Another Reason to Stay Subsonic

[rsterne]

Over the last few days we have been discussing the subject of Sonic Choking in the "Geek Gate".... I won't bore you with the details, as it took me quite a while to wade through the discussion and finally come to grips with the importance of the subject as it applies to PCPs when pushed to the limits.... If you want to look at the discussions in detail, you can read through these two threads....

http://www.gatewaytoairguns.org/GTA/index.php?topic=66737.0
http://www.gatewaytoairguns.org/GTA/index.php?topic=66866.0

If not, here is the Cole Notes version.... The concept of Sonic Choking, or Choked Flow is that once the flow reaches the speed of sound (known as Mach 1) in any part of a a pipe or tube (eg. a barrel) then no amount of additional pressure can push more gas (such as air) through that pipe, this is called Mass Flow Limiting.... In addition, there is an associated pressure loss across the choke point of at least 1.893:1 (for air).... This difference in pressure doesn't occur suddenly, but becomes significant in the transonic region (above Mach 0.8 ), reaching that value as the speed at the narrowest point in the system reaches Mach 1....

The air pressure at the base of the pellet times its area is the force available to accelerate the pellet faster as it travels down the barrel.... If you have a .25 cal pellet, the area of the base is 0.049 sq.in. so at 3000 psi there is 147 lbs. pushing on the pellet, accelerating it down the bore.... Once you start exceeding about Mach 0.8, the USABLE pressure starts to drop off significantly, even though there is still 3000 psi flowing through the valve.... By the time the pellet reaches Mach 1, the pressure on the base of the pellet is down to 1585 psi or less, and the force down to under 78 lbs.... By driving the pellet transonic inside the barrel, you are reducing the force available to accelerate it further.... By the time it goes supersonic, there is about half the force still available to continue pushing it faster.... No wonder the efficiency of a PCP drops off when we push the velocity too far....

Can we push a pellet supersonic, certainly.... I've seen over 1300 fps, and have watched a video where a .357 cal Delrin "bullet" reached 1475 fps.... However, the amount of air needed to reach those velocities is HUGE, and most of it is wasted.... Once you realize that the force available to continue to accelerate a pellet past Mach 1 is half what you had just a couple of hundred fps slower, it isn't hard to understand why....

Bob

[Tomg]

Yes, it confirms what my friend told me (engineer & ceo) of Alicat Scientific, they make flow meters for labs/high tech industries.
I talked to him about the physical limitation for gas flow inside tubes/containers.
He said the same thing, as you get closer to the speed of sound, the medium (gas) becomes a physical hindrance. And the only effective way to move gas any faster is using vacuum (not applicable to PCP) but basically (if I understand it correctly) if you transfer high pressure gas from one container through a pipe/valve, to another that are in a state of vacuum, you can achieve transonic speeds.

[rsterne]

You can achieve supersonic speeds in a barrel, don't get me wrong, and no need to pull a vacuum on the muzzle to do it.... HOWEVER, the efficiency drops like a stone once you enter the transonic region, and TANKS when you get the pellet moving past Mach 1 by the time it gets to the middle of the barrel.... The pressure to keep accelerating it just keeps dropping the faster you go.... while you keep using more and more air....

Bob

[Shovel74]

Is that etched in stone?  0.8Mach = 893.15ft/s  I shot with a guy at Adair the other day who said his guns seemed to like shooting at 900-925 FPS.  They were .177 guns, but I don't think size should make much difference to that. 

[rsterne]

Nope, all that happens is that as you increase the velocity beyond about Mach 0.8 the efficiency starts to drop off.... and past Mach 1 it REALLY drops off.... Supersonic velocities can be reached, it just takes a ton of air.... It makes little sense, because the same thing happens outside the barrel as well.... Past Mach 0.8 the drag goes up and the pellet starts losing velocity very quickly downrange as well.... That also increases the wind drift, which is at a minimum around the 900 fps mark.... Add to that accuracy problems when you get close to Mach 1 and it just makes no sense to go down that road, IMO....

Bob

[Frank in Fairfield]

Not unlike black powder rifles.
If you shoot your black powder rifle with a nice load of black powder over an area of snow you will see about half of the powder will be unused and fall, unused, on the snow.
What we want is efficiency whether using black powder or air.
You can load additonal grains of powder but what is the net gain for the additonal expense.
The same for air.
It might be 1400fps at the muzzle but by the time the pellet has reached 10 yards, it is coming out of the speed of sound and yawing excessively.
Accuracy above the speed of sound demands that you stay above that speed until you hit your target.
That is why I shoot .22-250s at long range prairie rats.
3800fps at the muzzle and still above 2500fps 500 yards later.

[PakProtector]

Not unlike black powder rifles.
If you shoot your black powder rifle with a nice load of black powder over an area of snow you will see about half of the powder will be unused and fall, unused, on the snow.
What we want is efficiency whether using black powder or air.
You can load additonal grains of powder but what is the net gain for the additonal expense.
The same for air.
It might be 1400fps at the muzzle but by the time the pellet has reached 10 yards, it is coming out of the speed of sound and yawing excessively.
Accuracy above the speed of sound demands that you stay above that speed until you hit your target.
That is why I shoot .22-250s at long range prairie rats.
3800fps at the muzzle and still above 2500fps 500 yards later.

you neglect proper grain size selection. just like tuning the dwell of the valve opening, the grain size selection should insure that it is all burned. though maintaining charge weight may prove hazardous to your health; a charge of 2F that leaves your un-burnt stuff all over the snow will likely seek to embed breech material in your skull if switched to 4F.
cheers,
Douglas
cheers,
Douglas