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My First Spud-gun. Failsafe question?

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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My First Spud-gun. Failsafe question?

Unread postAuthor: MikeB » Fri Nov 02, 2007 5:47 am

Hello everyone

Built my first spud gun last week. I've been between jobs so I thought I best make good use of my free time. It was a sort of rush job didnt do any calcualtions we just wanted to see if we could make one work. The barrel is about 4 feet long, 68mm downpipe, the chamber is maybe 2 feet of 110mm pipe, ignition from a bbq lighter and fuel is Tesco value hairspray 33p a can, bargain. I'll put some photos up when I get them off my mates camera its not much to look at though, pretty basic


Basically the gun works, we started shooting socks in my garden to test it, then took it out to a field and shot spuds out of sight. Smashed a wooden pallet up and bent some 3mm steel. Then disaster, the end cap blew off obviously the glue wasnt good enough. Luckily I was shooting from the hip and it just shot out from under my arm. This made us realise, if that hadnt blown off, the whole thing could have exploded, so in short whats the best way to add a failsafe to the gun. We thought maybe just glue end-cap on badly again so that will blow off before the gun shatters, but surely theres a better way?

Cheers
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Unread postAuthor: TurboSuper » Fri Nov 02, 2007 5:54 am

A pop-off valve is the only reasonable way to do this. But your end cap shouldn't be able to blow off if it's built properly (and the cannon shouldn't explode either), mabye the ammo just jammed?
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 02, 2007 6:12 am

A pop-off valve is the only reasonable way to do this


I doubt it would have enough flow to alleviate the problem.

The only reasonable way to do this is glue it properly ;)
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Unread postAuthor: MikeB » Fri Nov 02, 2007 6:12 am

Well It must be cause it was badly built Either the glue I used was crap or my glueing was bad, probably my glueing. The ammo still shot out when it blew off so I guess it wasnt that stuck.

Just a bit concerned reading all this about PVC pipe shattering and stuff, dont want to lose an eye or something.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 02, 2007 6:38 am

Pressure rated PVC should not shatter, and in any case, even if it wasn't pressure rated it should be enough to contain the pressures generated by a combustion.

Have a look here for a good how-to on gluing PVC.
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Unread postAuthor: Ragnarok » Fri Nov 02, 2007 7:47 am

It sounds like we probably have a UK user here, which may mean that the pipe is unrated. Rated PVC is rarer than hen's teeth in the UK.

The cannon exploding is improbable, as long as the friction in the barrel is less than the strength of the walls.
Still, I would recommend building a cannon from properly rated materials. Rated PVC is hard to find without looking on expensive internet sites, so many UK members use copper pipe.

Shamelessly advertising my own work, it is possible to create some impressive, yet safe (provided you are the right side of the muzzle) pneumatic launchers from copper, although most new members are put off by the high price tag of a good one.

I also use goggles and ear defenders when firing spudguns, even with my metal launchers.

If you have any UK specific questions relating to materials or such, drop in at UKSGC.
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Unread postAuthor: MaxuS » Fri Nov 02, 2007 1:56 pm

I disagree, you can buy a decent sized length of 10 bar rated pvc (110mm, 90mm,75mm...) for quite a reasonable price from hendersons or if you look into it you can probably find it cheaper. Making a large combustion cannon is alot easier and alot less expensive to build than a high pressure copper gun. Not to mention that if you wanted to fire large things, youre screwed if you want to use copper.
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Unread postAuthor: BigGrib » Fri Nov 02, 2007 2:41 pm

All I know is that youshouldn't be excited about the fact only the endcap blew off, rated or not if you're just shooting a spud you wont be building pressures high enough to shatter your non pressure rated pvc. take solace in the fact that noone was standing behing you and didn't get hurt, next time use a good solvent and a good glue, I prefer Christy's Red Hot Blue Glue but thats just me.
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Unread postAuthor: Ragnarok » Fri Nov 02, 2007 4:13 pm

MaxuS wrote:I disagree, you can buy a decent sized length of 10 bar rated pvc (110mm, 90mm,75mm...) for quite a reasonable price from hendersons or if you look into it you can probably find it cheaper.

Yes, but not everyone is fortunate enough to live near a Hendersons.

The word "Downpipe" is not usually an indicator of pressure rating.
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Unread postAuthor: clide » Fri Nov 02, 2007 4:23 pm

A flying end cap is very dangerous. In fact the only death I have heard of from a standard combustion with no dangerous fuels is from an end cap coming off and striking a man in the chest. It stopped his heart and killed him.
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Unread postAuthor: MikeB » Fri Nov 02, 2007 6:45 pm

thanks for advice. Im 99.9% sure the pipe is not pressure rated since its just wastepipe and downpipe why would it be. The wastepipe might be since it goes underground, dunno though I wouldnt bet on it anyway

The glue could be the problem, for glueing anything I usually swear by Araldite. It held my old car together (seriously, cracked sump and radiator both glued up with it). Couldnt find any when I was shopping so I asked the guy at the motor factors what else he had and he said the stuff I have is almost as good. Put more of it on this time so it should hold now

Out of interest whats the stance on greasing.oiling the barrel? Is this a done thing or does it just get sticky and make the situation worse, or catch fire? Just load an oily rag in there and drop it out of the bottom or something
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Unread postAuthor: ShowNoMercy » Fri Nov 02, 2007 6:50 pm

Depends on the type of oil and the temperature. Ever try pumping out a big gas engine in the cold? The oil could pass for glue, so I guess if your running at hot temperatures then try it, and I don't think oil is too flammable, combustible maybe.
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Unread postAuthor: Ragnarok » Fri Nov 02, 2007 7:37 pm

MikeB wrote:Out of interest whats the stance on greasing.oiling the barrel? Is this a done thing or does it just get sticky and make the situation worse, or catch fire? Just load an oily rag in there and drop it out of the bottom or something


That's much debated. Some people will say that some types of oil can damage PVC (although against a test with petrol, there was no observable change in the pipe's dimensions or strength), others say that reducing friction will add a reasonable velocity.

I have little personal experience with PVC, so I can't tell you much other than that I clean and lubricate my copper barrels occasionally, because I find that sometimes deposits build up in the barrel. And it can only help performance.
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Unread postAuthor: Novacastrian » Fri Nov 02, 2007 7:41 pm

Apparantly personal lubes such as K-Y can be used to lube your barrel, without the risk of damage to the PVC.
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Unread postAuthor: TurboSuper » Fri Nov 02, 2007 10:22 pm

Erm, to glue the cannon together you should only be using specialized PVC cement/primer (or ABS, depending on what you're working with). PVC cement is not an adhesive- it chemically melts the two pipes and resolidifies them as one pipe, ensuring an extremely strong bond.
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