0.8:1 C:B ratio less than optimum?

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Postby SpudBlaster15 » Tue Oct 24, 2006 1:03 am

I have been using a 1.5:1 C:B ratio on my cannon, and have quickly grown tired of the loud bang that results from firing, indicative of wasted energy. So today, as my barrel to chamber connection is threaded, I made a new barrel, using the ratio 0.8:1. Using the same propellant (right guard in the brown can or starting fluid) firing of the gun results in the potato being lobbed a slightly lesser distance, with an apparant decrease in muzzle velocity. It seems as if the potato stops accelerating before it reaches the end of the barrel. I realize that the 0.8:1 ratio may be optimized for higher energy propellants, but according to burnt latke's chamber fan tests, a 0.7:1 (!) ratio works extremely well when using starting fluid as a fuel, with some muzzle velocities exceeding 400mph. Should I cut my barrel down, or am I missing something?

Thanks.
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Postby Freefall » Tue Oct 24, 2006 1:56 am

A perfect mix is hard to achieve with aerosol fuel, and .8:1 is only approximately ideal for a well-mixed stoichiometric (not lean, not rich) mixture.

Since you're using aerosol, you're probably not getting a perfect mix. I'd suggest using a slightly shorter barrel. Cut off a little at a time until you start to get a little bit of a bang again.
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Postby SpudBlaster15 » Sat Oct 28, 2006 2:50 am

I cut it down to a 1:1 ratio and it works alot better now. Coupled with my triple arc ignition system, it shoots spuds MUCH faster than it did with a single arc ignition and a 1.5:1 C:B ratio. It is also much quieter.
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Postby nicholai » Sun Oct 29, 2006 11:37 am

since you've already read some stuff on latke's site id suggest using a propane meter to get your air/fuel ratio dialed in more accurate. While i love the simplicity of spraying into the chamber, every propane injected cannon ive made has been more powerful and reliable.
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Postby man_o_brass » Mon Nov 06, 2006 2:17 pm

Latke's .8:1 ratio was only tested for propane and one specific projectile. It won't hold true for different propellants and projectiles. Most aerosol propellants are less powerful than straight propane, so they require less barrel to optimize. As Freefall said, to really optimize any particular gun, start with an overly long barrel and cut it down bit by bit until you get the best results.
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Postby boilingleadbath » Mon Nov 06, 2006 4:16 pm

One specific projectile?
Pfft!

He did tests with tennis balls, potatos, and "gasket slugs"... I count <i>three</i>
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Postby man_o_brass » Tue Nov 07, 2006 10:23 am

True, but the .8:1 ratio only held true for gasket slugs. The tennis ball test yielded a ratio of .7:1. I never found a C:B test using spuds. My point was that each gun/ammo/propellant combination is different, and until better mathematical models are created (by someone smarter than myself), you pretty much have to test each gun the way latke did if you want to maximize performance.
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Postby FLONE » Mon Nov 27, 2006 2:22 pm

And 1 thing extra to note on the Latke tests. Go beyond the .8 thing, look at the barrel lengths associated with that point. Then look at the average velocities of both the slugs and spuds at the various points. Notice the wide range of points where the average velocities are very close to the same? Then look at the drastic changes at the points on either side. Very clear how the pressure curve builds, holds, then drops off. Then, notice the differences in barrel length within the points that have the highest average velocities. That is a lot of barrel where the spud is beibg pushed in excess of 500 feet per second!!
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