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Simming Airstarted Spin Motors

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Charles_McG

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Today I tried calculating the roll damping force of my 1/5 Talos Terrier Malemute stack.

I've been told (by Wildman Tim) that in real flight, the 4 x A10s I can put on the Talos interstage aren't enough to make it spin. (Assuming I can convince myself it would be safe to fly that way - and then convince an RSO. Not a great chance of that after the last test.)

As I understand it, as the rocket starts to roll, the fins see an apparent angle of attack that generates opposing lift. So the spin motors have to overcome that, which my static test doesn't simulate.

I see a roll dynamics section in the component analysis portion of OpenRocket. So I stick in some numbers from the sim for when I would have the spin motors airstarted. So where I think I can get my Proton to fire, alt = 75', v = 65m/s, or 0.19 mach. From the last spin test, I get 2 rev/sec.

Punching those numbers into the roll dynamics calculator, I get a C(l) of -1.02 +/-.2 for values of roll and speed around the target value,

So if lift = C(l) * rho * Area * vel^2/2. And rho = 1.23 kg/m^2. And total fin area = 284 in^2 or 0.18 m^2, I get a roll damping force of ~480 N. Which is way more than the 50N peak of 4xA10s.

That reinforces the advice I got.. Did I make an obvious mistake? Drastically abuse/misunderstand OpenRocket parameters?
 

Ez2cDave

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Charles,

You are a "brave man" . . . I wish you success, sir !

Dave F.
 
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