Explosive Reactive Armor - some history, some types

Explosive Reactive Armor (ERA) is a type of modern special armor. ERA is formed by using one or multiple layers of high explosive between armor plates.

ERA can be manufactured as add-on armor or integrated into the armor of a vehicle. Add-on ERA usually consists of a smaller tiles or boxes, which are attached to the vehicle by bolts. The use of smaller tiles reduces weakspots once the ERA has been detonated.

The working mechanism of ERA, patent drawing from Dr. Held

The working mechanism of the most common types of ERA follows a very simple design. A layer of high explosives is sandwiched between two metal plates, together this is forming a reactive element. When being penetrated by a shaped charge jet or another projectile, the high explosives will detonate and propell the two metal plates into opposing directions. This has two effects: more mass (metal) is moved into the path of the projectile and the converse movement of the metal plates will have a disrupting effect on the projectile, shattering the shaped charge jet.

To properly work however, the reactive element has to be inclined in relation to the penetration path of the shaped charge jet.

Heavy ERA is able to destroy or at least damage modern armor-piercing ammunition like the long-rod penetrators used in APFSDS ammunition.

The Bradley IFV is a prime example for bad ERA coverage

However ERA has in general a number of drawbacks:

• ERA is expended after one hit, leaving an exposed area on the tanks surface
• ERA can be detonated by precursor charges and other projectiles
• most types of ERA don't provide protection against tandem charge ammunition
• fragments from ERA are spread over a larger area, increasing collateral damage

On the drawback of poor coverage of a vehicles surface with ERA, the Swedish FMV came to the conclusion that in case of the T-80U the Kontakt-5 ERA should be considered "50% protection", because only about half of the frontal surface is actually covered by the ERA.

According to Russian sources, the Soviets started their earliest research in explosive (reactive) armor in 1949. However the whole Soviet ERA development ended without success or the adoption of any type of ERA, following poor design choices and accidents.

The ERA in the form as known today was invented - or popularized in the case one values the fruitless Soviet developments high - by the German researcher Dr. Manfred Held. Dr. Manfred Held traveled to Israel following the Six-Day War. He was an expert for shaped charge warheads and wanted to examine the effects of shaped charges on real Soviet-made tanks, like the ones used by the Arab nations in the Six-Day War. Following his investagions of damaged T-54 and T-55 tanks, he discovered in some cases, the detonation of ammunition has reduced the damaged caused by the shaped charge warhead. He developed a first type of working ERA and patented it in 1970 (German patent 2053345). This original ERA also provided protection against kinetic energy penetrators like APDS and APFSDS, but was impractical and complicated compared to more modern designs. It was designed as integrated ERA, which required the vehicle to have special cavities into which the reactive element and explosive was inserted.

Held then patented (German patent 2008156) a more refined version of ERA in the same year, which happens to be very comparable to modern ERA, consisting of a tile formed by two thin steel plates with an explosive material sandwiched inbetween.

While not having any success in persuading Germany or other NATO members to adopt his developments, things changed when Dr. Held came back to Israel in 1974 for further research following the Yom-Kippur War. After he presented his invention to a delegation of the IDF, the Israeli government issued Rafael to develop a type of ERA for series production.

The first type of explosive reactive armor adopted on a tank was Blazer ERA. Blazer used a rather primitve construction using one reactive element per tile. Each reactive element consisted of a three milimeter steel plate, a three milimeter thick layer of explosives and a further three milimeter thick steel plate. Blazer was first deployed in the Lebanon War of 1982.

Cross section of Blazer ERA mounted on a M48 Patton tank. Two reactive elements are bolted onto the steel case.

Blazer ERA uses an insensitive explosive filler, which does not detonate upon penetration by small arms and medium caliber ammunition up to 23 mm AP. Upon penetration by a shaped charge jet, the steel plates accelerate to a velocity of approximately 800 meters per second. Blazer ERA could reduce the penetration of an RPG-7 projectile from 300 to about 100 milimeters into steel, although the absolute figures are depending on the angle of impact.

IDF M60 tank fitted with Blazer ERA

However Blazer in it's original form had a number of drawbacks. It provided rather poor coverage on many vehicles (including the M48, M60 and Centurion tanks), leaving unprotected gaps in the layout. Due to using many different shapes of ERA tiles, Blazer was a logistical burden compared to other types of ERA.

The Slovenian M-55 upgrade used (Super Blazer) ERA for protection.

Blazer has been succeded by the Super Blazer ERA in the 1990s/2000s. Aside from being more effective than it's predecessor, Super Blazer also offers some amount of protection against kinetic energy penetrators and is not being initiated by KE ammunitions up to 23 mm AP and fragments from artillery and mortar rounds detonating as close as 2 metres away.

The Soviet Union was the second country to adopt explosive reactive armor in form of their Kontakt ERA, which was first introduced in 1984. In most forms Kontakt ERA uses two reactive elements, although there might have been versions with just one reactive element. The reactive elements are located in rather small boxes (314 x 148 mm base area) with three milimeter thick steel walls. The reactive sandwich with the designation 4S20 consists of a 2 mm thick steel plate, a 7 mm thick explosive and a 2 mm thick steel plate. A Kontakt ERA tile with two reactive sandwich plates has a weight of 5.7 kilograms without attachment bolts. About 151 tiles are used for a tank (although the number varies depending on tank type and surface area).

Kontakt-1 ERA tile showing the arrangement of the two elements.

Just like Blazer, the original Kontakt ERA did not provide any protection against armor-piercing ammunition and used insensitve explosives in order to prevent the detonation by small arms fire.

According to Russian sources, Kontakt ERA provides protection equivalent to up to 400 mm rolled steel armor (RHA) when hit by single-stage shaped charge warheads. Unlike Blazer, Kontakt provides protection at a greater amount of angles, because of the use of two reactive elements aligned at different angles. The smaller size of the Kontakt tiles is also favorable.

Kontakt-5 was the Soviet follow-up development to the original Kontakt ERA, which was designed to also provide protection against armor-piercing ammunition. Unlike the previous generation of Kontakt ERA, Kontakt-5 was considered "integrated ERA" by the Soviets, because of the mounting mechanism and the. Kontakt-5 is a type of heavy ERA.

