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New Failure Mode in Combustion Cannons

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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New Failure Mode in Combustion Cannons

Unread postAuthor: DYI » Sat Jun 23, 2012 10:41 am

I recently built a 1X combustion gun following the chamber layout of BigBang's "Crusader" (single ignition point at back of chamber, fan immediately after) for recreational use.

The fuel meter was plumbed into the front of the chamber through a double-thickness wall with a 1/8" NPT hex nipple, which was followed by a 1/8" -> 1/4" bushing, a 1/4" 90° elbow, a 1/4" hex nipple, and then the first ball valve for the fuel meter. The meter assembly was fairly standard, and clamped onto the wall of the chamber with two large hose clamps (with a spacer to keep the meter roughly parallel to the chamber).

The problem arose as the result of a shortcut I took during the initial assembly and testing phase combined with a low quality hex nipple. Seeing as how nothing between the ball valve and the chamber would be pressurized, except during the shot, I neglected to apply PTFE tape and tighten the nipple fully into the elbow to ease disassembly (the nipple was engaged four turns into the elbow).

The gun had been tested remotely six times with a rag jammed very tightly into the breech (needed to be knocked in with a hammer). I'd resorted to high friction rather than a heavy projectile due to laziness, and convinced myself of the safety of the launcher without fully comprehensive testing.

Yesterday, my assistant and I set out to a suitable outdoor test location in kayaks. We got there without incident, set up the launcher, loaded a potato, injected the fuel, turned on the fan, and pointed it out over the water. Instead of the heavy recoil I was bracing for, I was rewarded with a sharp punch to the gut, and the immediate realization that it was going to be a long, painful trip home. The 1/4" hex nipple had torn completely out of the elbow, knocking the entire meter pipe back by an inch or so. The resulting gas jet (consisting of the entire 5L chamber volume) sprayed across my belly, causing deep second degree burns in places and a first degree burn over the remainder, totaling ~3% of my body surface area. Not at all threatening, but quite annoying.

To add insult to injury, the sharp pressure drop behind the projectile caused the air pressure built up ahead of it to fire it back into the chamber, utterly destroying the fan. As of yet, I don't know if the endcap will need to be replaced.

RECOMMENDATIONS:
-Perform remote tests with all projectiles likely to be used, as well as closed chamber tests before handheld use.
-Where possible, perform tests above with oxygen enrichment aiming for 120% operational chamber pressure.
-Always wear gloves and at least a long-sleeved shirt while operating combustion launchers
-Design launcher so as to point chamber ports away from operator

The pictures below include one of a minor burn injury. Don't scroll down any further if you're pathetically squeamish.

Stripped thread on pipe nipple
Image


Burn with dressing:
Image


Burn without dressing, 1 day later (apologies for the quality, the low light required exposure long enough that a tripod should have been used):
Image
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Unread postAuthor: jackssmirkingrevenge » Sat Jun 23, 2012 11:05 am

Ouch... you live, you burn.

Hope it heals infection free and that you like chicks who dig scars.
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Unread postAuthor: sagthegreat » Sat Jun 23, 2012 3:23 pm

Spudding is dangerous, and hope it heals fast, and I hope you didnt loose intrest in the hobby
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Unread postAuthor: Fnord » Sat Jun 23, 2012 3:33 pm

...and I hope you didnt loose intrest in the hobby

Ha! He's been through far worse, remember...

_____________

4 turns worth of steel threads had been stripped?
It looks like it just backed itself out somehow, having much less than 4 turns thread engagement when the shot occurred.

I know I've made jokes regarding my lack of clothing when building/operating dangerous equipment, but I generally at least try to cover what's in contact with the device in question. I don't think I've ever *not* worn a shirt/pants when testing out a new launcher, or heavy gloves in the case of a hybrid.

As to scarring, my left forearm has small, barely visible white marks from a couple years ago. The loss of skin in that case was perhaps slightly worse than yours, so if you're concerned with scarring it should not be much of a problem.

