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Still Don't Understand C:B Ratio on Pnuematics...

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Ragnarok » Wed Jun 23, 2010 4:15 pm

Technician1002 wrote:In a proper springer this is not the case. If you have a fast piston at the end of the shot, that is wasted energy and LOW efficiency.

We're not talking about straight springer (and yes, I do know how they work). While in a springer, the spring piston is brought to a halt by the pressure building up ahead of it, in a "spring/pneumatic combination" the pressure is not building up ahead of the piston - it's decreasing.

I looked into the idea years back. You'd have to talk to btrettel, as he knows the subject matter better than I, but as far as I remember...

You all know how a regular pneumatic water gun works, I'll assume. However, there was a line of waterguns which had a piston between the water and the air in the pressure chamber. You could pump up the air side of this chamber, which meant that there could still be pressure, even when the chamber was mostly empty of water.
(This is opposed to a variety that used rubber tubing as the chamber, producing a similar effect, but that concept is less useful to us.)

It works for that, because the chamber doesn't empty that fast - usually about 50-100 ml of water per second, meaning the piston usually has a velocity of centimetres per second, and very little energy.

If you try applying the concept to a pneumatic, giving it a piston in the chamber to "conserve" air, then what you get is a piston moving at dozens of metres per second.
With an open valve, and the projectile already gone, there's no pressure build-up, and nothing to make it slow down other than the end of the chamber.

Unless you can add something to do it instead... and innovating that would be a big boost to spudgunning.

(Some people may relate this to the "spring piston" in the hybrid chamber like CS was talking about recently. However, that doesn't need the piston to build up any real speed - it's not part of maintaining pressure, just venting the chamber - so the problem doesn't exist there.)

jimmy101 wrote:No, high performance and high efficiency are indeed mutually exclusive.

Not so much. As btrettel putting it, it's about scaling the whole gun.

If you build around a C:B ratio that's 70% efficient, then scale the length of both chamber and barrel (keeping to that proportion) until you get the desired muzzle energy, then you can keep that 70% efficiency.

What it really is one of those "Three choices, pick any two" scenarios:
- High energy
- High efficiency
- Compact (compact being a relative term).

You can't have small, energetic and efficient launcher, but you can have one with high energy and high efficiency - it'll just be pretty long. But, as you say, most people tend to sacrifice the efficiency.
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Unread postAuthor: kydavies » Wed Jun 23, 2010 7:39 pm

if you use a burst disk you can have a small cannon with high energy (high energy in my terms). ive got a burst disk pneumatic thats 2 feet long (80% barrel) and shoots marbles out of sight at very low pressures (im guessing 20-30psi MAX)
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mikemurph wrote:yea, the ammo has been changed to a more aero dynamic projectile. it consists of the back half of a dart, and a hot glue ball
AHahahahah!!
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 23, 2010 11:32 pm

Ragnarok wrote:What it really is one of those "Three choices, pick any two" scenarios:
- High energy
- High efficiency
- Compact (compact being a relative term).

You can't have small, energetic and efficient launcher, but you can have one with high energy and high efficiency - it'll just be pretty long. But, as you say, most people tend to sacrifice the efficiency.


Pretty much the point I made here:

I think what he means is that to make full use of the energy available from the pressurised chamber, at the point where the projectile reaches the muzzle the pressure in the barrel behind it should be the same as atmospheric pressure. In practice this would result in really tiny chambers, or excessively long barrels, weak launchers in the former case or massive ones in the latter


Sadly it is always going to be a question of compromise, as you say most people build as big as they are comfortable with then up the pressure/chamber volume to achieve desired power levels and efficiency is quite literally blown out of the barrel :roll:

See what you're started BeaverRat, I hope you've gleaned something from this :roll: :D
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Unread postAuthor: Ragnarok » Thu Jun 24, 2010 8:54 am

jackssmirkingrevenge wrote:Pretty much the point I made here

Pretty much. I just felt like phrasing it in the whole "Pick any two" scenario.
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Unread postAuthor: BeaverRat » Thu Jul 01, 2010 1:08 pm

Ok, thanks guys for this crazy discussion...

Anyway, as of now, my laucher has a 2 inch x 35 inch chamber, and a 2.5 inch x 40 inch barrel (for tennis balls). The reason I kept the chamber so small was to allow hand pumping, but I just bought an air compressor, so that should not be a problem anymore. I moddled my cannon on GGDT (what flow coefficient should I use?) And discovered that with a fixed barrel length of 40 inches, I will get about 30 more FPS by adding 30 more inches of 2 inch chamber pipe. If I add another 30 inches of chamber, the FPS will only increase by 9. I think what I will do is increase the chamber to 65 inches, and finish off with a C:B ratio of 214:187 or 1.15:1.

Well, does that seam good to you guys? Since I have an air compressor, I just want to go to as much power as possible (within reason). Does this acomplish it with the barrel I have? The chamber cannot be any longer that 65 inches, but I suppose I could move up to 2.5 inch pipe. The difference becomes more and more neglegible though as GGDT and you guys have proven that chamber volume increases has a exponetially decreasing effect.

Thanks guys
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Unread postAuthor: velocity3x » Thu Jul 01, 2010 2:28 pm

I find 1:1 ratio to be best for my needs.
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Unread postAuthor: Gun Freak » Thu Jul 01, 2010 2:49 pm

Mine is 545:1107 lmao which is about 2:1 and it is loud as hell but it works.
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Unread postAuthor: BeaverRat » Sat Jul 03, 2010 12:42 am

What flow coefficient should I use in GGDT?
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Unread postAuthor: Crna Legija » Sat Jul 03, 2010 1:21 am

just use what ggdt's default, unless you buy a valve that states otherwise
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