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8ft 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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8ft spud gun

Unread postAuthor: nick753159 » Sun Feb 24, 2008 12:48 am

hey im working on my second potato cannon, first one was a basic combustion one, but it broke :(
now im trying to build number 2, same basic combustion system, but bigger!
now, keeping a 1.5 C:B ratio, would having a 4" x 20" chamber with a 1.5" x 84" barrel be dangerous because of the sheer size of the gun - over 8ft?? if not, how much bigger can i go?
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Unread postAuthor: elitesniper » Sun Feb 24, 2008 1:01 am

how did a basic combustion break?
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Re: 8ft spud gun

Unread postAuthor: Novacastrian » Sun Feb 24, 2008 1:21 am

nick753159 wrote:hey im working on my second potato cannon, first one was a basic combustion one, but it broke :(
now im trying to build number 2, same basic combustion system, but bigger!
now, keeping a 1.5 C:B ratio, would having a 4" x 20" chamber with a 1.5" x 84" barrel be dangerous because of the sheer size of the gun - over 8ft?? if not, how much bigger can i go?


Ah grasshopper, it is not the size of the wave that is to worry about, it is the motion of the the ocean that can sink vessels, not the sheer size of the vessel.
What you make combustion from? Any pics?
Also your c:b ratio makes no sense.
You can go as big as you want really, just make sure all of your fittings can withstand ~70psi and you'll be set for destruction!
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Unread postAuthor: nick753159 » Sun Feb 24, 2008 8:20 am

elitesniper wrote:how did a basic combustion break?

the barrel woudn't come unscrewed from the chamber, so i hit it against a wall to see if it would loosen it. bad idea. it snapped.
the combustion was basic hairspray and any other flammable aerosol i could get my hands on.
soryy i dont have any pics.

whats wrong with the C:B ratio? i thought 1.5 was the best one to have
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Unread postAuthor: Carlman » Sun Feb 24, 2008 8:30 am

nick753159 wrote:whats wrong with the C:B ratio? i thought 1.5 was the best one to have


0.8 : 1 is the optimum for spray and pray combustions
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Unread postAuthor: nick753159 » Sun Feb 24, 2008 8:48 am

ahhh thanks! burnt latke didnt mention that, or else i read too fast
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Unread postAuthor: TurboSuper » Sun Feb 24, 2008 10:04 am

Really? I thought 0.8:1 was for propane and 1.5:1 was better for aerosol ones.

Whatever, 1.5:1 makes a more satisfying kaboom, I say go for it :D

As for your question, size shouldn't make a difference in safety/reliability.
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Unread postAuthor: Carlman » Sun Feb 24, 2008 6:55 pm

TurboSuper wrote:Really? I thought 0.8:1 was for propane and 1.5:1 was better for aerosol ones.

Whatever, 1.5:1 makes a more satisfying kaboom, I say go for it :D

As for your question, size shouldn't make a difference in safety/reliability.


you may be more knoledgeable on combustions than me, im norm on the pnuematics side, so to the OP go with him ^^ lol
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Unread postAuthor: TurboSuper » Sun Feb 24, 2008 7:22 pm

Haha, I'm no expert either, I'm just regurgitating what was circulating around Spudtech back in the day.

Frankly, I doubt you'd notice the difference without any fancy chronographic equipment, so I say just go with what's louder.
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Unread postAuthor: STHORNE » Sun Feb 24, 2008 7:46 pm

like turbosuper said, "go for it!" lol

sounds like a nice idea, "upping" the scale, what do you plan on using for ammo and any future ideas of switching to propane?

good luck :D
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Unread postAuthor: nick753159 » Sun Feb 24, 2008 7:48 pm

yeah, well i built it today.
took a while to work out some problems but its really nice
im using potatoes right now, and id like to switch to propane sometime when i get some more money.

ill post the exact specs in the cannon forum
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Unread postAuthor: STHORNE » Sun Feb 24, 2008 7:52 pm

kewl, with a 1.5" barrel of that caliber, may i suggest using a heavier ammo?

suggestions:
*D batteries
*bouncy balls (the big ones)
*mini tennis balls (for dogs)
*or even make your own pvc rockets :D

--sthorne
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Unread postAuthor: Jared Haehnel » Mon Feb 25, 2008 8:40 am

Mine has a 2 "five foot barrel and a 4" 18 inch chamber which is probably closer to 20 with the fittings. Chamber fan and I use ether (a little goes a long way)

The ratio is pretty close to 1:5 to 1 and I have had excellent results for a combustion launcher. I haven't played around with C to B ratios too much but it has always made sense that barrel should be more volume then the chamber because of the expanding gases during combustion.

...but I've know to be wrong more then once too... if all else fails play around with it. Thats half the fun of building these things.
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Unread postAuthor: MaxuS the 2nd » Mon Feb 25, 2008 12:24 pm

Jared Haehnel wrote:t has always made sense that barrel should be more volume then the chamber because of the expanding gases during combustion.


You're right, but a barrel shouldn't be of much larger volume than the chamber because the pressure would decrease too much.
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Unread postAuthor: Jared Haehnel » Mon Feb 25, 2008 1:25 pm

Yeah the friction would slow it down. So it becomes a matter how much longer. However I'm afraid its going to a matter of trial and error... I don't think burnt lakes results apply in every case.

In interest of time and money 1.5 has always sounded reasonable to me...
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