I was wondering about building up a potato cannon, using carbide power instead of hair products..
I already have some drawings i made about the prototipe, i wish to know if there is anyone experienced in that topic that can give me some hints and maybe correct the errors i made being my first work with acethylene gun.
Thanks

You might want to provide some more detail of your design e.g. dimensions, construction materials etc.

Experience of acetylene powered spudguns isn't likely to be that great, as most people regard it as too dangerous apart from in special circumstances and don't bother.

my idea is making it from one piece steel tube, the type used for gas pipes underground, that is quite thick walls, closed at the bottom with a screw cap and then soldered.
an hole is performed in the bottom cap for the insertion of a spart plugh stopped by a big bolt from the inside to prevent it to be blown out, then is obtained a combustion chamber of around 15-20cm, at the lower top of the combustion chamber two holes are performed to divert part of the pressure of the explosion out to the atmosfere and to prevent excessive pressure build up while acethylene is generated.
Some water is inserted from the barrel and lays on the bottom om the combustion chamber, it shouldn't be enough to cover the spark plugh. A small piece of carbide is inserted then a potato is set up deep enough not to cover the pressure hole in the barrel, using a calibrated piece of wood.
After few seconds waiting to allow the acethylene to be fully generated a few spark are sent to the spark plugh so igniting the mixture. If too time passes from the carbide charge and fire the holes stop pressure build up that could spit out the potato.
The pipe used for all should be around 2''-2.5'' tube with 0.5-0.7mm walls, made of steel.
make me know what you think about that.

I don't think that 0.5mm - 0.7mm thick walls will be strong enough to hold the pressure. Also there will be flames coming out of the pressure vents so you'd have to take precautions against getting burnt ;).

EDIT - You'd also have to be careful about what type of steel you use. Higher carbon steels are brittle and can shatter.

My first thought is that you're probably going to want something other than potato for the projectile. Potato isn't very good under high pressures and tends to disintergrate. Also, steel piping tends not to have smooth walls, rough barrel walls will shred a potato leading to horrible blow-by.

Something more solid and dense would give far more impressive results anyway.

How are you going to know that you have the ideal fuel/air mixture inside your cannon?

that is a weak point, there is no way to know i've reached the optimal mixture inside the barrel, it can be seen only by tests.
You are right, i didn't think about that the inside walls should be smooth, i know there are some tools that can smoothen them using a drill, but i am not sure that will be enough..
i'd cover the inside walls with a thin layer of grease to avoid excessive friction, or find a stainless steel pipe, but that would be probably brittle.

Flames coming out of pressure vents, yes for sure there will be, but i am not gonna use the cannon on my shoulder, so the flaming won't be a problem, i should try what is the best number and dimension of holes, not too many to weaken too much the pipes, not too few that the pressure would be out of control..

Its not a good idea to use a very powerful propellant with a small chamber is a bad idea. I would definitely reccomend a steel chamber 3" diameter and AT LEAST .125" thickness, .25" would be preferable but that leads to weight. There is always aluminum, which I would also recomend for the barrel even if you use steel chamber, because it is a bit smoother. I sure hope you have a welder ;) . And don't use pressure vents, though if you can get a PSI pop off valve that might be good, but just open holes is a bad idea. Trust me, I know from experience. (the only reason I'm here to talk about it is because it was a tiny spudgun, only a handgun)

As long as the chamber is thoroughly reinforced any substance can be used as propellent. Instead of hairspray I used propance from a small bottle. I disassembled a small single mantle lantern and mounted it in the end cap of my launcher so I can turn on and off the gass as needed. Its the best design I've come across yet my digital cam needs no batterys so I'll link to picks when i get it. I reinforced the chamber with a piece of three inch stell. I can launch a spud with a 2 inch diameter about a hundred yards. I know not alot of distance but its easy fun and has some fierce muzzle velocity. I've shot several neighborhood strays and knocked then out cold.

Pneumatic spud guns are much more powerful than conventional hair spray or propane powered ones. Mine will put a frozen potato through 1/2" plywood, or send it 600 yards downrange.

Here (http://www.frii.com/~bsimon/pngun.htm) is the one I built. If I have time and the cash this winter, I am going to build a piston valve gun with a 2" bore and a 4" chamber.

At one time I was considering making a "cabbage cannon", a spud gun with a large chamber and an adaptor on the barrel to expand it to 6". I figured I could send a cabbage maybe 100 yards, and they would turn to cole slaw when they hit the ground. I decided not to build it when I figured that it would cost $100+.

