Thread by @JonHawkes275

Following up on my bar armour thread (bit.ly/3h8nZIV), a shortish primer on explosive Reactive armour (ERA). Other reactive armours (NxRA, NERA, SLERA, Electric) to follow another day. No exciting tropes to quash like bar armour, but perhaps some useful nuance to offer.

Usual disclaimer - this is Twitter, I don’t have much space and so some things are simplified or omitted for simplicity. This is a hugely complex science; I’m just giving a flavour of the considerations inherent in AFV design. With that out the way…

Discovered by Manfred Held in early 70s, ERA is simple and elegant. In basic form, a pair of steel plates sandwich a layer of high explosive. This sandwich is typically housed in a mild steel box to protect against damage and provide standoff from the vehicle hull as appropriate

Note that ERA was originally designed to defeat shaped charge jets (not EFPs). It has evolved to have efficacy against APFSDS long rod penetrators, but to do so designer has to accept reduced protection against jets in return. There is no one-size wonder ERA against all threats

When a threat projectile penetrates the box and the first steel plate it rapidly compresses the explosive layer. This rapid compression leads to detonation, propelling the plates apart as the jet penetrates through them

As the jet is travelling in a straight line through the moving plates, they are in effect continuously offering new material in front of the jet for it to penetrate, cutting a slot through the moving plates and presenting vastly thicker effective armour than actually exists

ERA can provide limited efficacy even in a normal incidence ‘head on’ impact, including effect of defeated material at front of the jet impacting tail as it continues to penetrate. For good efficacy in such an attack, explosive layer needs to be at least 2x as thick as the jet

The plates need to be moved at very high velocity to maximise new material being offered to the jet. The achievable velocity is a function of plate thickness and amount of explosive material used and is a key element in designing a high efficacy ERA module

Note that leading edge of penetrating jet (precursor) will often fully penetrate all layers of ERA before plates start to move and so escapes meaningful interaction. Size of the precursor is factor of plate obliquity and jet velocity and can be an issue especially for lighter AFV

In order to offer improved obliquity to a range of attack vectors, to allow repeat hit protection and defend against precursor penetration, some ERA utilise multiple sets of plates canted at differing angles, typically a ‘flat’ and a 12° angled 2-layer composition like Kontakt-1

Broadly speaking, normal incidence impact with a single ERA plate will see a circa 10% reduction in penetration compared to no ERA. Oblique impact sees anything from 50% to 75% reduction rising to 90% or more with a double plate V-profile configuration like Kontakt-1 above

The angle of the V between the plates is not hugely significant, with a sweet spot at a 13° but <5% total variation across all angles – i.e. across all V angles testing achieved 85-90% reduction compared to no ERA and so penetration is scientifically ‘not sensitive’ to V-angle

One benefit of ERA is individual cassettes can be easily affixed to welded on bolts or brackets without structural changes. Typical package uses 200-300 cassettes and weighs c. 2,000 kg. Some systems like Duplet see multiple tiles arrayed upon each other for increased protection

The first operational fielding of ERA was Israel with RAFAEL Blazer single plate system on M60 tanks in 1982. It claimed to defeat the Sagger ATGM at 60° and RPG-7 at all aspects. A full Blazer package added c. 1,000 kg to the tank

Russia toyed with ERA for a long time and after Israel’s success began rampant fitment of ERA to its tanks, continuing to this day. The first example, Kontakt-1, used two-plate arrangement shown earlier, which was claimed to reduce a 93 mm warhead’s efficacy by 98% (at 70°)

Additional design challenges are to ensure that the ERA is insensitive to heat and lighter projectile impacts that do not require initiation such as small arms fire, fragmentation or accidental collisions when driving.

