calculating CP for add on pods

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Adam Selene

Roving Rocketeer
Jan 18, 2009
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i have been unable to find any info on this. is it as straight forward as it seems?

i want to add external booster pods to a model. just cosmetic, not functioning. at least not at first. ;)

do i just figure the cp for the nose cones and add it in as if it was another transition?
Do you have RockSim version 7? If so, I wrote an article on this subject that appeared in the Apogee Peak of Flight Newsletter from January 27, 2004 visit:

If you don't have RockSim Version 7, or just need help with a side pod simulation, send me the details and I will create a RockSim file for your model that gives you the static CP.

Bruce S. Levison, NAR #69055
Adam, I think the short answer is, not so simple: figure the drag and CG of the pod & however it's attached (on the fin? an arm of some kind?). figure the torquing from the pod's drag & mass by multiplying them by the length of the lever arm - the fin or whatever.

in other words: a small pod at the end of the fin, or a big pod on the main airframe, will have the same effect.

long answer: remember the drag is the frontal area plus the "wetted area" over the length of the pod. combine this with the CP & CG of the fins and main airframe. check the moments at all roll angles of attack - it might be stable "right side up" but not sideways. rocksim 7 does all this for you. I love using the polar stability graph.

what the heck, just build it and do a swing test!! and post pix.

A swing test would work but i was thinking about doing this for my NARTREK GOLD, so i have to do the math. I just thought it would be more fun and more of a challenge to do a rocket more interesting than a nose cone, body tube and fins. and since i use a mac i can't run rocksim (and no, i'm not going to buy virtual pc just to run rocksim) and i would rather learn more about the math, with an eye to maybe writing something for the mac in the future.

i was thinking of just gluing a small tube to the body tube, no struts or anything. kind of like the parachute tubes on the Estes Gemini DC.