Sonic Choke? Maybe not. 2162 FPS

[rsterne]

The air going into the SCUBA tank is flowing through a hose, but not expanding (merely slowing down when it enters the tank), and it is compressing the air already in the tank.... This is a continual process.... When you fill the tank 1/4 full, at 750 psi, the incoming air continues to squeeze that into a smaller space.... When you add another 25%, the first air is now at 1500 psi.... and hotter than before....

That's how I understand it, anyway.... If it was entropy picked up in the hose, a microbore hose would heat up a gun faster than a large diameter one, no?.... I'm betting if you open up the valve on the SCUBA tank wide open, the reverse is the case, when the gun fills faster, it gets hotter.... or rather the air has less time to cool through contact with the tube walls, and by conduction and radiation, the air surrounding it....

Bob

[lloyd-ss]

I am glad you guys are willing to take on the math challenge because it is quite daunting. 
I took a shot tonight with a Teflon seal.  There was a technical difficulty and the air pressure never got high enough, but the performance was still reasonably close to what was predicted.

The projectile was long and heavy.  13.7 gns.  It needed a long stem so that the Teflon seal was deep enough into the barrel to seal properly. It is a 3-piece construction of a machined Teflon  U-cup style seal, with a 7075 aluminum head and stem, and a 7075 retention knob with a 6-32 thread  on the back end. The 3 piece construction was necessary for installing the Teflon seal.  The barrel was swabbed clean prior to firing.




I had a thermocouple on the breech block to monitor the fill temp.  The temperature of the breech block might have gone up a few degrees F, but that is all.
I had the projectile clamped in the breech mechanism, and started filling, and unfortunately, the retention knob slipped through the clam shell clamp at about 2500 psi.  I was disappointed at the early release.   BTW, By hand touch, I could not detect any temperature changes in the barrel or breech.
So here is the shot, 13.7 gn, .278 barrel, 47.5" long, 2500 psi, 55cc, 1688fps.

In the above picture, behind the chrony, you can see some plastic grocery bags sticking out of a cylindrical container. That is the bullet catcher, and it worked.
Here is the projectile after I retrieved it from the plastic bags. There were multiple layers of wad cutter circles of plastic bags on the front of the Projectile, but it remained fully in-tact.  There was no evidence of melting or fusing of the plastic bags to the aluminum front end of the projectile.


Here you can see the marks where the projectile pulled thru the retention clam shells and cut the test short.


Here is the projectile, after the shot, with the plastic bags peeled off. The Teflon looks perfect and the projectile is re-usable if I make a larger dia retention knob.


But even though the shot pressure was only 2500 psi, it is one more piece of data.
More refinements to the rig, and a few more HPA shots are needed before the nitrogen arrives.
Lloyd

[rsterne]

Great piece of data, Lloyd, I look forward to you trying again at higher pressure.... The fact that once again, you exceed the magic 1650 fps is interesting, because Maxwell's Demon (aka Steve_in_NC) has finally stopped using obscure language and spit it out that what you have accomplished is impossible....

http://www.network54.com/Forum/79537/message/1459063919/That%27s+a+vaguely+related+response.

IMO, any scientist who digs in his heels behind a flawed theory, without even the curiousity, or professional courtesy, to investigate what might be happening.... but just outright rejects the facts.... isn't worth our time or effort.... It is unfortunate, as he could have so much to offer, should he not be so stubborn.... We are totally wasting our time trying to convince him.... Best we simply leave him behind, stuck at 1650 fps.... It really is a shame....

Bob

[lloyd-ss]

Great piece of data, Lloyd, I look forward to you trying again at higher pressure.... The fact that once again, you exceed the magic 1650 fps is interesting, because Maxwell's Demon (aka Steve_in_NC) has finally stopped using obscure language and spit it out that what you have accomplished is impossible....

http://www.network54.com/Forum/79537/message/1459063919/That%27s+a+vaguely+related+response.

IMO, any scientist who digs in his heels behind a flawed theory, without even the curiousity, or professional courtesy, to investigate what might be happening.... but just outright rejects the facts.... isn't worth our time or effort.... It is unfortunate, as he could have so much to offer, should he not be so stubborn.... We are totally wasting our time trying to convince him.... Best we simply leave him behind, stuck at 1650 fps.... It really is a shame....

Bob

It is unfortunate that our friend on the other forum chooses not to enter into this discussion in a constructive manner.   He is very intelligent and could certainly help explain what is happening, and his contributions would be appreciated by the entire airgun community.  Who knows, maybe he already has the explanation, but just chooses not to share it.      
A boss I once had used to say, "when your wagons are in a circle to protect your position, no forward progress can be made."  I am glad that our wagons are not in a circle.

