Tanks, Explosive Reactive Armour and Defensive Aid Suites

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Update: If you’re new here, you might also be interested in learning how vehicle armour works (terminal ballistics) or busting the myth about HEAT “melting” through armour. Other ballistics articles include wound ballistics, intermediate ballistics (with a deep dive on silencers), internal ballistics, and external ballistics.

Hi again. I know I promised to finish with ballistics last week, but I realised while I was wrapping up that I still hadn’t spoken about explosive reactive armour (ERA). So here we are again, on the series that refuses to die, just like a tank which is fending off threats from all sides with modules of exploding armour.

It’s worth coming back to this topic, because ERA is another example of seemingly crazy (or at least counter-intuitive) ideas which, through the beauty of engineering, becomes a very sensible thing. Who would have thought that strapping explosives to the side of a tank would make it safer? But it does can¹, and as a result is a widely used technology in modern main battle tanks.

So let’s discuss how this can be true. Firstly, I’m going to go on a literal deep dive into the mechanics of how shaped charges and kinetic energy penetrators (KEPs) work against armour. Then I’ll describe how ERA cleverly sacrifices spare material to save the armour from such threats. I’ll also discuss ERA’s limitations and how weapons designers have met the threat. Finally, I’ll veer slightly off-topic and talk about defensive aid suites (DAS) for tanks.

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First, we need to understand hydrodynamic penetration

When discussing high explosive anti-tank (HEAT) warheads previously and in the context of terminal ballistics, I didn’t go too deeply down the rabbit hole of explaining how shaped charges erode armour, because I didn’t want to make your eyes bleed. Unfortunately, the time has come for eye-bleeding (you can skip past this section if you want to preserve them).

A shaped charge jet travels at around 8,000 m/s, with the tip reaching speeds of 14,000 m/s. This is fast. Remember last week: we compared velocities, and rifle bullets were under 1,000 m/s. Kinetic energy penetrators are much slower than shaped charges, but still clock in at close to 2,000 m/s.

Normally, shooting something faster will make it better at penetrating a target. This is true up to a threshold some way above 1,000 m/s, which is where the “hydrodynamic” regime starts. As I’ve said before, this is not about materials melting, but about materials moving so quickly that they don’t have time to behave like normal materials.

Specifically, the armour doesn’t have time to flex and bend and deform and do all the other things that absorb energy. Instead, the fast penetrator material just eats away at the armour, getting itself eaten away in the process. If the shaped charge lance lasts longer than the armour plate, then it perforates the tank. If the armour plate lasts longer, then there’s no perforation. This is difficult, though, because shaped charge lances get very long (eight times the diameter of the cone), and KEPs are long to begin with.

The actual depth of penetration depends on the length of the penetrator and the densities of both the target and penetrator material. That’s it: not velocity, not material strength, definitely not temperature. As ballistics equations go, it’s one of the simplest:

The purpose of this detour wasn’t to send you to sleep, but to explain how shaped charges work so well so that you can understand how ERA disrupts them. Let’s discuss this next.

ERA uses angles to waste the power of a shaped charge jet

Shaped charges penetrate deeply into armour, so it needs to be thick. Armour is heavy, though, so you can’t make it too thick or the tank won’t be able to move. What if we could temporarily make the armour thicker, but only when it needed to be? That, in essence, is the idea behind ERA.

When a shaped charge hits a module of ERA it detonates the explosive within. This accelerates a steel plate into the path of the shaped charge. Because the plate is at an angle to the shaped charge, it continually feeds fresh material into the path of the shaped charge jet, eroding both and keeping the main armour safe:

There are useful simulation animations of ERA on Dejmian XYZ Simulations channel on YouTube (although mostly with KEPs, see below)

ERA is extremely effective, reducing penetration by up to 75% at the right angle of impact (we’ll talk a bit more about this below):

I’ve you’ve been paying attention here you’ll also appreciate that the explosives used in ERA, being secondary (i.e. less sensitive) high explosives, will not initiate from the smaller shocks which a bullet or nearby explosion would impart. The only thing that will initiate the ERA is the high-speed shock impact of a shaped charge.

It has some limitations…

Explosive Reactive Armour, though brilliant, is not without its problems. The first, and most obvious of these, is that it’s single-use. Once a plate or module of ERA has been detonated then, obviously, it no longer offers any protection. This means the designer has to strike a balance. Make the tiles too big and you increase the risk of a second strike on a previously detonated and now unprotected area. Make them too small, however, and you run the risk of a strike along the boundary between two tiles, where there will be less or no protection.

You can see in the pictures above where the compromise line falls. Because they are normally bolt-on², it’s relatively easy to replace tiles which have been blown away.

Another limitation of ERA is its angle dependency, which you saw in the results above. The shaped charge has to strike the plate at an angle for the effect to work properly. You can’t progressively erode the shaped charge jet unless you keep feeding it fresh material, and the only way to do this is with a plate travelling at an angle.

