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CB ratio for coaxial gun

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CB ratio for coaxial gun

Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 3:39 am

Does anyone know how to optimise the diameter and length of the inside barrel on a coaxial design, and how that would affect the actual CB ratio? Any help of suggestions would be much appreciated.

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Unread postAuthor: jackssmirkingrevenge » Sat Jun 25, 2011 4:48 am

Image

- what are yout shooting
- how much power do you want
- how loud can it be
- can the chamber be as long as the barrel
- what pressure are you firing at
- what are you using to pressurise
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 5:00 am

- what are yout shooting
Most probably lead cast rounds from molds, then machined down on lathe. 15-20mm diameter

- how much power do you want
As much as possible from 150x propane air mix. Same duplex stainless as Lardas

- how loud can it be
Doesn't matter

- can the chamber be as long as the barrel
If that is the most efficient, otherwise probably no more than 4 times the chamber length, as measured from the outside

- what pressure are you firing at
Roughly 2200psi pre-ignition. Peak will vary

- what are you using to pressurise
Scuba
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Unread postAuthor: jackssmirkingrevenge » Sat Jun 25, 2011 5:10 am

Image

With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.

I would suggest a short fat chamber to optimise combustion as opposed to a thin one along the whole length of the barrel.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 5:46 am

Yeah I think that's very true. Although I was hoping of some way to figure out what barrel and chamber diameter ratio would be optimal, to save on using more air than needed.
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Unread postAuthor: jackssmirkingrevenge » Sat Jun 25, 2011 5:50 am

have you played with HGDT?
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Unread postAuthor: Crna Legija » Sat Jun 25, 2011 5:54 am

download HGDT read the usage tab, then plug in the numbers and find out what works best.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 7:14 am

That's what I'm looking for :D Thanks heaps guys
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Unread postAuthor: saefroch » Sat Jun 25, 2011 9:36 am

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HOLY SHITE!!!! This will be interesting... I'm curious to know why you want to do a coaxial though...
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Unread postAuthor: warhead052 » Sat Jun 25, 2011 11:12 am

Wow, I saw all the specs that you posted and I had the same reaction as everyone else. Let me see if I can find the link of one of the coaxials I have seen.


Edit, here ya go. This may turn some gears. http://www.spudfiles.com/forums/high-pr ... 12803.html
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Unread postAuthor: Crna Legija » Sat Jun 25, 2011 12:39 pm

warhead052 wrote:Wow, I saw all the specs that you posted and I had the same reaction as everyone else. Let me see if I can find the link of one of the coaxials I have seen.


Edit, here ya go. This may turn some gears. http://www.spudfiles.com/forums/high-pr ... 12803.html


the OP is taking about a hybrid not pneumatic,

Jolly Roger wrote:- what are yout shooting
Most probably lead cast rounds from molds, then machined down on lathe. 15-20mm diameter

- how much power do you want
As much as possible from 150x propane air mix. Same duplex stainless as Lardas

- how loud can it be
Doesn't matter

- can the chamber be as long as the barrel
If that is the most efficient, otherwise probably no more than 4 times the chamber length, as measured from the outside

- what pressure are you firing at
Roughly 2200psi pre-ignition. Peak will vary

- what are you using to pressurise
Scuba
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Unread postAuthor: warhead052 » Sat Jun 25, 2011 12:42 pm

I know, I was just saying...
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Unread postAuthor: DYI » Sat Jun 25, 2011 1:01 pm

With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.


Now let's not get too carried away here Jack - performance would be terrible with a ratio that low, it'd totally defeat the purpose of using high pressure.

In the case of a 20mm bore barrel (and a coaxial configuration), I suggest a chamber internal diameter of 60mm or so, probably 15-20cm long, for reasonably good performance (about 600-650 m/s for the slugs you propose). You'll not get firearm like performance at 150X, and 600m/s with full bore lead slugs is about the best you can aim for without the dimensions getting overly large.


Also, don't put too much stock in HGDT's results - just remember that it IS a 0-D simulator and therefore tends toward inaccurate output at high propellant gas densities and/or speeds.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 8:06 pm

DYI wrote:
With those specs, you will have awesome power with the tiniest of chambers, even a 0.01:1 ratio is probably overkill.


Now let's not get too carried away here Jack - performance would be terrible with a ratio that low, it'd totally defeat the purpose of using high pressure.

In the case of a 20mm bore barrel (and a coaxial configuration), I suggest a chamber internal diameter of 60mm or so, probably 15-20cm long, for reasonably good performance (about 600-650 m/s for the slugs you propose). You'll not get firearm like performance at 150X, and 600m/s with full bore lead slugs is about the best you can aim for without the dimensions getting overly large.


Also, don't put too much stock in HGDT's results - just remember that it IS a 0-D simulator and therefore tends toward inaccurate output at high propellant gas densities and/or speeds.


Yeah I think when approaching high mixes with a piston valve the amount of chamber ignition points become critical. With a low number of ignition points, the projectile will likely have left the barrel before maximum chamber pressure is reached, simply because the pressure release is gradual, compared to a burst disk which will release it all at once at a specified failure pressure (higher the better).

And DYI, you are fairly spot on there with the chamber ratios. I'm getting 50mm x 170 as the sweet spot. Anything below or above 50mm seems to reduce performance. But again I think that has to do with more space without ignition points.
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Unread postAuthor: saefroch » Sat Jun 25, 2011 8:15 pm

You can make do with fewer ignition points simply by designing the valve only to open very near maximum pressure. MrCrowley and SB15 did this by pressurizing the pilot.
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