Edge Effect on ERA

Full Paper Edge Effects on ERA– Sandwiches Manfred Held* TDW, D-86523 Schrobenhausen (Germany) DOI: 10.1002/prep.200600015 Abstract The momentum transfer of thick flying plates of an ERA sandwich and iterative disturbances of shaped charge jets by thin flying plates are shortly described. Hits on the sandwich plates, out of the centre, in the horizontal direction are not reducing the effectiveness, as long as the high explosive charge is promptly ini
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  Full Paper Edge Effects on ERA–Sandwiches Manfred Held* TDW, D-86523 Schrobenhausen (Germany)DOI: 10.1002/prep.200600015 Abstract The momentum transfer of thick flying plates of an ERAsandwich and iterative disturbances of shaped charge jets by thinflying plates are shortly described.Hits on the sandwich plates, out of the centre, in the horizontaldirection are not reducing the effectiveness, as long as the highexplosive charge is promptly initiated. In vertical directions theauthor has found out that in case of hits on top or the bottom,where only the front respectively the rear plates are interactingwith the jets, the reduction effects in penetrations remain alsomore or less constant.Changing the azimuth angle up to 30 8  the residual penetrationsare also not increased or decreased. Keywords:  Explosive Reactive Armor (ERA), Shaped ChargeProtection, Edge Effects, Side Hits, Jet Disturbances 1 Introduction Explosive reactive armors (ERA) generally consist of symmetricorasymmetricsandwichesofmetalplateswithaninternalhighexplosivelayer([1]and[2]).Bytheimpactofashaped charge jet the high explosive charge detonates moreor lesspromptly [3]and drivesboth attached metal plates inoppositedirectionswithwellpredictablevelocities[4].Ifthesandwich is arranged under an angle to the shaped chargeaxis and the plates are relatively thick, then the jet is mainlydisturbed by the transfer of momenta along the passing jet[5].Iftheplatesarethin,thentheimpactofthejettipcreatesa hole, which is larger than the jet diameter.The edge of thehole is touching the following jet segment after a short timeby the oblique movement of the inclined plate. By suchiterativecontactsradialbubblesarecreatedinthejet,whichare observable on flash X-ray pictures (Fig. 1).Thisiterativelyongoingprocessisdescribedagaininmoredetails. The touch of the jet at the hole edge of a thin flyingplateopensthisholealittlemore,thereforethefollowingjetsection can now pass the hole less or fully undisturbed, untilto the point, where the end of the slit in the oblique flyingplatewilltouchagain thejet,creating thenext bubblein thefollowing jet segment. The sequence of undisturbed jetsegmentsanditerativeradialeruptedsegmentsisverynicelyvisible on the flash X-ray picture of Figure 1, where a jet of 9 km/s velocity has hit a symmetric ERA sandwich 3/3/3with a 3 mm mild steel plate, 3 mm high explosive layer andagain a 3 mm mild steel plate under 60 8  NATO-angle. Theupperpicturewasmadeafter80  m  s,justashorttimeafterthedetonationofthehighexplosivechargelayer.Thesandwichplates are flying in opposite directions with about 800 m/svelocity. After further 86  m  s time difference, in total 166  m  sdelay time, the second lower FXR picture was gained. The jet is remarkably stronger disturbed compared to the firstflash X-ray picture.Theplateflyingagainstthejetshouldgeneratebubblesinthe upper direction. Such bubbles are only a little observed.After the pass of both plates only downwards bubbles arevisible, caused by the rear plate, which is flying with the jet.This does not mean that no bubbles are created from thefront plate. The main reason is, that they are filtered out bythe rear plate, flying with the jet. This is really a little bitworse. Because bubbles flying in the upper direction areconsuming some material of the rear plate and are reducingtheir interaction potential with the jet. It is surprising thatpeople are working for a long time now with ERAsandwiches, but these complicated interactions of 2 thinplates with shaped charge jets have not really taken intoaccount all these details, at least in the open literature.Naturally the rear plate, flying with the jet, is longer timeinterfering with the jet sections. Therefore the jet is moredisturbed than by the front plate, which has shorterinteraction times by the higher relative velocities and istherefore something less disturbing the jet.The residual jet is partially lifted on the second FXRpicture in the lower velocity range by the transferredmomentum [6]. 2 Horizontal Edge Effects The question arises, what is the residual penetration of asandwich arrangement, if the jet hits the plate just at one of the edges. * E-mail: manfred.held@tdw.lfk.eads.net98 Propellants, Explosives, Pyrotechnics  31 , No.2 (2006) 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim  The jet has hit a symmetric sandwich of 4 mm mild steel,2 mm layer of high explosive and again 4 mm mild steelunder 60 8  NATO-angle at different distances from the side.The used 96 mm shaped charge with wave shaper hadaround 9 km/s jet tip velocity. It was fired at two caliberstandoff against a 10 mm RHA plate under 60 8  NATO-angle and in further 100 mm perpendicular or 200 mm lineoff sight distance against the ERA sandwich. The mild steelwitnessblockswerearrangedatadditional500 mmdistance.The large distance was selected by a number of differenttests to get FXR pictures of the disturbed jets behind theERA sandwiches. Therefore the mild steel blocks werearranged in 1050 mm standoff from the charge to get theresidual penetrations (Fig. 2).The firings were done between 50 mm, 30 mm, 20 mm,10 mm, 5 mm and 0 mm distance from the side edge, but inthe middle of the plates in the vertical direction. At 0 mmdistancefromtheedgearesidualpenetrationof575 mmwasachieved, which is very near to the normal penetration of this charge at this standoff. In this case the reactive armorwasprobablynotinitiatedbythejettip,ifatall,thentoolate.But at 5 mm beginning from the side edge the penetrationsareintherangebetween120 mm and245 mm withnotrendin any direction (Fig. 2). These individual firings are singlevalueswiththenormalscatterofsuchtests.Butthesefiringsdemonstrate, if the high explosive charges in the sandwicharedetonating,thedisturbancesoftheshapedchargejetareequal at different hit positions in the horizontal plane. 