Double barrel combustion gun

Post questions and info about combustion (flammable vapor) powered launchers here. This includes discussion about fuels, ratios, ignition systems and anything else relevant to launchers powered by igniting things like hairspray or propane.

Postby mafia_mike » Wed Feb 07, 2007 2:40 pm

One reason why these don't work is because the projectiles wont be exactly the same (assuming its potatos or whatever), causing one of them to leave one of the barrels first releasing all the pressure so that it doesnt push out the other one either not as far or not at all. OR if you are planning on making your own projectiles that are equal in weight, size etc. then it might work. I guess if you take that route then we could discuss what AMMO would be apropriate (sorry, spelling) for this application.
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Postby sgort87 » Wed Feb 07, 2007 4:20 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by An Apple Pie
[br]Sgort, Weren't those tests done with propane. I have always heard .6-.8 for propane and around 1 for aerosol.
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">You're right! I have once again made an ass of myself through assumption. [:p] He did not say what his fuel was (as far as I remember) and so I assumed propane just because it's what I'm used to.

In the case of aerosol, there are no real numbers to my knowledge on specific ratio performance. It's just common to assume the popular "ideal ratio" of 1.5:1. So I guess his ratio would be a bit off if he went down the spray'n'pray path.
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Postby boilingleadbath » Wed Feb 07, 2007 5:12 pm

gort, there's alot of noise in the data, so it's not accurate to just take the maxium velocity of any specific ratio... my fitted curves (of type sqrt(CX/X+1 - CX) where the Cs are varius constants) of the latke data show maxiums at about .8:1 (C:B) for all ratios... maybe a bit longer, at .75:1, for the .75" data.

Of course, we are still left to wonder if my fitted curves are any good - but I expect they are better than latke's data for this application.
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Postby spuzi14 » Wed Feb 07, 2007 9:36 pm

1.5:1 all the way,at least for aerosols! It's loud and powerful, sort of like AC/DC?

Anyway, I personally say get a valve that directs flow from one barrel to the other and have a good attachment method for each barrel (camlocks?) so you can fire one and have a friend load the other barrel then he could fire that while you're loading your barrel. It'd be a good idea to hook this puppy up to a monster propane tank though...

Overall, have fun!
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Postby hi » Thu Feb 08, 2007 10:13 pm

why dont you make a huge chamber and attack 3 or 4 tees to it and have 3 or 4 barrels, thats what i would do. i haven't the slighest clue wheather or not it would work, but if it did it would be really cool. if you could make a stock for it you could shoot it like a gun, but there would be a huge recoil. just a thought.
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Postby Velocity » Thu Feb 08, 2007 10:26 pm

This could be more easily done with a pneumatic, and it would probably benefit from the extra power...
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Postby Numbuh 16 » Fri Feb 09, 2007 12:56 pm

Again, I interject.

Dude, what? This would be TOTALLY pointless, therin I see your reason for building this. Remember to add BOTH barrels volume into the .8/1 ratio. With two normal sized barrels (4" approx) that would take one big ass chamber.
Numbuh 16
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Postby Iowaengineer » Fri Feb 09, 2007 9:03 pm

Here's a little plan that might make this design work
<img src="">
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Postby boilingleadbath » Sun Feb 11, 2007 1:38 am

I find it interesting that if we assume that:
1) That two potatos in 1.5" barrels vary in mass by an average of 18% (seems reasonable, given the latke data)
2) That the projectiles start moving at the same time
3) That the speed of sound in the propellent gasses is 3000 fps (it's probably slower by the time the projectile is at the end of the barrel), that your barrels are 4 feet long, and that your (slower) projectile is going 400 fps

Then the first projectile to leave would exit some 10% sooner (on average) - or roughly 1 millisecond.
And that the "rarefaction 'wave'" (the pressure drop) would travel the 8 feet to the rear of the other projectile in about 2.6 ms (because it'd propogate at the speed of sound) - and before it arrives, the pressure at the rear of the projectile is exactly the same as if the other projectile was still in the barrel.

You might think of it as something along the lines of "the other gasses havn't heard the news yet, so they keep doing their thing".
Nothing really new - it's what limits the velocity of sound.

My suspicion is that, because 1 ms < 2.6 ms, the fact that one potato leaves the barrel first doesn't matter.

...given that the projectiles start moving at (or nearly at) the same time.
I'd install a burstdisk to increase the chance that they do.
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