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I setup a simple rocket in openrocket with a body to fit the engine and a nose cone. I did not add fins. This puts the center of pressure up in the nose cone way ahead of the center of gravity. What I am trying to do is setup a situation where you would use gimbaling to stabilize the rocket but in an environment where it may not be necessary so I turned off wind for the simulation. The rocket still goes unstable and I'm curious why. It seems to remain unstable until I make fins large enough to move the center of pressure behind the center of gravity.

It appears I can remove air pressure in which case the simulation does work, so I'm guessing there is some non-uniformity somewhere?

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If the center of pressure is ahead of the center of gravity, even the slightest difference will cause a divergence -- even a rounding difference or numerical representation in your simulation code. It is your second simulation that "works" that is really not working. You have what is known as an unstable equilibrium.

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    $\begingroup$ Good point about the numerical representation and rounding errors. $\endgroup$ – andy256 Apr 1 '15 at 8:15

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