Sonic Choke? Maybe not. 2162 FPS

[phoebeisis]

Lloyd
Thanks for the quick response
Version 3 sounds like a good plan.
Your machined projectiles were/are a lot of work
Thanks
Charlie

[lloyd-ss]

Here is a rough schematic of Max Vel Version 3.  This is similar to the pif-paf valve that LV showed, but with a central difference.


Balancing, or more correctly, un-balancing the ports and piston areas is the trick to getting this to operate properly.  There are design similarities to some paintball markers, but in general, the circuitry used in paintball markers deals with a few hundred psi and cannot be made robust enough to work with 3 or 4 thousand psi.  The tubular valve with the fixed solid center core (patented), shown in the schematic,  makes this valve more controllable by greatly reducing the reaction forces that the HPA applies to the valve.

Here is how the valve works:
The red piston (tubular valve) is the main moving part.  It is basically a double acting piston, that can shuttle between an open and a closed position.  The tapered front end of the valve can seal the breech of the barrel when it is forced tightly into it.  The front end of the valve is surrounded by the working volume of the HPA reservoir.  The rear of the valve also has the force of the HPA against it via a small bleed hole from the HPA reservoir.  There is a small knock-open valve (pilot valve) that provides a quick exhausting of the rear piston chamber. 
In the closed position, the force of the HPA on the rear annulus area of the piston (the end that is away from the pellet), is somewhat greater than the force on the outside margin of the tapered valve face.  This force imbalance keeps the valve closed.  The amount of force imbalance is based on the force necessary to keep the valve from leaking.

The blue center core can be removed when the gun is under pressure, thus it could actually be used as a bolt, allowing loading from the rear.

To fire the valve, the small knock open valve is hit, and it drains the rear piston chamber instantaneously.  The knock open valve drains the rear piston chamber much faster than it can be refilled through the tiny bleed hole from the front HPA reservoir.  The exhausting of the air from the rear piston chamber drops the closing force so that the opening force on the front margin of the valve  forces the valve rearward and cracks the valve open at the barrel breech. As soon as the valve is cracked, the pressure imbalance doubles and the valve slams open and fires the pellet.  Lag time is virtually nil. 

To end the cycle, once the tiny knock open valve closes, it allows the pressure to build back up in the rear piston chamber.  This increased force on the rear of the piston tries to close the valve, but the pressure imbalance from the front of the valve remains too great until the projectile leaves the muzzle.  Once the projectile leaves the muzzle, the pressure in the barrel drops enough such that the valve will close.   
This particular variation of the valve is well suited to a function as dump valve and will provide high velocities.  Getting it to CLOSE fast enough is the problem.  Easy to open..... difficult to close.  This is not a problem for the high vel gun, but in applications where air efficiency is required,  controlling the closing strictly by passive means is difficult.  Active circuits can solve the problem, but their incorporation is not easy.

Just as a side note, if you study the schematic, the center of the 3 visible o-rings might seem superfluous.  However, it serves to reduce friction and galling between the sliding parts, and also is needed to control the bleed re-fill rate to the rear piston.
 
The actual design is farther along than the schematic shows, and building will start very soon.  I have made several styles of this valve over the years but am keeping my fingers crossed for this new application.
Lloyd

   

[Scotchmo]

Lloyd,

That last video was very thorough. I won't post it on the yellow as there is no urgency. They should spend some time chewing on what they have recently heard/seen. I will revisit it over there in the future. At some point, for a demo movie, you should cut to a clock, after the fill, and then cut back several hours later to show the temp gauge had fallen back to room temp. That would eliminate one other point of contention for the skeptics.

For the next version that has an actual valve, have you thought about round projectiles for testing. Delrin balls, ABS .177 BBs, airsoft balls, etc. An actual valve and a projectile that could be purchased would simplify when doing multiple shots.

On another note - the spreadsheets:
After some research on fanno flow, I found there is a known limit at the inlet. It is not a velocity limit perse, but it would limit the mass flux of the system. The limits of the inlet into a straight pipe is Mach 1. Or 1128fps for 70F air. Knowing this we can adjust the conditions in the barrel behind the pellet to more closely match reality.

