SpudFiles

Well I'm home for the week without much to do so the only logical course of action was to start drawing designs for a hybrid piston valve. Of course I have no idea if I'll actually be able to build this any time soon since it looks like I'm going to be away from home during the most of the summer.

Anyhow... it's going to be made from mostly galvanized malleable iron and copper fittings for use with a 3-4x air-propane fuel mix. The piston will be sealed with an air-spring that can be pressurized for a desired burst pressure through the fill valve. Setting this up requires a simple calculation:

P_a = P_b(D_p²-D_s²) / D_p²

P_a = Necessary air spring pressure
P_b = Desired burst pressure
D_p = Piston diameter = 0.957 in = ID of 1" SCH80 seamless steel pipe nipple
D_s = Seat diameter = 0.875 in = OD of 3/4" copper barrel

If my math is right, a 325psi burst pressure (about optimal for 3x mix for my design) can be obtained by filling the air spring to about 53psi, and a 445psi burst pressure (about optimal for a 4x mix) needs roughly 73psi. Hopefully these numbers sound realistic.

Of course, the trick will be making an air spring that doesn't leak. Hopefully a 1" rubber gasket up against 1" SCH80 seamless steel pipe (ID=0.957") will do it.

I think it will work. Maybe some of you guys think otherwise.

Its always nice to see people do their math.

You will need a floating o-ring instead of the rubber gasket.
I look forward to see it in construction.

That math looks to be in the ballpark (too lazy to actually check it). I used a non-floating O-ring in my piston hybrid. Make sure you take out as much deadspace behind the piston as you can. I filled mine with caulk.

In this case, I think you need as much volume behind the piston as possible.
Larger volume -> more constant force from the airspring instead of ramping up quickly.

Yes, more volume would help, but dead space in the pilot won't. You want the pressure to ramp way up before the piston hits the back of the tube. Otherwise, you can expect your pilot to act like anything else that has a metal hybrid projectile launched at it.

Too much piston travel will eventually create dead volume in the main gun, allowing the gas to expand without moving the projectile.