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large hybrid

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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Unread postAuthor: artillerycreations » Wed Dec 23, 2009 7:55 pm

I was just wondering if .188 wal 6 inch galvanized steel would hold up to say a 3-5x mix and in what length would i have to worry about detonation. I plan on using a 20 foot 3 inch barrel with a steel union. Im not sure if this is the best option. Maybe i should use a 3 inch chamber and a 1.5 inch barrel or what. Any good power combinations i might me missing. Suggestions? Pipe is not an issue. I have access to hundreds of feet of galvanized steel in many sizes.
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Unread postAuthor: auxiliary » Wed Dec 23, 2009 9:04 pm

I wouldn't take it to 2x. MoonBoggs's cannon:
http://www.spudfiles.com/forums/the-cobra-venom-t17948.html
has a 4"ID chamber made of sch10? aluminum and he isn't comfortable with igniting it at a 3x mix. I think a .188" thick 6" chamber would not be as strong as the Venom's chamber because of its increased surface area. Because of this I would stay with 1x combustion.
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Unread postAuthor: D_Hall » Wed Dec 23, 2009 9:30 pm

6" pipe has an OD of 6.625". 0.188 wall implies an ID of (essentially) 6.25".

stress = load / area = (pressure*ID) / (2*wall) = 6.25P / .376.

Assuming mild steel, yield stress is approximately 36,000 psi.


So... .376 * 36000 / 6.25 = Pressure = 2100 psi.


But that's when we fail, that doesn't mean it's safe! If you have a well supported chamber and such you could probably only go with a 5:1 safety margin. IE, you should be good to go with 400 psi. So somewhere on the order of 4X should be acceptable as a top end.

But those are ROUGH numbers pulled from memory (yes, I do actually remember the OD of 6" pipe, I'm sick that way). I wouldn't trust my life to them without more review.
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Unread postAuthor: artillerycreations » Wed Dec 23, 2009 9:41 pm

Thanks for the responses. A a 1x mix this gun would be a monster. Im not sure if it even feasable because i dont have money flowing out of all of my bodily orfices. I just have an incentive because im kind of the go big or go home kind of guy and i have so much pipe i dont know what to do with it. As far as i know only a few hybrids would be able to compete with it in muzzle energy and size.
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Unread postAuthor: daberno123 » Wed Dec 23, 2009 9:49 pm

Have you checked out Killjoy's FEAR?

I believe he's taken his design to a 10x mix using 6" pipe for his chamber.
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Unread postAuthor: Moonbogg » Wed Dec 23, 2009 9:49 pm

Also, holes in the chamber weaken it significantly in that area. So if you have holes in the chamber and no bosses to reinforce them, cut your safety factor in half (seriously).
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Unread postAuthor: artillerycreations » Wed Dec 23, 2009 10:37 pm

Yes i have researched fear quite a bit and any holes drilled in the chamber would be through a 1/2 inch backing plate for the chamber.
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Unread postAuthor: D_Hall » Wed Dec 23, 2009 11:12 pm

Moonbogg wrote:So if you have holes in the chamber and no bosses to reinforce them, cut your safety factor in half (seriously).

Depends on the size of the hole... It could be worse!
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Unread postAuthor: Moonbogg » Thu Dec 24, 2009 1:02 am

D_Hall wrote:
Moonbogg wrote:So if you have holes in the chamber and no bosses to reinforce them, cut your safety factor in half (seriously).

Depends on the size of the hole... It could be worse!


What do you think the failure would look like? I imagine the hole would flare outward and whatever is screwed into it, if anything, would fly out. This is assuming a metal chamber of some sort. PVC of course would shatter outright.
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Unread postAuthor: SpudBlaster15 » Thu Dec 24, 2009 1:40 am

Also, threaded holes are even worse. I recall reading in an engineering textbook that even a small threaded hole in the side of a pressure vessel increases the stress on the material surrounding the hole by a factor of 4.

I always try to thread components through reinforced areas of the launcher, such as where a pipe and fitting overlap.
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Unread postAuthor: Moonbogg » Thu Dec 24, 2009 3:35 am

Sounds like sch40 steel might be a better option. It should allow you to drill and tap while maintaining a better safety factor. If you don't like the weight, consider sch40 aluminum maybe. It can be a pain to get a good balance between weight and strength while debating material choices.
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Unread postAuthor: JDP12 » Thu Dec 24, 2009 9:52 am

Ok just quick here sorry for a bit of a hijack- I'm almost done on my hybrid and it's essentially thesame as sb15s. With the spark plug threaded into the chamber, same materials. Is that gonnna be ok at higher mixed, in the 4-7x range? I'm assuming yes because the steel is strong and sb15 had no problems? I'd like to be sure before I fire it up. If necessary I'll get a new pipe and redrill through a fitting/pipe.
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Unread postAuthor: ramses » Fri Dec 25, 2009 9:26 pm

remote fire it to start.

Also, if you do see failure, look at electrically welded or DOM tubing (drawn over mandrel) if you can weld.

7" OD, .25" wall goes for ~$20 per foot, plus cutting and shipping. 7"x .5" DOM goes for $110 per foot, plus cutting.

or even 16" OD x .5" wall for $55 per foot. Then you just need a TON of plate steel for end plugs!

Discount steel has decent prices, IDK about their service.
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Unread postAuthor: artillerycreations » Tue Dec 29, 2009 2:24 pm

Ill let you guys know if anything comes from this but it seems uneconomical at this point.
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Unread postAuthor: artillerycreations » Wed Jan 20, 2010 4:49 pm

I just looked today because i had a recent renewal of interest in such a cannon. I have 20 feet of 3 inch sch 40 pipe. Both pieces have couplings on one end. I also looked at the 6 inch pipe i have and realized that it also has a welded seam. I modeled it on HGDT and got supersonic speeds (and hella muzzle energy) with an 8 ounce projectile, 48inch chamber, one igniter,and a 150 psi burst disk, 240 inch barrel, and at 4x. I wouldnt be hurt if it blew up while in use because i wouldnt be near it but it would be upsetting if all of my work went to waste. Im just wondering if i should buy/find some pipe with a thicker wall or what.
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