The solid fuel engines in model rockets are made out of a mixture of
materials very similar to gunpowder -- they contain some kind of
nitrate compound like potassium nitrate or sodium nitrate, carbon, and
sulfur (which gives them that distinctive smell). High-tech solid
rocket boosters used on the shuttle use aluminum powder instead of the
carbon and sulfur (yup, aluminum burns!) See
for a comparison of the composition of model rocket fuel and gunpowder.
The nitrate compound is there to supply oxygen -- the formula for
potassium nitrate is KN03, which helps to burn the carbon and sulfur
very rapidly without the need for an air intake. Model rocket engines
may be purchased at hobby stores. People have been injured and killed
mixing explosives -- don't do it yourself. There are laws governing the
use of model rockets and explosives that vary state-to-state -- be
aware of these before doing anything.
The thrust is the product of the rate at which mass is lost by the
engine as the combustion gases exit times the speed of the hot gases as
they escape the engine. You can estimate the first bit by weighing an
engine before and after it has burned and measuring with a stopwatch
how long it burns in flight. This won't be exactly right because the
engine will contain other components, like a delay charge and an
ejection charge for multistage rockets, but it will be close. The speed
at which the gases escape is much harder to measure -- if you know the
density of the gases (and this depends on how hot they are) you can use
the mass you obtained before and the area of the exhaust exit in the
engine to estimate the speed:
(change in engine mass/gas density)/((burn time)*exhaust exit area)
should give you the speed of the gases coming out (that's just the
total volume of the gases divided by the area and the burn time). Make
sure you do everything in consistent units! I recommend kilograms,
meters, and seconds for everything, and you should get the thrust in
Newtons. The hard parts are estimating the density of the escaping
gases and also the exit hole size because it usually will be a small
hole with a slowly increasing flare -- the gases will slow down as they
exit the engine exhaust structure.
Note: what is important is not only the thrust, but the burn time
-- if the engine supplies its thrust for only a short amount of time it
won't make your rocket go as fast as one that supplies the same amount
of thrust over a longer time. Thrust*time = impulse, and this is what
engines are usually rated in. Divide impulse by the mass of the rocket
(with the engine) to get the speed of the rocket, but be sure to
subtract off the acceleration due to gravity times the burn time
(gravity will slow it up!) to get the maximum speed -- there is also
the effect of air resitstance to make your life difficult calculating
the final speed of your rocket.
For a good site on model rockets, see together with some .
Due to the way the propellant is arranged in a typical engine, the
thrust it provides changes with time. The calculation above I suggest
only as an estimate if you would like to verify the measurements
independently.
Here's also a with specifications of various commercially available model rocket engines.
Tom
(published on 10/22/2007)
