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Spark Strip Help!!!

A place to ask general spud cannon related questions.
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Spark Strip Help!!!

Unread postAuthor: Xxplosive42o » Tue Dec 09, 2008 9:10 pm

I need some pictures of user's spark strips for their combustion cannons. I am building a rather long chamber that is why I ask. If you could list the parts used or a briefing of materials I would appreciate it grealy! Thanks!
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Unread postAuthor: starman » Tue Dec 09, 2008 10:03 pm

Sure thing. This is what I did for the Triple Thunder cannons. It's a sectional of 1" PVC pipe with a groove cut into it with a table saw and a steel rod epoxied into the groove. Then, with a dremel tool cutting disk, I sliced the gaps. This way you can place the gaps anywhere you want along your strip...and as many as you want.


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Unread postAuthor: Xxplosive42o » Tue Dec 09, 2008 10:15 pm

THANK YOU SO MUCH! NOW QUESTIONS! hehe

Does it matter how big of a cut you make?

And how far apart should I space the spark gaps if I am constructing a 16" chamber.

Is the two bolts on each side the only contact that is made with the metal strip or does the bolt somehow go through the rod?

Finally, what is the rod that runs through the middle of the pvc made of?
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Unread postAuthor: starman » Tue Dec 09, 2008 10:43 pm

Xxplosive42o wrote:THANK YOU SO MUCH! NOW QUESTIONS! hehe

Does it matter how big of a cut you make?

And how far apart should I space the spark gaps if I am constructing a 16" chamber.

Is the two bolts on each side the only contact that is made with the metal strip or does the bolt somehow go through the rod?

Finally, what is the rod that runs through the middle of the pvc made of?


Enough of a cut that you expose and open the bottom of the strip to the gap. It's important that the flame proceed under the strip as well as over it.

Since I have a 4" chamber, I spaced the gaps every 4". This allows 3 mostly spherical flames that touch each other at the same time they touch the chamber's edge. If you have a longer chamber you might try to squeeze another gap in there, although 3 gaps, even stretched out some should still be very effective. Just equally space them.

Yes, the 2 bolts are supporting the strip as well as conducting the spark voltage.

It's a steel rod that's used to hang dropped ceiling frames. You could use just about anything from copper to aluminum to steel as I have here.
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Unread postAuthor: Xxplosive42o » Tue Dec 09, 2008 10:50 pm

Thanks man I really appreciate it! Do the bolts themselves run through the metal rod or are their holes right beside to the metal rod.
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Unread postAuthor: starman » Tue Dec 09, 2008 11:00 pm

Xxplosive42o wrote:Thanks man I really appreciate it! Do the bolts themselves run through the metal rod or are their holes right beside to the metal rod.


Yes, what you don't see is that I drilled mounting holes toward the ends of the strip. I discarded the little 1/2" or so rod piece on the end. The holes are directly at the ends of the rod and lies directly under the mounting nut and makes electrical contact that way. Just tighten it down onto the rod.
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Unread postAuthor: Xxplosive42o » Wed Dec 10, 2008 2:26 am

Well thank you star.


And props to your tripple thunder setup's. A++ Deign & Quality
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Unread postAuthor: starman » Wed Dec 10, 2008 4:26 am

Xxplosive42o wrote:And props to your tripple thunder setup's. A++ Deign & Quality


Thank you very much sir! I hope to get some video shooting golfballs with one of those things sometime during the holidays...and maybe a surprise or 2... 8) ..we'll see...
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Unread postAuthor: jimmy101 » Wed Dec 10, 2008 1:01 pm

Just a thought, but you want to minimize the amount of "stuff" you have out near the central axis of the chamber.

Combustion guns loose a lot of power to heat loss to the PVC. That big hunk of plastic adds a fair amount of surface area and will suck up a lot of heat.

Furthermore, the plastic is dead center in the chamber, it has twice the surface area and is located right where all the heat is being generated.

Finally, the sparks are on one side of the strip. The flame front will have to migrate around the strip to get to the other side of the chamber. The "shadow" of the strip will slow down combustion. (In high speed photos of spark plugs firing in gasoline engines their is a pronounced flame "shadow" from the plugs upper electrode.)

The strip should be mounted up agains the chamber wall and not supported in the center of the chamber. The sparks should be near the chambers central axis.

