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Posted: 4/17/2012 10:01:15 AM EDT

There has been a lot of questions and discussion on the use
of PVS-14s mounted to guns and how much recoil can they take. Although we have
little hard data to go on (AR15 in 223 is OK
according to manufacturer statements and up
to 6.8 SPC may be OK) there have been some good attempts to estimate recoil
levels.

cj7hawk gave a very well thought out evaluation of recoil in
his now archived post

http://www.ar15.com/archive/topic.html?b=6&f=18&t=344044



If I remember correctly these were cartridge specific
calculations for G force using the max PSI for the cartridge multiplied by the cartridge
head area to determine the maximum instantaneous acceleration.  This is a reasonable way to estimate and
compare cartridges but I have been concerned that instantaneous maximum acceleration
over estimates the actual force seen by an attached scope.

As an alternative method I ran some calculations for acceleration
based on recoil velocity of the gun and the acceleration time (time of the bullet
in the barrel) to give average G force.



Here are the results.

These represent the ‘average’ acceleration seen by the gun
assuming linear acceleration as the bullet & gases travel down the barrel, not
peak instantaneous.

Calculations are from the below links, these are plug in
converters and easy to use. I chose to compare the following:

A 308 rifle shooting a 150 gr bullet @ 2700 f/s in two
weight configurations, 11 lb and 9 lbs

A 223 rifle shooting a 55 gr bullet @ 3300 f/s in two
configurations, 9 lb and 7 lbs

A 50 BMG rifle shooting a 700 gr bullet @ 2800 f/s in one configuration,
35 lbs

Free recoil velocity of the gun is from here:  http://kwk.us/recoil.html





 

 

 

 

 

 

 





 

 

 

 

 

 

 





 

 

 

 

 

 

 





 

 

 

 

 

 

 





 

 

 

 

 

 

 





 

 

 

 

 

 

 





 
Gun

 

 
Weight lbs

 

 
Barrel (in)

 

 
Bullet (gr)

 

 
Velocity f/s

 

 
Powder (gr)

 

 
Recoil velocity f/s

 

 
308

 

 
11

 

 
20

 

 
150

 

 
2700

 

 
46

 

 
7.7

 

 
308

 

 
9

 

 
20

 

 
150

 

 
2700

 

 
46

 

 
9.4

 

 
223

 

 
9

 

 
20

 

 
55

 

 
3300

 

 
26

 

 
4.9

 

 
223

 

 
7

 

 
20

 

 
55

 

 
3300

 

 
26

 

 
6.3

 

 
50 BMG

 

 
35

 

 
36

 

 
700

 

 
2800

 

 
230

 

 
11.9

 





Now using the recoil velocity from the table we need to
calculate the amount of time the gun accelerates. This is approximately the
time in the barrel from ignition to exit and can be calculated by dividing half
of the muzzle velocity by the length of the barrel, as an example:

223, 20 inch barrel muzzle velocity 3300 f/s

The average speed of the bullet in the barrel is half the
exit velocity (assuming linear acceleration while not exact is very close) give
1650 f/s.

20 inch barrel = 1.666 ft

1.66ft divide by 1650ft/s = 0.001 s or 1 millisecond
acceleration time. This calculation agrees very well with established barrel
times as seen here: (ignore the table, just look at the graph)




Same calculation for
the 308 in a 20 inch barrel gives 0.00123 or 1.23 milliseconds

For the 50 BMG with a 36 inch barrel, 2.1 milliseconds

Using the plug in convert from here we get the acceleration
in Gs








 

 

 





 

 

 





 

 

 





 

 

 





 

 

 





 

 

 





 
Gun

 

 
Weight (Lb)

 

 
Acceleration (G)

 

 
308

 

 
11

 

 
195

 

 
308

 

 
9

 

 
238

 

 
223

 

 
9

 

 
152

 

 
223

 

 
7

 

 
196

 

 
50 BMG

 

 
35

 

 
176

 



Interesting to see a 7 lb 223 has a similar G force as an 11
lb 308. The 50 BMG is less than 308 in average g force.



This is a calculation of the average acceleration seen by
the recoiling gun vs instantaneous peak calculated by cj7hawk’s method. Neither
of these tells you what a PVS-14 mounted to the gun will see in real Gs but
they do allow a comparison between guns and cartridges.



The open question is what size and duration of g forces will
damage a PVS? The duration of the G force as well as the absolute maximum value
are important.  Will 500G peak impulse
that lasts for 0.1 milliseconds cause damage? 350 Gs that lasts 10x longer? These
are good questions and no hard answers.  

In my personal opinion I think the average G force is
probably the better metric. I say this because the peak impulse only lasts ~
0.1 milliseconds (look at that chart above) and I believe there should be
enough elasticity in the gun, scope mount and PVS-14 body to dampen the peak Gs
to a lower impulse, probably closer to the average G force.

Until we have measured numbers for G force (accelerometer on
a mounted PVS-14) we can only calculate a best estimate. Even with the measured
value the limit for damage would need to be experimentally determined by exposing
several PVS-14s to increasing Gs until they failed.

I bet the big names (ITT) have done this, too bad that isn’t
public knowledge.

 
 
 

 



 
Link Posted: 4/17/2012 10:03:05 AM EDT
Repost of cj7hawk calculations:



Hi All,










As mentioned earlier, here's the guide. This represents the peak
recoil energy experienced by an approximately 4kg rifle when firing the
listed cartridge.










It's not complete and I included what few cartridges I could find.
I'll update it if anyone comes up with others ( I need to know the peak
chamber pressure as a component of the calculation )-










This is not meant to be absolute and I offer no warranty for the
accuracy of the information though where possible I've used the highest
pressure I could find for the cartridge specified. Not all data comes
from the same source and the numbers are expected to be out by quite a
bit.










