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Opinion on coaxail mortar

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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Opinion on coaxail mortar

Unread postAuthor: Sparow89 » Sun Jun 22, 2008 4:19 pm

Hey everyone this is my first post. I am not new to the spudding community, but I have only built one air cannon prior to my planned project.

I would like to build a coaxial cannon for use as a paintball mortar. I would like to use a 5"-6" dia. chamber with a 3" barrel. I was thinking about a 3'-4' chamber and a 5' barrel. I have read a lot about C:B ratio, but I am still not clear on the ideal ratio. Also since it is such a large diameter chamber I was thinking about setting the piston in a 4" tube set in the rear with a long enough piston that would not constrict maximum airflow instead of having a huge piston. Would that work?

Range is the key ingredient for success in this design.


Also the picture is very primitive.
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Unread postAuthor: DYI » Sun Jun 22, 2008 4:26 pm

Optimum C:B ratio is dependent on which variable is fixed, the pressure, the gas used, its temperature, the projectile weight, and a host of other, less important factors. Assuming average parameters in all areas:

With a fixed chamber size, C:B ~0.2:1 .

With a fixed barrel size, increasing chamber always increases performance (obviously), but a point of diminishing returns is encountered eventually, where all you're really doing is wasting more gas.

Why don't you just use GGDT for the optimisation, and spare us the trouble? It's a lot more accurate than any of our general figures can be.

As to the valve design: It should work just fine.
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Re: Opinion on coaxail mortar

Unread postAuthor: jackssmirkingrevenge » Sun Jun 22, 2008 4:45 pm

Sparow89 wrote:Range is the key ingredient for success in this design.


There isn't really an ideal ratio, just a point where for a given barrel volume and valve, any further increases in chamber volume fail to result in a significant increase in performance. As DYI suggested, downloading GGDT and modelling your launcher virtually will give you an idea of what this chamber limit is.
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Unread postAuthor: Ragnarok » Sun Jun 22, 2008 6:11 pm

I'd echo what DYI said, but as he's said it, I don't need to.

However, my personal preference for pneumatic C:B ratios is between 0.5:1 and 2:1, but generally, I tend to stick between 1:1 and 2:1, which gives a good compromise between muzzle energy and total efficiency, if a little noisy.

The efficiency of my designs tends to hover around 25-30%, not great, but decent, and the power is more than enough to placate my worries about that kind of thing.
Sure, I could get ~80% efficiency with a smaller chamber, but then I'd only have less than half the muzzle energy, and for another thing, the cannon wouldn't appear "balanced right".
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Sparow89 » Mon Jun 23, 2008 1:44 pm

Thanks for the replies. As far as noise goes the louder the better. I intend for it to be very intimidating. On the GGDT I was getting about 1.3:1 I believe. For some reason I cannot get the program to work on my Vista laptop, but it does work on the XP desktop.

I plan on giving this thing a nasty paint job as well as sights and I might even mount my calculator with the ballistics program I found on here. A range finder would be nice to, but I think that is a little to deep for my pockets.
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Unread postAuthor: DYI » Mon Jun 23, 2008 3:28 pm

The efficiency of my designs tends to hover around 25-30%, not great, but decent, and the power is more than enough to placate my worries about that kind of thing.
Sure, I could get ~80% efficiency with a smaller chamber, but then I'd only have less than half the muzzle energy, and for another thing, the cannon wouldn't appear "balanced right".


Wasn't the 3" bore SCTBDC getting some ridiculous efficiency figure like 90%? I always found it funny that such a stupidly impractical cannon was so efficient.
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