Barrowman vs Rocksim =advanced=

[Rocketmaniac]

Note to Mods: I don't know if this right forum.


Barrowman vs Rocksim for stability analysis. Can someone explain why they can be so different? Which one is more accurate? Or should you use an average of the two?

 

[illini]

They're different because they are different approximations of the full 6-degree-of-freedom equations of flight mechanics. Barrowman did a fantastic job of making the necessary assumptions to allow the average rocketeer to calculate Cp with just pencil and paper. The full equations are nasty non-linear, coupled partial differential equations that are not solved so easily. The Barrowman equations are out there, described in all their glory, and people have debated them, proposed additions to them, and worked with them for years. The RockSim method is, to the best of my knowledge, not really described anywhere in such a way that it could undergo similar scrutiny. That doesn't make it more or less accurate than the Barrowman equations. We just don't know for sure because we don't have the equations to study the same way we do the Barrowman equations. We *do* know that the RockSim equations give us answers for configurations where the Barrowman equations do not. But, again, that doesn't make them more or less accurate. Average the two results? No...that just wouldn't be appropriate. Use one or the other, but averaging wouldn't make the results any more accurate.

 

[GL-P]

Hear-say

Barrowman tends give a larger margin of error than Rocksim. Just what I heard though. It doesn't really matter.

 

[rstaff3]

On 'normal' designs, I personally trust the Rocksim numbers. On weirder ones, I often go the conservative route. On really weird ones you can't do Barrowman and have to trust Rocksim to some extent.

 

[rocket72175]

Some of the Rocksim method details are given in this Apogee publication (from the website):

"Apogee Components' Technical Publication #17 "Numeric Methods in Model Rocket Design" (P/N 36017) This report covers the methods of calculating the CG and CP of odd rocket configurations; which are used by the RockSim software."

I believe the basis of this report was Paul's NARAM R&D submission in 1998. I was there and listened to the presentation. I seem to remember that odd fins and such were discretized, like an FEA mesh.

Ken

 

[rstaff3]

By really weird I meant stuff like 2-fins, etc, where the Rsim program defaults to the Rsim method

 

[powderburner]

Originally posted by rocket72175
I believe the basis of this report was Paul's NARAM R&D submission in 1998.

Is this report posted anywhere on the net, so we could look at it?

 

[rocket72175]

A Google search only produced links to the Apogee publication. So, if you want it, you have to buy it.