Personally, I think several standard pairs of screws will work as well, if not better, and is easier to do. The wiring between the gaps is all external to the chamber and there is no added PVC in the chamber for the support.

Three spark gaps with minimal addition of surface area inside the chamber.
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Unread postAuthor: starman » Wed Dec 10, 2008 3:36 pm

jimmy101 wrote:Finally, the sparks are on one side of the strip. The flame front will have to migrate around the strip to get to the other side of the chamber. The "shadow" of the strip will slow down combustion. (In high speed photos of spark plugs firing in gasoline engines their is a pronounced flame "shadow" from the plugs upper electrode.)


You have mentioned this before and I have commented on it before...perhaps you missed it.

The gaps are sawn completely through, meaning the flame has direct access under the strip without having to "migrate around the strip". I designed them that way on purpose to avoid the migrating flame issue. Is it a perfect spherical burn..no, but it is darned close to it...certainly close enough.

About "all that plastic in the middle", I do plan future strips to use less than the almost 1/2 pipe that I used to build these strips, possibly 1/4 - 1/3 section of the pipe instead. However, cutting the gaps becomes a little more tedious.

These gaps also have the advantage of being narrow, about 1/16" or less, without a lot of mounting and adjusting and bending to prod into position.

Are they perfect? no. Ideal? close to it. Consistant? extremely. Easy to build? moderately. My shots are consistant and violent. If I'm losing energy in my shots, it would be noticable only under lab conditions, not in the field.
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Unread postAuthor: Xxplosive42o » Wed Dec 10, 2008 9:12 pm

Wow, interesting read guys. Thanks for all your added ideas! I plan on using starmans design and cutting off the extra "fat" per say. I will credit you in my write-up. My cannon should be complete by next weekend. Paint & All.
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Unread postAuthor: starman » Wed Dec 10, 2008 10:03 pm

Xxplosive42o wrote:Wow, interesting read guys. Thanks for all your added ideas! I plan on using starmans design and cutting off the extra "fat" per say. I will credit you in my write-up. My cannon should be complete by next weekend. Paint & All.


Yeah after writing my response, I thought about trimming the "fat" as you say from around and beside the wire areas, but leaving the gap zones fat to accomodate the gap slit. This would give you a look of 2 hour glasses back to back with the middle spark gap the dividing line.

While I don't believe the performace difference in the real world would be noticable, some small theoretical advantage probably exists.
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Unread postAuthor: Xxplosive42o » Thu Dec 11, 2008 5:23 am

Hey Starman. Did you remove the very end of the metal strip on each side in order to screw the bolt through the half moon pvc? Is the locking nut the only contact made with it?
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Unread postAuthor: starman » Thu Dec 11, 2008 11:19 am

Xxplosive42o wrote:Hey Starman. Did you remove the very end of the metal strip on each side in order to screw the bolt through the half moon pvc? Is the locking nut the only contact made with it?


Yes in both cases.
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Unread postAuthor: jimmy101 » Thu Dec 11, 2008 11:33 am

starman wrote:The gaps are sawn completely through, meaning the flame has direct access under the strip without having to "migrate around the strip". I designed them that way on purpose to avoid the migrating flame issue. Is it a perfect spherical burn..no, but it is darned close to it...certainly close enough.

About "all that plastic in the middle", I do plan future strips to use less than the almost 1/2 pipe that I used to build these strips, possibly 1/4 - 1/3 section of the pipe instead. However, cutting the gaps becomes a little more tedious.

These gaps also have the advantage of being narrow, about 1/16" or less, without a lot of mounting and adjusting and bending to prod into position.

Flames don't like to propagate through gaps. Too much surface area for the volume, resulting in too much heat loss. As I said, even the very small upper electrode of a spark plug gives a noticable shadow in the flame front. A gap is going to be much worse.

You are probably right though, the difference in performance is probably small, perhaps even completly unmeasurable.

I just don't see the advantage of the extra parts over just using several sets of screws through the chamber wall. Both techniques acocmplish the same thing. The screws are simpler and will be more effective (though the increased performance may be negligable).

The only real advantage is the ability to easily get the gaps correctly aligned. As you said, screw gaps can be a bit tricky to aline . The strip eleviates that problem.
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