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Quick Release Piston Blowback?

I have put a great deal of money into constructing a 6" air launcher, but to my dismay things didn't quite go as planned with the valve design.  I have attached a picture below of the valve's design.  The idea is that when the air is released from behind the piston, the piston should move back allowing air to exit through the 5" pipe.  The valve successfully holds pressure, but fails to fly back and I believe this is a result of the piston and barrel it is up against being equal in size.  I wouldn't have designed this in this way, but the large schedule 80 pvc tee only allows 1/4 inch around the sides of the piston at best.  I am curious if anyone has any ideas to make this piston valve work, I really don't want to pay 200 dollars to get a butterfly valve on it and would greatly appreciate any ideas that may make this work.  I promise videos of the beast once I get it up and running.  Launching 6" objects using up to 80psi will be a blast, literally if I ever get crazy enough to bring it up that high.  

Picture of Quick Release Piston Blowback?
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i'm no expert on those piston valves but I have seen quite a bit about them on spudfiles.com in the forum. I've also seen a build of a piston valve on youtube.

My next build will likely be a sprinkler valve type. Would also love to make a propane or mapp gas type with circulator fans and such. Ahh time and money. wish I had more of it.
I don't quite see why you are expecting the piston to move back at all. The pressure behind the piston will likely always be slightly higher than the pressure at the front. I think what you are trying to create is a a piloted, or servoed valve - like the ones used on urinal flushers - a tiny disturbance is amplified by the water pressure itself to open the main valve.
jj.inc (author)  steveastrouk3 years ago
http://www.youtube.com/watch?v=8zvUMFqUVcI
http://www.youtube.com/watch?v=75O71AHTIr4
this was my original inspiration http://www.youtube.com/watch?v=EdzfgMvQha4
I now have a piston which can fully seal which I will now be using, fingers crossed
Right. That confirms what I said.

It strikes me he ACTUALLY has a pretty GOOD FIT between the piston and the T - and in Part 2 he says he's aiming to use O ring seals between the piston and the T piece.
Done right, this IS a piloted valve.
jj.inc (author)  steveastrouk3 years ago
With my very generous calculations there will be 10-25 pounds pushing the piston back, and In an ideal situation I could get 50 pounds, as soon as the piston unseals that pressure will likely reach a couple hundred pounds. That is assume 20% to 50% effectiveness. I could get a better idea if I knew how to calculate the speed of effusion for air at 20 psi through a .8" hole
+1
RavensCraft3 years ago
I think you need to add a piston rod to the big piston. The other end of the piston rod should have a small airtight plunger which will move inside the air release tube.
Release.jpg
+1 on the linked pistons idea, the 1st piston blocks the main barrel so that the air can only act on the secondary piston which travels backwards.