3 Vertical Edge Effects With hits on the top or the right side of the sandwich afterFigure 3, only the plate flying against the jet is interactingwith the elongating jet. By hits on the bottom only the plateflyingwiththejetisinteracting.Themagnitudeofreductionis maybe a little different, but there were not found anynoticeable or remarkable vertical edge effects, if the semi-infinite target has at least 5 caliber distance from thesandwich.An interesting paper was recently published by Ismail [7]to this topic. He has experimentally found a little bitdifferentresults.Withasmallshapedchargehehasgot69%reductionoftheresidualpenetrationatahitonthetop,85% Figure 1.  Flash X-ray picture of a copper shaped charge jet with 9,08 km/s tip velocity of a 96 mm diameter shaped charge, disturbed bythe interaction with two 3 mm thick mild steel plates, flying with about 0,8 km/s velocity in opposite directions under 60 8  NATO–angle,photographed at 80  m  s and 166  m  s delay times to the warhead detonation. Figure 2.  Shaped charge test number, plate thicknesses of 10 mm armor steel spaced plate and 4/2/4 ERA sandwich on the top line andbeneath, all the distances up to the mild steel blocks with the achieved residual penetrations in 11 caliber standoff (shaped chargediameter 96 mm).Edge Effects on ERA–Sandwiches 99 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim  Prop., Explos., Pyrotech.  31 , No.2, 98–101  in the middle of the sandwich and 86% on the bottom afterFig. 3. But he explained this already, that at top a smallerdistanceexistsfromthe60 8 inclinedsandwich,probably3/3/3 to the vertical arranged witness block, compared to thebottom, where the distance is remarkably increased. Thesedifferent air gap distances from the ERA sandwich to thewitnessblockhavecausedthesmallerreductionvalueatthehits on the top. 4 Angle effect Innormalteststheshapedchargesarefiredperpendicularto the plane of the sandwich. In practice the shaped chargescan also attack reactive armor under some additionalazimuth angles. Therefore also few fundamental tests aredone with changing azimuth angle with a 60 8  NATO-angleoriented sandwich under the same test configurations asbefore. The definitions are shown in Fig. 4. The residualpenetrationresultsof0 8 ,10 8 ,20 8 and30 8 azimuthanglesarepresented in Fig. 5, where all the values are similar and notrend is recognizable. 5 Summary The results show, that hits away from the centre inhorizontaldirectionsarenotinfluencingtheprotectionlevelof an explosive reactive armor sandwich, as long as the highexplosive charge is promptly initiated.In addition it is surprising, that hits on the edges in thevertical direction are also not remarkably changing the Figure 3.  Hits on an inclined ERA sandwich at top or at the bottom with different arrangements of the witness blocks Figure 4.  To the 60 8  elevation angle  e  was added an additional azimuth angle  a .100 M. HeldProp., Explos., Pyrotech.  31 , No.2, 98–101   2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim  stopping power, even for hits in the down position of Figure 2 the jet is only interacting with the front plate or forhits on the upper edge only with the rear plate.For research tests the witness plate should be inclinedunder the sameangle as theERA sandwich, to getalso overtheir length constant distances between the disturbancemechanism and the witness blocks. This can be in practicesometimes different.This constant defeating behavior against shaped charge jets can be explained in the way that an already stronglydisturbed jet cannot be effectively reduced a second time inhis performance. 6 References [1] M. Held,  Schutzeinrichtung gegen Geschosse,  DE 2008156, 1970 , Messerschmitt-Bçlkow-Blohm GmbH, Mnchen, Ger-many;  Dispositif de Protection Contre Projectiles ou Corps Analogues  FR 2436361  1974 , Messerschmitt-Bçlkow-BlohmGmbH, Mnchen; Germany,  Structure for Protection against Projectile , GB 1581125,  1974 , Messerschmitt-Bçlkow-BlohmGmbH, Mnchen; Germany;  Protective arrangement against  projectiles, particularly hollow explosive charge projectiles , US4368660,  1983 , Messerschmitt-Bçlkow-Blohm GmbH, Mn-chen; Germany.[2] M. Held, Overview on Reactive Armour,  European ArmouredFighting Vehicle Symposium , May 28–30,  1996 , Shrivenham,UK.[3] M. Held, Initiation Phenomena with Shaped Charge Jets,  9thSymposium (International) on Detonation,  Portland, OR,August 28–September 1,  1989 , p. 1416 and p. 1432.[4] M. Held, Plate Velocities for Asymmetric Sandwiches,  Pro- pellants, Explos., Pyrotech.  1997 ,  22 , 218.[5] M. Held, Momentum Theory of Explosive Reactive Armors, Propellants, Explos., Pyrotech.  2001  26 , 91.[6] M. Held, Anti ERA Shaped Charge Warhead System,  18th International Symposium on Ballistics , San Antonio, USA, 484, 1999 [7] M. M. Ismail et al., Optimization of Performance of ExplosiveReactive Armours,  21st International Symposium on Ballistics ,Adelaide, Australia, Vol. 1, 227,  2004 . (Received July 21, 2005; Ms 2005/040) Figure 5.  The residual penetration is not changing with the small variations of the azimuth angles from 0 8  to 30 8 .Edge Effects on ERA–Sandwiches 101 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim  Prop., Explos., Pyrotech.  31 , No.2, 98–101
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