Aerodynamic Balance (Rocket Motor)

To be aerodynamically stable and remain on course, a rocket must be properly balanced, and the correct balance is determined by the relationship between the rocket’s center of gravity and its center of wind pressure. The center of gravity, abbreviated “CG”. is the point on the rocket’s body where the gravitational forces to the front equal the gravitational forces to the rear. To find the center of gravity, load the rocket with everything it will contain at liftoff, including the motor, the payload. and the parachute. Then hang it from a loop of string. Slide the string back and forth until the rocket hangs level. Cut a small arrow from a piece of masking tape. label it “CG”. and stick it to the body at the balance point (Figure 19-30). I

The center of wind pressure, or “CP”, is the point on the rocket’s body where the aerodynamic forces to the front equal the aerodynamic forces to the rear. In a simple and practical sense for most rockets, it is also the point where the front and rear forces exerted by a right angle side wind are equal. Since a right angle side wind encounters the rocket as a “straight-on” silhouette, you can simulate the relationship between the rocket and the side wind, and locate the CP. by cutting an exact silhouette of the rocket from stiff cardboard, and hanging it from a music wire hook with a paper clip.
To create the silhouette, tape the cardboard to a wall. and. with the motor removed, hold the rocket on a stick, parallel to. and as close to the cardboard as possible. Illuminate the rocket with a single, bare light bulb placed directly across the room, and as far away as possible. Then rotate the rocket so that the shadow cast by its fins is as wide as possible (Figure 19-31).
With a sharp pencil, trace the outline of the rocket’s shadow onto the cardboard, and cut it out as accurately as you can. With a paper clip, hang this cut-silhouette from a music wire hook, and slide the clip back and forth until the silhouette hangs level. Then mark the location of the clip, and label it with the letters “CP” (Figure 19-32).

Now hold the silhouette next to the rocket, and compare the positions of the two points. For proper balance, the CP should be located about one body diameter (called a “caliber”) behind the CG (Figure 19-33). If it isn’t. adjust the position of the CG by increasing or decreasing the weight of the rocket’s nose. Important note. If the CP is located more than 1 caliber behind the CG. (2 or more calibers), the rocket will be “overly stable”. In this condition, it will fly straight in calm air. but in windy weather it will tend to weathervane into any side winds it encounters on tire way up. In practice, because the rocket’s CG moves forward as its propellant is consumed, a starting position for the CG of 1 caliber in front of the CP (that’s 1 body diameter toward the nosecone) will usually result in a good, stable flight.
As a final preflight test, again load the rocket with everything it will contain at lift-off. Make a noose at the end of a 6 foot to 8 foot piece of string. Slip the string over the rocket’s body, and tape the noose firmly in place at the CG (Figure 19-34 ). Then swing the rocket on the end of the string around your head. If it Hies fonvard into the wind like a weathervane. it is properly balanced for flight. If it flies at an angle, flies sideways, or flies backward, add weight to the nose, and readjust the position of the string until it hangs level again. Then repeat this “swing-test”, and keep adjusting the weight of the nose until it flies forward, and the problem is corrected.