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Fully Automatic potato cannon project.

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: jackssmirkingrevenge » Sun Jul 11, 2010 12:20 am

Conrad2468 wrote:A fan won't work


Why not?
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Unread postAuthor: Conrad2468 » Sun Jul 11, 2010 12:22 am

I figure if were going to be firing this fast....the fan will melt....but were unsure...will it work?
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 11, 2010 5:51 am

what rate of fire did you have in mind?
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Unread postAuthor: POLAND_SPUD » Sun Jul 11, 2010 6:02 am

Have you considered using venturi nozzles to mix the chamber?? it seems it will help a bit

Also, I think it might be a good idea to purge the chamber after each shot with compressed air
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Unread postAuthor: Conrad2468 » Sun Jul 11, 2010 12:44 pm

We did it with a venturi nozzle....it worked better, but pure oxygen worked best...and since we can get oxygen tanks at the hardware store, well use that. I wanted to use compressed air, but that would take up wayyy too much space as opposed to a small bottle of oxygen. My rate of fire will be relatively high, the auto loader works in less than a second and as soon as the cannon fires, the only thing that would slow us down is the solenoids opening and closing (were using a regulated propane nozzle, if that isnt fast enough well cut it down to where its unregulated). so, were looking at a firing time of about a shot every 1-2 seconds. This is however, untestable since my solenoids havent arrived.
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Unread postAuthor: ramses » Sun Jul 11, 2010 9:26 pm

If you are going to use a lot of oxygen, you should really get a real oxygen cylinder, or an oxygen concentrator. It's a big initial investment, but those little oxygen cylinders will add up quickly.
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Unread postAuthor: Technician1002 » Sun Jul 11, 2010 9:32 pm

POLAND_SPUD wrote:
Also, I think it might be a good idea to purge the chamber after each shot with compressed air


Purge and fill as one operation works well. You only need to look to any 2 cycle engine with the piston down to see this. Exhaust port is wide open at the time the fresh fuel air mix is ported into the cylinder.

A simple propane meter AND air meter can be used together to deliver the fresh fill purge charge with the correct mix every time.
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Unread postAuthor: Conrad2468 » Sun Jul 11, 2010 9:33 pm

I dont think well use that much when we get this tuned in to the perfect ratio haha. Does anyone have the perfect Fuel (propane) to OXYGEN (keep in mind its pure) ratio???
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Unread postAuthor: Technician1002 » Sun Jul 11, 2010 11:14 pm

i won't answer that simply because of the danger. Since you asked the question in the first place indicates you have not studied the subject at all and are unaware of the factors that can make this lethal.

In that mix, there is no buffer gas (Nitrogen) so the burning fuel is not heating another gas in the mix. As a result detonation is very likely. If you are considering building out of any plastic, you will most likely be needing medical care to remove it from within your body. You have been warned.

On buying bulk Oxygen for welding, the price difference between a 40 and 80 fill is very minimal and is less than $5 for the extra 40 cubic feet. I have an empty 40 cubic foot tank and will be upgrading on my next fill.

Edit, I looked up how much Oxygen is in the disposable cylinders. They contain 1.1 cubic feet. That is a price of about $10 per cubic foot.
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Unread postAuthor: Conrad2468 » Mon Jul 12, 2010 7:54 pm

I dont like the sound of that :/ we have a T joint. do ya think if we put it on so it looks like the T is upside down and the left part of the T is in the cannon and the right is connected to the propane so when it goes past, it sucks air in. Will that work? P.S. Though not flammable, there will still be CO (or CO2 i forget) wouldnt that work for a buffer gas? And even if that didnt wouldnt adding just a "tidge" of O2 make a detonation so small that it wont harm anything?
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Unread postAuthor: Ragnarok » Mon Jul 12, 2010 8:29 pm

Conrad2468 wrote:Does anyone have the perfect Fuel (propane) to OXYGEN (keep in mind its pure) ratio???

Unlike Tech, I will answer this (as it's easy to find out with a simple Google search, so not answering isn't going to do much good), but I would advise you think twice before messing with pure oxygen. It's very dangerous stuff. Just about anything will burn explosively in the presence of pure oxygen.

Try looking up Thermal lances for example. Those work by burning iron in the presence of oxygen (try doing that in air), and produce temperatures hot enough to melt Tungsten - which has the highest melting point of all metals.

~~~~~

Anyway, the answer. Stoichiometric combustion of propane:

C<sub>3</sub>H<sub>8</sub> + 5O<sub>2</sub> => 3CO<sub>2</sub> + 4H<sub>2</sub>O

Given that all gases have the same molar volume (at the same temperature), the ratio is therefore 1 part Propane to 5 Parts Oxygen.

Obviously, the same ratio applies in air - 1 Propane to 5 Oxygen - but you need to account for the fact that air is only 21% oxygen.

~~~~~

But again. Oxygen is very dangerous. Even I don't mess with pure oxygen, and I'm an idiot.
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Unread postAuthor: Conrad2468 » Mon Jul 12, 2010 8:50 pm

Why not dull down the mixture? make it 3:1 or 2:1?
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Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 8:53 pm

you can't do that, it will not combust
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Unread postAuthor: Conrad2468 » Mon Jul 12, 2010 8:54 pm

ok, so how about a 1:5 mixture in a smaller quantity?
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Unread postAuthor: Gun Freak » Mon Jul 12, 2010 9:00 pm

Then it would still be a 1:5 mixture...
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