Spacecad Question

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Oct 28, 2004
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I'm trying to design a rocket using the free demo version of Spacecad, and the software is saying that the rocket is overstable (6.43 calibers.) Who or what is a "caliber" and is overstable necessarily a bad thing? I didn't think a rocket could be too stable. Thanks.
The diameter of your rocket is its "caliber." A rocket that is 6.43 calibers has the CP (Center of Pressure) and the CG (Center of Gravity separated by 6.43 times the diameter of your rocket.

Is overstable a bad thing? Oh boy, here we go.... if I were you, I'd do a search on "overstable" and read the many past discussions on the subject.

But, the short answer is... overstable is usually OK.
Overstable is usually OK..Unless you fly in windy conditions. Then you have a greater chance of the rocket weathercocking badly.

At least in my humble understanding. Experts please comment.
The recommendation most seen is 1 to 2 calibers of stability. Less than this is risky, resulting in a rocket that corrects too slowly or maybe not at all in some circumstances. More than 2 calibers is said to be prone to weathercocking (tipping into the wind). "Too stable" is far better than not stable enough or (gasp) unstable, but stable enough is best.

I was looking into my Big Bertha today, and it's about 3 calibers. I love the way that puppy flies. Takes off like it's on rails.

6.43 calibers does sound excessive. Ways to correct it include:
1.) Adding a bit of weight near the tail.
2.) Decreasing the fin size and or moving them a bit forward.
3.) Shortening the body.

If the fins come to the end of the body tube, moving them a bit forward can decrease interference drag, thus improving overall performance.

Be careful though with anything that moves the rocket toward being less stable. In this case, I'd do it, but check and recheck the stability once you do.
Originally posted by rabidsheeep
ive also heard that you could add small foward fins
Yep. I'd triple check the stability if I did that though. And attach the rear fins extra securely. Losing a rear fin is bad enough if it happens, but it's worse if forward fins are used. Forward fins are cool to look at, but proceed with caution.

If rear fins are made smaller, do it by decreasing their chord, i.e. by making them thinner from leading edge to trailing edge. Don't make them shorter from root edge to tip.

If the rocket has nose weight that can be removed, that's a good option.

If the body is shortened, don't do too much of it, lest the dynamic stability suffer.

"The Handbook of Model Rocketry" and "Model Rocket Design and Construction" both contain a lot of good info on rocket stability.