1. Anatomy of Light ERA
The first impetus to develop 'energetic' armours began in the 1960s after the expensive glass and ceramic armours proved defficient. The goal of such research was essentially to use the controlled release of energy to somehow destroy a forming HEAT jet. Logically, most of these ideas utilized the compact chemical energy stored in explosives to push some sort of metal plate into the incoming jet. One early idea incorporated the idea of using explosive 'pills' which were a metal plate backed by a thick layer of explosive. This explosive was confined or tamped by metal sidewalls, thus forming a metal pillbox over the explosive. This setup was then stuck on the surface of a tank and was detonated when the HEAT jet penetrated the cover plate, driving the plate into the jet. This idea was later abandoned by Rafael because the design proved unfeasible due to the large amount of explosive necessary to effect any damage against the jet.
Around 1969, a Norwegian working for Rafael by the name of Dr. Manfred Held discovered the drive-plate explosive sandwich design which later became explosive reactive armour. In this design two rectangular metal plates, referred to as the reactive or dynamic elements, sandwich an interlayer of high explosive. This 'box' is set at high obliquity to the anticipated angle of attack by the HEAT jet, usually 60°. When the jet penetrates the outer plate, the explosive is detonated by the pressures involved and the plates are rapidly forced apart; the acceleration is completed in around 6 us. The orientation of the plates to the explosive detonation front accelerates the front plate upwards in the x-y plane and slightly forwards and conversely forces the rear plate downward and slightly backward. The front plate is moving upward through the path of the jet and it exerts a destabilizing force on it, i.e. there are elastic longitudual waves travelling down the length of the jet. The destabilized jet, i.e. undergoing wave motion, then reaches the rear plate, which is moving in the opposite direction to the original plate. The force exerted by the rear plate is essentially a torque when taken with that of the front plate, and this causes the already destabilized jet to break up into many smaller pieces. These smaller pieces exhibit self-destructive behavoir - namely yaw (the equivalent of the high velocity impact belly-flop) and transverse velocity, which causes them to strike seperate areas of the target's armour.
This X-ray flash photograph shows the drive-plate explosive sandwich in action. The jet is moving from right to left. On the extreme left is the cylindrical steel target; on the extreme right is the slug remaining from the shaped charge. The two reactive elements are clearly visible as well as the atmospheric effects on their outer surfaces. The jet is fairly thick between the plates, but note how it is in many different pieces after it has passed through the rear plate. As a point of interest, the velocity of the flyer plates can be estimated at about 1 200 m/s through comparing the curvature in the plates to an assumed explosive detonation velocity (v = 8 000 m/s).
So what are all the destructive effects visited on HEAT jets by ERA? The largest and most obvious result is the break-up of the jet and rotation of its pieces. There are, however, also some secondary effects that should be kept in mind. The first secondary effect on the jet is mass loss. Essentially, the jet must penetrate (or, in reality, perforate) the ERA plates. While in 'light' ERA these plates are relatively thin, the transverse motion of the plates means that the jet must actually generate a 'slot' rather than a 'hole' in the plates. So if the jet must travel through a 3 mm plate set at 60° with an apparent height of 15 mm, the total amount of armour that must be penetrated is twice that (two plates) or 80 mm. However, since in reality the jet is perforating the plates rather than undergoing radial displacement penetration, this is really more equivalent to 60 mm. Still, it is an important factor. Another important factor is the damaging of the tip of the HEAT jet. The tip of a HEAT jet can be moving in excess of 8 000 m/s, while the outer edges may be closer to 3 000 m/s. The tip of a HEAT jet also acheives initial penetration of the target material, and initiates adiabatic phase penetration (target metal flow). Essentially, the tip of a HEAT jet is the most efficient part of the jet, and it allows the rest of the jet to efficiently pile into the hole it generated and force the armour material out of its path. Removing jet head will reduce the penetration of the jet by 30% or more, even though it is a relatively small part of the jet's mass.
