Cold War, Modern Day

Why are Some Tanks Covered in Blocks?

When looking at vehicles and tanks from the Soviet era you would have likely noticed that they are often covered in little blocks. These funky bricks are actually a type of armor that is known as ERA.

Shockingly, these blocks contain explosives. Their name, ERA, stands for explosive reactive armor.

But why would you cover a tank in explosives? Isn’t that exactly what your trying to defend against?

ERA is a very clever, real life example of fighting fire with fire.

ERA on a turret roof.
T-72B1 covered in ERA. Image by Vitaly V. Kuzmin CC BY-SA 4.0

ERA is placed on top of a tank’s pre-existing armor in an attempt to increase protection. It is highly effective against HEAT rounds, and can even stop some kinetic energy penetrators.

However, ERA is certainly not invincible, and has a number of flaws that can be capitalised on.



Explosive reactive armor dates back to the mid 20th Century Soviet Union, as a response to developments in anti-tank weapons, particularly HEAT ammunition.

HEAT stands for high explosive anti tank, a type of ammunition that completely transformed tank warfare and design.

HEAT projectiles contain a shaped charge, which is an explosive that focus its energy into a very small point.

In most anti-armor applications, this is usually done by surrounding a copper cone with an explosive charge. When detonated, the explosive energy compresses the copper, forcing it into an extremely fast-moving jet.

Heat round cutaway.
Cut away of a HEAT round. The copper cone is projected forward into a thin jet by the explosive charge behind it. David Monniaux CC BY-SA 3.0.

This jet can move at up to 10 km/s and has enough force to punch through thick armor. As the penetrating effects are generated within the projectile itself, it does not need to be fired at high velocities, unlike conventional armor-piercing rounds.

The explosive and high-temperature nature of HEAT rounds often means that a successful penetration results in the detonation of the tank’s ammunition.

In the post-WWII world HEAT rounds effectively rendered thick armor useless. In fact, they were so good that designers actually stopped using thick armor – like on the Leopard 1 – as it added huge amounts of weight but couldn’t stop HEAT rounds.

Results of a HEAT round impact.
Damage caused by HEAT rounds.

As these rounds were being developed by all sides during the Cold War, the Soviets created explosive reactive armor as a means to defend against them.

The research into this type of armor began in the late 1940s, with working prototypes undergoing testing in the 1960s.

How ERA Works

ERA consists of an explosive sandwiched between two metal plates. In a real life configuration, this is the blocks you see on the exterior of tanks. They are placed on top of the tank’s main armor.

When a projectile strikes a ERA block, it punctures the outer metal plate and detonates the explosives packed inside.

This is where the clever bit happens.

ERA Block Diagram
A diagram of a single block of ERA. As shown here, it is mounted on top of the tank’s armor.

The explosion forces the surrounding metal plates outwards, interacting with the high-speed copper jet heading towards the tank’s armor.

The motion of these plates increases the amount of metal the jet must pierce, as their movement feeds more and more metal into the jet’s path. This is because the blocks are usually mounted at an angle, so their outward movement is actually a sideways action relative to the copper jet.

With much more material to travel through, as well as the forces exerted by the moving plates, the ERA will hopefully disrupt and break up the jet. Even if the jet reaches the armor, the ERA would have slowed it down enough to prevent full penetration.

ERA Block Detonation Diagram
Starting from the left, the incoming jet triggers the explosives. Then, the ERA’s plate is propelled outwards through the jet, forcing it to cut through more material. Finally, the jet is defeated and the ERA block is destroyed.

ERA can also work against kinetic energy penetrators, although this requires thicker plates and more explosives. The ERA attempts to destabilise the projectile and reduce its armor penetrating efficiency, or potentially even shatter it.

Some of the best ERA can reduce an APFSDS round’s penetrating ability by 50 percent.

Speed is key for ERA, as the faster the plates move the more material they can feed into the incoming armor penetrators.

Additionally, the vehicle under the blocks must be sufficiently armored to survive the blasts from the ERA.

Not All Good

While extremely effective, ERA is not an invincible system. It is inherently weaker against kinetic energy penetrators.

But its biggest weakness is its inability to defend the same place twice. As ERA blocks literally explode, each block is single use only. Blocks detonated by an incoming round are now spent and must be replaced.

This area is now unprotected by ERA and is vulnerable to a second strike. In many cases hitting the same place twice is incredibly rare, but weapons have been created to exploit this weakness.

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Tandem-charges do exactly this. These devices contain multiple explosives and are ERA’s kryptonite. When a tandem-charge meets ERA, a smaller, first-stage explosive triggers the reactive armor and clears the way.

The second charge then detonates and strikes the tank’s actual armor in the same spot. This is a smart way of hitting the same place twice.

An Israeli Magach 3 with ERA.
M48 Patton Magach 3, fitted with explosive reactive armor. Image by Oren Rozen CC BY-SA 4.0.

Weapons like the FGM-148 Javelin contain tandem charges, and have proven to be ERA’s undoing.

Also, the armor is dangerous to people around it. Although the impact of an anti-tank round is a violent enough event to cause harm to nearby troops, the detonation of ERA blocks fling shrapnel at high speeds.

Therefore it is wise for troops to keep distance between them and an ERA covered vehicle when in combat.

The Soviets took a strong liking to ERA and applied it to many of their own vehicles, as well ones from the USSR’s republics. Even today, Russia and many eastern European countries equip their vehicles with explosive reactive armor.

Another Article From Us: What Does a Muzzle Brake Do?

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As always with arms and armor, there has been a constant back and forth between ERA and projectiles that can defeat it, and this will likely continue in the future.