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NERA basic principle


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#1 Sebastian Balos

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Posted 18 April 2006 - 0408 AM

What's the basic mechanism behind NERA armor?

Why are the plates bulging? Why does the rubber (or other polymer interlayer) push the plates apart? Is the rubber bouncing thanx to it's elasticity? Are there any fundamental papers on this? Paul, Jeff?

Is there a basic difference between HEAT and AP action of NERA sandwiches? Is a complete confinement neccessary to produce bulging?

Would the NERA consisted from rubber sandwiched between perforated plates be feasible? Would perforations prevent the plates from bulging? Perforations would certainly induce additional yaw and tricky internal stresses in the core, which, if bulging would occur, might have an even better effect. What would be the effect of such a combination against AP or HEAT ammo?

Regards, Sebastian

#2 TTK Ciar

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Posted 18 April 2006 - 1431 PM

What's the basic mechanism behind NERA armor?
Why are the plates bulging? Why does the rubber (or other polymer interlayer) push the plates apart? Is the rubber bouncing thanx to it's elasticity? Are there any fundamental papers on this? Paul, Jeff?

I believe it is because the rubber absorbs sufficient energy from the shaped charge jet to vaporize part of the rubber, and this vapor acts as a propellent. This is a good paper for NERA principles, the authors used rubber as their baseline nonexplosive fill to compare against other nonexplosive fills:

Combination of Inert and Energetic Materials in Reactive Armor Against Shaped Charge Jets by Holzwarth and Weimann

Is there a basic difference between HEAT and AP action of NERA sandwiches?

I think so, but I don't know for sure. I think long-rod penetrators impart insufficient energy to the rubber to cause it to vaporize.

Is a complete confinement neccessary to produce bulging?

Again I think so, but I'm not sure.

-- TTK

#3 Sebastian Balos

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Posted 19 April 2006 - 0414 AM

I believe it is because the rubber absorbs sufficient energy from the shaped charge jet to vaporize part of the rubber, and this vapor acts as a propellent.  This is a good paper for NERA principles, the authors used rubber as their baseline nonexplosive fill to compare against other nonexplosive fills:

Combination of Inert and Energetic Materials in Reactive Armor Against Shaped Charge Jets by Holzwarth and Weimann


I'm aware of that paper and it seems that the bulging does have something to do with vaporising the rubber. Do you have any more fundamental paper? IJIE 21 pp.297-305 A Parametric study of the Bulging Process in Passive Cassettes with 2D Numerical Simulations is a great and most informing paper, but the fundamentals are not explained. Some potentially interesting papers are given in it's Peferences, but these papers are from 1992/3 and I can't download them.

I think so, but I don't know for sure.  I think long-rod penetrators impart insufficient energy to the rubber to cause it to vaporize.


What about AP ammo? I understand EAAK is a NERA package, which is effective against HEAT and AP ammo (12.7 and 14.5mm). Now, I can't imagine that such a tiny penetrator from 12.7 and 14.5mm ammo could vaporise rubber. Perhaps rubber elasticity is the key. If so, maybe even perforated plate could be used.

Regards, Sebastian

#4 AlexVP

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Posted 20 April 2006 - 0915 AM

IMO bulging plates defeat mechanism has nothing to do with rubber vaporisation. The rubber (polyurethane actually) placed between metallic plates when penetrated acts as quasi-liquid. The inner plate is moved by hydroshock wawe in polyurethane. This way bulging armor effectiveness against both CE and KE can be explained.

So it looks like polyurethane must be confined and perforating plates is no issue.

#5 Sebastian Balos

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Posted 20 April 2006 - 1015 AM

IMO bulging plates defeat mechanism has nothing to do with rubber vaporisation. The rubber (polyurethane actually) placed between metallic plates when penetrated acts as quasi-liquid. The inner plate is moved by hydroshock wawe in polyurethane. This way  bulging armor effectiveness against both CE and KE can be explained.

So it looks like polyurethane must be confined and perforating plates is no issue.

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Interesting. So, in your opinion, if the rubber acts as quasi-liquid, it's hydroshock wave would be dissipated by the perforations in the plate? Even if the front plate (that the projectile strikes first) is perforated and the backing plate homogenous?

What about the plate that has only partially drilled holes, i.e., a plate with holes at the outside and plain surface at the inside, to confine hydroshock wave? Would this work? I guess it would, but the manufacturing of such partially perforated plate would be more difficult.