This isn’t a problem for the front of a tank and for most turrets, since their base armour is already usually angled. The reason it’s angled, by the way, is to present more material toward the area of greatest threat through the wonders of geometry:

For armour that’s vertical, such as on the sides of tanks, you need to find a way to mount the ERA tiles at an angle. An alternative, as with Russian Kontakt-1 ERA (seen on the T-72 tank which graces the cover of this article), is to angle the plates inside the ERA module. Kontakt-1 actually puts its plates in a V-shape, so that you’re more or less always guaranteed to have plates travelling at an angle to the shaped charge jet. This is even more effective than normal, reducing the penetration by up to 90%:

One final challenge of ERA relates to the fact that you’re strapping blocks of explosive and steel to the sides of your tanks. While these might be great news for the guys inside the tank, the folks outside the tank—you know, those chaps whom the tank is supposed to be supporting—are going to be less pleased.

As Paul Hazell explains on Bernhard Kast’s Military History [not] Visualized channel below, the fragment damage from ERA tiles might be much more of a threat to local troops than the detonation of the warhead (whose energy is deliberately going mostly toward a shaped charge to kill the tank). This aligns with what I’m constantly saying on this blog, which is that fragments are bad, m’kay?

…and threats have evolved to overcome it

Just as ERA has its limitations, so too have weapon designers found ways to negate its defensive effect. The first trick is to stack multiple warheads in one weapon. If the tank is going to put on extra armour, the logic goes, then why can’t the missile have an extra weapon? We saw this before when we looked at the make-up of the Javelin anti-tank guided missile:

The precursor warhead (the one nearer the front) initiates the ERA so that the plates are safely out of the way by the time the monster main charge arrives. Not much is surviving that. Add to this the fact that the Javelin can attack from above, where tank armour is weakest, and it’s clear that ERA or no ERA, the Javelin will soon turn a tank into a non-tank.

The other big way that designers have overcome ERA is to lean into kinetic energy penetrators (KEPs) such as APFSDS³. Although much slower than shaped charge gets, they are also much thicker and heavier. This means they just bash aside the lighter plates of ERA and get straight to the main armour where they start to do their erosion. New versions of ERA (such as Kontakt-5) are more effective against KEPs, primarily by incorporating heavier flyer plates.

The heavier plates break up and disrupt the KEP with their momentum, and this spreads the penetration effect over a wider area of armour, weakening its overall effect. This “disruptive” effect is also part of the reason that ERA is effective against shaped charge jets too, although the feeding of continuous fresh material is the more important one.

DAS disrupt rather than sacrifice

This section on defensive aid suites (DAS) or active protection measures (APS) doesn’t neatly fit in with our ERA tour⁴. I felt like any discussion of how tanks protect themselves would be incomplete without it though, and I’m damned if I’m coming back to do yet another encore for this never-ending ballistics series.

So I’m going to cover these quickly, which is just as well, because lots of the information around these systems is hard to come by and harder to share, for obvious reasons.

The basic principle of a hard-kill active protection measure is that it detects incoming anti-armour threats and fires some sort of interceptor to hit and therefore disrupt the threat. It can break the threat apart before it reaches the tank’s armour or, in the case of a shaped charge, detonate it early and in the wrong sequence so that a proper jet won’t form.

An example of an APS is the Trophy system:

Some DAS/APS (especially older ones) rely on shaped charges or EFPs embedded within the vehicle’s armour. In that sense they are very similar to ERA, albeit with a different initiation mechanism (a sensor instead of direct impact) and in bringing their effect to bear further away from the tank. The same limitations around danger to friendly troops apply to hard kill DAS/APS as with ERA.

Conclusion: ERA and DAS turn MBTs into explosive porcupines

At the outset I mentioned that ERA is one of those “things that only make sense to an engineer,” like firing narrow darts through giant tank guns. Another reason why it’s difficult, no, mind-melting, to think about ERA is because of the timescales involved. A shaped charge penetration happens on the order of microseconds, and yet we can design components that:

1. Sense the threat
2. Initiates own explosive
3. Accelerates sacrificial metal into the path of the shaped charge

ERA is a clever way to greatly reduce the threats from common anti-tank weapons. Despite its limitations, it is extremely effective, as evidenced by its use by both sides in the current (as of Oct 2025) Russo-Ukrainian War. There are newer variants such as non-explosive reactive armour (NERA) which pose less of a threat to troops in the open.

Strapping explosives to the sides of your tanks, it turns out, is a good idea. With the advent of hard kill DAS/APS systems, modern tanks are like porcupines (highly explosive ones, at least).

That’s it for this week, folks. Thanks for reading and please remember, if you haven’t already, to subscribe using the link below. Please also share this article with a friend. See you next week.

Featured Image: T-72M1R tank with reactive armour in Ukraine. May 2022. Photo credits: Buchko Volodymyr, via militarnyi.com

1. Important caveat, as we shall see. ↩︎
2. Or “appliqué,” to use the technical term. A term which is used, so far as I can tell, only for needlework and for vehicular armour. ↩︎
3. Armour-piercing fin-stabilised discarding sabot. ↩︎
4. Not to be confused with the Eras tour, a very different story. ↩︎