Kontakt-5 uses a heavy steel shell, into which the 4S22 reactive element is inserted. Depending on where the tile is used, one or two reactive elements are installed. The 4S22 reactive elements are using more powerful explosive fillers with a TNT equivalency of 0.33 kilograms compared to the 0.28 kilograms of the earlier 4S20 reactive element of Kontakt-1.

Kontakt-5 armor layout and 4S22 tiles

Nii Stali claims that Kontakt 5 ERA provides protection equivalent to 400-500 mm steel against RPGs and ATGMs, 200-250 mm steel against HEAT tank and artillery ammunition, and reduces the penetration of APFSDS by 20%. This is said to increase the armor protection of the T-72B by factors 1.2 against APFSDS and 1.9-2.0 against shaped charges. Nii Stali also claimed that a T-55 eqiupped with Kontakt-5 ERA is able to resist APFSDS ammunition with up to 400 mm penetration.

Kontakt-5 provides protection against APFSDS by using a combination of more powerful explosives charges and thick coverplates - this results in shattering of the penetrator or bending of the rod. However more modern types of APFSDS can defeat Kontakt-5 ERA without being destroyed or strongly deformed.

The DYNA reactive armor was developed 1988 by the Czech military repair facility in Novy Jicin together with the military technical institue of Slavicin. Unlike the previous mentioned types of reactive armor, DYNA does not use reactive elements formed by simple steel plates with explosives sandwiched inbetween, but rather uses rhombus shaped elements (◊).  These consist of a thin-walled hollow steel rhombus which is coated with a layer of explosives. The explosvies are covered by another thin steel layer.

Each ERA tile for the frontal armor of the local T-72M4Cz versions holds a number of rhombus shaped elements. Based on photographs it appears to be up to five rhombi in a DYNA tile

DYNA reactive armor showing the ◊-shaped steel and the explosive filler (white)

According to figures from the Czech military, when mounted onto a T-72M1 the protection is increased to 1.3 times the original protection against APFSDS, and to 2.2 to 2.7 times the original protection against RPGs and missiles.

The construction of the DYNA reactive armor used for protection the tank's roof and hull might be different, at least the thickness of the used ERA modules is different.

Nozh and Duplet reactive armor have been developed in the Ukraine. Instead of just using explosive materials sandwiched between simple steel plates, the Ukrainian engineers decided on using several linear shaped charges (LCS). The LCSs consist of either copper or aluminium and have a classic "half moon" shape [☾]. In the hollow center section, high explosives are inserted. 

Upon penetration, the explisve filler of the linear shaped charges will detonate and accelerate the liner material. This will form a shaped charge jet (or rather a bar), just like in a convential shaped charge as used on HEAT warheads.

Different reactive elements for Nozh and Duplet ERA.

Unlike many other types of ERA, the Duplet ERA consist of many different layers ontop of each other, the space between the different ERA layers is partly covered by solid steel elements. In case of the T-84M Oplot-M main battle tank, there are up to three layers of Duplet ERA at the turret and the hull sides, whereas the glacis armor utilizes only two layers.

Thai T-84M Oplot-M with three ERA layers at the turret front

As a result of Duplets multi-layer design - consisting of up to three reactive armor tiles sandwiched spaced by rather heavy, solid blocks of steel - Duplet ERA cannot be utilized on small and lightweight vehicles.Per square metre Duplet ERA weighs between 400 and 500 kilograms.

According to figures provided by the Ukranian manufacturer, Nozh and Duplet provide between 1.5 and 2.5 times as much protection as the "best world analogues" like Kontakt-5 and Blazer. Other, even more questionable figures from Ukranian sources claim that Duplet can reduce the penetration of unknown types of single stage shaped charge, tandem shape charge warheads and APFSDS by up to 90%. These values are most likely propganda or based on completely obsolete ammunition - the combat performance of Ukranian T-64 and T-72 tanks has been rather poor, despite using Nozh ERA.

The Polish ERAWA reactive armor is designed for extended coverage of the tank's suface in comparision with other ERA solutions. The tiles are quadratic and smaller than those of the previously mentioned ERA types. This means that ERAWA ERA tiles can be fitted to a tank nearly "seamless".

The ERAWA-2 tile and layout

ERAWA-2 is a multilayer ERA version of ERAWA, using two reactive elements in one tile. This provides higher protection against shaped charge warheads with a precursor charge or tandem warhead. It also offers some degree of protection against explosviely formed penetrators (EFPs) and APFSDS, however only against older types. 

The tiles of both ERAWA and ERAWA-2 have a size of 150 x 150 militmetres. ERAWA has a thickness of 26 mm and weighs 2.9 kg. It contains of one single but rather thick reactive element consisting of steel plates and explosive located inside a steel casette mounted on bolts.

The multilayer ERAWA-2 ERA utilizes a thicker tile of 46 mm containing two reactive elements. The coverplate is thicker and might include a non-metallic composite content. This leads to a weight of 4.7 kg per tile.

Polish BMP with CERAWA-1 ERA

CERAWA is a prototype type of ERA based on ERAWA, but designed to incorporate a ceramic tile into the reactive element instead of mainly relying on metalic materials like steel. The reactive element still contains a certain amount of steel though and is also making use of rubber in some applications. CERAWA provides protection against classic (small) RPGs, which have a penetration into steel in the range of 300 milimetres, and 14.5 mm armor-piercing ammunition when fitted to a BMP-1. Unlike other types of ERA, CERAWA does not require a large amount of base armor: in Polish trials, a 10 mm steel plate was neither penetrated by a RPG-7 nor by the CERAWA explosive reactive armor protecting it.

Switzerland's RUAG has developed SidePRO-CE, which is a composite explosive reactive armor design. The focus for SidePRO-CE was put on lower collateral damage. SidePRO-CE does provide protection against HEAT ammunition such as RPGs (incl. tandem shaped charge warheads) and kinetic energy ammunition.

SidePRO-CE ERA being tested against a HEAT warhead.

CLARA (Composite Lightweight Adaptable Reactive Armour) is a German composite reactive armor, which has been developed by Dynamit Nobel Defence (a company that traces back it's origin to Alfred Nobel, aka "that guy from the Nobel prize" and inventor of the dynamite). Unlike other types of ERA, CLARA is designed to be fragmentation free. This is achieved by using a composite fibre material instead of metal or ceramic plates for the reactive elements.

Depending on version CLARA has an areal density of 70 kilograms per metre to 270 kilograms per metre. It provides protection against single stage shaped charges (as used on most RPGs), EFPs and in case of the heavy version against kinetic energy penetrators up to 30 mm APFSDS. 

