1745 fps on AIR !

[rsterne]

Yes, you read that right.... Lloyd Sikes, using a setup designed to push the limits of PCP velocities, has just achieved 1745 fps using 4500 psi of AIR, not Helium.... Here is the link....

http://www.gatewaytoairguns.org/GTA/index.php?topic=102604.120

The .22 cal "bullet" is turned from a piece of aluminum, weighed only 7.5 gr., and is pulled through the 23.3" long barrel by an O-ring, and was driven by a dump chamber of only 26 cc.... Lloyd made two identical bullets, and the backup shot was 1714 fps.... The reason this is important is that it reliably demonstrates that the proposed "speed limit" of ~1650s fps theorized because of the RMS velocity of air molecules at 70*F has solidly been broken by this well documented experiment....

Congratulations, Lloyd.... This confirms the only previous shot exceeding 1700 fps I have ever heard of, conducted by a CAF member, with a .177 Condor shooting a 1.7 gr. plastic Sabot at 1705 fps, recorded on video a few years ago....

Bob

[Austringer]

I just read through the post link you provided Bob. Mostly way over my head, but I have to say, what you guys are doing is simply amazing. And that this forum works out to be literally an international, instantaneous method of comparing theories, tests and  results is equally amazing.

You guys are awesome.
troy

[SpiralGroove]

Hey Bob,
In light of the practical applications of your recent SSG modification, (which is spreading faster than the Zika virus in the air gunning community), what applications if any do you see for this speed breakthrough?  Or is this just disproving theoretical limits ..............

Kirk 

[Onebaddj]

That is awesome.

[nielsenammo]

Quite the breakthrough! Love it!!!

[rsterne]

This is simply testing to prove or disprove what some (including me, at one time) believed was the limit for velocity in a PCP.... The concept was that since the average (actually the RMS, or root mean squared) velocity of air molecules at 70 deg. F is 1650 fps, you couldn't exceed that because the molecules of air couldn't keep up with the bullet to provide any pressure to continue to accelerate it.... That made perfect sense to me, but when I posted it on the Canadian Airgun Forum a few years back, one of the guys said "I think I can break that", and using a .177 Condor shooting the plastic Sabot (only) from a lightweight pellet, video taped his Chrony as he sent the 1.7 gr. Sabot through at 1705 fps.... so, back to the drawing board for the theory, right?....

When I researched this deeper, I found that there is actually a velocity distribution around that 1650 fps number, with some molecules moving slower, and some MUCH faster.... This is called the Boltzman Distribution, and if you look at a graph of it, some molecules are going well over 3000 fps.... I reasoned that if a few molecules were moving fast enough, those could continue to bounce off the back of the bullet and accelerate it past 1650 fps.... It was kind of grasping at straws, but did explain the 1705 fps shot.... I have never seen another shot at over 1600 fps until Lloyd did it a month or so ago.... He hit 1605 fps using only 14.5 cc or air at 4500 psi in a 23.3" barrel, firing a 10.2 gr. aluminum "bullet" that is a .22 cal cylinder of aluminum pulled down the barrel by and O-ring.... It is held in the breech by a rotating shaft with a slot in it that matches another slot in the bullet.... Rotating the shaft releases the bullet, which already has the full 4500 psi behind it.... It's not a practical system, but excellent for this kind of testing to push the limits....

To our surprise, when we calculated the efficiency of that 1605 fps shot, it was nearly 70%.... on a par with your typical PCP.... There was no indication that we were approaching any kind of limit.... So we bugged Lloyd, and he made a lighter bullet (7.5 gr.) and used a larger chamber (26cc), and achieved the two shots at 1745 and 1714 fps.... The barrel length is still only 23.3".... Using various PCP Internal Ballistics spreadsheets, we have calculated that it should be possible to break 2000 fps, so a couple of us just sent Lloyd a few bucks as an incentive to continue.... The next step will be a piece of 5/8" OD, DOM tubing, with a 1/4" bore, and 48" long for the barrel.... and a bullet that is hopefully under 9 gr. in the larger caliber to decrease the Sectional Density and make even higher velocities possible....

These are really just "Research Toys", with no practical value.... However, the data we are collecting is invaluable in fine-tuning the spreadsheet models we have to predict PCP performance.... The more we understand, the easier it will be to go in the right direction with future developments.... Incidently, the atomic velocity for Helium is 4444 fps.... and the speed of sound in Helium, at 3000 psi, is about 3600 fps, compared to 1360 fps for air.... both about 2.7 times faster than air.... In addition, Helium is less than 1/7th the weight of air, so it takes MUCH less power to accelerate its own mass down the barrel.... We have only scratched the surface of the performance potential of Helium, IMO....

Bob

[avator]

At the risk of "dumbing" the thread up a bit, this is where I get lost understanding and keeping up... I thought the speed of sound was like 1050 fps. I read all the time that it's non-productive to push a pellet faster than that because the turbulence makes accuracy non-existent. I understand that these tests have nothing to do with accuracy... that's not what I'm questioning. My question is.. where did 1360 fps come from? Have I been misled or am I misunderstanding now?
BTW.. you guys continue to awe me. Thank you.

