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Critique the Design...

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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Critique the Design...

Unread postAuthor: D_Hall » Fri Jan 20, 2012 2:23 am

OK, so I've decided to build another gun. Many fine details are still TBD, but I think I've got the big picture nailed down unless somebody finds a gross problem with it.... Thus, this thread.

First... The basic layout.

It's a coaxial design.
Barrel: 6' x 1.68" bore (nominal 1.5" sch 10 stainless steel pipe).
Valve port: 1.5" diam.
Chamber: 12" x 5.6" (nominal 6" sch 80 steel pipe).

GGDT output:
Image

Basic idea of the gun:
Image

Concept of operation? It's an auto-loading golf ball gun utilizing a floating barrel. The valve is standard fare but the barrel is pushed forward by springs (breech open) and backward by pneumatic rams (breech closed).

More to the point...

1) As the gun sits ungassed and ready for operation, the valve piston is aft and the barrel is forward. A golf ball (not shown) is dropped into the feed tube (which also acts as a 6-shot magazine) and falls through the barrel shroud into the breech of the gun.
Image

2) 300 psi air is utilized to pressurize the chamber through the pilot vent (mechanism not shown). This drives the piston forward, obviously.
Image

3) As the chamber pressurizes, 100 psi air is fed through a regulator mounted to the forward chamber wall to the pneumatic rams. These push the barrel backwards and close the breech.
Image

4) Venting the pilot volume fires the gun like any other gun. The piston moves back.
Image

5) After the chamber is dumped, air is bled from the pneumatic rams through a check valve into the chamber (after which, the air simply goes out the barrel). This allows the springs to push the barrel forward again, opening the breech and returning the gun to it's original state.

Thoughts?
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 20, 2012 2:35 am

Why not modify it to have the piston and barrel connected, thereby avoiding the need for separate rams to govern reloading?

piston closed = breech open

pilot vented -> piston starts to pove back, pulling barrel back and sealing off the breech opening -> barrel vents exposed to chamber -> boom.

Refill the pilot, piston recloses, repeat as necessary.

It will give a lovely "ma deuce" effect when firing too :D

[youtube]http://www.youtube.com/watch?v=3-OmjCe6Ze8[/youtube]

Shame not to add a pop-off pilot actually...
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Unread postAuthor: D_Hall » Fri Jan 20, 2012 2:46 am

jackssmirkingrevenge wrote:Why not modify it to have the piston and barrel connected, thereby avoiding the need for separate rams to govern reloading?

Seems to me that would yield a decidedly slow acting valve given that the "piston" is now going to weigh in at 10 pounds or so. Also, unless a radically different seal is designed, you're going to be allowing gases to exit the feed tube as the valve will be fully opened long before the breech is sealed.

And finally, one thing I'm aiming for...

Notice that I've got a very large volume on my pilot but a reasonably small pilot vent? Granted, there'll be some real world tuning involved, but I'm actually wanting a small amount of pressure to remain in the pilot volume. If I do it right, the valve will close itself shortly before the projectile leaves the barrel thereby saving a lot of gas (chamber should still have about 200 psi in it). If I link the two I suspect my ability to tune that aspect of operation will be greatly reduced (although I may be kidding myself on the practicality of such operation).

Shame not to add a pop-off pilot actually...

Heh... I know where you're going with that.
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 20, 2012 4:08 am

D_Hall wrote:Seems to me that would yield a decidedly slow acting valve given that the "piston" is now going to weigh in at 10 pounds or so.


Agreed, in this case simplicity would be at the expense of performance.

Also, unless a radically different seal is designed, you're going to be allowing gases to exit the feed tube as the valve will be fully opened long before the breech is sealed.


Also true.

Notice that I've got a very large volume on my pilot but a reasonably small pilot vent? Granted, there'll be some real world tuning involved, but I'm actually wanting a small amount of pressure to remain in the pilot volume. If I do it right, the valve will close itself shortly before the projectile leaves the barrel thereby saving a lot of gas (chamber should still have about 200 psi in it). If I link the two I suspect my ability to tune that aspect of operation will be greatly reduced (although I may be kidding myself on the practicality of such operation).


It certainly true that with a spring loaded and non-airtight piston having a "momentary" pilot you can get more than one shot out of a single chamber. However, instead of large pilot chamber, why not small pilot chamber and hefty resetting spring? You should get faster valve opening time that way, it will be like a pressure operated hammer valve of sorts...

Heh... I know where you're going with that.


You sound like a man who can afford a lot of golf balls :D
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Unread postAuthor: al-xg » Fri Jan 20, 2012 8:25 am

Notice that I've got a very large volume on my pilot but a reasonably small pilot vent? Granted, there'll be some real world tuning involved, but I'm actually wanting a small amount of pressure to remain in the pilot volume.


Could you not just use a solenoid valve on the pilot side to control the dwell time ?


