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Power - C:B

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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Power - C:B

Unread postAuthor: Big D » Fri Jun 13, 2008 3:04 am

Many of us here are limited by the length of our barrels because of transportation issues but our chambers are generally not nearly as long as the barrel for obvious reasons.

The generally accepted C:B is .8:1 but should one be limited by a certain length barrel; will making a larger chamber make it more powerful than an optimized one? Currently i have a larger advanced combustion with everything that is required except fo much though into C:B. Right now it is sitting at a 3:1 ratio.(4" x 45" chamber, 2" x 60" barrel)

Would an optimized C:B (.8:1) with a 2" x 60" barell create a higher FPS for a potato than my current setup? I realize excess energy is spent as noise and its very loud but is this 3:1 cannon more powerful than a .8:1? My reasoning is more propane = more power but after thinking about it for a while im not really sure. (more propane and more power is assuming 4.2% propane in the .8:1 and the 3:1 with a chamber fan.)

This is assuming one would keep the same diameter barrel. I realize that a larger barrel could propel a heavier object farther (3"+) but i would like to continue shooting potatoes for costs' sake.

So... which would result in a higher FPS using a 2" x 60" barrel?
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Unread postAuthor: psycix » Fri Jun 13, 2008 4:36 am

There was a thread about this some time ago. Instead of calculating the most powerful barrel for a chamber, what we need, is to find the most powerful chamber for a barrel.

I think that a larger chamber (and certainly when it has multiple spark gaps) would be better then a chamber fitting in the 0.8 rule.
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Unread postAuthor: jimmy101 » Fri Jun 13, 2008 1:59 pm

A larger chamber for a fixed barrel (and ammo) will usually increase performance. The optimal CB, and how much performance you might gain, depend on many factors.

For very light weight and low friction ammo the optimal CB for a particular barrel may be less the 0.8. For this type of ammo the optimal CB is a pretty sharp peak, if you are off by a small amount in the chmaber volume the performance suffers significantly.

For intermediate weight and friction ammo (say a tennis ball), the optimal CB is often pretty close to 0.8.

For extremely heavy and/or high friction ammo (like spuds or tightly wadded bb's, marbles etc.) the optimal CB may well be 3 or 10 or ...

For very large optimal CBs you get into the range of diminishing returns, the difference between a practical CB of 2 and the optimal CB of 10 (or whatever) might be minimal, as in a couple percent in muzzle velocity.

HGDT will help you answer this question. I wouldn't consider HGDT to be completely verified yet but it appear to be getting all the trends correct and gives reasonable answers. Enter you barrel and ammo and chamber diameter then do a "sensitivity study" on the chamber length. You'll find that if the ammo is very low mass the optimal CB will be less than 0.8 and small errors in the chamber volume have a large affect on the muzzle velocity. For heavy and high friction ammo the optimal CB is much larger than 0.8 and the muzzle velocity is much less sensitive to small changes in the chamber volume.
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Re: Power - C:B

Unread postAuthor: jimmy101 » Fri Jun 13, 2008 2:33 pm

Big D wrote: So... which would result in a higher FPS using a 2" x 60" barrel?

HGDT says ~480 FPS for the CB 3 gun and ~370 FPS for the 0.8 gun.
EDIT: 100g spud, 10 lbs friction, single spark, 1.6" fan, 5 cfm
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