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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: MrCrowley » Mon Nov 29, 2010 3:32 pm

I think it was SpudBlaster15 who suggested I look for electrical conduit pipe, my initial search turned up nothing. I was thinking that if I found some thin-walled metal pipe I could try sleeve it inside the ABS. Chances of a good fit are small but it could be a possibility.

Another option is doing what SB15 did and sleeve the first meter or so of the barrel with another layer of ABS.

I'd like to back up DYI here in saying you really don't want to get hit with hybrid muzzle flash

Tell me about it, I have no doubt it would tear my hand apart :(
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 22, 2011 10:25 am

[youtube]http://www.youtube.com/watch?v=biNQzdMLrAY[/youtube]

Two things:

- a 1.5" hybrid is a much more appropriate namesake of the thunder god's hammer, as opposed to a barely audible suppressed bolt action rifle.

- saying it in a Southern American drawl really does take away the impact and mystique of a reference to Norse mythology :D
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Unread postAuthor: Tobin » Sun Mar 13, 2011 2:32 pm

Isnt there a risk in hybrids that the piston will fail and wont open and the gun explode?
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Unread postAuthor: SpudFarm » Sun Mar 13, 2011 2:33 pm

Tobin wrote:Isnt there a risk in hybrids that the piston will fail and wont open and the gun explode?


A gun should always be designed to handle the peak pressure of the combustion, so - no.
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Unread postAuthor: MrCrowley » Sun Mar 13, 2011 2:37 pm

Tobin wrote:Isnt there a risk in hybrids that the piston will fail and wont open and the gun explode?

Ha, no.

If my safety margin was that small, the cannon would be far too dangerous to use. I'm not sure why the piston wouldn't open, shall we say it gets jammed somehow but the pressure wouldn't spike for long since the heat would dissipate extremely fast and the pressure would drop just as fast. It's not like if the piston failed you would have 1000PSI in your chamber...
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Unread postAuthor: ramses » Sun Mar 13, 2011 7:38 pm

MrCrowley wrote:It's not like if the piston failed you would have 1000PSI in your chamber...


I'm pretty sure you would, if only for a second or so.
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Unread postAuthor: jackssmirkingrevenge » Sun Mar 13, 2011 10:59 pm

I've had burst disks fail to rupture several times in the course of my hybrid experimentation, a well made launcher will resist the pressure regardless.
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Unread postAuthor: MrCrowley » Sun Mar 13, 2011 11:54 pm

ramses wrote:
MrCrowley wrote:It's not like if the piston failed you would have 1000PSI in your chamber...


I'm pretty sure you would, if only for a second or so.

Ah I should've clarified, I meant that you wouldn't be storing 1000PSI for an extended period of time.

As JSR said, when burst disks fail to burst your cannon doesn't blow up, if it did it probably would blow up at some point during firings even if the burst disks do burst. When I vent a chamber after the burst disks have failed to burst, about the only pressure in the chamber seems to be from the smoke/steam as it doesn't seem like much once you release it all.
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Unread postAuthor: Moonbogg » Mon Mar 14, 2011 12:06 am

If a chamber isn't strong enough, even a very short pressure spike will break it. Do you guys take comfort in the fact that the pressure spike is very short lived? The way I see, pressure is pressure regardless of how long it lasts. Even if its really short, its still plenty long enough to burst a weak gun.
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Unread postAuthor: MrCrowley » Mon Mar 14, 2011 12:08 am

No, I don't take comfort in the pressure being short lived. I want the cannon to be able to take 1000PSI whether it is stored or only momentary. I think that if someone's cannon failed because the valve didn't open then their cannon would've failed at some point over a number of shots regardless if the valve opens.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 14, 2011 12:25 am

MrCrowley wrote:No, I don't take comfort in the pressure being short lived. I want the cannon to be able to take 1000PSI whether it is stored or only momentary. I think that if someone's cannon failed because the valve didn't open then their cannon would've failed at some point over a number of shots regardless if the valve opens.


Agreed, if the chamber is marginal even a heavy projectile might cause the pressure spike to be high enough to burst it, even if the burst disc ruptures.

If your hybrid chamber can't take the shock of unvented ignition, you shouldn't be using it
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Unread postAuthor: Ragnarok » Mon Mar 14, 2011 10:34 am

MrCrowley wrote:No, I don't take comfort in the pressure being short lived.

I would. It doesn't really affect the chances of failure, but it means it's a hazard for the shortest possible time.

