GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: rsterne on February 19, 2016, 02:26:21 PM
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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 (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
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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
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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
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That is awesome.
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Quite the breakthrough! Love it!!!
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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
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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.
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non-ideal gasses in compression have an increase in the speed of sound due to Pressure/density ratio.
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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....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/Speed%20of%20Sound%2020C%20with%20Helium_zpsbphc6tdo.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/Speed%20of%20Sound%2020C%20with%20Helium_zpsbphc6tdo.jpg.html)
Note that the velocity for Helium is on the right axis.... air and Nitrogen are on the left....
Bob
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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..? ;)
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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....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Wind%20Drift%20vs%20Velocity_zpsaxlbda63.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Wind%20Drift%20vs%20Velocity_zpsaxlbda63.jpg.html)
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
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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... :-[
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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
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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..?
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The velocity of the gas (air) does not accelerate the pellet, the PRESSURE it exerts on the pellet is what does the work....
Bob
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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..?
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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.
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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
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Has anybody drawn a vacuum on the barrel to see how that affects velocity? I would suspect it to provide a decent boost.
Mike
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Anybody familiar with the light-gas gun?
https://en.m.wikipedia.org/wiki/Light-gas_gun
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I don't think a vacuum would make a huge difference, after all, you are only talking about 14.7 psi on one side, and 3000ish on the other.... Same thing when you talk about the density of the air in the barrel, it could never be greater than the mass in there before you pull the trigger.... but the mass of the gas being accelerated behind the bullet is a huge percentage of the total mass (and is included in the spreadsheets, of course).... This is not to say firing the gun in a vacuum would make NO difference.... just that I would expect it to be quite small.... using the numbers given, about 0.5%.... To answer directly, I am not aware of anyone trying it....
Light gas guns are not airguns, because they use an explosive charge to drive a piston like a Springer, that then compresses a light gas (usually Hydrogen) to propel the bullet.... Comparing them to an airgun is like comparing apples to orange juice.... they are both kinda fruity....
Bob
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[url=https://www.youtube.com/watch?v=ERNlSDqeiTg]https://www.youtube.com/watch?v=ERNlSDqeiTg (https://www.youtube.com/watch?v=ERNlSDqeiTg)[/url] LOL
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I guess I was thinking the pressure in front of the pellet would dramatically rise as the pellet went down the barrel. Kind of like a fast moving piston.
Mike
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Have not messed around with a venturi, but have played with converging nozzles in an airgun...of sorts. ;)
The original experiment...
https://www.youtube.com/watch?v=UbnBsAt28Hc (https://www.youtube.com/watch?v=UbnBsAt28Hc)
Adding a converging nozzle @ 11degrees taper...inlet was something like .3??" velocities exceeding 1300fps were recorded @ 5'. When modeled with ggdt, the barrel is choking the flow, and the program does predict higher velocities than achieved...more reliable below 1000fps.
The fellow who wrote ggdt warned me that @400psi with room temp air I'd have chunks of dry ice forming...a bit of cooling going on there. ;D
Getting something...ANYTHING...to 1700fps with air is quite an achievement. Congrats and well done Lloyd. 8)
Al
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Please have patience Bob brainstorming in case the sonic choke is a sonic joke...
will do drawing to illustrate... but have to go for now...
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I can't WAIT until he has the demonstration to go with the hype on the video.... *LOL*....
With a 100% vacuum, the force on a .22 cal bullet is a massive 0.56 lbs.... If you have 3000 psi on the other side, that is 114 lbs....
Michael, pressure is nothing more than molecular collisions against an object, in this case the front of the bullet.... The average molecule is moving 1650 fps, so even if the bullet reaches that velocity, you have only doubled the velocity of the collisions against it.... The main effect, I think, is that you have to move that mass of air out the muzzle along with the bullet.... For a .22 cal gun with a 24" barrel, and a 3000 psi dump shot, and a 14 gr. pellet....
