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The Hammer Shotgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Spitfire » Thu Feb 21, 2008 1:01 pm

Why does the piston look like it is in the same place when Extended and in fireing mode? Also what pulls the thingy back in JSR's drawing?
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 21, 2008 1:49 pm

Spitfire wrote:Also what pulls the thingy back in JSR's drawing?


You open the schrader, the space behing the purple section empties. Now the pressure on the left of the purple bit is greater than that on the right, so it moves to the right, eventually hitting the red portion and opening the valve allowing the air to escape through the barrel and fire the projectile, voila :)
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Unread postAuthor: Ragnarok » Thu Feb 21, 2008 2:13 pm

It's an interesting concept, but I don't know how well it will catch on. It needs more space, is more complex, and once a valve gets fast enough, the gains in it's speed diminish greatly.

Just load GGDT, put a generic valve in, and fiddle with the opening times. When they get quick enough (under 10 milliseconds or so, depending on launcher geometry - but generally less than that at higher pressures), then the gains from increasing valve speed are limited.

It's a nice idea and prototype, but I think I'm going to stick with conventional piston valves for now.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 21, 2008 2:19 pm

It's beyond the capabilities of less advanced spudders to I don't expect it to become widespread, besdies many are happy enough with current performance without seeing the need to push the envelope.

I do believe there are significant gains in performance to be had, after all you're countering the same force that was keeping your barrel sealed, but with a much larger force thanks to a piston that's already had time to accelerate. Perhaps once Nova has it properly set up he could build a conventional piston and do some comparative penetration tests (unless he gets his mitts on a chrony in the meantime ;) )
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Unread postAuthor: Gippeto » Thu Feb 21, 2008 2:57 pm

Nova,

Two thumbs up for you, good job!

Ragnarok,

Any ideas as to why that may be?

JSR,

When can we expect the "How to cast a Hammer-Piston" tutorial? :lol:


No, seriously...when? :)
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 21, 2008 3:07 pm

Gippeto wrote:When can we expect the "How to cast a Hammer-Piston" tutorial? :lol:


Technically I already made a floating piston here ;)

Image


Image


Image


This one was made for another project, a scaled down version of the beast - the idea in this case was that the latch holding the piston in the closed position could afford to "give" a little without leaking air.

Image
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Unread postAuthor: Ragnarok » Thu Feb 21, 2008 3:16 pm

jackssmirkingrevenge wrote:Many are happy enough with current performance without seeing the need to push the envelope.

Quite. Even if HEAL's valve were to open instantly, I'd likely only yield a joule or two more of energy at most, and on top of a few hundred, that's not going to be noticed.

The complication of a new valve is another thing to go wrong. I think a similar effect can be created with a good pilot valve, a small enough volume, a piston check valve and the right diameter ratio. For now, that's what I'm going to stick with.

Personally, those couple of joules doesn't balance against extra complication, but equally, in some launchers it might be worth it.
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Unread postAuthor: clide » Thu Feb 21, 2008 3:33 pm

Novacastrian wrote:Any chance of seeing your creation? :P

Eventually
jackssmirkingrevenge wrote:Shown a penchant for ventilating ceilings? :D

Oh man, I think you may be right. I ventilated the ceiling in an indoor gun range not to long ago. :oops:
But hey, those ceilings shouldn't be acting so aggressive if they don't want to be shot.

As for the actual design, one thing to consider is that if the pilot can't vent faster than the pressure can push the piston back then it probably won't act much different than a normal piston valve, and could even be more prone to bouncing.

Personally I agree with Rag, even if this did open significantly faster than a normal piston valve then the performance benefits would still be very slim unless you are using a light projectile or very high pressure.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: jackssmirkingrevenge » Thu Feb 21, 2008 3:33 pm

I used to think that my mini-pistons were about as good as it gets, with less than 0.04" cubic inches of pilot volume, lightweight and low friction - but then I made my burst disk pneumatic, the performance of which was astounding.

With a similar chamber volume (before I increased it on the piston version), pressure and barrel length, for a 16 grain lead pellet I got around 565 feet per second from the piston version, compared to the 720 feet per second I got from the burst disk. A 31 grain pellet from the latter was clocked at 577 feet per second, that represends a very significant leap in muzzle energy.

I agree, it makes things more complex, lord knows I'd prefer to keep things simple (even though at the rate I find new things to complicate and fail at, you'd be led to believe otherwise :roll: ) but if it's sheer performance you're after, I guess it's the price you have to pay.

clide wrote:As for the actual design, one thing to consider is that if the pilot can't vent faster than the pressure can push the piston back then it probably won't act much different than a normal piston valve, and could even be more prone to bouncing.


I see your point, but my logic was that the piston is alredy moving, therefore it's carrying a lot more momentum than a piston that empties air through the barrel the instant it starts to move.

Nova, we need some hard data and I don't mean you should try and arouse a cyborg on board the Enterprise :D
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Unread postAuthor: clide » Thu Feb 21, 2008 3:40 pm

Burst disks usually get their large performance leap over piston valves from their greatly improved flow coefficient due to strait flow from the chamber.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: jackssmirkingrevenge » Thu Feb 21, 2008 3:49 pm

I wasn't saying that this design could better a burst disk, just making the point that pistons can afford to be improved. To what extend this idea actually does this remains to be quantified.
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Unread postAuthor: Novacastrian » Thu Feb 21, 2008 7:28 pm

I just make things like this because i can, not because i expect super dooper improvement from current designs.
Any hard data will have to wait until i get a chrony, i suppose i could do a rough test with acrylic sheet, i'll see what happens.
The piston has been modded so it actually works like it's supposed to now :D I will update the pics to reflect these changes this afternoon at witch time i should have some damage pics too, vids will have to wait until next week as me being my usual dumb-ass self left my digicam at home.
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Unread postAuthor: STHORNE » Thu Feb 21, 2008 8:03 pm

Any hard data will have to wait until i get a chrony


ebay for 56 bucks you can get one

Here's the Page
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Unread postAuthor: Gippeto » Thu Feb 21, 2008 10:25 pm

Jack,

Has anyone, EVER, gotten even half a step ahead of you? :)
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Unread postAuthor: mopherman » Thu Feb 21, 2008 10:42 pm

Gippeto wrote:Jack,

Has anyone, EVER, gotten even half a step ahead of you? :)

*correct me if i am mistaken* but the single recorded instance of somebody beating jsr to the proverbial punch is the "snap valve" by Btb.
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