So to use it, look up the cartridge and see what the peak
acceleration is. If it's higher than 400G's I'd say forget it. If it's
higher than 250 G's I'd say be careful. Don't assume just because
there's a low number that you're safe. I might have gotten the figures
wrong.  It's intended as a "don't try these" type document, to list
known high risk.










In case you're determined to risk everything though, remember that
you can pro-rata increase the bare weight of the rifle proportionately
over 4KG and change the acceleration. Eg, An 8kg gun, the maths are
4/8*G's where 8 was the gun weight. This means an 8kg rifle would
experience about half the peak acceleration.










I hope this helps. If it saves you making a mistake and breaking
your expensive NV scope just once, then you can buy me a drink if we
ever meet up sometime. :)










Regards



David

Sorry, the whole table doesn't fit on AR15... Give me some time to work it out. In the mean time, here's the brief version.

Note: Extended slightly to show pressure, caliber and calculated for light rifles.



Note: This is the fixed structural weight. NOT the add-on-bits weight. So bipods and the like don't count.  Rigid parts only.



Regards

David









































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































CartridgeCaliberPressure4Kg
Common NameInches(PSI)G's
.17 Hornet0.1753000174
.17 Mach 40.1760000197
.17 Rem0.1762000203
.204 Ruger0.259000278
.22 Short0.2221000115
.22 Long0.2224000132
.22 LR0.2224000132
.22 WMRF0.2224000132
5.6x35R0.2239000215
.22 Hornet0.2244000241
.218 Bee0.2246000248
.22 Rem Jet0.2240000220
.221 Rem0.2252000288
5.6x50R0.2249000270
0.220.2254000302
.222 Mag.0.2259000333
0.220.2262000350
.219 Zip.0.2241000223
.225 Win.0.2357000327
.22-2500.2265000357
.220 Swift0.2262000340
.223 WSSM0.2265000367
5.6x61R v.H.0.2255000303
5.6x52R0.2248000265
.22 Sav.0.2248000263
6x70R0.2438000240
6x50R0.2464000405
6x52R Bret.0.2448000304
.243 WSSM0.2465000435
.243 Win.0.2460000402
6 Rem.0.2465000411
.240 Wea.0.2464000418
6x620.2462000392
6x62R0.2462000392
.240 Fl.NE0.2446000300
.25-200.2539000276
.25-20 S.S.0.2522000156
.256 Win.0.2651000379
.25-36 Marlin0.2531000220
.25-350.2544000312
.25 Rem.0.2536000255
.250 Sav.0.2553000376
.257 Rob.0.2654000404
.25-060.2563000447
.257 Wea0.2664000479
6.5x52R0.2636000267
6.5 Jap.0.2643000319
6.5x50R0.2653000394
6.5x70R0.2641000304
6.5x52 Car.0.2641000304
6.5x53R0.2646000342
6.5x54 M-S0.2653000394
6.5x54 Mauser0.2644000327
.260 Rem0.2660000460
6.5x550.2655000408
6.5x58R0.2641000304
6.5x570.2657000423
6.5x58 Mauser0.2651000379
6.5x57R0.2648000356
6.5x72R0.2655000408
6.5 Rem Mag0.2663000468
.264 Win Mag0.2664000506
6.8 SPC0.2759000480
0.270.2765000537
.270 WSM0.2765000537
.270 Wea0.2764000529
7x50R0.2853000456
7-30 Wat.0.2849000422
7x72R0.2841000353
407620.2861000525
7x570.2857000491
7x57R0.2849000422
7 Fl.Mag.0.2848000413
7x65R0.2855000474
7x75R v.H.0.2860000517
.284 Win0.2864000585
.280 Rem0.2860000533
7 WSM0.2865000560
7 Rem Mag0.2862000534
7 Wea0.2864000551
.280 Fl.NE0.2843000382
.30 Carb.0.346000469
.30-357 AeT0.344000449
.30 Rem AR0.355000561
.30-300.346000469
.30 Rem0.341000418
.303 Sav0.339000406
.300 Sav0.353000541
.308 Marlin0.3147000506
.30 Fl.Purdey0.346000469
.30-400.347000480
.307 Win0.3160000641
.308 Win0.3162000667
.30-060.359000602
.30 R Blaser0.359000602
.300 Whisper0.339700405
.300 Whisper HP0.349600506
.300 WSM0.365000663
.300 H&H0.362000633
.300 Win0.364000653
.30 Fl.Mag.0.346000469
.300 Wea0.365000663
.300 RUM0.365000663
.300 Lap.Mag.0.368000694
.303 Brit0.353000552
7.62x390.351500526
7.62x54R0.357000582
.32-200.3230000348
8x72R0.3141000461
.32-400.3234000395
.32 Spl.0.3242000488
.32 Rem0.3243000499
8x50R0.3151000574
8 Lebel0.3151000574
8x57 JRS0.3148000540
8x57 JS0.3157000641
8x60RS0.3149000551
8x64S0.3159000664
8x65RS0.3159000664
8x75RS0.3155000619
8 Rem Mag0.3167000754
0.320.3248000550
.333 Riml.NE0.3348000604
.338 Fed0.3462000803
.338 Marlin0.3446000596
.33 WCF0.3344000543
.338-06 A-Sq.0.3465000842
.338 Win0.3464000829
.338 RUM0.3465000842
0.350.3546000632
.351 Win SL0.35300042
9 Luger0.3535000498
9x570.3541000584
9x57R0.3541000584
.38 Spl0.3822000360
.357 Mag0.3645000650
.357 Max0.3648000694
400/3500.441000744
.350 No.20.3548000667
.350 Mag Rigby0.3545000625
.35 Rem0.3540000556
.35 WCF0.3544000611
.356 Win0.3660000862
.358 Win0.3659000857
.350 Rem Mag0.3562000861
.35 Whelen0.3558000806
9x53R0.3549000698
.360 NE 2¼0.3636000529
9.3x72R0.3729000441
9.3x570.3743000654
9.3x620.3757000867
9.3x640.3764000973
9.3x74R0.3749000745
9.3x65R0.3755000836
.375 Win0.38640001021
.375 NE 2½0.3832000510
9.5x57 M-S0.3744000698
.376 Steyr0.3862000994
.375 H&H0.3862000989
.375 Wea0.38640001021
.375 Fl.Mag.0.3847000749
.369 NE0.3744000679
.378 Wea0.38640001037
.38-550.3835000573
.38-400.3817000278
.40-82 Win0.424000435
.41 Rem Mag0.4144000839
.400 Jef.0.441000744
450/400 3¼0.4543000987
.400 H&H0.4640001161
.405 WCF0.4136000670
.416 Rem.0.42650001276
.416 Rigby0.4247000922
.404 Jef.0.453000981
.44-400.4416000351
.44 Spl0.4416000351
.44 Rem Mag0.4441000900
0.440.44510001140
.45 ACP0.4521000482
.45 Colt0.4516000367
.454 Casull0.45570001332
.45-700.4529000666
.450 Mar0.4542000964
.458 SOCOM0.4635000833
.458 Win.0.46620001475
.458 Lott0.46620001475
.450 NE 3¼0.45440001010
.450 No.2 NE0.4541000941
.465 Belted0.47620001520
500/4650.5360001021
.480 Ruger0.48480001254
.475 Linebaugh0.48500001279
.475 Turnbull0.48420001075
.470 NE0.4739000977
.475 No.2 NE0.48400001023
.505 Gibbs0.51390001128
.500 Jef.0.5480001361
.500 NE 3"0.5410001162
.50 BMG0.5540001531
.50 BEOWULF0.533000936
.577 3" NE0.58360001359
.600 NE0.6360001470
.700 NE0.7400002223
4 Bore Rifle1360004082







