What is the limiting factor

[Guaps]

I am building a cannon with an interchangeable barrel so I can shoot anything from marbles (~1/2" barrel) to full potatoes (~2" barrel).

If I use the ebay QEVs mentioned in this thread, will that work for a 2" barrel? Is a 1/2" valve too small?

Thanks!

[Technician1002]

It depends on what is acceptable performance. Are you trying to launch spuds cross the yard or trying 3 football field lengths, or something in between?

Personally don't use barrels more than twice the diameter of the valve's orifice. If possible I use barrels no more than 1.5 times the diameter.

If your 1/2 inch valve has a 3/8ths inch orifice, a 3/4 inch barrel is about the largest you will get reasonable performance from.

[Guaps]

I'm hoping to put a potato into low earth orbit, but i'll settle for a few football field lengths.

Thanks for the 2x orifice rule of thumb. That's exactly what I was looking for.

Is there a similar rule of thumb for canister size and barrel size/length?

[jackssmirkingrevenge]

If your 1/2 inch valve has a 3/8ths inch orifice, a 3/4 inch barrel is about the largest you will get reasonable performance from.

Not sure about QEVs but the ball valves I have have a significantly larger orifice than their stated diameter.

[Ragnarok]

To expand on what Jack said, the nominally half inch "European"/Piston style QEV I've got has a 14.2mm (0.56") seat diameter.

Not that that will necessarily apply here, but they can easily be bigger than their nominal size.

[lozz08]

If I were you, I'd go with the larges QEV I could find. An interchangeable barrel of a smaller size can screw in via adapters and you wont lose any performance. Why not try making a larger piston valve yourself from a tee and such? Plenty of info on that here.

[Technician1002]

If going for competition performance and low earth orbit, the rule of thumb is use a valve with larger flow than your barrel so the restriction to flow is due to the barrel size, not the valve.

[Selador]

If going for competition performance and low earth orbit, the rule of thumb is use a valve with larger flow than your barrel so the restriction to flow is due to the barrel size, not the valve.

Are there any restrictions in THAT direction ?

I built a 1-1/4" piston valve.

Approximately 1" restriction at the barrel end of this thing.

Is something going to go "blooie", if I fit it with say a 1/2" barrel ?

[jackssmirkingrevenge]

Is something going to go "blooie", if I fit it with say a 1/2" barrel ?

Assuming it's attached securely you shouldn't have any problems.

[Technician1002]

I screw on a 1/2 inch barrel on my Marshmallow Cannon which is a piston valve with a 1 inch orifice. I shoot mini marshmallows, jawbreakers, and AA batteries. No problems.

Batteries shot from 1/2 inch barrel with a 1 inch valve.


I have found when using a larger valve, a larger barrel and sabot for the smaller projectile works very well.

AA battery shot from a 2-1/2 inch barrel on a 2 inch valve.


Sometime later I'll test AA batteries in a 1 or 1.25 inch barrel on the 1 inch valve Marshmallow Cannon to compare various barrel/sabot sizes on the 1 inch valve.

[Selador]

I didn't think there would be a problem.

But you never know.

So I thought I'd check with the collective wisdom of experience, here.

Thank you.