It also means you don't have to find a way to safely vent the full combustion pressure if it goes wrong. You'll still have to vent the pre-pressure (plus a bit with a propane/air mix, as you get 7 moles of combustion product for 6 moles pre-mix), but that's less of a problem.
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Unread postAuthor: Tobin » Mon Mar 14, 2011 3:38 pm

MrCrowley wrote:No, I don't take comfort in the pressure being short lived. I want the cannon to be able to take 1000PSI whether it is stored or only momentary. I think that if someone's cannon failed because the valve didn't open then their cannon would've failed at some point over a number of shots regardless if the valve opens.


Okay, im interrested because in my eyes it looks dangerous. But with preassure about 20 bar + propan and ignite: wont the pressure in a closed chamber have the potential to rise far above , lets say 150bar? Or do i overestimate
the power? If you lets say welded the piston in the gun, wont the gun explode, or does it handle the maximal pressure from the combustion?
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Unread postAuthor: MrCrowley » Mon Mar 14, 2011 3:55 pm

Maximum pre-ignition pressures in this cannon are more like 10bar, I've never exceeded that before and don't intend to. Maximum chamber pressures upon ignition are closer to 70bar. I've never modeled a cannon in HGDT which has pre-ignition pressures of 20bar with peak combustion pressures of 150bar so I wouldn't know how close your guess is.

We've already answered your second question about six times:

SpudFarm wrote:
Tobin wrote:Isnt there a risk in hybrids that the piston will fail and wont open and the gun explode?


A gun should always be designed to handle the peak pressure of the combustion, so - no.


MrCrowley wrote:
Tobin wrote:Isnt there a risk in hybrids that the piston will fail and wont open and the gun explode?

Ha, no.

If my safety margin was that small, the cannon would be far too dangerous to use. I'm not sure why the piston wouldn't open, shall we say it gets jammed somehow but the pressure wouldn't spike for long since the heat would dissipate extremely fast and the pressure would drop just as fast. It's not like if the piston failed you would have 1000PSI in your chamber...


jackssmirkingrevenge wrote:I've had burst disks fail to rupture several times in the course of my hybrid experimentation, a well made launcher will resist the pressure regardless.


MrCrowley wrote:As JSR said, when burst disks fail to burst your cannon doesn't blow up, if it did it probably would blow up at some point during firings even if the burst disks do burst. When I vent a chamber after the burst disks have failed to burst, about the only pressure in the chamber seems to be from the smoke/steam as it doesn't seem like much once you release it all.


jackssmirkingrevenge wrote:If your hybrid chamber can't take the shock of unvented ignition, you shouldn't be using it



I'm not sure if you just didn't bother to read all that or you didn't understand it, anyway the short answer is no.

No.

No.

No.


As we've stated several times, if the cannon failed because the valve didn't open then your cannon would fail at some other point in time even when the valve does open, simply because the safety margin is far too small and the difference between a closed valved combustion (valve fails to open) and an open valved combustion (valve does open) is minimal anyway.

No one on this forum would be dumb enough to operate a hybrid cannon which had a safety margin of ~100PSI, well I hope not anyway.


Edit: And yes, the cannon is dangerous but not because it could fail catastrophically. The galvanised pipe fittings would need several thousand (5000PSI+ I imagine) PSI to fail. The ball valve would fail at a much lower pressure but it wouldn't matter anyway because all that would happen is the seals would melt/fail and start to leak, no problem there. The piston would also fail in some way (damaged sealing face, leaking o-rings, sheared threads, bent rod, damaged washers etc) at a similar pressure to the ball valve as well.

So if anything did fail, it would be the ball valve or piston and it wouldn't matter to my safety if they did fail. Now that I think about it, the barrel could fail as well given the right circumstances. That could be dangerous if the barrel split and hot gases leaked out and burnt me or something. I'm confident if I don't jam a projectile in to the barrel too hard, it should be safe for the pressures I use it at.
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Last edited by MrCrowley on Mon Mar 14, 2011 4:02 pm, edited 1 time in total.
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Unread postAuthor: Zeus » Mon Mar 14, 2011 4:02 pm

If the pressure triggered valve is set close to the peak combustion pressure the difference should be minimal.

Sorry for the white space, Vista paint doesn't seem to have a resize. I'm spoilt by 7.

Download HGDT, you'll learn a lot just by playing with variables.

I'll even give you a link http://thehalls-in-bfe.com/HGDT/

If you're using Vista or 7, run it as an administrator (right click)

[Edit: Curse you MrC with your quick typing]
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Attachments
HGDT Example 2.jpg
With the burst disc set 8 PSI above the maximum pressure of combustion.
HGDT Example 1.jpg
With the burst disc set 2 PSI under the maximum pressure of combustion
/sarcasm, /hyperbole
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