Re: Air canon, sanity check on my calculations

thomas.greenleaf@xxxxxxxxx wrote:
I made a few quick calculations consering an air canon with the
folowing specks.
Barrel length = 2m long with inside diameter = 1cm
Air pressure = 8kg/cm^2
Ammunition is a steel bal with diameter = 1cm and mass=4.08g and a
frontal area (air resistance) of 0.000079m^2

I assume no friction between ball and barrel and I assume no drop in
air pressure as the ball travels down the barrel. I allso assume an
instantanious valve which puts ful air pressure on the ball right from
the start. I also assume that the barrel is not filed with air in
front of the ball which resists its movement. That may be the problem?

Highly optimistic.

If there is no friction between the barrel and ball there is going
to be no seal and lots of gas will go around the ball and the pressure
will be very different from your approximation.

The instantaneous valve could be a reasonable approximation depending
on the valve used and the assumption of no air in front of the ball
is likely a fairly reasonable approximation.

The no drop in pressure as the ball travels is probably way off
reality. Gas flow down the barrel is going to be contrained by the
inside diameter even if backed by an infinite compressed air tank.

I did a simulation which told me that if this contraption was to fire
the bal at an inclination of 35 degrees, it would travel over 1200
meters. The mussel velocity would be arround 233m/s = 838 km/t
That sounds fun, but also a little too optimistic.

Real world building of air cannons about 40 years ago (don't ask)
tells me your simulation is wildly optimistic.

A bonus question is what would hapen if I would make the ball spin
quicky arround the horisontal axis so that it would generate lift due
to the rotation and velocity. I had a vagua idea about placing strong
magnets along the top of the barrel to make the steel ball tend to
roll along the top and give it rotation. Silly idea?

Yes.


--
Jim Pennino

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