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First Combustion, PVC Questions...

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: TurboSuper » Sun Aug 12, 2007 11:40 am

boilingleadbath wrote:DWV is not pressure rated; combustions create pressure; ergo, no DWV for combustions!

Now, you may get away with it, and you may not... I personally have gotten away with clean-out plugs on pneumatics - but that's a dumb thing to do.



Erm, not exactly, they are rated for gravity pressure, and can generally tolerate up to around 70 PSI. Combustions produce around 60 PSI for a very short period of time, so they're quite safe. I agree with what you're saying though, no reason not to use pressure rated fittings if they're easily accessible.

Oh, and 1.5:1 is supposedly the best ratio for aerosol combustions. I say screw the math and just get a hefty chamber for the satisfying kaboom it creates.
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Unread postAuthor: DYI » Sun Aug 12, 2007 11:49 am

If the chamber on a combustion is too large, performance will begin to decrease, due to the slower rate of combustion. On metered propane combustions, a good ratio is about .8:1. Aerosol cannons can stand having a slightly higher C:B because of the lower pressures that they produce, but any ratio over 1.5:1 is not just wasting power, it is actually losing power.

I would say that DWV is fine for a combustion, as long as it is well built and has a chamber that doesn't cause DDT (ie. 2" in diameter and 40' long). If pressure rated materials are available, use them.
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Unread postAuthor: Modderxtrordanare » Sun Aug 12, 2007 12:27 pm

DYI wrote:If the chamber on a combustion is too large, performance will begin to decrease, due to the slower rate of combustion. On metered propane combustions, a good ratio is about .8:1. Aerosol cannons can stand having a slightly higher C:B because of the lower pressures that they produce, but any ratio over 1.5:1 is not just wasting power, it is actually losing power.


Use multiple sparks all around the inside of the chamber? I know there hasn't been much research that gives a definite answer, but I would assume more points of ignition inside the chamber would benifit it.

Edit: woot, 600th post.
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Unread postAuthor: Dornep » Sun Aug 12, 2007 2:42 pm

Cool deal guys. I've got just enough barrel to give me a 0.82:1 C:B. The chamber is complete with all wiring crimped and connected. I'm going to start gluing and will post some pic's as I go. Thanks for the help guys!
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Unread postAuthor: frankrede » Sun Aug 12, 2007 3:39 pm

TurboSuper wrote:
boilingleadbath wrote:DWV is not pressure rated; combustions create pressure; ergo, no DWV for combustions!

Now, you may get away with it, and you may not... I personally have gotten away with clean-out plugs on pneumatics - but that's a dumb thing to do.



Erm, not exactly, they are rated for gravity pressure, and can generally tolerate up to around 70 PSI. Combustions produce around 60 PSI for a very short period of time, so they're quite safe. I agree with what you're saying though, no reason not to use pressure rated fittings if they're easily accessible.

Oh, and 1.5:1 is supposedly the best ratio for aerosol combustions. I say screw the math and just get a hefty chamber for the satisfying kaboom it creates.
70 psi?No no no, thats a major exageration.
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Unread postAuthor: Dornep » Sun Aug 12, 2007 8:56 pm

I will try to contain my excitement but HOLY CRAP BATMAN!

I will admit I made several rookie mistakes while making this gun that I can't go back on now. However I see this gun as only the beginning of what I plan to do with spud guns in the future. The gun still has a few things to be done to 100% complete but it does shoot and shoots well. I have no idea what the range is as it was getting dark once we got everything ironed out with the metering setup. I have a few pictures and some videos of the firing so If you guys want to see them I will upload them to Youtube and link you to them.

These are a couple pic's of the gun
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Unread postAuthor: TurboSuper » Mon Aug 13, 2007 6:04 am

70 psi?No no no, thats a major exageration.


Hardly, if you were to read the failure log on Spudtech, people who were silly enough to use DWV in their pneumatics reported failure when they pushed them past 70 PSI.
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