 
 
Link Posted: 4/17/2012 10:12:29 AM EDT
Don't try mounting one on a spring piston 4.5mm air rifle....
Link Posted: 4/17/2012 10:37:58 AM EDT
Link Posted: 4/17/2012 11:17:19 AM EDT
Vic,





A quick calc using the Armalite 338 Lapua rifle for weight & dimensions.





Weight 12.3 lbs, add 1 lb for scope & rings = 13.2 lbs





Barrel = 26 inches





Load 250 grain @ 3000 f/s with 87 grains of powder





acceleration time = 1.44 ms


















 





 





 





 





 





 





 

















 





 





 





 





 





 





 

















 





 





 





 





 





 





 

















 





 





 





 





 





 





 

















 





 





 





 





 





 





 

















 





 





 





 





 





 





 

















 





 





 





 





 





 





 






















 




Gun










 

 




Weight lbs










 

 




Barrel (in)










 

 




Bullet (gr)










 

 




Velocity f/s










 

 




Powder (gr)










 

 




Recoil velocity f/s










 

 




338 Lapua










 

 




13.3










 

 




26










 

 




250










 

 




3000










 

 




87










 

 




12.3










 

 




308










 

 




11










 

 




20










 

 




150










 

 




2700










 

 




46










 

 




7.7










 

 




308










 

 




9










 

 




20










 

 




150










 

 




2700










 

 




46










 

 




9.4










 

 




223










 

 




9










 

 




20










 

 




55










 

 




3300










 

 




26










 

 




4.9










 

 




223










 

 




7










 

 




20










 

 




55










 

 




3300










 

 




26










 

 




6.3










 

 




50 BMG










 

 




35










 

 




36










 

 




700










 

 




2800










 

 




230










 

 




11.9










 

























 





 





 

















 





 





 

















 





 





 

















 





 





 

















 





 





 

















 





 





 

















 





 





 






















 




Gun










 

 




Weight (Lb)










 

 




Acceleration (G)










 

 




338 Lapua










 

 




13.3










 

 




265










 

 




308










 

 




11










 

 




195










 

 




308










 

 




9










 

 




238










 

 




223










 

 




9










 

 




152










 

 




223










 

 




7










 

 




196










 

 




50 BMG










 

 




35










 

 




176










 



















50 BMG = 176 G





338 Lapua = 265 G
Shorter barrel, higher velocity = faster acceleration in this instance.
Quoted:






Rich, thanks for the hard work putting this together.  I was especially interested in the 6.8SPC...We knew from the Industry bullet folks we have talked with over the last few years, this was just shy under 500g's, thus we have taken the cautious approach with this round in regards to the 14's..  You're study shows what we have seen and what we've been told.
I have several more questions regarding this chart, but need to digress a bit more in regards to your 50bmg vs. 338's.  Leupy did a study awhile back I need to re-examine from some notes I took but peak recoil time duration is also key.  The 338's are real optic killers due to the very sharp (short time recoil spike) vs. the 50BMG which has longer time duration recoil peak, not a sharp peak such as the 338's.  
Vic

 


 
 
Link Posted: 4/17/2012 2:10:03 PM EDT
Does it go without saying that a .458 socom or a Saiga 12 would be a no go for a gun mount?
Link Posted: 4/17/2012 3:53:59 PM EDT
So is a 12lb hk91 safe then?
Link Posted: 4/17/2012 4:04:19 PM EDT
Based on calculated average g force it would be similar to an AR15 in 223, using calculated peak G force it is in the 400 G range and in the marginal to dangerous zone.



Quoted:



So is a 12lb hk91 safe then?

 
 
Link Posted: 4/17/2012 5:59:26 PM EDT


Hi Rich_V - An excellent and well thought out analysis and an interesting way to look at things.







