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aluminum bust pressure

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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aluminum bust pressure

Unread postAuthor: ilovefire » Thu Sep 23, 2010 5:54 am

would anyone know what the burst pressure of 14mm ID 1mm walled aluminum tube and would like to know if it could take a 10-15x mix, it is what i am going to use for my hybrid cartridges and want to know if i should scrap that as the chamber or what mixes it would be safe at, thank you
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 23, 2010 7:08 am

If you're using epoxy endcaps, the caps will likely pop out long before the aluminium fails. Is it seamless tubing?
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Unread postAuthor: ilovefire » Thu Sep 23, 2010 8:02 am

jackssmirkingrevenge wrote:If you're using epoxy endcaps, the caps will likely pop out long before the aluminium fails. Is it seamless tubing?


good well iv put the holes in them like i did to my first one and yes it is seamless, only problem is iv ran out of epoxy :cry: but i would still like to know what the fail pressure is if anyone would know/have an idea/know where i could find out, thanks
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Unread postAuthor: Ragnarok » Thu Sep 23, 2010 8:45 am

For pure aluminium, a back of the envelope calculation puts the yield (not rupture, but yield is in many ways more important.) pressure at a surprisingly low ~10 bar. It surprises a lot of people, but when talking about pure metals, aluminium isn't that strong - a fraction of the strength of pure copper (and let's not even go into how it compares to steel). It's only when you start to alloy and process aluminium that it gains any real strength.

However, fortunately, it's unlikely to be pure aluminium. But without knowing which alloy and how it's been treated, it's not possible to give any definitive answer about its strength.
But assuming it's at least a decent alloy, Jack's answer is probably the most appropriate one - your bigger concern will be the epoxy end plugs than the tubing.
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Last edited by Ragnarok on Fri Sep 24, 2010 4:57 am, edited 1 time in total.
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Unread postAuthor: ilovefire » Thu Sep 23, 2010 8:07 pm

Ragnarok wrote:For pure aluminium, a back of the envelope calculation puts the yield (not rupture, but yield is in many ways more important.) pressure at a surprisingly low ~10 bar. It surprises a lot of people, but when talking about pure metals, aluminium isn't that strong - a fraction of the strength of pure copper. It's only when you start to alloy and process aluminium that it gains any real strength.

However, fortunately, it's unlikely to be pure aluminium. But without knowing which alloy and how it's been treated, it's not possible to give any definitive answer about its strength.
But assuming it's at least a decent alloy, Jack's answer is probably the most appropriate one - your bigger concern will be the epoxy end plugs than the tubing.



for a second there you got me a little worried, but that is also good to here, as for the epoxy plugs i think they will hold because i used 3500psi stuff and iv got at least 1.5cm plugs with four holes in the tubes for it to hold to and have sandblasted the surfaces :P
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 23, 2010 11:53 pm

My"micro" hybrid has a 13.8mm I/D steel chamber with 0.8mm wall thickness and took 17x without problems, of course aluminium is not as strong but you should be fine :)
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Unread postAuthor: ilovefire » Fri Sep 24, 2010 12:46 am

jackssmirkingrevenge wrote:My"micro" hybrid has a 13.8mm I/D steel chamber with 0.8mm wall thickness and took 17x without problems, of course aluminium is not as strong but you should be fine :)


what about 20x or is that pushing it a bit to far?
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 12:48 am

I can't get ignition with a piezo past 17x so I can't give you a fact based answer - but I was prepared to try a 20x mix in mine ;)

...says Jack's-not-so-well-known-for-his-safety-record-but-still-smirkingrevenge :roll: :wink:
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Unread postAuthor: ilovefire » Fri Sep 24, 2010 2:57 am

jackssmirkingrevenge wrote:I can't get ignition with a piezo past 17x so I can't give you a fact based answer - but I was prepared to try a 20x mix in mine ;)

...says Jack's-not-so-well-known-for-his-safety-record-but-still-smirkingrevenge :roll: :wink:


ha ha ha ha yea well i might try make the spark gap on my prototype smaller and try remote firing it but i dont think i will use the cartridges past say 10-15x because there going to be near my face, and for now im quit content with the power, btw i have almost finished my cartridges got some schrader caps with rubber washes in them and have drilled out some 5mm holes in the back to act as my onions but dont know when ill be ably to get to bunnings to replenish my epoxy supply.... i think im turning into you JSR i cant get enough of that stuff :D
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 3:10 am

Looking forward to seeing your results :) I still haven't found time to make a revised prototype with an o-ring friction breech, I have high hopes that this will be the definitive cartridge assuming I can make it in a way that will hold at least a 10x mix without popping from the mix pressure alone. Casting with a 4.5mm mandrel gave me a 50 psi pop-off pressure, without it given the o-rings 3mm I/D it should give me enough resistance.
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Unread postAuthor: ilovefire » Fri Sep 24, 2010 3:39 am

jackssmirkingrevenge wrote:Looking forward to seeing your results :) I still haven't found time to make a revised prototype with an o-ring friction breech, I have high hopes that this will be the definitive cartridge assuming I can make it in a way that will hold at least a 10x mix without popping from the mix pressure alone. Casting with a 4.5mm mandrel gave me a 50 psi pop-off pressure, without it given the o-rings 3mm I/D it should give me enough resistance.


yes well i think youll have better luck with the o-rings then with the rubber tubes i think it will be interesting to see what you come up with when you try it again and it will be good if you can make the o-rings replaceable, but for easy and simplicity in making i like the bust disks better :)
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 4:20 am

ilovefire wrote:I like the bust disks better :)


Fair enough, but I really didn't like the amount of weight they added to the cartridge, not to mention the hassle of having to cut burst disks etc.
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Unread postAuthor: ilovefire » Fri Sep 24, 2010 4:45 am

jackssmirkingrevenge wrote:
ilovefire wrote:I like the bust disks better :)


not to mention the hassle of having to cut burst disks etc.


hassle? what do you mean how do you cut them out?
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