Mes Amis, I would like to interject a little food for thought today. What would cause a rocket to rise into the air and then turn ninety degrees and fly sideways? This is not a question totally unrelated to the subject of fins. But it is one of concern, because I myself have seen it happen with a rocket that I constructed.
My rocket was considered to be a high-performance one, and yes, I built it from a kit. As much as I would like to say, I did not design this rocket myself. The fins for this rocket went straight to the sides, and were almost parabolic in shape. I added a small amount of lead to the nosecone of this rocket to increase the stability, and I thought it was well within the parameters of stability, but, sacrebleu, it was not. Approximately ten metres from the ground it abruptly turned parallel to the ground and flew out of sight, never to be found again.
I must say I was quite upset, because I had spent many hours constructing this rocket and painting it. But why did it not climb straight into the air?
As a rocket’s solid-fuel engines burn, the center of gravity of the rocket shifts to the front. My rocket seemed stable, even tested stable, but it was, as you say, on the ragged edge of stable. There was a considerable wind that day, and the wind acted to push the fins to one side. As the rocket fuel was burned, the rocket became more and more stable, and by the time the wind had turned my rocket horizontal, it became stable enough to continue on a straight course.
Thus, my rocket flew directly into the wind with much of its fuel remaining. It flew entirely too far to be found and retrieved. I had spent many an hour building this model, and I was quite saddened by the experience. But it was not the first rocket I had lost. NASA has lost many rockets, and do you think it has stopped them? No, I assure you, it has not.
The moral of this story is that there must be a stability margin of error. We saw before that the center of gravity must be ahead of the center of pressure, but how far? A practical answer is that it must be considerably forward of the center of pressure. But take care that you do not add so much weight to your rockets in the interest of stability that you exceed the lifting capabilities of their engines. I added 3.5 ounces of lead to the nose of a large rocket I built once, but its performance characteristics did not measure up to my expectations.
Au Revoir, mes amis.
