Sonic Choke? Maybe not. 2162 FPS

[rsterne]

Yes, extremely sharp tools are the key for soft plastics....

IIRC, UHMW is sensitive to humidity, and swells when it absorbs water from the air (which is does, it is hygroscopic, isn't it?)....  been a long time since I machined it....

Bob

[lloyd-ss]

OK guys.  I did it.
2031 fps, 9.1 gn, .278 cal, 47.5" barrel, 55 cc dump chamber, 4000 psi

There is more velocity to be achieved.  I still have yet to see any sort of limit at the velocities and pressures we are, or could be, using in our PCPs.

Here's the video.  I also put up a blurb in the PCP gate.
Lloyd

#

[rsterne]

WOW!.... Slightly better in relative terms than you did with the .22 cal @ 1745 fps....

GREAT JOB !!!

Bob

[lloyd-ss]

Thanks Bob.  I lubed the barrel with a little silicone oil and it left a pleasant burnt smell like at a race track.

[Buldawg76]

WooHoo Lloyd
I knew it could be done and at only 4000 psi so you still have another 500 psi to go up to your stated max of 4500 psi for another test and I believe you will break 2300 fps with ease.

Great job and superb results for all the nay sayers. Never a doubt in my mind.

Mike 

[Matt15]

Congrats Lloyd!!!

[lloyd-ss]

Thanks very much guys.  It is a lot more fun when you can share it.

This video was actually the 4th shot from this gun.  The very first shot was with a 15.3 gn projectile and went very well, the second was with an 8.5 gn and the o-ring extruded and popped.  The third shot was again with an 8.5 gn, but the projectile popped apart at the waist at approx 4000psi while still being clamped. I think about 7gns of it got launched, but because it was just the tip, it might have ended up cockeyed in the bore, PLUS, the air was effectively being flowed thru a .155 T-port which was the portion of the projectile that was still being held in the clamp.  I had drilled out the center to reduce the weight, but I guess I thinned the wall a tad too much.

Here is the data on all those shots.  I am comparing them to the previous 1745fps shot to see how they track based on my spreadsheet calculator's predictions.

Previous fastest shot:
1745fps,   7.5gn, .22 cal, 23.3", 4500psi, 26cc,  50.7fpe (full bore dia direct port)

1802 fps, 15.3gn, .278 cal, 47.5", 4500psi, 55cc, 110.3fpe (full bore dia direct port) (Predicted vel was 1804fps)

1778 fps, 7.0gn, .278 cal, 47.5", 4000psi, 55cc, 49.1fpe (.155 dia direct port, 31% of bore area) Projectile pulled apart and accidental release. (Prediced vel at full bore port was 2108fps, so this shot was at 84% velocity with a 31% port)

2031 fps, 9.1gn, .278 cal, 47.5", 4000psi, 55cc, 83.4fpe (full bore dia direct port) (Predicted vel was 2025fps, but if I misread the pressure gauge, 4100psi is a perfect velocity match. )

Looking at the numbers, I see absolutely no tapering off of the velocities.  No sort of ceiling or major speed reductions.  I would guess that we are working at velocities that are still well below anything that would cause such anomalies.

Lloyd

[rsterne]

I agree.... It appears we are lonely limited by barrel length and air pressure at these velocities....

Of course Helium would roughly triple the FPE.... *grin*....

Bob

[lloyd-ss]

Lucky for me, I do not have any helium available.   
Lloyd

[rsterne]

Titanium pellet to stand the heat of 3500 fps and the strain of the firing mechanism?....

Bob

[lloyd-ss]

Well, I do have some 110ksi yield titanium rod.   

[rsterne]

Hollowpoint it to reduce the weight further?.... *grin*.... Mach 2 on air (~2250 fps), here we come !!!

Bob

[Scotchmo]

as soon as you said "burnt smell" - red flag.

In order to make this a stick, there must be no question about combustion contributing to the exceptionally high velocity.

