16.4 Ounce Containers for a Combustion Cannon

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
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kjjohn
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Mon Jul 19, 2010 1:57 pm

The fan survives the combustion because it is very quick. Although the combustion produces a few thousand degrees of heat, it only does so for a small fraction of a second, so the fan can survive it. The only time a fan won't survive is in high-power hybrids.

I would not run your ignitor/chamber fan off of a car battery. First of all, you can achieve the same thing in a much more portable package with something like an RC car rechargeable NiMH or Li-poly battery, and if that's too expensive, you can always go with a BBQ igniter.

Secondly, yes, a car battery could drive an ignition coil circuit well, but I wouldn't risk it with a little CPU fan. I'm no electronics expert, but I'd wager that the amperage a car battery puts out would be enough to toast your fan after a few minutes.
lbfergus
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Mon Jul 19, 2010 3:22 pm

Can the tank be inverted or does it have to be stored 'right side up'.
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ramses
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Mon Jul 19, 2010 3:24 pm

Do not use the tank upside-down; the liquid propane will come out, freeze up your regulator, and cause problems.

I'm not sure about storage requirements, but I store mine right-side-up.
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Technician1002
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Mon Jul 19, 2010 3:35 pm

Storing them right side up is correct. That way the relief valve can vent vapor and lower the pressure to prevent an explosion if overheated. Inverted, it would vent liquid which would not appreciably lower the pressure and cause a fire hazard.
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Mon Jul 19, 2010 3:43 pm

lbfergus wrote:And I have another question, how does the computer fan not get toasted going through that many explosions?
Quite simply, because the heat capacity of the gases is insufficient for any 'toasting' to occur.

Although the temperature in the combustion chamber is very high during firing, the low density gases are only carrying enough energy to warm the solid plastic and metal fan by a few degrees. Thus, it survives the shot.
I was thinking to power the electrical I would make some leads to hook to my car battery with alligator clips. Could I use a chamber short with the battery like that? Just placing a narrow gap for the spark to jump, + on one side and - on the other?
Seems overly complicated and bulky to me. When you can buy small, compact R/C battery packs capable of supplying over 30A continuous current, there's no reason to use a car battery to power any spudgun circuitry.
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lbfergus
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Mon Jul 19, 2010 3:47 pm

Thanks for all of the help everyone!

My last (hopefully) question for this build, can I build a stand for this thing out of PVC? Or is the recoil too much for it to hold?

I plan on an overall length of about 8-10 feet.
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kjjohn
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Mon Jul 19, 2010 4:34 pm

8-10 feet? For the barrel, or just the chamber? Because if you mean the chamber, that's going to be one powerful combustion cannon! Usually these things don't have a whole lot of recoil. At least, not as much as you would expect. You could build a stand out of PVC, but make sure it has a large enough base, and if the recoil does prove to be too powerful, just put some weights on it, or use metal pipe towards the bottom of the stand.
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Jeeperforlife
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Mon Jul 19, 2010 5:43 pm

lbfergus wrote:Thanks for all of the help everyone!

My last (hopefully) question for this build, can I build a stand for this thing out of PVC? Or is the recoil too much for it to hold?

I plan on an overall length of about 8-10 feet.
It can be done. here is a picture of my coaxil on a PVC stand. It is more than strong enough and you could easily add 1" wooden rods to increase the sturdiness quite a bit.

Image

The gun is 4" PVC chamber 5' long and a 5' 2" barrel. Shot at about 135 PSI on average. It does kick pretty good but the stands flexibility helps to absorb it. With me standing behind it I absorb some recoil too.
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