SpudFiles

I built up the following spud cannon ...

http://24.22.111.106/wordpress/?s=spud

Just installed a 100kv stun gun and when you look down the barrel you can totally see a spark at the spark plug. I tried to spray some starter fluid in but not ignition.

Do I need to have a potato installed? How long should I shoot the propellant into the chamber?

mackconsult wrote:Just installed a 100kv stun gun and when you look down the barrel you can totally see a spark at the spark plug.

"and then the plans called for a spark plug mounted in a fixed cap"

anyways stoichiometry can be a bitch when your trying to get your first combustion spudgun to work. this design is just particularly annoying to work because it has no opening in the rear to vent or inject the fuel. you will simply have to do a lot of experimenting

most commonly people put too much fuel in their cannon, if you put too much fuel in your cannon you will have to blow the air out somehow (or let it dissipate very slowly), I'd advise not putting your face over the opening of a potentially explosive mix of gases to do this, you can try blowing into a long flexible tube inserted in the muzzle being careful to not inhale any starter fluid

I viewed the spark plug in action before putting fuel in .... I am a mechanical engineer and so now what is safe.

Yes load the spud and shoot a two second blast of fuel preferably aqua net hair spray.

Thanks for the quick responses. My boy so wants to see it fire.

Like others have said, too much fuel is the most likely problem. If you want to keep the current design you can fuel with a syringe and needle through a very small diameter hole near the rear of the gun (or even through the spark plug hole). Load the ammo to a consistent depth (since you don't have a defined chamber and barrel in that design), the injection hole can be used to help vent the trapped air as the ammo is shoved into the barrel (or loosen the spark plug), inject the stoichiometric amount of fuel (probably in the 50 to 100cc range), let the gun sit two or three minutes for mixing and you should be good to go.

A reasonable partition of the gun's volume between "chamber" and "barrel" would be 1:1 or so. Whatever you choose figure out how to consistently get the same ratio (perhaps with a marked ramrod). You'll also need that measurement to calculate the chamber volume so you can calculate the proper amount of fuel.

If you drill a hole for fueling, and it is just a bit bigger than a syringe needle, you can just cover the hole with tape. Surprisingly, tape on a sufficiently small hole survives just fine. A bit fancier would be to drill a bit bigger hole and tap it for a screw.

Syringes and needles are available for a buck or so at farm supply stores that cater to horse, cow etc. owners.

So I did drill and tap on the barrel down at the base for venting and also fueling.