Short question topic

[wyz2285]

With higher compression I think the heavy piston will match the lighter one maybe even more. Probabaly increase the felt recoil and noise too... I like the feeling of the light piston, but the heavy sounds so good atm plus I'm have 0 variation in FPS. One the lighter piston I don't have such absurd consistency (around 1 fps variation) even though both has perfect air seal.

[jackssmirkingrevenge]

I think with the lighter piston, the projectile friction at the breech becomes more critical and since this is prone to variability, it will cause a corresponding variation in velocity.

[wyz2285]

I always thought a light and fast trveling piston is better beacause it compresses air faster, reaching a higher peak pressure before the projectile overcomes the friction. I always imagined the projectile takes some time to start moving no matter how much pressure is behind it, therefore a fast piston can produce a peak pressure higher than the necessary for the projectile to move.

[jackssmirkingrevenge]

I always imagined the projectile takes some time to start moving no matter how much pressure is behind it

Look at the mass of the piston vis a vis the projectile, it's typically over 100 times heavier. Naturally acceleration is going to be much greater.

I always thought a light and fast trveling piston is better beacause it compresses air faster, reaching a higher peak pressure before the projectile overcomes the friction.

With less momentum, it will reach a lower peak pressure in a sealed chamber because its lower mass means that it takes less pressure to slow it down. It's sort of like the analogy between launching a ping pong ball vs a golf ball. Yes, the ping pong ball will come out of the muzzle much faster, but it will decelerate much faster as well and therefore not travel as much.

For the purpose of spring guns, projectile weight is the sliding scale that determines whether a heavier or lighter piston is optimal. A heavier projectile or high friction at the breech is closer to a sealed chamber so a heavier piston is optimal.

[wyz2285]

Ahhh, that makes much more sense now. I baffled by the quantity of variables one has to account to build the most efficient spring gun. Thankfully no such things for HPA... Just kidding, I'm still reading that GTA thread, the guys over there are just insane about the efficiency and details...

[jackssmirkingrevenge]

Thankfully no such things for HPA... Just kidding, I'm still reading that GTA thread, the guys over there are just insane about the efficiency and details...

It's mostly Bob Sterne, he's very meticulous and has a lifetime of experience with pneumatics. I highly recommend going through his articles published in Hard Air magazine, he really goes over all the variables when it comes to how PCPs function and how to tune them, backing it up with hard data.

[Cthulhu]

[youtube][/youtube]
I made this quick animation for a concept I have to make a burst disk holder that doesn't need a clunky union.

The bottom plate is threaded while the top plate is not. The aluminum foil is sandwiched between two flat rubber gaskets and held in place tightly by 8 screws.

Would this seal and function for the lower pressure range (up to 50 psi)?

[Moonbogg]

You should be able to add as many layers of foil as it takes to hold a wide range of pressures. What are the dimensions of that thing? Also, I wouldn't want to mess with 8 screws every time I wanted to shoot the cannon. Can you use a cam-lock instead? What mixtures you going to run?

[jackssmirkingrevenge]

I wouldn't want to mess with 8 screws every time I wanted to shoot the cannon.

That's what I would have thought, unions are "clunky" but they're quick.

[Cthulhu]

You should be able to add as many layers of foil as it takes to hold a wide range of pressures. What are the dimensions of that thing? Also, I wouldn't want to mess with 8 screws every time I wanted to shoot the cannon. Can you use a cam-lock instead? What mixtures you going to run?

The OD is 2.067 inches and the ID is 1 inch.
I'm trying to see if I can 3d print them for preloaded combustion cartridges loaded with oxy-hydrogen and butane at a 1x. HGDT showed a significant performance increase if I use a burst disk and I figured it would help keep the cartridges ready longer before the gas leaked out anyways. 8 screws probably is gonna be frustrating when I reload all the cartridges but at the same time I really do want the shells to look sleek.

[hectmarr]

You should be able to add as many layers of foil as it takes to hold a wide range of pressures. What are the dimensions of that thing? Also, I wouldn't want to mess with 8 screws every time I wanted to shoot the cannon. Can you use a cam-lock instead? What mixtures you going to run?

The OD is 2.067 inches and the ID is 1 inch.
I'm trying to see if I can 3d print them for preloaded combustion cartridges loaded with oxy-hydrogen and butane at a 1x. HGDT showed a significant performance increase if I use a burst disk and I figured it would help keep the cartridges ready longer before the gas leaked out anyways. 8 screws probably is gonna be frustrating when I reload all the cartridges but at the same time I really do want the shells to look sleek.

A suggestion. Open a publication with the ideas, drawings and everything related to your project, so it is simpler to be able to follow your development, and to be able to understand what you are planning.

[jackssmirkingrevenge]

A suggestion. Open a publication with the ideas, drawings and everything related to your project, so it is simpler to be able to follow your development, and to be able to understand what you are planning.

Seconded, it's what the forum is for.

[Cthulhu]

hahaha, alright. I always feel bad when I do that and my ideas turn into a pipe dream

[hectmarr]

hahaha, alright. I always feel bad when I do that and my ideas turn into a pipe dream

and I ... one of ten ideas works one and not exactly as I wanted ... at least here, we can make fun mistakes.

[jackssmirkingrevenge]

fun mistakes

The answer to the question "Jack, how would you describe your progress in this hobby?"