Mass of pellet = 0.0000621 slugs
3000 psi of air behind pellet = 0.0001260 slugs (the air is twice as heavy as the pellet)
Total of the above = 0.000188 slugs
Mass of air in front of pellet = 0.00000072 slugs (0.4% of what is already being accelerated)
Incidently, if you didn't have to accelerate the air in the barrel along with the pellet in Lloyd's latest test.... instead of hitting 1745 fps, he should have achieved 3300 fps....
PS, Al, your first shot was probably accurate, the bullet probably slowed that much in 5 feet.... :o
Bob
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Bob, I didn't put any math to it. I just remember a mythbusters episode where they got a ping pong ball to go supersonic using that same principle and relatively low pressure air. Don't remember all the specifics.
Mike
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Yes. However, the light gas guns rely a lot on initial adiabatic compression which increases the average speed of the atoms. The PCPs suffer from adiabatic cooling as the compressed air/gas expands.
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Mike.... Yep, saw it.... The lower the driving pressure on the back, the more important, in relative terms, is the vacuum on the front....
Yep, light gas guns are a minor miracle.... very impressive piece of equipment, for sure.... They do all the right things at the right time....
I would suggest that adiabatic cooling only occurs after the valve is closed in a PCP, or if the reservoir is not huge compared to the barrel volume.... If the reservoir is effectively infinite (ie many times the barrel volume), the amount of expansion while the valve is open is negligible.... so although it might technically still be Adiabatic the amount of cooling is so small the difference between that and Isothermic would likely be unmeasurable....
Bob
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Yes it uses gun powder to power the first stage, and either hydrogen or helium for the second stage.
I just thought that a similar two stage setup with a heavy spring piston powering the first stage and helium powering the second stage might be possible.
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Guys, for my 1700 plus fps, there was no magic or anything super special. Two things made the difference.
First was the converging cone delivering the air directly from the reservoir to the rear of the bullet. Just efficient air delivery with no obstructions or convoluted paths.
The second important thing was the release of the bullet. These bullets were locked in place until full pressure was applied directy to them, and then the release on the bullet was tripped. That eliminated the time usually needed for the initial buildup of pressure after the valve opens. In a normal pcp that means the bullet starts moving with only partial pressure behind it which gives poor acceleration. These bullets get 100 percent right out of the gate. It makes a BIG difference.
Lloyd
So that's it. Just 2 improvements on a (ab)normal pcp system.
Lloyd
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Work will continue this coming week on this project. Lighter "pellets" and a smooth bore longer barrel of slightly larger caliber will be built in hopes of achieving significantly higher velocities.
Then i will need a witness for a backup performance. :D
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BLUF: it is physically possible to accelerate a projectile above 2000fps.
Firstly CONGRATS on the achievement.
Secondly, I'm glad I'm no alone on this.
Thirdly, cudos on the clever test rig. Very simple/practical and well though out.
About a year ago while testing my own invention (a new type of an In-line valve) I achieved 623m/s (2043fps). Sorry...I'm a metric guy:)
Here is the date:
Cal: 5.5mm (0.22)
L_barrel=550mm
P_chamber=250atm
V_chamber=19.5cm^3
Propellant=air
Projectile=aluminum foil ball. Made simply by hand then rolled to snugly fit in the barrel. So mass of the projectile....no idea...very small. Who ever has scales up to 0.01g can repeat the experiment and will come up with close to what i had.
A bit on the project and the test rig.
The intent of my project was to make an in-line valve that would address the inherent deficiencies of a standard PCP:
1) get rid of 2x90* bends in the air duct (flow restriction)
2) minimize dead space of the transfer port.
As it turned out #2 is necessary for a proper operation of any valve type device -it lets the pressure build up while the valve opens to 100%. The only exception would be a burst disc or the test set that was used in this topic. The latter two however do not allow the main chamber to shut close while the projectile is still in the barrel. This will lead to poor overall efficiency.
