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Spark Strips

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 Strips

Unread postAuthor: Hubb » Thu Sep 10, 2009 10:41 am

When people think spark strips, they tend to think of Burnt Latke's version. The biggest debatable problem with this particular design is that it does not produce flame fronts in the center of the chamber. In theory, when a flame front is produced in the center of the chamber, it allows for maximum expansion rate of the flame. In Latke's strip, it produces essentially "half a flame front", that is, the expansion rate of the flame only proceeds in one direction.

The biggest advantage concerning Latke's spark strip is the fact that it is much easier to install and uninstall. With a modified version (such as something similar to starman's spark strip used for Triple Thunder) it is probably easier to build as well.

So, why can't we have the ease of installation of Latke's spark strip while maintaining the flame expansion properties of a centralized sparking system? Well, this is what I was hoping to achieve in my latest combustion launcher. I was thinking of installing two Latke style spark strips into the chamber that are directly opposite one another. Since Latke's strip only produces "half a flame front" wouldn't this produce "two halves" which should produce the same expansion rate as a centralized sparking system?

Now, before anyone asks "why" it is because of the design of the launcher. Any other questions as to "why" will be ignored.

Thoughts?
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Unread postAuthor: rcman50166 » Thu Sep 10, 2009 10:48 am

Well the way I see it, the flame front produced would be similar to a center mounted strip. The only downfall to it is the fact that it isn't nearly as efficient. You would need to jump 6 gaps as opposed to the 3 gaps needed for a center mounted strip. So its only 50% efficient in comparison. But if you have the means of achieving enough power to do it then more power to ya, literally :wink:
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Re: Spark Strips

Unread postAuthor: Technician1002 » Thu Sep 10, 2009 12:15 pm

Hubb wrote:When people think spark strips, they tend to think of Burnt Latke's version. The biggest debatable problem with this particular design is that it does not produce flame fronts in the center of the chamber. In theory, when a flame front is produced in the center of the chamber, it allows for maximum expansion rate of the flame. In Latke's strip, it produces essentially "half a flame front", that is, the expansion rate of the flame only proceeds in one direction.

The biggest advantage concerning Latke's spark strip is the fact that it is much easier to install and uninstall. With a modified version (such as something similar to starman's spark strip used for Triple Thunder) it is probably easier to build as well.

So, why can't we have the ease of installation of Latke's spark strip while maintaining the flame expansion properties of a centralized sparking system? Well, this is what I was hoping to achieve in my latest combustion launcher. I was thinking of installing two Latke style spark strips into the chamber that are directly opposite one another. Since Latke's strip only produces "half a flame front" wouldn't this produce "two halves" which should produce the same expansion rate as a centralized sparking system?

Now, before anyone asks "why" it is because of the design of the launcher. Any other questions as to "why" will be ignored.

Thoughts?


I've always wondered if the center strip is really the best choice for some chamber shapes. I know mult gaps perform better, just the same as they do in an airplane engine with dual ignition.

So which is better? Assuming a chamber twice as long as the diameter to only 1-1/2 as long, much like the vortex cannon video. Would say 3 gaps evenly spaced in the rear of the chamber produce a combining traveling wave front with higher peak pressure than say 3 gaps centered along the axis of the chamber?

Any thoughts? Has anyone tested this?

I'm thinking a later start of the barrel burn out phase can produce a higher average pressure behind a projectile as it travels down the barrel to reach a higher muzzle velocity with a slightly longer barrel.

Is there a way to achieve a higher average pressure in the barrel over a longer distance for more power?
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Unread postAuthor: niglch » Thu Sep 10, 2009 12:27 pm

What about a design where the spark strip is simply supported in the center of the chamber? You could just put holes in the strip and place the electrodes in the center of the holes. I wouldn't think the thin profile of the strip would interfere too much with the flame expansion (but I'm just assuming that).
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Unread postAuthor: Hubb » Thu Sep 10, 2009 12:35 pm

I've kinda always wondered that too. Start the spark at the rear of the chamber and have the flame front traveling forward. I'm thinking that, as the flame front travels, it burns more and more fuel, steadily increasing the speed of the flame front. Turbulence increases the speed of the flame front even further.

On the other hand, having a spark in the center of the chamber will produce two flame fronts, one going forwards and one going backwards. The forward flame front begins pushing the projectile before all of the fuel is used up.

This whole concept was actually the basis for the design for the sparking system I used on my hybrid.

Of course, I don't really know if this is what goes on during a combustion in a sealed container and, if this is what is going on, I would be more inclined to think that it happens so fast the any increases would be negligible.

What about a design where the spark strip is simply supported in the center of the chamber? You could just put holes in the strip and place the electrodes in the center of the holes. I wouldn't think the thin profile of the strip would interfere too much with the flame expansion (but I'm just assuming that).
That's the technique starman uses in Triple Thunder. Seems to work.

