SpudFiles

As I am working out the finial preparations to start building my new cannon I ran a simulation with two piston valves and noticed a considerable increase in power...in the order of 75 to 100 fps.

If you remember I'm dealing with about 1300 cubic inches of air so the increased flow would add considerably to power. And I think I can get by with shorter barrels with increase performance. However the restriction is the the increased flow is choked by a barrel smaller then 4 inches.

I'm wondering has anyone done it and is it worth it?

I did a quick search on the forum and didn't come up with any immediate results so if its been discussed before please just link me to it.

The piston valves are four inches in diameter with a 3 inch seat.

if you want velocity you want a little choke because when the flow is choked the gasses have to accelerate and that is what you want in a small bore.
in a large bore you don't need all that velocity when the protectile has a great amount of mass.

you choose

There's really no problem with flow choke, as long as it's the barrel pipe that does the choking. You are wasting power, but it could allow for rather impressive velocities none the less.

With two valves, assuming the seat is as big as your barrel, you're not gaining any real flow advantages. What you are obtaining however is a higher rate of flow increase.

Coaxial pistons provide their maximum flow after travelling 1/4 of the barrel diameter, but obviously the projectile will start to move before this theoratical maximum is reached. Whith two similar piston valves, they only need to move back 1/8th of the barrel diameter for maximum flow, so essentially what you have is equivalent to a faster opening valve.

There could be other issues at hand, like increased turbulence and dead space, that would conspire to decrease performance, but overall you should see a gain in velocity for a given projectile weight.

It's probably not worth the trouble compared to the ease of making a burst disk though, if it's performance you're after.

Choosing seems to be the hard part. Does the increase cost and labor out weigh the increase in performance?