Weird Idea again

[rsterne]

I use 1 cc per FPE for the plenum whenever possible.... You can get away with half that, but be prepared to increase the regulator setpoint about 10% to compensate for the pressure drop during the shot.... Going smaller than that really hurts power, efficiency and shot count....

Bob

[Jim Holmgren]

I use 1 cc per FPE for the plenum whenever possible.... You can get away with half that, but be prepared to increase the regulator setpoint about 10% to compensate for the pressure drop during the shot.... Going smaller than that really hurts power, efficiency and shot count....

Bob

Offtopic maybe, but do you recomend 1 cc per FPE on airguns with dump valve?

I know using the the half to full barrel volume as the dump volume is a good rule of thumb, but what if one want to squeeze out the last bit of power?



What would the driffence be from using 1 cc per FPE to use the Barrel volume?

If we take YEMX airgun as exemple at 250 fpe range in .357.
250 cc vs 125 cc vs 45 cc vs 23 cc?

[YEMX]

I use 1 cc per FPE for the plenum whenever possible.... You can get away with half that, but be prepared to increase the regulator setpoint about 10% to compensate for the pressure drop during the shot.... Going smaller than that really hurts power, efficiency and shot count....

Bob

Excellent, that's what I thought, I just wanted to confirm.  The reservoir will be only a few inches short of the barrel, so I can mount an LDC.  I'll lose a bit for the end cap, but if I'm going to go off the USFT's specs, that'll be quite the reservoir.  It'll be more than sufficient. 

Offtopic maybe, but do you recomend 1 cc per FPE on airguns with dump valve?

I know using the the half to full barrel volume as the dump volume is a good rule of thumb, but what if one want to squeeze out the last bit of power?

What would the driffence be from using 1 cc per FPE to use the Barrel volume?

If we take YEMX airgun as exemple at 250 fpe range in .357.
250 cc vs 125 cc vs 45 cc vs 23 cc?

No worries about getting off topic here, this is kind of a free-for-all post.  I like seeing where things go, reminds me of the smoke pit when I was in the service.  Veeeeeeeeery interesting conversations were had!

[rsterne]

Jim, that 1 cc per FPE does not apply to dump valves.... It refers to a PCP with a plenum between the regulator and the valve, and a valve that is "timed" so that it closes while the pellet is still inside the barrel.... usually when the pellet has travelled 20-33% of the barrel length, and almost always before it is halfway to the muzzle.... During the time the valve is open, the reservoir pressure is dropping, and if the plenum is too small, the pressure drop reduces the power.... Let's say you have a 250 cc reservoir, and the barrel volume is 50 cc.... If we close the valve at halfway, that 250 cc of air (at 3000 psi) has expanded to (250 + 50/2) = 275 cc (at 2727 psi)…. If the reservoir is only 125 cc, same valve dwell, then we have 125 cc @ 3000 expanding to (125 + 50/2) = 150 cc @ 2500 psi.... It is pretty obvious you can get more power from the larger plenum, with an average pressure during the shot of 2864 psi than the smaller plenum with only 2750 psi.... In both cases, you have used up 25 cc of 3000 psi air, however....

With a dump valve of 25 cc (half the barrel volume) at 3000 psi, you use all of it (same volume as in the above PCP).... However, when the pellet is halfway down the barrel, that 25 cc has expanded to (25 + 50/2) = 50 cc @ 1500 psi.... Double the valve to equal the barrel volume and you have (50 + 50/2) = 75 cc @ 2000 psi when the pellet is halfway to the muzzle.... Note that even with a bore volume valve, the pressure is much less when the pellet is halfway to the muzzle than with a PCP, and since most of the acceleration occurs in that first half of the pellet travel, you end up with less power with a dump valve, even when using twice as much air....

If you think about it, having a dump valve that is half the barrel volume is kind of like closing the valve when the pellet is halfway to the muzzle.... in terms of what happens to the efficiency if you exceed that.... Too big a dump valve.... or too much dwell.... both put the efficiency  in the dumpster....

Bob

[rsterne]

I just used Lloyd's Internal Ballistics spreadsheet to compare your 4 volumes, using a 125 gr. bullet in .357 cal at 3000 psi with a 28" barrel (barrel vol. = 46 cc).... For the PCP, I closed the valve at 50% barrel travel, and adjusted the overall efficiency to give 250 FPE with the largest reservoir....

250 cc plenum.... 250 FPE @ 0.90 FPE/CI
125 cc plenum.... 240 FPE @ 0.94 FPE/CI

23 cc dump valve.... 188 FPE @ 0.65 FPE/CI
46 cc dump valve.... 223 FPE @ 0.38 FPE/CI
92 cc dump valve.... 248 FPE @ 0.21 FPE/CI

In actual fact, the efficiency will drop even further with the dump valves, reducing the FPE as well.... I doubt if dumping the entire 125 cc plenum in the PCP would actually give you more than 250 FPE.... Please note that these numbers are not an actual prediction of the FPE of YEMX's gun, they are just for comparison purposes.... However, my .357 Hayabusa with a 0.28" TP and 28" barrel maxed out at 253 FPE at 3000 psi with a 132 gr. bullet, unregulated on a 460 cc reservoir.... Dialed back for decent efficiency, it shot 230 FPE....

Bob

[YEMX]

.However, my .357 Hayabusa with a 0.28" TP and 28" barrel maxed out at 253 FPE at 3000 psi with a 132 gr. bullet, unregulated on a 460 cc reservoir.... Dialed back for decent efficiency, it shot 230 FPE....

Bob

Honestly, I could live with 230 fpe.  I hope this rifle performs as well as your Hayabusa Bob.