CLARA was originally developed for the Dutch-German Fennek scout car deployed in Afghanistan, but it was never adopted on the Fennek due to weight concerns and changes in priorities. After being successfully trialed on the Marder infantry fighting vehicle (IFV), CLARA is being used to protect the German Puma IFV in certain areas. In this configuration, each ERA tile has a weight of about 40 kilograms. It has also been tested on the Boxer APC. According to Dynamit Nobel Defence CLARA offers a tenfold increase in protection per weight compared to "traditional passive armor", the latter is understood to be steel armor.

Sweden's armor upgrades

Recently the Försvarets materielverk (FMV), the Swedish military equipment procurment agency, announced that the Stridsvagn 122 (Strv 122) and the Stridsfordon 90 (CV90) will be upgraded within the next years.

The contracts for the upgrade have an estimated total value of $300 million USD and will be carried out by two main contractors: the CV90 upgrade will be made by BAE Systems, which happens to be the company that acquired Hägglunds (original CV90 manufacturer). The contract for the Strv 122 upgrade will be carried out by the German company Krauss-Maffei Wegmann (KMW). 

Half of all vehicles will be refurbished by the two companies roughly at the same time, while the other half will be renovated at FMV's own workshops with maintenance kits to be acquired by FMV. The first refurbished and upgraded vehicles are expected for 2019, the last will be delivered in 2021.

Swedish CV9040B

A total of 262 CV90s will be refurbished and upgraded. Worn-out components will be replaced, new batteries and a new battlefield management system will be installed. The old Kulspruta m/39 machine gun from 1939 will be replaced with a relatively modern MAG from the Belgian company Fabrique Nationale Herstal (FN). The number of 262 vehicles to be upgraded is distributed among the following variants: 172 infantry fighting vehicles, 40 command post vehicles, 22 scout vehicles, 16 self-propelled anti-air gun vehicles, and 12 recovery vehicles. The scout and air-defence vehicles will additionally receive the current generation of thermal imagers used already on the CV9040C.

The new BMS

The Strv 122A is currently the main version of the Leopard 2A5 in Swedish service. Compared to the basic German version, the Strv 122 has been fitted with the full armor configuration of the TVM max prototypes, which includes additional MEXAS-H composite armor at the hull and features enhanced roof protection against top-attack bomblets and heavy artillery fragments. Furthermore Sweden decided to adopt their own battlefield management system and the French GALIX smoke grenade discharger system. All this results in an increased combat value and a greater weight of about 62.5 metric tons. The upgraded Strv 122A will be called Strv 122C.

The Strv 122B is a version of the Strv 122 fitted with the additional mine protection kit developed by Krauss-Maffei Wegmann on behalf of the representation of the Leopard 2 user countries. In Germany the same mine protection kit was adopted with the Leopard 2A6M. It includes an underbelly armor plate, a decoupled seat for the driver and a different, shock-proof ammunition rack for the hull.

The complete details of the improved Strv 122C are unknown, but it has been written that 77 Strv 122As and 2 recovery vehicles will be upgraded. Worn-out and/or obsolete parts will be replaced, including the old batteries and electrical components. Most likely the old PERI R17A2 commander's sight will be upgraded to the new PERI R17A3 version, which exchanges the Ophelios-P thermal imager with a new ATTICA thermal imager. The ATTICA thermal imager is a third generation device, which provides higher performance. The Ophelios-P has to be replaced on the long run, as a component supplier has stopped providing spare parts. The R17A3 also includes a laser rangefinder, further enhancing hunter-killer capabilites and system redundancy.

EMES-15 upgrade possibilities

The old EMES-15 could also be upgraded with the ATTICA thermal rangefinder aswell as an improved laser rangefinder, a similar upgrade has been purchased by Canada and Denmark already.

The Strv 122B will be upgraded to the Strv 122D, which probably will be identical to the Strv 122C aside form the mine protection kit. A total of 11 Strv 122B will be upgraded to the new Strv 122D configuration.

Strv 122 Evolution

A possible upgrade path for the Strv 122D or both versions would be the Strv 122 Evolution tank upgrade developed by Åkers Krutbuk, a subsidiary of IBD Deisenroth of Germany. The Strv 122 Evolution utilizes IBD's Advanced Modular Armor Protection (AMAP) system in order to achieve a higher level of allround protection while keeping the weight rather constant. 

Iran's new tank prototype

Despite arms and military goods being subject to an international embargo for decades, Iran still has a steady output of new prototypes and military goods.

T-72 Khorramshahr tank, the most advanced Iranian tank to date?

Pretty much every locally made weapon system is based on foreign technology, which has been reverse engineered as good as possible. Iran has created three prototype versions of the Zulfiqar (Zolfaqar) main battle tank (MBT) based on technology from the M47/M48/M60 Patton tanks and the Soviet T-72M1/S MBT. It is unknown if and to which extend the Zulfiqars have actually entered service.
The rare Khorramshahr tank (only one prototype?) is probably still the best Iranian tank, mating the welded turret of a T-80UD/T-84 including Kontakt-5 reactive armor with a T-72S hull.

Sabalan tank

In 2014 the Iranian Army revealed the Sabalan tank prototype, an improved version of the M47 Patton tank. Unlike the original Patton, the Sabalan uses a welded turret fitted with a local version of the rifled 105 mm British L7 tank gun. The frontal hull armor is also welded and the bow machine gun (MG) has been eliminated. Thin steel skirts were added over the tracks, providing additional protection against MG, autocannon and RPG fire.
The Sabalan seems to feature a rather advanced fire control system (FCS) for Iranian standards, it seems to be identical to that of the Zulfiqar 1 prototype tank. The Zulfiqar 1 is fitted with the EFCS-3 FCS from the Slovenian company Fotona. This computerized system allows accurate firing on the move and featurs a laser rangefinder, "second generation" nights sights and an independent sight for the tank commander, which is fitted to the commander's cupola.

The new Tiam tank prototype uses the Sabalan hull

Based on the Sabalan hull, the new Tiam (or Teeiam) main battle tank was revealed in 2016. The Tiam however differs in a number of points compared to the previous Sabalan: the hull has received larger sponsons, reducing the driver's vision to the left side at least. A new turret (apparently a version  of the T-55/Type 59 turret) has been fitted, which is also armed with a L7 gun copy. However the sights locations have been altered and the fire control system might be different, but this is not confirmed yet. However the Tiam features an (infrared) searchlight.

