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6x... look what happened

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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6x... look what happened

Unread postAuthor: Fnord » Wed Dec 19, 2007 1:40 pm

Well, I got the valve fixed(leaks) on my hybrid, so I decided to have some fun today.

I've never tried anything past 4x, so I figured I might as well give it a go.

I tried 5x a few times, embedding a lead ball into a tree and blowing though 2 1/4 inches of wood planks in the process, and so I finally did a couple blank fires at 6x (indoors :)).

-The noise almost made my ears ring... with earmuffs on.
-Gave a muzzle flash like a gun, not the normal blue-orange propane flame. A white-yellow one.
-Ya know those videos of people shooting .50 cal rifles in iraq? The ones where the room fills with dust and chunks of plaster fall off of the walls? That is what it was like. Not that I have plaster walls, but a couple garbage bags and a hunting jacket that were 10 feet in front of me were blown back from the blast.


Getting to the point... I put a coat of primer on the valve and found this after it had dried.
A crack had spread around the reducer from the piston impact. I put a big heavy spring in there as a bumper, but it did no good. Unless "good" is being smashed into a spring-shaped pancake.


I need to find an uber-tough bumper that will absorb heavy impacts and not be damaged. Any ideas?

I'm also open to 2" piston ideas.
I'd make a teflon piston, but I don't have any 2" bar stock or a way to cut o-ring grooves.

Pictures. The uploader managed to put every single one in a different order than I uploaded them in.
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Here's a damage shot at 5x
000_0223.jpg (32.44 KiB) Viewed 1610 times
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spring.jpg (23.26 KiB) Viewed 1610 times
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The crack goes all the way around...
Easy fix, but it probably would have hurt a bit if it flew out and hit me.
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Unread postAuthor: Hotwired » Wed Dec 19, 2007 1:45 pm

Hey hey, sounds good, cracking looks discouraging.

The kinetic force of a piston flying back seems to be very rarely considered until it blows out the back of the valve :o

First time I've heard of metal cracking but just goes to show. Do you think you could increase the piston travel and thus allow a larger buffer to slow it down?
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Unread postAuthor: Maniac » Wed Dec 19, 2007 1:49 pm

holy crap glad i wasnt there cant belive a piston cannon did all that.
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Unread postAuthor: Fnord » Wed Dec 19, 2007 2:04 pm

With some interesting ggdt usage, I figured that the piston (~6 oz) accelerates to over 150 fps in the 1.5" of travel it has currently. With figures like that, I'm not sure if adding additional travel would help even with a bigger spring to slow it down... there's just too much energy there.
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Unread postAuthor: Hotwired » Wed Dec 19, 2007 2:24 pm

Is this a barrel sealing piston here?
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Unread postAuthor: Fnord » Wed Dec 19, 2007 2:27 pm

Yes, the same one I've been using, just with a reducer instead of a modified cap on the end. Do you want some pictures?
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Unread postAuthor: Hotwired » Wed Dec 19, 2007 2:34 pm

If you like.

The reason I'm asking is because at very high pressures it's worth considering that a barrel sealer has a very much larger potential area for the chamber pressure to act upon than a chamber sealer.

A chamber sealing valve could have the same diameter sealing face as the ID of the barrel.
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Unread postAuthor: Fnord » Wed Dec 19, 2007 2:48 pm

Surface area = 3.14 in^2 * 500-600 psi = A lot.
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Unread postAuthor: Hotwired » Wed Dec 19, 2007 3:28 pm

Mmm...

By comparison, a 1" diameter chamber sealer only has 0.785 sq in and a 3/4" diameter: 0.442 sq in face (not sure which barrel size you're using).

But still, even a 1" chamber sealer has FOUR times less area than a 2" barrel sealer for the pressure to act on.

Thats 392.5 pounds of force @500psi instead of 1570 pounds of force and you can still have the same porting :o
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Unread postAuthor: benstern » Wed Dec 19, 2007 3:38 pm

Hopefully this will give people an idear about how powerful hybrids are.
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Unread postAuthor: boilingleadbath » Wed Dec 19, 2007 3:49 pm

Is that a metal piston? A lighter piston would hit the endcap with a good deal less force.

Secondly, I think I see a scar on the endcap where the corner of the piston hit it. I am prone to think that the piston hit the endcap directly.
Any bumper is better than no bumper; you might try a ring of polyethylene or rubber. (solid rubber)
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Unread postAuthor: rna_duelers » Wed Dec 19, 2007 6:09 pm

A Valve spring out of a car?They are tough little buggers.
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Unread postAuthor: Killjoy » Wed Dec 19, 2007 7:19 pm

Wow that is actually kind of scary how flat that spring got.
A chunk if rubber might work well as a bumper...like a rubber stopper from the hardware store or something.
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Unread postAuthor: Novacastrian » Wed Dec 19, 2007 8:12 pm

May i ask why you made a piston Hybrid over a burst disc design, seems to me the piston is an added danger.
Nice damage too :D Do you think that a higher mix would have blown it apart altogether?
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Unread postAuthor: Fnord » Wed Dec 19, 2007 9:25 pm

Why I made it? Well, does anyone else have one? To my knowledge there are only 2 in the world (galfisk has the other one). BTB could easily convert one of his guns to work as a hybrid, though.
And it isn't necessarily less safe, I just haven't figured out how to control it yet :D

I'm guessing if I had done 7x the results would have been similar, but a little
worse. The crack doesn't go all the way through the metal, it's just on the outside, so I don't think it would have completely separated. 8 or 9x would have probably done it though.

I'm gonna try a bumper made of rubber next, unless someone has a better idea. I managed to find enough fittings to repair it, so no trip to the hardware store for now.
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