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In need of opinions

A place to ask general spud cannon related questions.
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Unread postAuthor: Pete Zaria » Sun Jul 01, 2007 4:49 pm

@SpudBlaster15,

My over/under combustion is cell core DWV abs (4" x 20" chamber, 1.5" x 60" barrel) with metered propane, chamber fan, ball valve venting, spark strip, etc...

It's seen several hundred shots with zero problems, no signs of stress or fatigue...

My last one was also 4" ABS cell core DWV and I'm sure it saw over 500 shots before I took it apart.

I pressure-tested some 4" ABS cell core DWV fittings to 125psi overnight before I built this gun... nothing happened to them. I feel very safe using it.
If it does fail, it should rip open very cleanly without throwing off shrapnel.

Having said that, I wouldn't feel safe taking it over 100psi regularly, and I definitely would not use it for a pneumatic or (God forbid) a hybrid.

Sorry your gun failed on you. I'm guessing there were some unusual circumstances - maybe the pipe spent too much time in the sun, maybe there were some hairline cracks or bad seams from molding, maybe your solvent welds weren't solid. But I've never seen ABS fail under normal combustion circumstances.

Peace,
Pete Zaria.
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Unread postAuthor: SpudBlaster15 » Sun Jul 01, 2007 5:00 pm

The launcher was very similar to yours, though the pipe was solid wall ABS. I inspected everything for wear before assembling the gun, and everything checked out ok. The gun was kept inside, away from the sun. I blame the failure on a manufacturing flaw.

You might get way with firing an unrated gun 500 times without a failure, and you might get away with pressurizing a piece of unrated pipe with air for 12 hours, but that doesn't mean it won't fail on the 501st shot, or after 12 hours and 30 seconds. Trusting your safety on something that is not designed to hold ANY pressure at all is idiotic. Use only rated parts.
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Unread postAuthor: Pete Zaria » Sun Jul 01, 2007 5:27 pm

SpudBlaster15 wrote:The launcher was very similar to yours, though the pipe was solid wall ABS. I inspected everything for wear before assembling the gun, and everything checked out ok. The gun was kept inside, away from the sun. I blame the failure on a manufacturing flaw.

You might get way with firing an unrated gun 500 times without a failure, and you might get away with pressurizing a piece of unrated pipe with air for 12 hours, but that doesn't mean it won't fail on the 501st shot, or after 12 hours and 30 seconds. Trusting your safety on something that is not designed to hold ANY pressure at all is idiotic. Use only rated parts.


You do have a point. However, propane combustion guns won't generate more than 86psi max at atmospheric pressures - I tested to 125, giving a small cushion there. However, I didn't account for the pressure spike.

I look at it this way: The hot, expanding combustion gases are going to follow the path of least resistance out of the gun. As long as pushing the projectile out the barrel requires less force than ripping the pipe, you should be fine. Obstructing or blocking the barrel would cause a failure, though.

Of course using NSF-PW is preferred when it's available and cost-effective. But around here, no hardware stores carry NSF-PW fittings, and plumbing supply stores only sell stuff in bulk. Building this launcher out of sch40 nsf-pw pvc would have cost me twice as much.

Additionally, when cell core ABS does fail, its failure characteristics are very clean, usually producing no shrapnel at all.
I think sch40 nsf-pw PVC is probably just as likely to fail in a combustion situation, because it's much more brittle if dropped, is more sensitive to UV, doesn't "give" at all (ABS does), etc....

My point is just, your case is a rare one, and probably was, as you said, due to a manufacturing flaw.

Just my opinions though.

Peace,
Pete Zaria.
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Unread postAuthor: origin unknown » Sun Jul 01, 2007 7:50 pm

A ball valve or sprinkler valve pneumatic would be a great starter gun. In my mind, combustion cannons seem more trouble to build (i.e. more components) than a simple pneumatic, but pneumatics require more precision work in making them. If you are good at solvent glueing pvc pipe and don't like slopping things together, making a good pneumatic should be no problem for you.
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Unread postAuthor: Spedy » Mon Jul 02, 2007 9:43 pm

jimmy101 wrote:Spedy, if you have access to a laptop, or to an old desktop PC, you can make your own chrony for a couple bucks like this.

BTW, combustion vs. pneumatic is a long standing debate. For the two guns you linked to I wouldn't be surprised if the combustion out-performed the pneumatic.


Wow, thanks for that link. This is going to be really useful. Thanks!
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