1745 fps on AIR !

[rsterne]

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

[oldpro]

    [url=[/url]  LOL

[michaelthomas]

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

[Gippeto]

Have not messed around with a venturi, but have played with converging nozzles in an airgun...of sorts.

The original experiment...



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.

Getting something...ANYTHING...to 1700fps with air is quite an achievement. Congrats and well done Lloyd.

Al

[K.O.]

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...

[rsterne]

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.... 

Bob

[michaelthomas]

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

[I_like_Irons]

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.

[rsterne]

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

[39M]

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. 

[lloyd-ss]

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

[lloyd-ss]

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.   

[madeInLV]

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.

[madeInLV]

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.

[rsterne]

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

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

[zandrew]

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.

[lloyd-ss]

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.

......................................

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

The next testing will be with a .25 cal 48" barrel with 4500psi.  Any predictions?   

Lloyd
 

[madeInLV]

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.

[rsterne]

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

You input "air at 300 bar".... and it spits out 465 m/s.... pretty close to my estimate, HUH?....



Now THAT is a really cool little widget.... 

Bob

[lloyd-ss]

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