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Problem with propane potato gun

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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Problem with propane potato gun

Unread postAuthor: Kennymester » Fri Aug 15, 2008 11:01 pm

I just built my first spudgun today with mixed results. My measurements are as follows.

Combustion chamber - 4in PVC 24in long
Projectile barrel - 2in PVC 48in long

According to my calculations this should get my to around a 1.75:1 ratio of combustion chamber to launch barrel. My problem is that I cannot get it to launch at all. I had it go about three times, but I cannot find a common reason why. I'm pretty sure my problem is the way I have my propane hooked up. I bought a propane regulator meant to be used with a small screw in tank. From what it seems ,when I open the first valve to fill no propane comes out, only when I open both valves and turn on the regulator do I actually hear the gas. I think that the regulator is not designed for such a use, but I could be wrong. That could be one problem, another could be how the propane is brought in. The way I bring in the propane is through a 4in coupler with a t'ed reduction to 2in. I'm not sure if propane floats or sinks in air. It could be that the propane is staying in the fittings. Also for the grill igniter I used the stock piece how it came. It uses a 3 walled cage to shield the spark. This could be a problem as well, but I'm not sure. If you find any of those to be mistakes any suggestions would be great as well as any other concerns.

Thanks for your help,

Ken
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Last edited by Kennymester on Sat Aug 16, 2008 12:14 am, edited 2 times in total.

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Unread postAuthor: frankrede » Fri Aug 15, 2008 11:24 pm

Your calculations on your meter are whats off.
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Unread postAuthor: Kennymester » Sat Aug 16, 2008 12:10 am

So I have a 8in meter pipe in there now should I change it to 7in or 6in? I guess I have to much in there to achieve combustion. I think propane is not able to combust after about a 9.7% mixture.

Edit: After looking at my calculations again it seems like I could even go for a 5in pipe.
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Unread postAuthor: BigGrib » Sat Aug 16, 2008 1:15 am

I'm confused can i see a picture of your meter cause i'm really lost
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: psycix » Sat Aug 16, 2008 6:26 am

You should aim for a 4.2% mixture.
If your above 9.7, then your meter is way too large.
You can also decrease the volume of it without cutting by putting stuff (like an AA battery) in it.
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Unread postAuthor: Fnord » Sat Aug 16, 2008 8:02 am

If you're using a regulator that goes with a grill or something similar, you'll only get about 1 psi out of it. If all you need it for is a tank adapter, I suggest you mutilate its insides with a drill so it won't regulate properly.

If it's a pressure-adjustable reg, I don't know what the problem is. Make sure you're not putting liquid propane through it.
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Unread postAuthor: Kennymester » Sat Aug 16, 2008 2:02 pm

Its a grill adapter, so I think that's the problem. I already ordered a tank adapter so it should be ready by next week.

Thanks for your help.
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Unread postAuthor: Kennymester » Sun Aug 17, 2008 1:50 am

I figured it out tonight! =)

So I found out first that the propane adapter I got only works when exiting to a lower pressure, i.e. it only works if the meter isn't used. I will fix that in the future with a new piece. I also added a fan the back of the unit. The both of those combined got my cannon to shoot about 200 yards or so. All I have to say is that it was amazing. I'll see if I can upload some pics of it tomorrow.
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Unread postAuthor: psycix » Sun Aug 17, 2008 4:39 am

I had problems with a "regulate to max 15 millibar" thing on my butane tank. Which basically only lets flow trough, but no pressure.
I couldn't find any fittings to couple it directly, so I HAD to use that reg, just for its fitting.
Solution: mod it.
After ripping one reg to pieces with the necessary equipment, I saw how it worked, bought another one, and slammed a nail into the front port, destroying the shut-off valve, so it now can't block off anymore and lets all pressure just go.
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