The Tiam is protected by steel armor and Kontakt-1 ERA

The Tiam is fitted with Kontakt-1 explosive reactive armor (ERA) or a local copy of it. The ERA is fitted to the turret front, sides and roof aswell as the hull front and among a small sponson section at the hull sides. As Kontakt-1 and derivates do not provide any sort of protection against kinetic energy penetrators such as APDS and APFSDS ammunition, the overall protection level against AP(FS)DS should not be higher than that of a T-55. Kontakt-1 also does not protect against HEAT missiles with tandem charge warheads.

It's intereting to see wether the Tiam will end a single prototype like the Sabalan or enter service in a bigger number. Compared to the M47 it's quite an upgrade, but compared to modern vehicles it's mostly 30 years outdated technology.

First Boxer Cargo delivered...

The Koninklijke Landmacht (Royal Netherlands Army) has taken over the first Boxer cargo vehicle on the 8th April of 2016. This is the fourth of the five versions ordered by the Dutchmen.

Boxer Cargo vehicle, photo from Artec by ESUT

The Boxer Cargo can transport up to 2500 kilograms (5511 lbs) of load under very high armor protection. The Dutch Army has ordered a total of 27 Boxers in the Cargo configuration among the 200 vehicles of their first batch. A crew of three (commander, driver and gunner) operates the Boxer Cargo. The mission module of the Cargo variant has a usable transport volume of 14 m³.

Honestly, let's be real. What sense has a heavy armored, mine-resistant cargo vehicle? What type of cargo needs to be protected against bomblets and light/medium types of top-attack ammunition and is required on the battlefield?

MAN truck with AMAP-B armor and ADS active protection system

With the exception of high value medical equipment, optronics or electronics, there is nothing what seems to require the additional protection provided by a Boxer vehicle. Then one also has to ask the question, how likely a larger amount of this equipment is needed directly at the frontline (otherwise the armored protection of the Boxer wouldn't be needed) and in what quantity (smaller quantities can be carried by other vehicles or within Boxer APCs, larger quantities will be too much for a Boxer Cargo variant). Protection of cargo can be provided by armored containers, so there should be no need to armor the whole vehicle plattform.
Mine-resistant belly plates, additional balllistic protection and even active protection systems are available for larger trucks. Rheinmetall & MAN have shown their uparmored truck solutions on different exhibitions like Eurosatory 2012. The Integrated Armour Cabin (IAC) and Modular Armoured Cabin (MAC) concepts from Rheinmetall provide driver and gunner/commander of a truck with heavy armor protection, while the cargo bay can be armored with a lower protection level. Other companies have come up with similar solutions.

British MOD version of MAN truck with net and slat armor against RPGs

The Koninklijke Landmacht seems to have very odd requirements and might not be very efficient with distributing funds among multiple different vehicle programs. After downsizing the CV9035 fleet (and making sure that no applique armor is fitted during training, in order to reduce the fuel consumption) and decommissioning the Leopard 2 tanks (the negative impact of this has been slightly equalized by letting a Dutch unit joining the German Army), one simply has to be astonished or shocked by the decision of introducing the Boxer Cargo into service.

New T-64 version with unmanned turret in the making?

According to Ukrainian newspaper "Defense Express", a new version of the T-64 MBT is being developed.

A first 3D rendering of the new tank, image taken from SNAFU Solomon's blog

The new version of the T-64 will supposedly be more advanced and better than the current T-64BM Bulat main battle tank (MBT) operated by the Ukrainian Army. Like the T-14 Armata the new T-64 version will utilize an unmanned turret. The crew of three (commander, driver and gunner) will be moved into the hull, an armored bulkhead behind the crew compartment separates the crew from the highly explosive ammunition stored in the standard T-64-style autoloader.
It has been claimed that the Azov volunteer brigade is responsible for the development of the new T-64 version. The Azov unit already developed the Azovets urban combat vehicle based on the T-64 chassis.

Details of the new tank upgrade

The new T-64 version will feature a remote weapon station (RWS) with a 12.7 mm heavy machine gun (HMG) located on top of the turret bustle. Like on the T-90MS, the commander's independent periscope also functions as main optronics for the RWS. The turret features four banks of smoke grenade launchers, each holding 4 smoke or multi-purpose grenades. Apparently the RWS can also be fitted with up to four smoke grenade launchers. The tank is protected by composite armor and additional Duplet heavy ERA. The ERA is also fitted to parts of the turret, something that hasn't been done on the T-14. Unlike the T-14 however the upgraded T-64 is not fitted with an active protection system (APS).  As main armament the tank will feature a 125 mm smoothbore gun, probably the localy built KBA3 or KBM1M tank gun.
At a weight of 39 metric tons the upgraded T-64 will be a lot lighter than the T-14, however the overall physical size of the tank seems to be quite a bit smaller. However it seems that the hull height had to be raised in order to make enough room for enough space for the seats, controls and equipment for the crew of three. There are also only two sets of containing three vision blocks each, thus either the commander or gunner can only utilize the optics fitted to the turret. The T-14 Armata's design includes a seventh vision block at the hull.

PS: As the cover of Defense Express reveals, despite the new turret the vehicle will keep an IR illuminator like the original T-64.

Merkava 4 damaged during training

Last week video footage and photographs from a training accident of a Merkava 4 unit was released. During a firing excersice, a Merkava 4 main battle tank (MBT) drives in front of another one, which was shooting a 120 mm M392 APAM round. APAM stands for anti-personnel/anti-materiel and is the designation given to the current high-end multi-purpose ammunition of the IDF.

The M392 APAM consists of a 17 kilograms ( ~37 lb) heavy projectile, which is accelerated by a reduced propellant charge to a muzzle velocity of 900 metres per second. The maximum chamber pressure while firing a APAM round is only 340 MPa (3400 bar). The APAM  projectile has a modular design and can contain up to 6 sub-munitions (warheads), which usually contain a high explosive (HE) filler. Thanks to a programmable electronic fuze, APAM can be used for a variety of task in different modes including a mode for detonation at impact, air-burst and detonation after sub-munition ejection.

Merkava 4 after APAM impact

According to Israeli sources no member of the tank's crew was injured.

