Ye Olde Rocket Forum - View Single Post - Saturn V with clear fins

Quote:

Originally Posted by mojo1986

If the clear fins are left off and you just go with the small scale fins, any idea where the centre of pressure would be?

I have a K-39 that I built with 5 BT-5 engine tubes. If I knew the centre of pressure I could install 5 mini A engines and then add sufficient nose weight to bring the CG ahead of the CP.

For the Estes Semi Scale Saturn V K-39 kit RockSim calculates a CP of 11.7602 inches from the tip of the nosecone on the escape tower with just the scale fins in place, an unstable situation by -0.50 calibers with a C6-5 motor installed. I had to put 1.5 ounces of weight into the BT-52 section to get a margin of 1.13, see attached RockSim file. I tried to create a simulation with the 5 x BT-5 motor tubes installed and using 2 centering rings; RockSim calculates a margin of stability of 1.11 calibers with 5 x A10-3T motors installed and 1.75 ounces of weight installed at the top of the BT-52 tube at the bottom of the balsa section. Since there is some uncertainty in this simulation, I urge you to use caution when making this flight for the first time, 1.75 ounces still might not be enough added nose weight for a stable flight especially on a windy day.