This is a totally impossible valve. When it appeared on my drawing board I couldn't see how it could work but neither could I see how it couldn't work. So I made it, just to see what would happen. I doesn't appear in the book because it's far too dangerous. What happens is this. There is a long copper barrel, and it is brazed straight through a short fat copper air reservoir. The reservoir is capped, top and bottom, with meaty plugs silver-soldered in place. The only ingress to air is via the Schrader valve, and the only egress is via a (lateral) hole that sits in the side of the barrel, inside the air reservoir. 
A valve with two O-rings, one at each end, is pushed in from the back of the gun, such that one O-ring sits on either side of the barrel's lateral hole. The bullet, duly wrapped in thin paper, is muzzle-loaded down to the valve. In the photo, both bullet and valve have been pulled out of the barrel and lie to the side, exactly where they lie inside the barrel when the blessed thing's loaded.You pump up the reservoir via the Schrader valve. You use your bicycle track pump for this, and if you haven't got one you hie yourself down to the bike shop and buy one - they aren't expensive - because then you get a pressure gauge and can pretend to be doing physics whereas actually you're just making big bangs and annoying the neighbours.Now what next happens is incredible. You put on a stout glove, grab the back of the valve, and pull it backwards. As soon as the front O-ring clears the lateral hole in the barrel, the air tries to blow the bullet out. It also tries to blow the valve back where it was. A sort of unlikely sucking. And you can see why I put on the stout glove, because whatever you think, you cannot imagine that the valve will not be blown backwards just as hard as the bullet is blown forwards.But it isn't. It's sucked back into place, though the reservoir is emptied before it gets there. So at least you don't have a valve blasted through your wrist.At least, that's what always happened until last night when another physicist turned up and I had to demonstrate. The valve is fairly stiff, and I couldn't pull it out easily, so I got him to hold the barrel on a stool, the muzzle pointing at some rags that I keep for the purpose, and taking pliers to the back of the valve, I gave it a sharp blow with a mallet. This sent the whole valve all the way back completely past the hole, so there was now a bullet on one side of the barrel, and the entire valve on the other side. Whereupon, of course, there was an equal and opposite reaction so whereas the bullet hit the target, the valve blew across the workshop and snapped the back of it off on the door. (Which is why, in the photo, it is now so short.)
A valve with two O-rings, one at each end, is pushed in from the back of the gun, such that one O-ring sits on either side of the barrel's lateral hole. The bullet, duly wrapped in thin paper, is muzzle-loaded down to the valve. In the photo, both bullet and valve have been pulled out of the barrel and lie to the side, exactly where they lie inside the barrel when the blessed thing's loaded.You pump up the reservoir via the Schrader valve. You use your bicycle track pump for this, and if you haven't got one you hie yourself down to the bike shop and buy one - they aren't expensive - because then you get a pressure gauge and can pretend to be doing physics whereas actually you're just making big bangs and annoying the neighbours.Now what next happens is incredible. You put on a stout glove, grab the back of the valve, and pull it backwards. As soon as the front O-ring clears the lateral hole in the barrel, the air tries to blow the bullet out. It also tries to blow the valve back where it was. A sort of unlikely sucking. And you can see why I put on the stout glove, because whatever you think, you cannot imagine that the valve will not be blown backwards just as hard as the bullet is blown forwards.But it isn't. It's sucked back into place, though the reservoir is emptied before it gets there. So at least you don't have a valve blasted through your wrist.At least, that's what always happened until last night when another physicist turned up and I had to demonstrate. The valve is fairly stiff, and I couldn't pull it out easily, so I got him to hold the barrel on a stool, the muzzle pointing at some rags that I keep for the purpose, and taking pliers to the back of the valve, I gave it a sharp blow with a mallet. This sent the whole valve all the way back completely past the hole, so there was now a bullet on one side of the barrel, and the entire valve on the other side. Whereupon, of course, there was an equal and opposite reaction so whereas the bullet hit the target, the valve blew across the workshop and snapped the back of it off on the door. (Which is why, in the photo, it is now so short.)
What ho. Read the book, then found the blog. Great book, I renjoyed it immensely (so not just me the is how I felt about that) and can empathise with some of the near misses...and I view my catapault with new respect, or will do when I've changed the elastic.
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