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Help with fueling setup

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: Crna Legija » Tue Dec 07, 2010 9:13 am

did you look at the pic its steel
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Unread postAuthor: cfb_rolley » Tue Dec 07, 2010 2:26 pm

skyjive wrote:Just out of curiosity, how high a mix is it safe to use sch40 PVC with? I assume its easily safe for 1X.


Just sayin'...
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Unread postAuthor: saefroch » Tue Dec 07, 2010 6:28 pm

cfb_rolley wrote:pressure rated PVC is always better in my opinion because you know what it can take.
You can look up pressure rating values for steel pipe, the pressure rating is very high compared to PVC, and the failure mode is far preferable.

In answer to the previous question about PVC safe at 1x, yes it should be unless it's very damaged or cold. But I personally wouldn't push it past 2x (though some have, I bet), just for fear that the burst disc doesn't rupture or the valve jams, and the PVC ruptures, potentially sending shrapnel into the operator.
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Unread postAuthor: cfb_rolley » Tue Dec 07, 2010 7:20 pm

Damn should have clarified. I meant pressure rated PVC is better than DWV PVC in any kind of launcher.

At 2x, what sort of pressures would be made if the burst disc didn't rupture? Would it be possible that it would be safe (or rather, "not so prone to failure") to use at higher mixes if the wall thickness is greater or is it one of those things that should never really be experimented with?

Anyway back on topic, maybe you could incorporate a thin burst disc separating the chamber so that you can open it up and check to see if ignition happened just by seeing if the burst disc broke or not?
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Unread postAuthor: saefroch » Tue Dec 07, 2010 8:05 pm

HGDT says 215.4 psi from a 2x mix, generated by 3,600F combustion products. Most PVC is only rated to 240psi, at 73F.
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Unread postAuthor: ramses » Tue Dec 07, 2010 9:47 pm

The big issue PVC has in (properly designed) hybrids is not the pressure; it is the pressure spike, as well as the forces from recoil.
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Unread postAuthor: skyjive » Wed Dec 08, 2010 12:31 am

Right, I would imagine there's a large difference between a slowly applied load (filling with compressed air) and a shock load (combustion), you can't just read off the pressure rating.

Also, in case anyone is curious I redesigned my apparatus to be a shorter section of 1.5" pvc with a 1" x 3/4" "barrel" that will spit out a little plug of metal just to see if it worked. Fill and spark plug will be tapped into opposite sides of the center of the pipe.
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Unread postAuthor: ramses » Wed Dec 08, 2010 3:20 pm

I'm not sure it's the sudden shock of combustion; the relative dP/dt compared to a normal combustion should only be about equal to the mix number. Perhaps it's the sudden rise in pressure when the burst disk bursts, or a shock of decompression (faster muzzle velocities, mach number of gas is close to the same)

You may find that the little plug of metal might be expelled with surprising force.

Make sure the sparkplug isn't too recessed ; I have mixing trouble with my hybrid, and it has the spark plug in an isolated little section of 1/2" pipe.
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Unread postAuthor: cfb_rolley » Wed Dec 08, 2010 4:02 pm

I think a high speed of a failing PVC hybrid may help shed light on to how exactly they fail. Has anyone done that before?
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Unread postAuthor: saefroch » Wed Dec 08, 2010 6:17 pm

There has been high-speed video of PVC failing before, and plenty of evidence afterwards to demonstrate how, I'd imagine the failure mode from most internal pressure sources is the same, but I can't be sure. IIRC, it fragments in all known failures.

Prove me wrong now, why doncha...
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Unread postAuthor: cfb_rolley » Wed Dec 08, 2010 7:20 pm

Haha I don't think I'll be trying to replicate PVC shrapnel any time soon.

What I meant was the different reasons for the failure of PVC in a hybrid, Ramses suggested it could be from the sudden shock of decompression, if you watched a high speed video of that would it show the swelling of the pipe followed by it shattering as the projectile leaving the barrel? Or would it just swell until it cracked?

I don't know much about the fuel mixes in hybrids or their failure modes, that's why I'm asking lots of questions.

Also, I'm with Ramses on the metal plug thing, I'd probably not have it pointed anywhere thatit can deflect back at anything, or you.
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Unread postAuthor: saefroch » Wed Dec 08, 2010 8:31 pm

I'm guessing under high pressure, swell then grow a spiderweb of cracks like glass, then shatter into a bunch of pieces. Under more gentle pressure load, no significant bulging before failure. Just a guess though...

Metal usually has a small section that's weaker than the rest, a bulge forms there then tears down the length and releases the pressure, but I've never seen a metal hybrid chamber failure, the failure I'm looking at is from hydro-testing.

skyjive: Have you considered making this thing into a hybrid? You've pretty much already got all you need...
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Unread postAuthor: jimmy101 » Wed Dec 08, 2010 9:22 pm

saefroch wrote:HGDT says 215.4 psi from a 2x mix, generated by 3,600F combustion products. Most PVC is only rated to 240psi, at 73F.

The temperature is irrelevant since it is HEAT that matters and not temp. Since the density and heat capacity of gases (even at higher hybrid ratios) is so small, relatively little heat is actually transfered to the gun. For a 1x gun the temperature rise is only a couple degrees C. For higher hybrid ratios you get more heat transfered but most of that heat transfer probably occurs after the ammo has left the barrel.
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Unread postAuthor: saefroch » Wed Dec 08, 2010 9:34 pm

I am fully aware that it is the heat that matters (feel free to calculate that for yourself exactly, but it's about ), but the temperature is NOT irrelevant if the valve does not open, which I believe was what I was discussing...
cfb_rolley wrote:At 2x, what sort of pressures would be made if the burst disc didn't rupture?
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Unread postAuthor: Grignacke » Wed Dec 08, 2010 9:42 pm

It takes a special kind of stupid to research the topic and leave thinking that air/propane at 1X might rupture SCH40 steel.


That's the funniest thing I've heard in awhile :D

Otherwise, the problem you seem to have (as others have indicated) is too long of a chamber and improper mixing. Give it some more time to mix. Or install a spark strip if you'd like. [/quote]
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