Same tank, photo taken from a bit closer

The photographs from the accident show a number of different interesting aspects:

• APAM managed to "peel off" the composite armor of the tank on an area of ~1.8 m² (for reference, the roadwheels including rubber rim have a diameter of 700-710 mm)
• The composite armor at the hull sides is made of several smaller modules, which have a metal frame and contain several bolts, which probably hold the armor elements
• Two side skirt armor modules are held by a single bolt. An unlucky hit directly at the bolt or close to the bolt (like in this case) means that both skirt armor modules will fall off
• The turret rear (which was facing the APAM detonation during impact) is hidden behind a canvas cover and probably damaged too
• Hitting the edge of two composite armor modules at the hull side will cause high damage to both (see how parts of the steel frames are missing at the lower and at the upper edge)

The Merkava 4 is known for relying on adapted special armor instead of integrated special armor. Most conventional tanks - including the M1 Abrams, the Leopard 2 and the T-90 - utilize integrated composite armor or a combination of integrated and adapted armor. This means the steel casings for turret and hull are manufactured with double walls, which are spaced apart. The composite armor modules are then inserted into the hollow sections between the steel walls, which is closed thereafter by welding/bolting a roof section ontop.
The steel walls of the turret and hull casing provide structural support and additional armor protection, however they add weight and make repairs more cumbersome.

Merkava 4 turret construction showing the lack of integrated armor

The Merkava 4's turret is made from steel, but only has a single steel wall. This is usually a design commonly used on lighter armored vehicles (like IFVs and APCs) or older vehicles later retrofitted with special armor. The composite armor is mounted in form of separate modules at the exterior of the tank. This allows an easier replacement and easier upgrades of the composite armor modules.

Damaged M1A1HA and Merkava 4 after being  hit by large calibre ATGMs

However it seems that the implementation of the adapted special armor design on the Merkava 4 has some flaws. The Merkava's composite armor consists of many thin spaced sandwich plates, which is a common design for protection against HEAT warheads. However the lack of a thick coverplate providing additional protection against KE, detonating warheads before they enter the composite armor and providing additional structural support causes some issues not found on other tanks.
Compared to other tanks, the Merkava 4's armor (and similar armor used to upgrade older tanks) takes considerable amounts of damage after a single hit. Above is a comparision of a M1A1 HA hit by a Maverick ATGM (an attempt to destroy the immobilized tank in order to prevent the tank being captured) and a Merkava 4 after being hit by a 9M133 Kornet (AT-14 Spriggan) missile. The difference at the exterior is huge - while the M1A1HA has only a single hole (~2-3 inch diameter) in the armor's cover plate, a large portion of the Merkava 4's armor is damaged and some elements are even missing. This means a larger portion of the tank's armor is weakened and allows the enemy to specifically target the weakened sections of the tank (at closer ranges at least). Most of the damage to the Merkava's armor is not caused by the metal jet formed by the shaped charge's copper liner, but rather the detonation of the high explosive used to form it. Ironically the British engineers of the FVRDE already knew of this problem in 1969 when developing a Chobham armor upgrade for the Chieftain. The Merkava 3's different armor design also does not seem to share this problem.
Thus it appears that this is a known disadvantage, intentionally accepted in order to keep the tank's weight at an acceptable level.
The M1A1 HA's composite armor is only damaged by the copper jet formed by the shaped charge's liner. The HE used to accelerate it does only cause cosmetic damage to the exterior.

The steel wall of the integrated armor design prevents major damage to the composites

As previously written, APAM uses a reduced propellant charge and operates only with a pressure of 340 MPa. This is considerable less than a DM12 HEAT round (M830 in US service), which operates with a full charge at 480 MPa. Given the weight and velocity, it seems likely that the current German multi-purpose ammunition - the DM11 (also used by the USMC and others) - should operate at a similar pressure level. Soviet/Russian 125 mm HE rounds have comparable kinetic energy. So seeing how a APAM can nearly penetrate a Merkava 4's armor (here is a Merkava hull without armor) or maybe even penetrate it (official statement only says nobody injured), it seems very likely that other multi-purpose ammuntion could completely penetrate the side armor of a Merkava - depsite there being an approximately 300 mm thick composite armor module (~1 foot of armor module). As HE/multi-purpose ammunition should only penetrate lightly armored vehicles and structures (a double-layered concrete wall offers less protection than 70 mm steel armor) this raises some questions about the IDF's priorities and the combat value of the Merkava in other types of conflicts.

To be fair most other tanks don't have armor at this exact location where the Merkava 4 was hit - that is the result of most tanks being optimized for tank-vs-tank combat or high intensity conflicts at least. Such conflicts require thick and heavy frontal armor, so that not much weight can be utilized for side armor. The early Merkava tanks (including the intial model of the Merkava 3) also followed this design, as they were primarily meant to fight the numerical superiority of the tank force of the Arab nations. The Merkava 4 however was designed primarily for a new type of warfare, to fight in the assymetrical warfare against HAMAS and allies. This meant that heavy frontal armor was not necessary and allround protection against RPGs and ATGMs was only required.
The Merkava 4 seems to have sacrificed conventional armor protection in order to meet the current requirements of the IDF, but how well would it fare against the new tanks of the region: M1A1s in Egypt, Challenger 1s in Jordan (with upgrades being in development) and T-90s in Syria?

PS: According to some people on the internet claiming to belong to the IDF, the training rounds fired might not have been APAM rounds, as originally claimed by other sources, but an inert HEAT-TP rounds. Unfortunately it's impossible to prove or disprove any of the stories, but informations posted by random persons on the internet always have to be taken with care.

Upgraded Abrams to feautre XM360 gun and guided ammunition?

