What Is ERA(Explosive Reactive Armor)

M3A2 ODS "Operation Desert Storm" Bradley w/ERA, Explosive Reactive Armor 

Reactive Armours are a type of armour that reacts to the impact of a weapon to reduce the damage to the vehicle. It is very effective in protecting against shaped charges and specially hardened long rod penetrators. The most common type is explosive reactive armour (ERA), but variants include self-limiting explosive reactive armour (SLERA), non-energetic reactive armour (NERA), non-explosive reactive armour (NxRA), and electric reactive armour. Unlike ERA and SLERA, NERA and NxRA modules can withstand multiple hits, but a second hit in exactly the same location will still penetrate.

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.

To be effective against kinetic energy projectiles, ERA must use much thicker and heavier plates and a correspondingly thicker explosive layer. Such "heavy ERA," such as the Soviet-developed Kontakt-5, can break apart a penetrating rod that is longer than the ERA is deep, again significantly reducing penetration capability.

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.

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).

Reference
T-54 and T-55 Main Battle Tanks 1944-2004 By Steven J. Zaloga
Explosive Effects and Applications By Jonas A. Zukas, William Walters, William P. Walters
Tanks and Infantry Carriers 1985-2004 By Marsh Gelbart
http://en.wikipedia.org/