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My newest cannon fail... Any help out there?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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My newest cannon fail... Any help out there?

Unread postAuthor: jaythedogg » Mon Sep 28, 2009 11:03 pm

Here's the vid I made, any suggestions would be greatly appreciated.

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Only thing I can think of is that maybe I need to move my two leads to the center & make them longer so they spark in the center of the chamber.

Thanks in advance.
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 28, 2009 11:10 pm

Potential problems:

spark gap too small/large

wires from the ignitor too close to eachother outside the chamber, creating a spark outside

fuel-air mixture too rich/lean

Improper chamber ventilation

The location of the spark isn't really relevant to go/no go functioning.
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Unread postAuthor: grock » Mon Sep 28, 2009 11:13 pm

it could be that if you were letting the cannon sit for too long, the fuel all settled to the bottom of the chamber, making one part with too much fuel to fire and one part with not enough fuel to fire. did you try shaking/rotating the chamber? that might help mix it a bit. also, it might be hard to vent without compressed air, because of the long barrel and the small fill port, try giving it a blast of air from a pump or compressor
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Unread postAuthor: Moonbogg » Tue Sep 29, 2009 12:35 am

My best guess is that, as previously stated, the fuel mix is bad, or the fuel not mixed enough with the air. Stick a wad of foil or paper in the chamber and shake it around and see what happens then.
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Unread postAuthor: Technician1002 » Tue Sep 29, 2009 12:50 am

Stand it on end and drop in a lit match. If it is too rich the interface between air and the rich mix will light. Don't look down the barrel when trying this if you value your eyes. At night it may light up and go whoosha such as the common water jug videos online. They are rich. If they were leaner, they would go bang.

http://www.youtube.com/watch?v=koDpDRpN4pY Too rich burns like this and very difficult to light with a spark from the other end.
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Unread postAuthor: jimmy101 » Tue Sep 29, 2009 3:41 pm

How do you measure the fuel? The most common "no fire" cause is the wrong mixture and it is usually too rich.

When refueling after a fire, or a "no fire", you must properly ventilate the chamber. Even in a no-fire you typically still consume some of the oxygen in the chamber. Pull the trigger multiple times and you'll use up all the oxygen in the chamber even if it doesn't fire.

It's possible you aren't getting a spark. You can double check by just shorting the contacts with your finger. If there is an electrical short you won't zap yourself.

Propane, butane etc. won't "settle". The longer you let it sit the better the mixing will be. The only time when a long wait is a problem is when you have gross leaks (like nothing in the barrel).

Without a fan it takes a surprisingly long time for fuel to mix. A couple of packing peanuts in the chamber are another way to help mix things.
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Unread postAuthor: jaythedogg » Tue Sep 29, 2009 3:51 pm

Well, since I am lacking matches (LOL) I tried airing it out, putting the wadding back in & using a syringe (1sp -5mL) to meter butane, it still didn't work.

I injected the butane, capped it immediately, & tried to fire. Nothing.

Tried rotating the cannon, didn't fire. Tried everything else under the sun, didn't fire, but when I uncapped the fill cap, I got a tiny blue flame with my lighter.

So I didn't overdo the butane, just won't fire.

The only thing I can figure is that my spark is too far back & at the bottom where heavier gas will sit.

I am going to cut the leads, make new ones, plug the old holes with epoxy & try putting the leads in the top of the canister. With epoxy. Lots of epoxy.

Too bad I am out of epoxy putty, I will have to use gel mix.

Think the holes in the center of the canister will cause weakness? I don't think so, as propane/butane only makes what? 100 - 150 psi?

Any feedback from the gurus or less experienced alike will be greatly appreciated.
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Unread postAuthor: jaythedogg » Wed Sep 30, 2009 11:32 am

Well, update I guess.

I cut the leads & stripped back the last inch of wire for each lead.

I attached them to the blunt ends of two finishing nails, soldered them & put a few tightly wrapped layers of electrical tape for insulation between the leads & the canister/chamber.

I coated them in liquid epoxy, inserted them after drilling two holes, then further epoxied the holes & leads on the outside, using a bit of hot glue to keep them properly spaced on the outside.

I have spark, but it has to be a fuel issue now. I sealed up the old lead holes, I metered some butane with a syringe, nothing.

I pulled the paper wadding AFTER getting angry & filling the chamber with tons of butane (to no avail) by blowing through the fuel port until the paper wadding came out of the barrel.

Then I rammed the paper wadding back down, pressed the ignitor & got a faint whoosh... I forgot to cap the fuel port.

So here is the down on it. I over fueled it, blew it out & pushed the tight paper wadding back down the barrel & got ignition.

Could the canister be far too small in volume, therefore needing extremely small amounts of fuel?