In the document "The US Army - COMBAT VEHICLE MODERNIZATION STRATEGY" published by the Army Capabilities Integration Center of the U.S. Army Training and Doctrine Command (TRADOC) in 2015, there is an interesting passage about the future of the M1 Abrams tank:

"Next-Generation Large Caliber Cannon Technology. The XM360 next-generation 120mm tank cannon integrated with the AAHS will provide the M1 Abrams a capability to fire the next generation of high-energy and smart-tank ammunition at beyond line-of-sight (LOS) ranges. The XM360 could also incorporate remote control operation technologies to allow its integration on autonomous vehicles and vehicles with reduced crew size. For lighter weight vehicles, recoil limitations are overcome by incorporating the larger caliber rarefaction wave gun technology while providing guided, stabilized LOS, course-corrected LOS, and beyond LOS accuracy"

The 120 mm XM360 tank gun was originally designed for the XM1202 Mounted Combat System tank component of the canceled Future Combat Systems (FCS) program of the US Army. It retains the original dimensions of the M256 smoothbore gun (aside from being fitted with a muzzle break to reduce the recoil impulse/force) and is ballistically equivalent to the M256 gun when firing the same ammunition. Due to having an increased pressure limit the XM360 gun can however utilize more powerful KE ammunition. An ammunition data link (ADL) allows the usage of programmable ammunition or more accurate firing (i.e. when the propellant temperature can be taken into consideration by the fire control system). Parts of the barrel and many other components are made of composite materials, which reduces the weight compared to a conventional design. The XM360 is 2100 pounds (952.5 kilograms) lighter than a M256.

XM360 prototype on a mock-up XM1202 hull during Army trials

The XM360E1 is a version of the XM360 specifically designed for the M1 Abrams. Some components of the former M256 gun like the rotor are reused to reduce cost and increase commonality to older M1A2 tanks. It seems very likely that the XM360E1 is actually meant, when the article from TRADOC speaks about the XM360 gun for the M1 Abrams. From the renderings of the XM360 and XM360E1 posted above, the XM360E1 appears to have a slightly longer barrel. However this is no secured information and no official data on the XM360E1 has been published yet. The fact that the XM360E1 looks longer might be result of the perspective or the claims that the XM360 has the same length as M256's are including the muzzle break in the XM360's length, albeit drawings implying otherwise. The XM360E1 might have a barrel length of 46 to 48 calibre then.

The second part of the quote is implying that the US Army has yet to loose interest in developing guided tank ammunition. This means that the Army has either reactivated a formerly canceled development program like the XM943 STAFF or the XM1111 Mid Range Munition (MRM), or the US Army has started a new development program for long range/beyond-LOS guided tank ammunition.

While upgunning the M1 Abrams is certainly a move in the right direction - the Leclerc with it's GIAT CN-120-26 and the Leopard 2A6 with it's Rh 120 L/55 tank gun have already introduced longer barreled tank guns designed to sustain higher operating pressures more than a decade ago - the question remains if the XM360 is enough for the future. The German company Rheinmetall is currently working on an improved 120 mm smoothbore and a new 130 mm smoothbore gun for the Leopard 2 and the future Franco-German main battle tank (MBT), while the new Russian T-14 MBT is being fitted with either a new 125 mm high-performance gun (2A82) or an 152 mm 2A83 gun. It seems that the US tanks might be lacking in the gun department, despite receiving a new gun in the near future. 
The fact that the XM360E1 gun's barrel is rather short might be related to the oscillation and stabilization issues found on the M1A2, when investigation upgrading the M1A2 tank with the Rheinmetall L/55 (M256E1) gun. From 1998 to 2000 the US tested three modified Rheinmetall L/55 guns (and a further 12 barrels produced under licence by TACOM/Watervliet Arsenal) under the designation M256E1. While the Rheinmetall gun was designed to specifically fit into a modified gun mount of the German Leopard 2 tank, the different gun mount and stabilization systems of the M1A2 Abrams had troubles handling the longer and heavier barrel. This would result in a reduced accuracy and required major modifications (including a completely new stabilization system) to fix, which results in inappropriately higher costs for simply upgunning. The same issues arrived when testing the 120 mm XM294 L55 smoothbore gun prototype on the Abrams between 1996 and 1998.

The IDF's Pereh missile carrier is a lot better suited for launching ATGMs than a conventional tank

As far as the guided ammunition is concerned, one should take into account that this has to fit into the XM360 smoothbore gun and into the ammo racks of an M1A2 tank. This means the maximum missile diameter can be 120 milimetres (mm) and the maximum length of a hypothetical gun-launched missile for the Abrams can not exceed ~990 mm (39 inches). Unfortunately the limitations on the missiles size have a negative impact on the possible effectivness: currently all ATGMs are relying on shaped charge (HEAT) or explosively formed penetrator (EFP) warheads. Modern ATGMs such as the Hellfire, Javelin, PARS 3 LR (TRIGAT), Spike-LR, Kornet, etc. all exceed a diameter of 120 mm, mostly by a rather large amount. The length of long-range high performance ATGMs exceeds 1.5 metres (59 inches). Thus it seems rather unlikely that guided LOS/BLOS ammunition for the M1 Abrams can defeat the frontal armor (in some cases even the side armor) of a current and next-generation tank. Therefore in order to have a high chance at defeating current generation armor, the guided ATGMs have to follow a top-attack design. Even then, a number of current tanks already has enhanced roof protection against top-attack ammunitions (e.g. every Soviet/Russian tank since the 1980s has roof mounted explosive reactive armor). Some variants of the German Leopard 2 aswell as the Israeli Merkava tanks have been fitted with passive roof armor against shaped charges and/or EFPs.

So overall there are good news for the Abrams, but are they good enough?

First rendering from OBRUM's Borsuk IFV design released

A computer-generated rendering of the current stage of  the Borsuk infantry fighting vehicle (IFV) development has been released by the Polish state-owned OBRUM. The Borsuk will replace the BWP-1, which happens to be the local designation for the Soviet BMP-1.

The vehicle is fitted with the ZSSW-30 turret from HSW. This is equipped with a Mk 44 Bushmaster II autocannon from Alliant Techsystems (ATK), a dual-missile launcher (probably designed for Spike-LR, which is already operational with the Polish Army), two banks of four smoke grenade launchers each, an independent commander's sight located on the roof and the GOC-1 Nike gunner's sight from the Polish company PCO on the left of the gun (hidden behind an armored cover when not in use). A co-axial UKM-2000C machine gun is also installed.
The Mk 44 Bushmaster II gun is an electrical driven chain gun, usually chambered in 30 x 173 mm. There is however an option to swap barrels for a 40 x 180 mm "Super Forty" supershot round with enhanced penetration (by 30% according to older presentations from the development) and payload.

The Nike gunner's sight installed in a ZSSW-30 turret

The GOC-1 Nike sight for the gunner incorporates a Polish thermal imager, a set of color cameras and an eyesafe laserrangefinder. The commander is provided with a GOD-1 Iris sight from PCO, incorporating a thermal imager, a daylight camera and a laser rangefinder. The Janus sight is also protected by an armored cover at the sides and rear.
The turret has been criticized by Polish defence magazines due to ATK Bushmaster II gun is incompatible with programmable ammunition from other manufacturers (such as Rheinmetall) and the Spike missile integration being still work-in-progress.