If we ignore all friction and fudge factors, once the velocity down the barrel reaches 1128fps, pellet is at a choke point. Mass flux cannot go higher. So the inlet is maxed out. In order for velocity to continue increasing, for the next increment, we start to expand the air behind the pellet to maintain the same force. The temp will need to rise in that increment to account for the expanding air. And that continues all the way out. Mass flux is choked, and in each subsequent increment, velocity continues to increase, density decreases, and temperature increases. I think this will accounted for a big chunk of the inefficiencies from friction without having to actually use any friction coefficients.

And I think I can do this without having to start from scratch.

[Buldawg76]

Lloyd
I like the design of the knock open valve in your drawing and it appears to be a very simple operating system in concept but as you said the subtle differences in pressures on either side of the tubular valves will be the key to its proper operation and rapid release of the HPA chambers full pressure in an instant to truly act as a full dump valve.

It will be some very subtle fine tuning to get it working as needed but I know you will get it working better than expected and further impress us all with your achievements here in proving that there is "maybe" a limit to how fast a projectile can be accelerated with HPA or it may be all depended on just how much pressure and for how long it acts on the projectile that is truly the only limit in the velocity threshold.

I am curious to see if commercially available ammo when used in the max version 3 valve can withstand the velocities as well as forces placed on it by the full dump system without damage or disintegration from the hyper velocities it will be subjected to that is far above the norm for its intended use.

Keep up the great work and as Bob has said "To infinity and beyond" 

Mike

[lloyd-ss]

Mike,
I am not sure how the pellets will hold up, or if I will need to stick with cast bullets.  My guess is that if I am going to eventually incorporate any accuracy testing into this set-up that I need to stick with bullets.  The fastest I have ever shot a pellet was a .25 cal H&N match wadcutter 21.6 gn, 1354 fps, using a 24" LW barrel.  I still have that barrel.  That was shot using a valve similar to the one in the schematic above.
Lloyd

[phoebeisis]

Lloyd
Wow-cool setup.
If you use a common caliber-
firearm or pellet-it will certainly save lots of work-
lotta choice in firearm projectiles

[lloyd-ss]

Lloyd
Wow-cool setup.
If you use a common caliber-
firearm or pellet-it will certainly save lots of work-
lotta choice in firearm projectiles

Charles,
Agreed on the calibers.  Particularly pistol calibers where there might be a better selection of lighter weight bullets and molds.
Lloyd

[Scotchmo]

I think I just made some big breakthroughs on the spreadsheet. I was able to keep the integration by barrel increment, but I switched all calcs to metric, so as to make it easier to keep things straight. For familiarity, I still allow some of the inputs in imperial units, and I have a separate output sheet that displays the familiar imperial units. I was up late last night working on it, and back up early this morning. My mind too busy, I could not sleep.

I'm using the "OpenOffice Calc" spreadsheet. Spreadsheets sure have a lot more capability than when I once used them. I learned about it's logic functions and conditionals last night. It's almost like a complete programing language (I did about 15 years of duty heavy LISP programming). And this morning I discovered an iteration tool in the spreadsheet that simplified the whole process of what I was trying to do. It took care of the recursion errors, without having to reorganize the spreadsheet. I was contemplating a very deep multi-layer spreadsheet when I stumbled on that capability.

My solution to the simulation model was to assume a fanno flow model with the addition of a pellet mass at the front of the air stream. Sounds simple enough.

Bottom line - it is matching Lloyd's data very closely, with no additional friction coefficients or fudge factors at this point. I may add a small friction or fudge factor later. It won't take much. The predicted velocities are running just a little ahead of Lloyd's tests. And that makes sense because I'm not including any pre choke-point friction. Up until Mach 1 at the inlet, the only resistance is force needed to accelerate the mass of air and pellet. I can also display the temperature behind the pellet at any point in it's travel down the barrel.

There is a "Sonic Choke". It limits the mass flux to Mach 1 at the inlet. Carry that mass flux through the whole simulation model and voila!

It still needs refinement and further checking, but it appears to be a much closer simulation than any of my previous versions. I'm jazzed!

I also shoot FT competition and our small club hosts some big matches. We are hosting a big piston FT match this coming weekend. And I still have lots of preparation to do. So I won't be able to work on the spreadsheet as much as I'd like for a awhile.

[lloyd-ss]

Scott,
Hot dog !  You pumped or something!!  That sounds really good.  Is that similar to what Bill G was talking about a while ago about maximum mass flow through an orifice, or something different?  So there is a limit to the maximum mass of air that can flow into the breech of the barrel, and that occurs at Mach 1?