Or, let's suppose a similar arrangement, but with a regular perforated plate, followed immediately by a thinner homogenous plate, rubber and the other, e.g. thinner homogenous plate?

All these ideas might work better if the whole NERA package would be angled.

Do you have any references about hydroshock wave theory, perhaps some papers or so?

Regards, Sebastian

#6 TTK Ciar

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Posted 20 April 2006 - 1950 PM

IMO bulging plates defeat mechanism has nothing to do with rubber vaporisation. The rubber (polyurethane actually) placed between metallic plates when penetrated acts as quasi-liquid. [.. snip ..]


Note that we're talking about two slightly separate effects, here. Soviet-style bulging armor uses a stiff outer layer, a PU rubber interlayer, and a flexible inner layer. The inner layer bulges inward, in the direction of the penetrator's travel, and at an angle to it. The T-72B's turret armor and the Enigma modules are of this type. NERA like in Holzwarth and Weimann's paper, and presumably in MEXAS, uses a slightly different mechanism to bulge either both outside layers or just the outer layer. In the latter case, the outer layer is bulged outward, opposite the direction of the penetrator's travel, and at an angle to it.

However, you are right that it doesn't appear to have anything to do with vaporization of the rubber interlayer. From Holzwarth and Wiemann:

In case of an inert interlayer the plate motion is caused by the elastic pressure transferred into the interlayer by the jet. Then the plate motion is only a bulging of the plates.


I apologize for the misinformation.

-- TTK

#7 AlexVP

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Posted 21 April 2006 - 0346 AM

Sebastian, TTK Ciar

I have no any hard evidence to support "hydroshock theory". It is just a guess after looking at its description on NII Stali website - HERE

Exact picture
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The page has a section devoted to "Armor with honeycomb filler" what uses hydroshock effect of some liquid or quasi liquid inside of this honeycomb structure. Then I recalled what exactly was used as a filler in actual armor composition in soviet tanks (polyurethane). So the material has a quasi liquid properties. Thus it is the reason it was put between plates. At least it looks logical for me :)

#8 Sardaukar

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Posted 23 April 2006 - 0815 AM

Years ago I did read that it's also possible to add "inert explosive/propellant" to rubber, making the reaction better. I don't remember it if was tested in Germany or Israel. It was confidential source then and I don't have access to papers any more now.

#9 Paul Lakowski

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Posted 23 April 2006 - 1339 PM

Years ago I did read that it's also possible to add "inert explosive/propellant" to rubber, making the reaction better. I don't remember it if was tested in Germany or Israel. It was confidential source then and I don't have access to papers any more now.

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Is this the paper?

"Combination of Inert and Energetic Materials in Reactive Armor Against Shaped Charge Jets"; Holzwarth and Weimann

http://uploads.armsa...rg/TB611523.PDF

#10 Sardaukar

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Posted 24 April 2006 - 0846 AM

Might well be....teaches one to actually read whe it said on forum posts... :lol:

#11 Sebastian Balos

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Posted 25 April 2006 - 0517 AM

Sebastian, TTK Ciar

I have no any hard evidence to support "hydroshock theory". It is just a guess after looking at its description on NII Stali website - HERE

Exact picture
Posted Image

The page has a section devoted to "Armor with honeycomb filler" what uses hydroshock effect of some liquid or quasi liquid inside of this honeycomb structure. Then I recalled what exactly was used as a filler in actual armor composition in soviet tanks (polyurethane). So the material has a quasi liquid properties. Thus it is the reason it was put between plates. At least it looks logical for me  :)

View Post


This's the paper which might give some answers:

H. S. Yadav, Study of Jet Interaction with Interlayer Material of Bulging Armor, Propellants, Explosives, Pyrotechnics, Volume 29, Issue 6 , Pages 349 - 353

The only problem is that I don't have access to this journal. However, here's the abstract:

The paper presents an analytical approach for determining the effects of different properties of inert material of bulging armor sandwich on its efficiency against a shaped charge jet. The crater and shock wave parameters have been investigated. A theoretical relation for calculating the energy of the shock wave, produced by the jet impact, in the inert material has been derived on the basis of continuity of pressure and particle velocity at the interface of jet and inert material. This relation reveals that the higher the velocity of the jet the greater is the amount of energy in the shock wave. The energy of the shock wave also increases with increase of density and decrease of strength of the inert material. The shock energy has been found to further depend on the constants of equation of state of the inert material, (us=a+b u).

Regards, Sebastian




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