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Aluminium Chamber(Ignition Ideas????)

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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Unread postAuthor: SPUD-U-LIKE » Mon Jan 05, 2009 5:30 pm

cannon will end up being fully welded and mirror polished for looks and strength,we've been engineering with this type of material for over 40 years between us and have always over engineered projects,so once it's built there is no downtime fixing and bodging things,i've seen the damage this type of apparatus can cause first hand and it's nasty,propane gas at atmospheric pressure then compressed air added to the equation seems fairly dangerous,the aluminium were using is of a known quality and strength to us and having never worked with ABS,PVC this seemed like the obvious choice
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Unread postAuthor: Moonbogg » Tue Jan 06, 2009 5:14 am

Using aluminum, even 6063-T6 and assuming its super strong is dangerous. Just because the tensile strength of the material is high, doesn't mean squat if the design has high stress points, such as holes drilled in the chamber, etc. These things must be taken into account as they will cut the yield strength down a LOT .

For instance, the chamber i'm working on right now is of 6061-T6, and with an intact chamber the FOS is 4.38 at 500psi. Thats super safe you might say. Well, after adding my holes for the wiring and all that good stuff, the FOS drops to 1.3. Thats only 29% of the original value. So this gun i'm making will only be used with pressures no higher than 300max psi for a safety factor of at least 2. Thats assuming all the welds are clean and with full penetration also.

I hope I didn't insult you, but i'd rather piss you off than have you take aluminum shrapnel to the face.

OMG I almost forgot to tell you dude...and you might already know this. But when you weld T6, it returns to near condition 0 near the welds, which is weak and soft as butter. You need to send it out to be re heat treated back to T6 if your using it as a hybrid.
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Unread postAuthor: john bunsenburner » Tue Jan 06, 2009 7:35 am

he is making a combustion, nothing too fancy, alli is not going to explode at 100psi and not at 150, besides if he made it from alli foil or something, If pvc will hold it so will alumium!
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Unread postAuthor: jimmy101 » Tue Jan 06, 2009 12:22 pm

Radiation wrote:ok another question, if you are making a standard combustion spudgun why use/make a chamber that is rated for 300+ Psi? Any standard combustion is only capable of 100-120psi max. The rest is just unnecessary and quite honestly pointless ovrerkill.

1. Because that is the material that is most commonly available that has a pressure rating greater than equal to what is required.

2. Most chambers are rated to more like 240 or 280 PSI not "300+ PSI" (NSF-PW for 3" and above pipe). That gives a safety factor of about 2, in addition to the rating's intrinsic safety factor.

3. Since NSF-PW pipe is rated for static loads of a non-compressable liquids (i.e., very little explosion potential) but is being used with compressed gases it is prudent to increase the safety factor. If a water pipe fails at 120 PSI it's no big deal. It makes a big mess but releases very little nergy. If a pipe containing 120 PSI compressed gases fails, and you happen to be holding it in your hands, that is a big deal. The mess the failure makes will involve blood and body parts.

4. The cost (in the US) is essentially identical for NSF-PW (pressure rated) vs. non-pressure rated pipe.

5. Better "overkill" on the pipe specs than any "kill" associated with a chamber rupture.
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Unread postAuthor: btrettel » Tue Jan 06, 2009 1:53 pm

In addition to what jimmy101 has said, let me remind everyone that more than the material matters to strength. The geometry (thickness, shapes, etc.) matter a lot in strength. That is why you don't see box shaped pressure chambers very often--they are weaker. Something made of aluminum can easily be weaker than something made of plastic depending on the geometry, flaws in the material, and other specifics.

And as I said in my thread about lightweight air tanks, any appreciable gas pressure is nothing to joke about. That's a lot of energy. If you don't respect that, you can easily be killed.
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Last edited by btrettel on Tue Jan 06, 2009 2:43 pm, edited 2 times in total.
All spud gun related projects are currently on hold.

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Unread postAuthor: john bunsenburner » Tue Jan 06, 2009 2:27 pm

if i start praying to 150psi then what do i do with 3000, i dive, i have 3000-4500psi around me constantly, and the tanks are ""only" an inch or so thick. I fill the tanks i screw on the regulator, i do it all and i respect them, but dont exagureate. What he is doing is an over kill to explain it to everyone who is new or not used to spudguns:

It is like using 50 condoms because your scared one might burst, no need to elaborate is there?

A combustion CANNOT under any cirumstances generate more than 120psi under earthly conditions. Using aluminium for that, besides for the coolness factor is just plain overkill! Yes making a square chamber is REALLY dumb, but chances are he wouldnt do it without mentioning it, and then atleast 10people will try and stop him, so not really a consern...
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Unread postAuthor: SPUD-U-LIKE » Tue Jan 06, 2009 2:44 pm

thanks for your comments guys,yep after welding and before polishing i'll anneal the cannon using just acetylene to give a black soot all over and then add the oxygen and heat all over gently until the sooty deposits disappear,now at T0,then heat treat to T4 followed by T6 the following day to achieve 48/55 R/B

were gonna start building in the next week or so and i'll try to post progress pictures at each stage

what sort of distance are you guys getting from your cannons so we've got some sort of benchmark to go by
anyone gone down the two smokin barrel route(could be interesting)

better safe than sorry ay guys
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Unread postAuthor: Moonbogg » Tue Jan 06, 2009 2:46 pm

john bunsenburner wrote:he is making a combustion, nothing too fancy, alli is not going to explode at 100psi and not at 150, besides if he made it from alli foil or something, If pvc will hold it so will alumium!


Read his first post, he sais he includes a compressed air inlet. Is he using that for ventillation? I hope so. If its for hybrid use, weld it at your PERIL! Unless you heat treat =)

Edited: Oh awesome he just replied and he's gonna heat treat. Can't wait to see this beast your building!
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Unread postAuthor: john bunsenburner » Tue Jan 06, 2009 3:02 pm

you changed your first post three times, i stoped paying attention after the second, a hybrid it is, how many x? And there are some very good simulations programs for hybrids and combustions like HGDT (http://thehalls-in-bfe.com/HGDT)
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Unread postAuthor: Combustion Monkey » Tue Jan 13, 2009 7:35 pm

SPUD-U-LIKE wrote: twin ISO valves for greater fuel air mixture ratio and compressed air injection any comments welcome
regards wayne :) :)


I know i'm new here but doesn't this mean that its a hybrid due to the compressed air? And therefore subject to a whole different set of rules?

Sorry for the redundant post, the big dogs beat me to it.
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Unread postAuthor: SPUD-U-LIKE » Wed Jan 14, 2009 1:18 pm

this is a combustion cannon using propane but boosted up to a higher pressure before ignition using compressed air
cheers wayne
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Unread postAuthor: psycix » Wed Jan 14, 2009 1:42 pm

So you use a 1x mix propane, but with more air?
Whats the point of that may I ask? You will have unspent oxygen in your chamber.
Then simply add less air, but a bit more fuel. Making it a hybrid.

Im not sure about the burn rate and effectiveness of too less propane in too much air either.
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