Anyway...the intent is to make a .22LR alternative on air with appealing dimensions and a reasonable shot count for hunting application.
To do this the device needs to be able to accelerate 2.6g of lead to min of 285m/s. I can get it to 300m/s with 20cm^3 @200atm. That's a .22LR in my book.
A gun based on this valve is currently in the making. Stay tuned:)
The 623m/s shot was more of an accident rather than a planned test. That's what heppens after many many.... many test shots and experiments...you just want to introduce some fun into the whole endeavour again.
I suspect that the actual velocity at the muzzle was higher since the chronograph was positioned about 30cm away from the muzzle+10cm between the optical gates of the chrony. Since the sectional density is very small by the time the projectile crossed the second opt. gate is must have lost some m/s.
I had a bit more foil laying around enough for one more shot which resulted in 614m/s serving to me as a proof that the 623 was not a glitch.
Thought this would be helpful and very excited to see what the test rig achieves.
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also, to those truly interested in this subject here is some VERY VALUABLE literature.
Seigel A E 1965 The Theory of High Speed Guns
oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0475660
I must warn that the math is somewhat intense however it is packed with useful info.
Also, the authors actually made a compressed air gun with a burst disc to test the math and achieved something insane...2300fps is I'm not mistaking. Read the report if you are interested. The key here is the burst disc.
Also...the choking phenomenon was something that puzzled me as well. None of my teachers could clarify this either. It was clear that that PCP can go supersonic while they shouldn't due to M=1 limitation when the flow gets choked.
Seigel's report addresses this as well. A it turns out Chamber+barrel IS NOT equal to camber+nozzle.
Lastly...there was a graph of "speed of sound vs pressure" presented here. I would really love to see the source of this information if the author doesn't mind.
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Congrats on breaking 2000 fps with a foil ball I a PCP....
Data for speed of sound in N2 and O2 is here.... http://www.peacesoftware.de/einigewerte/einigewerte_e.html (http://www.peacesoftware.de/einigewerte/einigewerte_e.html)
I used the geometric average (78:21) to get the values for air.... That calculator has air density and all the other values, at various pressures, but for some reason omits speed of sound....
Bob
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I know this will be hard to believe but I have a talon .22 that had a 18" barrel on it. When I first got it I had an ldc that I use on my hatsan at44s. I was shooting it and it sounded like a 22 magnum. I put it on the chrony and it was shooting JSB mediums at 1400 fps. I still do not know why but I am guessing something was oily and caused it. It would absolutely smoke a pellet.
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BLUF: it is physically possible to accelerate a projectile above 2000fps.
Firstly CONGRATS on the achievement.
Secondly, I'm glad I'm no alone on this.
Thirdly, cudos on the clever test rig. Very simple/practical and well though out.
About a year ago while testing my own invention (a new type of an In-line valve) I achieved 623m/s (2043fps). Sorry...I'm a metric guy:)
Here is the date:
Cal: 5.5mm (0.22)
L_barrel=550mm
P_chamber=250atm
V_chamber=19.5cm^3
Propellant=air
Projectile=aluminum foil ball. Made simply by hand then rolled to snugly fit in the barrel. So mass of the projectile....no idea...very small. Who ever has scales up to 0.01g can repeat the experiment and will come up with close to what i had.
A bit on the project and the test rig.
The intent of my project was to make an in-line valve that would address the inherent deficiencies of a standard PCP:
1) get rid of 2x90* bends in the air duct (flow restriction)
2) minimize dead space of the transfer port.
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Hello Alex !
Glad to have another contributor who is willing to challenge conventional "theories."
Congrats to you, also on your 2043 fps. The bar keeps getting lifted higher, and that is a very good thing.
I will be interested to see how your in-line valve project progresses, but I know very well that projects that like can take a very long time.
