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assorted hybrid questions

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: rp181 » Sat May 31, 2008 1:48 pm

same here, but you can't just go in and expect to draw a perfect bead.
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Unread postAuthor: ramses » Sat May 31, 2008 1:51 pm

King- I'm going into highschool next year, and can't take any electives that year (and they won't MAKE me take shop). I could have older students do it, but my school has an anal non violence, hurting people, gun=bad mindset, and I'm not sure I'd trust people just learning how to weld learning on something that may become a frag grenade while I'm holding it. that's probably not likely, but i don't want to take the risk.

great idea though!

[/img]
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Unread postAuthor: Lentamentalisk » Sat May 31, 2008 2:13 pm

I am so jealous of all of you who have metal working shops at your schools! If we had any shop class what so ever, that would be all I took, all four years, but alas all they have is ceramics...
There is, however, a welding shop around the corner from my house, and I am a good friend of the owner of the building (who got me my firefighter's air tank and fridge compressor), who is a good friend of the welder.

I would really like to learn how to do it myself though...
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Unread postAuthor: rp181 » Sat May 31, 2008 2:40 pm

im going into highschool too next year. In 9th grade, we can choose electives, but theres this thing called "project lead the way" where you design something on CAD and make it.
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Unread postAuthor: ramses » Tue Jun 03, 2008 12:49 pm

i am about to just give up and go with copper, but how can i attach this to threaded steel? could I just solder it into a steel female threaded coupling and expect it not to blow off? if not, how is this done?

also, just to make sure is 2" sch40 welded steel okay for a 15x MAPP/propane-air chamber, assuming all forged steel fittings (3000 psi)?

just to make sure, 150psi malleable iron should be able to take 200psi easy, right?

thanks again, ramses
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Unread postAuthor: DYI » Tue Jun 03, 2008 7:41 pm

i am about to just give up and go with copper, but how can i attach this to threaded steel? could I just solder it into a steel female threaded coupling and expect it not to blow off? if not, how is this done?


Copper threaded adaptors. They have a solder connection on one side, and threads on the other.

also, just to make sure is 2" sch40 welded steel okay for a 15x MAPP/propane-air chamber, assuming all forged steel fittings (3000 psi)?


It's okay if you're at least 100 feet away from it when you set it off. In any other case, absolutely not. That's higher pressure than even I have ever achieved in a spudgun. It's doable, but for your own sake, get some proper pipe. You do realise that 15x MAPP/air could reach over 1.7kpsi, and that the burst pressure of 2" SCH 40 steel (assuming a perfect weld) is about 2.3kpsi, right? And just out of curiosity, how do you plan on reaching 215 psi with air? Homemade pump, kickass compressor, or bottled gas?
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Unread postAuthor: ramses » Wed Jun 04, 2008 9:28 am

ok, maybe 15x was a little excessive... a lot. in HGDT, you're right, it can reach 1.7kpsi if i use a 1.7kpsi burst disk. right now, i was going to use around 500psi, which it tells me around 800 psi peak. i know HGDT is inaccurate (especially because it thinks the stoichiometric ratio for MAPP is half that of propane), but that leaves my safety factor around 3. SCH 80 isn't that much more, so i may go with that.

i was just going to use a shock pump, but i suppose i will lower the mix to 14x so i can keep the 200psi meter i was going to use. or i could just get an oxygen regulator...

thanks,
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Unread postAuthor: SpudFarm » Wed Jun 04, 2008 9:53 am

oxygen regulators don't put much pressure out. DYI modded his reg and that can get ugly if it fails on you
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Unread postAuthor: ramses » Wed Jun 04, 2008 2:08 pm

but for 15x with oxygen, I'd only need 30psig since the air is only ~21% oxygen. still, that's another tank, another high accuracy gauge, another component to manipulate, and probably a buttload more money. i think I'll go to SCH 80 instead...

edit: i decided to go with a 2"x18" chamber. will cost around $50 less in fittings alone...
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Unread postAuthor: DYI » Wed Jun 04, 2008 4:52 pm

Firstly, 15x using pure oxygen as an oxidiser would be roughly 50psig.

right now, i was going to use around 500psi, which it tells me around 800 psi peak


What the hell does that mean? If you fuel to 500psi, I can guarantee you that the combustion pressure will be far in excess of 800psi. Try something more like 3.5k, or more.

oxygen regulators don't put much pressure out. DYI modded his reg and that can get ugly if it fails on you


That's generalising. I have a very poor quality reg. at the moment, and had to mod it for it to be useful. The one I'm getting in the summer is rated at up to 200psi output, and some oxy. regs can output from 0psi to the entire tank pressure.
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Unread postAuthor: Lentamentalisk » Wed Jun 04, 2008 5:59 pm

Ya, I just got an oxy reg from a "connection" that I am going to use for my SCBA tank that goes from 4000 to 400psi.
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Unread postAuthor: ramses » Thu Jun 05, 2008 1:11 pm

DYI wrote:
right now, i was going to use around 500psi, which it tells me around 800 psi peak


What the hell does that mean? If you fuel to 500psi, I can guarantee you that the combustion pressure will be far in excess of 800psi. Try something more like 3.5k, or more.



yeah, i have to agree that that doesn't make much sense. i meant that the burst disk would burst at around 500psi, and that HGDT said my peak chamber pressure would be around 800psi. the peak pressure could reach 1.7kpsi if the burst disk held to 1.7kpsi (which is rather unlikely). Whoever is wrong (probably me), you convinced me to go with SCH 80 pipe.

in any case, i am too cheap to use oxygen. I'd rather double the barrel length...

sorry for my momentary retardation.
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