There are countless commonly-cited myths floating around regarding what high explosive anti-tank (HEAT) warheads are and how they work, most of which are completely inaccurate. In this article, the author provides a crash course in the field of shaped charges and related protection systems, demystifying HEAT warheads as well as many commonly-cited countermeasures, and explaining how they really work.
The most popular myths surrounding HEAT warheads typically include descriptions of the weapon as ‘unleashing a jet of molten metal’ or sometimes also ‘burning its way through armour’ and often state that a small amount of standoff spacing – such as that offered by bar armour, side skirts, spaced armour, or overhead protection – most notably the early versions of so-called ‘cope cages’ – provide protection against this threat. None of these myths are true. To understand why, we will first need a proper explanation of what a HEAT warhead is and how it actually works.
What is a HEAT warhead?
HEAT warheads belong to a class of high explosive warheads known as shaped charges.[1] Of the multiple designs available in this class, designers working on military applications have typically been concerned with three main sub-types:
- High Explosive Anti-Tank (HEAT);
- Explosively-Formed Penetrators (EFPs);
- Linear Shaped Charges (LSCs).
In very simple terms, the above warheads consist of a container with a high explosive filler (often common high explosives, such as PETN or RDX) with an angled cavity inside it, and typically a liner (usually made of a ductile metal, but many different materials can be used) conforming to the shape of the cavity. The exact shape of the cavity and liner will depend upon which of the three main designs referenced above are used. For instance, HEAT warheads will use a steep-angled cone-shaped liner, while LSCs will use an extended V-shaped liner, and EFPs will use a much flatter, shallow-angle, bowl-shaped or plate-shaped liner to create a short but high-velocity projectile or ‘slug’. If no liner is present, this is known as a ‘hollow charge’ or ‘unlined-cavity charge’.[2] There are also variations on some of these designs, which will not be covered here for the sake of simplicity.
Of the shaped charge types listed above, HEAT and LSCs operate on the basis of the ‘Munroe effect’, wherein a cavity within an explosive material is used to focus blast forces and (if present) liner material along the axis the charge is facing. If a no liner is present, a crater on the target surface is produced by high pressure, high velocity gas erosion. If a liner is present, this results in a much deeper crater on the target surface.[3] EFPs on the other hand operate according to the ‘Misznay-Schardin effect’, in which a slightly bowl-shaped liner is accelerated by a high explosive charge against its convex face, forming a penetrating projectile known as a ‘slug’.[4] Since HEAT warheads are the primary focus of this article, and have tended to generate the most myths around their function, special attention will be paid to the former.
The Munroe effect was named after Charles E Munroe, an American chemist who at the time worked at the US Navy’s Naval Torpedo Station; Munroe has been popularly credited with discovering the effect in 1888 during experiments with nitrocellulose (also known as guncotton). However, this in itself is not entirely accurate – deeper examinations into the subject have typically credited a German named Max von Foerster with discovering the shaped charge effect five years earlier in 1883, during experiments with compressed nitrocellulose.[5] Consequently, the effect has sometimes been referred to in Europe as the ‘von Foerster effect’.[6] Added to this, in 1886, two years before Munroe’s reported discovery, a German named Gustav Bloem filed a US Patent for a type of explosive which used a hollow cavity to concentrate the blast energy in an axial direction. This is the basic principle along which all shaped charges work. However, there was one key component missing.
While earlier shaped charges had used a simple hollow cavity set into the explosive filler, what made Munroe’s discovery different is that he was the first to successfully demonstrate the use of a metal-lined cavity in a shaped charge, during an experiment he conducted in 1894. Munroe’s lined warhead was extremely crude by modern standards, comprising a tin can with sticks of dynamite wrapped around it. Nonetheless, during an experiment it managed to defeat a metal safe made of iron and steel plates with walls 121 mm (4.75 inches) thick – in which, according to Munroe’s writings, “a hole three inches in diameter [76 mm] was blown clear through the wall”.[7]
With this discovery, Munroe had demonstrated the potential of this technology for armour-piercing applications. However, the physics behind Munroe’s discovery would require decades to be understood properly. According to a 1983 paper titled: ‘History of the Shaped Charge Effect: The First 100 Years’ by Donald R. Kennedy, “Munroe was apparently unaware of the effect of the tin can, and the importance of the liner was not to be recognised for another forty-four years.”[8] The date Kennedy references is 1938, which coincidentally is the same year Munroe died.
Our understanding of shaped charges has come a long way since Munroe’s experiment, and modern warhead manufacturers have for years built warheads capable of defeating armour over 17 times thicker than what Munroe’s crude warhead managed. The warhead design community now has a strong understanding of the key roles plated by liner angle, liner material, explosive wave shaping, precision construction of the liner, and standoff. Designers have also developed increasingly sophisticated warheads in response to advances in armour technology. This has all been made possible through deepening our understanding of the key physical principles at work in this field.
How do HEAT warheads work?
The basic principle behind all of these shaped charge weapons is to use the chemical energy of the explosive to impart a large amount of kinetic energy onto the liner, which then imparts this kinetic energy onto the target. In a HEAT warhead, this is achieved by using the power of the explosive to effectively ‘squeeze’ the liner and also push it forward at very high speeds. In the process, this shapes the liner into a focussed high-speed ‘jet’ of high-speed metal which focusses its kinetic energy onto a small point on the target.