That is higher than I thought it would go after taking into account fluid friction. I'll adjust my model and am willing/desiring to see this new record stand. But the burning smell makes me suspicious of the results.

[lloyd-ss]

Scott,
The burnt smell was very mild, and from the o-ring friction in the barrel.  Like I said, burnt rubber smell like the burnout box at a drag strip, but faint.  The breech block itself is 1 lb 10 ozs of aluminum and absorbs most of the heat out of the air as the reservoir is filled.  I did not feel the aluminum with my hand specifically for heat during the fill, but I can do that next time I take a shot.  The only lube I used was a 30wt certified silicone shock oil made by LOSI for RC cars.  Given that information, I still hold that the smell was from a simple friction burn from the o-ring.  After all, o-rings are rated to maybe 10 fps in reciprocating applications, and this was 200 times that fast, so the o-ring is sacrificial.  I do not see how any combustion could be contributing to the shot.

I've shot wooden dowels out of barrels and there were scorch marks on the dowels from the lands, and a burnt wood smell, but certainly nothing that contributed to the energy in those shots either.  I see the same situation here, just rubber instead of wood. 

Scott, Give me your thoughts on this, in particular what you think might alleviate your concerns. 
Lloyd

P.S.  I could try fabbing a projectile with a Teflon seal, which shouldn't leave a smell from the friction burn.

[rsterne]

I would think that the Silicone lube would be pushed ahead of the O-ring, and the likely cause of the smell was the O-ring itself, smoking after leaving the barrel.... but anything is possible.... Interestingly, when I run the numbers through Lloyd's spreadsheet, leaving the efficiency at 70% like the 1745 fps shot, I get 2024 fps, which is within 0.3% of the results.... ie it is right where it should be, IMO.... The first shot with the new barrel, at 3500 psi with a 15.3 gr. pellet was within 0.1% of predicted....

I think any temperature effects will cancel out.... If the temperature is higher after the fill and before the shot, the molecular velocity would be higher, but only until the shot occurs.... Then Adiabatic cooling would drop the temperature of the air in the barrel and chamber by 53*C for a 2:1 expansion of the air.... I suspect if you could average out the molecular velocity throughout the shot it would be less than 1650 fps is anything, not more....

Bob

[Scotchmo]

Scott,
The burnt smell was very mild, and from the o-ring friction in the barrel.  Like I said, burnt rubber smell like the burnout box at a drag strip, but faint.  The breech block itself is 1 lb 10 ozs of aluminum and absorbs most of the heat out of the air as the reservoir is filled.  I did not feel the aluminum with my hand specifically for heat during the fill, but I can do that next time I take a shot.  The only lube I used was a 30wt certified silicone shock oil made by LOSI for RC cars.  Given that information, I still hold that the smell was from a simple friction burn from the o-ring.  After all, o-rings are rated to maybe 10 fps in reciprocating applications, and this was 200 times that fast, so the o-ring is sacrificial.  I do not see how any combustion could be contributing to the shot.

I've shot wooden dowels out of barrels and there were scorch marks on the dowels from the lands, and a burnt wood smell, but certainly nothing that contributed to the energy in those shots either.  I see the same situation here, just rubber instead of wood. 

Scott, Give me your thoughts on this, in particular what you think might alleviate your concerns. 
Lloyd

P.S.  I could try fabbing a projectile with a Teflon seal, which shouldn't leave a smell from the friction burn.

That's a tough one.

The heat sopurce is probably friction. Whether it's enough to ignite anything, I don't know.

With high oxygen content, anything that can burn can act as fuel. Silicon oil might have combustible carriers. Whether high pressure air and polymer or rubber o-rings would be adequate to add additional propellant, I don't know.

Conditions such as these make me think of a hybrid rocket motor.
https://en.wikipedia.org/wiki/Hybrid_rocket

2050fps gives 30fpe more than some sonic threshold proponents thought possible. If the process is being fueled by the o-ring, than the o-ring would deteriorate rapidly as percentages of it get consumed with each shot. If the velocity is repeatable using the same o-ring, that might rule that out as a source of combustion.

