Just because a rocket looks like it will fly, doesn't mean that it will or if it does, it may only be stable while the motor has fuel. It is very important that you understand rocket stability before attempting to scratch build your own rockets. We highly recommend using computer software such as RockSim or SpaceCAD to help you design your rockets (see links page). Here are some guidlines to help you design sucessful scratch built rockets.

The two key components that affect rocket stability are the center of gravity (CG) and the center of pressure (CP).

Center Of Gravity: This is the point at which the rocket can be balanced WHEN LOADED WITH A MOTOR. A simple way to find the center of gravity of any object is to balance it on one finger of each hand, and then move your hands togeather (being careful not to drop the rocket/object) until your fingers touch. The point at which your fingers meet, assuming the rocket is still balanced on your fingers, is roughly the center of gravity.

Center Of Pressure: The center of pressure is the point at which all aerodynamic forces converge upon the rocket. The center of pressure is relative to the surface areas of the rocket.

As a general rule, a rocket will be stable if the CP is one airframe diameter behind the CG.

Moving the CG: You can move the CG forward by adding weight to the nose, or move it aft by shortening the airframe tube.

Moving the CP: You can move the CP forward and aft by altering the diameter of the airframe or by adding/removing fins and also by changing the design of the fins (adding/removing surface area).

Proving Stability: You can prove a rocket to be stable by loading the rocket with the desired motor then locating the CG. Tie a piece of string around the rocket at the CG and swing the rocket around above your head. It will either fly as it should or it will twist and turn which will show the rocket to be un-stable.

The Apogee Rockets web site has many links on this subject, including more technical information.