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Could Use Some help With My Gun's Ratios

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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Could Use Some help With My Gun's Ratios

Unread postAuthor: stevenmw » Sun Apr 28, 2013 4:23 pm

I am going to build my first potato gun.

Obviously the idea is to have the projectile leave the barrel just as combustion ends.

I'm using the 1.5:1 ratio. With it I've determined the lengths for three possible guns.

1. barrel 96.36" (8.03') - combustion chamber 18"
2. barrel 107.04" (8.92') - combustion chamber 20"
3. barrel 64.2" (5.35') - combustion chamber 12"

I'd appreciate it if someone could tell me if my math is correct, but i don't expect anyone too.

What is an ideal or average barrel length of a potato gun? Would a gun with an 8.92 foot barrel be absurd or is it common? I'm wanting to make something with a slightly smaller barrel but get as much out of it as possible. (maybe something between 5 and 6 foot) Appreciate any advise.

Is there a list somewhere with common chamber and barrel lengths listed using the 1.5:1 ratio?
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Unread postAuthor: RBurke83 » Sun Apr 28, 2013 4:39 pm

depends on many factors, is your plan to be hand held? if so, an 11 foot gun might be unwieldy. just check out other combustion guns in the showcase area, chances are you'll find the info you seek in previous builds.
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Unread postAuthor: Labtecpower » Sun Apr 28, 2013 4:46 pm

If I were you I would have a play with hybrid gun design tool by D-Hall, it gives a lot of information about optimal sizes, and gives you an idea about the power your gun will produce.

There have been some tests on the ideal barrel length, i believe they were done by burntlatke.
I'm sure you can find them with google.
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Unread postAuthor: King_TaTer » Sun Apr 28, 2013 10:57 pm

1. Find the volume of your barrel of desired length
2. Multiply this by 1.5 to get the chamber volume
3. Divide this by the cross sectional area of your chamber
4. Bingo, chamber length has been found.

Now you're going to have to compensate for the irregularly shaped volume between the chamber pipe and barrel if you wish to keep a 1.5:1 ratio. It really won't matter though because that extra volume will give you just a bit more velocity anyway. And who doesn't want more velocity!?!

When it comes to calculating your fuel mix however, it will matter. Oh and there's really no common barrel lengths established. It just depends what you want, but for me, I wouldn't want a handheld barrel more than 6-7 feet long. It's all up to you.
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Re: Could Use Some help With My Gun's Ratios

Unread postAuthor: jimmy101 » Mon Apr 29, 2013 3:01 pm

stevenmw wrote:1. barrel 96.36" (8.03') - combustion chamber 18"
2. barrel 107.04" (8.92') - combustion chamber 20"
3. barrel 64.2" (5.35') - combustion chamber 12"

I'd appreciate it if someone could tell me if my math is correct, but i don't expect anyone too.
You math might be correct, or it might be wrong. Hard to tell since you didn't give the diameters of the chamber or barrel.
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