> I have just been given a large cardboard tube (not sure of the measurements, but
> at least 3" diameter) by a friend and am going to scratch build some sort of rocket.
> The thing is, it's got two wee slits about 5 mm wide, one on either side of one end
> of the tube. I was originally going to make a 3 fined design, but I will now probably
> make a 4 fined one.
> However, the real question is, would this thing be stable with just two fins? One fin
> in each of the slits (with a bit of modification to make them longer). Or are two fined
> rockets inherently unstable?
Assuming the rocket doesn't roll at all, two diametrically opposed fins
won't produce stability in the plane of the fins -- that is, if you see
the fins as "wings" the rocket would be stable in the pitch axis, but
unstable in yaw.
The solution to that is to put opposing tabs on the fins (I don't know
if opposing airfoils would be enough) to cause the rocket to roll; that
will ensure that by the time a disturbance can cause the rocket to
tumble in that unstable yaw axis, it's rolled enough to put the nice
stable pitch axis into the disturbed plane. Effectively, you can then
make two fins do the work of three or four.
You may or may not gain any performance over a three-fin design -- it'll
likely depend on how much drag you add with the roll inducers -- but
you'll certainly have a rocket that doesn't resemble anything else at
the field that day! B)
Better freedom and a chilly road than a warm hearth and servitude.
Donald Qualls, aka The Silent Observer NAR # 70141-SR Insured
Rocket Pages http://members.aol.com/silntobsvr/launches.htm
Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.