Your explanation of the molecular speed being additive to the moving column of air in the barrel, seems most plausible to me.  Similar to cannon fire from a fighter jet.  The projectile speed is equal to the cannon MV plus the speed of the plane. Not only is that explanation plausible, but it is palatable, too.   
Lloyd

[rsterne]

In the words of the immortal Buzz Lightyear.... "To Infinity and Beyond !"

Bob

[Buldawg76]

Lloyd and Bob
I agree with both of you in that it is indeed a shame the he (Steve) cannot or will not accept that theory is in fact just that and only serves as an example of where to start or design from not cold hard facts or a impassible boundary that is set in stone. The mere fact that the 1650 fps threshold has been broken with very well defined and proven data should in itself be sufficient to get him at the very least curious as to whether his math and calculations are incorrect or flawed in some respect.

The example of a fighter plane and its cannon fire is evidence and highly proven fact that indeed any projectile is not limited to a velocity of 1650 fps since if that were the case cannon fire out of a fighter jet moving at only 1650 fps with the jet at Mach 2 or above would never leave the barrel of the cannon or if it did the plane would be running into it own projectiles in flight. Then you have PB guns shooting at close to 4000 fps in this day and age so how are we stuck at 1650 fps as an impassible velocity.

Being no engineer or even much of a math guy other than simple measurements and basic formulas I still realize that nothing ever ventured is nothing gained and to say its impossible just drives those who want to know even harder to disprove that statement. It is a true shame that he cannot/will not join those that are only trying to improve and expand or hobby/sport to the fullest and most rewarding that it can be for all who care to enjoy and learn from other like minded individuals.

I myself know that without those of you here that put countless hours into the experiments and research that you do at considerable costs in money and time would be far less knowledgeable and have more struggles with my projects and not enjoy the fruits of my labor near as much as I do now all thanks to those of you who give so much of your time and money to help improve this for the rest of us.

I thank you all very much.

Now Lloyd I want to see you break Mach 2 with your new design projectile as I have all the faith it can be done. Its not a matter of IF but WHEN it happens.

Mike

[rsterne]

I thought it might be helpful to show where we are for modelling what happens inside a PCP at the present time.... This is what I get using Lloyd's current (J4) version of his PCP Internal Ballistics Spreadsheet, set up to analyze his current test gun.... To recap, the barrel is .278" bore, 47.5" long, and fed with a 55 cc chamber.... The solid lines are the projected maximums, at the pressures stated, using an infinite reservoir, no bore drag, but including the mass of the air used to create the shot.... The dotted lines represent the best model I have of what he is accomplishing, using the 55 cc reservoir at 4000 psi, 1 lb. of bore drag, and 70% overall efficiency....



Both graphs are plotted vs. the weight of the projectile (in grains).... It predicts that at 4000 psi, we would need a projectile down around 4 gr. to hit Mach 2.... That is less than half the weight of the lightest tested so far.... Further increases in pressure will have some effect, while increases in reservoir size, at this point, have nearly no effect.... Changing to Nitrogen should produce a slight gain.... I think that reaching Mach 2 (2250 fps) is possible, with this test gun.... but it won't be easy, even with Nitrogen.... Helium, of course, would move us up towards Mach 3.... 

Bob

[rsterne]

Here is the diagram that the 1647 fps speed limit for air at 20*C is based on.... from  http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html#c4 .... 502 m/s = 1647 fps....



The V rms value of 1647 fps, as you can see, is not the velocity of all the molecules, it is the Root Mean Square average.... Many molecules are going faster, and many slower.... some are going over twice that speed.... If those are the ones colliding with the projectile, surely it could be accelerated well past 1647 fps.... As I said several years ago, when I saw that first shot at over 1700 fps, maybe it only takes a FEW, pushing on the pellet to create the speeds we are seeing.... That first 1705 fps shot was something like 3% efficient, based on the total volume of air released, IIRC....

It may be nothing more startling than the barrel FOCUSING the fast air molecules down the middle, while the stragglers get left behind along the edges of the flow.... The sum of all the velocities is under the 1647 fps RMS value.... but the fast molecules in the center still have enough push left to propel the pellet well over 2000 fps.... Other, more complicated, or elegant solutions may be the answer to what is taking place.... but having the velocity profile inside the barrel focusing the fast molecules down the center to accelerate the bullet may be all it takes to keep everyone happy.... Think of the barrel as Maxwell's Demon directing traffic if you like.... The detailed explanation of how and why can come later....

Bob

[rgb1]

I tried seven years ago to gently show the troops on the YF that the proposed
1650 limit was nonsense because the thinking behind it was faulty. Made a
post showing several pressures and associated speeds, the highest was
1800+..........it was ignored. Oh well, maybe Lloyd finally has his attention.

                                                                                      Ron

[Scotchmo]

Lloyd's test's were certainly the impetus for me looking at it more closely.

