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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 spuzi14 » Sat Feb 17, 2007 12:28 am

With a snow storm comes two days off school. With that and parents refusing to take you somewhere comes bored. So I whip a spudgun.

I made it out of 27 inchs of SCH40 3/4 inch PVC for the barrel and a coffee can for the chamber. I cut a hole into the bottom of the coffee can and then just used a male adapter to screw it in. Instead of being average (and to take up more time) I took 3/4 CPVC and put to screws in it, one above the other. Then I wired a BBQ igniter onto those screws and fished the wire through the end of the CPVC which was then capped off. I was shooting 3/4 inch plastic (derlin I think) rods that were about 1.5 inchs long. They don't fit PERFECT in the barrel leaving probably a little less than 1/8 of an inch gap between it and the barrel. None the less, I have succesfully fire it out of the same barrel I was using here. I fueled this with some old Right Guard I have.

So I got this thing all together and I spray some Right Guard in and fire it. I'm inside the basement and the gun is by the barrel in a vice on a workbench. I'm firing at a piece of wood. When I hear the fuel ignite, I see (well barely see) the lid half come off and then go back on. I had used electrical tape to hold it on. Really, I didn't see it BUT I did see orange flames coming out. I was looking at the barrel more and seen the white rod about to fly out but then it stopped! It came about halfway back into the barrel, I was pissed off!!

Can anybody tell me what happened? I'm guessing the flame was able to escape faster out the barrel but also able to escape both ways. This could have caused a faster movement out the barrel for a fraction of a second until the flames out the lid equalized the pressure and allowed the rod to come back a bit. I hope that made sense and I hope somebody can tell me what happened.



I know I broke a crapload and a half of safety rules. Please, if you're less than 21 years of age always do the exact opposite of what you read in this post or you might look like this :? or this :x causing maybe people to look at us like this :evil:then we will look back like this :evil: and overall it will just be ugly.
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Postby Iowaengineer » Sat Feb 17, 2007 2:03 am

I'm pretty sure you got that right. My thermodynamics intuition tells me that the air inside the chamber and the barrel was able to come out at a very high rate, when your lid came half-off, those combusted gasses escaped really fast allowing a vaccume to form, shutting the lid. With a vaccume in the chamber and the lid shut, the only way that air could get back in to stabilize the pressures was through the barrel. This explains the rod going half out then coming back in.

I think that could have happened, I don't exactly know. Also, I love snow days!
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Postby spuzi14 » Sat Feb 17, 2007 12:03 pm

If I got that right then I don't know what to say. That's crazy because I've never taken phyisics (except reading on here) and I'm only in chem. I guess I underestimate myself.
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Postby Duodec » Sat Feb 17, 2007 2:47 pm

I've had similar experiences, it usually happened for me when the combustion was weak (granted my chamber was small for the type of cannon). In your case it sounds like your lid is working like a check valve thats only letting gasses out.
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Postby sgort87 » Sat Feb 17, 2007 3:38 pm

This happened to me ALL THE TIME with that stupid 16' Pumpkin Launcher.

What happens when the mix is not correct, is the combustion will first off obviously not reach the optimum expansion, but it will get you some. The projectile often makes it to the tip of the barrel just as the gasses stop expanding and at that point they rapidly cool off and thus shrink. This creates that vacuum and it sucks it back into the barrel about half way or so.

This is especially annoying in large cannons with fixed barrels and tight projectiles because of this vicious cycle: Projectile stuck - can't vent. Cant vent - can't fire. Can't fire - can't get the [currentelly intercourseing] projectile out. [:(!]
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Postby spuzi14 » Sat Feb 17, 2007 4:16 pm

So my barrel is too long in some cases and the lid is a piece of junk in other cases? I say the lid because the projectile didn't even make it to the tip of the barrel today. It actually ended up in the chamber.

This happened to me 4 more times today (I got the ignition fixed) and it's actually humerous. I'm expecting a rod to fly out of the barrel and I just hear a PHOOMP! and that's it.
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Postby sgort87 » Sat Feb 17, 2007 4:44 pm

Try adding something like a PVC cap into the chamber so you can shake it around to mix the fuel. Then work on getting the mix right. If that isn't doing it then chop the barrel a bit.
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Postby spuzi14 » Sat Feb 17, 2007 11:58 pm

Well I'm getting a computer fan off a friend so I might be able to use that. Could I use some loose changes or scrap metal instead of a PVC cap?
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Postby sgort87 » Sun Feb 18, 2007 1:28 am

You sure can. I just find that a PVC cap or something similar in shape moves air better than small flat things.

Actually, you might want to try soda bottle caps or glass bottle caps. They would move air considerably better than coins and I'm sure you must have some around.
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Postby spuzi14 » Sun Feb 18, 2007 11:13 am

I got a ton of shoda(bad spelling intended) caps laying around. I gotta do some snow shoveling today and then I'll experiment. Perhaps I should have built a snow powered snow blower...
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