Who cares about stability?

The mentality of ‘Let’s shoot some rockets!’ just doesn’t work. Your rocket must be stable or it will rotate incorrectly in the air and it won’t go where you want it. Ideally you want your rocket to go straight up and return to the ground about where you shot it off from. However, we don’t live in a perfect world. Wind, air resistance, and angle of launch are just a few factors that are going to blow your rocket off course. The last thing you want is a rocket that halfway in the air turns and shoots back down at the ground. For that reason, your rocket must be stable.

What makes a rocket stable?

Actually there are two factors that determine stability: center of gravity and center of pressure. The center of gravity of an object is where that object will rotate. For example, the center of gravity of a hammer is just below the head. This can be determined two ways: throw the hammer and figure out where the entire hammer rotates around (ie – the handle and head of the hammer both rotate around this point); the second way is to balance the hammer on your finger. The point at which the hammer balances is the center of gravity.

What is the center of pressure?

Ah yes. Basically center of pressure is the same as center of gravity but in a two-dimensional world. Center of pressure is completely based on surface area instead of the distribution of mass along an object. How do you determine center of pressure? Make a cardboard cutout of your rocket, and balance the cardboard on your finger. The point at which the cardboard balances is the center of pressure.

Who cares?

Well if you want to hav a stable rocket you better care! The center of gravity must be at least the diameter of your cylinder ahead of the center of pressure. Why? If it isn’t, your rocket will start rotating during launch and it won’t go straight up — it’ll go sideways, or maybe even back down at you. The moral of the story? Put your center of gravity closer to the nose than your center of pressure. One of the best ways to get around this is to have a heavy nose cone (as you’ll see I do) — you’ll sacrifice distance upward but your rocket will be stable.