igniter placement inside chamber

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Postby jimmy » Sun Dec 03, 2006 11:36 am

Spudblaster: Still have the gun? How about a controlled test of one versus multiple sparks so we have real data.

Requirements for believable results:
- chamber fan (specially now that it is cold outside)
- fuel meter
- consistent projectiles (or at least the mass of the individual projectiles which is pretty easy to do)
- method to reproducibly set the launch angle
- 5 (or more) shots for each spark configuration
- a reliable way to measure the range of each projectile (perhaps markers every 50' and a spotter at about the expected range but 50 feet off the trajectory)

Data from a controlled prospective study is the only way to say for sure if mutliple sparks do anything.
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Postby benstern » Sun Dec 03, 2006 12:03 pm

Ok, we need high speed camera clips of combustion in a clear spudgun......
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Postby SpudBlaster15 » Sun Dec 03, 2006 4:11 pm

I can do some controlled tests as soon as I locate a syringe to allow me to meter the fuel. I can control every other variable quite easily, so I may do the tests next weekend provided everything goes well. I have a 300 meter wide lake covered with an 18 inch sheet of ice to work with, so comparing distances will be very easy.

Since the gun has a spark gap placed 1/4 of the way from the front, and another 3/4 of the way toward the rear, I can also determine what effect spark gap placement has on a gun using only a single gap.
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Postby jimmy » Mon Dec 04, 2006 1:49 pm

Spudblaster15: It would be really cool if you can get the data. Spud gunning is long on theories but painfully short on hard data.

One point though, what if a single spark at 1/4 is essentally identical to a single spark at 3/4 but the optimal spark location is 1/2?

My WAG is that 1/2 is optimal and 1/4 and 3/4 will be nearly identical.

Are you going to use spuds for shells or something more consistent? If you do use spuds the mass of each spud is critical. I believe a lot of the variation in muzzle velocity people see is due in part to variation in mass of the individual shells. In your case, using range as a replacement for muzzle velocity, the mass of the round will still have a big affect on the carry distance. An easy way to get the mass of a spud shell is to;
1. Weigh the spud before loading.
2. Collect the spud shavings made by the cutter.
3. Weigh the shavings.
4. Subtract the weight of the shavings from the original weight of the spud before loading.


Is it too cold in Canada to be safely firing a combustion gun?
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Postby SpudBlaster15 » Tue Dec 05, 2006 12:01 am

My gun is made of ABS, so although it is hovering about -5*C, it is safe to fire.

I plan to use either potato slugs, each maesured to be a constant weight using my gram scale (accurate to one tenth of a gram), or slugs made of wood with a 1/2" bolt ran through them, cut to an exact length and diameter using a hole saw.
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Postby SpudBlaster15 » Wed Dec 06, 2006 1:43 am

I performed some rough tests today, using hang time as the determining factor. I only measured 3 shots using each configuration, but it was enough to get me an average. I used an alligator clip to close off either the front or rear gap, and measured the time with a stop watch. Surprisingly, using only a frontward spark or a backward spark made almost no difference at all. The frontward gap averaged 12 seconds, and the rearward gap averaged 13 seconds. When I used 2 gaps, however, the average time jumped to 17 seconds. Keep in mind that this was a very rough test, and the results could have been affected by many variables. I need to perform more controlled tests to get an accurate measure of the actual difference between configurations. I will try to get some pictures and video too.
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Postby jimmy » Thu Dec 07, 2006 11:37 am

Spudblaster: Interesting results. Even with the limited number of shots there is a huge difference between a hang time of 12~13 seconds and 17 seconds. That's probably what, a factor of two or so in muzzle velocity?

Was this with spuds or your wooden slug(s)?

Can't wait for you complete set of results.
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Postby SpudBlaster15 » Thu Dec 07, 2006 7:28 pm

Those tests used spuds, which along with the launch angle was the only variable I made much effort to control. Each slug was weighed on my digital gram scale, and all fell between 95 and 100 grams. The propellant which was starting fluid was simply sprayed in, so the results even with identical configurations varied from shot to shot, but the increase in average velocity afforded (presumably) by an additional spark gap outweighed any of those variations by quite a bit. I am currently working on building a system to meter starting fluid, but I will need to regain lathe access to build the adapter I have in mind. I did find a syringe, but it's capacity (50cc) is not large enough to properly fuel my 200ci chamber using butane or propane. I could try using 2 sperate shots of butane/propane.
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Postby cjaddy » Fri Dec 08, 2006 8:09 pm

So if the question is weather or not correct spark placment helps just get a stun gun for 20 bux and make a spark card that runs the length of the chambe and all problems are solved. If you're serious about getting properly applied spark energy it's the only way to go. If two sparks gave you that much more hangtime think of what 20 or 30 sparks could do. Of course you get exponetialy deminishing returns with each added spark over 3 or so.
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Postby SpudBlaster15 » Fri Dec 08, 2006 9:32 pm

My smaller gun uses 3 spark gaps. Honestly, I dont notice any difference in performance over 2.
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