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Is my cannon cursed?

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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Is my cannon cursed?

Unread postAuthor: Radiation » Tue Jan 06, 2009 10:38 am

Ok I know it's not cursed, but consider the following scenario:
After my rebuild my spark gap works perfectly.
My fan works perfectly.
When the fan is on the spark gap and fan both work perfectly.
Chamber sealed, fan on, spark gap all work perfectly.
Chamber sealed, fan on (or off), propane injected (proper amount), spark gap stops working

It starts to freak out like it does when the gap is too big and absolutely refuses to jump the gap. I can hear the spark jumping all over the place along the surface of cables, the cannon etc. :?

I have an idea of what is wrong, but I wanted to ask all of you for input.
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Unread postAuthor: JDP12 » Tue Jan 06, 2009 10:39 am

well what sort of ignition is it? stun gun, piezo ignitor?

could you provide a picture of the spark gap?

It could be sparking at another path of lesser resistance- make sure it isn't doing that
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Unread postAuthor: Radiation » Tue Jan 06, 2009 10:51 am

it's a stun gun 100,000 volt

here's a pic of the gap.
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Unread postAuthor: starman » Tue Jan 06, 2009 11:09 am

Believe it or not, the gaps probably add up to close to the limit. That second one from the right looks noticably wider than the others. As a test, a short couple of the gaps out and test shoot it.

Additionally, the propane will change the resistance of the air gaps. The added pressure in hybrids will also cause a change. Many hybrids will barely fire with one very tiny gap, much less multiples.

I say close those gaps up a bunch...and check to make sure all of you electrical connections were put back in securely, not causing another gap to have to jump.
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Unread postAuthor: jrrdw » Tue Jan 06, 2009 11:14 am

Add to Starmans post, change your electrodes to points and that will give you some gain of control of the arc. That matters under fueled conditions.
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Unread postAuthor: starman » Tue Jan 06, 2009 11:53 am

Oh and....make sure all your lead-in wires are well seperated from each other and there's not a common piece of metal that the spark could jump from one wire and into the other...ie. hose clams, etc.
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Unread postAuthor: jimmy101 » Tue Jan 06, 2009 12:10 pm

Like jrrdw said, change the gaps to points. Perhaps remove the washers and replace with pieces of small diameter copper wire wrapped around the threads. Bring the ends of the wire out to form the spark gap(s).

Or, remove the bults completly and replace with say 14G copper wire(no insulation). Sharpen the ends of the wire and bend them so they form the spark gaps.

Between blunt electrodes the break down voltage of air is about 3KV/mm. Between sharp electrodes the break down voltage is about 1KV/mm.
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Unread postAuthor: starman » Tue Jan 06, 2009 12:20 pm

While his gaps aren't ideal, keep in mind they worked great before he tore his gun down and rebuilt it. When he says "I can hear the spark jumping all over the place along the surface of cables" tells me there's a good chance something is different with his lead in cabling and/or its co-positioning to something metalic or electrically conductive.
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Unread postAuthor: Radiation » Tue Jan 06, 2009 12:52 pm

starman wrote:Believe it or not, the gaps probably add up to close to the limit. That second one from the right looks noticably wider than the others. As a test, a short couple of the gaps out and test shoot it.

Additionally, the propane will change the resistance of the air gaps. The added pressure in hybrids will also cause a change. Many hybrids will barely fire with one very tiny gap, much less multiples.

I say close those gaps up a bunch...and check to make sure all of you electrical connections were put back in securely, not causing another gap to have to jump.


Bingo that was my assumption the slighlty higher resistance of a propane enriched atmosphere is enough to cause the gaps to misfire. Surpisingly enough this is the same exact spark gap and stun gun that I have been using with no problems whatsoever. I am using this as an excuse to rebuild my spark gap. I will probably end up doing something more central in my chamber as this current SG is slightly off center about a half inch. I don't know, I'm just overwhelmed right now by this whole thing. I tore my gun down to rebuild the chamber 3 days ago thinking it was going to be a pretty simple task. I am now at day three with many scratches in my paint job, a spark gap that doesn't work, leaky regulator, crooked scope, shorted out fan, and I'm thinking to myself "why did I do this?"
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Unread postAuthor: starman » Tue Jan 06, 2009 3:18 pm

Radiation wrote:I am now at day three with many scratches in my paint job, a spark gap that doesn't work, leaky regulator, crooked scope, shorted out fan, and I'm thinking to myself "why did I do this?"


LOL...this is actually a good exercise for you. These are the kinds of things product manufacturers go through constantly, building in product reliability, safety, performance, etc. We only see the final result and it's easy to think they were always just that way.

Don't be discouraged. Take your time with it, repair or improve the things that are causing problems, one at a time. You'll be glad you did. I fiddled with triple thunder for 6 months before you actually saw them, went through several spark strips and way too many handle designs before I was happy.
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Unread postAuthor: Pilgrimman » Tue Jan 06, 2009 3:57 pm

If those are washers around the bolt heads you might just have a bad connection between the washers and the shafts. weird that it failed all of a sudden...
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Unread postAuthor: Radiation » Tue Jan 06, 2009 5:44 pm

starman wrote:LOL...this is actually a good exercise for you. These are the kinds of things product manufacturers go through constantly, building in product reliability, safety, performance, etc. We only see the final result and it's easy to think they were always just that way.

Don't be discouraged. Take your time with it, repair or improve the things that are causing problems, one at a time. You'll be glad you did. I fiddled with triple thunder for 6 months before you actually saw them, went through several spark strips and way too many handle designs before I was happy.


So true, I guess I forgot all the mental strife I went through making this gun originally because it was spread out over the course of 5-6 months. Now that it's complete and awesome it just aggrivates the hell out of me that it is non-functional when most of the variables are the same as they were when it worked. I will get it working, that's not in question. It's just my sanity. Ugh...

If those are washers around the bolt heads you might just have a bad connection between the washers and the shafts. weird that it failed all of a sudden...


That's the thing, it is working under all conditions except when propane is in the chamber the spark fails to jump the gap. I'll be rebuilding the spark gap this weekend.

Oh a side note I just found out. when I was working on the gun last night, I accidentally knocked my wifes PDA off it's charger. I put it back on the charger and didn't think about it again. This morning she calls me freaking out because the PDA is completely non-functional and asked if I did anything to it... :roll: She's a doctor and a lot of her notes are on the damn thing. I really do think this gun is cursed now. :(
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Unread postAuthor: Xxplosive42o » Wed Jan 07, 2009 5:21 pm

I understand your pain Radiation. The strip design I built had to undergo a few changes before I got it right. When I soldered the wire to the electrodes on the stun gun the plastic holding them in place melted a little causing the short out electrodes to shift. They shifted so much that they wound up getting closer than the gaps on my strip. I wound up having to cut (highly un-recommended) the material back on the short-out electrodes. Now it works just peachy. I didn’t come to this conclusion until after about a week of scratching my head saying, "I measured this sh*t out before, why is it not working now!!" But in the end it always works it self out.

Sorry to thread jack, but I can relate in this situation.
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