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Need advice on a new build

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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Need advice on a new build

Unread postAuthor: ski0724 » Tue Jul 11, 2017 8:11 am

I'm about to build my second cannon and was hoping some of you veterans would offer some advice and/or stop me from ending up with a bunch of shrapnel in my head.

Design is fairly simple (I think)-
Expansion chamber is 4" x 15" Sch40 PVC
Barrel is 3" x 30" Sch40 PVC
12v fan
5 Point pulse grill igniter

The 3" barrel will be shrouded within a 4" section of cell core pvc to give the entire gun a uniform diameter. I'm trying to mock up an AT-4.

Any thoughts would be greatly appreciated.

Thanks!
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Re: Need advice on a new build

Unread postAuthor: Cthulhu » Wed Jul 12, 2017 3:56 am

It should work just fine, but you could boost the power by trying to get the combustion chamber to barrel ratio closer to 1.5:1

Just remember, a measured amount of a gaseous propellant (propane, butane, methane) will always be 100x better than just spraying in hairspray.

Other than those bits of advice, you're good to go! Be sure to post pictures of the build!
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Re: Need advice on a new build

Unread postAuthor: jackssmirkingrevenge » Thu Jul 13, 2017 3:51 am

Cthulhu wrote:Just remember, a measured amount of a gaseous propellant (propane, butane, methane) will always be 100x better than just spraying in hairspray.


Hear hear!

Also, a couple of holes at the end of the barrel allowing gas to bleed into the shroud would absorb some noise, especially if you packed some heavy gauge steel wool swabbed with lithium grease... if one were so inclined.
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Re: Need advice on a new build

Unread postAuthor: ski0724 » Fri Jul 14, 2017 8:30 am

Duly noted, although I'm new enough at this to still consider the noise part of the attraction. Will post some pics and firing data after the weekend.

Thanks guys!
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Re: Need advice on a new build

Unread postAuthor: ski0724 » Fri Jul 14, 2017 12:23 pm

20170714_121828_crop_876x291.jpg
A pic
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Re: Need advice on a new build

Unread postAuthor: ski0724 » Sat Jul 15, 2017 5:34 am

Regarding the 1:5:1 chamber/barrel ratio-

If I understand correctly (and can do math) a 10"x4" chamber with a 30"x2" barrel would be 1:5:1, correct?

Also, any suggestion as to what I might fire from a 3" barrel? I was considering small grapefruit or oranges. Potatoes obviously, if I can find some with adequate size.
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Re: Need advice on a new build

Unread postAuthor: jrrdw » Sun Jul 16, 2017 6:01 am

ski0724 wrote:any suggestion as to what I might fire from a 3" barrel? I was considering small grapefruit or oranges. Potatoes obviously, if I can find some with adequate size.


Anything you can fit down the barrel... :bigsmurf:
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Re: Need advice on a new build

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Re: Need advice on a new build

Unread postAuthor: Cthulhu » Sun Jul 16, 2017 6:56 am

ski0724 wrote:Also, any suggestion as to what I might fire from a 3" barrel? I was considering small grapefruit or oranges. Potatoes obviously, if I can find some with adequate size.


You can put almost anything down the barrel with some paper towels acting as a wad.

Personally, I like to shoot Easter eggs filled with flour for a nice poof of smoke when they hit the ground. Maybe shove a bunch of them down the barrel wrapped in a paper towel?
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Re: Need advice on a new build

Unread postAuthor: jackssmirkingrevenge » Sun Jul 16, 2017 7:53 am

Looks good!

For a 3" barrel, an empty soup can wrapped with tape to built it up to the correct diameter makes a great sabot, light yet strong:

Image

This will allow you to launch virtually anything that fits in the barrel - even irregularly shaped objects like for example your little brother's favorite He-Man action figure - without losing any power from blow-by.
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Re: Need advice on a new build

Unread postAuthor: ski0724 » Mon Jul 17, 2017 12:49 am

Thanks to all for your help. After the first couple shots, I had already redesigned the gun, so I made a gift of it to a friend; I've been trying to generate some local intetest in pumpkin chucking and it seemed like a good way to start.

The new gun will be essentially the same, only with a 2" barrel, a 5 point electric pulse ignitor, and a closer eye toward the 1:5:1 chamber/barrel ratio. Also: no more oranges- they fly great but the juice makes a horrific mess when you stuff them down the barrel.

A final question - would the addition of burst discs be a significant benefit to this design? I'm not quite confident enough of my abilities yet to attempt a metered propane gun yet.

Thanks to all!

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Re: Need advice on a new build

Unread postAuthor: jackssmirkingrevenge » Thu Jul 20, 2017 4:54 am

ski0724 wrote:A final question - would the addition of burst discs be a significant benefit to this design? I'm not quite confident enough of my abilities yet to attempt a metered propane gun yet.


Good stuff! For this kind of launcher, a burst disc won't have a huge effect. You can meter propane with only a large syringe and some basic math, no need to make a complex manometric setup. Give it a go, it is the best way to optimize performance:

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