During the
flight
of a model rocket
small gusts of wind,
or thrust instabilities can cause the rocket to "wobble", or change
its attitude in flight. Like any object in flight, a model rocket
rotates about its center of gravity cg,
shown as a yellow dot on the figure. The rotation causes the axis of
the rocket to be inclined at some angle a to the flight path. Whenever
the rocket is inclined to the flight path,
a lift force is generated by the rocket body
and fins, while the aerodynamic drag remains
fairly constant for small inclinations.
Lift and drag both act through the center of
pressure cp of the rocket, which is shown as the black and yellow dot in
the figure.
If the center of gravity is located above the center of pressure,
the rocket will
return
to its initial flight conditions if it
is disturbed.
Engineers call this a restoring force because the
forces "restore" the vehicle to its initial condition
and the rocket is said to be stable.
Such a flight condition is shown on the left of the figure.
If the center of pressure
is above the center of gravity, the lift and drag forces maintain
their directions but the direction of the torque generated by the
forces is reversed. This is called a de-stabilizing force. Any
small displacement of the nose generates forces that cause the
displacement to increase. Such a flight condition is shown on the right
of the figure where the rocket is unstable .
For a stable model rocket, the center of
pressure must be located below the center of gravity.
To increase the stability of your rocket, add weight to the nose,
or increase the area of the fins.
There is a relatively simple test that you can use on a model rocket to
determine the stability. Tie a string around the body tube at the
location of the center of gravity. Be sure to have the parachute and
the engine installed. Then swing the rocket in a circle around you
while holding the other end of the string. After a few revolutions,
if the nose points in the direction of the rotation, the rocket is
stable and the center of pressure is below the center of gravity. If
the rocket wobbles, or the tail points in the direction of rotation,
the rocket is unstable. You can increase the stability by lowering
the center of pressure, increasing the fin area, for example, or by raising the
center of gravity, adding weight to the nose.
NOTE: Modern
full scale rockets
do not usually rely on
aerodynamics for stability. Full scale rockets
pivot
their exhaust nozzles
to provide stability and control. That's why you don't see fins on a
Delta, Titan, or Atlas booster.
Guided Tours
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Guidance System:
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RocketModeler III:
Activities:
Paper Rocket: Grade 6-10
Pencil Rocket: Grade 6-10
Related Sites:
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