These two secondary effects are actually pretty substantial, contributing as much as 50% to the effect of ERA. Part of the reason for this is that jet breakup - the primary defeat mechanism - is a pretty common phenomena. A HEAT jet is a piece of metal undergoing extremely rapid severe plastic destortion, so any tiny defect in the construction of the cone will be magnified by the enormous forces involved, resulting in critical failure of the material during the formation of the HEAT jet, and hence, some (limited) break-up. It wasn't actually until the late 1970s that we were able to design well constructed cones which would produce a continious jet.
This first generation light ERA generates about 350 - 400 mm RHA worth of protection against large calibre warheads for the vehicle equipped with it. This implies an efficiency multiplier of about 20, which is incredibly high. However, ERA is not some magical shield. It will not completely stop the HEAT jet from a RPG - a backing layer of armour is still necessary to absorb the remains of the HEAT jet.
2. Light ERAs Deployment History
Around 1978 concurrent with the deployment of the M111 'Hetz' APFSDS round, an ERA package called 'Blazer' was produced for the Israeli Defence Force's Mag'lach (M60A1 & M48A3) and Sho't (Centurion) tanks. Later, versions were also produced for Ti-67S (retrofitted T-55) tanks. The package for the Mag'lach massed about 1 000 kg and the package for Sho't massed about 850 kg.
The Israeli application of ERA was rather crude, using large blocks which left large null zones in the armour after detonation. However, it still proved to be quite a marvelous applique during Israel's invasion of Lebannon in 1982.
After the demonstration of ERA in Lebannon, Russian planners deployed their own Kontakt EDZ armour starting with the T-80BV in 1983. Kontakt EDZ was not a copy of Israeli Blazer ERA. Kontakt was developed by the Soviets cocurrently with Rafael's developments, but was not initially fielded because of concerns over safety. This was in 1978. The abbreviation EDZ stands for "Elementy Dinamicheskoi Zashity", this translates into something like "dynamic protection elements". Two types of Kontakt blocks exist, the standard 'brick' as well as the 'wedge' which has only a single fixed reactive element. The wedge is used to cover null zones and it partly relies on the overlap of its neigbouring bricks for its effectiveness. By about 1985 all Soviet model tanks in Grouping Soviet Forces Germany had EDZ packages.
The T-80BV usually carried a 210 - 222 block array of Kontakt EDZ which was layered over the turret front and side, as well as the top. The hull was covered over the glacis and two thirds of the way down the sides. The T-64BV, the other tank in service with GsfG at the time, only carried a 115 block array of charges which provided mainly frontal protection. After front-line forces had been equiped with EDZ, T-72A and T-72B tanks, and later T-62M and T-55AM1 tanks began to receive ERA packages. Unlike the T-64B and T-80B tanks, which usually have the suffix 'V' (vzryvnoi - explosive) added to indicate EDZ such as T-64BV, the T-72 when fitted with EDZ is usually not distinguished in this fashion.
Kontakt EDZ was more advanced than Blazer ERA in a couple respects. Firstly, the blocks are on the order of 40% the size of Blazer blocks, which is considerably more demanding in terms of technology of the explosive interlayer. This also means that the amount of underlying armour exposed after a detonation is less. Secondly, Kontakt is a little more clever in its configuration. The brick is assymetric in its explosive interlayer, meaning that one end is thicker than the other. This induces rotation in the plates as well as separation, and as a result the armour is effective against HEAT jets at a wider variety of angles.
3. Kontakt-5 Heavy ERA
The development of Kontakt EDZ logically led to the development of a later version, called Kontakt-5, which was optimized to be effective not only against HEAT jets, but also APFSDS long rods. It was first deployed around 1985 on the first T-80Us. It is claimed that Kontakt-5 provides about 300 mm RHA equivalent of additional protection against APFSDS rounds, which corresponds to an increase of about 160% over the base armour of the T-80U (~720 mm total).
