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Combustion Competition

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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Combustion Competition

Unread postAuthor: xXunderXcoverXx » Sun Dec 09, 2007 1:55 pm

So this is my first post and new to spudfiles but I'm not new to spudding. Ive built a pneumatic gun that will be on here soon and as human male instinct It has to get bigger and better. I made a bet with my girlfriend and her dad that I could a better spud gun then they could and so the competition began. Her father owns a hardware store and I figured If I better win. I'm thinking about a 96" 2" sch. 40 barrel and a 36" 6" sch. 40 Chamber with metered propane and chamber fan. which give the c:b ration about 3.2-1. Please questions or comment would help a lot Thanks
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Unread postAuthor: bigbob12345 » Sun Dec 09, 2007 2:03 pm

Make it a pneumatic they are a lot more powerful
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Unread postAuthor: xXunderXcoverXx » Sun Dec 09, 2007 2:08 pm

Ive done some research and Ive found that pneumatics are a lot more powerful but part of the bet was it has to be a combustion gun
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Unread postAuthor: cwazy1 » Sun Dec 09, 2007 2:09 pm

you definitly dont want a c:b ratio of 3.2:1, you want .8:1 for a combustion to yeild the most efficiency.3.2:1 would be more of a ratio for a pneumatic. why are you using 6" sch40?? why not just 4"?and you understand that with a chamber your size, 1017cu-in you will need alot of propane per shot? i recommend that you go with a more reasonable chamber volume like along the lines of 300-500 cu-in.

and i would hope that you know the mix for propane to air is a 4.2%. and the way to find out how much propane you need in a meter of "x" size, you should use burntelakte's calculators.
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Unread postAuthor: octane89 » Sun Dec 09, 2007 2:25 pm

Like cwazy said, the best ratio for combustion is an .8:1 ratio. You will have faster muzzle velocity and distance too.
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Unread postAuthor: xXunderXcoverXx » Sun Dec 09, 2007 2:29 pm

Yes Ive been to burntlakte and read all their stuff. I know the c:b ratio is High but I'm not really looking for efficiency. I guess it could be alot lower like 1:1 but I figued if I waste a little potential energy then the potato would be at maximum kinetic energy by the end of the barrel.
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Unread postAuthor: octane89 » Sun Dec 09, 2007 2:33 pm

With 3.2:1, it will just be loud and have plenty of kick. Thats why .8:1 delivers a perfect match because your combustion is complete when it leaves the barrel, and the potato will be at "max kinetic energy" like you said.
If you wanted to boost performance, use MAPP instead of propane but the mix is 4.6%
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Unread postAuthor: cwazy1 » Sun Dec 09, 2007 2:42 pm

yeah as octane said, 3.2:1 would be loud.

mapp gsa does boost preformance, but it is alot more expensive and you get alot less of it per bottle. over where i live, its 3$ for a small tank of propane, while 9$ for a small tank of mapp and it has less mass. it all reall depends on what you perfer, if you like loud, well then, you can stick with your 1000cu-in chamber and a 3.2:1 ratio (make sure to wear hearing protection.)
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Unread postAuthor: octane89 » Sun Dec 09, 2007 2:45 pm

Mapp here is $5, and propane is $3. I just like it because I get a bit better muzzle velocity.
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Unread postAuthor: cwazy1 » Sun Dec 09, 2007 2:48 pm

oh lucky, i've been reluctant to try mapp cuz of its price. ahahha im a cheapo
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Unread postAuthor: BC Pneumatics » Sun Dec 09, 2007 3:07 pm

His planned ratio will preform better with a 96" barrel than a .8:1 ratio with the same size barrel. Lowering your C:B ratio will only help if you do it by lengthening the barrel.

Of course, you are paying a lot for the relatively small increase in power that the oversized chamber gives you.
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Unread postAuthor: SpudBlaster15 » Sun Dec 09, 2007 3:24 pm

Make it a pneumatic they are a lot more powerful


Eh... That depends entirely on the launcher. Too many people automatically assume that pneumatic launchers are more powerful than combustion launchers in general.

A stoichiometric mixture of air and propane has the potential to produce CO2 and gaseous H2O at a temperature of ~4200*F, and a pressure of ~120PSIG in a closed chamber. According to my calculations, this chamber contains approximately the same amount of energy as a pneumatic chamber with air at room temperature and ~200PSIG inside. Now, it is unlikely that this reaction will produce such high values in practice due to heat losses and the "partially closed" nature of a combustion chamber, but with multiple ignition points and a good burst disk, 100PSIG and 2500*F are certainly possible. This chamber contains approximately the same amount of energy as a pneumatic chamber containing room temperature air at ~180PSIG. Combustions can be much more powerful than their pneumatic counterparts, especially when most pneumatic launchers operate at the maximum pressure capability of a standard air compressor.

xXunderXcoverXx, you are on the right track with the thought that a large cannon will be required to ensure victory. 96" of 2" Sch40 pipe should be effective as a barrel, but 36" of 6" diameter Sch40 pipe will yield a HUGE excess of chamber volume. Using 36" of 4" Sch40 pipe and multiple ignition points will yield roughly the same performance, and 4" pipe is much easier and cheaper to obtain fittings for.

In addition to a chamber fan and a fuel meter, you will want to add a union or cam locks to hold a burst disk. Setting the burst pressure at 50 - 60PSIG will substantially increase the chamber pressure, and thus performance of the launcher. Replacing propane with MAPP gas will also increase muzzle energy by several percent.
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Unread postAuthor: cwazy1 » Sun Dec 09, 2007 3:53 pm

^^ you dont have to have a burst disk
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Unread postAuthor: LikimysCrotchus5 » Sun Dec 09, 2007 4:05 pm

Yea he knows it isnt neccesary but the benfit of having one is that it locks the pressure up until it bursts at a specific point and all of that pressure shoots out at once, therefore giving better overall performance.
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Unread postAuthor: MrCrowley » Sun Dec 09, 2007 4:46 pm

Golfball barrel.

If your competition is on distance, golfball should win it, if it's on power or destruction golfball should win it.

I believe you can buy them from BC Pneumatics website BCARMS, just click the link in his signature.
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