designing my first pneumatic cannon

[geardog32]

THIS IS NOT MY DIAGRAM BUT ITS A GOOD ONE!


my cannon based off of it.

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[Technician1002]

When you build those, you will want to use a design where the piston can be removed for service. Leaks, dislodged sealing faces, a little dirt shed from projectiles, etc make this a must. A bumper is also required to prevent or reduce the possibility of the piston breaking the rear of the housing.

Performance is much better if the plumbing between the chamber and the T is built with the full diameter pipe, not reduced to a smaller size. Any restrictions in the high flow areas of the launcher hurt performance. This is most important for lighter higher speed projectiles and less so on heavy slower projectiles.

[Ragnarok]

In answer to Tech's question, assuming still air, a golfball needs to be doing in the 500 fps range to be reasonably confident of exceeding 400 yards range. You might get that velocity down a bit with a hop-up system (and thus lift from backspin), but not by a huge amount.

And for that, I don't think a sprinkler valve cannon has the guts. However, yes, a piston valve (or QDV) certainly could manage that. Or, as Mr Crowley says, a healthily sized advanced combustion would also be an option.

Unless you're really set on a pneumatic, I'd recommend the combustion. It will more than do the job, but as you've implied you'll probably be away from a convenient compressor, refuelling a combustion is a lot less effort than pumping up a pneumatic chamber each time.

(It's also a bit less effort to build, as piston valves can be a bit troublesome to make the first couple of times.)

[wolfie22]

how much do 1.5 and 2 inch valves run? i always had it in mind that they were relatively cheap?

i'd like to stick to something pneumatic, as they seem safer (please correct me if i am wrong) than combustion. would a very basic combustion gun be capable or would i need all sorts of modifications?

[Technician1002]

2 inch is expensive. The local plumbing store has them for about $80.
http://www.sprinklerwarehouse.com/Weath ... f-20-h.htm

Inch and a half are about half the price.
http://www.sprinklerwarehouse.com/Hunte ... gv-151.htm

[Gun Freak]

Pneumatics are WAY more dangerous than combustions. They hold pressure for a longer time than combustions, since the combustion only lasts like a few miliseconds, and you can do higher pressure in a pneumatic.

[Ragnarok]

i'd like to stick to something pneumatic, as they seem safer (please correct me if i am wrong) than combustion.

Well then, I need to correct you.

As far as safety, a combustion spudgun has one major advantage over a pneumatic, in that it's not pressurized other than while it's firing.
A pneumatic is pressurized while it's waiting to fire. If a seal blows, or it's dropped (which can damage the chamber), then all of that pressure energy is there waiting to do damage.

The only thing that will make a combustion fire is a spark or other ignition source. A pneumatic can theoretically (although it is unlikely) go off on its own. As such, I take more precautions with my pneumatics than I do with my combustions.

Other than doing something stupid, it's much harder to have a (dangerous) accident with a combustion than a pneumatic. The fact that it uses an "explosion" is very misleading.

would a very basic combustion gun be capable or would i need all sorts of modifications?

No, I'd recommend a launcher with propane metering, a chamber fan, and perhaps dual ignition. Not that that's complicated per se, but I can't lump it under "very basic".

Inch and a half are about half the price.

Also about half the flow.

[Fnord]

You can occasionally get a large valve at a reasonable price. I and several others once bought 1.5" aluminum diaphragm valves on ebay for $10 each. Look up Asco valves and irrigation valves; you'll occasionally find one cheap.

[Jimmy K]

I bought 2 Toro P-220 3" diaphragm valves on eBay for $80 for the cannon I'm building right now. Those things are huge.

[Technician1002]

My 2 cents worth on safety, depending on the cannon design, either can be safer. The frog in the face shows that a loaded combustion is capable of accidentally discharging. Piston air cannons suffer the same hazard. If you decide not to shoot the cannon, QDV cannons and chamber fill sprinkler valve cannons can be made safe by venting the air from the chamber if so equipped. A combustion remains fully armed until the chamber is vented.

Due to the locations the cannons were intended to be used, Safety was a prime concern. A trigger that had to be pulled instead of pushed was part of the considerations for safety. The cannon sitting on end on the trigger is very unlikely to accidentally discharge. If it did discharge, it is aimed in a safe direction.

[Ragnarok]

The frog in the face shows that a loaded combustion is capable of accidentally discharging.

If you put the igniter in the end cap, then set it down on its end, yes.

Still, my wording didn't say "combustions can't fire accidentally", it said "it won't go off on its own"*. If you press the trigger (even unintentionally), it hasn't gone off on its own.

*Yeah, well, it could, but in a well sealed combustion chamber, the chances of any other ignition source are very small.
Unless there's a major leak, a naked flame outside the launcher shouldn't have any effect - the only moderately likely thing is static charge, and even if that was energetic enough (I'm not sure it is), that's easily warded against by insulating the spark terminals on the outside of the chamber. Which you should do anyway - else you will shock yourself with the igniter.

[Technician1002]

This static discharge danger goes up with added oxygen and easier to ignite fuel. Here is the result of a mix in a balloon that was set off by a static discharge.

Safety is relative to the conditions.


Here is the article on the accident. Always plan on treating it as loaded any time it is loaded.
http://www.thedenverchannel.com/news/67 ... etail.html

[Ragnarok]

I'm not talking about oxyacetylene for this - propane/air will easily do the job.

Always plan on treating it as loaded any time it is loaded.

Yes, and I wouldn't argue that either combustions or pneumatics should be treated with the slightest laxness. But, if asked which was more likely to go off without anyone actually pulling the trigger, I'd say the pneumatic. Still unlikely, but less unlikely.

[wolfie22]

so if i were to go the combustion route, you dont think i could get away with something even easier like hairspray as my fuel? i know you guys are way more advanced than that, but propane and all of that seems like a bit more than i am looking to put into this. i know i'm asking for fairly big results (450yrds) but if it can only go 300 and is simple, i may have to settle for that.

[MrCrowley]

even easier like hairspray as my fuel

No, hairspray will be awful for performance. An advanced combustion would be easier than a piston valved pneumatic. It really isn't that hard, just do some research for a week or two.