Also, straight butane seems to displace air really quickly, so I am wondering if I should go back to propane as I did with my previous cannon experiments (2-liter & PVC). I think the reason it was so successful at firing was because I used a torch tip which premixes air before it leaves the tip, therefore if it displaces the air in the chamber it should still fire, right?

Any feedback would be appreciated. Since I don't have a propane torch at the moment, I will try my butane torch which works essentially the same.

Will post back.
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Unread postAuthor: Technician1002 » Wed Sep 30, 2009 12:15 pm

jaythedogg wrote:Well, since I am lacking matches (LOL) I tried airing it out, putting the wadding back in & using a syringe (1sp -5mL) to meter butane, it still didn't work.

I injected the butane, capped it immediately, & tried to fire. Nothing.

Tried rotating the cannon, didn't fire. Tried everything else under the sun, didn't fire, but when I uncapped the fill cap, I got a tiny blue flame with my lighter.

So I didn't overdo the butane, just won't fire.

The only thing I can figure is that my spark is too far back & at the bottom where heavier gas will sit.

I am going to cut the leads, make new ones, plug the old holes with epoxy & try putting the leads in the top of the canister. With epoxy. Lots of epoxy.

Too bad I am out of epoxy putty, I will have to use gel mix.

Think the holes in the center of the canister will cause weakness? I don't think so, as propane/butane only makes what? 100 - 150 psi?

Any feedback from the gurus or less experienced alike will be greatly appreciated.


I'm sure if you are creative and have a lighter, you can probably find a substitute for a match. Got Q-Tips?
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Unread postAuthor: jimmy101 » Wed Sep 30, 2009 12:26 pm

It takes a long time for fuel to mix. For complete mixing in your somewhat long and skinny chamber it'll take ... a long time. Fuel and let it sit for 15 minutes then spark it.

Purge the chamber between shots by blowing air through the fill port. A compressor would be a big help.

Propane = 4.0% of the chamber volume
Butane = 3.1% of the chamber volume

What is your chamber volume and what percentage of the chamber volume is the 5ml you injected with a syringe? Looking at the photo's the chamber is about 1 liter? 4% of 1L would be 40ml.

Oh, did I mention that it takes a long time for fuel to fully mix? (Packing peanuts and several inversion will help.)
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Unread postAuthor: Moonbogg » Wed Sep 30, 2009 3:52 pm

jimmy101 wrote:It takes a long time for fuel to mix. For complete mixing in your somewhat long and skinny chamber it'll take ... a long time. Fuel and let it sit for 15 minutes then spark it.

Purge the chamber between shots by blowing air through the fill port. A compressor would be a big help.

Propane = 4.0% of the chamber volume
Butane = 3.1% of the chamber volume

What is your chamber volume and what percentage of the chamber volume is the 5ml you injected with a syringe? Looking at the photo's the chamber is about 1 liter? 4% of 1L would be 40ml.

Oh, did I mention that it takes a long time for fuel to fully mix? (Packing peanuts and several inversion will help.)


Chamber fans are a beautiful thing. No doubt about that. If not, then packing peanuts are the logical step down I suppose :wink:
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Unread postAuthor: psycix » Thu Oct 01, 2009 5:13 am

The option between the packing peanuts and a fan would be a chamba-chain.
A chain attached to the side of the chamber, shake the cannon and the chain will mix it.
Makes sure you wont have to replace the peanuts every shot.

Chamber fan is highly recommended though.
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Unread postAuthor: jaythedogg » Thu Oct 01, 2009 8:52 am

Ack, can't do a chamber fan now that everything is sealed up so I will have to do that next go around.

Instead of packing peanuts, what about soft air BBs? Think they would melt?

Also, I assume chamber fans only work because the heat is so instantaneous that it doesn't harm them?
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Unread postAuthor: Technician1002 » Thu Oct 01, 2009 10:47 am

jaythedogg wrote:Ack, can't do a chamber fan now that everything is sealed up so I will have to do that next go around.

Instead of packing peanuts, what about soft air BBs? Think they would melt?

Also, I assume chamber fans only work because the heat is so instantaneous that it doesn't harm them?


Airsoft BBs would work. Due to the smaller cross section, it would take longer shaking to mix the contents, just like the time it takes to properly shake a can of spray paint after the contents settled. The marble in a paint can is considerably larger than the airsoft BB. A dozen or so may work more quickly. Styrofoam peanuts can be a poor choice because they may pick up a static cling and either stick to the side or possibly cause ignition, but the static charge is often too low energy to ignite the gas. In either case, fully rub down your choice of mixing articles with an anti static dryer sheet.
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Unread postAuthor: psycix » Thu Oct 01, 2009 12:19 pm

Also, I assume chamber fans only work because the heat is so instantaneous that it doesn't harm them?

Yes, the heat spike is so short they won't melt.
However, when placed near the barrel entrance, the extreme amount of flow will break the blades, so always place them in the middle or the back.
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