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rtuinila
11-03-2013, 09:59 AM
Hi Folks,

I have a question about the Cp location on just a plain body tube. I would have thought the Cp of a body tube would be at the mid-point of the tube at the same location as the Cg. But when I put a body tube into a new sustainer design in Rocksim the Cp is at the extreme end of the tube. Is that right or is my Rocksim Cp calculator broken? Thanks!

Roy

PS What brought this on was I downloaded a Rocsim file of the Estes Pathfinder and when I opened it the Cp marker was located at the tip of the nose cone

RandyT0001
11-03-2013, 10:18 AM
Hi Folks,

I have a question about the Cp location on just a plain body tube. I would have thought the Cp of a body tube would be at the mid-point of the tube at the same location as the Cg. But when I put a body tube into a new sustainer design in Rocksim the Cp is at the extreme end of the tube. Is that right or is my Rocksim Cp calculator broken? Thanks!

Roy

PS What brought this on was I downloaded a Rocsim file of the Estes Pathfinder and when I opened it the Cp marker was located at the tip of the nose cone

RocSim uses modified Barrowman equations to determine the CP location. People must remember that the Barrowman equations were themselves simplified equations derieved from more complex equations of aerodynamics. The Barrowman equations were based on the rocket having a nose cone, body tube and fins to function. If one of these parts is missing then the CP location cannot be confediently determined with great accuracy.

Royatl
11-03-2013, 02:16 PM
Hi Folks,

I have a question about the Cp location on just a plain body tube. I would have thought the Cp of a body tube would be at the mid-point of the tube at the same location as the Cg. But when I put a body tube into a new sustainer design in Rocksim the Cp is at the extreme end of the tube. Is that right or is my Rocksim Cp calculator broken? Thanks!

Roy

PS What brought this on was I downloaded a Rocsim file of the Estes Pathfinder and when I opened it the Cp marker was located at the tip of the nose cone

It would be at the midpoint if the tube were flying at 90 degrees angle of attack. The Barrowman equations assume the rocket is flying at 0 degrees AoA.

At 0 degrees AoA, and at low airspeeds, I would think an open tube would have essentially no center of pressure. At the very least it wouldn't matter where the cP was because the normal force at that point would be close to zero.

Note that when you put a nose cone on, the cP of the combination is basically the cP of the nose itself.

Or, alternatively, if you just put a fin set on, the cP of the combination is basically the cP of the fins themselves.

Therefore, the body doesn't have much effect at zero AoA.