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Just going to ask all my questions at once

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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Just going to ask all my questions at once

Unread postAuthor: Pyro » Fri Dec 05, 2008 6:32 pm

Did some searcing on here and didn't exactly find all the answers I was looking for, so here goes.

#1 My potato gun uses a 6" diameter chamber and a 2 1/2" diameter barrel. The barrel is 32" long, how to I figure out the correct chamber size?

#2 Propane, MAP gas, or ether ethanol? Currently I use the ether ethanol (starting fluid) but the problem with this stuff is it likes to burn too long and doesn't mix in the chamber well. I'm considering switching mine over to propane, but am curious is MAP gas would be a better idea. Also considered acetylene/oxygen...not so sure on that one, hehehe!!!

#3 Fans. What's the real story here, do in chamber mixing fans really help? I'd imagine they would, but has anyone done comparisons to actually find out?

#4 And lastly, how do you actually know how much propane (for example) to use in the chamber? I've seen the metering systems use, but what is the correct way to figure out the correct mix?

Love the site guys, I'd appreciate any help here!

Joe
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Unread postAuthor: jrrdw » Fri Dec 05, 2008 6:52 pm

Wow Joe, every question you ask can be easly found here on the forums. We here at Spudfiles can tell the differance between some one who reads what they look at, and the one's that skim what they look at.

So where going to give you some more time to figure it out.

Welcome to Spudfiles.... :wink:
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When life gives you lemons...throw them back they suck!
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Unread postAuthor: Pyro » Fri Dec 05, 2008 6:58 pm

Haha, on other forums it's usually me saying that! I'm still digging around, kinda finding some answers..
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Re: Just going to ask all my questions at once

Unread postAuthor: Ragnarok » Fri Dec 05, 2008 7:01 pm

My suggestion to you is to go and read around Burnt Latke - everything you ever wanted to know about combustion cannons.
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Unread postAuthor: jrrdw » Fri Dec 05, 2008 7:02 pm

If you have a design in mind, post it up.
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Unread postAuthor: Hubb » Fri Dec 05, 2008 7:02 pm

1 - I'd assume you're asking about the volume? If so, you could search for the volume formula or, a more accurate way, you could fill the chamber with water and measure the water.

2 - Propane. MAPP is a bit more powerful, but nothing you would notice. Also, it costs more.

3 - Firing mine with the fan on and the fan off: There is a huge difference. Put the fan in the chamber.

4 - http://www.spudfiles.com/forums/propane-injection-math-t8017,lighter,0.html
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Unread postAuthor: Pyro » Fri Dec 05, 2008 7:15 pm

Yup, just figured out the chamber vs. barrel volume math, pretty interesting article on it in spud files.

Looks like propane really is the way to go then.

I'll do the fan mod!

Thanks for the link, checking it out now!


Thanks man!
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Unread postAuthor: nibbler125 » Fri Dec 05, 2008 7:20 pm

i would say 7 or 8 inch long combustion chamber would work fine
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Unread postAuthor: Pyro » Fri Dec 05, 2008 7:23 pm

Reason I'm doing all this is I already have a potato gun but I wanted to rebuild it a bit, try to make it better.

Right now it's way out of proportion, using a 6" diameter chamber that's around 1.5 feet long and a 2.5" diameter barrel that's 32" long. Looks like the chamber will be getting a big reduction in length and a much longer barrel will go on. Glad I save the piping from the original build!

Just for the heck of it I'll mention my means of ignition. It's a single farad capacitor, a Ford TFI ignition coil, an old Split Fire ignition wire, and a Champion spark plug that's a bit modified to work in it better.

It's heavy. I have pics if anyone is interested.
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Re: Just going to ask all my questions at once

Unread postAuthor: MrCrowley » Fri Dec 05, 2008 7:25 pm

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Unread postAuthor: starman » Fri Dec 05, 2008 9:18 pm

I second all the other comments. I did notice you mentioned a 2.5" barrel...that's too large for potatos, perfect for tennis balls. 32" for a barrel is going to be short for optimum performance. Also, 6" dia chamber is overkill for a handheld potato shooter...4" is plenty.

I'm assuming here of course that a potato is what you're wanting to shoot.
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Unread postAuthor: Pyro » Sun Dec 07, 2008 5:51 pm

Does it have to shoot potatos? haha

Also Starman, I see you're avatar is an Iowa class BB, are you a BB fan?
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Unread postAuthor: starman » Sun Dec 07, 2008 6:53 pm

Pyro wrote:Does it have to shoot potatos? haha

Also Starman, I see you're avatar is an Iowa class BB, are you a BB fan?
'

You betcha. I've toured the USS North Carolina in Wilmington and the USS Wisconsin in Norfolk Va. several times in my life...quite an amazing set of technology, especially considering the design was essentially a 1930s design. The analog computer system, the stereo optical range finding as well as later addition of radar ranging that was used for fire control would dazzle any but the most hardened of you.

You wouldn't want to be on the receiving end of one of those 16" rounds. They say the sound of an incoming round is absolutely horrifying as the mass alone of the projectile is roughly that of a VW beetle....all coming at you at about mach 2.5 (about 2700 fps... :shock: ). Then of course you deal with the explosive charge that accompanies.

Take a good look at one sometime...it's a thing of beauty.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 7:04 pm

starman wrote:They say the sound of an incoming round is absolutely horrifying... <snip> all coming at you at about mach 2.5

There seems to be a a disjoint here - you can't hear something coming at you if it's supersonic.
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Unread postAuthor: Pyro » Sun Dec 07, 2008 8:14 pm

Reason I asked was because I'm a huge BB fan, especially the Iowa's. I've visited the Missouri when it was brought to Pearl and the New Jersey just over the summer. Wish these beauties were still in comission! Hope they find a good home for Iowa soon...

So back to the potato guns. I decided I'm going to just build a whole new spud launcher using 4" pipe for the chamber and 2" pipe for the barrel, it'll be a dedicated spud launcher and I'll use my old one for whatever else I want to shoot. Going to use propane for propellant, put a fan int he chamber, and use a flashback transformer to create the ignition while using duel spark gaps. Nothing that hasn't been done 100 times now it seems!
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