Simple over/under tennis ball launcher

[yman]

I'm looking at building my first pneumatic cannon and would greatly appreciate any advice. I'd like to keep it very simple for a first build, and I'd also like to keep the size and weight on the small side to make it easier for the kids to use. The most likely use would be to launch tennis balls, although I'm considering a threaded end with interchangeable barrels to fire other items.

My initial idea was something like the CSV19 (picture below), using a modified 1 in. Rain Bird sprinkler valve, but I have a couple of questions:

1) The CSV19 has a 16 in. barrel (2.5 in.) and an 8 in. pressure chamber (4 in. ). Is this a good, general ratio (2:1 barrel/chamber)? Would I gain much distance by making the barrel or chamber longer?

2) Does anyone know where to find 2.5 in. fittings? I've tried HD, Lowes and a big, local place, with no luck. Maybe have to order online?

3) I was going to include a 100 psi, safety "pop-off" valve on the tank to make sure it couldn't get over-pressurized. Any other ideas to keep it as safe as possible?

4) I already bought some Schedule 40 pipe, including hard-to-find 2.5 in. for the tennis ball barrel. The plumbing guy said all the fittings were Schedule 40, which meant they were also pressure rated, but there's nothing stamped on them. Is this true? Are all Schedule 40 fittings pressure rated and, if so, to what general range?

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[saefroch]

4) I already bought some Schedule 40 pipe, including hard-to-find 2.5 in. for the tennis ball barrel. The plumbing guy said all the fittings were Schedule 40, which meant they were also pressure rated, but there's nothing stamped on them. Is this true? Are all Schedule 40 fittings pressure rated and, if so, to what general range?

Schedule 40 or any schedule represents a wall thickness, not a pressure rating. However, corresponding pressure ratings can be found on a very handy site, here.

[Technician1002]

If you hand pump the chamber, the small chamber is fine. A chamber twice the size will require twice the effort to pump it up. I have even a greater C:B ratio in my version with my one inch valve launcher. It works well. A sprinkler valve will flow less than the launcher below. If you want maximum performance, use as large of a valve as you can get.

The Marshmallow Cannon with the Tennis ball or T shirt barrel. It launches rolled up t shirts about 200 feet with 100 PSI of air.

[jsefcik]

Sch 40 2.5 inch is easy to find depending on where you live,I use it for my tennis ball barrel

[Technician1002]

Schedule 40 is a tight fit on a tennis ball. If you get the thinwall pipe instead, you won't need a ram rod to load the barrel. You can see the thin wall pipe loading in this video.
http://www.youtube.com/user/Technician1 ... 0l5t7D0FDM
Schedule 40 pipe will require a ram rod to load it.

[yman]

4) I already bought some Schedule 40 pipe, including hard-to-find 2.5 in. for the tennis ball barrel. The plumbing guy said all the fittings were Schedule 40, which meant they were also pressure rated, but there's nothing stamped on them. Is this true? Are all Schedule 40 fittings pressure rated and, if so, to what general range?

Schedule 40 or any schedule represents a wall thickness, not a pressure rating. However, corresponding pressure ratings can be found on a very handy site, here.

Thanks - the pipe itself is marked, but it was the fittings (unmarked) that concerned me. I'll have to keep that site bookmarked for future reference, though!

[yman]

Schedule 40 is a tight fit on a tennis ball. If you get the thinwall pipe instead, you won't need a ram rod to load the barrel. You can see the thin wall pipe loading in this video.
http://www.youtube.com/user/Technician1 ... 0l5t7D0FDM
Schedule 40 pipe will require a ram rod to load it.

Thanks - I found Schedule 40 in 2.5 inch pipe near me, but haven't been able to find the thin-walled stuff. I'll definitely keep looking, though.

[jackssmirkingrevenge]

My initial idea was something like the CSV19 (picture below), using a modified 1 in. Rain Bird sprinkler valve

You want the sprinkler to be at the barrel end, not the chamber. This reduces the dead space and will considerably improve performance.

[Technician1002]

If super performance is not a concern and very compact size is important the short chamber and valve together work well as shown in the original post. As JSR mentioned, the valve at the breech of the barrel will perform better, but it will considerably alter the overall length.
Use GGDT to model both configurations to see if the trade off in size is worth it.

[yman]

If super performance is not a concern and very compact size is important the short chamber and valve together work well as shown in the original post. As JSR mentioned, the valve at the breech of the barrel will perform better, but it will considerably alter the overall length.
Use GGDT to model both configurations to see if the trade off in size is worth it.

Thanks - never heard of GGDT before your post - neat program. For some reason, when I increase the dead space (which I assume would be the result of having the valve closer to the tank, muzzle velocity increases somewhat. Strange.

[Gun Freak]

That is very strange. In real life it's probably not true.

[saefroch]

Can we have a screenshot?

[yman]

Can we have a screenshot?

I haven't actually run numbers to get an accurate idea of "dead space", but here is an example with (very) rough estimates. The only variable I changed was the amount of "dead space".

3 cubic inches of 'dead space" - valve on barrel end yields 152 fps muzzle velocity

15 cubic inches of "dead space" - valve on tank end yields 164 fps muzzle velocity

Am I missing something?

[Mr.Tallahassee]

Actually, dead space affects velocity like that because of the hammer effect. It's very similar to water hammer. Theoretically, there is a perfect balance of dead space that optimizes velocity. The air pressure has more time to build and that causes the air to heat inside the dead space. Higher temperatures equates to higher velocity. Another odd thing about physics.

[Technician1002]

For that to be very effective, a high flow valve needs to be used so the pressure can rise quickly behind the projectile and then the moving mass spring off the compressing air in the dead air space. Light gas guns use this effect with a light gas. The extra weight of a heavier gas is detrimental to this effect.