The spreadsheet programs are so darn powerful and I think most of us only touch the tip of the iceburg as far as there capability goes.  I don't think I have used the iteration tool.  What is that?
Lloyd

[Scotchmo]

Scott,
Hot dog !  You pumped or something!!  That sounds really good.  Is that similar to what Bill G was talking about a while ago about maximum mass flow through an orifice, or something different?  So there is a limit to the maximum mass of air that can flow into the breech of the barrel, and that occurs at Mach 1?

The spreadsheet programs are so darn powerful and I think most of us only touch the tip of the iceburg as far as there capability goes.  I don't think I have used the iteration tool.  What is that?
Lloyd

Lloyd,

That may have been what Bill G was referring to. In the case of fanno flow, the orifice is the full diameter pipe/duct inlet, continuing with a constant diameter down stream.

We are now in the realm of computational fluid dynamics (CFD).

The combination of spreadsheet, numerical integration, and automatic iteration (thank you OpenOffice!). It's a powerful tool.

During my research, I noticed that AutoDesk has a CFD simulation program out now. Years ago, I used to write engineering applications for AutoCAD. In the late 1990's, AutoDesk bought out one of my main competitors and started to offer applications similar to mine. The merger mostly killed that part of my business. I kept at it for a few years, but found it increasingly difficult to compete directly with them. I continued to do some engineering work but, got out of engineering for hire about 13 years ago. All work that I do now is for my own enjoyment and knowledge.

Here is a good read on fanno flow. Skip the math parts (save that for later):

http://www.pua.edu.eg/PUASite/uploads/file/Engineering/Fall%202012/ME254/lec%2010.pdf

[Buldawg76]

Mike,
I am not sure how the pellets will hold up, or if I will need to stick with cast bullets.  My guess is that if I am going to eventually incorporate any accuracy testing into this set-up that I need to stick with bullets.  The fastest I have ever shot a pellet was a .25 cal H&N match wadcutter 21.6 gn, 1354 fps, using a 24" LW barrel.  I still have that barrel.  That was shot using a valve similar to the one in the schematic above.
Lloyd

Lloyd
Yea I am concerned about common pellets holding up to the hyper velocities as well but would be interested to see if indeed they do or not just as a curiosity stand point. It will likely require real bullets for the tests with the new valve setup if not still custom made projectiles also.

How did that H&N wadcutter hold up at the 1354 fps and was it shot into a trap or backstop that caused deformation or into a soft trap to try to save its integrity. just curious is all.

Mike

[Scotchmo]

I'm not the expert at charts (not like Bob). But I decided to try and produce a couple using the spreadsheet:





Here is the amazing thing: I have no fudge factors, not even any pellet friction. The only resistance in system, is the pellet and air mass. And the energy needed to push back the "Sonic threshhold". I assumed adiabatic expansion. Once I put in a little pellet friction, and maybe a little heat loss, we should be right on.

If you look at the graphs, you'll see the blue velocity curve, and a red temperature curve. The temperature actually falls a little at first and then climbs fast as it pushes through the "threshold".

The long barrel is predicted to perform well at lower pressures. Nearly 2000fps should be possible even at 3000psi, when using the long barrel, and .278, 7.5gr pellet. The "choke point" just pushes a little farther out.

The initial velocity spikes quickly. The starting velocity is the average speed over the first increment. I'm currently using 1mm increments. I can easily reduce the increment to get a smoother take off, but it cuts the barrel length proportionally, unless I add a lot more rows. And then it becomes cumbersome to scroll.

I can't stop playing with simulations. OK - I've got to start prepping for the match.

[rsterne]

So what I want to know, is if the air inside the firing chamber is 413*F, as Steve says.... how is it possible that the pressure doesn't drop more than 100 psi after 8 hours?.... Anybody?....

Bob

[Scotchmo]

So what I want to know, is if the air inside the firing chamber is 413*F, as Steve says.... how is it possible that the pressure doesn't drop more than 100 psi after 8 hours?.... Anybody?....

Bob

Bob,

It's not possible. But it does not matter.

The leader of the 1640 movement appears to be SteveNC. He won't entertain the type of question you asked because it does not support his theory. He would rather assume faulty experiments in every case. I'm starting to think he cares more about having others perceive him to be right, rather than seeking the truth. I'm done over there for now. It's entertaining and fun to play, but I'll hang out with the adults for now. Though I'll still be on the yellow/green field target forum.

