Fins and the nose

A rocket must not just go up. It must keep pointing the right way. Two special points decide whether it does.

• The centre of gravity (CG) is the balance point of the rocket’s mass.
• The centre of pressure (CP) is the point where the sideways force of the air acts, if you replace all its tiny pushes by a single push.

Suppose a gust turns the rocket a little, so that air now hits it from slightly the side. The air’s sideways force acts at the CP. That force makes the rocket rotate around its CG. Which way it rotates depends on where the CP is.

• If the CP is behind the CG, the air pushes the tail sideways more than the nose, and the rocket turns back into the airflow. It is stable, like an arrow or a weather vane.
• If the CP is ahead of the CG, the air pushes the nose sideways more than the tail, and the turn grows bigger and bigger. The rocket tumbles.

That is the reason for fins. They are flat surfaces at the back that catch the sideways airflow, so they move the CP backwards. Making the fins bigger moves the CP further back. Fins do not lift the rocket. They only steer it back when it points the wrong way. They cost a little extra drag, so you want fins big enough, not huge.

The other side of the problem is the CG. Putting a bit of extra mass in the nose moves the CG forwards, so the CP ends up further behind it and the rocket is more stable. That is why a rocket with a heavy nose is steadier than one with a light nose. The nose cone itself has a different job. A pointed or rounded shape lets the air slip past with less drag.

A water rocket has an extra effect. The water sits mostly at the back, near the nozzle. As it leaves, the CG moves forwards, so the rocket becomes steadier during the burn. Real rockets change their CG too, because their propellant is used up as they fly. How air flows around shapes and creates lift or drag is the subject of the Flow Lab.