I think that Steve over on the yellow forum will come around. He is smart. I'm not always the best at explaining, so I'm taking several approaches. I'm hoping he gets that moment where:


On this forum, I have not detailed all of my recent thinking, since it's "preaching to the choir". But if your interested, follow this thread:
http://www.network54.com/Forum/79537/thread/1459019853/last-1459138839/Expand+Thread

[Scotchmo]

If you are using an % efficiency in your spreadsheets, here is something you may want to add:

My spreadsheet accounts for the adiabatic expansion which is significant with small reservoirs. So there can be a significant temperature drop. I have a column showing that.

What about the % loss (1-%efficiency) or flow losses? We know that losses are mostly heat. So we could include it as entropy production.  My flow loss compensation does not work any better than your efficiency compensation, so just use % losses. Convert it to heat and add it back into the process at each step. Rather than get a temperature drop, you will likely see a net temperature rise.

Now make a new column with v(max) showing v(max) = 172 194 * sqrt(Q/(m+Z*Q)). Base Z on the new temperature at that point in the process.

The v(max) column will always be a larger value than your predicted velocity. So you never actually violate that formula.

[lloyd-ss]

If you are using a % efficiency in your spreadsheets, here is something you may want to add:

My spreadsheet accounts for the adiabatic expansion which is significant with small reservoirs. So there can be a significant temperature drop. I have a column showing that.

What about the % loss (1-%efficiency) or flow losses? We know that losses are mostly heat. So we could include it as entropy production.  My flow loss compensation does not work any better than your efficiency compensation, so just use % losses. Convert it to heat and add it back into the process at each step. Rather than get a temperature drop, you will likely see a net temperature rise.

Now make a new column with v(max) showing v(max) = 172 * sqrt(Q/(m+Z*Q)). Base Z on the new temperature at that point in the process.

The v(max) column will always be a larger value than your predicted velocity. So you never actually violate that formula.

Scott,
Yes, I see that I have changes to make to my spreadsheet, beyond the VanDerWaals conversion that is in-process.

Thank you again for presenting this information to folks over on the yellow.   You have managed a difficult task very well, and presented some intriguing new thoughts about what is happening inside the airgun .  I had not considered much of what you discussed.
Thank you.
Lloyd

[lloyd-ss]

Latest test shot data as of 28 March 2016

Here is a spreadsheet of the last 8 of the 59 high velocity shots that I have taken.  It includes 2 new shots that I took last night.


A couple of things about the spreadsheet: 
Shot number 52, 1745 fps at 4500psi, 7.5 gn, .22 cal, 23.3" barrel, is what I am calling the baseline shot.  It is the first shot that significantly exceeded the 1640fps velocity limit.  The column labeled "predicted velocity" is calculated using my spreadsheet with a 70% efficiency factor and a 1 pound pellet drag.
I think it is significant, that in 6 of the next 7 shots, the velocities were within an average of 1.3 percent of the predicted velocity.   The one slow shot was an anomaly because it failed at the clamping point and "self launched" with the propulsion air essentially going through a .155 dia orifice.

Here are the 7 latest significant shots:
1745 fps, .22 cal, 7.5 gn 4500psi, 23.3" barrel
1714 fps, .22 cal, 7.5 gn, 4500psi, 23.3" barrel
1802 fps, .278 cal, 15.3 gn, 3500psi, 47.5" barrel
2031 fps, .278 cal, 9.1 gn, 4500psi, 47.5" barrel
1688 fps, .278 cal, 13.7 gn, 2500psi, 47.5" barrel
1884 fps, .278 cal, 13.7 gn, 3700psi, 47.5" barrel
1938 fps, .278 cal, 10.3 gn, 4000psi, 47.5" barrel

In the .278 cal, the first 2 shots used an o-ring seal and the last 3 shots used Teflon seals in a clean, dry, bore. The Teflon seals weigh about 1.2 grains more than the o-ring seals and make it difficult to keep the weight of the projectile low.  Also of significance is the fact that shots are all so close to the predicted velocity, regardless of the type of seal.  One might surmise that the seal friction and resulting drag, along with any aspect of combustion of the Buna-n o-rings are rather insignificant. 

Next order of business before the nitrogen is utilized, is figuring out a way to make the projectile lighter weight.  The last projectile was only 10.3 gns, but it really needs to be closer to 8 grains.

Here are  pics of the last last 2 shots, 1884 fps and 1938 fps.

