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Stratigic Spark Placment

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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Stratigic Spark Placment

Unread postAuthor: CannonCreator » Sat Aug 18, 2007 1:31 am

I was wondering. Does it really matter were ur spark in your chamber is coming from?

And im starting to think it might. If you place the bolts or spark strip near the back of the cannon then the gas will ignite in a forward motion and the gas at the back will force out the gas out the front will ignting it. While placing a spark at the front would light the gases in a backyards motion wasting the pressure the gas at the back could be already creating. So I personally think place a spark at teh back would be the most efficent. Am I correct or is this just something that dosn't really matter?
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Unread postAuthor: hi » Sat Aug 18, 2007 2:06 am

it does matter, but only slightly.

you are right, in the back is ideal, but if you have mutiple sparks thats best because it theorecticaly ignites all the fuel at once, but it happens so fast chances are you wouldnt really notice.
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Unread postAuthor: CannonCreator » Sat Aug 18, 2007 2:15 am

ya, ok thanks very much, I was just wondering
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Unread postAuthor: MrCrowley » Sat Aug 18, 2007 2:17 am

There are loads of topics that answer this question. please search in the future.
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Unread postAuthor: SpudBlaster15 » Sat Aug 18, 2007 2:18 am

hi wrote:but it happens so fast chances are you wouldnt really notice.


"Fast" in a combustion launcher is relative. To the human eye, the fuel may appear to burn quickly no matter the spark gap placement, but to the spud accelerating down the barrel, small differences in the rate of combustion make large differences in launcher performance.

As to the physics behind optimal spark gap location, one must remember that 1) The pressure in the chamber is roughly equal in all areas, regardless of the flame front position, and 2) The chamber is not completely open, nor is it completely closed, meaning open pipe and closed chamber combustion studies done by organizations are not a dependable method of determining the optimal gap placement. All we are looking for is the fastest burn rate. The faster the fuel burns, the better the launcher performs. IIRC, jimmy's combustion launcher model predicts that the optimal spark gap location is between 40% and 50% of the way from the rear of the launcher. Exceed this range, and performance drops of dramatically.
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Last edited by SpudBlaster15 on Sat Aug 18, 2007 2:24 am, edited 1 time in total.
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Unread postAuthor: CannonCreator » Sat Aug 18, 2007 2:24 am

Thanks a lot dude, I never heard about that thanks! :D
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Unread postAuthor: noname » Sat Aug 18, 2007 11:27 am

Using a 1" SDR 21 chamber and having a sprak about 1/3 from the front, I could see the flame front traveling backwards, which is what you don't want.
Just an extra tidbit of info, I know the question was already answered.
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Unread postAuthor: CannonCreator » Sat Aug 18, 2007 11:50 am

well ya if the spark comes from the front its going to ignite backwards, which is like have a stack of blocks and unstacking them 1 by 1 starting from the top. But if the spark starts at the back it will ignite going forward. Which is like have a stack of blocks, holding the blocks from bottom block and throwing it up in the air, Which gives much faster, more powerful combustion.
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Unread postAuthor: jimmy101 » Sat Aug 18, 2007 12:43 pm

As to the physics behind optimal spark gap location, one must remember that 1) The pressure in the chamber is roughly equal in all areas, regardless of the flame front position, and 2) The chamber is not completely open, nor is it completely closed, meaning open pipe and closed chamber combustion studies done by organizations are not a dependable method of determining the optimal gap placement. All we are looking for is the fastest burn rate. The faster the fuel burns, the better the launcher performs. IIRC, jimmy's combustion launcher model predicts that the optimal spark gap location is between 40% and 50% of the way from the rear of the launcher. Exceed this range, and performance drops of dramatically. SpudBlaster

Very well put. Closed and open chambers give some useful information but neither really explains the complexity of a combustion spud gun.

I've been reading up on gasoline engines and it appears that the combustion stroke may be a pretty good model for what happens in a combustion spud gun. It would be an excellent model for a hybrid since ignition occurs in a pressurized air + fuel mix.

Interestingly, in a gas engine, the rate that the fuel burns (or the flame front propagates) is dependent on the speed the engine is running at. Higher RPMs give faster flame rates. This suggests that the flame front speed is coupled to the movement of the piston/projectile. In other words, combustion accelerates once the piston/projectile starts to move. Perhaps because movement of the projectile allows movement of the gases, that creates turbulence, which converts the flame front from a slow moving laminar one to a much faster turbulent one.

CannonCreator, or if the spark is at the front of the chamber...
The pressure rises throughout the chamber, including near the breech plug. The spud starts to move, the gases at the breech start to move towards (they expand towards) the flame front, the unburned gases are being pumped into the backwards moving flame front. Just an alternate theory.

I don't think anybody really knows where the optimal spark location is or how much of a difference it makes. I would think the lack of any hard data is weak evidence for it not mattering very much. If a particular spark location was much much better than another, someone would probably have noticed by now.

The flame front has essentially zero mass, hence zero momentum and kinetic energy. The direction the flame front is moving, all by itself, won't make any difference to the performance of the gun. The combustion products can move in all directions, it is just hot air, it can go forwards, backwards, sideways, whatever, equally well.

The direction the front is moving may have an effect if the unburned gases are moving as well.
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Unread postAuthor: VH_man » Tue Aug 21, 2007 9:04 am

i know that in mini's, if i place the spark gap as far away form the barrel as possible i get the best preformace........

the BEST preformance ive gotten was when i placed the barrel through the combustion chamber and then had the spark gap at the front, where the barrel was attached to the chamber. im not sure why this was better, all i know is that it was...........
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Unread postAuthor: paaiyan » Tue Aug 21, 2007 9:12 am

I agree with putting the spark in the back, though I like my own idea better. Build an induction coil capable of producing a spark that runs the entire length of the chamber. Place one electrrode at the front, one at the back. Problem solved!
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Unread postAuthor: DYI » Tue Aug 21, 2007 9:59 am

Yeah, but then you'd need to figure out how to insulate something generating 100s of KVs.

I know that at some point someone ( I think it was BigBang from the SGTC forums) placed the spark gap behind the fan in a metered MAPP gun to induce turbulence and accelerate the flame front. He claimed that this caused a significant increase in performance.

As for me, I just space out 3 or 4 spark gaps through the chamber.
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Unread postAuthor: paaiyan » Tue Aug 21, 2007 10:00 am

DYI wrote:Yeah, but then you'd need to figure out how to insulate something generating 100s of KVs.


:twisted:

I know.
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Unread postAuthor: Rock » Tue Aug 21, 2007 11:00 am

Just an idea, correct me if i'm wrong.
If the combustion starts in the back of the chamber, the volume of the gas in this area increases. This means that it will push the still unburned part in the front of the chamber down the barrel, together with the projectile.
If the flame travels that fast that this case doesn't occur, it would be no problem to have the spark gap located in the front part of the chamber

This is why i put my spark gap exactly in the middle of the chamber (btw- it works very good). This means there is the flame spreading out in both directions.

I don't know very much about physics but this sounds logical to me.
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Unread postAuthor: noname » Tue Aug 21, 2007 11:21 am

I have a smalll combustion which I'll post soon, with a 5" long 3" diameter chamber. I put the spark in the middle of the chamber, because that's where I'd get the most even burn. In my huge combustion, I put the 4 sparks each 9-10" apart, starting 4" from the back of the chamber.
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