There are two ways to look at recoil - One is as acceleration over time. The other is as momentum ( Newton's laws ).







In the case of momentum, the concept is simple - M1V1 = M2V2 - That the mass of the bullet, gas and other ejecta multiplied by the muzzle velocity should effectively equal the mass of the rifle and all attachments multiplied by the recoil velocity.







Starting with this assumption, you've worked your way back to average acceleration and assumed that it may not be linear, but at least you have some idea of what it is.







This is still a perfectly valid way of examining the situation and you've done a good job of it. It's also valid, mechanically, for calculating fairly static accelerations and to some extend is reasonable for working out how much damping is required for a mitigating design to function effectively.







It's also the most common and effective way to look at recoil alone.







However there are a number of reasons I chose the other method ( Instantaneous acceleration )-







The first is the milspec. In this case, the relevant figures were shock as caused by recoil, but where calculations are based on half-sine-wave shocks ( eg, firing shock ) and for which there are clear amplitude limits within the specification.







The second was that at the time of the calculation, it was clear that something else was going on - people were being surprised by the recoil-induced damage and the manufacturers genuinely seemed to be caught unaware by this issue.







As a result, the research was also conducted in parallel to research and investigation of known recoil damage situations which concluded that autogating played a significant part in the overall recoil damage experienced by a tube. This it turns out is explained fairly well by the natural resonance of the MCP itself with respect to the recoil.







As a result, it was possible to determine that the peak recoil shock was actually the cause of the issue to begin with, not the total acceleration.







However, the damage sustained by PVS-14's overall ( and other scopes ) is not entirely related to this, hence your calculations have some validity to non-resonant recoil damage which is more related to the overall acceleration when it occurs within a somewhat elastic environment.







There's another factor that is absolutely critical to the understanding also, which was almost completely lost in the original articles I wrote some time ago.







This is the way in which the peak chamber pressure is calculated.







Now as you might guess, there's not a mechanical pressure gauge in the world that would be able to pick up a peak pressure that only lasted for 0.1ms. Most adjust in a matter of seconds, not microseconds.







To get around this, there are two methods for measuring peak chamber pressure - the understanding of which is critical for cartridge manufacture.







Originally, I believe they used a deforming piece of copper  (I think it's copper ) between the cartridge and the bolt. The total deformation tells exactly how much pressure the copper was under as a maximum.







Now, they have a much more accurate electronic method. They put a piezo effect device on the bolt face. This measures the extent to which the catridge presses on the bolt face during firing as a voltage. From this, they can determine how much pressure the cartridge was under. That's how they create graphs of such extraordinary accuracy.







When you think about that for a moment, the logic is clear. By measuring the force on the bolt face, which is almost the entire transfer point of recoil to the rifle, they are measuring recoil and are using that to determine what chamber pressure was necessary to generate that level of instantanous recoil.







What I did wasn't all that clever. I just reversed the calculation to find out what they measured as exact instantaneous recoil in the first place.







This is one of those interesting things - Although it looks like we're looking at pressure charts when we examine the one you posted, we're actually looking at the recoil, as measured at the bolt face, translated into pressure on the Y axis. If you want to see the original chart, just switch "Pressure" for "Newtons"







And finally, to work out G's ( for shock, as per the milspec ) we substitute A=F/M to know what shock acceleration was transferred to the system.







The final consideration, which is why I usually say "300 G's OK, 400 G's At Your Own Risk, 500 G's Definately NO" is because we also need to take lash into account.







Lash will allow the rifle to achieve greater velocity than the NV Monocular at first before the elasticity in the system is exhaused and rigid components generate their own secondary shock. This causes a much higher level of shock to be transferred to the tube and is why I recommend ensuring that there is no measurable lash in the components at shock levels above 300G's.







Regards


David

 
Link Posted: 4/17/2012 6:09:58 PM EDT
David,

When you use the term shock, is that analagous to jerk, i.e. the derivative of acceleration with respect to time or the third derivative of position with respect to time?
Link Posted: 4/17/2012 7:09:36 PM EDT



Quoted:


David,



When you use the term shock, is that analagous to jerk, i.e. the derivative of acceleration with respect to time or the third derivative of position with respect to time?



No. I'm referring to shock with relevance to the period ( time ) over which the acceleration occurs, not the change in rate of acceleration which is jerk or jolt, depending on which side of the atlantic you come from.



And in particular, I'm referencing the Milspec MIL-STD-A3256363D which is the milspec for the MX11769 tubes. ( Omni VII ).



This defines the following test;





 

3.4.16.3  Shock.  With no radiation incident on the photo-cathode, the operating assembly shall

not be damaged (see 3.5.9) nor suffer degradation of performance (see 3.5.31) when subjected

to 6 shock impacts parallel to the optical axis and 6 shock impacts perpendicular to the optical

axis with no more than 2 flashes in each axis.  Impacts shall be half-sine wave with a minimum

peak amplitude of 500g's (see 3.5.5) and a duration of 300 +/- 50 microseconds measured at

the 10 percent amplitude points.




( ITAR Disclaimer - The above information was released into the public domain by the US government )




Of particular note here is the duration - 300 milliseconds - very different from many other tube specs which often look at a different period, also the specification of 500 G's.




So the critical aspects of shock here are the amplitude ( 500 G's ) and the duration ( 300ms, with the 10 percent points at each side ).




Regards

David



Link Posted: 4/17/2012 7:21:26 PM EDT
I posted this in a different thread, but this one seems most relevant so....

How are you guys accounting for the weight of the propellant and the jet effect / thrust it creates exiting the muzzle?   Does peak G happen b4 that so it doesnt matter?

