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 engine performance. 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 charge which sets the amount of time for the
coasting phase of the flight.
On this page, we discuss the factors that
affect model rocket engine performance.
At the top of the page we show typical performance curves for several different
rocket engines. We plot the thrust of the engine versus the time
following ignition for each engine. 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.
To the right in the figure, we show a
typical engine schematic which is used to explain why the thrust changes. The
of any rocket engine depends on how fast and
how much hot exhaust gas passes through the nozzle.
at the surface of the propellant and
the surface burns away as the propellant turns into a gas.
You can then imagine a flaming surface that moves
through the propellant.
The flaming surface is called the flame front.
At any time and at any location
the amount of hot gas being produced depends on
the area of the flame front. The greater the area, the greater the thrust.
As the propellant burns, the shape and area of the flame front change
and that causes the thrust to change.
Here is a computer animation of the movement of the flame front
for a typical engine
In the animation, we show the shape and location of the flame front
for a C6-4 engine. Engine designations are explained on
The schematic is two dimensional while the real engine is three dimensional.
So a three dimensional cone surface appears as a two dimensional angle on
The flame front is shown as a red line that moves through the propellant
as the engine burns. The hot exhaust is shown in yellow.
The time is noted on the plot by a moving red line.
On a typical model rocket engine,
a small cone is formed in the propellant on the nozzle
end of the engine.
As the propellant burns, the size of the cone increases until
it hits the engine casing (about time = .2 on this engine).
The increasing cone surface area causes the large increase in thrust between
time = 0 and time = .2 on the plot.
Between time = .2 and .5, the
shape of the cone flattens out and the area and thrust decrease.
By time = .5, the
cone has become a flat flame front which proceeds on down the engine until
the propellant is used up at time = 2. Between .5 and 2, the thrust is
constant because the area of the flame front is constant.
At time = 2 the propellant is completely burned and the thrust goes to zero.
Immediately, the delay charge begins to
burn. Even though the amount of the delay charge is smaller than the propellant,it burns longer because it is made of a different material. For this engine
we show a 4 second delay. At time = 6
the ejection charge is reached and ignited and blows out the front of
NOTE:This animation is not time accurate. The fuel burning is shown every
.1 seconds, while the delay charge is displayed every .5 seconds. In
reality, the fuel burn is very fast and the delay burn relatively long.
Considering the various engine plots, we see a burn pattern similar to the
previously discussed C6-4, but with some variations in the amount of thrust.
We have seen that the shape of the thrust curve is affected by the shape of the
flame front. Designers of solid rockets can produce the given thrust curves
by changing the total amount of propellant placed in the engine, by varying the
the angle of the cone in the propellant, and by varying the diameter of the
propellant and casing.
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