SpudFiles

A buddy of mine (Theairgunman) likes the simplicity and the trigger feel of using a ball valve.

Using a 30 frame/sec video camera, he told me that he was able to determine that his spring actuated 1/2' ball valve opened within a single frame.

His said that his shots 'pop' and feel powerful.

If you could open a ball valve in approximately 0.030 seconds how much performance would you sacrifice to quicker opening piston or QEV valves?

This video is of his older version gun with internal spring:

[youtube][/youtube]

boyntonstu wrote:If you could open a ball valve in approximately 0.030 seconds how much performance would you sacrifice to quicker opening piston or QEV valves?

GGDT can of course be used to model this.

As I don't have the dimensions of his launcher (I don't know if it's in the video - haven't the time to today) I've done the modelling with the same specs I used here: http://www.spudfiles.com/forums/viewtop ... tml#231686

A 30 ms opening time yields 172 m/s and 295.8 Joules.
Compare to the actual piston valve: 236.7 m/s and 560.2 Joules.

... that's nearly twice the energy, simply for the faster valve.

Of course, it does depend on pressure (piston valves are slower at lower pressures) and this is a little more complex than just that. This just assumes the valves have the same flow characteristics when fully open - false - and that they have the same (linear) opening curve - also false.

Still, it demonstrates the overall difference. So, I think that the answer is that it's not fast enough.

As a general rule, for "real pressures", I think 5 ms is about the first "fastest" enough opening time, because at that point, the differences between that and instant opening are rather negligible.

Still, I'd personally want times down to closer to 1 or 2 ms, because a piston valve that's as slow as 5 ms probably has its own issues.

Ragnarok wrote:

boyntonstu wrote:If you could open a ball valve in approximately 0.030 seconds how much performance would you sacrifice to quicker opening piston or QEV valves?

GGDT can of course be used to model this.

As I don't have the dimensions of his launcher (I don't know if it's in the video - haven't the time to today) I've done the modelling with the same specs I used here: http://www.spudfiles.com/forums/viewtop ... tml#231686

A 30 ms opening time yields 172 m/s and 295.8 Joules.
Compare to the actual piston valve: 236.7 m/s and 560.2 Joules.

... that's nearly twice the energy, simply for the faster valve.

Of course, it does depend on pressure (piston valves are slower at lower pressures) and this is a little more complex than just that. This just assumes the valves have the same flow characteristics when fully open - false - and that they have the same (linear) opening curve - also false.

Still, it demonstrates the overall difference. So, I think that the answer is that it's not fast enough.

As a general rule, for "real pressures", I think 5 ms is about the first "fastest" enough opening time, because at that point, the differences between that and instant opening are rather negligible.

Still, I'd personally want times down to closer to 1 or 2 ms, because a piston valve that's as slow as 5 ms probably has its own issues.

Good analysis.

If a spring trigger can open a ball valve in 0.030 seconds, does it make sense to use it to trigger a piston valve?

Another way to answer the "fast enough" question is to consider the valve opening time versus the ammo's barrel transit time.

Obviously you want the valve fully opened before the ammo exits the barrel. Lets take that as a minimum opening speed (faster may be better but this is a nice simple reference point).

How fast do you want the ammo moving and how long is the barrel? Lets say it's a pretty good gun with a 500 FPS muzzle velocity and a 3' barrel.

Transit time is about (2)(3ft)/500FPS = 12 milliseconds
So a 30mS valve opening time is much to slow for this barrel and velocity.

5mS might be OK since during the 5mS much less than half of the barrel will have been used.

At 1mS very little of the barrel will have been used by the time the valve is fully opened.

The other thing to consider is the mass of the ammo and ammo to barrel friction. The higher both of those are the slower the valve can be without the valve speed affecting performance too much.

Spring opened ball valves are perfectly valid triggers for piston valves.

Actually, from some perspectives, they're particularly good. Their opening time will be repeatable (unlike a manual valve), so they should help the cannon's consistency.

Of course, that's only a major concern if you really need reliable muzzle velocities, which most people don't, but if you do, a mechanically opened trigger has its virtues.

jimmy101 wrote:Another way to answer the "fast enough" question is to consider the valve opening time versus the ammo's barrel transit time.

Obviously you want the valve fully opened before the ammo exits the barrel. Lets take that as a minimum opening speed (faster may be better but this is a nice simple reference point).

