About what psi is behind the pellet of a break barrel when it starts to move?

[subscriber]

Found link to paper on internal airgun ballistics:

https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

https://www.researchgate.net/profile/Domingo_Tavella/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns/links/552473050cf2b123c51740aa/Internal-Ballistics-of-Spring-Piston-Airguns.pdf

[George Schmermund]

A simple method of "pellet pushing" is described in the early posts of the CP-2 thread.  https://www.gatewaytoairguns.org/GTA/index.php?topic=137566.0

Any number of variations on the theme can be added. Also, consider all of the information offered up in the "hacking" threads as only being suggestive and not authoritative. There's plenty of room left for others to be creative with their own testing methods.

PS - I just looked at Subscriber's links posted above. I'll have to confess that I live in my own little world. Therefore I can make up my own rules... don't calculate what you can measure!

[subscriber]

don't calculate what you can measure!

You don't have to calculate what you can measure. 

When it comes to value, calibrated measurements are much more meaningful than calculations based on (often incomplete) theory.


In my first job, I remember calculating the total flow area between a piston rod and guide of a suspension strut; then predicting the effect of that leak-path on damper resistance.  As this incidental flow area was several times larger than that of the orifice used control low speed damping, I predicted that a rod to guide clearance that was too wide by .005" would drastically reduce the low speed damping, but choke-flow at high speed and do almost nothing to the high speed damping.  Actual damping dyno results showed the exact opposite to what I predicted.  A 25 year veteran with the company told me that my "theories were no good".

I thought about it for a while; then figured out why the hydraulic system behaved that way:  The total area of the rod to guide leak path was large due to the 63 mm rod circumference.  However, the flow path was still very narrow and very long compared to the control orifice.  This meant that almost all the flow area was subject to "boundary conditions", where the oil attaches to the rod and guide surface.  Only a very small area in the center of that annular flow path could actually shear the oil fast enough to matter.  So the actual additional leakage flow volume was small.  Hence, hardly any effect on low speed damping.  At high speed, the pressure was great enough to "strip" the boundary layer completely as if it were an overloaded fine pitch thread; resulting in a massive additional leak path.

For those of you who had their eyes glaze over two paragraphs up; this flow area example is similar to air flowing through 10,000 .005" diameter holes, VS  one .5"  diameter hole.  They may have the same area, but the small holes will flow way less air because almost all their area represents "edge conditions".  Now, instead of an orifice plate, stretch that to an inch long tube; both .5" in diameter; and .005" in diameter.  Then the flow losses for the 10,000 small holes would be even higher.

In the end, I said to my wise colleague; my theory was not wrong, it was incomplete.  So, while first order calculations are a good starting point, but due to the crudeness of most math models, nothing beats empirical results...

[lloyd-ss]

don't calculate what you can measure!

You don't have to calculate what you can measure. 

When it comes to value, calibrated measurements are much more meaningful than calculations based on (often incomplete) theory.


In my first job,..........................................
................................................................................
In the end, I said to my wise colleague; my theory was not wrong, it was incomplete.  So, while first order calculations are a good starting point, but due to the crudeness of most math models, nothing beats empirical results...

Subscriber, some good words of wisdom in that post. I approach it slightly differently by saying that I only do the testing to verify that my calculations were correct.
And I like the "not wrong, just incomplete" euphemism. One of the designers where I used to work was always diligent in applying labels to just what kind of fix was being done. So, was it a "correction or a change?"  And the ever popular "opportunity for improvement", LOL.


Opposed piston, balanced springer.
 I don't think it was mentioned, but our own QVTom built a beautiful opposed piston springer, starting it when he first became a member. The gun was beautiful but Tom wasn't happy with the performance. Tom is one of the sharpest guys I know , and if he didn't get it working to his satisfaction, it had to be a tough challenge!

Compression pressures and temperatures
This thread is about springers (mainly), but any and all compression of air raises its temperature. What happens to the heat, where the heat goes, is what makes the difference in usable power output. Isotropic Isothermal (edit)  or Adiabatic processes, or a blend of the two.  (Thermodynamics was a weapon used to weed out wannabe engineers.    ) And other possibilities, too. The subtle variations in these thermodynamic processes can be really difficult to calculate because sometimes (often) you can't be 100% sure of what process is occurring.  That is where the quote at the top of this post really rings true. You can try calculating the heat, but if you can accurately measure it, you are so much better off.

This compression heat occurs in pcps, too, but there is a continuum of temperatures throughout all the plumbing in a pcp when the gun is filled or shot. Pretty much impossible to figure out exactly what is going on. I had an eye opening experience regarding compression heat in a pcp recently. The kind where you have to sit down and ask yourself what the heck just happened. But i'll do a different thread on that.

Domingo Tavella, who is referenced earlier in this thread, got me to thinking about heat of compression and what it does in airguns. You might say he poked me with a stick.     Bob is good at that too, LOL. For example, i didn't really understand that a multistage air compressor will output cooler air than a single stage air compressor with the same input and output. There is always more to learn!



