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Miscegenation: the hybrid piston hybrid.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Miscegenation: the hybrid piston hybrid.

Unread postAuthor: Daltonultra » Tue Mar 27, 2018 3:11 pm

Okay, so I've got the design for something truly strange sitting in my head here, and I wanted to run it past you guys to see if I'm completely nuts, or if this might work.

I'm picking up fittings next week to build a real monster of a coax pneumatic, and I realized that it would be very easy to change the design slightly and make it a coax piston hybrid. Then I had the thought that I could actually have the best of both worlds, and build a coax piston hybrid that can be fired pneumatically, just by adding a very small pilot valve.

My concept is a piston hybrid with a pop-off sealing piston at the back, like a regular piston hybrid, but with a small pilot valve installed in the side behind the piston. A tiny drop in pressure would be enough to start piloting, crack the rear seal, and allow the pilot chamber to exhaust the same as if it were popped by the combustion pressure-spike. Has anybody done anything like that before?
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Last edited by Daltonultra on Tue Mar 27, 2018 4:30 pm, edited 1 time in total.
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: Daltonultra » Tue Mar 27, 2018 4:06 pm

The other questions I have to ask, since the math makes my skull split: What volume of propane at what pressure would I need for 10x in a 10,179ci coax chamber at 120psi? Would 6" sch80 PVC(280psi rating) be strong enough for that mix? Would I be taking a chance on collapsing the barrel inside that chamber?(3" sch 40) Would firing pressures in the barrel evert a can of Ravioli?
Is DDT a huge risk in a chamber that size?
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: wdr0 » Wed Mar 28, 2018 4:59 pm

There’s absolutly no way you could run 10x in that set up, it would generate upwards of 1200 psi.
You might get by with 2x but pvc is also rather brittle and at that size I wouldn’t even try to go hybrid.
In my experience over 4” sch 80 pvc steel pipe is cheaper and can handle much higher pressure.
Only issue with the steel is the likely need for welding, although something I’d once considered for a solution is victaulic if you can find it.
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: Daltonultra » Thu Mar 29, 2018 3:17 pm

Well, so much for that idea, then. At least, a coax is completely out of the question in steel. If I want to try it, I'll have to use a T piston valve and run a smaller chamber size. Or wait months to set aside enough spare cash for the kind of pipe size I really want. 6" steel is expensive... Well, at least I can try out hte basic piston design in a 6" tee and see what 200psi through a 4" port will do...
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: wdr0 » Thu Mar 29, 2018 4:36 pm

As I mentioned I found steel to be cheaper than pvc. More specifically sch 40 steel cheaper than sch 80 pvc and will withstand the pressure needed for 10x. Roughly I think I bought 4” butt weld steel elbows for around $20. The sch 80 pvc elbows I believe were toward $40. These were new fittings from a piping supplier.i don’t see why coaxial is out with steel?

Not specifically saying you need to move away from pvc, just I don’t see how you are saving cost and it would allow you a lot more design freedom in the direction you want to go with steel.
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: Daltonultra » Fri Mar 30, 2018 10:54 pm

I'll have to shop around some, I guess. McMaster is showing steel prices higher than PVC. I'll check out my local supplier on Monday.
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: wdr0 » Sat Mar 31, 2018 1:44 pm

McMaster I found to be much higher on steel than a lot of places, their pvc is high but not quite as bad, ok for a couple fittings. But yeah, I would look for a commercial plumbing supplier, mine ordered both the schedules 80 pvc and steel fittings for me no problem.
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Re: Miscegenation: the hybrid piston hybrid.

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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: Daltonultra » Tue Apr 03, 2018 5:57 pm

Well, so much for ANYTHING... My damn Jeep fried the bearings in the rear axle. If it's just the bearings, it'll get warrantied, but if the ring and pinion is damaged, they'll say the gears caused the bearing failure, and I'm up crap creek without a dollar. And it needs a steering rack and a transfer chain either way, so no cannon for quite a while.
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Re: Miscegenation: the hybrid piston hybrid.

Unread postAuthor: GalFisk » Fri May 25, 2018 2:05 am

Too bad about the car.
My old 1" copper tube piston hybrid (actually the first ever piston hybrid, afaik) can fire pneumatically, and has in fact almost only been used as a pneumatic, because of fuel mix issues I never got the time to resolve. It's an over/under rather than coax though. Coax gets expensive quickly as you size up, and can be tricky to align. Definitely use metal for hybrids, several PVC hybrids blew up in the early days.
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