SpudFiles

Right before i went on vacation, my mom decided "wouldn't it be cool to build a potato cannon?"
Once we got there, my mom spent the morning looking for designs online. She described to me some tin-can cannons she use to make for tennis balls. After hours of research, she went to the hardware store and got a 2 feet piece of 2 inch PVC tube. She took an empty soup can and duct taped it to the bottom, then covered the whole thing in duct tape and poked a hole in the side of the can.
This afternoon when we tried to fire it, it would not work. We were using lighter fluid and hair spray as our propellant. i need some ideas for a couple quick-fixes so we can get this thing working

Haha, sounds like you have a cool mum!

First off... the whole duct-tape approach is frowned on a bit here. It's really not particularly safe. However, for a small cannon like what you've made it probably won't fail immediately unless you jam something really tight in the barrel. And the touch-hole method of ignition is a very easy way to burn yourself.

As for the problem, I'm betting you are either using way too much fuel, or not venting out the fumes between attempts. You only need a very small amount of propellant, or it won't ignite at all.

I do recommend that you give the whole duct tape approach a miss and build one properly...

What insomniac said.

The most likely cause of no-fire is too much fuel. For a chamber the size of a soup can the proper amount of liquid fuel is probably down in the 1 drop range. Too much fuel, or too little fuel, won't ignite. For more info on liquid fuels see http://www.inpharmix.com/jps/Liquid_Com ... fuels.html

For more info on gaseous fuels, like propane, hair spray etc. see the spudwiki page on fuels at http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns

A 2' piece of 2" diameter pipe on a soup can? Sounds like the chamber is way to small. As a typical starting point you want the volume of the chamber to be about the same as the volume of the barrel. A common configuration is a 2" diameter barrel on a 3" diameter chamber (both made from PVC). A 1:1 volume ratio is barrel about 2.25 times longer than the chamber. Your local hardware store should have 2" and 3" pipe and the reducer that'll join them. Also get a 3" clean-out adapter (a threaded fitting and cap) for the back end of the chamber. Properly applied PVC primer and glue is the only way to safely join the parts.

Have a look through here if you haven't already. It'll give you some ideas for improvements and future builds.

I wouldn't use lighter fluid as a fuel for a combustion cannon, its vapor pressure is too high. The components of hairspray you're combusting is probably the propellants, butane and ethanol.

The duct-tape approach will hold for a few shots on a spray 'n' pray (experience speaks ) but it is NOT recommended, because a failure will result in burns (experience speaks again). The duct tape melts after exposure to the heat.

Quick-fixes? None, really. Go back to the hardware store and buy some lengths of PVC with primer and cement, with an endcap you can unscrew. Drill holes in the chamber and install a piezo-electric igniter from a BBQ igniter. Fuel from the back, screw the cap on, and spark the igniter.

Dude, your mom is amazing.