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Do I need to mod my spud gun?

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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Do I need to mod my spud gun?

Unread postAuthor: garrettee » Thu Dec 28, 2006 8:09 am

Ok guys, I built a 4' by 1 1\2'' barrel and 2' by 3'' chamber combustable (soon to be stun gun ignition) spud gun. I live in a neighborhood, so I stuffed an old rag down the barrel , sprayed some static guard in the chamber, and used an old cap from an old gun of mine that has a lantern flint in it to fire. It didn't have the kick I expected. The max amount of static guard I could spray was about 4 to 5 sec. worth before it choked out. The rag was packed pretty tight but I thought I maybe could be losing pressure. My other concerns are the lenth of the barrel (because it's too long), or bad chamber size.I'm open to any and all help you all can provide.
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Unread postAuthor: garrettee » Thu Dec 28, 2006 8:11 am

Oh yeah I almost forgot to ask. Will the stun gun ignition make that much difference?
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Unread postAuthor: experament-u2 » Thu Dec 28, 2006 8:18 am

it probly will make it more reliable
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Unread postAuthor: medievalman » Thu Dec 28, 2006 9:02 am

the best c:b ratio is around 0.8:1, while the c:b ratio on your gun is 1:1. the barrel is not to long, in fact you could make it a couple of inches longer, but it wouldnt improve preformance enough to be worth it.

the stun gun will definetly make it more reliable, and it will probably increase preformaqnce a bit too, because the principle of combustion spud guns states that the more violent the explosion, the more pressure, and the farther the spud will go. with a stun gun, you will probably be able to create a bigger spark which will create a more violent and instantaneous explosion. it should increase preformance a lttle, but you might not be able to notice it. the biggest difference it will make is reliability.

another suggestion i have is to use propane for fuel. it will make your gun alot more powerful and reliable if you use metered propane, instead of aerosol propellants, because it burns much faster, again resulting in a more violent explosion, you always get the perfect amount of fuel as there is no human variable, and since it is 100% flammable, unlike aerosol fuels, it wont gunk up your chamber no matter how much you use it.

my final suggestion is to add a chamber fan which will mix the fuel and air so that it is perfect for burning, and it will also expel the gases after you shoot. you could also use a chamber chain or throw in a bunch of pennies and you just shake the gun to mix the fuel. but this method doesnt help expel gases after you shoot, it only helps mix them.

i hope you find this information useful.

and please dont double post, use the edit button. thank you.
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Unread postAuthor: Bluetooth » Thu Dec 28, 2006 9:38 am

Don't give him false information :x . The best c:b ratio is 1.5:1. To make sure of this go here http://www.burntlatke.com/ratcalc.html . If your going to use metered propane study on how it works in the wiki or here http://www.burntlatke.com/lp.html .
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Unread postAuthor: medievalman » Thu Dec 28, 2006 9:44 am

really? i am more of a pneumatic guy, but i have heard that the best c:b ratio for combustion is 0.8:1. im really sorry about that. then your barrel is a lttle too long, i would cut off about 6 inches-12 inches, but if you want you could just leave it. once again, i apologize for my mistake.
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Unread postAuthor: Bluetooth » Thu Dec 28, 2006 9:46 am

Just enter it into the link to ratcalc I gave you and keep changing the length of the barrel until you get 1.5:1 and then cut it off that much.
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Unread postAuthor: frankrede » Thu Dec 28, 2006 12:50 pm

1.5:1 in fact isn't a good ratio, its an ok ratio but not the best so before you go and be a smarty about it make sure you know what your talking about.
http://www.geocities.com/man_o_brass2/main_page.html
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Unread postAuthor: boilingleadbath » Thu Dec 28, 2006 1:16 pm

1) His current ratio is about 1.8:1 (on account of 1.5" pipe being 1.59")
I'm surprised you people don't remember that the ratio goes by volume, and thusly, that you have to <b>square</b> the diameter of the pipe.

2) Sometimes I wish that burnt latke had never done those 'optimum ratio' tests; people get alot of false notions because of them.
The most <i>efficient</i> ratio is about .8:1

However, efficiency isn't really our goal. (well, most of the time - efficient cannons are quiet - which is sometimes desireable)
Rather, our goal is to produce the most power in a certain sized package.

A bit of fiddling with evbec shows that a .85:1 (chamber <i>length</i>):(barrel <i>length</i>) is best in this regard... assuming that combustion takes the same time in both chambers.
Which it doesn't. (so that's a fairly useless number)
It's worth noting that preformance is fairly intact out to a .3:1 length ratio. (at which - assuming equal combustion times - it's 10:11<sup>ths</sup> of the .85:1 number... which means it's acctualy better than that.)

In other words, your chamber is fine - although it could be a bit shorter. (say, 1.5 feet instead of 2)

3) Fueling is most likely our culprit.
As a widely accepted (and proven - check burntlatke.com) fact, preformance is lower when there is too much fuel in the chamber. You stated that you could inject 4-5 seconds of propellent before it "choked out" (which I'm assuming means "stopped working").
I'd try cutting the amount of fuel injected precisly in half (the upper flamability limmit of most fuels is about twice the ideal amount). In other words, spray 2 to 2.5 seconds of fuel.

************
This isn't to say that a chamber fan wouldn't help - it would, immensly.
Nor is this saying that a stungun or multiple ignition points wouldn't help - they would.
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Unread postAuthor: garrettee » Thu Dec 28, 2006 11:32 pm

boilingleadbath and everone else, if my chamber could be shorter , would my performance improve if I shortened it , and would I save propellant by doing this ?
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Unread postAuthor: Insomniac » Fri Dec 29, 2006 12:01 am

Shortening your chamber won't help performance. It will make the gun quieter though. The way you have it now, the explosion shoots the spud, but all the energy isn't used up. So when it comes out the barrel, the gas expands then collapses on itself as the gas cools, making a loud BOOM!
If you shortened the chamber, almost all the gas would have stopped expanding by the time the spud left the barrel, and there would be much less noise.

And yes you would save a tiny tiny tiny bit of propellent, but it unless the fuel you are using is really expenseive, it shouldn't really make that much difference.
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