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Larda's Coaxial Piston Valve (injector!) pneumatic.

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: spud yeti » Wed Jul 18, 2007 11:59 pm

Holy moley! That thing is awesome! Those damage pics tell all.... Give him a hug for me for building that beauty :D
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really good quote/phrase here
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Unread postAuthor: Pilgrimman » Thu Jul 19, 2007 12:34 am

Wish your friend luck from me! Railguns are my absolute favorite weapon! May he destroy lots of crap and achieve high velocities!
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 19, 2007 1:55 am

very nice, you're lucky to have such sophisticated machining facilities and materials at hand :) looks like it's good for at least 300 psi.

Presumably your projectile will look something like a flat arrowhead or similar, will you eventually have a rectangular barrel? Do make it as long as you can as base area will likely be very small for effective power transfer, in that regard, you're better off with a high pressure low volume chamber than the opposite system most amateur spudguns use.
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Unread postAuthor: mopherman » Thu Jul 19, 2007 1:57 am

jackssmirkingrevenge wrote:very nice, you're lucky to have such sophisticated machining facilities and materials at hand :) looks like it's good for at least 300 psi.

Presumably your projectile will look something like a flat arrowhead or similar, will you eventually have a rectangular barrel? Do make it as long as you can as base area will likely be very small for effective power transfer, in that regard, you're better off with a high pressure low volume chamber than the opposite system most amateur spudguns use.

I notice that you say "amature" spudgun. what would you refer to as a "profesional" spudgun?
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 19, 2007 2:43 am

A "professional" spudgun for me is something made out of high grade materials not easily available off the shelf that involve a considerable amount of machining and custom parts.

I'm not saying you can't make a very effective and complex launcher using standard PVC plumbing, indeed there are people on here that can do just that who I would consider to be professionals - but let's face it, not all spudguns are created equal ;)
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Unread postAuthor: wannabie » Thu Jul 19, 2007 5:03 am

OMFG thats sweet i love it

how strong is your piston because if you use it at high preasures the piston may shatter becuase of the presure on the back of the piston
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 19, 2007 6:03 am

wannabie wrote:how strong is your piston because if you use it at high pressures the piston may shatter becuase of the pressure on the back of the piston


That's not as big an issue as this:

Disadvantages of use of acetal based plastics

low impact strength
low melting temperature
more rigid than nylons


Larda, make sure you have a pretty good bumper behind your piston ;)
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Unread postAuthor: Larda » Thu Jul 19, 2007 6:20 am

Thanks everyone for the encouraging comments.

jackssmirkingrevenge wrote:very nice, you're lucky to have such sophisticated machining facilities and materials at hand :) looks like it's good for at least 300 psi.

Presumably your projectile will look something like a flat arrowhead or similar, will you eventually have a rectangular barrel? Do make it as long as you can as base area will likely be very small for effective power transfer, in that regard, you're better off with a high pressure low volume chamber than the opposite system most amateur spudguns use.

The projectiles i planing to use will be similar to the ones used in the French-German Pegasus railgun, with uses metal fiber brushes. More info Here and Here
I think you are right that i will get better performance with higher pressure/low volume and its likely that i will go that way, you can see this one as a Proof of concept. :)
Which type of valve do you thing would be best suited to deal with the high pressure (300psi+) and still be fast enough if you don't have to consider the difficulty to machine the valve?
So fare i haven't seen any damage to the piston, but if it fails i just machine a new one out off aluminium instead. :)
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 19, 2007 6:48 am

I've made quite a lot of smaller calibre launchers (including, it could be argued, the smallest calibre launcher ever :D ) and I consistently find that the smaller you go, the base area of your projectile decreases exponentially, so to keep up performance you have to up the pressure rather than the volume.

With the facilities you appear to have at hand, I would say stay with the piston valve. Remember, the area of the piston doesn't have to be much bigger than the calibre of your barrel, and for maximum efficency, the piston only has to move back 1/4 of the barrel diameter (in the case of a cylindrical barrel) for maximum flow. This gives you a smaller pilot volume (therefore more efficiency) and at the same time gives the piston less space to accelerate, making it less likely to damage itself. Still, some sort of rubber shock absorber or spring would be very useful to prolong piston life.
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