What is that part at the base of the magazine tube ?
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Unread postAuthor: jrrdw » Fri Jan 20, 2012 8:33 am

D_Hall wrote:unless a radically different seal is designed


Have anything in mind for that? I often think about this as a upgrade and better way of porting/venting to improve power...
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Unread postAuthor: Fnord » Fri Jan 20, 2012 8:57 am

5) After the chamber is dumped, air is bled from the pneumatic rams through a check valve into the chamber (after which, the air simply goes out the barrel). This allows the springs to push the barrel forward again, opening the breech and returning the gun to it's original state.


If I do it right, the valve will close itself shortly before the projectile leaves the barrel thereby saving a lot of gas (chamber should still have about 200 psi in it)


I think you may be dumping the pneumatic rams uphill a bit, then? If they're regged down to 100psi...
For a single shot per fill it obviously wouldn't be an issue but otherwise it kinda eliminates the 'valve tuning' you describe.



If it were me building something of this nature, I might be tempted to get rid of the pilot port, make the whole floating barrel a 'hammer' and use the rams for resetting it after each shot. The piston could even be 'counter-balanced' for additional power as in BTB's design. Cycling might be slow but the ram speed would likely be the limiting factor.

But, that's just me. I'm probably not seeing everything you want to accomplish.
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Last edited by Fnord on Fri Jan 20, 2012 9:14 am, edited 1 time in total.
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Unread postAuthor: POLAND_SPUD » Fri Jan 20, 2012 9:08 am

I think that an unbalanced QDV would suit this design better (and it could be breech loaded)
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Unread postAuthor: D_Hall » Fri Jan 20, 2012 11:03 am

al-xg wrote:Could you not just use a solenoid valve on the pilot side to control the dwell time ?

I could and I may still do it that way, but I'd like to keep the gun 100% mechanical.

What is that part at the base of the magazine tube ?

I'm not sure what you're asking about, but I'm going to guess that it's a nylon/PE (material not yet decided) ring that provides a sealing surface for the breech (barrel pushes up against it).

jrrdw wrote:Have anything in mind for that? I often think about this as a upgrade and better way of porting/venting to improve power...

You could seal up the breech of the barrel axially and then build a giant spool valve using the breech itself as a spool. I think performance would suffer, however.

Fnord wrote:I think you may be dumping the pneumatic rams uphill a bit, then? If they're regged down to 100psi...
For a single shot per fill it obviously wouldn't be an issue but otherwise it kinda eliminates the 'valve tuning' you describe.

You caught that, eh? Yeah, the operation of the rams was "set" in my mind for quite some time. I didn't contemplate re-using some chamber air until last night. I saw the issue shortly after I posted but decided to leave it in case somebody saw a solution to it I didn't.

But, that's just me. I'm probably not seeing everything you want to accomplish.

Well, one thing I've got in the back of my mind... I'm wanting to be able to change out just a few parts and find myself in possession of a valved hybrid.
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Unread postAuthor: al-xg » Fri Jan 20, 2012 3:38 pm

I'm not sure what you're asking about.

I was referring to the the purple/grey/brown part seen on the 1st picture. Maybe it is actually on the flanged front plate.

I once turned one of my QEV canons into a multi-shot version by adding a chamber after the pilot valve and then venting it once the pilot valve was closed again. One could just use a 3/2 valve to do this in one action. Hotwired had something like this back on the UKSGC forum.
Could be an option for a controlled pressure drop in the chamber.
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Unread postAuthor: Brian the brain » Fri Jan 20, 2012 5:26 pm

Please tell me you'll be using springs to close the breech and you'll be using gas fed from the barrel to feed the forward extending rams....


Seems like a way better design.

Or do the exact opposite like Polandspud did..

Either way...
I see no reason not to have it all automated in a simple way..
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: D_Hall » Fri Jan 20, 2012 6:24 pm

jackssmirkingrevenge wrote:instead of large pilot chamber, why not small pilot chamber and hefty resetting spring?

Missed that comment the first time through. Answer is easy...

If I do the return with a heavy spring, then I'm married to a given return force. Doing it with the gas in the pilot chamber means that the return force scales with chamber pressure.

More to the point: I'm designing the gun to run at 300 psi, yes, but I'd like it to run well all the way down to 80 psi.

A spring that gives me the return I want when running at 300 likely won't open like I want when running at 80 psi.

al-xg wrote:I was referring to the the purple/grey/brown part seen on the 1st picture. Maybe it is actually on the flanged front plate.

The purple part is the feed tube/magazine. It's nothing but a piece of 2" sch 40 pipe welded to the barrel shroud which is itself a piece of 2" sch 80.


Brian the brain wrote:Please tell me you'll be using springs to close the breech and you'll be using gas fed from the barrel to feed the forward extending rams....

It certainly be done that way but I'm not seeing any strong reason to opt for one way over the other. Your tone, however, indicates that you DO see such a reason. Care to elaborate?

edit: Wait, you said "fed from the barrel". I don't see how you could get that to work efficiently and reliably. That implies that the barrel must move all the way forward while the projectile is still in the barrel. That in and of itself isn't a huge deal, but you're opening up the breech when you do that. That means gas leakage in a big way.
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