The vehicle in this configurations will be rather light and is meant to be amphibious. Indicators for this are the rubber band tracks and the water blades. Unlike on most amphibious vehicles, the splash guard seems to be mounted at the lower hull front and probably will be elevated before amphibious operations. The drivetrain includes six roadwheels and two return rollers per side. Like most IFVs the Borsuk will have a front mounted engine.
Maybe this is caused by the odd perspective and the lack of track skirts, but the tracks and supsension unit seems to be relatively small compared to the overall height of the hull. This might be affiliated to the amphibious design, but it also might imply a lower suspension performance compared to other vehicles.

Based on the fact that it is an amphibious vehicle, the weight and thus the protection level will be limited. The ZSSW-30 turret has a baseline protection in accordance with the STANAG 4569 requirements for level 2 (protection against 7.62 x 39 mm API fired from 30 metres distance). When fitted with additional ceramic composite armor, the turret can meet the level 4 requirements for ballisitc protection (protection against 12.7 mm AP and 14.5 mm API). Given the weight limitations required for an amphibious vehicle, it seems reasonable to assume that a higher protection level than STANAG 4569 level 4 - in best case level 5 protection frontally - cannot be achieved.
 
The current Borsuk design however seems to be a bit lackluster in a number of different aspects. First of all there is the big question of why the Polish Army demands amphibious capabilities for the new IFV, when there are several hundred amphibious Rosomak vehicles with the same armament available. While previously the adoption of ceramic composite armor meant that the amphibious capabilities of the Rosomak where lost (the amphibious baseline version did not offer allround protection against 14.5 mm AP ammunition), recently the Rosomak-M was revealed at MSPO 2015. Thanks to new armor from German company IBD with reduced weight compared to the older armor from Israeli Rafael, the Rosomak-M provides a high level of protection while being amphibious.

Rosomak-M at MSPO 2015

The current configuration of the Borsuk hull is a lot less than ideal. The driver is only provided with one single sight block, while the commander has two smaller sight blocks. Aside of the gunner's sight in the ZSSW-30 turret, the gunner has no other means of observation. This means that the visibility for the driver, commander and gunner is limited; specifically the driver seems to have an unnecessarily small field of view. While cameras and digitial optronics (like the gunner's and commander's sight) can increase the visibility, the problem withsuch optics is that they are not fail proof (once the power delivery is gone, they won't work) and easier to damage than conventional armor glass/transparent ceramic vision blocks.

It has to be noted that the Borsuk is still in the design and conception process, so most things can still change. That the ZSSW-30 turret will be used seems to be guaranteed however.

Bigger guns are not always better

When it comes to armored fighting vehicles, there is always a tradeoff between vehicle size, weight, ammunition, gun calibre and armor protection.
This is really important when it comes to medium calibre guns and infantry fighting vehicles (IFVs): armor penetration, lethality against infantry, rate of fire, ammunition load and the availability of a large amount of ammunition at the gun are indispensable factors for a well performing IFV.

Balancing all these factors is critical, but hard to achieve. Different countries have come to different solutions in accordance with their doctrines. Increasing the gun calibre will lead to a higher lethality against infantrymen and most likely increase armor penetration (although the actual pressure, penetrator design and barrel length matter here too). However the rate of fire and the ammunition stowage will most likely be reduced. Using an anti-tank missile launcher will increase costs and weight of the vehicle, but allows the usage of an autocannon with lower penetration.
In the end choosing the main armament of an armored fighting vehicle (AFV) should be considered as an optimization problem: How can one make the AFV most lethal for a given weight and size (and cost)?

In order to take the different effectivness of the different calibres into account, the amount of "stowed kills" is measured or estimated. The idea behind this is rather simple: one compares the lethality (using different metrics like armor penetration, after armor effects or the amount and spread of fragments) to the amount of ammunition stored inside the vehicle and at the gun.

The Combat Vehicle 90 (CV90) is a great example for the positive and negative impacts of larger calibre ammunition, due to the larger amount of different guns adopted on it.
The CV9040 uses the 40 milimetre Bofors L70 gun, the CV9035 the 35 milimetre Bushmaster III autocannon, the CV9030 the 30 mm Bushmaster II autocanon, the CV90105 prototype light tank a M68E tank gun and the CV90120 with a 120 mm Compact Tank Gun from RUAG.

Dutch CV9035

In case of the CV90 the ammunition available at the gun varies:

• The CV9040 has a total of  24 rounds available at the gun (three rows of eight rounds) with a further 24 rounds being located in a carousel magazine used as ready racks.
• The CV9035 has a total of 70 rounds available at the gun, consisting of two belts a 35 rounds.
• The CV9030 has a total of 160 rounds available at the Bushmaster II gun.

This clearly shows the benefit of utilizing smaller calibres in combat vehicles. While the CV9040 has only 24 rounds directly available at the gun, just by using the slightly less powerful 35 x 228 milimetres calibre the amount of rounds available at the gun is nearly tripled. The 30 x 173 mm calibre still more than doubles the ready ammunition compared to the 35 mm calibre! Given that the 40 mm Bofors gun currently does not offer more armor penetration possibilities, the Bofors compares unfavourably to the Bushmaster guns. While the actual armor penetration of the 40 mm Bofors with APFSDS ammunition is higher, the added armor penetration does not allow engaging heavier armored targets: all three claibres can defeat current generation IFVs frontally and MBTs from the side - the only advantage gained by the larger calibres is additional ranges, which only matters under limited circumstances.  
In a similar manner the total ammunition load is affected by the calibre of the main gun:

• Including the 48 rounds stored in the turret, a CV9040B has a total combat load of 234 rounds of 40 mm Bofors ammunition.
• A CV9035 has storage options for a total of 203 rounds of 35 mm ammunition. The lower number compared to the version armed with the Bofors gun is the result of the less optimal ammunition storage and the belted ammunition.
• The most ammunition is stored in a CV9030: up to 400 rounds of ammunition, nearly twice as much as on the other versions, can be stored inside the vehicle.
• The CV90105 TML carries 40 rounds of 105mm ammunition inside the vehicle.
• The CV90120-T carries 45 rounds of 120mm ammunition.

