Scratch Golf Ball Rocket
On the other end of the size spectrum, trying to build the smallest rocket is a great way for children to learn rocket science.
Kids start with the assumption that a bare Estes rocket engine should go a long way, but it skips randomly through space bouncing all around you. Next thought: it needs fins and a nosecone. So, we add a nose cone and some fins to a motor, and it is still totally unstable (but now with a sharp point on it!).
You need a proper balance of weight and thrust vectors. A long rocket tube shifts the center of gravity forward, like a lever arm or see-saw. With a short rocket, you need a lot of nose weight to shift the balance point forward. So the shorter the rocket tube, the heavier the nose needs to be. My young son had the clever idea to tape a stray golf ball onto the nose of his 6” rocket made from the remains of others… and it flies perfectly straight every time. It also launches the golf ball at apogee instead of popping a chute…. So he can take on Big Bertha with his little rocket driver…
It’s Rocket Science redux! CP < CG
To be stable, the rocket’s CP (Center of Pressure) should be one or two body diameters behind the CG (Center of Gravity).
The fins are there to streamline the flow of air and provide a large surface area to help to keep the center of pressure below the center of mass of the rocket.
For more on stability, check out this page.