces and
the response of vehicles to external forces.
Like an airplane, a model rocket is
subjected to the
forces of weight,
thrust, and aerodynamics
during its
flight.
The thrust force is supplied by a small solid rocket engine.
There are two main categories of rocket engines; liquid rockets and
solid rockets. In a
liquid rocket,
the fuel and the source of
oxygen (oxidizer) necessary for
combustion
are stored separately and pumped into the combustion chamber of the
nozzle
where burning occurs.
In a
solid rocket,
the fuel and oxidizer are mixed together into a solid propellant
which is packed into a solid cylinder. Under normal temperature conditions,
the propellant does not burn; but the propellant will burn when exposed to
an external source of heat.
Some type of igniter is used to initiate the burning
of a solid rocket motor at the end of the propellant facing the nozzle.
As the propellant burns, hot exhaust gas is
produced which is used to propel the rocket, and a
"flame front" is produced which moves into the propellant.
Once the burning starts,
it will proceed until all the propellant is burned.
With a liquid rocket, you can stop the thrust by turning off the flow of
fuel or oxidizer; but with a solid rocket, you must destroy the casing to stop
the engine. Liquid rockets tend to be heavier and more
complex because of the pumps used to move the fuel and oxidizer,
and you usually load the fuel and oxidizer into the rocket just
before launch. A solid rocket is much easier to handle and can sit for years
before firing.
The relative
safety
of building and flying model rockets is the result of
the production and availability of pre-packaged solid model rocket
engines. The engines are produced by several manufacturers and
are available in a variety of sizes with a range of
engine performance.
The engines can be bought at most hobby stores and some toy stores for
a modest price (average current price is 3 engines for $5). The engines
are used once and discarded; a new engine is inserted into the
rocket for the next flight. Before these
engines became available, many young rocket builders lost limbs or life in
the process of mixing rocket fuels. With these engines, you can still
have the fun of building and flying rockets, learn the fundamentals, and then
move on to the more dangerous and complex problems of propulsion.
On this slide we show a drawing of the parts of a model rocket engine
so that you can learn how it works.
We have laid the engine on its side,
and "cut" the engine in half so that we can see what is inside.
Never disturb, cut, or modify a real model rocket engine. The propellant
can ignite at any time if there is a source of heat.
The engine is installed in a rocket shown by the dashed lines on the figure.
The engine casing is a cylinder made of heavy cardboard which contains
the nozzle, propellants, and other explosive charges.
At the right side of the engine is the
nozzle, a relatively
simple device used to accelerate hot gases and produce thrust. Model rocket
nozzles are usually made of clays or ceramics because of the high
temperature of the exhaust. The hot gases
for a model rocket are produced by the solid propellant, shown in
green. An electric igniter is used to launch a model rocket.
As the flame burns through the propellant, the rocket experiences
powered flight.
When the flame front reaches the far left of the propellant, thrust
goes to zero, and a delay charge, colored blue, begins to burn.
During the
delay, no thrust is produced and the rocket
coasts up to its maximum altitude.
The length of the delay varies between engines from 2 to 8 seconds and the
amount of the delay is listed on the engine casing.
When the delay charge is completely burned through, the ejection charge,shown in red, is ignited. This produces a small explosion which ejects
hot gas out the front of the engine through the engine mount, ejects
the nose cone, and deploys the parachute for a safe
recovery.
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