They are spinning their wheels if they keep chasing that 1640rms theory. I don't know what it applies to, but it is not airguns. The limit is Mach 1 (1128fps), and it applies to the mass flux of the inlet. What happens after the inlet is variable.

If I'm going to be convinced otherwise, I need to be provided with more than I've been offered. And believe me, If I am wrong, I'll admit it and adjust my thinking. All I'm getting over there is remarks suggesting:
the firing air was a "hot potato"
semantic arguments
The chrony is inaccurate
the lights are bad
the chrony moves
etc.

Maybe Lloyd should use a PaperChrony. 

I'm ready to be here and let them stay in the "dark ages". They are having fun. I'll let them be.

[rsterne]

Scott, I see you are chasing the Sonic Choke idea, and I wish you luck with that.... Please note, however, that Mach 1 in 4000 psi air is NOT 1128 fps, but 451.1 m/s (1480 fps).... Here is the calculator....  http://www.wolframalpha.com/widgets/view.jsp?id=b63c87b0a41016ad29313f0d7393cee8

type "air at 4000 psi" in the box and hit "submit".... If you want a higher temperature, you can enter both.... eg. "air at 4000 psi and 200 C" will give you 516.5 m/s (1695 fps)....

I agree, what they are doing on the Yellow now is playing with being Trolls.... better to leave them behind and try and figure out a new theory that fits the facts, rather than stick your head in the sand....

Bob

[lloyd-ss]

So what I want to know, is if the air inside the firing chamber is 413*F, as Steve says.... how is it possible that the pressure doesn't drop more than 100 psi after 8 hours?.... Anybody?....

Bob

Bob,

It's not possible. But it does not matter.

The leader of the 1640 movement appears to be SteveNC. He won't entertain the type of question you asked because it does not support his theory. He would rather assume faulty experiments in every case. I'm starting to think he cares more about having others perceive him to be right, rather than seeking the truth. .................................................
.................(sections omitted by Lloyd)..................................................
the firing air was a "hot potato"
semantic arguments
The chrony is inaccurate
the lights are bad
the chrony moves
etc.

Maybe Lloyd should use a PaperChrony. 

I'm ready to be here and let them stay in the "dark ages". They are having fun. I'll let them be.

Scott,
I think you summed it up pretty well.  Its the old NIH philosophy... Not Invented Here.....
They are just being silly and having fun making up straw arguments against real evidence. 
I no longer wish to actively share any of my work with them.  It is disappointing, but they are a not worth my time. Let them believe what they wish.
Lloyd

[Scotchmo]

Scott, I see you are chasing the Sonic Choke idea, and I wish you luck with that.... Please note, however, that Mach 1 in 4000 psi air is NOT 1128 fps, but 451.1 m/s (1480 fps).... Here is the calculator....  http://www.wolframalpha.com/widgets/view.jsp?id=b63c87b0a41016ad29313f0d7393cee8

type "air at 4000 psi" in the box and hit "submit".... If you want a higher temperature, you can enter both.... eg. "air at 4000 psi and 200 C" will give you 516.5 m/s (1695 fps)....

I agree, what they are doing on the Yellow now is playing with being Trolls.... better to leave them behind and try and figure out a new theory that fits the facts, rather than stick your head in the sand....

Bob

Bob,

Thank you for that info. The higher the value of the speed of Mach 1, the higher the mass flux. So it will end up with less temperature gain, and higher velocity. I currently start with 70F air., and adjust Mach 1 for temperature. I'll need to look at your info more. If needed, I will definitely use a pressure compensated Mach speed after I figure it out. Regardless of what the speed of Mach 1 is, it is not a fixed obstacle and will change during the process. A higher speed for the Mach 1 "sonic choke" simply means that the choke happens farther downstream before it is encountered.

I re-ran it it with a fixed c=452m/s (1480fps). Not really how it will be, but this is it:



The simulation starts to raise the temperature but never goes very high.

I'll quit using the 1128fps reference and stick with the Mach 1.

