CG and CP

[Trmbne2000]

Hello all,

Would there be a way to calculate both the center of gravity and the center of pressure without using software such as RockSIM? I have a reallly cool rocket designed, and I am ready to start construction, but I have to figure out if it will fly right.

Thanks,
Andrew
[email protected]

 

[KenParker]

Oh jeez.... you had to ask that. I'm sure that Math Professor Carl Tulanko will jump in here at any moment and provide you with all the formulas. Carl is the only guy I know of that not only used about three different computer programs to verify the CP/CG of his L3 rocket, he also did all the calculations by hand.

C'mon, Carl.... start professing.....

 

[Austin]

Andrew...

Yes, I do calculate CP manually on occasion as it helps me keep my math sharp and is fun to do, at leat I think so! Here are two sites for info, the first one is the actual Century Rockets Manual Written by James Barrowman himself and cand be found here:

https://www.dars.org/JimZ/tir-33.htm


The second site is actually one of the manual calcs I did myself using the above said manual:

https://www.rocketryforum.com/amraam/equations/barrowman.htm


As far as CG goes, that is measured by balancing the rocket...just insure it is at least one body tube diameter in front of CP.

Enjoy,

Carl

 

[eugenefl]

I'm surprised no one has mentioned the cardboard cutout method to figure out CP. I personally have never used it, but it makes some good sense.

You can read about it here:

https://www.apogeerockets.com/education/newsletter18.asp

 

[Trmbne2000]

Thanks again guys. I did the calculations for the CP and that CG; along with some help from the 6th edition of the Handbook of model rocketry. Thanks again for all of the help. When I start construction and or launches, I'll try to get pics to show.

Thanks,
Andrew
[email protected]

 

[Brian Barney]

Originally posted by eugenefl
I'm surprised no one has mentioned the cardboard cutout method to figure out CP. I personally have never used it, but it makes some good sense.

You can read about it here:

https://www.apogeerockets.com/education/newsletter18.asp

The cardboard cutout method is mostly useless compared to barrowman in normal flight conditions. The barrowman method gives the CP whne the rocket is flying with no wind at all. The cardboard cutout method will give the CP when the rocket is flying sideways It is important to use when the takeoff speed is expected to be low, or the wind speed is expected to be very high at the launch site. Barrowman simulations get confused with boattails, so use Rocksim for those simulations... For my 8" diameter V2 that I launched on an L1120, barrowman said that it would be unstable, but rocksim said that it had plenty of stability, and I have heard from other V2 people that rocksims calculations are much better at V2's.... so I flew it, and it went perfectly straight even though barrowman said it would be unstable

-Brian Barney