CpTn_lAw wrote:I agree with Rag, 1200 psi is a lot of pressure...1000 fps is achievable with far less pressure than that, so i think it'd be entirely possible.
Yes, but the Eclipse has a reasonably high bore and short barrel, which makes it much harder to achieve a given velocity.
As projectile mass scales as the cube of the barrel diameter, then a doubling in barrel diameter takes 8 times the force to reach the same velocity (with a projectile of a given shape and density).
Fortunately, force from pressure scales with the square of the barrel, so in this case, 4 times the force at the same pressure.
However, that leaves us falling behind by a factor of two, so the Eclipse needs twice as much pressure to achieve a given velocity with the same shape and density projectile as would a 20mm launcher with the same barrel length.
To bring HEAL into this briefly (being an example of a 20mm launcher that achieves high velocities) - its barrel is around twice as long as the Eclipse's, which then calls for the Eclipse to double it's pressure again to keep up.
So basically, 1200 psi in the Eclipse is roughly equivalent velocity-wise to 300 psi in HEAL. You can't neglect the importance of launcher dimensions to velocity.
And thanks for the helium info; i knew that SoS is around 1300 m/s at 27°c, but the actual difference in projectile velocity past 150m/s, that i didn't know.
Actually, at typical temperatures, Helium's SoS is about 1000 m/s. It's Hydrogen that has the 1300 m/s SoS.
The difference after 150 m/s is about when helium's disadvantageous ratio of specific heats (henceforth RoSH) starts to be cancelled out by it's lower density (and thus lower momentum, detracting less from the energy of the projectile.)
Interestingly, it's helium's high RoSH that makes it's SoS relatively close to that of hydrogen's. SoS is proportional to the square root of the RoSH, so with a RoSH of 1.4, it would have an SOS of closer to 920 m/s.
I assume a lot of launchers here could benefit from pressurising with helium instead of air, as a lot of these launchers tend to push projectiles past 150 m/s
The change is still fairly minor at 150 m/s - you'd need to start to push beyond that a bit before it would become a meaningful change.
On a similar note to above, hydrogen is the real "Ultimate gas". Lighter than even Helium; a RoSH of k=1.41, so about the same decompression efficiency to air; and a very good specific heat (Cp = 14.15 kJ/kg/K).
Obviously, air beats it by being non-flammable and being more available, but for sheer performance, hydrogen is epic. If I could produce it efficiently and in large amounts, you could bet I would be using it for occasional shots with my cannons...