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Milk Jug Failure Modes

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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Milk Jug Failure Modes

Unread postAuthor: auxiliary » Mon Apr 10, 2017 7:01 pm

I was playing around with my hybrid, trying to fine tune the optimum mix pressure for my volumetric meter setup and I noticed I got three failure modes when using milk jug burst disks. My union is 1" and I was at 4X. For correct mixtures (at least according to my calcs) and slightly rich mixtures I got a burst like example #1. Sort of a linear tear and bulging of the surface subjected to the pressure. Note this was the loudest bang as well. For slightly lean mixtures I got #2, a nice circular opening with the center still attached like a door. And sometimes with any mix the disk would deform and slip out like #3.

Do any of you have experience with these and/or have any suggestions as to what these failure modes mean?
My guess is that #2 is optimum because of greater flow and indicates that the combustion was fastest because the shock caused a more brittle failure. # 1 looks more like a slower build up in pressure until the material undergoes ductile yielding and ruptures. #3 looks like I just didn't tighten the union enough.
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Re: Milk Jug Failure Modes

Unread postAuthor: Zeus » Tue Apr 18, 2017 4:27 am

Consider going for a material that yields in a more abrupt fashion, you're losing performance clearly. A 1" union should be large enough that you can use light duty aluminium baking trays, I've had good success with aluminium tape, though your union is significantly larger than mine.

As we know from internal combustion engines, lean mixtures burn faster and are more inclined towards knocking.
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/sarcasm, /hyperbole
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Re: Milk Jug Failure Modes

Unread postAuthor: jackssmirkingrevenge » Tue Apr 18, 2017 7:11 pm

Zeus wrote:Consider going for a material that yields in a more abrupt fashion


Hear hear, you want something that breaks as opposed to tearing. My favorite material for burst disks was always photo paper, not really practical for larger calibers but I would go for thick aluminum foil before plastic in this case.
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