center of pressure - defined and explanied?

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dr wogz

Fly caster
TRF Supporter
Feb 5, 2009
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Land of Poutine!
Hi again, another question. I have been doing some reading (see other post) and come across the term 'center of pressure' somewhat often. I understand it to be the center point of thrust, and should be "1 caliber" behind teh CG.

But how is that determined? COG is easy.. balance it.
COP? How / where do you get it? What influences it?
The center of pressure is the effective central point where the sum of the external aerodynamic corrective forces act.

You can make a crude (conservative) estimate using the cardboard cut-out method. Draw a scale image of the profile of your rocket, including fins, on a sheet of paper or cardboard. Cut out the profile and balance it to find the c.p.

Check out this old thread:
(go to the last half of the posts)

And this one too:

Note that there is some question as to whether you get a true measure of stability with the cardboard cutout. I think it works OK, and probably gives you a little extra margin.

You should also go to the Apogee Rocketry website and download the free version of their rocketry design/analysis/simulation software (RocSim). You will not be able to create files to keep (and will have to re-load your data each time you operate the demo) but you will be able to perform an alternate stability computation.

As a third approach, find a copy of the 'Handbook of Model Rocketry' (by G. Harry Stine) and there will be an appendix in the back with the Barrowman equations for computing stability. Or look on the internet, I think the equations are posted in a couple of websites.

you don't need to go too into depth, just make sure your CG is in front of the published CD for your kit. if you get carried away with adhesives or paint, the position of the CG might change compared to the intended design.

when you check CG, make sure to have the chute, wadding, and the largest motor you intend to use in it all loaded and ready to fly.

and ^^^^^^^what he said^^^^^^