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Stability Conundrum

  • Claude Maina
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3 months 4 weeks ago #8611 by Claude Maina
For those that saw my launch of the 24mm cluster/rear ejection, on Saturday, you saw a couple of nice loop-de-loops! For the those that didn't, the rocket went about 100' to 200', then did a couple of loops before ejecting its parachute. The rocket did land safely with no damage. However it definitely looked 'unstable'. According to RockSim, it should have been stable. The calculated margin of stability is 1.64 - with both D12 motors inserted. I've attached a RockSim illustration. Any ideas?

I do need to modify the design as the fins were burned during the motor burn. It was a bad design to have the fins hang below the motor tubes. So replacing the fins with something with more surface area is not a problem, if that indeed is what is needed.

Thanks.
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3 months 4 weeks ago #8612 by Guy Wadsworth
Is it possible the motor pods are disturbing the flow of air over the fins?

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3 months 4 weeks ago - 3 months 4 weeks ago #8613 by Boris Katan
I believe Guy's statement is very likely correct.
Sim programs have quite a challenge modelling airflow over complex forms.
For some of my more unusually shaped projects, in RockSim I simplified the rocket shape to a round form with accurate total cross section at each point of the rocket. Then added fins to that to attempt to gauge stability.

I would suggest conventional trapezoidal fins that extend well beyond the pods. Only the area of the fins that extends beyond the pods is likely in clear air and contributing effectively to stability. If the rocket ends up moderately over stable, this just helps bring the recovery closer to the pad.
Last Edit: 3 months 4 weeks ago by Boris Katan.

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  • Claude Maina
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3 months 4 weeks ago #8615 by Claude Maina
Thanks Guy & Boris. That level of aerodynamics is well above my understanding - which isn't much. Isn't any, in fact.

The rocket is a somewhat upscale of a US Rockets model - Dual 18mm Rear Ejection . The fins on that rocket extend directly out from the three tubes and is probably more stable than my arrangement. My plan right now is to replace the Big Bertha fins with simple trapezoid fins attached to the four seams of the three tubes. Hopefully that will do it. If I get my act together, it should be ready by the May 6th MMMSC launch.

Thanks again.

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