I actually built a spark system that puts three sparks in the center, plus holds a fan at the same time. The entire unit is one piece and removing it from the chamber, clears the chamber out. More detail on this system can be found here. Of course, I have since improved this design and made it much easier to build, adjust, and use. I just can't use this particular one on the launcher I'm building at the moment.
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Unread postAuthor: Technician1002 » Thu Sep 10, 2009 2:37 pm

Hubb wrote:I've kinda always wondered that too. Start the spark at the rear of the chamber and have the flame front traveling forward. I'm thinking that, as the flame front travels, it burns more and more fuel, steadily increasing the speed of the flame front. Turbulence increases the speed of the flame front even further.


I was thinking more along the lines of the rise in chamber pressure will accelerate the projectile. The max speed in the transition from chamber to barrel is only SOS. As the flame front reaches the transition, the temperature rises raising the SOS value. This would continue into the barrel until barrel burnout. A projectile could have very high speed if the projectile is light.
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Unread postAuthor: Hubb » Fri Sep 11, 2009 9:08 am

This would be a nice theory to test.
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Unread postAuthor: jimmy101 » Fri Sep 11, 2009 3:01 pm

Didn't someone make a strip similar to Latke's except there were heavy wire (like coat hanger) electrodes up to the chamber's center line? There was very little added surface area near the chambers center, just the heavy pieces of wire that looked like a square wave with breaks in the tops.

Spark position has been discussed before and I don't thing there is definitive data on what is the best positioning. Models have said that the center is best (or just slight towards the breech). Multiple sparks are also predicted to be better. None of the models have been able to handle a spark that isn't on the center line of the chamber so there isn't any model data on how much that hurts (or helps) things. (IIRC, there isn't a simple formula for the volume of a sphere truncated by a cylinder where the sphere's center is not on the cylinders axis. That makes modeling an arbeit.)

The best data I know of on spark number is Latke's. In his data there wasn't any difference between two and three sparks.

I think it is pretty hard to predict things like a rearward spark having some advantage as it chases the combustion gases toward the muzzle. Or a forward spark that compresses the unburned fuel against the breech plug as the flame front moves backwards. And in this second case, as the unburned gases expand from heating they are forced forwards and into the backwards moving flame front. Very complicated stuff that the current models really can't deal with very well. The models (mine and HGDT) do take into account the affect of the gas's movement on the flame front movement but only in a fairly simple way.

I don't think SOS is ever an issue in a typical combustion gun chamber. All the gases in the gun are hot, even the unburned gas, so the SOS is more than 1100 FPS everywhere in the gun, especially when enough fuel has been burned to get the gases moving that fast. By the time the flame front gets to the chamber to barrel junction the SOS is probably up around 2000 FPS and no standard combustion gun is ever going to get the gases moving that fast since the ammo has to be moving at least that fast as well.
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Unread postAuthor: Hubb » Fri Sep 11, 2009 3:47 pm

Didn't someone make a strip similar to Latke's except there were heavy wire (like coat hanger) electrodes up to the chamber's center line? There was very little added surface area near the chambers center, just the heavy pieces of wire that looked like a square wave with breaks in the tops.
Yes, except he used like 1/4" thick copper. Then there's starman's spark strip, which is more suited to my taste.

The whole concept of spark position would be a great thing to test if one ever had time.
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Unread postAuthor: jimmy101 » Sat Sep 12, 2009 12:07 pm

This is what I was thinking of.
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Unread postAuthor: D_Hall » Sat Sep 12, 2009 6:19 pm

I'd say niglch's sketch is probably about the best you're going to get from the bang for the buck perspective.

But so what?

It's not like it isn't reasonably simple to set up an arbitrarily large number of ignition points if you're using a spark strip (or strips). High voltage circuits are cheap and easy to come by.


Personally, what I'd like to see somebody try out is a jet igniter. I actually have one at the office (even got a patent for that particular one)... Hmmm.... Maybe I'll have to give it a whirl someday.
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Unread postAuthor: theBOOM » Mon Sep 14, 2009 7:10 pm

Jimmy I like your design but the only problem I see is that it would be hard to service the sparkstrip if your using a small diameter chamber....
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Unread postAuthor: jimmy101 » Wed Sep 16, 2009 10:12 am

theBOOM wrote:Jimmy I like your design but the only problem I see is that it would be hard to service the sparkstrip if your using a small diameter chamber....

Actually, I figured it would be done the way Latke did his. There's a screw at each end of the strip the goes through the chamber wall. The screws also supply the electrical path.

If you remove the 2 screws the whole thing will fall out'a the breech.

The strip probably needs to have either threads, or nuts permanently mounted to it, since it'll be nearly impossible to get into the chamber far enough to hold nuts while the screws are installed.
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Unread postAuthor: Hubb » Wed Sep 16, 2009 5:50 pm

jimmy, that was the exact way I built the spark strip that went into the combustion I linked to above. Works great, easy to use, and all that good stuff. Like I also mentioned, I have since redone this particular spark strip to make it even better, but that's for a later topic.
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