ERA has an inherent danger, and this has been a major factor in its more reserved uptake in the West. Whilst ERA kits exist for many vehicles, they are not regularly fitted –TUSK/BUSK kits for US M1/M2, WRAP for UK Warrior and CLARA for Leclerc for example are not seen too often

UK fielded Challenger 1 in Iraq in 1991 with Royal Ordnance ROMOR-A ERA only fitted to the lower glacis in large part because this projects flyer plates into the ground and radically reduces risk of high velocity fragmentation hitting nearby forces, civilians or infrastructure

These cultural/political concerns led to development of low fragmentation ERA such as Dynamit Nobel Defence CLARA, which uses no metal components, instead using composites that retain efficacy in the defeat of a jet, but disintegrate into chaff as the explosion develops

Modern ERA increasingly use low volatility or non-explosive (SLERA/NERA/NxRA types) designs to contain the explosion and produce zero collateral. Here RAFAELs Armour Shield KE having initiated against two APFSDS rods and although bulged, has not projected any fragmentation

Similar concerns around hard kill active protection systems have stalled their uptake by Western armies, preferring instead non-kinetic soft kill solutions like MUSS on the German Puma.

That said, we are now seeing a surge of users starting to pursue hard kill more resolutely, with US M1, German Leopard 2, Dutch CV90 and soon UK Challenger 2 and Australian L400P3 to all field hard kill systems

That’s ERA from a shaped charge point of view, what about APFSDS? Defeating a long rod penetrator is hard. The rod is 20-30mm in diameter, not the 2-3mm of a copper jet and are composed of heavy tungsten-nickel alloys or deleted uranium, not comparatively light copper

This ERA must be heavier with thicker flyer plates and explosive. Russia claims high efficacy of its ERA against APFSDS. Rods are defeated by momentum transfer fragmenting and then deflecting fragments. They retain significant energy and substantial backing armour is still needed

An issue is that of triggering the ERA under these circumstances. ERA explosive is formulated such that only the unique circumstances of a shaped charge jet are sufficient to trigger it. Blunter/slower force of APFSDS/EFP may not be sufficient and allow the threat to penetrate

Heavier ERA therefore alter explosive and plates to better tune to anticipated APFSDS threat. Relikt for example has microspheres throughout the explosive that collapse and generate heat under impact of an APFSDS, initiating the explosive layer even without 'low' forces exerted

Kontakt-5 is probably the most famous example, fielded widely on T-80 and T-90, and Is suggested to represent an effective additional 300 mm of RHA armour equivalent protection against APFSDS. It uses a 15mm front flyer plate, contrasted with just 3mm in early designs like Blazer

Experiments with very heavy ERA showed that a 40/10/25 design (front plate/explosive/rear plate) reduced residual APFSDS penetration into an RHA backing plate from 800mm to 190mm. This testing was from 1999, so efficacy is even higher now with modern heavy ERA designs

Some more recent ERA designs such as the Russian Malachit used on T-14 are suing a smart initiation, with each tile sensing changes in the magnetic field around it to detonate responsively to the location and size of threat coming towards it and at the optimal distance

By predetonating rather than waiting for impact, Malachit can use less explosive and increase, claim Nil Stali, the efficacy of the defeat when acting against an APFSDS.

It would not likely be able to detect and act in advance of a shaped charge jet, but on T-14 a hard kill APS is intended to defeat such threats away from the vehicle and ERA is thus optimised for APFSDS defeat

At #IAV2021 this year RAFAEL showed off its Armour Shield KE ERA for countering APFSDS threats, with live fire tests producing a 48-54% reduction in penetration. THey too spoke to using APS for missiles and RPGs, and optimising ERA against KE threats

Note that ERA has limited efficacy against EFPs, which are typically used in top attack missiles and off-route mines. EFPs are short fat projectiles less inclined to be disrupted by plate movement than long and thin jets. AS a result, heavy APFSDS oriented ERA has some utility

And that’s a short primer on ERA. Again, it’s the subject of many books and papers, so this was much simplified and abbreviated, but gives some basics. Type explosive reactive armour into Google Scholar for a lifetime of further reading #miltwitter #tanktwitter /end