Idea for auto guns: make the propane do the work

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Postby GalFisk » Sun Jan 28, 2007 6:22 am

Something I just thought of: in many semi or auto design ideas (inclusing mine), the power for actuation of the various mechanisms is usually drawn from the combustion process, or comes from an external source such as electric motors. But with metered propane, the expansion of the fuel when charging the chamber can also be used to power the mechanism(s). A gun with a large chamber needs more propane, and will therefore have more gas available to do work. A high meter pressure is also preferrable, so that the gas expands a lot when let out. The advantage to this method is that the power is constant and regulated, and the actuation doesn't happen at the moment of combustion so there are less timing issues.
I think it would be possible to devise a mechanism where both reloading and venting is done by a single back and forth stroke of one piston, but you can get more (though shorter) strokes or use two or more pistons in sequence by using a smaller meter pipe, and doing several fills.
Example (here be metrics):
A 5 liter chamber needs 5000*4.2%=210 ccs of propane.
At a meter pressure of 4 bar, that gives us a 210/4=52.5 cc meter pipe.
If we have a piston with an area of 1 cm<sup>2</sup>, the gas will push 1 kg at 1 bar, so if that's the minimum force we need we can let the propane expand to 210*.75 = 157.5 cm<sup>3</sup>. With the pston area 1 cm<sup>3</sup> the piston can then move 157.5-52.5= 106 cm (this because the meter pipe is part of the volume), which should be more than enough. A bigger piston will give more force and less distance, More pistons can be actuated at once, but keep in mind that the one with the smallest load will move first.
When the piston has one its job, a valve should be opened to allow the propane into the chamber (the piston could do this as well), and the piston returned via a spring. This way all the gas in the cylinder is pushed into the chamber.
With a piston inside the chamber for venting, and a metering system similar to that of the Tippmann C3, I think it would be possible to make a simple fueling/venting system powered entirely by the propane pressure, and triggered by actuating one valve or releasing a catch holding the mechanism in a ready state. This small job could easily be done by recoil or gas operation for a full auto cannon.
The advantage to these both is that the force is much smaller when the ammo is used up, and this can be used to stop the cycling.
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