Technician1002 wrote:In a proper springer this is not the case. If you have a fast piston at the end of the shot, that is wasted energy and LOW efficiency.
We're not talking about straight springer (and yes, I do know how they work). While in a springer, the spring piston is brought to a halt by the pressure building up ahead of it, in a "spring/pneumatic combination" the pressure is not building up ahead of the piston - it's decreasing.
I looked into the idea years back. You'd have to talk to btrettel, as he knows the subject matter better than I, but as far as I remember...
You all know how a regular pneumatic water gun works, I'll assume. However, there was a line of waterguns which had a piston between the water and the air in the pressure chamber. You could pump up the air side of this chamber, which meant that there could still be pressure, even when the chamber was mostly empty of water.
(This is opposed to a variety that used rubber tubing as the chamber, producing a similar effect, but that concept is less useful to us.)
It works for that, because the chamber doesn't empty that fast - usually about 50-100 ml of water per second, meaning the piston usually has a velocity of centimetres per second, and very little energy.
If you try applying the concept to a pneumatic, giving it a piston in the chamber to "conserve" air, then what you get is a piston moving at dozens of metres per second.
With an open valve, and the projectile already gone, there's no pressure build-up, and nothing to make it slow down other than the end of the chamber.
Unless you can add something to do it instead... and innovating that would be a big boost to spudgunning.
(Some people may relate this to the "spring piston" in the hybrid chamber like CS was talking about recently. However, that doesn't need the piston to build up any real speed - it's not part of maintaining pressure, just venting the chamber - so the problem doesn't exist there.)
jimmy101 wrote:No, high performance and high efficiency are indeed mutually exclusive.
Not so much. As btrettel putting it, it's about scaling the whole gun.
If you build around a C:B ratio that's 70% efficient, then scale the length of both chamber and barrel (keeping to that proportion) until you get the desired muzzle energy, then you can keep that 70% efficiency.
What it really is one of those "Three choices, pick any two" scenarios:
- High energy
- High efficiency
- Compact (compact being a relative term).
You can't have small, energetic and efficient launcher, but you can have one with high energy and high efficiency - it'll just be pretty long. But, as you say, most people tend to sacrifice the efficiency.