There are two points on a rocket one must look at when dealing with stability:
The center of gravity is simply the balancing point on the rocket. The center of pressure is the point where the drag forces are equalized. This can be visualized by imagining a weathervane:
THUS, IN ORDER TO MAKE A ROCKET FLY STRAIGHT THE CP SHOULD BE FARTHER AFT THAN THE CG.
But how large should the margin between them be?
There are a few guidelines to go by for a stable rocket:
However, it won't help any to have the stability be more than 2 or 3 calibers. In fact, it could hurt stability (but that's getting into dynamic stability, which would have to be another post).
-There are 3 ways to increase the margin between the CG and CP: 1) Add more weight to the nose (not a very good way, unless you're dealing with a scale model) 2) Increase the fin are (Tip: The best way is to increase the span, not the length. The farther from the rocket body, the less turbulent the air, so the fin is more effective). 3) Lengthen the body tube.
So how do you find out what the CG/CP margin is on a rocket? Well, to find the CG all you have to do is balance the rocket on a straight edge. But finding the location of the CP is a little more complicated...
One not very accurate way is to use the center of lateral area as the CP. To do this, make a 2D cardboard cutout of the rocket, and then balance it to find the CG. Then compare it with the CG of the actual rocket. This is an okay way to calculate stability. However, it turns out that the CP is even more complex than that.
You see, the center of lateral area is the CP when the rocket is flying at an angle of attack of 90 degrees (i.e. the aiflow is perpendicular to the rocket). That's the concept of dynamic stability: When the rocket is exposed to real flight conditions, the CP location will travel, depending upon the angle of attack.
There are equations that determine the position of the CP under actual flight conditions, known as the Barrowman equations. To learn how to use these equations, click here. They look complicated at first, but they're pretty straightforward if you know how to use simple arithmetic. Just plug in the numbers and the location of the CP appears.
As a third alternative, download some rocket design software to aid you with stability calculation. Rocket design software usually does much more than this, but they are an excellent and pretty reliable way of determining stability. To learn more about rocket design software, click here.