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Mjöllnir - 1.5" porting Handheld Piston Hybrid

Built a hybrid cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Technician1002 » Sun Nov 28, 2010 11:01 am

Moonbogg wrote:If the pressure is directing force straight back, what would cause pipe flexing? Hope i'm not getting too inquisitive here.


Some barrel flexing is influenced by fluid dynamics and chaos. This is the same thing that makes a flag wave in a steady breeze flowing in one direction and smoke curl as heat rises in still air.

At high energies, this needs to be supported.
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Unread postAuthor: SpudFarm » Sun Nov 28, 2010 2:53 pm

My opinion is as simple as it gets, that barrel is dangarous.

We have seen sch80 hybrids fail on low mixes after repeated shock from the combustion.

Get a aluminum barrel!
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Unread postAuthor: MrCrowley » Sun Nov 28, 2010 6:43 pm

We have seen sch80 hybrids fail on low mixes after repeated shock from the combustion

The barrel isn't Sch80, it's PN12 ABS, so it is rated to 12 bar. I can only recall DWV ABS failing before and I think DYI may have even used DWV ABS as a barrel on his 20x hybrid, i'm not 100% on that though.
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Unread postAuthor: SpudFarm » Sun Nov 28, 2010 7:30 pm

The barrel isn't Sch80, it's PN12 ABS, so it is rated to 12 bar. I can only recall DWV ABS failing before and I think DYI may have even used DWV ABS as a barrel on his 20x hybrid, i'm not 100% on that though.


I am completely sure I have seen a 4" chamber constructed out of SCH80 PVC fail drasticly at a 4x mix. And those pipes are rated for 13.3 bar..
Even if it is ABS I would absolutely not trust it in a 10x hybrid, do you really think it is more then TEN times stronger then the DWV pipe that's barely accepted in a combustion?

DYI used IIRC a 1.5" ABS pipe cast in a 3" diameter epoxy reinforcement for his 20x shots and guess what, the barrel got destroyed ;)

My opinion is still as clear as the midday sun in your eyes on ecuador :)
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Unread postAuthor: MrCrowley » Sun Nov 28, 2010 7:33 pm

ABS can handle the temperature and shock a lot better than PVC. My ABS barrel is actually 2mm thicker in wall thickness than PN15 PVC as well. I would think the barrel would only crack if it failed anyway, so I think it is safe in that regard.

I certainly would never use it as a chamber though :D

But yeah, the barrel is my main concern for firing a 300g cannon ball from the cannon :P
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Unread postAuthor: SpudFarm » Sun Nov 28, 2010 8:01 pm

ABS can handle the temperature and shock a lot better than PVC


I see, that's why DYI had his barrel melt? :P

I am not trying to be a jerk, just a piece of advice :)
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Unread postAuthor: MrCrowley » Sun Nov 28, 2010 8:09 pm

Hahaha I didn't read that bit :P

I am not trying to be a jerk, just a piece of advice

Yeah I hoped you weren't taking it personally, Jeg har fortsatt å møte søsteren din ennå :D

To be fair, he was running 20x mixes. Though, I have noticed the breech of my barrel has melted very slightly at where it glues in to the threaded adapter.
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Unread postAuthor: Technician1002 » Sun Nov 28, 2010 8:18 pm

In testing ABS and PVC, I have found that PVC becomes soft like a garden hose. If heated much beyond that, it scorches and burns.

ABS on the other hand does turn to liquid and melts.

I can soften and shape PVC. This is not true for ABS.

Photo of a test bend in PVC.
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Unread postAuthor: DYI » Sun Nov 28, 2010 11:24 pm

To be fair, he was running 20x mixes. Though, I have noticed the breech of my barrel has melted very slightly at where it glues in to the threaded adapter.


Actually, I was only running a 10X mix when the barrel melted, if I recall correctly. However, I was using a 4" diameter, 2.5L chamber. There had never been a problem when using the "normal" chamber, which was 2" diameter and a litre or so. Softening won't be an issue here. Any "reasonably" dimensioned pipe will explode in these conditions long before it melts as badly as mine did (remember, mine's was 1.6" ID, with 1" walls composed mostly of resin bonded glass fibres). As to repeated damage from the heating cycles, I can't offer a useful opinion there; I never ran a barrel through enough cycles to notice those effects.

I'm well acquainted with your problem in barrel choice, Crowley. It was what drove me to that reinforced plastic barrel in the first place, which ended up disappearing in a $200 puff of black smoke when I increased the chamber volume. There's no easy solution for most of us without ordering online. The best I can do is recommend a fibreglass reinforced DWV copper tube barrel (the tube itself is generally rated to 200-300psi). Weight concerns would probably prevent you from using a seamless steel pipe barrel like I did on the CGG-50, even if you could acquire one.

