Sonic Choke? Maybe not. 2162 FPS

[lloyd-ss]

Lloyd, In the two shots you had a 31fps difference. This setup is meant to eliminate variables and make things consistent and predictable. So is this 31 fps seem within expectations? I guess it's only like 1.8% variance.

I'm not criticizing your setup or your efforts in any way. With all things considered, it seems your setup should offer closer shot variance? Am I just picking at stupid details or is this 31fps good for the speed of the projectile and I'm really just seeing nothing out of the ordinary?

Happy to answer, no offense taken at all.  I think 1.8% is actually very good for this project.  1.8% is generally considered pretty decent on an airgun, correct?   On a 900 fps gun that is only +/- 8fps.

Think of these numbers.  For the air pressure, I am relying on the compressor shut off, but also reading the gage. With markings every 100 psi, reading to +/-30 psi is good, and that is a total of 1.3%.  The gauge itself might have a 0.5% repeatability.  With the weight of the projectile, the scale I have has a resolution of .1 gn.  With a 7.5 gn projectile, there is another 1.3%.  On the machining of the projectile itself, the center core that controls the o-ring compression is .115 in diameter.  If I am off .001, that is almost 1% which will affect the velocity somehow.  So if you add up the percentages that I have mentioned, 1.3 + .5 + 1.3 +.8 =   3.9%.  So yes, I am very happy with a 1.8% variation, especially since it spanned the high and low side of my spreadsheet calculations.

Lloyd

[rsterne]

IMO, this disproves the idea that the velocity of a PCP is limited by the RMS velocity of air molecules, which is ~ 1650 fps at 70*F....That is, after all, an average velocity, and some of the molecules in fact are travelling many times that fast.... Ever since I saw a video of a .177 Condor shooting a 1.7 gr. plastic Sabot at 1705 fps, I pretty much discarded the concept of a hard speed limit.... I just double checked the 1605 fps shot that Lloyd had previously at 4500 psi with a 14.5 cc chamber and 10.2 gr. bullet, and using 1 lb. drag and 2 lbs. breakaway, the spreadsheet balances at 70% efficiency, which is quite remarkable.... Changing the bullet weight to 7.5 gr. and the reservoir to 26 cc, and leaving the efficiency at 70%, the velocity prediction is 1745 fps.... To me this indicates that we are not approaching any particular limit, or you would expect the efficiency "fudge factor" to be dropping off rapidly.... Lowering it to 68% predicts 1719 fps.... so the average of the 2 shots is 69%....

My thoughts are that if a bullet can be made with a lower Sectional Density.... or by using a larger reservoir or longer barrel.... the velocity can be pushed even higher.... We are approaching velocities where increasing the chamber size pays very small dividends.... Using the average efficiency of 69%, I get the following....

26 cc = 1732 fps
100 cc = 1752 fps
400 cc = 1758 fps

On the other hand,  lightening the bullet is more productive.... using the current 26 cc reservoir and 69%....

7.5 cc = 1732 fps
7.0 gr. = 1756 fps
6.5 gr. = 1782 fps
6.0 gr. = 1809 fps

The most productive method for increasing velocity, however, is a longer barrel.... using the current 7.5 gr. with a 26 c reservoir at 69%....

23.3" = 1732 fps
30.0" = 1810 fps
36.0" = 1865 fps
42.0" = 1907 fps

It would take a 6 gr. bullet, using a 100 cc reservoir, and a 42" barrel, to break 2000 fps, assuming the efficiency could be held to 69%.... Incidently, if you hold the SD constant, the caliber doesn't affect the results.... The only benefit of going to a larger caliber is if it would be easier to make a bullet with a lower SD.... or if the efficiency increases because of lower air resistance in the barrel (or other reasons)....

BTW, I agree that 1.8% ES is pretty good for such a test, particularly when the results agree with theory.... Once again. Lloyd, congratulations on a job well done !!!

