Combustion Launcher Explosion

Post questions and info about combustion (flammable vapor) powered launchers here. This includes discussion about fuels, ratios, ignition systems and anything else relevant to launchers powered by igniting things like hairspray or propane.

Postby mcook » Mon Feb 26, 2007 12:13 am

I have built and since destroyed my 6" PVC combustion cannon. It had a chamber volume of over 61,000 ml. Each chmber had a fan and a 6" screw cap on the end. The ignition used a model T Ford coil and car battery for ignition. The cannon ended up exploding as the screw caps blew off and shot through my neighbor's windows and walls. The fans blew out of the chambers and left an imprint on the inside of the screw caps. It was the loudest explosion I have ever heard. I am puzzled on why the explosion traveled this way and not out the barrel. If anyone has any reasoons why this happened, please let me know.
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Postby sgort87 » Mon Feb 26, 2007 12:29 am

You used thinwall pipe fittings.
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Postby TurboSuper » Mon Feb 26, 2007 7:01 am

Yeah, or you used acetylene.
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Postby fullmetaljacket » Mon Feb 26, 2007 8:43 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">...shot through my neighbor's windows and walls.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

[:0]

Could you elaborate on that? *Through* the walls, you have to show us photos :D

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">I am puzzled on why the explosion traveled this way and not out the barrel. If anyone has any reasons why this happened, please let me know.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

The explosion attempts to "travel" in all directions, and nature always chooses the pass of least resistance. In this case, the inertia required to get your projectile moving was greater than the force require to blow off the caps. Did you use an excessively tight/heavy projectile?
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Postby M15 » Mon Feb 26, 2007 8:19 pm

Sounds like somebody used an Oxy/fuel mix...
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Postby mcook » Mon Feb 26, 2007 11:23 pm

I used propane only as the fuel. The fittings were schedule 80. There was a loose rag in the barrel opening. This would have been the path of least resistance. Do you think the location of the fuel injection compared to the spark and fan location had anything to do with this explosion?
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Postby hi. » Tue Feb 27, 2007 12:09 am

you said that the end cap came off? I do know that if the cap isnt tight the threads melt. trust me, ive had this happen, but they didnt shoot off. so you probly should look at the caps and see if they are melted. If so, they probly just didnt get a grip, or the plastic wast weak and simply broke. I also had a simular experience with my cannon. Only instead of the end cap, i had the bell adaptor pop off. it only went about 2 feet, but the 5 foot barrel along with my hand were attached, no dammage done, but my wrist hurt for about a day.
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Postby Velocity » Tue Feb 27, 2007 12:12 am

The fittings could not have all been SCH 80. You say you used a screw cap? I believe that you mean a cleanout cap, which is probably the weakest PVC fitting I have personally ever encountered. Maybe I am wrong, and you did use a SCH 80 Plug. If this is true... Then for 1, I am amazed you could find one that large and afford it, and 2, it seems very unlikely for it to blow off. I use a 4" one on one of my piston valve cannons, and I can tell you, its a durable thing. I have a piston that must weigh over a pound slamming into it with incredible force, and I have not been worried at all about it exploding.

Mind posting some pictures of your cannon? I suppose it might be hard to retrieve the screw cap...
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Postby mcook » Tue Feb 27, 2007 12:37 am

I need to correct myself. The caps were sch 40 but the male adapters were schedule 80. One of the chambers had part of the sch 80 male adaptor threads torn off. The caps were not on tight put had been threaded on pretty far. I was unaware that the threads could melt. I can post a drawing of what was built but no pics as I needed to have it destroyed.
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Postby sgort87 » Tue Feb 27, 2007 1:57 am

I'm gonna have to call BS on this one.
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