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Spark Strip Advice

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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Spark Strip Advice

Unread postAuthor: Xxplosive42o » Sun Jan 04, 2009 6:11 pm

Hello, I just finished building what will be the spark strip for my cannon. I would appreciate any input and commentary.

It is comprised of a 15 1/4" piece of 1" Sched. 80 PVC that has been cut length wise. Then I used a mill to cut a troth into it that was 1/4" in diameter to fit a 1/4" piece of copper. I then took the copper piece and divided it into 5 pieces that measure 3" in length. The pieces were then lathed on each end to form a point; excluding the end pieces. The end pieces were cut flat on the ends and drilled to support the electrodes. Finally; the piece of PVC was drilled where each point meets to allow the spark to ignite gasses in 360 degrees and then the pieces were spaced 1/16" to get a good spark. Finish.

Edit: I forgot to mention the basic elements were borrowed from starman's design. JFYI

<a href="http://s42.photobucket.com/albums/e307/surferboy226/?action=view&current=strip2.jpg" target="_blank"><img src="http://i42.photobucket.com/albums/e307/surferboy226/strip2.jpg" border="0" alt="Photobucket"></a>

<a href="http://s42.photobucket.com/albums/e307/surferboy226/?action=view&current=strip1.jpg" target="_blank"><img src="http://i42.photobucket.com/albums/e307/surferboy226/strip1.jpg" border="0" alt="Photobucket"></a>

<a href="http://s42.photobucket.com/albums/e307/surferboy226/?action=view&current=strip4.jpg" target="_blank"><img src="http://i42.photobucket.com/albums/e307/surferboy226/strip4.jpg" border="0" alt="Photobucket"></a>

<a href="http://s42.photobucket.com/albums/e307/surferboy226/?action=view&current=strip3.jpg" target="_blank"><img src="http://i42.photobucket.com/albums/e307/surferboy226/strip3.jpg" border="0" alt="Photobucket"></a>
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Unread postAuthor: VH_man » Sun Jan 04, 2009 7:14 pm

sexy......

Id imagine it will work great.
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Unread postAuthor: starman » Sun Jan 04, 2009 7:17 pm

Wow, that's a lot of copper there...and a 4 gapper at that.

Here's some thoughts:

- I would have spaced the gaps where they are equidistant in the chamber, not equidistant on the strip. This would take into consideration the space ahead and behind the strip and mean your first and last element would have been somewhat shorter and elements in between a little longer. However, I doubt you will notice much difference in practice.

- get those gaps as small as possible without shorting.

- the fat copper is way overkill, but I like the sharp points!

- I assume you're using epoxy or JB weld to attach the copper elements. Just make sure it's a very solid attachment. Those masseous copper elements could shake loose.

- An idea I'm working for my next strips....those 1/4" spark holes in the PVC....make em bigger...like 5/8" or 3/4"...will more emulate gaps out in mid space.

This should really rock as a spark gap source...nice work. :wink:
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Last edited by starman on Sun Jan 04, 2009 7:24 pm, edited 2 times in total.
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Unread postAuthor: MaxuS the 2nd » Sun Jan 04, 2009 7:23 pm

Are you using a BBQ Ignitor?

I'd use a flyback IMO.
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Unread postAuthor: Xxplosive42o » Sun Jan 04, 2009 7:53 pm

VH Man, Thank you! I appreciate it!

Starman, my chamber length is 20" and the gaps are every 3" of 15" of that space (that only leaves 5" left-over), so the flame front shouldn't suffer much, as you noted. I wanted to make larger holes underneath but I didn’t want to take away too much of the material. The strip is rather long and by clunking the cannon around or accidental drops might result in it snapping. I was merely taking precautionary measures; it should be effective as is. Once again, thank you for inspiring me to build this. I just fired it up and it arcs beautifully. The 1/16” gaps work perfectly without any shortage.

Maxus, I'm using an 80,000 volt stun gun, enclosed into a hobby box. The wires that connect to it are heavily insulated and the electrodes are 8/32 brass bolts.
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Unread postAuthor: starman » Sun Jan 04, 2009 7:59 pm

Xxplosive42o wrote:I just fired it up and it arcs beautifully. The 1/16” gaps work perfectly without any shortage.


Awesome, I'm looking forward to seeing this beast in action. Yeah that stun gun should make short work of jumping those gaps... :wink:

Edit: In looking at it again, I believe the gaps are within the middle 12", not 15" ....4 gaps...every 3"... You're going to be fine though.... :wink:
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Last edited by starman on Sun Jan 04, 2009 8:57 pm, edited 1 time in total.
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Unread postAuthor: MaxuS the 2nd » Sun Jan 04, 2009 8:34 pm

Xxplosive42o wrote:Maxus, I'm using an 80,000 volt stun gun, enclosed into a hobby box. The wires that connect to it are heavily insulated and the electrodes are 8/32 brass bolts.