Bob

[lloyd-ss]

Bob,
You are teasing me with the numbers, so why stop now, huh? 
It looks like I can get some smooth bore .245 I.D. tubing in long lengths for a whole lot less than a barrel, so I might try that. Hopefully it won't need to much polishing in the I.D.   A 9.3 gn projectile in .245 would have the same S.D. as the 7.5 gn in .22, so I would have to make  a projectile lighter than 9.3 gn.  I think that is possible. 
The plot thickens, he, he, he. 
Lloyd

[rsterne]

Lloyd, for a constant SD, with a cylinder, the length stays constant.... If the cross-groove remains the same diameter, you should be able to shorten the length.... Of course you can always bevel the base, it looks like there is material on the back, on the side opposite the notch, that is just going along for the ride and not required for strength....

Bob

[Airsnipe]

Lloyd, thanks for the explanation. I knew it was very good for a normal airgun. Thinking of all the variables you eliminated in the setup, I was just shooting expectations to high in my mind. Like you explained, there are still several variables you are contending with.

Great job on the whole project and I can't wait to see more. This topic is fascinating to say the least. I'm glad this community has such great and talented people to discover and share this info.

Lets see Mach 2  I have faith 

[Scotchmo]

Scott,
...

Regarding your fluid friction Xfactor, I am assuming that you use that to reduce the force in the F=ma calculation of each iteration, correct?

Lloyd

Exactly.

At zero pellet friction:

If I look at the 23.0" position in the spreadsheet, the remaining pressure is 2431psi, and velocity is 1721.0. At that point, the empirically derived fluid friction is:

2436 x 1721/100000 = 41.9 lbf.

For 26cc chamber volume, the fluid friction peaks at about 7"-8" down the barrel and then slowly falls off. Pressure is falling, and velocity is increasing only marginally. A larger dump chamber results in more pressure farther down the barrel, but also more fluid friction and more air mass.

At .22 caliber and 23", not a lot to be gained with a larger dump chamber. I'm assuming that 4500psi is about the limit of your equipment.

We need a longer barrel, and/or lighter pellet to get explore 1800fps, 1900fps, and 2000fps.

My spreadsheet predict 1888 FPS if you increase the barrel length to 48" and change nothing else.

40cc dump, 4500psi, 48", .25 caliber, 9.2gr = 2006 FPS according to my spreadsheet.

My fluid friction formula was derived from your .22 caliber barrel, and barrel diameter is not factored in (yet), but hopefully it's close enough.

Drawn over mandrel tubing might have a consistent enough internal diameter to use as a smoothbore barrel.

In the interest of advancing the knowledge, I'm willing to help fund the 48" "barrel blank". It will be about $74 shipped. Anybody else want to chip in?
https://www.speedymetals.com/pc-3423-8242-58-od-x-0188-wall-dom-steel-tube.aspx

[rsterne]

I'll throw in $25 towards that.... just give me a PayPal address...

Bob

[Scotchmo]

Lloyd, thanks for the explanation. I knew it was very good for a normal airgun. Thinking of all the variables you eliminated in the setup, I was just shooting expectations to high in my mind. Like you explained, there are still several variables you are contending with.

Great job on the whole project and I can't wait to see more. This topic is fascinating to say the least. I'm glad this community has such great and talented people to discover and share this info.

Lets see Mach 2  I have faith 

Mach 2 (2232fps) is a tall order. For 7.5gr, .22, we'll need to increase the dump chamber to 300cc and barrel length to 16 feet.

A lighter pellet would help a lot but may be tough given the restraining/releasing mechanism and limits on material strength. Next step should probably be a 48", .25 smoothbore. And a more modest goal of 2000fps.

Lloyd, I'll throw in $25 as well. PayPal address?

[Scotchmo]

Here is an interesting conundrum:

No matter how long of a barrel, or how much pressure, or how light a pellet. I can't reach Mach 3 (3300fps) in the spreadsheet.

[lloyd-ss]

There is no backing down now is there?   I am up for it. But i am in the airport now and won't be home till monday. Not really necessary for anyone to chip in,but if you do, it is lloyd500 @hotmail.com
Thanks for the generous offers. This is one of those projects that just Needs to be done.
Lloyd

[Hobbyman2007]

Congrats Lloyd on your achievement. I have been reading this since the beginning and know how much work went into it. cant wait to see what a smooth bore and bigger caliber might bring.

[rsterne]

Done.... have a good trip !!!!

