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Crazy Differences

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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Unread postAuthor: Fnord » Thu May 24, 2007 12:07 pm

I generally feel that the number of spark gaps that are used will have different effects depending on the size of the gun.
As a real world example, you dont use fine grain gun powder in a large-bore gun, just as you dont use rifle powder in a 9mm bullet. The larger bore guns use slower burning powder to make sure the bullet doesnt slow down because the powder finishes burning before it leaves the barrel.

In a large spudgun, a single gap may be more efficeint than 2 or more, while a mini may benifit from 4 gaps.
There really is no hard evidence either way unfortunately.
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Unread postAuthor: SpudBlaster15 » Thu May 24, 2007 7:15 pm

jimmy101 wrote:As to your spark gap results.... As I pointed out earlier, lots of theories, very little hard data. In your case, "splat" versus "dent" is not terribly quantitative. Is that a 100% difference in KE or 30%?


Not quantitative, but qualitative to some extent. I am unsure of the exact figure, but the difference between the two results seems rather significant.

jimmy101 wrote:Besides, spark strips would be expected to behave a lot differently than a basic "two-screws" gap. The problem with a spark strip is that the flame propagates from the strip as a ~hemisphere (at least immediately after ignition). With screws for the gap the flame propagates as a ~sphere. Rough estimate would say two gaps on a strip are about the same as a single gap made with screws.


I think I should have worded this differently:

My propane metered over/under combustion with an 8 foot long 1.5" diameter barrel originally had a 220ci chamber with a 4 gap, central mounted spark strip.


The spark strip was positioned directly at the center of the chamber, such that the flame front would propagate as a sphere. I took this into account when designing the theoretically optimal ignition setup.

jimmy101 wrote:Of course, it also depends on where the strip is in the chamber (near the axis or the wall), how far apart the gaps are on the strip (two gaps separated by 1/4" are probably ~identical to a single gap), how wide the strip is etc. For best results the distance between the gaps should probably be about the diameter of the chamber. Separations that are significantly less than that will start to behave as a single gap. What were the dimensions of your spark strip; length, gap to gap separation, width? (The actual gap distance is probably not important.)


My chamber was ~15.5" long, and so I placed each gap 3.5" apart to prevent the supposed spherical to dome flame front shape transition. I am not stupid.

jimmy101 wrote:Besides, in the comparison you give you increased the chamber volume of the second gun by 36% (220 ci vs. 300ci). How much of the difference in performance is because of the larger chamber? Did you also resize the barrel? The 300ci should have had a longer barrel than the 220, enough longer to make a fair amount of difference in the velocity?


The barrel length was left alone. Obviously a great deal of the power increase resulted from the larger chamber, because with a larger chamber, more energy is available to be transferred to the projectile. My argument is not that a single spark gap works more effectively than multiple gaps, but that a single properly placed gap (along the axis of the chamber, 2/5 of the way from the back) will work just as well.

jimmy101 wrote:I'm not sure I follow your point about oversized chamber and burst disk. I believe I have always said* that a grossly oversized chamber is a bad thing in a combustion gun. ("grossly oversized" being C:B >1.5 or 2). I too have been flamed for saying that. People always come back with "but a larger chamber has more fuel, hence more energy, therefore it must outperform a smaller chamber". I suspect that is wrong. A large chamber burns slower than a smaller one. A properly sized chamber will outperform an overly large chamber. However, as with many things in combustion guns, the difference in performance is frequently pretty minor. If an 0.8 gun shoots at 400 FPS than a 2.0 gun might shoot at 350 FPS. The difference can be pretty hard to detect if you are measuring performance by range, or hang time, or by how big of a dent you can make in a refrigerator. And, the spud to spud variability in mass and friction can pretty easily obscure a 50 FPS difference. Bottom line though is that you don't gain anything with an grossly oversized chamber and, most likely, the performance actually drops a bit.


As far as a larger than optimal ratio decreasing performance, I believe this depends largely on the barrel length. The fuel's burn rate as a function of heat and pressure is (for the most part) fixed, but a spud's travel time down a barrel of variable length is not. If a barrel is long enough to allow for complete combustion of the fuel in a larger than "optimal" chamber before the projectile leaves the barrel, then the performance of the launcher should increase. I believe this is responsible for my large ratio cannon experiences.

jimmy101 wrote:A burst disk gun is completely different than a conventional gun in terms of the affect of the C:B ratio. I don't believe I have ever posted that a burst disk gun can not utilize the greater energy in an oversized chamber. Isn't that partly the whole point of a burst disk? As SpudBlaster said, in a properly ratio'd cannon a burst disk doesn't really help much. (Not sure I completely believe that, would love to see chrony data. But I can believe that the benefit of a burst disk is not huge on a properly sized gun.)