Using a muzzle brake on my .300 win Mag makes a very huge obvious difference in felt recoil.  On a .223 it can almost eliminate felt recoil, but the basic F=MA with regards to the rifle and the projectile only is the same.   The cartridge pressure is also the same.   A supressor acts similarly to a brake.   Does it matter in this?

Do these calculations assume the rifle is suspended in mid air?   For the sake of argument, if the rifle and stock were made from hardened steel and it were up against a hard steel, essentially immovable object (i.e. a firing fixture that does not move), the force due to acceleration experienced the optic would be near zero since it would not move.  Reality being somewhere in between and varying depending on how the gun is held and fired....

Is there any actual accelerometer measured data available on any of this?
Link Posted: 4/17/2012 7:29:41 PM EDT
The reason cj7hawk's numbers are the ones you need to look at is because his method calculates peak instantaneous recoil, and that's what will damage a tube. Rich_v's numbers are not invalid, but they deal more with describing "felt' recoil. A muzzle brake is a good example of why felt recoil is a poor indicator when it comes to tube damage. The shooter doesn't feel the same recoil however the optic is still getting the peak instantaneous recoil.

Rich_v did some great work getting numbers together for this thread, but they are not indicative of recoil damage as far as I2 tubes are concerned. cj7hawk's numbers are the correct ones for this given circumstance.
Link Posted: 4/17/2012 7:36:33 PM EDT
Quoted:
The reason cj7hawk's numbers are the ones you need to look at is because his method calculates peak instantaneous recoil, and that's what will damage a tube. Rich_v's numbers are not invalid, but they deal more with describing "felt' recoil. A muzzle brake is a good example of why felt recoil is a poor indicator when it comes to tube damage. The shooter doesn't feel the same recoil however the optic is still getting the peak instantaneous recoil.

Rich_v did some great work getting numbers together for this thread, but they are not indicative of recoil damage as far as I2 tubes are concerned. cj7hawk's numbers are the correct ones for this given circumstance.


So  peak acceleration does happen before the  bullet leaves the muzzle and the effects of a brake have a chance to make a difference?   It makes sense to me that a ms of high acceleration could damage a tube, but that a cumulative/average recoil would be more what we would perceive as felt recoil...
Link Posted: 4/17/2012 7:45:20 PM EDT



Quoted:



How are you guys accounting for the weight of the propellant and the jet effect / thrust it creates exiting the muzzle?   Does peak G happen b4 that so it doesnt matter?





Correct. It doesn't matter. Peak recoil occurs before any gas has left the system, so it's all simple piston physics at this point. Some force is lost to friction - but not enough to make it worth calculating or to significantly affect the outcome.



As for can it be measured by an accelerometer? It depends on what you mean by accelerometer, but that's pretty much what they are measuring at the bolt face... They measure force to be more accurate, but most accelerometers only measure force over time as exerted on a known quantity of mass.




Of course, other factors such as elasticity come into it, but it's good enough (close enough) for the calculations we're doing to simplify things.




Regards

David









Link Posted: 4/17/2012 7:47:59 PM EDT



Quoted:



Quoted:

The reason cj7hawk's numbers are the ones you need to look at is because his method calculates peak instantaneous recoil, and that's what will damage a tube. Rich_v's numbers are not invalid, but they deal more with describing "felt' recoil. A muzzle brake is a good example of why felt recoil is a poor indicator when it comes to tube damage. The shooter doesn't feel the same recoil however the optic is still getting the peak instantaneous recoil.



Rich_v did some great work getting numbers together for this thread, but they are not indicative of recoil damage as far as I2 tubes are concerned. cj7hawk's numbers are the correct ones for this given circumstance.




So  peak acceleration does happen before the  bullet leaves the muzzle and the effects of a brake have a chance to make a difference?   It makes sense to me that a ms of high acceleration could damage a tube, but that a cumulative/average recoil would be more what we would perceive as felt recoil...


Well said :)



This is why so many people have trouble understanding that a brake might help your shoulder, but won't help your tube.



 



Regards

David
Link Posted: 4/17/2012 9:28:21 PM EDT






Just to add to this, here ( for comparison ) is the MX-10160A/AVS-6 specification and how they address the requirement for Weapons Rated tubes. The below are from the performance specification PS/07/805/0xx
















3.4.16.3 Shock.
With no radiation incident on the photo-cathode, the operating
assembly shall not be damaged nor suffer degradation of
performance when subjected to 6 shock impacts parallel to the
optical axis and 6 shock impacts perpendicular to the optical
axis with no more than 2 flashes in each axis.  Impacts shall be
half-sine wave with a minimum peak amplitude of 75g's (see 3.5.5)
and a duration of 6 ± 2 milliseconds measured at the 10 percent
amplitude points.
















3.4.16.3.1 Weapon
Shock.  The Image
Tube, when installed   in Night Vision Weapon Sights, must be
able to   withstand the shock of the following weapons:  300 Win
Mag, MK46, MK48, M4A1, M2
















(
ITAR Disclaimer - The above information was released into the public domain by the US government  )
















So this pretty much gives you an idea of what a tube rated for weapons use can handle. I would imagine the F9800WG would conform to this specification.
















Take note though that not all MX-10160A/AVS-6 tubes are weapons-use rated. It's usually a special order.














So taken as-is, if you work out the average recoil acceleration over a 6 millisecond period, the max is not 500 G's, it's 75 G's unless it's a weapon's rated tube ( normal tubes omit the "Weapon Shock" criteria in the specification ) - So for the method of calculation that Rich_V provides, there is a valid solution, but it can only be compared against 75 G's as per the Milspec.












Regards





David















 
Link Posted: 4/18/2012 6:08:21 AM EDT



Good to see a lively discussion on this topic as that was
part of what I hoped my post would produce.



