Detailed Analysis of Rocket Stability
With A Focus On How It Relates
Water Rockets
There are three points along a rocket that are important in calculation of flight stability:

Center of Gravity (CG):  the point at which the rocket balances.
Center of Lateral Area (CLA):  the point along the rocket where, if you were to attach a pivot and then hold the rocket in the wind by that pivot, the wind forces on either side of the CLA are equal, so the rocket wouldn't point either into or away from the wind: it would be perpendicularly "wind balanced."  

Center of Pressure (CP): a point that is often referred to as the aerodynamic center. This is the point where the aerodynamic forces acting on the rocket in front of this point is equal to the forces acting behind this point during normal flight. The only way to truly calculate this point is through the use of a wind tunnel. Generally CP is and CLA are very close on rockets that are shorter and wider. The CP is usually located far behind the CLA on long skinny rockets. To stabilize a rocket the CG is typically placed 1 or more rocket diameters ahead of the CP.

Variables you can adjust to increase stability during ascent.

A. Add mass to the front (moves CG forward)
B. Decrease mass in the back (moves CG forward)
C. Increase fin size (moves CP & CLA backward)
D. Increase number of fins (moves CP backward with little effect on CLA)
E. Move fins further back (moves CP and CLA backward)
F. Lengthen rocket (moves CP back in relationship to CLA)
G. Add helical twist to fins (moves CP back and induces inertial stability)
H. Check Body Alignment (Cone, Extension, Pressure Vessel)
I. Check Fin alignment
J. Make sure mass is center through the cross section of the rocket

Discuss basics of passive deployment                

Passive deployment depends on a parachute escaping from its containment area at or near apogee and opening rather quickly with out the aid of timers or kick out devices. To achieve this, a rocket must have initial stability but must have some design characteristic to make it deploy its parachute. Some different designs that have worked in the past include:.

A. Rocket falls slowly through apogee and tall cone separates from the rocket deploying parachute.
B. Rocket falls backward at apogee and short cone separates from rocket deploying parachute.
C. Rocket falls backward at apogee and drone chute on front of rocket pulls out a main chute.
D. Rocket falls backward at apogee and goes into a horizontal glide and uses no chute.
E. Spinning rocket destabilizes at apogee as spin slows down. Cone & chute are freed.

Making a rocket move slowly or backward through apogee.

Rocket can be designed so that they will have initial flight stability but will fall backward or move slowly through apogee. The angle that they fall backward can be controlled through design. As early as 1938 Robert Goddard observed a peculiar phenomenon. During a test flight of his model L-16, a tall, slender rocket, problems were encountered and the rocket fell from apogee horizontally. The phenomenon was used in rocket designs over the years but went virtually unexplained until 2000 when Robert & Peter Always published a research and development project for the National Association of Rocketry. (  They determined that a backward slide from apogee no longer need be an accidental quirk, but can be a deliberate design feature. By designing a rocket so that the CG is located between the CLA and the CP, a backward movement can be expected after achieving apogee. Through experimentation, I have found that movement through apogee can be slowed down, by applying the same principles.

These changes can be attempted but may destabilize your rocket.

A. Decrease mass in the front (moves CG backward)
B. Increase mass in the back (moves CG backward)
C. Move Fins further forward (moves CLA & CP forward)

These changes will not decrease stability but will increase the likelihood of slowed apogee or backslide since the CG is more likely to fall between the CLA and CP.

D. Decrease fin size & increase the number of fins (moves CLA forward & moves CP back)
G. Lengthen Rocket  (moves CLA forward & moves CP back)