I'd use no lube for starters. Swab with dry patches.

If the same o-ring can repeatably shoot at anywhere near that velocity, without deterioration, and without lubes, that would eliminate any combustion products that I would have flagged.

Another option would be to use nitrogen instead of air. Similar properties as far as density/heat-content/compressing/expanding but it does not support combustion. Nitrogen would have about the same sonic threshold (if the threshold exists), as air. Then lubes/o-rings are fine as is, since they won't burn in pure nitrogen.

[lloyd-ss]

Scott,
I think I can assure that a Buna-N o-ring is not going to be in any kind of condition to work for a second shot.  That amount of friction will wear the surface of the o-ring off to a degree that it won't seal for a second shot.  Whether or not the amount of o-ring that gets scrubbed off actually burns at all or burns fast enough to contribute any force, is anybody's guess, and I find it rather a stretch to think that it would contribute to the velocity.  I don't have nitrogen to use, so that change is not a possibility.  About all I can change might be to use a dry barrel and a Teflon seal. 
Lloyd

[Scotchmo]

Scott,
I think I can assure that a Buna-N o-ring is not going to be in any kind of condition to work for a second shot.  That amount of friction will wear the surface of the o-ring off to a degree that it won't seal for a second shot.  Whether or not the amount of o-ring that gets scrubbed off actually burns at all or burns fast enough to contribute any force, is anybody's guess, and I find it rather a stretch to think that it would contribute to the velocity.  I don't have nitrogen to use, so that change is not a possibility.  About all I can change might be to use a dry barrel and a Teflon seal. 
Lloyd

A single Nitrogen test that matched the air test would seal the deal. If it gave approximately the same result, than I would have more confidence when using air for further data collection.

Outside of that, there will always be a question of combustion, even if minor. You would be surprised at what can act as fuels under the right conditions. I'm not sure if Teflon would be safer from a combustion standpoint.

[rsterne]

Unfortunately, I see no way to dismiss the fear of combustion-controlled assist without using Nitrogen.... If we see the same, or slightly increased velocity (which we should) then we could revert to air.... Otherwise, people will always doubt the results (even though I am 100% comfortable with them)....

Old ideas die hard.... Just look at the continuous arguments about dieseling in Springers.... No amount of "no lube" or "dry lube" or "non-combustible lube" experiments will satisfy those who believe the only way to disprove the Cardew's experiment of filling and firing a Springer in dry Nitrogen is to duplicate it.... BTW, I'm not saying they didn't obvserve controlled combustion in their experiments.... but I still find the idea that it can be so consistent over hundreds or thousands of shots to be mind-boggling.... Please don't let these observations drag this thread off course, I was just making the obvious comparison....

Bob

[lloyd-ss]

Scott,
I think I can assure that a Buna-N o-ring is not going to be in any kind of condition to work for a second shot.  That amount of friction will wear the surface of the o-ring off to a degree that it won't seal for a second shot.  Whether or not the amount of o-ring that gets scrubbed off actually burns at all or burns fast enough to contribute any force, is anybody's guess, and I find it rather a stretch to think that it would contribute to the velocity.  I don't have nitrogen to use, so that change is not a possibility.  About all I can change might be to use a dry barrel and a Teflon seal. 
Lloyd

A single Nitrogen test that matched the air test would seal the deal. If it gave approximately the same result, than I would have more confidence when using air for further data collection.

Outside of that, there will always be a question of combustion, even if minor. You would be surprised at what can act as fuels under the right conditions. I'm not sure if Teflon would be safer from a combustion standpoint.

Scott, like I said, I can not test with nitrogen.  You loan me the gauge set and I will see what I can do.
 
Regarding the Teflon, Teflon does not support combustion and is mandated as wire insulation in most aircraft and naval installations requiring maximum fire resistance.  I am not aware of any seal material that is more non-combustible than Teflon.  A few tests done with a Teflon seal, in my opinion, should eliminate the argument about combustion.
That's as good as I can do with what I have.
Lloyd