SpudFiles

I am about to build my first Cannon. I'm a plumber, so I dont think I'll have too much trouble with construction. I plan to make a FEW changes... 80kv instead of 30, 3 gap strip instead of 2, and 12v A23 battery's instead of 9v. Anybody ever build one of these? Any and all info would really help. Thanks, Fman.
http://aircannonplans.com/pdf/propane-p ... -plans.pdf

Welcome! Those are some amazing plans. A lot of work involved, that's for sure. Your changes seems fine to me. Keep us posted on your progress and good luck!

They are making plans now I'm sure you know as a plumber that regular PVC is NOT rated for gas pressure.

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That is TRUE, in 35 years I never installed a PVC gas system. I have installed Polypropylene. PVC is not used on gas because it is FLAMABLE. SCH.40 PVC IS pressure rated to about 220lbs and SCH 80 to about 320lbs. OF course the Explosion of Propane probably creates more than those pressures, which worries me. I am going to use SCH 80, if I even build one. Pricing the parts is close to $200. that's a lot of cash just for a "TOY" and a dangerous one at that. That is why Im looking for someone that has built one of these. Thanks for your input.

Moonbogg wrote:Welcome! Those are some amazing plans. A lot of work involved, that's for sure. Your changes seems fine to me. Keep us posted on your progress and good luck!

This is not my design, I found them on the net. I really do like the Whole concept.

To be fair, that explosion didn't sound like propane and air to me. It sounded really sharp (and highly energetic), which is something you often get with oxygen added. Also, the inside of the pipe was black, so that didn't look typical either. I could be wrong though since I wasn't there.
On the other hand, for someone (me) who's constantly doubting the strength of even 1" thick aluminum for spud gun use, the thought of using PVC for anything these days is out of the question.
If I were to make a plastic cannon again, I'd use schedule 40 cellular core ABS pipe and ABS fittings. I simply trust ABS more than PVC to not shrapnel if it fails. The good old ABS vs PVC debate. Does it ever get old? I think not. Let us discuss.

LOL this dude made a cellular core ABS pneumatic! Haha! And it works, lol. That takes balls, man.

Fman wrote: I plan to make a FEW changes... 80kv instead of 30, 3 gap strip instead of 2, and 12v A23 battery's instead of 9v. Anybody ever build one of these? Any and all info would really help

If you're changing the voltage and batteries simply because you have easy access to the appropriate components, knock yourself out. But if you're doing so thinking you're going to get better performance out of the overall system simply because your ignition system is a bit more robust, understand that you're kidding yourself.

Moonbogg wrote: It sounded really sharp (and highly energetic), which is something you often get with oxygen added. Also, the inside of the pipe was black, so that didn't look typical either.

You can get plenty of soot buildup using propane and air if you screw up your mix. Happens quite a bit at the office on some propane-powered toys we have. We usually clean it up by injecting some oxygen, but that's not something that is advised around here.

Anywho, I'm not saying you're wrong, I'm just saying that a bit of soot on the inside doesn't really mean anything more than somebody sucked at getting their mix right.

^
Good eye on the rich mix soot. I wish people would include details when showing a video of something exploding that wasn't supposed to explode. Although they did look like they were about 15 years old, so I suppose we get what we get. I certainly wouldn't expect my 15 year old self to be smart enough to not kill myself. Sometimes it's better to be lucky than good! That kid could have easily taken some PVC through the neck. LUCKY ASS KID!

He was lucky for sure.

Fman wrote:That is TRUE, in 35 years I never installed a PVC gas system. I have installed Polypropylene. PVC is not used on gas because it is FLAMABLE. SCH.40 PVC IS pressure rated to about 220lbs and SCH 80 to about 320lbs. OF course the Explosion of Propane probably creates more than those pressures, which worries me. I am going to use SCH 80, if I even build one. Pricing the parts is close to $200. that's a lot of cash just for a "TOY" and a dangerous one at that. That is why Im looking for someone that has built one of these. Thanks for your input.

1. Both PVC and polypropylene are flammable.
2. It is impossible to light PVC with the short term combustion in a spud gun. Heck it'll take a while to get a PVC pipe to actual light using a gas torch.
3. Even in a closed PVC chamber the amount of heat generated by the combustion process is only enough to raise the temperature of the PVC by a couple degrees Celsius. The combustion process generates high temperature but the gases have very little mass and contains relatively little heat. (Heat and temperature are completely different things.)
4. Propane + air (1X) in a closed chamber only generates a peak pressure of about 130 or 140 PSI. In a closed chamber the pressure will drop back to basically atmospheric pressure in less than one second after ignition. Combustion in a typical sized chamber takes about 50 milliseconds (1/20th of a second).
5. In a spudgun the ammo starts to move before the peak theoretical pressure is reached resulting in an actual peak pressure of 50 to 70 PSIG or so. (HGDT will calculate the peak chamber pressure.) With light ammo that has low friction, the peak pressure might not even reach 10 PSIG.
6. Burning propane in air is a pretty slow process. 50 milliseconds sounds fast but that is glacial compared to say a gun powder. (And gunpowder is slow compared to a high explosive.) Combustion is so slow the ammo generally has moved quite a bit before combustion completes. For reference, in a 3 foot barrel with a 300 foot/second muzzle velocity it takes about 20 milliseconds for the ammo to transit the barrel. So in many 1X combustion spudguns the ammo is leaving the barrel about the time the combustion finishes. But at that point the volume in the gun has increased from the chamber volume to the chamber+barrel volume.
7. I have a many years old cell core PVC gun (DWV, non-pressure rated) and even with a chamber fan and precisely measured fuel there really isn't a significant risk of chamber failure. For 1X propane in air pressure rated PVC is fine.

Welcome Fman and good luck with your first project!

D_Hall wrote:If you're changing the voltage and batteries simply because you have easy access to the appropriate components, knock yourself out. But if you're doing so thinking you're going to get better performance out of the overall system simply because your ignition system is a bit more robust, understand that you're kidding yourself.

Hear hear.