As David and others have mentioned the effect of recoil
impulse on scope failure is not something that can be boiled down to a simple
number or calculation. It exhibits a very dynamic multicomponent presentation
of force on the weapon system. I’ll try and break it down to some of the
components as I understand them. I’m not an expert on this but I am a trained
scientist (chemist) and I am familiar with the physics. The opinion and expertise
of our members is welcomed.







               To
start, look at the graph in my first post. The curve is the pressure rise over
time in the gun chamber/barrel as measured by a strain gauge. The rise and fall
in pressure corresponds directly with the acceleration of the bullet and the
recoil imparted to the gun. It is not a 1 to 1 linear correlation but as a
first approximation it’s very close. Now if the gun was a perfect inelastic
system (high rigidity no flexing/compression) then all parts would experience
the same rise and fall in acceleration as traced by the curve, a steep rise to
a maximum then a more gradual drop in acceleration. The entire acceleration and
the corresponding strain impulse takes place over a time frame of 1-2
milliseconds and in that time the gun system has reached it’s maximum recoil
speed. To put this in perspective if the recoil velocity of the gun is 10 f/s
then the gun has only moved 0.065 - 0.125 inches during the acceleration phase
and is now just coasting into your shoulder. This is why perceived recoil has
little to do with the mechanical impulse experienced by the gun. The party is
over by the time you feel the kick.



OK so much for the imaginary perfect ridged system, what
happens with real guns?



The time frame for the acceleration does not change but the
delivery/prorogation of the impulse is modified by the compression and bending
of the parts. In reality a gun is far from being a rigid piece of steel we
imagine and when fired experiences a complex series of flexing and oscillating
motions. Watch this short video of a 50 BMG and notice how much flexing is
experienced, particularly the scope and rail mount.




What you observe is any part that can bend is stimulated by
the impulse and begins to vibrate/oscillate at a frequency and amplitude dictated
by its mass and elasticity/compression constant. Think of a tuning fork and how
the pitch is modified by the weight and length of the arms. This happens to
every part on a gun and the amplitude and duration will depend on the geometry
and rigidity of the components.



A real life example of this would be hitting a baseball with
a bat. A solid hit is felt as a simple sharp impulse on the hand, do that with
a cracked bat and the impulse feels like an electric shock. The sharp impulse
is converted to high frequency oscillations because the bat can vibrate between
the two masses separated by the crack.



Once you digest that
thought it becomes apparent that just quoting a maximum or average G force for
a gun/cartridge combo is just a small part of the equation that describes how any
part of the gun will react to the recoil impulse.



That’s the first part of the problem, complex motion of the
gun and its part.



The second part is the failure mode of the optics, in this
case a PVS-14. My knowledge here is far below some of the resident experts so I’ll
only comment briefly.



From what I have read here and elsewhere the main concern is
the impact of the light amplification ‘plates” on thin film tube designs. These
plates are electrically charged to a high voltage and when the thin gap flexes/compresses
they touch and cause a burn mark. This is seen as black spots in the image and
is permanent.



The question then is how much instantaneous force/acceleration
is required to compress the components to the point of touching. This would presumably
be influenced by the instantaneous peak acceleration of the gun.



As we saw in the video the recoil impulse stimulates the
parts to oscillate and this could also be a factor for damage to the image
amplification tube and its plates. The oscillation amplitude and frequency will
be governed by the overall geometry of the parts and the magnitude/duration of
the impulse.  Hence how and where you
mount the optic could greatly influence the frequency and amplitude of the oscillation.




A good example is the action of a whip. When flexed
correctly you form a high amplitude low frequency wave that propagates down a gradually
thinning shaft. As the wave propagates the mass of the shaft decreases so the
frequency increases and you get an increase in speed. The gain in frequency and
speed is so great that the tip is accelerated to the speed of sound and the
crack is a sonic boom. That’s something like a 100x gain in speed.



An extreme example for sure but the principle is the same.
In our case if see a doubling of the impulse amplitude we could turn a ‘safe’
recoil into a broken scope.



So it turns out that from an engineering/physics point of
view we are trying to describe a very complex system to come to a simple
answer, "can I mount my PVS-14 on XYZ gun and be safe”.



As is usually the case for complex systems you need to test
the system, measure the effects and make your conclusions based on the testing
results. That was my conclusion in my first post, now you have the reasoning
for my opinion.



So would I mount my PVS-14 on a 20 lb 308? Yes



On an 8 lb 308? No



On a 8 lb 223? Yes



Would I mount it on a 5 lb 223? NO



You will have to decide where to draw the line between those
examples because as of today no one "in the know” is providing the details.












 
 
 
Link Posted: 4/18/2012 7:45:56 PM EDT
David, I hope you have an insurance policy on your brain ! All I have to contribute to this thread...
Link Posted: 4/19/2012 3:24:17 PM EDT





Quoted:










You will have to decide where to draw the line between thoseexamples because as of today no one "in the know” is providing the details.







     



Hi Rich_V,

 






It's not so much that no-one is in the know. When a recoil-capable tube is required by the military, they order it. But that's a specific milspec and it's not even correct to say "Milspec" tubes can handle recoil, because it depends on the specification to which a particular tube was made.







In the end, people facing this issue can always go to someone like Vic and ask for a weapon rated tube and pay for it, but the question of recoil and PVS-14's is not just the tube, it's the housing as well. And while you can get a mount that may or may not be stronger, the PVS-14 itself was never intended for higher recoil applications.







So in the end, the people "in the know" are people such as yourself who push the boundaries a little and further the known research. If you want to see how far we've come, visit the archives and see how much this forum has learnt in the past few years. I still cringe at some of the things I wrote when I started out. Especially back when I didn't really know the difference between Gen1 and Gen2 and what the heck was Gen3?  :) It's taken me years of studying what information is out there to gain what little I now know. We're all in the same situation to some extent.