[MichaelM]

I remember a few years ago watching some college kids launching ping pong balls through several pop cans using a setup that that placed the barrel in vacuum and then using relatively low pressure filling the pressure chamber with air till a thin layer of plastic ( like garbage bag thin) burst sending a ping pong ball with enough velocity to blow right through soda cans ....... theory being the vacuum pulled the projectile while the pressure pushed it to reach speeds......

at the sake of sounding ignorant(which I mostly AM when it comes to this type of maths) could something like this be happening to help with the velocities? wouldn't the atmospheric pressure in the barrel be at a relative vacuum to the pressure chamber making for a lot more force being applied to the projectile both pulling and pushing?


sorry if thats been covered.... I will admit I glossed over a few pages in this thread as the maths was making my eyes go a bit crossed at a few points LOL!!

[39M]

Could you not drill out the inside of the bolt to lighten it?

[39M]

Don't have a formula or graph, but I'd guess that exceeding the threshold is accomplished by changing the environment through a higher pressure than which the threshold was originally theorized.
Just as we have a so called constant speed of gravity on earth, if the mass of the gravitational force were increased so would be the speed of gravity.

[lloyd-ss]

I remember a few years ago watching some college kids launching ping pong balls through several pop cans using a setup that that placed the barrel in vacuum and then using relatively low pressure filling the pressure chamber with air till a thin layer of plastic ( like garbage bag thin) burst sending a ping pong ball with enough velocity to blow right through soda cans ....... theory being the vacuum pulled the projectile while the pressure pushed it to reach speeds......

at the sake of sounding ignorant(which I mostly AM when it comes to this type of maths) could something like this be happening to help with the velocities? wouldn't the atmospheric pressure in the barrel be at a relative vacuum to the pressure chamber making for a lot more force being applied to the projectile both pulling and pushing?


sorry if thats been covered.... I will admit I glossed over a few pages in this thread as the maths was making my eyes go a bit crossed at a few points LOL!!

Hi Michael,
The vacuum on the ping pong ball keeps coming up but I never did the math on it to see what was going on.     If you pull a vacuum in front of the ping pong ball you really are just applying a pressure of about 14.5 psi (atmospheric pressure) behind it.  The PP ball weighs 2.7grams = 41.7 grains.  The caliber is 1.575.   If you use a 24" long tube/barrel you have the equivalent of a 766cc tank in front of the PP ball.
Doing the math with my spreadsheet, because the PP ball is so light compared to its caliber, at the end of the  24" barrel, it should be going about 460 fps.  You better be about 20 feet back from the table because that is a 313 mph serve! 
Basically, it is 1.575 cal, 41.7 gn, 766 cc dumptank at 14.5 psi, using a reasonable 60% system efficiency, gives you 460 fps.  Unfortunately, adding that extra 14.5 psi on top of 3,000 or 4,000 psi in a regular pcp, wouldn't be noticeable.
Lloyd

[match]

I would go back to the Buna-n O-ring projectile and just make sure to clean the barrel well after each shot.

A very light lube would help in the cleaning - it should not have much effect on the actual velocity. 

[lloyd-ss]

After I get the nitrogen, I will be switching back to a size 007 o-ring, possibly Aflas in an 80 duro.

I also have to re-do the breech end of the barrel.  While deburring the entrance to the breech, I managed to get it oversized several thou for about a half inch.  The tube material was softer than I am used to.  That is why some of the projectiles were so long.  They needed to reach in past the oversized bore to get to the proper diameter for sealing.   

[rsterne]

I have to take my hat off to Scott for suggesting that the missing 30% is changing to heat, and offsetting, or even reversing, the Adiabatic losses during the shot.... I agree that the spreadsheets could be revised to take into account this effect, predicting the temperature at each increment.... I would like to point out that not only does Z change according to temperature, but also it is not a linear function relative to pressure because of the VanDerWaals effect.... In fact, at 6000 psi, the air density is 20% less than predicted by Boyle's Law.... I have been looking at that the past couple of days, and when used in Steve's formula (the 172 constant is incorrect because of a missing PI/4 term in the numerator), it raises the predicted maximum velocity, and the higher the pressure, the more the effect....

The correct constant in the above equation is 194, but the value of "Q" must then be changed to include PI/4, with Q being the actual volume of the barrel (CI) times the pressure (PSIA).... In addition, as I mentioned, "Z" decreases as pressure increases, although we can ignore it below 3000 psi, as the error at the pressure is only about 4%, increasing to 13% at 4500 psi and 20% at 6000 psi.... I have a version of the equation where "Z" is calculated from the equation of an Excel Trendline fo a plot of Density vs Pressure....

While doing the detailed calculations in increments (ie spreadsheet integration) is the proper way, I think a quick estimate of the amount of heat generated by that 30% loss in energy (conversion to heat) is in order, and then using the specific heat of air at pressure, it should be possible to calculate the temperature rise.... I am NOT that familiar with Thermodynamics, but hope to get a rough idea of the magnitude of that effect later today....

Oh, and Lloyd, congratulations on two more high speed shots.... I didn't mean to lose your real-world accomplishments in the shuffle.... 

Bob