another potato cannon thread (and a question)

[timmeh87]

well, somewhat inspired by the other guy around here who made a cannon (forget who), me and my friend jeff decided to build a potato cannon out of plastic pipe.

we decided to run it on propane by design, since propane is readily avaliable, chemically safe, and clean-burning. I personally want to try to run it on hydrogen-oxygen later on. (:D)

but it dosent even fire well yet.

here is what we have glued together so far. (this cost more than $90 in parts from home depot)

(long picture)

the entire gun screws apart into two pieces where indicated.

there is a 1/2" hole drilled in the end cap, that we originally had a spark plug screwed into. we hacked it up to a barebecue lighter, and it did spark, but it failed to ignite anything we put in there (it was easily ignitable with an open flame held up to the hole. )

after trying several ideas, we moved to a makeshift igniter system. A 1/2" rubber stopper with two small wires going through it is shoved in the hole, and some fluffs of steel wool are tangled around the exposed ends of the wires. we charge up a disposable camera "zapper", then connect the other ends of the wires directly to the capacitor.

in open air, it usually explodes spectacularly into sparks. but we have been having trouble getting the cannon to go off with it...

we added more air to the fuel by cutting a hole in the hose at the regulator, which seemed to solve some of issues with poor ignition (we had previously been refreshing the air in the gun with a vacuum cleaner after each shot..). however, we are not sure if our mix is quite right yet... another problem is that the ignition system is unreliable and complicated to operate.

so right now we are looking for a better way to ignite the cannon, working around our existing 1/2" hole. you can be creative and come up with something, but what i am would really like is some documentation of a design that has already been proven to work, or at least a firsthand account. (i havent googled this yet, but im throwing it out there)

also, any advice on air/fuel mixture?

if you were curious, so far we have succeeded in firing a small soft ball almost 6 feet!

one of our longest shots was with the hole unplugged though (you should have seen the jet of flame come out the back). i think that would have been a good shot if we had had our ignition sorted...

[Triton]

sounds like your getting a poor air/fuel mixture, your using propane right? the sparkplug was a good idea, widen the gap a bit and try it again, what were you using as the ignition source? how much airspace between the back of the endcap and projectile when loaded, fuel line might be too far foreward on the gun. your design is wrong for a chemical activated gun, you would have been better off with a straight pipe in all honesty, like a real gun, combustion chamber is HUGE, ineffective. and whatnot

[timmeh87]

yeah we didnt do much research into real guns... the size could be adusted inwards a fair bit by replacing the (aldredy destroyed) female thread connector.

the spark gap was. i dunno. about standard for a car. it was sparking. the spark came from one of thoe long barbecue lighter things that was wired (soldered) directly to the spark plug.

total air voume of chamber? umm.. a little over 450 cubic inches. (ish). if we are injecting the right air/fuel mixture in the first place (we arent yet), would it mater where it comes from?

[SavageLych]

Not to sound like a bung-hole, but, YOUR USING PROPANE IN A ENCLOSED TUBE, doesn't anyone else see how that could end badly? Try compressed air before you become the first fourm member to lose fingers by potato cannon....

[Sparkfist]

First with a flamible gas you only need a little added to the air. Something like a quick spray in the chamber would work fine. Also be sure the sparkplug is positioned so that it's near the middle of the chamber, that will maximize the combustion. If neccessary you could replace it with what the guy at xinventions.com did.

Also combusion cannons are not very powerful. The pressure from the burning propane isn't going to match say 70+ psi of compressed air. Combustion cannons I've read only generate about 20 to 30 psi. The key advantage you have with a combustion connon is that all you need is the fuel and battery to make the spark, much more easier to move then a big compressor.

[Ness]

Out of curiosity, what is so big with people making potato guns/launchers???

[Sir Games-A-Lot]

A small recomendation of mine would be to use a smaller projectile with a discarding sabot to increase range.

[Sparkfist]

Out of curiosity, what is so big with people making potato guns/launchers???

You're saying that you don't enjoy the want 'n destruction a potato cannon can do? Anything like tha is fun. Seeing a potato go through 1/2 ply/partical board is cool, or as another person did put a marble through a few glass bottles.

In addition to Sir Games-A-Lot's post, you can also try to use thing that will have minimal friction on the barrel. Well apples and potatos are nice they have a lot of frition on the barrel and so a lot of energy is used just gettting it pasted the mussle. I'd say if you want to try something fun, use super balls.

[Shadeslayer]

I use a latern igniter. They work great and fit perfectly in a small whole. They only problem is they break very easily.

[timmeh87]

i actually pulled a late night last night (up to 4am) and built two ignition systems, the "mark I" and "mark II" as i shall call them. i only took pictures of the mark I.

it is basically a homemade relay designed to fit in the hole on the back of my cannon. a 9v circuit closes a 300v circuit inside the barrel, and, well, the rest is history (300v coming from a disposeable camera still)



the 300v system runs through the screw and the piece of guitar wire. the problem with this is that the guitar wire slowly erodes away, and eventually it wont spark anymore.

the mark II has much larger electrodes, it only uses guitar string forr its springy properties and not for the actual spark. the solenoid is also threaded so you can remove it, and adjust the gap... kinda hard to describe i guess. maybe pictures later

[khaag]

What you need to do is build is a way of measuring how much propane you are putting into the gun. Build a Propane meter

And please do not use hydrogen or oxygen! The pipe is in no way capable of withstanding that kind of force! Exploding shrapnel piercing your body is not a good thing. If you use those fuels... you will probabally die...

Anyway, Im glad i could inspire someone! http://benheck.com/phpBB/viewtopic.php?t=13816

[vb_master]

YOUR USING PROPANE IN A ENCLOSED TUBE, doesn't anyone else see how that could end badly?

Tippman did this with a paintball gun and it worked fine.

[Metroid fan]

Just use this site http://www.advancedspuds.com/index.html And having the end cap with the hole in it is a bad idea it's better to put it in the barrel.

[SavageLych]

YOUR USING PROPANE IN A ENCLOSED TUBE, doesn't anyone else see how that could end badly?

Tippman did this with a paintball gun and it worked fine.

Aah yes, but tippman is getting payed to do it, they have experiance with gases, only reason i said that is because a kid I know made a pipe bomb with matchheads and a lead pipe, and accidently blew off him fingers.....

[Triton]

well, that kid you know is a retard for making a pipebomb to begin with, if your cautious bad things tend to happen less often