Science
Objective:
Determine the Stability of a rocket
Terminology Covered:
 Center of Gravity
 Center of Pressure
 Disturbance Force
 Restoring force
Background:
A model rocket uses physics to guide its trajectory. Since it does
not have a system to steer its fins to keep it pointed straight, it must then
rely upon its fins to provide a Restoring Force that will overcome any
forces that try to make it go off course (Disturbance Force).
Procedure:
Take a model rocket with an engine (your bottle) in it. Tie a string
around the rocket body and slide it back and forth until you find the point at which
it balances. This is the Center of Gravity.
Hang the rocket in front of a strong fan and turn the fan on to High
Speed. Turn the rocket to one side so that the nose is pointing about 30 degrees
off course and then let go of the rocket.
Results:
If the rocket is stable, the rocket will point the nose back into the
wind even though it was originally pointed off course. If it is unstable, the rocket
will swing back and forth crazily or even rotate so that it is backwards in the
airflow.
Why:
The rocket fins provide a restoring force because the rocket Center
of Pressure is behind the Center of Gravity. This creates what is called a
moment arm that allows forces on the rocket fins to correct the rocket course. As
shown in the drawing below, the rocket will always rotate around its Center of
Gravity.
FORCES ACTING ON A STABLE
ROCKET
Disturbance
Forces
Center of Gravity
Restoring Forces
FORCES ACTING ON AN UNSTABLE
ROCKET
Disturbance
Forces
Aerodynamic
Forces
Center of Gravity
A Rockets “Center of Pressure” is Important:
The CP is the point on the rocket where the surface areas on each
side of the rocket are equal. It is also the point at which the aerodynamic forces
act on the rocket. Therefore in a stable rocket, the position where the
Aerodynamic Forces are applied = Position of its Center of Pressure.
You can find the CP of the rocket in two ways:
1. place the rocket in front of a fan when it is hanging from its CG point.
The point where the rocket STAYS sideways in the airflow is the CP.
2. The other method is to cut out an outline of the rocket from cardboard and
find the CG of the cutout. The point where the cardboard cutout of the
rocket balances is the point that has equal areas of cardboard on each
side of it. This is the CP of the rocket!
A Stable Rocket Design:
A stable rocket design of course swings back into the wind even
when the nose of the rocket had started to go off course.
The ability of the rocket to continuously correct its flight is the
Restoring Force provided by its fins.
MAKE SURE that the Center of Pressure is BEHIND the Center of
Gravity! The further the CP is behind the CG the better.