# CP calc discrepancy RockSim vs. Barrowman

• posted

Running RockSim 7.0.1

Utilizing RockSim in my current project; a "toy-bashed" Mini Crayon rocket I've noticed a significant difference in the CP calc performed by the proprietary RockSim method and the classical Barrowman. In my design, with an Estes E9-6 loaded, Barrowman yields a CP with a static margin of -.04 while RockSim yields a static margin of 1.64. On a short stubby rocket such as this (10.5 " x 2.5") we're talking about the difference between instability and a comfortable margin of safety.

Who should I believe?

I've glanced at a few RockSim files and this discrepancy does seem to be most pronounced on such short stubby rockets. Am I right here?

... Bill

p.s. remove SPAMFILTER on e-mail reply

• posted

Both. Neither.

Short stubby rockets don't recover well from disturbances. They can be OK one flight, and not the next. TO be safe, add the nose weight and make them more stable.

One of the reasons I recommend against short stuby rockets particularly for HPR cert flights.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

• posted

Search Apogee's e-zine archive. I remember some technical articles on the stability of stubby rockets.

Ken

• posted

Bill K wrote in news: snipped-for-privacy@4ax.com:

Keep in mind that both numbers are approximations, not exact calculations. Both methods have their own sets of simplifying assumptions, and the farther your design gets from the assumptions, the more error there will be in the result.

• posted

Yes, except that both are nearly exact calculations of semi-emperical methods using input data that is aproximated to the model(s). The farther your design gets from the comutational model, the more static margin you should use.

Alan

• posted

Actually, if you check on the Barrowman equations, they are only good approximations for rockets with 11 to 1 length to width ratios. You're running about a 4 to 1. If you want to check the Estes Fat Boy, you'll find that the stability margin is around 0.1 yet this rocket flies quite well. The Fat Boy will fly quite well on the end of a string in a swing test. My last stubby rocket got weight added to the nose until it swung well. The launch was near perfect (it was my recovery that was the problem).

Rob

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