Stability

When designing a rocket, you must be sure that the rocket will be able to fly straight. This ability is called stability. There is one basic rule: keep the center of pressure ahead of the center of gravity. You must keep your fins large enough to do this.

With every model you design (and even those you build from kits), you must test their stability before launching them. This can be accomplished safely with a swing test.

Start by finding the center of gravity. It is usually right on or near the launch lug.

Second, tie a string around the body tube at the center of gravity so that the rocket balances perfectly.

Third, swing the model around your head by the string. Be sure you are in an area free of other people and objects!

If the rocket is stable, the rocket will 'fly' with the nose pointed directly into the line of flight (or 90º to the string).


Test the model again, but this time with the nose pointed ten degrees down. The model should still fly perpendicular to the string.

Do NOT fly rockets that have not passed the swing test!

---

Two different measures can be taken to stabilize a rocket that has not passed the test: move the center of gravity forward, or move the center of pressure backwards.

The easiest way to move the center of gravity fowards is to add weight to the nose. This can be accomplished most easily by placing a small ball of clay inside the nose cone.

Another way is to increase the fin size. This moves the center of pressure backwards, helping to stabilize the rocket.

More on stability can be found in the aerodynamics section.

This page, its contents, and the Hitchhikers Guide logo are copyright 1997 by:
Nicholas Burlett, Nathaniel Grady and William