The reference document you cited looks very interesting. I down loaded it and started reading, and as you said, the math gets intense very quickly. But just skimming thru, there seems to be a tremendous amount of good reference information, much of it directly applicable to PCPs. Would you agree that the PCPs are similar to the PP guns referenced in the document, except, without the high heat? I only got into it a few pages but I noticed that the author also stated that for maximum velocities to be achieved, the projectile must be restrained until full pressure is allowed to build up behind it. That seems like common sense to me ;) , and is what I am doing in my test rig. I think some of K.O.s comments are also addressed in this document.
You probably already know this, but the actual test bed and experimentation is being done in this thread:
http://www.gatewaytoairguns.org/GTA/index.php?topic=102604.180 (http://www.gatewaytoairguns.org/GTA/index.php?topic=102604.180)
The next testing will be with a .25 cal 48" barrel with 4500psi. Any predictions? ;)
Lloyd
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Bob,
thank you for the link. Useful info.
I do not however understand how you got to the speed of sound. I cannot find any literature that would address speed of sound being a function of anything other than TEMPERATURE. To me it makes intuitive sense-higher pressure->smaller distance between molecules=> faster moment exchange between the molecules BUT i can't find any literature proving my point of view.
Zandrew,
I have not been able to make a PCP diesel. I wish i had:) But...no luck so far. Do you have contradicting info/experience?
lloyd-ss,
yes, the document is basically all we need. I call it the PCP bible, because it explains the FLUID DYNAMICS effects taking place DURING the shot. I honestly had no idea there was so mach to it. I have attempted to go through the paper multiple times but it's like grinding a stone...you get a small bit every time. I wish i had the brain power to take it in all at once. It's one of the reasons why i might persue a MS in CFD (computational Fluid Dynamics) some day. My BS in ME just is not enough...
It actually has all you need to make a computer code that would predict the final values we are looking for. All you would need is a mathematician and perhaps an IT expert. If somebody does end up making one....please share:)
BUT at this point I believe i have a clear picture on what is taking place during the short at what variables are at play here. In PCP there is not much we can change to be honest.
In order to get the best PCP you need:
Infinite volume firing chamber
Infinite barrel length
Infinite pressure.
Reality forces us to go with a reasonable barrel length (18 to 25"), reasonable chamber volume, and 200-300atm of pressure. So we are quite limited. Additionally you would need a valve that opens and closes while the bullet is still in the barrel. Combining the latter with an in-line valve turned out to be VERY challenging.
You are correct about the pressure. It is actually quite simple. Kinetic energy=work=Force*distance=P*A*L_barrel=P*dV.
Now….dV is basically fixed , so you want to maximize your potential by increasing P (whose dropping rate depends on the V_chamber). If the bullet travels some distance (decreasing dV) under P that is not P_max…your efficiency drops. That is actually quite noticeable in my system. Thus it is well known that a good PCP has a valve that opens VERY quickly and closes when the bullet is in the middle of the barrel.
The PP gun you are talking about is basically a PCP with hot gas. Our gas (air) is at room temperature, that's the only difference. Still…the level of detail the author goes into is amazing.
Nevertheless it would be wise to study the material and thus get a sense of what is going on AND apply this knowledge to a CFD simulation which will give you a high resolution of the flow phenomena around your geometry. Now that would be a killer. Do we have any CFD people on the forum?
Lastly…and this IMHO is very important.
If you have the will and the time here is what you could do: Modify your test rig to resemble a conventional PCP, that is…introduce 2*90deg bends in the duct. The intent here is to empirically compare the energy and efficiency of the system in in-line vs traditional configurations (given that the V_chamber, P_chamber, L_barrel=constant. (and compare the gain/loss vs power of the system (mass flow rate))
Both schools of thought are currently present in the airgun community-some think in-line is a waste of time while some (myself included) are absolute convinced there is a lot to be gained. NOBODY can give you hard evidence proving/disproving their position. Your test rig has the potential to put an end to this debate.
