*Originally posted by moocrew *

**How do you determine weather or not the rocket you are launching will be fast enough for the fins to take effect at the tip of the rod?**

You know the swing method of stability testing, by swinging it around you on a string?

Start out slow and go faster very gradually until it goes stable. Swing at that rate several times and find out how many circles it makes in a given number of seconds. Divide by the number of seconds you test, and that's how much of a circle it's going per second (almost certainly a fraction of a circle per second if you're using a long enough string).

Now multipy the length of the string by pi to get the circumference of the circle.

Then multiply that fraction-of-circle-per-second number by the circumference, and you feet per second. That's how fast it was going when it went stable. That's how fast it needs to be going when it leaves the rod, no matter what the length.

Now you can use SpaceCad or another method to calculate the speed with a given engine at whatever time steps you're calculating with (SpaceCad does tenths of s second, I think), and see at what time/altitude it's going the necessary speed. That altitude is the needed rod length.

I trust SpaceCad's speed calulations OK, but not so much the stability estimates. I always swing test.