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First spud gun and I'm having issues.

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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First spud gun and I'm having issues.

Unread postAuthor: dan-27 » Fri Sep 10, 2010 10:43 am

Hi there I was in the Sates this year for the 4th of July and saw one of these beasts for the first time and knew I had to build my own when I got back to England.

I have sourced all the parts and cemented them together. It's very simple 1.5 foot X 4" down to about 4 foot X 2".

The ignition is the issue. I salvaged a sparker from a gas heater and have it sparking between the electrode and a 8mm bolt. It sparks really well before any gas goes in I've tried Right Guard and cheap hair spray and I'm finding that as soon as any gas enters the chamber the sparking stops.

I have cleaned the electrode, changed the gap achieving larger and smaller sparks, different amounts of propellant anywhere from half a second to about 4 seconds. But I can't get it to spark when it's required.

Any advice would be great, thanks guys

Dan
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Is this the right fuel?

Unread postAuthor: Technician1002 » Fri Sep 10, 2010 11:20 am

Make sure your fuel is propane or butane propellant based. The environmentally friendly ones that you have to pump are lacking the flammable propellants and the water in them short out the spark gap.

Spray your fuel of choice at a lighter flame and see if it burns well.
The cheap stuff in a spray can is the stuff you want. The stuff in the plastic bottle with a spritzer pump is the wrong stuff.

The simple test in the video will show if you are using the correct fuel.
Video removed by jrrdw, dangerous example!
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Unread postAuthor: Fnord » Fri Sep 10, 2010 12:08 pm

Maybe something in your propellant is conductive, and shorting out the gap by allowing the charge to follow the chamber walls?
Engine starting fluid is ideal if you want to use a spray can propellant.
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Unread postAuthor: jrrdw » Fri Sep 10, 2010 1:37 pm

If you want to test any type of fuel, spray a small amount on concrete then roll up a piece of news paper and lite it on fire and touch it to the small puddle of fuel to be tested, if it ignites it should work in your launcher.

There is also a user generated list of fuels you can use in the Wiki, the button is at the top of the web page.
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Unread postAuthor: Fnord » Fri Sep 10, 2010 1:46 pm

By the way jrrdw, it's usually the gas mixture in an aerosol can that is flammable, not the stuff that the gas mixture itself. You'll have to spray it into a bucket rather than out in the open air to ignite it easily.
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Unread postAuthor: jrrdw » Fri Sep 10, 2010 1:55 pm

Fnord wrote:By the way jrrdw, it's usually the gas mixture in an aerosol can that is flammable, not the stuff that the gas mixture itself. You'll have to spray it into a bucket rather than out in the open air to ignite it easily.


Good point...
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Unread postAuthor: Technician1002 » Fri Sep 10, 2010 2:21 pm

I guess i should have found an outdoor example where the lighter is not hand held.. Sorry.

JRRDW, can you find a better example of testing the flammability of an aresol spray? I don't mind the edit for safety as an unsafe indoor test. A suggested alternative would be nice.
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Unread postAuthor: jrrdw » Fri Sep 10, 2010 2:32 pm

Fnord wrote:By the way jrrdw, it's usually the gas mixture in an aerosol can that is flammable, not the stuff that the gas mixture itself. You'll have to spray it into a bucket rather than out in the open air to ignite it easily.


Never ever test indoors for safty reasons. This part of the discussion brings up a very good topic.....BRB.
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Unread postAuthor: Fnord » Fri Sep 10, 2010 2:37 pm

(Flamethrowers are legal in the US, by the way, so long as state or local laws don't forbid them. If that's what you're getting at...)
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Unread postAuthor: jrrdw » Fri Sep 10, 2010 2:42 pm

As far as testing fuels safely....here.
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