Flying model rockets is a relatively
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
The weight and aerodynamics are determined by the design of the
model rocket components.
The thrust is provided by a replaceable
solid rocket engine
which can be purchased at local hobby or toy stores.
Model rocket performance (
how fast) depends a great
deal on the rocket
There are several different ways to
characterize rocket engine performance. Model rocket engines come in a variety
of sizes and weights, with different amounts of propellant, with
different burn patterns which effects the thrust profile, and with
different values of the delay time.
At the top of the page we show typical performance curves for several different
rocket engines. We plot the thrust versus the time
following ignition for each engine with solid lines. You will notice that when comparing
engines, there is a great difference between the levels and shapes of the
plots. For any single engine, the thrust changes with time.
We can specify a time-averaged thrust of the engine by
adding up the product of the thrust over some small time increment times
the amount of the time increment and then dividing by the total time.
The result of this averaging is shown by the dashed lines on the plot.
When purchasing model rocket engines, you will notice a label on each engine
in the format; letter number - number. On the figure, we show a C6-4.
The first number indicates the average thrust in Newtons.
A C6-4 has an average thrust of 6 Newtons. The average thrust times the
burn time of the engine is called the
of the engine.
The letter gives the maximum total impulse of that
class of engine. An "A" engine has a maximum impulse of 2.5 Newton-seconds,
a "1/2A" has 1.25 N-sec, a "B" has 5.0 N-sec, a "C"
has 10.0 N-sec, and a "D" has 20.0 N-sec. If we compare the curves for B6 and
the C6, we find that both engines have the same average thrust (6 Newtons),
but the "C" engine burns almost twice as long for double the total impulse.
The second number indicates the length of the delay time in
A C6-4 has a delay time of 4 seconds between the engine cutoff and the firing
of the ejection charge.
The delay time determines the length of the
coasting phase of the flight.
If the delay time is too short relative to the optimum coast of the vehicle, the parachute
deploys on the way up and stops the flight. If the delay time is too long, the vehicle
might hit the ground before the parachute deploys.
The engine designer can affect the thrust and the total
impulse of an engine by changing the diameter of the propellant (and casing).
Typical "1/2A" engines are 13 mm in diameter, typical "A", "B" and "C" engines
are 18 mm in diameter, and typical "D" engines are 24 mm in diameter. This is
important to remember because a model rocket designed for a "B" engine will
not accept a "1/2A" or a "D". The engines will not fit into the
fixed engine mount
of the rocket.
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