Re: .30 cal Condor

[Rescue35]

Nice work. For some reason I feel like I "need" a Condor.

[rsterne]

I test with no tank and tether to a big tank so I don't have to fill and drain a tank

You say "tether", so I assume you are using a regulator between the big tank and your gun to supply a constant pressure.... either a separate regulator that stays with the big tank, or you blank off the inlet on the regulator shown in your photo of the chamber and feed through the Foster?....

First I turn off the tank then bleed the line. Then I close the bleed valve and the hose is still hooked up. Then I took a shot and basically it doesn't loose any fps.

so you are working with JUST the air in the chamber for the shot, correct?.... The regulator is between that chamber and the tank (big or small)....

If that is correct, then it doesn't matter if you have a small tank, a big tank, or a hose (bled off or not) before the regulator, in all those situations you are making the shot with ONLY the air inside the chamber.... This is pretty much proof of what I have been saying all along, that the regulator can't open quick enough to top up the chamber during the shot.... It also means that testing with and without the tank won't tell you if the chamber is large enough or not.... because the chamber and the tank are separated by the regulator at all times.... The only way to know how much you are losing if the chamber is too small is to remove the regulator from the gun and connect another tank or chamber (much larger) to your existing chamber, with a large enough passage between the two so that the small chamber acts like a much larger one.... Perhaps this graph will explain what happens as you reduce plenum size....



The red dot on the graph is 1cc per FPE.... You are running 1600 psi and a 50ish cc chamber on a gun shooting about 90-100 FPE, so let's call that 1/2cc per FPE.... That would be roughly the first blue dot on the graph to the left of the red dot.... The actual numbers will change depending on the setpoint pressure, but a lower pressure (eg. 1600) trying to provide a lot of FPE (eg. 100) will be even worse than shown.... Basically, compared to if you had a huge tank at 1600 psi with no regulator, you are giving up about 10% of the FPE you could get.... In other words, if you tried your 62 CI tank, connected directly to the gun, with no regulator between, you should gain about 50 fps (and greater efficiency as well).... I hope that explanation helps....

Bob

[dyotat100]

Yeah I forgot the reg doesn't fill fast enough. I was just doing it because I had to drain it anyways. That's when I started messing with it. I have a cap I screw onto the reg threads that has a fill nipple on it so I can shoot the gun and see if I'm happy with what it's set at before dropping it in the tank.

I will pull the valve out and put it in a tank. Then tether to my reg setup to change the psi to see what it does on 1600 psi.

[rsterne]

Good test.... I'm curious how close to your gain my 50 fps guess is....

Bob

[dyotat100]

Ok. 400 cc tank tethered with digital gauge to get exact psi. Gauge is +- .025%.

1606 psi is 945-950 fps
1410 psi is 908-912 fps


So you are real close. Now I have to figure out how to double the size or the chamber without adding to the LOP.

How much will I gain if I add .5" in length and .5" in diameter?

Right now it is 2" OD X 1.56" with .120" wall.

[rsterne]

2.0 - (2 x 0.120) = 1.76" ID.... 1.76^2 x 1.56 x 3.14 / 4 = 3.8 CI.... However the dimensions you gave me before were 1.74" ID x 1.25" long inside, which was 3.0 CI.... minus the guts....

2.5 - (2 x 0.120) = 2.26" ID.... 2.26^2 x 2.06 x 3.14 / 4 = 8.3 CI.... If we assume the increased inside length is only 1.75" we get 2.26^2 x 1.75 x 3.14 / 4 = 7.0 CI minus the guts.... ie more than double what you have now.... Just increasing the diameter by 1/2" at the same length would be 2.26^2 x 1.25 x 3.14 / 4 = 5.0 CI minus the guts....

You could double the volume by increasing the diameter by 1/2" and the length by 1/4", working with inside measurements.... I have not done the math on the strength with a 0.120" wall, nor on the end plates (or welding)....

Cool how the theory works out, eh ?!?.... Don't forget that going from 1/2 cc per FPE to 1 cc per FPE won't get you the whole 50 fps you got with the 400 cc tank (4 cc per FPE), it will get you about half the difference (ie 25 fps) at 1600 psi.... or allow you to drop the pressure about 100 psi for the same velocity.... Maybe your 1/2" increase in both ID and length is a good idea....

Bob

[dyotat100]

Yes it is.

At 2.5" od and .120" wall with  50,000 for strength it says 1800 psi working at 3 to 1 safety factor. 

I'm going to build it. I will go .5" longer and .5" larger to make sure I'm covered. I  will build the etac .5 longer to clear the frame.

The weld is only sealing it. Not holding it together.
Plus now I will be in the 1400 psi area or I could keep it at 1600 and shoot 50 gr around 920 fps and 45 gr around 950 fps.

[QVTom]

Doug, Are you running a burst disc on these plenums? If not they need to be safe at tank pressure.  I get as SF of 1,6 at 3k using the 50ksi value.

Tom

[dyotat100]

Yes. But they are 3k. I need to find a 2k.

If I use 4340 would that get me close enough to 3000 psi safety?

[rsterne]

I had a look at your efficiency, and here is what I got, based on the numbers you gave.... 60 shots at 85 FPE (50 gr. @ 875) = 5100 FPE total.... a 62 CI tank from 3000 psi down to 1600 is 1400 / 14.5 = 97 bar x 62 = 6014 CI.... That works out to 5100 / 6014 = 0.85 FPE/CI which is OK but not great....