[match]

non-ideal gasses in compression have an increase in the speed of sound due to Pressure/density ratio.

[rsterne]

The speed of sound in air, outside the barrel, is about 1126 fps (depending on temperature).... Inside the barrel, it increases with pressure.... not linearly, but more rapidly as the pressure increases.... At 3000 psi, and 70*F, it is about 1360 fps....



Note that the velocity for Helium is on the right axis.... air and Nitrogen are on the left....

Bob

[K.O.]

What this means in practicality...

Maybe, the Bobs White out is be a good way to reach for 200 yards accuracy but in the future may with a lot of figuring may be bested by a lighter round that stays supersonic pushed out of a long barrel with Helium..? 

[rsterne]

I wish that were true, but staying Supersonic to 200 yards is in reality VERY difficult, and requires a heavy round and a high muzzle velocity.... Those two things are to a large extent mutually exclusive in airguns, we simply don't have the pressures available to achieve both at the same time.... Even if you do manage it, you are then working in the very WORST possible part of the drag range, which increases wind drift a lot over what we get by staying Subsonic.... Here is a chart showing wind drift vs vellocity for three vastly different bullets....



Note that they all exhibit minimum wind drift at 800-900 fps, and don't get back down to that number until over 2300 fps, if ever.... So, not only does it take huge amounts of air (or Helium) to get the bullet Supersonic, once you push it there it slows quickly.... and that slowing pushes the bullet off course.... You suffer a double whammy....

Bob

[K.O.]

Could some thing patterned after the 168g Match king make it to 2200 fps out of a long barrel on helium..?

I do tend to look at  things  and then go with what my gut says, huge frackin plenum inefficient dump valve... to get it up where its B.C. is .424

or even 1600 fps & ~ .4 for the b.c. ... my gut did equate the better bc to less drift...

Sorry dreaming of 850-1800 fpe... I do wonder how big of a bottle and how long of a barrel...

But yep the match king was basicly what I was thinking and a new 4500 psi bottle every shot...

My  imagination gets away with me sometimes...

[rsterne]

Lloyds spreadsheet predicts 1325 fps (655 FPE) with a .308 cal 168 gr bullet in a 36" barrel at 4500 psi with a dump shot from a 500 cc reservoir.... The reservoir size doesn't make a huge difference....

300 cc = 640 FPE
500 cc = 655 FPE
1000 cc = 667 FPE

These are at 75% efficiency, which is a good guess for Helium, but it might be lower.... For 1000 cc reservoir and 100% efficiency, you are still only at 890 FPE (1544 fps).... Increasing the pressure to 6000 psi at 100% efficiency, still only gets you to 1754 fps (1154 FPE).... it would take a 49" barrel (and 100% efficiency) to hit 2000 fps.... So, basically, even Helium, at 6000 psi, in a 4 foot barrel, at 100% efficiency (impossible, of course) won't give you what you want....

Bob

[K.O.]

Thanks Bob  my mind did jump to the what if of staying supersonic and that 1600 fps would do it (my gut not math) down to 1350 or so at 200 yards( once again by gut)...

but starting out at about 1300 or 1400 did not have a chance...

3/4 of the way is not enough....

but back down to the very light stuff  say 7.9g-35g and very high psi... Does it mean that maybe by harnessing the venturi effect additional efficiency could be found... by boosting initial charge velocity...

sure once the charge hits the round it would slow down but now we know that the charges speed limit is higher

get the fpe with a lighter faster charge at the beginning of acceleration..?

[rsterne]

The velocity of the gas (air) does not accelerate the pellet, the PRESSURE it exerts on the pellet is what does the work....

Bob

[K.O.]

The velocity of the gas (air) does not accelerate the pellet, the PRESSURE it exerts on the pellet is what does the work....

Bob

In my mind Bob it is the density/speed of the charge...

a 1500 psi charge and .1 ci plenum with the barrel length needed to use that charge, slower/heavier charge

a 3000 psi charge and a .05 ci plenum with the barrel length needed, faster/lighter charge

both have the same end fpe one with higher initial acceleration...

Light pellets respond well to that faster initial acceleration...

so fool the pellet into thinking it has the faster lighter charge for longer with the venturi effect because that speed limit on the charge has been found not to be what it was thought to be...

The venturi makes it think it has a larger plenum for  a while..?

[K.O.]

oops said that wrong both charges weigh the same... but the higher pressure at half the volume the air itself is being stored at a higher energy state.

[rsterne]

IMO, a "venturi" is much more likely to suffer from choking than straight bore-size porting.... When (if) a choke forms (which occurs when the velocity at the venturi reaches Mach 1).... there is a 47% minimum pressure drop across the choke.... While the downstream velocity of the air can exceed Mach 1, you still have less pressure available to accelerate the pellet....

IMO, and in any tests we have done.... any restriction in the system is going to limit power, and is a bad thing, not a benefit....

Bob

[michaelthomas]

Has anybody drawn a vacuum on the barrel to see how that affects velocity?  I would suspect it to provide a decent boost.

Mike

[39M]

Anybody familiar with the light-gas gun?
https://en.m.wikipedia.org/wiki/Light-gas_gun