[phoebeisis]

Scotmo/scott
Is that STEVE NC the same as this  STEVE-  the one who made this  post-yellow forum- Nov 12  2008-with the EXPLANATION of WHY it is IMPOSSIBLE to go over 1640 fps

I'm reposting the yellow forum 1640fps explanation
-  Steve yellow forum Nov 12 2008- of why anything over `1640fps  IS IMPOSSIBLE

anyway the below is their WHY  of why anything over 1640 is impossible
Steve seems to think the WHOLE column of air behind the pellet has to be accelerated along with the pellet-as if it was a solid rod-or a liquid like water
Sorry I was unclear back in  #361 when I first posted this cut from YELLOW FORUM NOV 2008 -this is Steve yellow forum nov 2008


Nope. No matter how high the pressure, ~1640fps remains an absolute limit for cold air.   November 12 2008, 4:24 PM

This simply because as pressure rises, so does the mass of the air filling the bore behind the pellet. Since this air has to be accelerated along with the pellet, as pressure rises without limit, eventually all of the available energy goes to accelerate air and not lead.

The velocity at that limit is 1640fps.

To go any faster requires, not more pressure, but a propellant with a lower molecular density.

Steve


Charlie-phoebeisis-once again-
Steve- NOV 2008 seems to insist that the  barrel air column must  be accelerated as much as the pellet-
like it was a solid rod-or a water column-
Steve is clearly wrong-to go faster requires LIGHTER molecules-not because they are less dense-but because they are FASTER-
NASA used H2 in their super guns not because it was LESS DENSE- but because at the same TEMP-they are faster than heavier molecules-higher peak speeds higher average speeds-
I would bet that the N2 and the H2O might be slightly more concentrated at the leading edge of the column-actually I wouldn't bet-since it is a pure guess-but it a one molecule thick disc  of air was released into an empty  barrel-no pellet in front-no air in front -H2O N2  would win the race over O2 CO2 etc

[rsterne]

Yes, Charles, this is pretty much the whole point of the thread.... to prove that it is possible to exceed not only the Speed of Sound, but also the 1640 fps RMS Molecular Speed of air at room temperature.... after all, RMS means Root Mean Square, which is essentially an average speed, and as shown by the Maxwell Speed Distribution diagram, many molecules are going much faster, some more than twice as fast.... Looked at from a couple of ways, it has been suggested that twice the RMS Speed may in fact be the limit, based on the speed of the molecules at the temperature present during the shot.... Yes, Steve_in_NC is the primary proponent of the theory that 1640 fps cannot be exceeded.... and rather than put any effort into suggesting a mechanism that could explain the high velocity shots done by Lloyd AND OTHERS, he is picking and choosing ideas to present that support his case, even though those same numbers create roadblocks for his ideas.... The latest was to propose that the air inside the shot chamber was heated to 413*F on filling, while ignoring the fact that when left to cool for nearly 8 hours the pressure only dropped 100 psi, not the 1000+ psi that would occur had the temperature been that high.... The fact that the pressure didn't drop by over 1000 psi is proof positive that the temperature after filling was not elevated anywhere near that high.... In fact, the temperature of the air at the end of the fill can be calculated from the pressure drop as follows....

Starting pressure 3530 psi.... ending pressure 3430 psi.... ending temperature 72.1*F (531.8*R).... Starting pressure = (3530/3430) x 531.8 = 547.3*R = 87.6*F....

Had the temperature inside after filling been 413*F (873*R), the pressure after cooling would have been (532/873) x 3530 = 2150 psi....

Bob

[rsterne]

Scott, how are you calculating the maximum mass flow rate?.... During a discussion in another thread we multiplied the air density at the pressure (kg/m^3) used by the orifice area (m^2) times the speed of sound at that pressure (m/s) to get the maximum mass flow rate (in kg/s).... This was then derated about 5-10% to allow for losses.... You can use the calculator I linked to above for the speed of sound and air density.... http://www.wolframalpha.com/widgets/view.jsp?id=b63c87b0a41016ad29313f0d7393cee8/

For example, at 4000 psi and 20*C, we get Mach 1 of 451.1 m/s and a density of 302.7 kg/m^3.... for an orifice of 1 cm, we get....

451.1 x 302.7 x 1 x  PI/4 / 100 / 100 = 10.72 kg/sec.... which is 10.72 grams/mSec.... or 165.4 grains/mSec.... less a derating factor of course.... 150 gr/mSec seems about right....

The only way to increase the mass flow at that temperature is to increase the inlet pressure, it is completely independent of outlet pressure.... In addition one of the conditions for choked flow is a pressure drop of 47% across the choke.... Here is the discussion, if you haven't gone through it yet.... Jim_Hbar's Post #69 is particularly revealing.... http://www.gatewaytoairguns.org/GTA/index.php?topic=66737.60

How does that compare to what you are doing?....

Bob