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A place to ask general spud cannon related questions.
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Unread postAuthor: jimmy101 » Fri Mar 26, 2010 8:17 pm

For a quick gun that doesn't cost much start with butane as the fuel. It's a lot cheaper and easier to control than are aerosols like RightGuard or engine starter.

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The needle isn't needed, just use a longer piece of hose (e.g., wire insulation) so you can stick it into the chamber.

A butane lighter ($1) supplies fuel pretty slowly. You can get a rough measure of in<sup>3</sup>/sec by just bubbling the gas into an inverted measuring cup filled with water. Unlike most aerosols, the lighter will probably take a couple seconds to deliver the right amount of fuel.

The right amount of fuel is critical. The combustibility limits only span a range of about 3% to about 9% by volume. Outside that range the fuel won't ignite.

The chamber won't pressurize since the stoichiometric (i.e., the correct) amount of fuel is 3% propane by volume (4% for propane). A 100in<sup>3</sup> chamber needs just 3 in<sup>3</sup> of butane. You just let the butane push out that amount of air from the chamber.

A $12 BBQ piezo will be much more fun than a flint based sparker. You can easily build the piezo into a pistol grip. With the flint sparker you have to dedicate one hand to operate the sparker. That can be a problem since a typical size gun has a couple times the muzzle energy of a 0.22 cal rifle.

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jimmy101
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