# CP

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#### SHH

##### Member
Anybody, anywhere have a simple, perhaps new, method to determine the Center of Pressure. My rockets are sorta large and the string thing won't work. . .and neither will the card board cut out idea.

#### BMcD

##### Well-Known Member
Open Rocket if it is not a tube finned or otherwise oddly configured design.

#### samb

Nothing new from me I'm afraid. The cardboard cutout method will work on a smaller scale profile of your model. Also this site has a spreadsheet that you can plug your measurements into. Hope this helps.

http://www.rocketmime.com/rockets/Barrowman.html

#### Sooner Boomer

##### Well-Known Member
What do you do if you have a rocket that is asymetric in cross section? For example, a rocket that is oval (or eliptical) in cross section?

##### I don't do spirals
TRF Supporter
As my dad would have said "Son, sometimes you just have to eyeball it"

#### JoePfeiffer

##### Well-Known Member
TANSTAAFL. Your choices are easy but so inaccurate as to be useless (cardboard cutout), harder but reasonably accurate (Barrowman Equations, either by hand or through a program like RockSim or OpenRocket -- and really, they are not hard to use), or bite the bullet, learn to do CFD, and get very accurate. Easy and accurate does not exist.

#### mjennings

##### Well-Known Member
@Sooner Boomer you can do cardboard cut out of multiple profiles. ie 0 and 90 degrees, more if you have lots of asymmetric parts Taking the worst case should be sufficiently conservative.
Cardboard cut out is just finding the centroid of the shape. For large birds you can do this mathematically.

#### JoePfeiffer

##### Well-Known Member
@Sooner Boomer you can do cardboard cut out of multiple profiles. ie 0 and 90 degrees, more if you have lots of asymmetric parts Taking the worst case should be sufficiently conservative.
Cardboard cut out is just finding the centroid of the shape. For large birds you can do this mathematically.
The biggest problem with cardboard cutout is it vastly overestimates the effect of the body tube and so for most rockets is 'way too conservative (in the sense that it estimates the CP as being much too far forward).

#### dhbarr

##### Amateur Professional
Open Rocket if it is not a tube finned or otherwise oddly configured design.
Vanilla tubefins work fine in OR

#### mjennings

##### Well-Known Member
The biggest problem with cardboard cutout is it vastly overestimates the effect of the body tube and so for most rockets is 'way too conservative (in the sense that it estimates the CP as being much too far forward).
Not saying it's a perfect method by any means. But learning and affording cfd software isn't for everyone. There are things that Barrowman equ's can't handle, and RockSim and OR definitely have their limits. If I was RSO at a launch and someone came in with CFD plots for a highly unique design I'd have hard questions before I let it on the pad about how the work was verified. I've seen way to much GIGO on analysis models to take the word of a pretty plot. The important point I think we're both trying to make is to think about the assumptions the analysis is based on.

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#### icyclops

##### Well-Known Member
Not saying it's a perfect method by any means. But learning and affording cfd software isn't for everyone. There are things that Barrowman equ's can't handle, and RockSim and OR definitely have their limits. If I was RSO at a launch and someone came in with CFD plots for a highly unique design I'd have hard questions before I let it on the pad about how the work was verified. I've seen way to much GIGO on analysis models to take the word of a pretty plot. The important point I think we're both trying to make is to think about the assumptions the analysis is based on.
Ditto......totally agree.
For the odd roc’s I have designed, sims are very limited or useless. I have surprised a few RSOs in my time with how some of them fly....they are sometimes too conservative also until you show them. There is nothing bad about a “heads up” launch.

#### Jeff Lassahn

##### Well-Known Member
TRF Supporter
If the problems you're having are just because your rocket is too large, remember that CP location doesn't generally vary much with size. You can test a smaller mockup of the same geometry and get a useful answer.

#### Ez2cDave

##### Well-Known Member
TRF Supporter
Back in the day, circa 1995, a Tripoli rocketeer, named Bruce Lee, developed a simplified method to manually calculate Center of Pressure.

His 10-year old son was able to do the computations with ease.

Dave F.

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#### Alan15578

##### Well-Known Member
Back in the day, circa 1995, a Tripoli rocketeer, named Bruce Lee, developed a simplified method to manually calculate Center of Pressure.

His 10-year old son was able to do the computations with ease.

Dave F.

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That is nice, but a downloadable PDF would be better. I'm not sure I like the fill in the blanks spreadsheet approach. Back in the early 80's I published a blank CP worksheet in my newsletter. In any event, I remember being frustrated, staring at those equations from the Handbook, while in Junior high. My math teachers were unwilling to help me and were only interested in advancing their own agenda. I just needed a little intro to algebra.

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