The CV90105 TML and the CV90120-T however require a larger crew of 4 soldiers and have lost all infantry carrying capactity, so the ammunition load for an IFV version with 105/120mm gun would have been drastically lowered.

While the total ammunition stowage of a CV9040 is actually higher than that of a CV9035, this is related to the different gun design: a dual-belt fed externally powered gun loaded with belts of 35 rounds is larger but also a lot more capable than a 40 mm Bofors L/70 gun.
Above is a graphic from CTA International showing the advantages of their "space efficient" 40 mm Cased Telescopic Armament Systems (CTAS) gun. This graphic illustrates nicely how the gun size is affected by larger calibre ammunition, albeit it is a bit "unfair" and biased. The 40 mm CTAS is not fitted with any sort of gun mantlet protection, whereas the Bushmaster III gun at least is fitted with one. The Bushmaster guns are all externally powered guns and are including parts of the ammunition feed mechanism, whereas the 40 mm CT(AS) gun's external powered motor and feed mechanism are not shown completely. The CTAS gun is at least quite larger and heavier than the 30 mm RARDEN and 30 mm Mauser MK30 guns, which are/were used in the original versions of the Warrior and ASCOD vehicles to be equipped with the CTAS gun in British service.  
In general larger calibre ammunition has the following negative impacts on vehicles, which need to be taken into account by the vehicle designers and manufacturers:

• weight
• size
• gun overhang
• internal space
• costs

Different medium calibres used by IFVs

For further reference here is a small listing of AFVs and stored ammunition:

• Schützenpanzer Lang HS.30 - 2000 x 20 mm rounds
• Marder 1A3 - 1250 x 20 mm rounds (503 rounds ready to use), 4 MILAN ATGMs (1 ready to use)
• Marder 2 prototype - 287 x 35 mm rounds (177 available at gun)
• Bradley - 900 x 25 mm rounds (300 available at gun), 7 TOW ATGMs (2 ready to use)
• Warrior - 300 x 30 mm rounds (2 clips of 3 rounds at the gun)
• Puma - 400 x 30 mm rounds (200 available at the gun), unkown number of missiles (2 ready to use)
• BMP-1 - 40 x 73 mm rounds, 4 missiles (1 ready to use)
• BMP-2 - 300 x 30 mm rounds, 4 missiles (1 ready to use)
• BMP-3 - 500 x 30 mm rounds, 40 x 100 mm rounds/missiles
• AMX-10P - 760 x 20 mm rounds (325 ready at gun), 10 MILAN ATGMs (1 ready to use)

The Canadian Army and the US Army both did reject upgunning their LAV IIIs and Bradley IFVs respectively to a higher calibre, because of the lowered amount of "stored kills". The Canadian military also pointed out that a larger gun due to the limited amount of ammunition has to be used differently - the 25 mm Bushmaster gun of the LAV III was "used like a machine gun" by firing short salvos. This is not really possible with larger calibres due to the increased size and therefore reduced combat load.

Figures from the US Army Research Laboratoy on investigating the adoption of a 35 mm gun on the Bradley come to the results pictured above: While a standard 35 mm point-detonating high explosive (HE) round has a higher lethality per round than a 25 mm HE round, the actually amount of stowed kills is considered to be worse at short to medium combat ranges, where the smaller fragmenting effect of the 25 mm HE round doesn't matter as much, because the accuracy is still very respectable. Only at longer ranges - i.e. above 1500 metres/one mile - the 35 mm point-detonating HE ammunition is favourable. Depending on terrain and combat scenario this can be satisifactory or not - during the Cold War the average combat distance in Central Europe was considered to be less than 1500 m, which means that a 35 mm Oerlikon or Bushmaster gun does not offer more lethality against infantry in this situation.

There seem however to be two major factors speaking for the adoption of autocannons of larger calibres on infantry fighting vehicles:

1. programmable ammunition
2. armor penetration

As the figures from the test and simulations made by the US Army Research Laboraty show, programmable ammunition can greatly increase the lethaliy against infantry. Programmable ammunition also can provide considerable better results when used against aircrafts, main battle tanks (by damaging all optics) and infantry hidden in structures (by exploding within the building).
As the current electronics and fuzes required for programmable ammunition cannot be fitted into small calibres such as 20 mm and 25 mm ammunition without reducing the payload beyond to an unreasonable small amount, calibres of 30 mm and above have gained popularity.

As far as armor penetration is considered, this is always a trade-off depending on the user's doctrine: it mostly comes down to a simple design decision: For what targets will the main gun of the vehicle be utilized and against which targets are other (and better) weapon systems available?

The CV9040 was designed with rather specific requirements, which are not shared by many other countries. The 40 mm Bofors gun was chosen for a number of reasons:the same calibre was already in use with the Swedish Navy and was used for anti-aircraft weapon systems (including the CV9040AAV self-propelled anti-air gun later developed, based on the CV90 chassis). A major factor however was the demand to penetrate the side armor of (ex-)Soviet main battle tanks such as the T-55 and T-72: these tanks have 80-90 mm thick steel armor over the sides of turret and hull, penetrating this with a 20-30 mm gun at medium ranges and certain angles of impact is not possible. The Swedish requirements however saw no adoption of an anti-tank missile system on the CV90; the main gun was intended as sole weapon to engage enemy armor and thus the demand for armor penetration was high.
Other infantry fighting vehicles like the M2 Bradley, AMX-10 and Marder are designed with less emphasis on main gun penetration, as the autocannons were only intended to defeat infantry units, APCs and IFVs. For heavier armored targets these vehicles were equipped with anti-tank missile launchers that offer greatly improved penetration even over a 40 mm gun and can be effectively used against a wider variety of targets (thanks to the availability of multi-purpose warheads for modern ATGMs).

In a presentation on the 40 mm CTAS gun, the manufacuters implies with a graphic, that three rounds of the new 40 mm case telescopic ammunition have the same lethality as 21 rounds of 20 mm or 30 mm ammunition. Unfortunately the resolution of the image is poor and it seems to be the result of photoshop work. However the total combat load of vehicles fitted with the CTAS gun is really poor, which means despite being optimized for being "small", the stowed kills of a Warrior or Scout-SV with CTAS are still limited compared to other IFVs with smaller guns and missile launchers. This shows that bigger guns are not always better.