We've done a lot of work to analyze how effective Kontakt-5 is and by what methods it defeats the incoming APFSDS rounds. The results of the analysis are quite impressive in their own rough and limited way. We assumed that the Kontakt-5 brick was 10.5 cm wide by 23.0 cm long by 7.0 cm thick, with a mass of 10.35 kg. We arrived at a total mass of 2.8 t for the array. We later found out from Steven Zagola's literature that the array is supposed to be around three tonnes, so we were pretty happy. Assuming the use of Semtex for the interlayer, I found that the configuration was most likely a 15 mm plate up front, backed by 35 mm of explosive, and then a 20 mm plate. This assymetrical configuration had improved effectiveness because the APFSDS rod could still 'catch' the retreating rear plate while the front plate would retain a charateristic high velocity. This is completely opposite to the model that the US Army used in the late 1980s to discribe 'heavy' ERA. In their model, the front plate was on the order of 60 mm thick and the rear a standard 5 mm plate. They thought that the thick plate simply moved up into the path of the incoming long rod and forced it to make a 'slot' (thickness x height) rather than a hole (thickness). This is bogus; the front plate would tamp the explosive and would be barely set in motion.
Anyway, back to the point. Without getting into the actual math, after a couple of analyses, we arrived at our conclusion as to what defeat mechanisms were being imployed. These conclusions have not yet been conclusively proved and we hope to do that soon. We assumed that the massive areal density of the long rod perforated the thin plates with relative ease. Actual ablatic penetrator mass loss was set at about 2%. What we found was that we had these two plates, each individually with about 60% the momentum of the long rod penetrator, were moving oppositely up/down to each other, and that the path of the penetrator was such that it was moving between them. The forces exerted on the penetrator are apparently very large, so large in fact that they were in the region of plastic failure for most (read: all) metals. Essentially, when the penetrator touches the rear plate, the front plate guillotines off the first 5 - 6 cm of the rod. For a round such as the 120 mm M829A1 this represents a loss of about 8% of the total mass. More importantly, the nose is blunted. You would not believe how important that sharp point on the penetrator is. The difference in penetration between an equivalent hyper-sonic spike tipped penetrator and a blunt nose one is at least 20% (to a maximum of around 30%). This is mainly because a blunt nose is very inefficient in the initial phase of penetration before the ablatic shear phase can begin. The penetrator has to actually sharpen itself to the optimum Von Karam plastic wave theory shape for penetration of the target material before it can begin radially displacing the target material. This resolves itself in the form of a lot of wasted work and thus penetrator mass. The blunted penetrator also suffers structural damage and more mass loss as a shock wave travels down its length and blows spall off the tail. The main secondary effect of Kontakt-5 EDZ against APFSDS rounds is yaw induced by the front plate before contact with the rear plate is established. The total is about two to three degrees of yaw, which suddenly becomes a lot more in a denser material such as steel. Reduction in penetration due to a 2° yaw is about 6% and it grows exponentially worse from there, and on the 67° slope of the front glacis of the T-64/72/80/90, this is increased to about 15%.
Total loss in penetration amounts to about 2% + 8% + 22% + 6% = 38%, or in other words the penetrator is now only capable of penetrating 62% its original potential. Conversely we could say that the base armour is increased by the factor of the reciprocal of 62%, which is - surprise! - 161%.
So was I surprised by the results? Not really. I had expected penetrator yaw to be the primary defeat mechanism, but otherwise we had verified the effectiveness of Kontakt-5 before it became general public knowledge, which is great bragging rights.
Of course, now the goal is to do a rigorous mathematical proof.
Jane's International Defence Review 7/1997, pg. 15:
"IMPENETRABLE RUSSIAN TANK ARMOUR STANDS UP TO EXAMINATION
"Claims that the armour of Russian tanks is effectively impenetrable, made on the basis of test carried out in Germany (see IDR 7/1996, p.15), have been supported by comments made following tests in the US.
"Speaking at a conference on Future Armoured Warfare in London in May, IDR's Pentagon correspondent Leland Ness explained that US tests involved firing trials of Russian-built T-72 tanks fitted with Kontakt-5 explosive reactive armour (ERA). In contrast to the original, or 'light', type of ERA which is effective only against shaped charge jets, the 'heavy' Kontakt-5 ERA is also effective against the long-rod penetrators of APFSDS tank gun projectiles.
"When fitted to T-72 tanks, the 'heavy' ERA made them immune to the DU penetrators of M829 APFSDS, fired by the 120 mm guns of the US M1 Abrams tanks, which are among the most formidable of current tank gun projectiles.
"Richard M. Ogorkiewicz"
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