Hi, I've just started a temp job of teaching at Radley College, and they have asked me to make a spud cannon. They have loads cartridges of N2O, and acetylene and O2 in bottles. I've ordered a N2O dispenser, so I can get the gas out at a regulated rate. I'm thinking one of these "hybird" cannons - charge a chamber to high pressure, then detonate it to create an even higher pressure. Does anyone have experience building on of these?!??! The budget is fairly generous, and they have specified using steam pipe! Thanks, Profqwerty
They really aren't hard to make. You can't go wrong - we used to make them as kids, spraying hairspray into a piece of metal pipe, drop in a potato, followed quickly by a match. The only thing you need to worry about is trying to do too much pressure - even homemade crappy ones can easily break bones. Make sure you're aiming it at something you don't mind hitting and be sure to do the math to be certain.
I have a decent amount of experience airsmithing/working with compressed gases (HPA, CO2 & a little N2) and I've built a HPA spud gun. First and foremost, remember that anything you assemble has to be able to survive the pressures you're putting it through, not to mention the explosion. Seal the breech, and perhaps you'll want burst disks on the sensitive sections - regs etc. Steam pipe, as in iron? How are you going to ignite it? Piezo spark? Spark plug? The little widget out of an oven (forgot the name)? Breech loading? Have you considered the best fuel to oxygen mix? And how you'll achieve it? Out of interest, is it a whip cream dispenser?
here's a design i quickly made in CAD: 1 - barrel - 1.5" steam pipe 2 - flange - connects the barrel to the combustion chamber. sheets of foil/plastic/tape/whatever are sandwiched between the flanges (held together with bolts thru them), allowing the pressure to build up, until it ruptures in one go. 3 - bolts holding base to the combustion chamber. allows chamber to be opened to clean/adjust things 4 - plate welded on top of the combustion chamber wall - 5mm thick steel or so 5 - combustion chamber wall - 5" steam pipe. 6 - ignition system - several spark electrodes in series, creating multiple sparks in one go for better ignition. 7 - base - same as the top 8 - filling port - 90 degree ballcock - has a quick connect connector on one side, and the other screwed onto the base. the aceylene hose can be connected to this, and the system filled to one pressure, then the N2O dispenser connected to it and emptied into the chamber. it's then closed so the system is sealed. there will be a hole tapped somewhere for a pressure guage aswell ( this may have another ball valve to protect it from the massive detonation pressure). Can anyone tell me the pressure inside one of those N2O cartridges - I tried the various mole calculations, but kept getting weird results (43atm??). I figured it had about 4 litres of gas inside (at 1 bar). I also figured the stoiciometry (sp?) to be 3N20 + C2H2 -> 2CO2 + H2O, so the ratio of pressures is 3:1??? There shouldn't be a chance of this exploding, as people seem to be using PVC piping, and I'm using steel steam pipe + welding. thanks for the help! profqwerty
at 70f, co2 liquefies at ~850 psi. this is VERY temperature-dependent though. my marker tends to shoot hot when it gets above 85f out, and that is with a remote line and the tank in a 4+1 pack on my back, not on the marker. as for n2o, i couldn't tell you. i would do the test firing behind a double thick stack of cinder blocks. shrapnel, you know.