That's one of the reasons why debate and discussion on this forum is so important. We're the ones looking to find out whether a normal tube can withstand more than the recommended recoil and if so, how much?







We do have some anecdotal evidence and from what's been learnt from those who have sacrificed their tubes for our knowledge ( or at least those who have been good enough to tell us when they've suffered recoil damage ) is that the peak recoil seems to be critical to the argument due to other effects, harmonics and such and that with thin film, the "safe" level of peak acceleration with a typical AR15 is around 300 G's. 400 G's you will often get away with and 500 G's is too high a risk. We've also learnt that even with these, some tubes are stronger than others so it doesn't work that way for everyone, however based on the level of specification we get, I try to publish only information that will let people work within safe parameters even though I know some will be able to push the envelope.







This is not the end of this topic - it will continue and later on will come back over and over and I think that's a good thing. I certainly enjoy talking about it :)







And it's clear you've got a firm grasp of physics so I hope you'll keep looking at this topic and digging deeper. The next thing I would recommend if you're able to experiment is to look at the influence that autogating has on this topic - and the issue of harmonically induced oscillation caused by coulumb's law acting in-phase with recoil, which I am pretty sure is why the thin-film tubes are so much more affected than the older thick film tubes, because the thickness of the film itself doesn't appear to have any significant contributing factor that I can find reason to explain.







This forum needs people willing to experiment and dig into these topics so that those who come here looking for reliable advice can find it :)







Regards


David











 
Link Posted: 4/19/2012 5:30:03 PM EDT
David thanks for your insight on this, while I have a grasp of the physics it is in the mode of tube failure that I have much less understanding.

When I wrote my comments I was thinking of the standard PVS-14s with tubes not tailored to weapons mounting. These seem to be what most people own and ask about in regards to recoil myself included.

As you know I have been dabbling in the design of recoil mitigating mounts so understanding if it is the very short peak impulse or the longer period but lower intensity average that is responsible for damage makes a significant difference in the design. Since simple mechanical failure of the housing and other components can happen and not just tube damage I have tried to address both in my designs.

In regards to failure modes you mention the increased recoil sensitivity in autogated systems and suggest harmonically induced oscillation may be the cause. I assume you are referring to the voltage switching causing the differentially charged MCP and Photocathode to oscillate? If that is the case then a badly timed recoil impulse that was in phase with the 'contraction' vibration node would make a normally safe recoil cause damage. If true then would you expect more damage under high light environments since there would be more switching/gating from the power supply?

Rich

Quoted:

Quoted:

You will have to decide where to draw the line between those examples because as of today no one "in the know” is providing the details.

     

Hi Rich_V,  

It's not so much that no-one is in the know. When a recoil-capable tube is required by the military, they order it. But that's a specific milspec and it's not even correct to say "Milspec" tubes can handle recoil, because it depends on the specification to which a particular tube was made.

In the end, people facing this issue can always go to someone like Vic and ask for a weapon rated tube and pay for it, but the question of recoil and PVS-14's is not just the tube, it's the housing as well. And while you can get a mount that may or may not be stronger, the PVS-14 itself was never intended for higher recoil applications.

So in the end, the people "in the know" are people such as yourself who push the boundaries a little and further the known research. If you want to see how far we've come, visit the archives and see how much this forum has learnt in the past few years. I still cringe at some of the things I wrote when I started out. Especially back when I didn't really know the difference between Gen1 and Gen2 and what the heck was Gen3?  :) It's taken me years of studying what information is out there to gain what little I now know. We're all in the same situation to some extent.

That's one of the reasons why debate and discussion on this forum is so important. We're the ones looking to find out whether a normal tube can withstand more than the recommended recoil and if so, how much?

We do have some anecdotal evidence and from what's been learnt from those who have sacrificed their tubes for our knowledge ( or at least those who have been good enough to tell us when they've suffered recoil damage ) is that the peak recoil seems to be critical to the argument due to other effects, harmonics and such and that with thin film, the "safe" level of peak acceleration with a typical AR15 is around 300 G's. 400 G's you will often get away with and 500 G's is too high a risk. We've also learnt that even with these, some tubes are stronger than others so it doesn't work that way for everyone, however based on the level of specification we get, I try to publish only information that will let people work within safe parameters even though I know some will be able to push the envelope.

This is not the end of this topic - it will continue and later on will come back over and over and I think that's a good thing. I certainly enjoy talking about it :)

And it's clear you've got a firm grasp of physics so I hope you'll keep looking at this topic and digging deeper. The next thing I would recommend if you're able to experiment is to look at the influence that autogating has on this topic - and the issue of harmonically induced oscillation caused by coulumb's law acting in-phase with recoil, which I am pretty sure is why the thin-film tubes are so much more affected than the older thick film tubes, because the thickness of the film itself doesn't appear to have any significant contributing factor that I can find reason to explain.

This forum needs people willing to experiment and dig into these topics so that those who come here looking for reliable advice can find it :)

Regards
David


 


Link Posted: 4/20/2012 1:45:04 AM EDT
Heady stuff.

So if I wanted to run a 5lb 5.56, or 6.5lb 7.62, what g forces am I dealing with?
Link Posted: 4/20/2012 5:48:52 AM EDT







Quoted:




David thanks for your insight on this, while I have a grasp of the physics it is in the mode of tube failure that I have much less understanding.
When I wrote my comments I was thinking of the standard PVS-14s with tubes not tailored to weapons mounting. These seem to be what most people own and ask about in regards to recoil myself included.
As you know I have been dabbling in the design of recoil mitigating mounts so understanding if it is the very short peak impulse or the longer period but lower intensity average that is responsible for damage makes a significant difference in the design. Since simple mechanical failure of the housing and other components can happen and not just tube damage I have tried to address both in my designs.
In regards to failure modes you mention the increased recoil sensitivity in autogated systems and suggest harmonically induced oscillation may be the cause. I assume you are referring to the voltage switching causing the differentially charged MCP and Photocathode to oscillate? If that is the case then a badly timed recoil impulse that was in phase with the 'contraction' vibration node would make a normally safe recoil cause damage. If true then would you expect more damage under high light environments since there would be more switching/gating from the power supply?
Rich













Hi Rich,










Under recoil, the rifle moves approximately 28 thousandth of an inch... So an effective buffer against recoil would allow at least that much movement under recoil alone without hitting too much resistance.