I have a suggestion for you about the pressure.... I ran your numbers through Lloyd's spreadsheet, and it came up with your valve closing when the pellet is 18" down a 30" barrel, which is the reason for the relatively low efficiency....  Leaving the pressure at 1600 when you install the larger plenum will allow you to close the valve sooner, improving the efficiency.... Here are a few scenarios I ran through the spreadsheet, all using the 50 gr. pellet which was 875 fps above.... but using a 100 cc plenum (6.1 CI)....

Larger plenum, no other changes.... 910 fps, 0.76 FPE/CI, ~ 50 shots....
Larger plenum, 1500 psi, same dwell.... 885 fps, 0.80 FPE/CI, ~ 59 shots.... (at the same velocity you should get about the same shot count - even though your setpoint is lower)
Larger plenum, still 1600 psi, reduce dwell.... 875 fps, 1.02 FPE/CI, ~ 72 shots.... (valve closing at ~12" instead of 18" - quieter and more efficient)....

I would therefore suggest that you leave the pressure at 1600 psi and drop the velocity back by reducing the dwell on the valve (hammer strike).... The increased efficiency from the larger plenum will increase your shot count once you detune to the same power you have now.... or you can trade off power and shot count from there....

Bob

[dougroundup]

Man , I love this stuff! I am glad to hear that Doug can get some solid advice and numbers from you Bob!
It just keeps getting better and better what you guys keep accomplishing with furthering this hobby of ours.
KEEP UP THE GOOD WORK GUYS

[Hanabata808]

Man , I love this stuff! I am glad to hear that Doug can get some solid advice and numbers from you Bob!
It just keeps getting better and better what you guys keep accomplishing with furthering this hobby of ours.
KEEP UP THE GOOD WORK GUYS

Exactly this. Always really interesting to read and I learn a lot from these discussions

[rsterne]

I actually ran the numbers on Doug's .30 cal up to an 1800 psi setpoint, and either 1600 or 1700 are the best, the shot count starts to drop again at 1800, assuming that 100 cc plenum and 85 FPE power level.... Change either of those and the optimum pressure would change as well.... What is happening is that as you increase the pressure, for a given FPE level, you have to back off on the hammer strike, saving air by increasing the efficiency.... However, on the other side of the coin, you have less "headroom" between the 3000 psi fill pressure of the tank and the regulator setpoint pressure.... At 1800 psi, you only have 1200 available, at 1600 you have 1400, and at 1400 you have 1600 psi you can use between full and falling off the reg.... These two factors (efficiency vs. psi range available) fight each other, and according to Lloyd's spreadsheet, based on the results Doug has posted so far, the "best" pressure to run to get the most shots at 85 FPE is between 1600-1700.... Either side of that range, the shot count will drop....

It's really not QUITE that simple, of course, as if the pressure and hammer strike get too far out of balance, the you won't be operating on the "knee" of the curve, ending up with a noisy air hog (running too low a pressure for the FPE you want) or a gun that is quiet and efficient (high pressure, light hammer strike) but the velocity will actually INCREASE when the pressure falls below the setpoint.... When all is said and done, there will only be one BEST solution (combination of setpoint pressure and hammer strike) that will give you the flattest velocity string and the highest shot count.... If you want more FPE, you will have to run more pressure and more hammer strike to get back to the knee of the curve....

Bob

[KnifeMaker]

Man , I love this stuff! I am glad to hear that Doug can get some solid advice and numbers from you Bob!
It just keeps getting better and better what you guys keep accomplishing with furthering this hobby of ours.
KEEP UP THE GOOD WORK GUYS

Me as well DougRoundUp! Can't get enough of it. Mine will be switched back to a Pellet shooter this week until Doug has time to fashion a new valve stem for it. Or the New Barrel! Woo-Hoo!!!

I don't think I will be enjoying going back to pellets. I'm loving the .257 far too much for that. LOL!

Knife

[rsterne]

Tom, did you run the numbers on the safety factor for the ends of the chamber?.... I don't know how to do that, but my gut tells me that a flat surface (as in the end plate) has totally different stresses across various points than a cylinder.... I envisage increased stress at the corners/edges compared to the middle because of the leverage factor from pressure in the flat center portion.... Doug would have to give you the thickness of the end plates and how they are attached, of course.... Totally beyond my simple math, but I seem to remember your CAD program did stress analysis?....

Bob

[dyotat100]

It's .125" on the one I'm running right now. It is a solid piece of bar that I hollow out.

[rsterne]

Doug, my gut tells me that the weakest point is the corner where the flat end meets the cylindrical wall.... but I don't have the math skills, or knowledge, to calculate the stress in that corner.... I would suggest you PM QVTom if he doesn't see this question, as I am pretty sure his CAD software can do a stress analysis.... My gut tells me you need a thicker wall on the end plug, at least at the corner, and an internal radius as well.... You would do well to find out before testing it and possibly finding out the hard way....

Bob

[QVTom]

I could take poke at the analysis.  I need some details.  The end plate sits inside the tube?  its .125 thk and made of?  Does it sit on a step?  What is the weld depth?

[QVTom]

2.5 dia x .120 wall mild steel.  End cap .125 mild steel.  Pressure = 1600 psi

[rsterne]

I'm not too familiar with stress diagrams but it appears that the stress in the middle of the end is over 32x greater than the stress in the tube wall.... If we take a look at the chamber on the previous page, however, there is are rings roughly 1" in diameter on each end, to mount the regulator and valve.... In very rough terms, that would be about where the inner lime green circle is on the end, so the highly stressed yellow and red area inside that isn't there.... That might drop the highest stress down to about 45-50 ksi, but that is still much too high for normal materials, isn't it Tom?....

Doug, we need to know the diameter of the rings on each end.... Tom, what happens if you double the wall thickness of the ends to 1/4" ?

Bob