I've also pushed one ABS barrel to the point that it failed from overpressure alone. As one would expect, it did not shatter or throw fragments. It would, however, have caused significant injury to anyone standing near the breech, much less holding onto it while it was fired. Think of the effects of removing the barrel, and holding your arm in front of the burst disk while pulling the trigger - that's essentially what vents out the rip in a failing barrel.
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Unread postAuthor: MrCrowley » Sun Nov 28, 2010 11:53 pm

Hmm now I'm a little more worried, especially if I added that extra 40cm to increase the chamber volume to almost 2.0L. As you've said, seamless steel pipe is a bit difficult to come by, it was my first choice but I never found anything suitable. I'm not too worried about the weight really. I think the most difficult part would be attaching it to the cannon, I would either need to get it threaded or weld it to a threaded adapter.

What about if I sleeved the ABS barrel with a steel/aluminium barrel? I know PN12 PVC would sleeve perfectly but that is definitely not suitable.
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Unread postAuthor: DYI » Mon Nov 29, 2010 12:40 am

DOM tubing can be had in the same external diameter as plumbing pipe here, but I don't know about the situation in NZ. I was fortunate enough in the case of the CGG-50 to find a brand new 20' section of 2" SCH40 seamless steel pipe at the welder's shop, but that was sheer luck. If you can get appropriately dimensioned DOM, major hardware stores should be able to thread it (with a great deal of trial and error, no doubt :roll: ). Aluminum would also be nice, just keep in mind that DOM isn't actually intended for use in pressure vessels in most cases. Leave a generous safety factor and do some higher pressure remote testing if you do end up using a non-rated barrel.

Hydraulic tubing may be another option, if you live near an "industrial" city. It's generally seamless, and makes an excellent barrel material for "medium-pressure" launchers (i.e., launchers where the barrel doesn't have to be custom gundrilled out of bar stock... ).

Sleeving the ABS in steel or aluminum would help the user safety issue, but it doesn't address the "boiling barrel" problem I ran into. However, since you're not using a large diameter chamber or a valve with the flow of a 2" burst disk, you may not have to worry about that.
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Unread postAuthor: Matt_NZ » Mon Nov 29, 2010 12:50 am

I don't know about Auckland, but down south a workshop in town wanted about $50NZ to thread a 2" stainless steel pipe. This would probably be more than you would like to pay.
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Unread postAuthor: Moonbogg » Mon Nov 29, 2010 1:36 am

Crowley, if you are able to order online, you can find threaded aluminum pipe on McMaster that should be a direct replacement of your current barrel. If you can't order online, give a friend some cash and have them order for you.
If its any help at all, my 2x cannon uses a barrel with only .065" (1.65mm) wall thickness. Thats super thin and it hasen't budged yet. Shots have been taken at 3x with it as well. If you use a SCH40 aluminum barrel, you should be good to go.
One more thing though, and I probably shouldn't judge here since I don't use them myself. But with your barrel situation I am very concerned about your lack of safety glasses.
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Unread postAuthor: DYI » Mon Nov 29, 2010 10:21 am

@Matt_NZ: Major hardware stores can generally thread up to 2" NPT (they may say 1.25", but the thread pitch is the same for everything from there up to 2"), and they tend to do it for less than $10. Still, $50 is not a significant cost considering the benefits if that's all that's available.

@Moonbogg: McMaster may accept international orders, but they will not ship them. You'll get an email message a week or two later saying that they don't do exports, if they still operate like they did when I tried to order from them. The best work-around is to have a contact in the US buy the parts and ship them to you, assuming that they can't be acquired elsewhere.

Considering the use of up to 10X here, SCH40 aluminum would be somewhat worrisome. It'd likely hold in the case of a jammed projectile, but I certainly wouldn't put my face near it.
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Unread postAuthor: Fnord » Mon Nov 29, 2010 1:42 pm

Hey Crowley,

Can you get steel thinwall electrical conduit pipe where you live? Unfortunately 1.5" steel has a seam and is too small for golf balls, but you should be able to sleeve some abs inside a section of 2".
You may have trouble connecting it to the breech though.

I'd like to back up DYI here in saying you really don't want to get hit with hybrid muzzle flash. I was screwing around blank firing 5x indoors, and one time I shot at a popcan from a few inches away. The pop can was obviously deformed considerably, but what I didn't expect was the placemat under it being torn apart. It wasn't even in the path of the muzzle flash and it was missing a good 9" circle after that.
It had two layers of fairly tough stretchy plastic that was probably a good ballistic subsitute for human skin.
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