(This was not a spudding related injury, but sun-burn that was abused for weeks and had no type of treatment applied whatsoever. No, I'm not telling the story.)
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Unread postAuthor: jsefcik » Sat Jun 23, 2012 4:09 pm

Fnord wrote:
...and I hope you didnt loose intrest in the hobby

Ha! He's been through far worse, remember...

_____________

4 turns worth of steel threads had been stripped?
It looks like it just backed itself out somehow, having much less than 4 turns thread engagement when the shot occurred.

I know I've made jokes regarding my lack of clothing when building/operating dangerous equipment, but I generally at least try to cover what's in contact with the device in question. I don't think I've ever *not* worn a shirt/pants when testing out a new launcher, or heavy gloves in the case of a hybrid.

As to scarring, my left forearm has small, barely visible white marks from a couple years ago. The loss of skin in that case was perhaps slightly worse than yours, so if you're concerned with scarring it should not be much of a problem.

(This was not a spudding related injury, but sun-burn that was abused for weeks and had no type of treatment applied whatsoever. No, I'm not telling the story.)



after my golf ball incident i take much higher precautions! now!
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Unread postAuthor: mattyzip77 » Sat Jun 23, 2012 4:24 pm

Damn dude, hope they gave you some percocets!!!
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Unread postAuthor: MrCrowley » Sat Jun 23, 2012 5:02 pm

Goddamn I hate burns, hope they gave you some prescription pain-killers :wink:
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Unread postAuthor: DYI » Sat Jun 23, 2012 5:21 pm

It looks like it just backed itself out somehow, having much less than 4 turns thread engagement when the shot occurred.


This is certainly what it looks like. However, the meter couldn't make more than half a turn before the ball valves hit the support. I was at loss as to how exactly this happened. The threads on the elbow were certainly not of high quality, but this didn't explain how only a single turn of the brass nipple's thread was stripped off. A localized overdriven detonation could explain what happened, but I thought that unlikely, so I took another look.

The internal threads in the elbow have a flaw which prevents a properly cut pipe nipple from being threaded in more than 1.5 turns. This quite likely means that I was tired or distracted when assembling the meter, possibly counting half turns as whole turns.


Spudding is dangerous, and hope it heals fast, and I hope you didnt loose intrest in the hobby


Why would I lose interest in the hobby?
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Unread postAuthor: AircannonSweden » Sat Jun 23, 2012 5:30 pm

I hope that you will be alright dude! greetings from Sweden
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Unread postAuthor: POLAND_SPUD » Sat Jun 23, 2012 7:03 pm

Goddamn I hate burns, hope they gave you some prescription pain-killers
OMG dude grow some balls - pain is weakness leaving the body
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Unread postAuthor: MrCrowley » Sat Jun 23, 2012 7:35 pm

I was wondering if they gave him something like Vicodin; I wasn't concerned about whether they were relieving any pain :wink:
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Unread postAuthor: ramses » Sat Jun 23, 2012 10:00 pm

Off topic, but is there a particular reason you date-stamp your pictures? I find that I usually have to crop them out for printing and stuff. Date and time taken is usually in the EXIF data.
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Unread postAuthor: Moonbogg » Sat Jun 23, 2012 11:53 pm

Well this is unfortunate. Of all the high powered experiments you do it sure is ironic that a simple combustion got the best of you lol. Goes to show the hobby is inherently dangerous and is not to be taken lightly. Makes me wonder what would happen if my 2X cannon took a crap on me during use :shock:

Glad you are OK.
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Unread postAuthor: Zeus » Sun Jun 24, 2012 5:17 am

Well done DYI, stripping threads with a combustion.

Burns suck, I've cooked my hand one, and healing burns on a moving area takes a while. Anyway, try aloe vera gel if it gets bad, cools it down.
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/sarcasm, /hyperbole
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Unread postAuthor: Petitlu » Sun Jun 24, 2012 5:45 am

Wouaaaa ....
I hope you heal very quickly!
We do a dangerous passion, do not forget ..
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