The closest thing to hard data you are going to get at this time is probably this.
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Unread postAuthor: aturner » Fri May 25, 2007 10:27 am

jimmy101 wrote:Could be. But has anyone ever observed it to be not true?

That is flawed logic my friend. Let me expose it for you...

Let's suppose I think that farting into a potato gun is better than metered propane for launching spuds the fastest or furthest. I can also concoct some rationale having something to do with Methane having a higher flame speed blah blah blah. Then you ask me to back up my claim with evidence, and preferably chrony data. So I turn around and say...but has anyone ever observed it to NOT be true?

So back to multiple sparks and placement. Someone needs to show that multiple centered sparks improve performance before we accept it as the absolute "truth". Because.....if I claim a single spark at the rear of a combustion chamber is the best, then can you tell me if anyone has observer it to NOT be true?

I'm not sure I follow your point about oversized chamber and burst disk. I believe I have always said<sup>*</sup> that a grossly oversized chamber is a bad thing in a combustion gun. ("grossly oversized" being C:B >1.5 or 2). I too have been flamed for saying that.


Good! Because I was flamed for saying it too, and I'm 99% sure you and manObrass took me to task in the process! It was well over a year ago, but regardless I'm happy to see a smart person who agrees with me on this general idea. Better late than never! :wink:

Oh, and I didn't say anything about a burst disk in this topic. That was someone else.
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Unread postAuthor: jimmy101 » Fri May 25, 2007 12:12 pm

This is fun!

Spudblaster15
: Could you post a drawing of your spark strip? I'm having trouble envisioning exactly what you did. My default "brain picture" for a spark gap is basically what Latke did (http://www.burntlatke.com/strip.html).
Image or Image
A fairly wide piece of plastic with several spark gaps placed relatively close together, mounted on the wall of the chamber.

You said "The barrel length was left alone." So if you changed the chamber volume but left the barrel alone. What was the C:B ratio of the two guns? Can the difference in performance be explained as resulting form a larger chamber and a better C:B ratio?

"The fuel's burn rate as a function of heat and pressure is (for the most part) fixed," That is basically true but irrelevant. The burn rate is a function of the temp and pressure. But the temperature and pressure as a function of time is dependent on the size of a chamber. A small closed chamber burns faster than a larger one even though both chambers will reach the same final temp and pressure (ignoring differences in the rate of heat loss to the chamber). A small gun's chamber burns faster than a larger gun's chamber. Usually the large chamber has a larger barrel, hence the importance of C:B ratio.

Aturner: No that is not flawed logic. It is merely stating that not only has no one ever proven it wrong, no one has ever even TRIED to measure the affect. I agree that it would be flawed logic if I said "AAA is true" and regardless of (insert link to reasonable well conducted study that proves AAA is false) I still think it is true until someone shows, in all cases, that it is not true.

If you can point to a single chrony study comparing one central spark to several well separated sparks and show that there is little or no change in performance then, and only then, would the "advantage of multiple sparks" be disproven. The fact that no one has attempted to measure the affect can hardly be used to disprove the hypothesis.

I still don't recall flaming you for "oversize chamber decreases performance". I found <a href="http://www.spudfiles.com/spudtech_archive/viewtopic.php?t=5348&postdays=0&postorder=asc&start=0">this post</a> at the SpudTech archives. I posted once in the six pages of the discussion but didn't say anything about the hypothesis. I just suggested an alternative way of optimizing the C:B for a particular gun. I'm not sure of exactly what to search for in the SpudTech forums so I may have missed something.
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Unread postAuthor: SpudBlaster15 » Fri May 25, 2007 8:43 pm

My spark strip was similar to the one in the first photo, but it was mounted on the end of two 2.5" long screws threaded into the chamber, effectively placing it in the center, rather than against the wall.

The C:B ratio of the gun was originally 1.2:1, and increased to 1.5:1 upon the chamber switch.
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Unread postAuthor: jimmy101 » Mon May 28, 2007 3:41 pm

SpudBlaster15 wrote:My spark strip was similar to the one in the first photo, but it was mounted on the end of two 2.5" long screws threaded into the chamber, effectively placing it in the center, rather than against the wall.


So for the early part of the burn the flame fronts are hemispherical, not spherical. Once the front propagates far enough then it will start to "wrap around" the plastic to the back side of the strip.

Not the same as a spark strip along the wall, not the same as a standard gap in the center. Be a real PITA to model.

I wonder if there would be much of a difference if the spark strip had gaps on both sides of the plastic? You would get nearly spherical "fronts" when you consider the flame front from the top and the bottom of the strip together.
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