Calculating center of pressure of a complex rocket

[air.command]

Hi All, we are trying to work out the CP for this booster, but because of its complex shape, I don't think it can be accurately modeled in OpenRocket. Unless someone can recommend a way to do it in OpenRocket, or at least a reasonable enough approximation? Is this something that Rocksim could do? We have a 1/3 scale model of the booster. Do we build wind tunnel, do we strap the model to a car on a frame and then adjust the pivot point until we find the CP or is there a better way? Any ideas welcome. The ultimate goal is to figure out the size of the fins so that we can correctly position the CP relative to the CG. The overall length of the booster is ~2.3m (7.5 feet).




Then we'll then need to do it again with the sustainer fitted.

 

[kjhambrick]

I don't know RockSim but I do know that a single cylindrical airframe does not affect the Barrowman Equations.

I wonder if you could you sim the three booster tubes as a single tube with the same diameter as the three tube configuration and then treat the three nose cones as a single transition to get an approximate CP for the full stack ?

I wonder how the CP would shift as you vary the diameter of the 'faux booster tube' from the full max diameter down to some minimum ?

How big are the fins on the sustainer ?

Maybe add them as 'canards' to the approximate model when calculating the CP for the booster + sustainer ...

Interesting design -- the booster is acting as a launch tower for the sustainer when it lites

-- kjh

EDIT: my brother still does his BEq calcs in Excel. He got the maths either from Barrowman's Thesis or from James and Judith's NARCON Paper

EDIT[2]: oops, I can't link these from my machine ! Papers are attached as PDFs

HTH and Good Luck !

Please post your videos

 

[ReynoldsSlumber]

A water rocket where the first stage is the launch tower for the second stage, wild!

 

[cls]

Cardboard cut out method.

It looks fine!

 

[neil_w]

You could certainly enter such a design into OpenRocket, and I would tend to trust the results. Probably same for Rocksim. Your choice.

 

[air.command]

You could certainly enter such a design into OpenRocket, and I would tend to trust the results. Probably same for Rocksim. Your choice.

Ahh perfect! I was using an older version of Open Rocket and was not able to get the right configuration. I downloaded the latest and we have boosters and pods! I will have a go at modelling the booster now. Thanks!

 

[air.command]

I don't know RockSim but I do know that a single cylindrical airframe does not affect the Barrowman Equations.

I wonder if you could you sim the three booster tubes as a single tube with the same diameter as the three tube configuration and then treat the three nose cones as a single transition to get an approximate CP for the full stack ?

I wonder how the CP would shift as you vary the diameter of the 'faux booster tube' from the full max diameter down to some minimum ?

How big are the fins on the sustainer ?

Maybe add them as 'canards' to the approximate model when calculating the CP for the booster + sustainer ...

Interesting design -- the booster is acting as a launch tower for the sustainer when it lites

-- kjh

EDIT: my brother still does his BEq calcs in Excel. He got the maths either from Barrowman's Thesis or from James and Judith's NARCON Paper

EDIT[2]: oops, I can't link these from my machine ! Papers are attached as PDFs

HTH and Good Luck !

Please post your videos

Thanks for the suggestions and reference papers. The fins on the sustainer are quite small and don't extend outside the booster segments.
Yes the booster acts as a launch tower. This was the only way we could think of being able to support the really long sustainer at the peak 50G acceleration of the booster. As neil_w pointed out, the latest version of OpenRocket does support multiple body tubes, so I will have a go with that to see what it predicts for Cp.