Thats whats an ass about pneumatics...good ones can be very expensive compared to combustions, and I know from experience they are a pain in the ass to get them to work right sometimes (I've been working on my pneumatic piston valve for a while but it refuses to work and I've given up on that for a while)

As long as the chamber is thoroughly reinforced any substance can be used as propellent.

This kind of thinking will leave you preforated some day. First of all, any PVC or plastic polymer cannon user should be very wary of fuels, because many flammable things are conveniently solvents for PVC. Second, different chemicals have different expansion times. Propane is pretty harmless if you're using SCH 40 or greater PVC, but if you want to use stuff like ether, acetylene, or hydrogen (I'm assuming you're suicidal), then metal is the only way to go. We spudgunners don't want any more incidents like the tard who shot himself in the face with a frog, or worse, an exploding cannon.

As far as safety gos, when you want to test a new fuel in a PVC cannon (I use ABS personally) fire remotely. I use a stun gun for electric ignition so I just mount it and lead off some speaker wires to a safe distance.

Someone earlier in this thread said that Pneumatics are difficult to make.
I disagree with this as I have made 3 Pneumatic cannons (all solenoid powered as I have found a brand of solenoid that opens REALLY fast) and have not had any trouble making any of them even though I had never used PVC pipe for anything else before.

True, Pneumatics cost about twice as much as the average combustion but who wouldn't be willing to pay this for the greater safety and 4 or 5 times more power.

However, a pneumatic will never be as powerful as a combustion gun that uses pure oxygen and a fuel instead of air and a fuel. These kinds of cannons usually have to be made from steel though. I hope to eventually get round to making a hydrogen/oxygen powered one from steel and using golf balls as projectiles.

Yes I use a 'star post driver', which for those of you not in aus is a piece of 3" pipe with 1/4" walls and a handle on both sides, this has a piece of solid steel for the weight in one end and the other end has a piece of 1/4" plate welded all the way around to seal the chamber. the barrel is some scrap pipe that a golf ball fits loosely into, the barrel is welded through the 1/4" plate. At the rear of the gun there is a 1/2" and a 5/8" threaded holes. For the fuel insertion the is a threaded plug that screws into the 1/2" hole, and for the ignition a normal spark plug threads in. to power the spark plug is a 6 volt lantern battery and a motor bike ignition coil switched by a push button switch on the handgrip.
Now, for the fuel I use an oxy/actylene set. First I light the torch and then tune it to a proper cutting flame then I push the torch flat up against a piece of timber thus extinguishing the flame.
The gas mixture that is coming out of the torch is placed into the chamber via the 1/2" plug for no more than 2 seconds!!! Emphasis on 2 (two) seconds, 4 seconds knocked me off my feet and left my ears ringing for about 2 days after. The plug is replaced, the battery is connected, and a golf ball is dropped down the barrel (there is a rubber ring that stops the golf ball rolling into the chamber and holds the golf ball still).
The gun, cannon is placed onto the users shoulder, aim is taken and the trigger is pulled. The ignition is ALWAYS instantaneous (unlike most aerosol spray mixtures).
The only problem with the cannon so far is that a golf ball although being aerodynamic, does not seal to the barrel like the way that an orange does to PVC, but still this is better than jamming and goodness knows what happening to the user.

Heh, you bastard I wish I had steel pipe. :cool: I had a solenoid cannon for a while, it was fun but the solenoid hampered the performance a ton, so I tried to modify the gun to accomidate a piston valve, which to date hasn't worked. Anyone know of something that is pretty solid yet light, fits well in 3" pipe, and makes a nice seal?

What kind of seal are you after? Cut two steel circles of 2.5" diameter and then cut some 1/2" sheet rubber into a 3" circle using a hole saw, this will leave you with a center hole. To trim the rubber use a bench grinder or dremel, then put the circle of rubber between the 2 steel circles. Put a bolt through the whole lot and you should have an effective seal.

Ammonal, that is an excellent idea to use a star picket hammer as the chamber, I will almost certainly use that in the one I hope to build.

Goodluck xyz, and one thing- wear some ear muffs/ hearing protection when you do fire it.

I probably won't get to build it for a while but all the parts are available and I have the necessary welding skills. I intend to wear earmuffs and remote fire the cannon the first 10 or so times.

