Does anybody know why can't pellets travel straight after they pass the sound barrier?
April 2 2006 at 12:07 PM
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I mean, What is the ballistic difference between an ordinary bullet and a pellet which makes the pellets thumble in the air
at supersonic speeds?Why can't the pellets maintain their smooth
flight after they surpass the speed of sound?
Is it because they are lighter or not as well made as the normal bullets? Or what??
Ok, so it's clear that the problem does not occur from going through the sound barrier (since that happens in the barrel i think), but things get ugly when the bullet slows down and has to pass the barrier again when the pellets slows down, only this time in mid-air making the pellet unstable or even tumble ...
I've seen a slowmotion capture of a fighter jet going through the sound barrier once, and its quite a wall of air (pressure) the thing went through.
Lots and lots of turbulance, so the heavier and aerodynamic shaped, the better.
Airgun pellets are light, and actually have a terrible aerodynamic shape, mostly because of the pinched waste.
(maybe the logun penetrators behave a little better since they are longer then regular pellets and have no (or a lesser) pinched waste ...
What actually happens is that already at about 800fps the air around the pellets is becoming increasingly unstable,meaning the air molecules gets pushed apart too quickly, at 1000 to 1100fps the air molecules collide with each other because the pellets is moving so fast and it pushes the air out of the way. At approximately 1100fps and above (depending on sea level) there will a sonic boom becuase the air molecules cannot move out of the quickly enough.
The first problem with pellets is that they are not very areodynamic,secondly is that their weight hence kinetic energy is very low,so even small differences in pressure etc can have a big influence on stability, the third and I think most critical point of difference between pellets and centrefire weapons is that centrefire projectiles move way above the sound barrier so the time it spends at the unstable 1100fps mark is very little (compared to a slow moving airgun pellets that just reaches 1100-1300fps) another crucial difference is that centre fire bullets reaches their target way before the "ripple" of the sonic boom(the disturbed air) can catch up, where as with pellets that do go above the sound barries the sonic booms ripple catches up quite quickly and before the pellets reaches its target, unless the target is at point blank range. I am working on an idea for a pellet that is very stable and aerodynamic at high velocity, who knows maybe we will see it in the near future.
While andrem has some valid points, I would point out that many centerfire pistol bullets are fired at initial velocities of between 800 and 1400 fps. This is true for .38 Special, for which I found a flyer of a "light" load that produced about 530 fps. Notable is the .45 ACP, the standard by which many other cartridges are measured. Many possible loads for the .45 ACP are "subsonic", producing velocities in the range of 900 to 1000 fps.
As andrem points out, what these bullets have in their favor is mass. They are less susceptible to the vagaries of wind and weather, as their masses range from 150 to 260 grains (in .45 ACP). They are also less susceptible to the impact of internal ballistics such as barrel imperfections. A 15 grain pellet with a mar from sub-standard rifling will be affected much more than a solid projectile of more than four times that mass.
Another aspect to consider is the projectile shape. Artillery shells and high-power rifle bullets frequently use a boat-tail design. That is, the bullet narrows toward the base. This allows a more aerodynamic profile, so the projectile suffers less from possible buffetting brought on by the vacuum being filled at the tail of the bullet as it moves forward. This is important as the projectiles have to travel long distances and must minimize detractors to their accuracy during time of flight. Not many handgun bullets are available in boat-tail designs, but neither are they generally intended to accurately cross long distances.
I weighed several of my low-end .177 and .22 pellets last night, just to see how consistent the manufacturers were (I can provide the excel chart to anyone who's interested). The point to highlight here is that none of the pellets weighed more than 16 grains. Quite a weight difference between 16 grains and 71 grains (a typical bullet weight for a .32 Auto).
The combination of greater bullet weight and aerodynamic shape provides for less observed instability. Pellets, being lighter and oddly-shaped, provide more opportunity for instability to insinuate itself. This instability can be observed at the point of impact. Did you hit what you were pointing at when the pellet left the barrel, or did you not?
So, it's probably appropriate to attribute some of the accuracy problems of pellets to their shape, which likely emphasizes the effect of supersonic to subsonic transitions.
Ultimately, two points: 1. your pellet travelling at 1000 fps is not travelling slow when compared to most centerfire pistol rounds. It's actually a fairly decent speed. It's just the mass and shape that cause problems. 2. andrem has the opportunity to design a more stable pellet, and I might recommend a sabot with a pointed or rounded-point nose, and a flat or boat-tail rear. I would definitely be interested to see whatever it is he is cooking up.
I reload pistol ammunition, and my data came from the book "Modern Reloading" by Richard Lee. Not the final authority, but certainly a good reference. As a side note, I found most centerfire rifle cartridges listing speeds between 2500 and 3600 fps, with the lion's share coming in at, or just below, 3000 fps. I'd like to see a pellet travelling that fast!
Back in the 80's,there used to be pellet(well bullet actually),shaped with a pointed nose and a boat-tail end.They were expensive and rather fiddly to put in the breech.Further more,they were terribly inaccurate, probably due to the build up of plastic debris from the sabot,in the rifling.
I think if I were to design a new shape for a pellet,it would be without any form of carrier(sabot) and rely on some form of annular rings of metal soft enough around its centre of gravity, to pick up the rifiling and form a seal for the air behind it,have a boat-tail and be highly polished.
David.
With the diabolo shape being unstable I began to wonder about "Perfect Rounds" the lead balls, whats the skinny on their ballistics, weight, and trajectory,accuracy etc.....
Trans-sonic turbulence occurs between .85 and 1.1 times the speed of sound
April 12 2006, 9:13 PM
Any projectile within that velocity range will experience more "bumpiness" due to that turbulence. Ideally, you don't want the pellet to pass thru a sound barrier as it slows down. If it starts out too close to speed of sound it will get bumped around as well. Same thing with firearm bullets only you're talking mach 2 or mach 3 that you don't want to be near as the bullet slows down. For airguns figure anything over .85 X 1100 = 935 fps to be starting to get into the marginal range. Some pellets will work ok somewhat above that speed and others just won't.
