Multishot cannon wont ignite.

[whitewolf]

Hi all. I have built a gun here, a conversion from a pneumatic that worked but did not live up to its multiple shot ambitions. (hence the barrel)


This is my first combustion, and I cannot for the life of me figure out why it wont combust at all. The spark is working. It's a small spark, but a spark no less. The propane is definately getting into the gun, but no matter how much or how little I squirt in it wont ignite. By sealing both valves, the gas should mix with the air because it has nowhere to escape. The only time I did get it to explode was by accident, and this was without any butane: the pvc epoxy had evaporated inside the gun and the spark ignited it. Hehe that was unexpected.

Edit: propane, butane, the stuff that you refil lighters with, forgot what its called.

[jackssmirkingrevenge]

How big is the spark gap? And could the wired be touching or close enough to spark outside the chamber?

[whitewolf]

the spark gap is about 7mm. Its definately sparking inside the chamber, I un-glued the holes into wich the electrodes are inserted and you can see it sparking inside.

[jrrdw]

Try making your spark gap smaller. Put something inside your chamber like a small ball, cork something to shake around to help mix the butane, (that stuff that you refill lighters with).

Welcome to Spudfiles.

[ralphd]

I think you are charging too much propane at first then increasing it every time you inject smaller amounts. Do you have a fan? If not then get one in there. I couldn't believe the increased performance when I was talked into doing the same. The fan will also allow you to vent the launcher with fresh air between shots. First move between barrels and open the breech valve, next inject the propane and close the breech valve.

[Radiation]

a few notes, in a fanless design it is very hard to get a proper iginition to begin with. You are making the situation much worse for yourself with the location of your spark gap and the U design you are using (which essentially is folding a really long chamber, long chambers are notoriously hard to get the mixture right as the gases have to travel all the way to the end). It will be extraoridnarily hard to get anything close to an even mixture with the way you have your chamber configured.

Let's talk about your spark gap a little more. The location of it would be almost ideal in a design that utilized a fan, but in your design it can be a cause for much headache as it will require a relatively even mixture to even ignite. This is because the butane goes in the end, is extraordinarily rich and can take a very very long time to mix in the chamber. On the other end you have oxygen that is unmixed and therefore serving only as dead air space.

Also you are using what appears to be butane which can be great, but it is very finicky. What ever amount you're using I can almost gaurentee it's too much (even the slightest amount. I used to use a spray bottle like yours and I would barely have to compress it to get a mixture that was too rich, then I would have to blow into the chamber a bit to literally get the air mixture correct.

Anyway as I see it the U design coupled with no fan and too much fuel, honestly IMO you are fighting a losing battle with this design, not knocking it just speaking from experience.

As a general rule of thumb if you are going fanless it is better to have a wide and stout chamber as the mixture will naturally be better. A long curved chamber can be a nightmare in a fanless design.

[starman]

I second radiation's thoughts here. Also, you don't indicate if you are even metering the butane or not....just that you "squirt" a little then a lot trying to get it to fire. You're going to have to find a way to reasonably precisely meter your fuel, especially in this small and wierd shaped chamber.

It also looks like you may not be getting fresh air in the chamber. This is as critical as the proper amount of fuel.

[jimmy101]

parrot what others have said... need a fan, gases take a long time to mix, especially when injecting fuel at one end of a long skinny chamber with a bend in it.

You should calculate the volume of the chamber then the volume of the butane (or propane) needed. That'll give you an idea just how small the volume of fuel needed is.

Do you have a plastic syringe? Perhaps from a printer ink cartridge refill kit. Or, you can get a 60cc plastic one for about $2 from a decent farm supply store. With a syringe it is easy to get the fuel ratio dead on. Once you are sure the fuel ratio is OK then you can trouble shoot the problem down to poor/no spark versus slow mixing. Properly fuel the gun and let it sit for half an hour. If it doesn't fire then mixing isn't the problem. Even on a cold day, ane even with your chamber geometry, half an hour for mixing should be enough.

[whitewolf]

Hehe, this is a lot more complicated than I thought. Thanks for the imput though guys.

I'm not metering the butane, no. In my naivety I didnt reallize it was so important to get the amount right.

I'd like to ask though, how important is it to make it airtight? Wont the butane simply leak out of the fan before I can spark it? (yeah the fan blows air in, not out, but butane is lighter than air.)

So where would you people suggest I put in the fan? How about in a T junction between the bottom elbow and the canister, is that ok?

And yeah I have a syringe, but I'm not at all sure I could get the butane injected into it what with the nozzle on the cannister being what it is.

[TurboSuper]

Well...the fan should be inside your sealed chamber, I don't see how anything would leak anywhere.

And a meter isn't absolutely necessary. Sure, it helps, but if you're willing to experiment you can probably get it to work without one. Lighter fluid has the disadvantage of being sprayed in liquid form, so it will create a dynamically changing fuel/air ratio as it evaporates. This may be problematic in a small chamber.

[whitewolf]

Ok, so I added a chamber fan, made some modifications to the spark gap, etc. but didnt actually glue the fitting containing the fan onto the rest of the gun, just friction fitted. I was clicking away, then suddenly BOOOM. Boy, did it explode. Scared the living **** out of me. The fan section blew right of, and just for a split second the flames flew around my hands. Hell, is this even safe? Has anyone been injured from an exploding cannon? Haha I guess you get used to it.

[Hotwired]

Is firing an unglued cannon safe? Hell no.

Injured by an exploding cannon? Yes, but that's not going to happen if its made properly.

You can get a nasty burn from being in the flame jet from the chamber, it's like a very large blowtorch for a second
:violent3:

[Radiation]

I'm glad to hear it's working, I'm disappointed that you did it in such an unsafe manor. Any spudgun is potentially dangerous and igniting a mixture in an unglued chamber is just plain stupid. No way to sugar coat it, that was just moronic of you.

[whitewolf]

Yes yes, but hear me out first, its not that simple. If I had of glued it, where would all the force have went? The force would have had one way to escape: the small hole that is the muzzle. My muzzle is much smaller than the ones I see on this site.

What would have happened if all the pressure could not have escaped through that little hole within the fraction of the second that the explosion took place? It would have blown the cannon up. That would entail either fragments of pvc flying everywhere (I have seen this happen on youtube), or hot gasses spurting out of a single rupture, focusing it all in one spot, which might have been my eyes or my hands.

By allowing the the gasses to escape through the full diameter of the pipe when the front section blows off, I avoided both the possibility of the whole thing disintegrating into fragments, and allowed the gases to spurt harmlessly away from me. So in fact the way I did it was more safe than if I had of sealed the whole thing up. I am more worried about it exploding AFTER I seal it up, because in that situation there is no telling which part of the cannon is going to fail, and how. By friction fitting the front section on, it can function as a sort of a "fuse". If the gasses cannot force their way out of the muzzle with the projectile loaded, rather than explode, the front section of the cannon will simply pop off.

[TurboSuper]

Well, your chamber also has smaller diameter than most (combustion) cannons on this site, so ratio-wise it still pretty much works out.

They sell safety pop-off valves if you're worried about that sort of thing, having part of the cannon blow off every time is a kind of bizaare way of going about it.