How fast do you want the ammo moving and how long is the barrel? Lets say it's a pretty good gun with a 500 FPS muzzle velocity and a 3' barrel.

Transit time is about (2)(3ft)/500FPS = 12 milliseconds
So a 30mS valve opening time is much to slow for this barrel and velocity.

5mS might be OK since during the 5mS much less than half of the barrel will have been used.

At 1mS very little of the barrel will have been used by the time the valve is fully opened.

The other thing to consider is the mass of the ammo and ammo to barrel friction. The higher both of those are the slower the valve can be without the valve speed affecting performance too much.

This point was a major design goal when designing the competition t shirt launcher and is the primary reason the light projectile marshmallow launcher is using a fast high flow valve. A slow valve is ok for dense slow projectiles, but a fast valve is more critical to competition launching of lightweight projectiles.

Interesting chrono data and GGDT modeling of a ball valve:

http://www.airgununiverse.net/community ... 147.0.html

Theory and practice coming together.

"Back to the ball valve airgun, I remembered taking a shot with a marble and larger diameter barrel. The actual speed was 467.6 fps, GGDT says 478 fps, again pretty close."

Why does the projectile go as fast with a slow ball valve?

In case you missed it, I am bumping this post.

IMO It is quite important.

Well, firstly, I'm seeing some lack of knowledge about GGDT - clearly, the FAQ/Guide has not been read.
An input of 99.61% into GGDT's valve coefficient actually only translates to 82.3% - it halves the difference of any valve efficiency above 65%.
Still high (too high) for the circumstances, but not an impossible efficiency.

Secondly, it pays to understand that GGDT does not model ball valves particularly well, because it models all generic valves as opening linearly.

I've got some equation (that I can't remember off the top of my head) for modelling how ball valves open. Either way, their opening curve (particularly in sprung loaded versions) is slow to start, then gets faster.

Whether this curve is an advantage or a disadvantage will vary depending on exact criteria of the launcher, but it will make a difference over linear opening, particularly in slow valves.

~~~~~

I tend to reckon that spring loaded ball valves can be fast. I'm not too surprised to see them under 20 milliseconds when I've measured them, which is a dramatic improvement over a manually opened valve... but that's still a lot to lose to a faster valve.

Project 3vo's valve and dead volume are designed to come together in such a way that a typical projectile will have scarcely moved by the time the valve is opened. That's how a valve should be if you want to eke out as much power as you can from the launcher.

When a valve takes so long to open that the projectile is gone by the time it's finished... well, you can guess the difference.

Suffice to say, I have no plans to switch to using ball valves as main valves. They have their uses, but in the split second events we're talking about here, they're just too slow.

Ragnarok wrote:Well, firstly, I'm seeing some lack of knowledge about GGDT - clearly, the FAQ/Guide has not been read.
An input of 99.61% into GGDT's valve coefficient actually only translates to 82.3% - it halves the difference of any valve efficiency above 65%.
Still high (too high) for the circumstances, but not an impossible efficiency.

Secondly, it pays to understand that GGDT does not model ball valves particularly well, because it models all generic valves as opening linearly.

I've got some equation (that I can't remember off the top of my head) for modelling how ball valves open. Either way, their opening curve (particularly in sprung loaded versions) is slow to start, then gets faster.

Whether this curve is an advantage or a disadvantage will vary depending on exact criteria of the launcher, but it will make a difference over linear opening, particularly in slow valves.

~~~~~

I tend to reckon that spring loaded ball valves can be fast. I'm not too surprised to see them under 20 milliseconds when I've measured them, which is a dramatic improvement over a manually opened valve... but that's still a lot to lose to a faster valve.

Project 3vo's valve and dead volume are designed to come together in such a way that a typical projectile will have scarcely moved by the time the valve is opened. That's how a valve should be if you want to eke out as much power as you can from the launcher.

When a valve takes so long to open that the projectile is gone by the time it's finished... well, you can guess the difference.

Suffice to say, I have no plans to switch to using ball valves as main valves. They have their uses, but in the split second events we're talking about here, they're just too slow.

There is a 2 shot test that would be definitive:

Same projectile, same pressure, chamber size, barrel length, etc.

1> Fast piston valve

2> Spring ball valve

Compare chrono readings.

Or to put things in GGDT terms.

Input all variables and average valves.

See how close a spring ball valve compares.

In the ballpark or out?


I am building a new piston cannon today using a side swing breech. Think of loading a revolver with nail darts.

The design came to me in a dream.

.