 |
             
  
|
|
Rocket Stability Determination
A rocket that flies straight through the air is said to be a stable rocket.
A rocket that veers off course or tumbles wildly is said to be an unstable
rocket. The difference between the flight of a stable and unstable rocket
depends upon its design. All rockets have two distinct "centers." The first
is the center of mass. This is a point about which the rocket balances.
If you could place a ruler edge under this point, the rocket would balance
horizontally like a seesaw. What this means is that half of the mass of
the rocket is on one side of the ruler edge and half is on the other. Center
of mass is important to a rocket's design because if a rocket is unstable,
the rocket will tumble about this center.
The other center in a rocket is the center of pressure. This
is a point where half of the surface area of a rocket is on one side and
half is on the other. The center of pressure differs from center of mass
in that its location is not affected by the placement of payloads in the
rocket. This is just a point based on the surface of the rocket, not what
is inside. During flight, the pressure of air rushing past the rocket
will balance half on one side of this point and half on the other. You
can determine the center of pressure by cutting out an exact silhouette
of the rocket from cardboard and balancing it on a ruler edge.
The positioning of the center of mass and the center of pressure on
a rocket is critical to its stability. The center of mass should be towards
the rocket's nose and the center of pressure should be towards the rocket's
tail for the rocket to fly straight. That is because the lower end of
the rocket (starting with the center of mass and going downward) has more
surface area than the upper end (starting with the center of mass and
going upward). When the rocket flies, more air pressure exists on the
lower end of the rocket than on the upper end. Air pressure will keep
the lower end down and the upper end up. If the center of mass and the
center of pressure are in the same place, neither end of the rocket will
point upward. The rocket will be unstable and tumble.
Stability Determination Instructions
- Tie a string loop around the middle of your rocket. Tie a second string
to the first so that you can pick it up. Slide the string loop to a
position where the rocket balances. You may have to temporarily tape
the nose cone in place to keep it from falling off.
- Draw a straight line across the scale diagram of the rocket you made
earlier to show where the ruler's position is. Mark the middle of the
line with a dot. This is the rocket's center of mass.
- Lay your rocket on a piece of cardboard. Carefully trace the rocket
on the cardboard and cut it out.
- Lay the cardboard silhouette you just cut out on the ruler and balance
it.
- Draw a straight line across the diagram of your rocket where the ruler
is. Mark the middle of this line with a dot. This is the center of pressure
of the rocket.
|
|