Add to this any spring buffer and calculate compressive force as acting on the mass of the PVS-14.










So in simple terms, a two stage buffer would do it, or for single stage, probably any material with a deformity of at least, say 0.05" such that when compressed to that extent, the force it applies to a mass the same as a PVS-14 would result in an acceleration of less than 75 G's...










Or simply put, any material that when compressed by 0.05" from neutral exerts a force over the working area of approximately 288N










( That I've already thought that through to some extent gives you an idea what I'm working on also. )










Additionally, you have to make sure that the buffer acts in an elastic way, that it has no properties that might cause issue - such as shear-thickening properties ( ie, goes hard under shock, transmitting the shock impulse ) and that it won't lose elasticity over time - so the modulus of elasticity or shear modulus must be considered.










So the selection of a buffer material while relatively straightforward should still be carefully considered - and it's not necessary to choose something that compresses as low as 0.05" - after all you can select anything that gives you the correct compression as long as it's at least 0.03" when exerting less than 288N of force. Personally, I'd say use 0.05" as a safety margin.










AND that's based on 5.56mm in a 9lb rifle... The calculations for different calibre, recoil and rifle weight will affect the calculation, but you get the general idea - that's the optimum for a M4 or AR15.










Also, give a little thought as to where you're putting this pressure. The PVS-14 case and mount point isn't designed to take the pressure either :)










On the other matter, yes, that observation is what has been observed anecdotally when damage has occured, so at least anecdotally, there appears to be a correlation there. Also, it's not just high light environments - it's when the level of high light is such that the duty cycle on the autogating is close to 50%.










Regards



David






















 

 
Link Posted: 4/20/2012 6:07:43 AM EDT



Quoted:


Heady stuff.



So if I wanted to run a 5lb 5.56, or 6.5lb 7.62, what g forces am I dealing with?


Halve the mass, double the G's...

 



eg, Since I used an 8.8lb rifle for my calculations, then 8.8/5*300 = ~440G's.. Too much to safely attach a PVS-14 to.




Regards

David
Link Posted: 4/20/2012 7:25:53 AM EDT
Quoted:

Quoted:
Heady stuff.

So if I wanted to run a 5lb 5.56, or 6.5lb 7.62, what g forces am I dealing with?

Halve the mass, double the G's...  

eg, Since I used an 8.8lb rifle for my calculations, then 8.8/5*300 = ~440G's.. Too much to safely attach a PVS-14 to.

Regards
David


Exactly why I said in my post I would not run a PVS-14 on a 5 lb AR15.

The decision to use a PVS mounted on a gun is not as simple as 223 is OK to use  and 308 is not. The weapon weight PLUS the cartridge must both be considered when making your choice.

Newton's second law:
A = F/M
Acceleration = force divided by mass.
Big force, small mass = lots of G force
Big force, big mass = not a lot of G force.

Mounting a PVS-14 on a 50 BMG is not a problem if the gun is heavy enough and  many of them are heavy enough that the max G force is no worse than an average weight AR15 in 223.
Link Posted: 4/20/2012 8:04:27 AM EDT
DT  

Link Posted: 4/21/2012 1:36:29 AM EDT
I see this as an overall picture now, and can only wonder if a recoil reducing stock would mitigate the initial recoil pulse and spread it out to soften the blow to the monocular?

Are g force/shock meters readily available to measure this accurately from one rifle to the next?
Link Posted: 4/21/2012 2:13:40 AM EDT
Quoted:
I see this as an overall picture now, and can only wonder if a recoil reducing stock scope mount would mitigate the initial recoil pulse and spread it out to soften the blow to the monocular?

Are g force/shock meters readily available to measure this accurately from one rifle to the next?


You're thinking along the right lines now but at the moment I am unaware of any commercial scope mount that does this.
Link Posted: 4/27/2012 3:00:15 PM EDT
Not intended for NV use, but there is a scope mount for piston airguns called a Dampa, IIRC.  I have one on my beeman R1.  it wouldn't be hard to design one that is rail mount, but it would take up space.  Basically just bolt the PVS to a block of rubber and bolt the rubber to the gun.  The rubber will flex rather than transfer the full G force, yet it will appear rigid to the user.  A glock frame may be a good analogy.

Link Posted: 4/27/2012 10:30:03 PM EDT



Quoted:


I see this as an overall picture now, and can only wonder if a recoil reducing stock would mitigate the initial recoil pulse and spread it out to soften the blow to the monocular?



Are g force/shock meters readily available to measure this accurately from one rifle to the next?


No. There are companies that will build to spec - but if you have to ask, "How much?" - you can't afford it.  I will be seeing my friend with an EE degree tomorrow and see if I can get him to commit some time to a proposed G meter project I first raised up to him before Christmas.



I am willing to put a reasonable amount of money into building a G meter and have another person who can do custom machinery work.



Other projects have been competing for everyone's time.



 
Link Posted: 4/28/2012 4:18:09 PM EDT


Shockwatch produce suitable G-meter devices for this application. Use the 75G devices if you intend to do this.




Recoil must be mitigated at the mount ( requires much higher level of mitigation ) or inside the housing ( less mitigation required )




Regards

David
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