SpudFiles

That's what I'm thinking, even though GGDT is adamantly telling me it won't. Oh well. HPA first.

Wait... sexy piston?

One of the reasons besides muzzle velocity is impact forces. A steel piston has about 8X the mass of a HDPE piston. consider the bumper requirements.

I use a steel hammer to drive nails, not plastic.

Impact force is mass*change in velocity/time, I got that, and how a lighter, more flexible piston will certainly decrease impact force... but what does that have to do with valve opening time? It stands to reason (F=ma) that there is an exponential valve opening speed increase with a lighter piston. But again, how do bumper requirements and impact force relate to valve opening speed?

This was a benefit other than opening speed. You are less likely to have a piston in your shoulder with a lighter piston. Many PVC cannons have failed because the piston broke the rear of the cannon.


As you use longer barrels and heavier projectiles, the opening speed matters less. As you go to a shorter barrel and lighter projectile, the opening speed becomes a factor in performance. Play with the numbers in GGDT to see this. I open full cans of pop by hitting them with a marshmallow.
http://youtube.com/watch?v=TW3bUP-Fov4

Technician1002 wrote:One of the reasons besides muzzle velocity is impact forces. A steel piston has about 8X the mass of a HDPE piston. consider the bumper requirements.

Actually, I have to disagree with you here. It might be intuitive to think "Heavier objects hit harder", but that's learned from gravity - where the forces scale in proportion with the mass involved. That isn't the case with valve pistons.

Think about the energy stored in the piston. Force x distance. Force will be pretty much the same either way (pressure x area, of course). Distance is also going to be the same. So, a heavier piston has no more energy than a lightweight one.
However, it will be moving more slowly, which gives any bumper more time to deform and absorb the impact - rather than the shorter, sharper shock of being hit by a faster piston.

Heavier pistons are actually easier to stop!

Now, before you start arguing with me over the same not necessarily holding true for spudgun projectiles, there's two reasons for this.
a) Lighter projectiles are usually less strong. As a result, they're far more likely to disintegrate on hitting a target. As pistons preferably stay intact, energy lost in permanent deformation or destruction of the piston is not a factor.
b) Transonic losses. As projectile mach number goes up, the force on the projectile falls. As pistons are relatively slow compared to most projectiles (with a speed maybe in the tens of metres per second, not hundreds), these losses are fairly irrelevant.

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Also, when I talk about reductions in opening time being moot past a certain limit, it's because they are. I threw some numbers at the problem here to show that "faster than fast" doesn't really do much.
What that valve speed usually means is: No noticeable improvements in velocity, but increased demands of the piston bumper. I'd actually usually recommend a bit of weight in any piston for that reason.

Sure, if you're using really really light projectiles in a very short barrel, then super fast opening times may become a slightly more valid concern, but to be honest, that's not really a launcher you build if you're looking for maximum performance.

If I am to trust GGDT on this, at my new capable pressure of 600 psi, I will see a total of 1 fps muzzle velocity increase if I replace my piston with UHMWPE. I can see how this could be either right or wrong, but am hoping somebody can give me a little more clarity.

Most of the time for me, changing the piston mass has little or no change in performance at all, even with the bigger valves.

How loud is that thing at 600 PSI? The pressure drop is only around 150 PSI, so you've got four and a half cubic inches of 450 PSI air blasting out the barrel behind the projectile.

I really can't say. One, I had it poked out a door, so most of the sound was prevented from coming back at me. Two, no projectile. I can say however that when I used to operate it at 250psi, I was disappointed by the lack of a condensation cloud. Now I get one about 1.5 or 2 feet long.

I plan on finding out this weekend when I have oodles of free time, heck I might even make a video..

its not a colax so you dont need to put anything in the ''inner dia'' spot it should be 0

Thanks! Didn't know that's what it was for...

Well unfortunately that still doesn't address the lack of change in muzzle velocity... I guess I'll just assume GGDT is right.

Bumping for another damage pic, more to come soon. Read the descriptions...

Well my piston was getting stuck in the returned position a lot, and just now it got stuck in the returned position so bad I couldn't force it out by air pressure just taking off the valve housing. So I decided to open it up and have a look.



Will be replacing that brass bushing with steel.

In addition, this proved the effectiveness of my o-ring piston attachment, as the greater diameter of the o-ring was slamming into the nipple the rest of the piston sits in. O-ring still happily seated.

Could you explain what we are seeing, there ?

Thank you.

I think you are looking into the pilot valve which has had a few too many energetic piston collisions due to the lack of a proper bumper.

The piston in the T has hit the ball valve behind it very hard.