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Aluminum combustion advice

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Aluminum combustion advice

Unread postAuthor: Moonbogg » Fri Nov 28, 2008 4:29 am

I have received some good advice on this forum about combustion cannons. I want to make one capable of handling pure oxy/acetylene fuel mixtures. Do people already do this? If so, what material do they use? I want to make a powerful combustion spudder that has all the goodies, like chamber fan (replaceable) full length spark strip or equivalent ( 3-4 spark gaps) and adequate ventilation fan. I would also like breach loading since jamming a tennis ball down a 3-4 ft barrel can be a pain. Anyway..on to my main question. I want to make a simple combustion spudder out of aluminum with these features. HOW THICK should it be? I was thinking 1/4" wall thickness, not only so it won't explode, but so that it won't easily dent if dropped or banged. Aluminum is kind of soft after all, but relatively cheap to machine/weld. I don't have much money to spend on this or else i'd just draw something up and have it made and consider revisions later. Oxy/Acetylene capable thickness with aluminum...how thick? Do I need aluminum? Maybe SCH80 PVC? I dunno.
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Unread postAuthor: ralphd » Fri Nov 28, 2008 9:18 am

Try this place...

http://www.engineeringtoolbox.com/pipes ... -t_17.html

I use it quite a bit for different things...
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 28, 2008 10:09 am

There are pneumatic airguns with aluminium chambers that take 3,000 psi easily, with a thick enough wall you should be fine. Just make sure your ignition system is properly insulated.

As to whether a chamber fan as we know it in combustion terms would survive and oxy-acetylene explosion, hmmm...
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Unread postAuthor: starman » Fri Nov 28, 2008 10:37 am

If I may ask, why are you pursuing acetylene? Acetylene has this ugly tendancy to self ignite when blended with O2 and has typically been discouraged from use as a spudding fuel.

I'm very sure you haven't exhausted all of your power possibilities with MAPP or propane yet seeing as you are still in the process of building your first advanced combustion.

Also, take a look at a high-end metal spudder that's been around for some time, the legendary Taternator II.
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Last edited by starman on Fri Nov 28, 2008 11:39 am, edited 1 time in total.
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Unread postAuthor: SEAKING9006 » Fri Nov 28, 2008 11:17 am

If it's your first, don't do it. If you are experienced, don't do it. If you're Larda.... Go right ahead. But in all honesty, pursuing acetylene may be a form of suicide. Acetylene does not burn, it explodes. In poorly designed hybrid guns, their controlled burns sometimes turn into explosions, a phenomenon called DDT (deflagration to detonation transition). DDT usually spells the death of the cannon. If you fired a simple combustion using acetylene, you would be experiencing the effects of DDT (while not DDT itself) on every shot. There are very few people who have the resources to build cannons suitable to take that kind of punishment.


Summary; Don't do it, acetylene is an incredibly dangerous fuel.
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Unread postAuthor: ralphd » Fri Nov 28, 2008 5:50 pm

just stick to propane or mapp. It's not worth it. After all, this is only for for, right?
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Unread postAuthor: daberno123 » Fri Nov 28, 2008 8:02 pm

I plan on making a low (2-3) mix hybrid out of aluminum tube with a 4" diameter. The walls are 1/16" thick. With a 3x air/propane mix the highest theoretical pressure (according to HGDT) is 333.3 psi which gives me a safety margin of about 4 using Barlow's formula to calculate the tubes theoretical burst pressure.

In reality though, with the burst disks I'll be using the pressure won't exceed 200 psi at most and usually alot less than that.

If I were you though, I'd just buy the thickest aluminum tube you can afford. Check eBay, that's where I found mine.
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Unread postAuthor: Moonbogg » Fri Nov 28, 2008 9:07 pm

the reason I was thinking acetylene was because I wanted something with excellent power, yet simple to use. From what i'm seeing it sounds like acetylene is just nto a good fuel to use in any case really. Sounds like for more power i'll just have to make a well optimised advanced combustion. Maybe the simple addition of pure oxygen alogn with the propane would significantly increase power.
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Unread postAuthor: starman » Fri Nov 28, 2008 9:52 pm

Moonbogg wrote:Maybe the simple addition of pure oxygen alogn with the propane would significantly increase power.


Be very careful doing that as well...it will certainly work..duhh... :shock: Considering the danger and until you have some more experience under your belt, there's really no reason to use O2 in a 1X spud gun. Good ole air has plenty in it and the nitrogen acts as an inert buffer gas during the burning helping to prevent DDT. A pure propane/MAPP and O2 chamber is a potential DDT bomb just waiting to go off.
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Unread postAuthor: D_Hall » Sat Nov 29, 2008 1:07 am

Want more power? Build a hybrid. Voila, all the power you could ask for without trying to kill yourself messing with things you clearly do not understand.
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Unread postAuthor: john bunsenburner » Sat Nov 29, 2008 7:31 am

WE sould like all togeda try to build the ultimative hybrid oneday...
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Unread postAuthor: D_Hall » Sat Nov 29, 2008 1:22 pm

Group projects involving people from all over the place work well for software projects. Not so much for hardware. ;)

Besides, I'm already doing that.
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Unread postAuthor: john bunsenburner » Sat Nov 29, 2008 1:41 pm

Hahaha, once i made a few pneumatics maybe i will try a hybrid one day, i can cast nearly any part out of aluminum for virtually nothing!
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Unread postAuthor: frankrede » Sat Nov 29, 2008 2:32 pm

cast aluminum? If thats anything like pot metal, then thats no good.
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Unread postAuthor: rp181 » Sat Nov 29, 2008 4:00 pm

By cast he means melting ignots and molding them in sand molds. Even so, it wont be too good, because of air bubbles and impurities.
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