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preloaded pneumatic cartridge

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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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Fri Nov 10, 2017 9:04 am

Had a powderburning shooting session recently, very pleasant in spite of the rain...

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Naturally it spurred on thoughts about pneumatic equivalents, here's a couple of thoughts. I would really love for a launcher that uses these to be semi-automatic, so apart from ease of manufacture, the critical performance elements are a fast gas dump to generate enough pressure to be able to recock the mechanism and load another cartridge through direct blowback, and a relatively weak force required to actuate firing in order to reduce the amount of blowback force needed for reliable operation.

Now that I have an efficient HP pump, pressures of a couple of thousand pounds are within the realm of possibility, increasing the potential blowback force but also the amount of force needed to actuate a valve, which requires some thought.

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Coaxial exhaust valve somewhat simplified to exhaust through the "barrel". Since the spool is balanced and small diameter, even at high pressures it should need a relatively small effort to open. Filling can be done though the "barrel", either by connecting the mouth of the cartridge to a pressure source of by placing it in a pressurized chamber, similar to the fill rigs I had devised for my hybrid cartridge experiments.

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Mechanically locked valve with a small difference between sealed diameters on the piston to limit the force needed to unlock it, similar in concept to the Mad Bull SS6 airsoft shotshell:

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A proven design but definitely a little more complex to implement.
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Re: preloaded pneumatic cartridge

Unread postAuthor: Cthulhu » Fri Nov 10, 2017 4:06 pm

I'm really glad you brought up the madbull shells, it's a mechanism I've been pondering over for a long time.
I would recommend taking a look at the airsoft ICS 40mm shells, they are apparently a mechanism with only 2 O-rings, no ball bearings, and no need to reset the valve with performance rivaling madbull 40mm shells. Perhaps they would be easier to manufacture?
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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Fri Nov 10, 2017 4:58 pm

The ICS shells are a simple balanced spool valve, conceptually a larger version of the APS Shotshells I described here.

Essentially it would be a similar idea to this, however the problem with balanced spools is that even while the pressure forces are in equilibrium, the pressure forcing the o-rings against the tube causes a lot of friction which needs a hard knock to overcome, particularly at the pressure levels I wish to operate.

This is why a mechanism that uses the chamber pressure itself to actuate the valve, such as an exhaust driven piston, is of interest if the intention is to make the launcher capable of self-loading.
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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Mon Nov 13, 2017 7:11 pm

Perhaps a non-percussion system can be used to release air from the cartridge. It would be possible to use a pilot to draw the air from behind the cartridge, this would work just like the opening pistons by force difference. I imagine that the trigger system, when one presses the trigger, would open a valve that needs little force to be activated. It is just a thought.
To increase the recoil force it would be possible to inject water, just a little bit, into the cartridge, or some other liquid. The increase in mass of what is expelled would give more "push" to achieve backload, although it would reduce a little the amount of air inside.
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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Tue Nov 14, 2017 7:46 am

hectmarr wrote:It would be possible to use a pilot to draw the air from behind the cartridge, this would work just like the opening pistons by force difference. I imagine that the trigger system, when one presses the trigger, would open a valve that needs little force to be activated. It is just a thought.


Not sure I understand, could you draw a quick diagram?

To increase the recoil force it would be possible to inject water, just a little bit, into the cartridge, or some other liquid.


In my experience with hybrid cartridges there is a lot of recoil generated, a high pressure air cartridge (I'm thinking of at least 1000 psi) should be similar:



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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Tue Nov 14, 2017 10:44 am

I was referring to this system that is so common.
I was referring to this system that is so common. The mass that must be moved to open the pilot's exhaust is much smaller than a complete axis. The required force required, being the pressure of about 150 bar, the diameter of the pilot valve shaft of 2mm and the exhaust hole of 3mm, would be about 6 kg.
Because of the recoil force of these air cartridges, in relation to a hybrid, you're right. I thought about increasing the mass of the expelled fujo. because I thought it was a problem the little backspace generated in the escape action. In these air cartridges, you would have more backward movement, because the air is denser because it is cooler than when it comes out of a hybrid, and therefore there should be more reaction in the opposite direction to that of the cannon. That being the case, I do not see the problem that I can automatically reload the firing system. What would be interesting is to measure how much force these cartridges exert when expelling the air at such a high speed ...
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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Tue Nov 14, 2017 10:52 am

Impressive the power that is seen in the videos, :shock: which I have already seen very carefully. In my small new hybrid only at 6X, the 4 mm screw that I used to keep the bolt system in place is bent ... I have already replaced it with a 5mm and 8th grade, the high strength ones.
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Re: preloaded pneumatic cartridge

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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Tue Nov 14, 2017 1:19 pm

A conventional coaxial piston valve was more or less the original plan but at high pressures the forces are considerable.

