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Hybrid Barrel Explosion

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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Hybrid Barrel Explosion

Unread postAuthor: whoa044 » Mon Sep 27, 2010 7:59 pm

No actual barrels exploding, sorry.
I was interested in knowing what kind of barrels you guys use.

Currently, my only option is copper. It's light, and can be bought in long sections. Anything plastic is out, since I'm guessing it'll shatter into a thousand pieces. HGDT shows pressures in the barrel will be slightly greater than pressures in the chamber. I'm using 2" forged steel, and I think using forged steel as a barrel would be way too heavy and costly. My point is, does barrel material really matter? Or can it withstand the pressure?

On another pressure-related topic, do any of you guys use gauges designed to measure maximum pressure? I've seen those designed for diesel engines, but cost around 100$ and require an engine run time of 5 seconds for a reading.

Will a check valve on a high pressure gauge work?
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Unread postAuthor: Gun Freak » Mon Sep 27, 2010 8:01 pm

Copper will be fine for your hybrid barrel. But you might want to change the title to something mroe like " Hybrid Barrel Material" or something.
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Unread postAuthor: Technician1002 » Mon Sep 27, 2010 8:10 pm

There are a few copper barrels that had a DDT event and it created a bulge or blowout. Use copper at low mixes.


Hmm can't find the picture at the moment. I'll have to search more later.

Edit, found one.

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Last edited by Technician1002 on Mon Sep 27, 2010 8:14 pm, edited 1 time in total.
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Unread postAuthor: saefroch » Mon Sep 27, 2010 8:11 pm

Nobody here has ever tried to measure their maximum pressure, we just assume HGDT is correct.

Your barrel will only be subjected to those pressures for a tiny fraction of a second. Copper should hold up quite fine, some plastics might even stand up to it, like in SB15's handheld.
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Unread postAuthor: SpudBlaster15 » Mon Sep 27, 2010 8:23 pm

The suitability of a material for your application will depend on the mix range you plan to use.

PVC is obviously out for anything >2x.
Solid core ABS will be fairly safe at up to around 5x, or higher if reinforced.
Copper will withstand any mix you can achieve using a standard hand pump or air compressor, but a barrel of any significant length will be heavy and cumbersome, especially if this is going to be a hand held launcher.

Another potential option is thinwall galvanized steel EMT conduit. It's quite readily available, but I've never seen it adapted for use as a launcher barrel.

If you're open to ordering online, definitely look into 1.5" SCH10 aluminum. It has a relatively high burst pressure, is reasonably lightweight, and fits golf balls well.

EDIT: While we're on the topic of exploding hybrid barrels, here's a good example of the failure characteristics of ABS in excessive pressure conditions (4x oxy/propane), courtesy of a certain member who also hails from Canuckistan.

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Unread postAuthor: D_Hall » Mon Sep 27, 2010 8:29 pm

saefroch wrote:Nobody here has ever tried to measure their maximum pressure, we just assume HGDT is correct..

I believe Jimmy has.

And 2 weeks from now I will have (assuming all goes according to plan).
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Last edited by D_Hall on Mon Sep 27, 2010 8:38 pm, edited 1 time in total.
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Unread postAuthor: D_Hall » Mon Sep 27, 2010 8:36 pm

SpudBlaster15 wrote:but a barrel of any significant length will be heavy and cumbersome, especially if this is going to be a hand held launcher.

Granted, mine is a pneumatic but... My use of 6 feet of sch 10 stainless steel 1.5" pipe for golf balls says otherwise. The gun is heavy, but it's not because of the barrel it's because of the chamber. Use a smaller chamber (as hybrids do) and it will be fine.

And said pipe should be good through about 8X (with a roughly 4:1 safety factor).
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Unread postAuthor: whoa044 » Mon Sep 27, 2010 10:17 pm

The gun will be expected to fire at up to 15x mixes, possibly 20 if I have faith in the 2" forged steel pipe. At 10x HGDT calculated barrel pressures to be 1000psi +/- 50, depending on how strong the burst disks are.

Thanks for the suggestion on aluminum. Is sch 10 safe enough to go at these pressures, or should I go for higher thickness?
I figured the title may attract some people
>.>
<.<
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Unread postAuthor: Fnord » Tue Sep 28, 2010 11:30 am

With 1.5" sch10 aluminum I'd wrap the first 1/3 to 1/2 of the barrel with some fiberglass cloth, if I was planning on standing nearby. I figure it's about as strong as low-to-medium grade aluminum, so a 1/8" layer should add plenty of saftey factor.
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Unread postAuthor: whoa044 » Wed Sep 29, 2010 3:40 pm

I just bought two barrels today. One 7ft long copper, 1/2 pipe, and one 5ft galvanized steel 1/2. They both have different inner diameters, for the two ammo types I will use.

I have a few reducers for the copper pipe going from the 1-1/4 bushing, down to the 1/2 pipe. Since I don't want it as far from the union, I will slide it a few inches past the reducer it's connected to, and towards the union. This helps with more power directly into the barrel, and breech loading (I won't have to reach in)

Edit:
What would you guys suggest to be the highest mix I would go with a 2 inch nipple, 0.225 inch wall thickness? Will post pictures in a bit.
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Unread postAuthor: D_Hall » Wed Sep 29, 2010 3:56 pm

whoa044 wrote:Edit:
What would you guys suggest to be the highest mix I would go with a 2 inch nipple, 0.225 inch wall thickness? Will post pictures in a bit.


Keeerist, are you really so helpless that you can't google for pipe failure equations and break out a calculator? Here, I'll even do the google part for you....

http://www.scribd.com/doc/9724087/Press ... sel-Design
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Unread postAuthor: saefroch » Wed Sep 29, 2010 4:06 pm

Just a side comment to D_Hall... You'd be surprised how much Googling I've done for newbies on this site. Yes they often are that helpless....

Be careful of driving too close to the failure level of anything, hybrid combustion is not a normal load.
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Unread postAuthor: kenbo0422 » Wed Sep 29, 2010 5:58 pm

Try this one:

http://www.engineersedge.com/calculator ... t_calc.htm

and this one is useful as well:

http://www.engineersedge.com/calculator ... es_pop.htm


A general all around online help is found here:

http://www.engineeringtoolbox.com/
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Unread postAuthor: whoa044 » Wed Sep 29, 2010 8:19 pm

^ Thanks for the links!

I would feel safe to work with the steel even if it had a tensile strength of 30,000 psi. It's bursting pressure would be 7000
If it was around 55,000, which it should be, it'd be 13,000 ;)
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Unread postAuthor: inonickname » Wed Sep 29, 2010 8:36 pm

You should use the yield strength, not the failure one. You also need to add in a safety factor. Finally, the seam on that pipe and the galv (galvanising is done hot, and tends to soften the metal) will drop the strength by a lot.
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