jhalek90 wrote:So, what your telling me is that gasses will not expand faster than there SOS?
Yes. If at any time a flow starts to go supersonic, its own shockwave will instantly slow it back down.
SOS is the eventual limit on these things.
The hotter the gas, the faster its SOS... right? or am i wrong?
Correct. This is the main reason why the vast majority of supersonic cannons on this site are hybrids. Not because of the extreme pressure, but because of the extreme heat of the gases.
The actual increase in speed of sound comes when a gas has a high pressure for its density. This can arise either because of high temperatures, or a lighter gas, like steam, helium or hydrogen (and to a lesser extent, pure nitrogen instead of air).
So, pressure causes gas(Air) to heat up and gain a higher SOS... correct?
Compressing air (the more rapidly, the better) causes it to heat up. Pressure alone will not result in a hot gas.
The compressing is typically done by some kind of piston - this can either be solid (as in a springer), or one made out of gas itself (as in supersonic PCPs).
The rapid expansion of gas (through a valve ext) cools it, and thus lowers its SOS and therefore the speed at which it can expand?
Precisely. It's this cooling that generally means it's very rare to see a pneumatic put out a velocity of more than about Mach 0.95 (in the surrounding air, not the pneumatic's own gas).
Now, we have seen some supersonic pneumatics, by virtue of extreme pressure and lucky design causing heating of the gas in the dead volume, and actually making the launcher's internal SOS higher than the atmospheric SOS. Still, I stress the importance of pressure on achieving this effect.
Meaning that the air in my cannon can only expand as fast as its SOS at any given time...
As an upper limit. Obviously, if the projectile is (reasonably) airtight, then the gas can't expand faster than the projectile is moving, whatever Mach number that may be.
and the only way i would get higer FPS is to raise the gases SOS??
Higher max velocity, as I explain above. Although increasing the gas' speed of sound will help to some extent in a cannon that's shooting subsonic, the real bonuses come by raising the SOS velocity, and the velocity where transonic flow occurs.
Transonic is an inefficient area. As the name "transonic" suggests, the flow is fluctuating between where parts of the flow are subsonic and other parts are trying to go supersonic (and as I said at the start, thus creating a shockwave that slows them back down again).
To illustrate the efficiency issue, although it's not quite the same thing, you can see the effects of transonic flow on the drag co-efficient of various shapes on the graph below.
The regime tends to occurs between about Mach 0.8-1.2, but it depends on exact projectile shape (or in the case of launchers, internal design)
As is evident, the transonic regime results in a very large increase in drag coefficient, which actually makes it, per metre of flight, the most wasteful velocity regime for a projectile in terms of percentage of velocity retained. (It's also a region that negatively impacts accuracy because of the sonic shocks causing instability in the projectile.)
Drag coefficient slowly recovers past the transonic area, but it's still behind subsonic flow.
Similar things happen in launchers, causing lots of losses in the transonic region. So, by being able to bring up the transonic velocity, you can yield notably improved performance in the 900-1100 fps range. By being able to bring up the SOS, you can potentially get past the sound barrier in the surrounding air.
Still, I remind you, it's not called the sound
barrier because it's easy to beat...
There is a reason for the huge fast valve.