Ahh good stuff. :D
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Unread postAuthor: ALIHISGREAT » Mon Jan 05, 2009 3:26 am

oooh i like those tips on the copper, looks very professional :wink:
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Unread postAuthor: jimmy101 » Mon Jan 05, 2009 5:36 pm

Nice job but it seems like way overkill. Too much plsatic and metal up near the center of the chamber, does nothing but suck up heat.

The actual conductors don't need to be anywhere near that big, 40G wire (hair thin) would work just as well if not better. Small diameter wire will have a sharper point (which will spark at a lower voltage) and will suck up much less heat.

So, looks good, but it won't accomplish much. Heck, it might not be as good as a single spark.
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Unread postAuthor: Xxplosive42o » Mon Jan 05, 2009 7:07 pm

How do you figure Jimmy? It puts 4 sparks in the middle of the chamber, leaving very little area within the chamber un-sparked. The spark fires through the strip; not just on one side. I also voltage tested the material and its still above 75,000 volts. Each piece is as sharp as a pin head JFYI which defeats your idea that thinner gauge wire provides a sharper point. I would have used a smaller rod but I didn’t have the material off hand. It may absorb a bit of heat but is that really going to make performance suffer? :?

Thanks for the input though gentlemen. :lol:
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Unread postAuthor: Radiation » Mon Jan 05, 2009 8:01 pm

That is a fantastic spark gap! It will probably require a bit of maintenance down the road as copper oxidizes when exposed. Also it oxidizes when current is passed through it... And in a moisture rich environent such as the inside of the spent chamber of a combustion gun. Better get yourself some Brasso!
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Unread postAuthor: jimmy101 » Tue Jan 06, 2009 1:10 pm

Xxplosive42o wrote:How do you figure Jimmy? It puts 4 sparks in the middle of the chamber, leaving very little area within the chamber un-sparked. The spark fires through the strip; not just on one side. I also voltage tested the material and its still above 75,000 volts. Each piece is as sharp as a pin head JFYI which defeats your idea that thinner gauge wire provides a sharper point. I would have used a smaller rod but I didn’t have the material off hand. It may absorb a bit of heat but is that really going to make performance suffer? :?

Well, a typical combustion spud gun is about 10% efficient. Of the 90% energy that is lost a BIG chunk of that is heat loss. If the greater mass and surface area of your spark strip results in the loss of just 1% more energy as heat transfer then the performance of the gun will drop by 10% in KE, 5% in muzzle velocity. 2% more heat loss and the KE is down 20%. So yes, small increases in heat loss result in much larger decreases in performance. Heat is mostly lost to the chamber walls and the flame front takes a while to actually reach the walls. Your spark strip is dead center in the combustion volume. From the moment the fuel is ignited you will be getting significant heat loss to the strip. In high speed photographs of car spark plugs there is a clear indication of slower flame front propagation in the vicinity of the upper electrode. The electrode cools the flame front resulting in slower propagation.

How big is the affect? Hard to say. Chrono the gun. Disable all but the central gap and chrono it again. Remove the strip and replace with a single centrally located gap (done the usually way of a pair of screws through the chamber wall) and chrono again. Those three numbers will give you an idea of the heat loss associated with the central strip and what performance gains the multiple gaps are giving.

Thinner wire makes a sharper point much easier than does thicker wire. With thin enough wire you wouldn't even need to try to make a point. Just cutting the wire would give a very sharp "point".
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Unread postAuthor: Xxplosive42o » Tue Jan 06, 2009 6:22 pm

Well I applaud you for your vast knowledge in flame propagation and heat absorption. There is not much at this point that I can do to avoid it. I will try to cut away some of the excess plastic on the strip but as for the copper, well I'm up the creek without a paddle per say. I hope that it doesn’t affect the performance as much as you claim, but through testing we will find out.

Jeez, I thought I had something here. :?
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Unread postAuthor: Radiation » Tue Jan 06, 2009 6:31 pm

You do, if you didn't you wouldn't be getting so much attention. :D

I think it's one of the most bada$$ spark strips I have seen and I'm wishing I had it instead of my non-functional one!

All this talk about flame propogation and such is a bit beyond my scope of 'things I care about' but I will tell you that oxidation may be a problem, and that's about all I know. :)
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