[rsterne]

No reason why not, Tom.... Pressure, barrel length and port size pretty much govern the FPE.... If you drop below a 250 cc or so plenum, you will need either a higher setpoint pressure (>3000), or more dwell (less efficiency) to get there, however....

Bob

[YEMX]

No reason why not, Tom.... Pressure, barrel length and port size pretty much govern the FPE.... If you drop below a 250 cc or so plenum, you will need either a higher setpoint pressure (>3000), or more dwell (less efficiency) to get there, however....

Bob

If I am able to make a reservoir that's 1.61" or so ID, that's 19" long (actual reservoir space), that's around 630cc's...  Should be good.  That'll also have around 4" of barrel past the end cap...

[rsterne]

IIRC, the USFT used quite a low fill pressure (1500 psi?).... To have a reservoir that is 1.61" ID strong enough for 3000 psi you will need a very thick wall.... ballpark about 1/8"-3/16" depending on if CrMoly (over 4 lbs.) or 2024-T3 Aluminum (over 2 lbs.).... Make sure you know what you want/need before you get too deep into planning....

Bob

[Motorhead]

LOL ... if history tells / teaches us a thing about Field cannons is this .... BEST BUILT over WHEELS !

[YEMX]

IIRC, the USFT used quite a low fill pressure (1500 psi?).... To have a reservoir that is 1.61" ID strong enough for 3000 psi you will need a very thick wall.... ballpark about 1/8"-3/16" depending on if CrMoly (over 4 lbs.) or 2024-T3 Aluminum (over 2 lbs.).... Make sure you know what you want/need before you get too deep into planning....

Bob

Oh believe me, I've been keeping that in mind! 

This is probably a stupid question, but do pressure vessels need to get bigger with more volume?  The Mrod reservoir's wall isn't that thick, and it can hold 3k.  So with more volume, I'm assuming you have to up the wall thickness?

[YEMX]

LOL ... if history tells / teaches us a thing about Field cannons is this .... BEST BUILT over WHEELS !

I might have to! 

I'm thinking I'll mostly be shooting from either a bench or tripod...  Good thing my tripod can hold up to 19.5 lbs! 

[rsterne]

For the same MSWP, if you increase the diameter, you must increase the wall thickness by the same proportion.... It is the thickness/diameter ratio that determines the strength.... If you double the diameter, you need to double the wall thickness, assuming the same material.... The length really doesn't factor into the calculations.... If you double the diameter and wall thickness, you will have 4 times the volume (assuming the same length) and also 4 times the weight....

In addition, if you double the ID, you increase the force on the valve and end plugs by a factor of 4 times (force proportional to area x pressure)…. so the fasteners need to be 4 times as strong.... Same pressure assumed for all of the above comments....

Bob

[YEMX]

Excellent explanation Bob- I kinda *facepalmed* when I re-read my post, I hadn't worded the question quite right...  I'm glad you understood my question anyway.  Safety is definitely at the fore-front of this build, believe me.  I want to enjoy this hobby for a very long time to come!

[KnifeMaker]

Food for thought. I never even considered this.


Once again, Mr. Bob to the rescue!


Mike

[Jim Holmgren]

I just used Lloyd's Internal Ballistics spreadsheet to compare your 4 volumes, using a 125 gr. bullet in .357 cal at 3000 psi with a 28" barrel (barrel vol. = 46 cc).... For the PCP, I closed the valve at 50% barrel travel, and adjusted the overall efficiency to give 250 FPE with the largest reservoir....

250 cc plenum.... 250 FPE @ 0.90 FPE/CI
125 cc plenum.... 240 FPE @ 0.94 FPE/CI

23 cc dump valve.... 188 FPE @ 0.65 FPE/CI
46 cc dump valve.... 223 FPE @ 0.38 FPE/CI
92 cc dump valve.... 248 FPE @ 0.21 FPE/CI

In actual fact, the efficiency will drop even further with the dump valves, reducing the FPE as well.... I doubt if dumping the entire 125 cc plenum in the PCP would actually give you more than 250 FPE.... Please note that these numbers are not an actual prediction of the FPE of YEMX's gun, they are just for comparison purposes.... However, my .357 Hayabusa with a 0.28" TP and 28" barrel maxed out at 253 FPE at 3000 psi with a 132 gr. bullet, unregulated on a 460 cc reservoir.... Dialed back for decent efficiency, it shot 230 FPE....

Bob

Thanks for doing the math, I appreciate that.
Quite intressting results and it shows at least fo me why the are so few big bores with dump valve.

[YEMX]

Thanks for doing the math, I appreciate that.
Quite intressting results and it shows at least fo me why the are so few big bores with dump valve.

I only knew of one big bore with a dump valve that was made, and it was a Quackenbush...  Do you know of others? 

[rsterne]

it shows at least for me why the are so few big bores with dump valve.

If you have ever had a tank dump with a Condor, or other Big Bore, you would know exactly why they don't use dump valves on a Big Bore.... The mass of air released makes for HUGE recoil.... Air at 3000 psi weighs 60 gr. per CI.... The air released by a 46 cc dump valve (bore volume for the gun in question) would weigh 168 gr., or more than the bullet....

Bob

[MJP]

I tested a dumb valve on the 20mm, it was too exiting to test it only at 90bar.
So it's not practical on a big bore.

Marko

[Jim Holmgren]

Thanks for doing the math, I appreciate that.
Quite intressting results and it shows at least fo me why the are so few big bores with dump valve.

I only knew of one big bore with a dump valve that was made, and it was a Quackenbush...  Do you know of others?

The Quackenbuch like you said.
Allan Dicke's airguns which was of the multi-pump type, making around 400 fpe.
Some airguns that uses air cartridges, example MAC and the new Aea precision airguns company that have shown some designs.