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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
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
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. |
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Posted: 4/17/2012 10:03:05 AM EDT
[#1]
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
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Posted: 4/17/2012 10:12:29 AM EDT
[#2]
Don't try mounting one on a spring piston 4.5mm air rifle....
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Posted: 4/17/2012 10:37:58 AM EDT
[#3]
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 |
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Posted: 4/17/2012 11:17:19 AM EDT
[#4]
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
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 |
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Posted: 4/17/2012 2:10:03 PM EDT
[#5]
Does it go without saying that a .458 socom or a Saiga 12 would be a no go for a gun mount?
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Posted: 4/17/2012 3:53:59 PM EDT
[#6]
So is a 12lb hk91 safe then?
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Posted: 4/17/2012 4:04:19 PM EDT
[#7]
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? |
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Posted: 4/17/2012 5:59:26 PM EDT
[#8]
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 |
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Posted: 4/17/2012 6:09:58 PM EDT
[#9]
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? |
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Posted: 4/17/2012 7:09:36 PM EDT
[#10]
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? 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 |
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Posted: 4/17/2012 7:21:26 PM EDT
[#11]
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? |
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Posted: 4/17/2012 7:29:41 PM EDT
[#12]
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. |
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Posted: 4/17/2012 7:36:33 PM EDT
[#13]
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... |
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Posted: 4/17/2012 7:45:20 PM EDT
[#14]
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
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Posted: 4/17/2012 7:47:59 PM EDT
[#15]
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 |
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Posted: 4/17/2012 9:28:21 PM EDT
[#16]
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 |
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Posted: 4/18/2012 6:08:21 AM EDT
[#17]
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. |
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Posted: 4/18/2012 7:45:56 PM EDT
[#18]
David, I hope you have an insurance policy on your brain ! All I have to contribute to this thread...
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Posted: 4/19/2012 3:24:17 PM EDT
[#19]
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 |
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Posted: 4/19/2012 5:30:03 PM EDT
[#20]
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 |
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Posted: 4/20/2012 1:45:04 AM EDT
[#21]
Heady stuff.
So if I wanted to run a 5lb 5.56, or 6.5lb 7.62, what g forces am I dealing with? |
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Posted: 4/20/2012 5:48:52 AM EDT
[#22]
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 |
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Posted: 4/20/2012 6:07:43 AM EDT
[#23]
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
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Posted: 4/20/2012 7:25:53 AM EDT
[#24]
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. |
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Posted: 4/20/2012 8:04:27 AM EDT
[#25]
DT
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Posted: 4/21/2012 1:36:29 AM EDT
[#26]
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? |
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Posted: 4/21/2012 2:13:40 AM EDT
[#27]
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. |
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Posted: 4/27/2012 3:00:15 PM EDT
[#28]
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.
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Posted: 4/27/2012 10:30:03 PM EDT
[#29]
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. |
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Posted: 4/28/2012 4:18:09 PM EDT
[#30]
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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