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Potato Cannon Barrel Too Short?

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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Unread postAuthor: trigun » Mon Feb 16, 2009 11:43 pm

Yea, that can all be done later
Right now it just won't get the max amount of power. Also might be a bit louder than usual.
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Unread postAuthor: tape fist » Mon Feb 16, 2009 11:45 pm

yes but when dealing with combustion you really want to use primer because PVC shard don't show up on an X-ray.
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Unread postAuthor: trigun » Mon Feb 16, 2009 11:47 pm

Actually if he didn't glue it correctly it will simply blow the ends off.
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Unread postAuthor: tape fist » Tue Feb 17, 2009 12:03 am

That's like saying if you don't put oil in your car its just going to break down but you are right the ends will just come off.no fair i want to be a staff sergeant. :)
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Unread postAuthor: trigun » Tue Feb 17, 2009 12:07 am

Well if he has any more ? I would be happy to answer them because he actually had enough commen sense to realize what he did wrong.
Yea not that parts blowing off is that much better than shards. It is probally a spray and pray so I would assume it doesn't generate enough psi to blow 3" parts off.
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Unread postAuthor: GeekFreak » Tue Feb 17, 2009 12:27 am

To keep the lantern lighter (my lighting method) in place and to seal it, could I use a silicone based sealant that is made for plumbing?
P.S. Thanks so much for all the help (except the part about PVC not showing up on X-rays... thats scares me...)
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Unread postAuthor: trigun » Tue Feb 17, 2009 12:38 am

Silicone will work I use it to secure my fans in my advanced combustions.
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Unread postAuthor: GeekFreak » Tue Feb 17, 2009 12:50 am

Would it help to seal all the attachments with the silicone stuff?
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Unread postAuthor: tape fist » Tue Feb 17, 2009 1:13 am

no i think you will be fine for know just take all the advice your getting an apply it to the next cannon your going to build.Any chance for some pictures.
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Unread postAuthor: GeekFreak » Tue Feb 17, 2009 1:30 am

I'm trying to find my camera. When I do I'll post some pics!
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Unread postAuthor: jimmy101 » Tue Feb 17, 2009 2:09 pm

Folks have implied, but not really said ...

The length of the barrel will really have no affect on the chances of the gun blowing up. If anything a grossly too short barrel will limit the maximum pressure the gun will reach to a pretty small value, compared with a longer barrel.

So, go ahead and fire it with the short barrel. Then, as others have posted, chop off most of the barrel and add an adaptor. Personaly, I would add a threaded adapter to you can use different length and diameter barrels, each with their own threaded adaptor.

2" diameter barrels are pretty standard for shooting spuds from a combustion gun. A 1.5" diameter barrel will make it much easier (and cheaper) to get big enough spuds since the maximum diameter of the spud needs to be bigger than the barrel diameter. 2" diameter spuds are harder to find and more expensive than smaller 1.5" ones. You'll get more energy into the larger diameter spud but for a first gun I wouldn't worry too much about that.

It is unfortuante that you didn't use primer or cleaner. My feeling is that you are probably OK as long as you put copious amounts of glue onto both pieces being joined and you got the pipe fully seated into the fittings. Standard (or advanced) combustion guns can't generate more than about 120 PSIG even if the round jams in the barrel. In normal operation, combustion spud guns rarely exceeds about 60 or 70 PSIG.

Next trip to the hardware store (for the new pipe and fittings) you should get a can of purple primer (or cleaner/primer).

If you are going to paint the gun you need to scrub it clean before painting. Paint won't stick to PVC very well. Add some dust and oil from sitting in the hardware store for a year or so and spray paint won't stick worth a damn. Take a scrubing pad and liquid dishsoap and give the cannon a good scrubing. Let it dry then spray paint. Paint will take several days to fully cure on PVC so be gentle with the gun for a few days to keep from flaking the paint off.

If your ignitor is threaded (I believe most are) you can just drill a slightly undersized hole in the PVC then let the sparker thread the hole as you screw it in. If done correctly you shouldn't need any sealant. If you really want to seal it, and not increase the mechanical strength, then you can use pretty much anything, even hot melt glue.

Other things....
Did you sharpen the muzzle?
What fuel will you use?
How will you measure the fuel?
Did you install a chamber fan?

Don't even joke about pointing this thing at a person. A combustion gun this size will launch a spud with 2 to 3 times as much kinetic energy as a 0.22 rifle. (100 gram spud at 300 FPS is 500J, 0.22's are about 150 to 200J)
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Unread postAuthor: GeekFreak » Wed Feb 18, 2009 1:23 am

Ok, I can't find batteries anywhere for my camera, so no pics for awhile... I did a test shot with the cannon on the ground with 1 of my feet on it to make sure nothing would pop off, and it was fine! But, after that, my lantern lighter broke (sort of) so I might look for a new ignition source. I did sharpen the muzzle and I'm using hairspray as fuel. Considering the fact that I'm using hairspray, I'm not going to measure the fuel or put in a chamber fan. I'm probably only going to do about 1 shot per couple minutes.
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Unread postAuthor: GeekFreak » Fri Feb 20, 2009 3:01 pm

Well I put on a 3 foot barrel and tested it yesterday. I got 2 or 3 shots out of it, when the back of the chamber blew of, hit my arm, they continued into some bushes maybe 25 feet away... lol. It wasn't damaged, so I just cemented it back on and tested it again. This time the chamber cap just shattered... again, lol. So I got a heavy duty cap and then got about 8 shots off before it got too dark. The only thing is that it went furthest the first couple shots. I think it is because the holes that I drilled for the sparkers aren't sealed (when I fire, I see smoke and my friend said he saw fire). I wonder if that is the cause? Any ideas for quick fixes to make it shoot them further? On my last couple shots, the potato only went maybe 100-175 feet...
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Unread postAuthor: jimmy101 » Fri Feb 20, 2009 5:04 pm

Two failures in a dozen or so firings....

Time to chuck that POS into the trash and start again. A basic combustion should never fail.

Get PVC primer (or cleaner/primer) and PVC glue.

Read the Wiki on the proper way to glue PVC.
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