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Alex, if you use the calculators for N2 and O2 on that site.... put in 300 bar and 20*C, click "Calculate", and scroll to the bottom, you will see the Speed of Sound.... For N2 it is 482 m/s, and for O2 it is 414 fps.... I used the geometric average for air (78:21) to get 468 m/s.... They give the Speed of Sound for every gas in that list, except air, I guess because it is a mixture.... For most physical and thermodynamic properties of mixed gasses, using the Geometric average (ie using the proportions of the mix) give the correct result.... so that is why I used that method.... I would certainly suggest that it would give a much closer result than ignoring the pressure, and using temperature only.... which would give 343 m/s....
HEY, guess what!.... I just found a widget that will do the calculation at 20*C and 300 bar....
http://www.wolframalpha.com/widgets/view.jsp?id=b63c87b0a41016ad29313f0d7393cee8 (http://www.wolframalpha.com/widgets/view.jsp?id=b63c87b0a41016ad29313f0d7393cee8)
You input "air at 300 bar".... and it spits out 465 m/s.... pretty close to my estimate, HUH?....
(http://www4b.wolframalpha.com/Calculate/MSP/MSP1371i8dhddgg41d704700002cb45g0c2ii0ddg3?MSPStoreType=image/gif&s=6&w=245.&h=240.)
Now THAT is a really cool little widget.... 8)
Bob
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Reality forces us to go with a reasonable barrel length (18 to 25"), reasonable chamber volume, and 200-300atm of pressure. So we are quite limited. Additionally you would need a valve that opens and closes while the bullet is still in the barrel. Combining the latter with an in-line valve turned out to be VERY challenging.
......................................................................................
You are correct about the pressure. It is actually quite simple. Kinetic energy=work=Force*distance=P*A*L_barrel=P*dV.
Now….dV is basically fixed , so you want to maximize your potential by increasing P (whose dropping rate depends on the V_chamber). If the bullet travels some distance (decreasing dV) under P that is not P_max…your efficiency drops. That is actually quite noticeable in my system. Thus it is well known that a good PCP has a valve that opens VERY quickly and closes when the bullet is in the middle of the barrel.
...........................................................
Lastly…and this IMHO is very important.
If you have the will and the time here is what you could do: Modify your test rig to resemble a conventional PCP, that is…introduce 2*90deg bends in the duct. The intent here is to empirically compare the energy and efficiency of the system in in-line vs traditional configurations (given that the V_chamber, P_chamber, L_barrel=constant. (and compare the gain/loss vs power of the system (mass flow rate))
Both schools of thought are currently present in the airgun community-some think in-line is a waste of time while some (myself included) are absolute convinced there is a lot to be gained. NOBODY can give you hard evidence proving/disproving their position. Your test rig has the potential to put an end to this debate.
Hello Alex,
Let me address three of the topics in your abbreviated quote, above.
In number 1, yes, I agree, in unconventional valves, it seems that getting the valve to open quickly is the easy part, but getting it to close quickly and precisely, is another matter. Like you said, after the bullet has left the muzzle and the back pressure is gone, closing the valve becomes much easier.
In the second section, the forces acting on the pellet are adequately addressed with regard to theoretical forces, but the variables that detract from the motive forces are the ones that seem to be hardest to define and mathematically model. Based on our own spreadsheet models that include reductions for static and dynamic friction, T-port volumes, weight of the moving air column, expansion characteristics of the air, and diminishing pressures prior to, and after valve closure, we are still finding that the actual velocities indicate that the system efficiencies are only in the 55% to 75% range. That means that something is happening to 25% to 45% of the input energy. That is the part of what we are struggling with, and I hope that maybe Mr. Seigel addresses it in his manuscript. That would be wonderful!
For your third point, about doing some comparison testing with 2 90 degree bends forming a U-turn, I agree, that would be very nice. I will add it to the list. ;) Whether or not that would put the discussion to rest among airgunners, I have my doubts, LOL. Old beliefs, even when clearly dis-proven, die hard!
Lloyd