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CB ratio for coaxial gun

A place to ask general spud cannon related questions.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 8:41 pm

saefroch wrote: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.


Yeah I thought of doing that however around 27x mixes is as far as I can go to bring the rear chamber pressure close to the max peak chamber pressure. This is because the scuba tank I'm using to fill it will be around 3000psi (when full) which is the max I can fill the rear chamber, and the front chamber peak pressure will reach a max of around 3100psi at 27x. Technically I can still fill the rear up more than the front for higher mixes, although it won't be very near the maximum peak pressure. So as the mixes goes up, the efficiency of the piston valve goes down. Unless I can get a higher pressure tank of course.

However if I use this concept along with plenty of ignition points, then we should be all good.
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Unread postAuthor: saefroch » Sat Jun 25, 2011 8:50 pm

You don't have to bring the pressures equal. A piston valve triggers when there is a substantial drop in the pilot pressure. They begin to open the moment the forces on the piston equalize, and as I understand it (this confuses me a bit, but I'm just regurgitating), they trigger when chamber pressure times the cross-sectional area of the piston minus the port area equals the pilot pressure times the cross-sectional area of the piston.

So you see? You don't actually have to match the pressures, far from it. Run some calculations again (assuming you already didn't take this into account) and see what you can come up with a max mix number with a pressurized pilot.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 9:53 pm

Yeah I understand what you're saying, I was just pointing out that by bringing the rear chamber near the chamber peak pressure (assuming surface area on the piston is the same), I am enabling it to open near peak pressure.

Of course I don't have to get it near the same pressure, however that is what I am aiming for as it is more efficient and will increase performance. Problem with the scuba is that I can only take the rear pressure up to 3000 psi, which is near the 27x peak chamber pressure of 3100psi. That is the highest mix I can go to, while keeping the gun at its most efficient. It will work fine with higher mixes, it just wont open as close to peak pressure.

However if I increase the surface area of the piston in the rear chamber, I won't need such as high a pressure in the rear chamber.
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Unread postAuthor: Jolly Roger » Sat Jun 25, 2011 11:43 pm

saefroch wrote:Image
HOLY SHITE!!!! This will be interesting... I'm curious to know why you want to do a coaxial though...


It's actually a coaxial piston valve hybrid design, intended to accommodate for semi-auto firing.

Edit: Sorry guys, double post :(
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Unread postAuthor: saefroch » Sun Jun 26, 2011 2:30 pm

Jolly Roger wrote:However if I increase the surface area of the piston in the rear chamber, I won't need such as high a pressure in the rear chamber.
Or if you just have a very large port diameter... I remember for my Trident (in my sig) I calculated that I needed a pressure drop of 400psi (out of 1,000psi) or something like that for the valve to open. That was with a .364" port in a .5" piston valve... Even if you can't prevent it opening until the maximum pressure is reached, you can still probably hold it shut for a while... Enough to really make a difference with a bunch of ignition points.
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Unread postAuthor: Jolly Roger » Sun Jun 26, 2011 11:44 pm

Yeah I think that'll be the go. I'm drawing up some ideas at the moment for how to install the ignition points
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