For a schrader valve at 1000 psi for example, you need a force of at least 4.9 lbs/2.2kg to knock it open in spite of the fact that it's a tiny 2mm diameter opening. That was the logic behind designing the balanced alternative. That being said in a typical CO2 pistol like the Crosman 2240 you're looking at around 25 lbs of pressure keeping the valve shut and the hammer springs tend not to be that strong, perhaps such innovation is unnecessary.
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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Tue Nov 14, 2017 4:42 pm

Maybe by making the force necessary to open the piston that lets the air escape, have a sense perpendicular to the axis of the cartridge, and balanced totally in the longitudinal direction, you only need to overcome the resistance of two or ring. It would be something like that, as I see it, like a swinging valve. Just by moving through the firing system, the tube that opens the hole would be everything.
I'm not sure if you understand me, if not, please tell me ...
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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Wed Nov 15, 2017 6:23 am

Your design would work, one thing to consider however is that while the "trigger" portion is balanced, the fact that it is large diameter means that it will have a lot of friction proportional to the pressure. A smaller diameter like this would require much less force to trigger for a given pressure:

Image

Note also that instead of having an o-ringed piston with an equalization hole, it makes more sense to simply have no o-rings and a tight fitting piston.
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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Wed Nov 15, 2017 7:17 am

jackssmirkingrevenge wrote:Your design would work, one thing to consider however is that while the "trigger" portion is balanced, the fact that it is large diameter means that it will have a lot of friction proportional to the pressure. A smaller diameter like this would require much less force to trigger for a given pressure:

Image

Note also that instead of having an o-ringed piston with an equalization hole, it makes more sense to simply have no o-rings and a tight fitting piston.

I agree Jack. Without a doubt, it works better that way. Less force is needed with less oring friction.
You have to get a cartridge of these, I mean of those that are already manufactured, for sale to the public, and design a small experiment to obtain the data that backspace they produce, when they are filled to the pressure that you are going to use. This information is fundamental. With this data you can try a system for loading and ejecting cartridges for your semiautomatic weapon :)
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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Wed Nov 15, 2017 8:01 am

.22LR rimfire develops more than enough pressure for straight blowback, here's a pressure curve I found for the chamber:

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The critical advantage for a pneumatic cartridge would be that the outside diameter subjected to pressure is larger than say the rimfire cartridge which is more or less the same diameter as the bullet.

Comparing a 1/4" diameter cartridge with a chamber pressure of 8000 psi, a 3/4" outer diameter cartridge would only need 890 psi for the same blowback force. Of course this is a gross oversimplification, there's a question over how long the impulse is delivered to the cartridge and bolt, but at least it suggests that I'm in the right area. With my fancy new Chinese pump and machined construction, working pressures of at least 2000 psi seem very feasible making this more likely to work than when I first attempted it with epoxy construction and a small shock pump.
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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Wed Nov 15, 2017 8:46 am

Good date. I imagine that the mass of the air cartridge is greater than that of a bullet.22. Would it affect this? Even so, it should be sufficient to recover the spring if it has a compressive force according to what is necessary. Have you already devised the trigger system? or what ideas do you have to build the prototype? This is interesting ! It is very difficult to solve this with numbers, they are only indicative. With your "super Chinese air pump" :D you have an important tool.
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Re: preloaded pneumatic cartridge

Unread postAuthor: jackssmirkingrevenge » Wed Nov 15, 2017 10:05 am

hectmarr wrote:Good date. I imagine that the mass of the air cartridge is greater than that of a bullet.22. Would it affect this?


Sure, but the bolt can be lighter to even things out.

hectmarr wrote:Have you already devised the trigger system? or what ideas do you have to build the prototype?


What I have in mind is a simple open-bolt design like the Sten/M3 Grease Gun/MP40 type submachineguns:

Image

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Basically the bolt is held back by the trigger, and at the moment of firing strips a cartridge from the magazine and loads it into the breech, then fires it when it is seated. The blowback then reloads the bolt, ejecting the cartridge in the process allowing the cycle to be repeated.

The alternative would be to have a separate firing pin so it would fire from the closed bolt but for what I need it's an unnecessary complication:

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Re: preloaded pneumatic cartridge

Unread postAuthor: hectmarr » Wed Nov 15, 2017 4:58 pm

This system is just what you need. I await news about this project, which I really like.
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