Bob

[Bill G]

Man!  Nicely done Lloyd.  I'll be off the grid as of Monday.  I have to travel for work and its a pain in the rear to keep up on the smart phone.  Now this is getting over the top interesting and I'll be all torn up in suspense.  My numbers aren't as close as you and Scott with out fudging the efficiency manually.  I'm working on integrating a regression that will adapt the efficiency based on Port, psi and Barrel length.  Still rolling the ideas around in my head as too how it all influences each other.  Based on the models that I've run, the port and psi seem to have the largest influence with regard to flow efficiency.  I'm really at the limit of my mathematical ability so it slow going.  LOL I had some old text books out doing remedial learning the other day.  I might be getting too old for this.    Scott thanks for the lead on the flow friction, I'll see about implementing that.  Looking forward to what the future offers.   

Great job!
Bill
Bill   

[K.O.]

Hi Lloyd I have been thinking about this but have not read thru this thread yet...

but

I have a theory that no this does not disprove sonic choking but it shows how sonic choking can be beneficial by inducing Sonic packing...using the shock wave that is produced by supersonic flow...

post #123 in the below thread about theoretical max velocity (rather than copy and paste)...


http://www.gatewaytoairguns.org/GTA/index.php?topic=94054.msg994804#new

?

[K.O.]

ok just read the thread and  Bills explanation of sonic packing is what I was trying to explain... I think...

[Bill G]

My lack of terminology leaves me wanting when it comes to articulating my thoughts on the subject.

[lloyd-ss]

OK folks, with our feet back on the ground, we are gearing up for the next round of testing with a
                          Version 2 Really Really Fast Pellet Gun     

The previous, kinda clunky test bed, now dubbed Version 1, got us to 1745 fps, so a job well done. It had an offset cam to clamp the projectile on one side until the reservoir was at full pressure, and then the projectile was released.
That was .22 cal, 7.5 gn, 4500 psi air, 26cc dump tank, 23.3" barrel.  End result, 1714 and 1745 fps.

Version 2 uses a projectile that resembles a conventional pellet with a pinched waist, but again, has an o-ring for sealing. The clamping in Ver 2 is balanced and clamps from both sides on the waist of the pellet.  Release should not be so sluggish, LOL, and should be clean and crisp, and as my father used to say, the pellet should "take of like a ruptured rubber duck". Another advantage of Version 2 is that the pellets are a lot easier to make without the milled notch.  Also, they should be less susceptible to damage during launch.

The picture at the end of this post shows how the clamping will work.  The little levers will be external to the block, and the two rotating cam clamps will be made from 5/16 dia rod, similar to the single one in Version 1.  The levers might be switched to the forward position for "pinch them together to clamp", "release them to launch."

Here are the specs, as of this moment.
Barrel, smooth bore DOM steel tubing with a nominal I.D. of  .245". 
Barrel length TBD, but starting at approx 47" for maximum go power.  But that might be too long. There will also be a 36" barrel so that the gains from 36" to 48" can be determined. That barrel will probably be progressively shortened to collect additional internal ballistic data.
Pressure, up to 4500 psi air.
Pellet weight, approximately 6.5 grains, made of 7075 aluminum, with size 005 70 duro Buna-N o-ring.
Dump reservoir volume TBD, but at least 36cc, to equal 100% of the barrel volume.

Predictions
Based on the 1745 fps shot, if the new Version 2 shot were as follows:
.245 cal
6.5 gn
4500 psi
36cc dump shot

With a 24" barrel, I would be somewhat confident in predicting something over 1850 fps, although the spreadsheet shows 1882fps.
With the 47" barrel, the spreadsheet shows 2081 fps with the 36cc reservoir, but I wouldn't bet much money on that.
We'll just have to wait and see.   

I will say one thing, if Version 2 works as expected, I will bring it to the Funshoot in Kentucky this May. 
 
Here is the Version 2 pellet.




Here is a sketch of the mechanical set up for Version 2.



 

[Buldawg76]

Lloyd

I have faith you will indeed get to 2000 fps with version 2 and beyond.

Let the suspense begin !!!

Mike

[Scotchmo]

Right now, I'm on the deck of a cruise ship off of Catalina Island. Mai tai's, burgers, now I need a nap. I can't run Lloyds next configuration through my spreadsheet simulation. At least not until Saturday. But his planned setup should break 2000fps by a comfortable margin.

[rsterne]

I agree.... 2000 fps will soon be in the mirrors....

and I can't wait !!!

Bob