On Sun, 29 Nov 2009 20:11:29 -0500, "Ralph Mowery"
wrote:

You do not need the projectile to be very tight.
I just used some electrical tape to make it fit beter. Used orange as I
though it might make it easier to find .

Yech. One of the local hams was trying to use one and had the same
fit problem. Somehow, I got involved. The first thing I did was toss
a little household flour into the launch tube so I could see if the
ball was leaking. Someone appeared up with a camcorder that would do
about slow motion. Kinda like that TV show that shows things in slow
motion. The tennis ball leaked really badly.

Time to fix the leak. I decide to make a new projectile in the shape
of a Civil War Minie Ball.
http://en.wikipedia.org/wiki/Mini%C3%A9_ball
The idea was to have a conical head for improved aerodynamics and an
expanding base for a good seal. Of course, friction had to be
minimal. To improve the seal, I found some close cell urethane foam.
Using a hot nichrome wire, I cut a simple cylinder that was slightly
smaller than the bore diameter, and about the length of a beer can.
Well, it was the exact size of a beer can because that was what we
were launching nearby with a propane powered canon. Anyway, I also
used the hot wire to form a somewhat conical cavity in the base. Air
pressure would expand the base outward towards the barrel wall,
forming a better gas seal. The length of the foam would keep it from
wobbling. As long as the foam was a loose fit, there would be little
added friction. I never got around to building a conical projectile
and just dumped the tennis ball on top of the foam forming a sabot.

As I recall, we got about 60 ft with 50 lbs pressure using only the
tennis ball, and about 90 ft at 50 lbs using the foam Minie Ball.
Videos of the trajectory showed that the air resistance of the tennis
ball was only a little better than if it had been dragging a
parachute. End of flight droop was severe. A better aerodynamic
shape would certainly have been a major improvement. However, I never
tried.

It would also be fairly easy to add spin stabilization, but that would
require a one piece conical projectile.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558