calculating CP for add on pods

[Adam Selene]

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?

 

[teflonrocketry1]

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:

https://www.apogeerockets.com/education/downloads/Newsletter119.pdf

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 Selene]

Thanks, but i want to do it with paper and pencil.

 

[cls]

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.


Cliff

 

[Adam Selene]

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.

 

[teflonrocketry1]

You could do this with VCP using Mark Fisher's technique, or you could use his technique (RMS frontal area) with the Barrowman equations from Randy Culp's site here are the Links:

Mark Fisher's site (Warning a VERY pop up intensive site!) look under rocketry software then tips and techniques:

https://fly.to/mrhq

Randy Culps Equations:

https://my.execpc.com/~culp/rockets/Barrowman.html

and:

https://my.execpc.com/~culp/rockets/rckt_eqn.html

Bruce S. Levison, NAR #69055