more hybrid questions

[ramses]

I will hopefully be making a piston hybrid soon, and have some questions.

my hopeful, made up specs:

• -3"x12" steel pipe for chamber EDIT: now hopefully 3"x18"
  -piston for a 1.5" "barrel" in a 2.5"tee*
  -120"x 3" sch80 pvc barrel.
  -11x propane/air temporary max mix.(until I buy a pump that can go above 150psi.
  -chamber pressure propane meter.
  -Ignition through tee at front of chamber

* the piston is over-sized to let the valve open at a lower pressure and save the pvc barrel from a pressure spike.
now for the questions:

• 1) I know we don't know much about DDT, but is it likely in the 3"x18" chamber?
  
  2) How much is performance degraded by using a piston instead of a burst disk?
  
  3) How can i pass thought the sealing face of the "barrel?" my first thought was a fully threaded nipple, but mcmaster doesn't carry 1.5" fully threaded nipples long enough to protrude through the reducing bushings and into the piston's tee. Is there any way besides welding or threading the pipe further?
  
  4) I HGDT'd my gun launching a very heavy projectile (20oz soda bottle full of concrete, and the max pressure barrel was under 400psi with my "burst disk" opening at 350psi. I know this is out of spec, but I am fine with a safety factor of 6 vs. the 7 I would have had before. slightly Can sch80 withstand the shock load of this firing? I am mainly worried about the threads that attach pvc and steel ripping off. The projectile will be getting over 700g's after all 8)
  
  5) Are malleable iron fittings up to this job? forged steel is REALLY expensive.
  
  6) will this launcher be somewhat safe to be near when fired. I know that 10000 ft/lbs isn't something you can really hand hold
  
  7) how would you recommend bracing this cannon so it doesn't recoil into something important. Will a malleable iron end cap and tee be sufficient

If all goes well, it will be getting muzzle energies over 12000 ft/lbs, 8.1x that of the good old ak47

[frankrede]

Why not just use a burst disk?
Pistons aren't even that common in hybrids.......

[ramses]

I wanted to use a piston to be different and to cut reload times. A burst disk is my "fall back."

[MrCrowley]

Fair enough.

I was thinking, instead of having a spring to make the piston 'normally closed' and contain the uncombusted fuel mix, couldn't you use compressed air?

Say your piston is o-ringed 100% air tight. Now you pump in air behind the piston, enough to keep it closed long enough to build up the pressure, if you then placed a pop-off valve at the back of the piston like a pilot valve, would the air compress, build up the pressure and then trigger the pop-off valve during the combustion peak to let the piston actuate and fly back like a regular piston?

[SpudUke5]

Fair enough.

I was thinking, instead of having a spring to make the piston 'normally closed' and contain the uncombusted fuel mix, couldn't you use compressed air?

Say your piston is o-ringed 100% air tight. Now you pump in air behind the piston, enough to keep it closed long enough to build up the pressure, if you then placed a pop-off valve at the back of the piston like a pilot valve, would the air compress, build up the pressure and then trigger the pop-off valve during the combustion peak to let the piston actuate and fly back like a regular piston?

Yes but the pop off really would "pilot it", it would just release the air behind the piston. The combustion peak pressure should push back the piston all the way, while the pop off disperses the air. But really im not sure, it should work though.

[MrCrowley]

Yeah i'm not sure either, it was just a brainfart really.

[ramses]

I think they both work, but the pop-up removes any cushioning effect the air behind the piston would have. You end up with a piston with several hundred pounds of force slamming into a hard steel/iron fitting. I think that would put considerable wear on the piston/tee. I would rather not use an O-ring, but if the propane/air mix behind the piston ignited through the equalization hole it would be the end of almost any cannon. Now i just have to think of how to make an o-ring seal with the irregular surface inside the tee...

any input on the cannon would be helpful

[Fnord]

1. Chances are, you won't even come close.

2.
Depends of the design of the piston. Tell me the precise specs of it and a mix number and I'll tell you about what pressure it'll open at.

3. Thread a 2nd pipe nipple into the back of the bushing.

4. I'm just going to take a guess and say 11x would shred sch80 pcv. Personally, I'd use 3" thinwall steel conduit (Might even be cheaper).

5. Theoretically, yes, but the piston will be your failure point. You'll need a hell of a bumper.

6. I wouldn't stand next to it at 11x, if that's what you're getting at.

7. Go with the "trash can full of rocks" approach to recoil absorption.

[ramses]

lol trashcan full of rocks. Sounds effective. I obviously wouldn't be anywhere near it the first time it goes off, but after it is proven "safe" I might. Would the threads on the pvc barrel be able to stand the force of firing. Aka: would the barrel pull out? Would you suggest the oring/ separate pressure chamber behind the piston to control the exact opening pressure. Should i use a safety pop off valve behind the piston? How should I determine it's rating? I think I have the opening pressure calculation down.

Thanks for the input.

[Fnord]

The threads would probably be the initial failure point, but after a crack forms during the first 10ms of combustion the rest of the barrel will fragment as the pressure wave travels down it, propagating the cracks as it goes.

I don't think you'll be able to control the exact opening pressure, but an oring should let you get it in the +/- 30 psi range.

a 100% seal, 2.5" piston with a seat diameter of 1.9 would open at around 230 psi with an 8x mix (not relative to initial pressure). Sorry I can't give you a formula because I don't use one, and I can't seem to remember what numbers I'm throwing around while I'm figuring it out:)

Is this a traditional barrel-sealing design we're talking about btw?

[ramses]

Can you think of a way to attach the pvc without a weak point? Where would I find this conduit and how would I attach it? I can't weld and I wanted this cannon to be able to be taken apart. Is there a way to have 2 shorter pieces of conduit attached to each other somewhat seamlessly, for transportation and storage?

As for piston opening pressure, it doesn't have to be exact, and I have a good idea of how to figure it out.
Thanks for the help!

edit: standard barrel sealing, yes

I just checked McMaster and their conduit only goes up to 2." I really wanted to shoot 20oz soda bottles...

[Fnord]

You'll need to buy the conduit locally; it'll be really expensive with shipping.
A 20ft piece of 2" steel conduit will probably be around $40 from home depot, but if you want 3" you'll have to get it from a pipe supply company.
It will most likely come threaded, so you can just screw it in..

[ramses]

Would the conduit be NPT threaded? EDIT: I just checked wikipedia and it says that EMT (thinwall conduit) isn't threaded, it's connected with clamp fittings. I would use RMC (thickwall) instead, but it's expensive and is threaded NPSM, not NPT.

Is it safe to thread the 2nd nipple into the chamber like that? I would think it would hold on less strongly because of less threads and maybe leak. Would I need to use a forged steel bushing because of the fewer active threads?