I want to totally fill the chamber and remote fire the cannon just to see what kind of range/power will be obtained.

Has anyone seen the site where the guy has a steel cannon that uses 3 ounces of pyrodex to fire a BOWLING BALL at 500fps? I will see if I can find the link to it

EDIT : Here is the link to the kickass bowling ball mortar. (http://www.docsmachine.com/nonPB/mortar.html)

xyz, the biggest charge I fired through mine was 10 seconds worth of gas - the result was beyond spectacular. The golf ball which was aimed at an old outhouse went straight through the door and out the other side and kept going skyward! This is a serious weapon! (kind of thing to stop an Armaguard truck) and the damage to the front 1/4" chamber plate had a very large bulge. Then there was the noise that firing it made- I sometimes wonder if I may live my life with all my fingers and limbs but with no hearing?! I would strongly suggest you use nothing less than a 1/4" thick steel and even then that is to keep the weight managable.
Enjoy!

i live in australia and i have made only one pneumatic cannon, but i was thinking of making a solenoid operated one using a airbrake valve from a truck. i dunno if it would work but they seem to open pretty quick and let alot of air through. also i was thinking of making a smaller one that you could pump up with a bike pump and carry in a backpack. oh and my nextdoor neihbour used to weld stainless steel tee peices for an oil company, they got tested to 30,000 psi.

I had an epiphany while i was making balloon animals. what if someone used a two chamber barrel? my cannon is basic because of a severe lack of green, but I thought about this design for a while now. The gun would use pipe joins and seals to allow the gun to break in the middle, like a shot gun. This lets the exaust out of the fuel chamber, allows you to fill the chamber with your propellant, then makes use of your ammo (tennis ball) to stop gas from escaping. simply rejoin seal, clamp a lock on side if nervous, and push trigger. pop!
Weak points in the joints can be addressed with heavier material. The removeable barrel uses one pipe nested in another, and filled with a non-flammable insulant. Very heavy, but safe, and allows it to be use more like a bazooka than a mortar (over shoulder rather than placing the butt in the ground and plotting a ballistic trajectory). Q's or Comments?

I don't know how to help with design as I find it highly risky, but I assume it is possible to be made carbide powered cannon. This is slightly OT but I had to post it. In a suburban area not very far from me there WAS a tradition during winter holidays to "launch a barrel" (best translation I come up so far) based on that principle. Actually it is not the cannonball that flies but the barrel itself (action/reaction). If I understood well barrel is turned upside down and dig-up inside the land (it is swampy area so it works good as seal). They use carbide to make gas from reaction with water (again swamp) and kill me if I know how they lit the charge. Anyway this year there was no such thing since the autorities became grumpy. One of the guys that do such things (leave that stuff to pros;)) said the reporters that police stated that if they even hear the blast he and couple of other pyros flies behind the bars. Also he confessed that claimed reason for banning this was "public safety" with one claimed material damage. He said that few years back local goverment representative tried to stop them, as a retaliation they aimed his house with a barrel, "we missed with first one but the second went stright to the office"...needless to say I cried my eyes from laughing. Guys deserves a scout merit badge in artilery skills even cannons make sure hit in three rounds.

I feel I should share my recent experiences , not to promote the use of calcium carbide , but to hopefully avoid accidents with the people who are going to use it anyway.

I have recently built a carbide powered spud gun.

The chamber is made from a schedule 80 pvc coupler with reducers glued inside the ends giving almost ¾ “ wall thickness I would not consider anything less than this adequate , steel would definitely be safer
Weighing the CC is the most reliable way of ensuring a “safe” charge.

Consider this :

I gram of calcium carbide will produce 349 cc of acetylene, therefore, in a sealed 1000ml chamber by simple displacement approx 1 gram of carbide will only raise the pressure by about 5 psi this is way too much gas for good combustion (poor power lots of soot)

4 grams of carbide will produce enough gas to raise pressure to the critical 15 psi or so and could result in all the horrible things that go with violent chemical explosions

I try to get close to a 10 : 1 air fuel ratio this requires only about 0.5 grams of carbide and I get a reasonably clean burn and frightening amounts of power

As for oxy acetylene don’t even think about it, I am speaking from personal experience (not spud guns but other toys ) this stuff is lethal , the explosions in unsealed containers are violent and deafening, in a plastic spud gun, death or serious injury would be the likely result