Aerodynamic Stability

A stable rocket is like a weather vane, it will always point into the relative wind. If it doesn’t, then it’s not aerodynamically stable and it will quickly fly off course. So, how do you make a rocket stable? There are two factors to consider. First, where is the rocket’s center of gravity? This determines the point about which the rocket will pivot. Second, where is the rocket’s center of pressure? In other words, if air pushes against the side of the rocket, at which point will the force of the air pushing on the front part of the rocket equal the force of the air pushing on the back part of the rocket. The center of gravity for a stable rocket will be forward of the center of pressure. The further aft the center of pressure is from the center of gravity, the more stable the rocket will be.

Now for water rockets, there’s an interesting problem related to rocket stability and recovery. If the center of pressure is very much behind the center of gravity, the rocket will be aerodynamically stable and continue to fly in good form right through apogee and then point downward and continue to fly very efficiently right into the ground with a very high speed! If the center of pressure is moved nearer the center of gravity, you reduce the stability of the rocket, but at apogee, the rocket will tend to lie sideways and drift slowly back to Earth. Again, this can be a delicate balance between flying efficiently and having safe, slow, recoveries. The center of pressure can be moved forward by decreasing the size of the rocket’s Fins, or the center of gravity can be moved aft by adding weight to the rear of the rocket. This can be accomplished by fastening weights (washers) to the rear of the rocket, effectively moving the center of gravity rearward. However, the added weight will affect the performance of the rocket and will reduce the maximum attainable altitude.