Model Rocket Design Failure
Date: Summer 2012
I am working on a science fair project. My project deals with fin designs on the rocket. I have already done my test by buying 3 rockets an changing the fin design on two of them...the other is the control. The fist rocket I made has a fin with a ring around the rocket. The other one I made the fins longer up the tube. The control was the best. I was wondering why the ring around the rocket did not work as well.
How did you determine which rocket design was better? I assume it's how straight the rocket flew, or in other words, its aerodynamic stability. The most stable rocket designs are the ones which have the center of gravity furthest forward (toward the nose) and the center of pressure most rearward. The reason this works is because the rocket will tend to rotate around its center of gravity. The center of pressure is the point where the net aerodynamic pressure is acting. Aerodynamic forces are pushing on the surface area of the rocket nose, body, and fins and creates pressure on these surfaces. If the center of pressure is behind the center of gravity, the aerodynamic forces will tend to keep the center of pressure right behind the center of gravity, or in other words, it will keep the nose going straight instead of tumbling around. But if the center of pressure is further forward than the center of gravity, any tilt of the rocket will be further magnified and it will be very unstable.
Determining the center of gravity is easy -- tie a thread around the body of the rocket and slide it to the position where the rocket balances horizontally. Determining the center of pressure can be much more complicated, and if you want to be exact, involves some intensive mathematics. Fortunately, one approximate way to do this is to make a 2-D drawing of the rocket design on stiff cardboard, cut it out, and balance that on a string. The point where it balances will be the approximate center of pressure.
So I think the reason why your rocket with the ring didn't perform as well is because it probably didn't move the center of pressure far enough behind the center of gravity to make it stable. But try the methods described above and see if they hold true. The NASA web page below will explain more about how this works. Good luck!
John C Strong
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Update: June 2012