Compression pressures and temperatures in springers
But back to Earl's original question about what kind of pressures are inside a springer. (BTW, I met Earl at one of the funshoots in Kentucky when he was nice enough to buy lunch for several of us. A real gentleman, and a mountain mover, too, but that's another story.)
Many years ago I had a Gamo 1000 springer. It was powerful, but I knew I could make it MORE powerful. Well, I messed it up pretty good  , but I did learn a lot  .

When air is compressed it heats up. Its the law, LOL. If it is compressed very slowly, all the heat dissipates into the surroundings, and the air stays the same temperature.  That is isothermal (more or less) compression. The opposite process is where all of the heat stays in the compressed air because everything happens so fast that the heat can't get out of the air. These two processes yield very different results in temp and pressure. The springer process is arguably 100% adiabatic and the air stays hot and delivers a lot more power to the pellet.

Here is how. We've all accidentally fired a springer with no pellet loaded and cringed when the piston slammed violently against the end of the compression chamber with a big bang. The piston is traveling full distance.  The Gamo had a 1"diameter piston and a 3.7" stroke. The average spring force during the 3.7" stroke was about 140 pounds, so the piston was moving pretty fast when it crashed into the end. But when a pellet is loaded, and acts like a cork in the barrel, it is much smoother and it feels like the piston either does not bottom out, or that it hits gently. Let's say (guess?) the piston comes within .010" of bottoming out. And I think we can also assume that the pellet has traveled at least an inch or 2 down the barrel.
If we do a quick pressure calc, P1*V1=P2*V2, the pressure looks way too low. Using the position of the piston at .010" of full stroke and the pellet 2" down the barrel, we calculate a pressure of only 515psi (right end of row 10, below). Kinda low. (Sorry, but in the chart I used a .22 barrel instead of a .177, but the concept is the same.)

So, using a benign isothermal process where all the compression heat just vanishes, we get only 515 psi and low power. But the heat can't just vanish!
So, if you calculate it as an adiabatic process using a gamma exponent of 1.4 (standardly used for air) you get very different results.
If you use the same .010" piston headspace and pellet 2" travel down the barrel, you get 2151psi and 1771F for the air. Maybe that is too aggressive, but maybe not. The chart shows different combos of head space and travel down the barrel. Sorry for having to make so many assumptions, but I think somewhere within these calcs is the real-world performance. But its hard to find.

For anybody interested, here are the final pressure and temperature formulas that I used in the chart. (CR is compression ratio)

P2= P1*(V1/V2)^1.4    T2 Kelvin=297K*CR^0.4

If you've stuck around to this point, you deserve a medal!
It's good to be back.
Lloyd

[Tom Tucker]

The math is way over my head, but I still stuck around, does that count?

BTW, I've found this program to be a heck of a lot of fun to play with.  Not sure how accurate it is, but it seems pretty close.

http://ballistics.eu/airgun.html

[lloyd-ss]

Tom, Sticking with it always counts!
That calculator is very interesting; thanks for providing the link, and I will definitely check it out.
In the example you cited, it looks like the pellet broke free at 7ms, and after quite a bit of pressure had built up. Nice.

[Tom Tucker]

LOL!  The fun part is being able to zoom in on the graphs.  In this example, the pellet begins to move at 6.4ms, and peak pressure is achieved at 7.1ms, at which point the pellet has traveled about 17mm.  The piston comes to a stop 0.05mm from the end of the cylinder before bouncing backwards 1.6mm.

[subscriber]

I had an eye opening experience regarding compression heat in a pcp recently. The kind where you have to sit down and ask yourself what the heck just happened. But i'll do a different thread on that.

I wish you could give us a hint, Lloyd.  Looking forward to it regardless!

If you've stuck around to this point, you deserve a medal!

You posts are a gift; with cream on top and underneath.  Who wouldn't want to make it to the end?

[subscriber]

BTW, I've found this program to be a heck of a lot of fun to play with.  Not sure how accurate it is, but it seems pretty close.

http://ballistics.eu/airgun.html

Wow, Tom.  Someone put a lot of effort into creating that program.  Then sharing it for free.

[grand-galop]

LOL!  The fun part is being able to zoom in on the graphs.  In this example, the pellet begins to move at 6.4ms, and peak pressure is achieved at 7.1ms, at which point the pellet has traveled about 17mm.  The piston comes to a stop 0.05mm from the end of the cylinder before bouncing backwards 1.6mm.

Wow!  That is more than a starting point  as far it goes in the calcalations..  If we take in account  it is made for steel spring piston, the margin of error would be negligable..

[lloyd-ss]

Tom Tucker,
That program you noted in reply #44 is a great find! I just downloaded it and am looking forward to trying it out.
It looks like there is some connection between the author of the free airgun springer program and some of the NATO STANAG specs for interior and exterior ballistics of all sorts of military guns.  I know that, in general, the development and revisions of the STANAG specs is well financed and the quality of the document is excellent. Maybe the free airgun programs are somehow an unofficial off-shoot from someone on the team of the STANAG specs? Whenever you can find a subject that is covered by an existing, accepted, specification (Mil, Fed, STANAG, DIN, ASTM, etc), it  is certainly worth paying attention to.