SpudFiles

Hi:

New to the forum, got lots of great ideas for my son's science fair project here. Trying now to help him interpret the results. We built a compressed air tennis ball cannon using the simplest design I could come up with based upon examples I found referenced on this site. We used a 4" PVC section with a Tee for the compression chamber with a schrader valve tapped into it for pressurizing. Attached to the center leg of Tee is a reducer to a 2" manual ball valve. The other end of the ball valve threads onto a 4 foot long 2.5" barrel.

The goal of the experiment was to determine the launch angle to have the furthest flight distance. Generally the experiment was a success, with some expected variation due to ball differences, unpredictable spin etc. The result was that 30 degrees was the optimal angle.

Now to the question:

We decided to increase the launch pressure from 30 PSI to 60 PSI to see how the distance varied with a constant launch angle. Strangely at 60 PSI the ball went no further than at 30 PSI, around 300 feet. I'm puzzled as to why this would be, wondering if its related to the barrel length as a limitation, or is it possible that the large amount of air resistance on a fuzzy, light tennis ball prohibits greater launch velocities?

Any ideas?

Also curious from this groups experience how barrel length effects launch velocities? I'm used to thinking about longer barrels in guns being better but that is due to rifling etc so probably not applicable in a smooth bore.

thanks for your insights.

Peter

First off, welcome to the forum and props for supporting your son with his project!

2" manual ball valve

Opening time is a critical variable for projectile velocity and therefore range. A manual ball valve opens as fast as the individual operator opens it, which means you have zero consistency even with one operator.

I strongly suggest you switch to a modified sprinkler valve that will not only give better performance for a given pressure but will be a lot more consistent in terms of operation.

We decided to increase the launch pressure from 30 PSI to 60 PSI to see how the distance varied with a constant launch angle. Strangely at 60 PSI the ball went no further than at 30 PSI, around 300 feet. I'm puzzled as to why this would be, wondering if its related to the barrel length as a limitation, or is it possible that the large amount of air resistance on a fuzzy, light tennis ball prohibits greater launch velocities?

A tennis ball is certainly far from ideal in terms of a long range projectile, having poor sectional density and aerodynamic shape. Terminal velocity is only around 90 feet per second which you more than likely have a achieved with your launcher. Increasing muzzle velocity therefore will give you diminishing returns.

Also, did you consider the prevailing wind? A higher launch velocity might result in a greater time for wind to carry the projectile back towards the launcher. This may sound a little silly but consider using an artillery game to see how launch angle, velocity and wind would affect a projectile in flight.

Also curious from this groups experience how barrel length effects launch velocities? I'm used to thinking about longer barrels in guns being better but that is due to rifling etc so probably not applicable in a smooth bore.

Unless the chamber pressure is too low or chamber volume too small, a longer barrel generally means higher velocity, not related to rifling but rather the projectile being accelerated by the gas for a longer time.

Consider downloading Gas Gun Design Tool to model your launcher virtually and play with the variables to see how they effect performance. The program also features a Ballistic Calculator that will give you a rough idea of what range to expect for a given velocity and launch angle.

Thanks for the insights. The terminal velocity suggestion makes a lot of sense. I ran the Gas Gun Design tool, and once I figured out how to correctly input the chamber data, the results from the Ballistic Calculator are reasonably close to what we measured as well as the overall flight time. Very cool.

thanks.

No worries, feel free to post images and results for the record.

Also regarding consistency, I forgot to mention that it is possible to spring-load your ball valve in the manner done here:



This could be a modification to make to your existing launcher instead of replacing the valve entirely, however it is mechanically a more complex undertaking than modifying a sprinkler valve.

That's nice 8)

farcticox1 wrote:That's nice 8)

It is somewhat your style isn't it?

Like I made it and forgot I made it