Here we encounter the first set of myths, which are some of the most enduring about HEAT warheads, and often go as follows:
- ‘During detonation, the liner metal becomes molten or liquid.’
- ‘During detonation, the liner material gets vapourised and transformed into some form of super-hot plasma.’
- ‘The penetrative effect is achieved due to the high-temperature jet melting through the armour.’
None of these are accurate. As neatly summarised by William Walters, writing during his time as a scientist at the Weapons and Materials Research Directorate of the US Army Research Laboratory:
“[T]he jet is not a “cutting plasma”, it is not a liquefied or molten metal jet, the cone does not impact the armor intact, the jet temperature is not 20,000 [°]C, and the density of the jet is not several times that of steel, and the jet does not burn its way through armor, as reported in many newspaper, TV, and even semi-technical journal articles. Some confusion may arise due to the fact that shaped charge devices are sometimes called HEAT rounds. HEAT is an acronym for High Explosive Anti-Tank and does not relate to thermal effects”.[9]
Not (that) hot, not (usually) liquid
All three of the above myths have been disproven in laboratory tests which have shown that nearly all metal liners do not get hot enough to melt at all, let alone become some form of super-heated plasma, or hot enough to burn or melt through the high-carbon steels and/or ceramics often used in modern armour. In fact, the average temperatures on the surface of a typical copper liner HEAT jet usually range from around 400-600°C (on average 20-50% of the melt temperature of the liner), depending on liner and explosive material used, although temperatures at the core and the tip can be greater.[10] Various different materials have been tested as candidates for shaped charge liners over the years, but overall the typical jet temperature reached is well below the melting point for most materials commonly used as warhead liners, see table 1:[11]
Table 1: Liner material melting points | |
Material | Melting Point |
Aluminium | 675°C |
Copper | 1,083°C |
Molybdenum | 2,610°C |
Nickel | 1,452°C |
Steel (Mild) | 1,410°C |
Tantalum | 2,996°C |
Tungsten | 3,422°C |
Uranium (Depleted) | 1,133°C |
Zirconium | 1,852°C |
NOTE: The above is not an exhaustive list, with various other metals and non-metals, including Gold, Silver, Titanium, various alloys, bi-metallic combinations, glass, Lucite/Plexiglass, plastics, and water being trialled as candidates for shaped charge liners. Some, such as water (as the liner in an LSC), have been adopted for niche, low-collateral damage applications such as car bomb defusal, or breaching buildings. |
While it is certainly possible to create fluid jets, for instance through the use of liners made from low melting point elements such as Lead, which melts at 327.5°C, or even to use liquid liners, the overwhelming majority of shaped charges encountered on the battlefield will be designed to create coherent, ductile jets. This requires a liner made from materials with a melting point above the expected jet temperature, and relatively good ductility and density.
Nonetheless, historically, circumstances have sometimes required that imperfect materials are used. Aluminium, which sits at the low end of the melting points typically used for liners, is a relatively uncommon choice for a shaped charge liner material nowadays, due to its very low density – though it is worth noting it has seen use in the past, notably on the AGM-65 Maverick warhead.[12] Mild Steels were also commonly used in the early days of shaped charges, notably during World War II, in large part because steel was more available than Copper, which was in high demand for various other purposes, such as manufacturing brass for bullet and shell casings. For the past several decades, however, Copper has been by far the most widely-used liner material for HEAT warheads, owing to its combination of ductility, density (8.9 g/cm3), availability, and cost.
This does not mean that Copper is necessarily the best choice for achieving high penetration, with Molybdenum gaining in popularity in recent decades due to its good ductility coupled with a slightly higher density (10.2 g/cm3) than Copper. When coupled with modern high explosives, this can allow for greater penetrative performance than traditional Copper liners. Elsewhere, in the realm of EFPs, Tantalum is also becoming a popular choice for modern designs, due to its good ductility and significantly higher density (16.6 g/cm3) than Copper.
So what is the jet if it isn’t (usually) liquid? The technical term would be ‘in a state of high-speed plastic flow’. In essence, the pressures imparted onto the liner by the explosive are so high, that they overcome the yield strength of the liner material and cause it to undergo extremely rapid plastic deformation, resulting in elongation of the liner by 1,000% or more, thereby forming what is called the ‘jet’.[13] Probably the best analogy is that the force applied to it becomes so great that the liner effectively becomes flexible like plasticine to the explosive, forming a long, stretched shape akin to a long spike or lance, with a thinner tip, and becoming progressively wider toward the rear end. This jet moves incredibly quickly, but has an uneven speed along its length, with the tip moving the fastest (in modern examples, it can travel in the order of 8-10 km/s or more) and having the greatest penetrative potential, while the middle and rear (sometimes known as the ‘slug’) move at slower speeds and have less penetrative potential. Due to the presence of this velocity gradient along its length, the jet will keep stretching until it particulates (stretches itself into a column of particles).[14]
Nonetheless, the jet remains a solid, albeit one which has been stretched under extreme pressure to the point where it behaves a bit more like a liquid. It is for this reason that hydrodynamic equations are commonly used to calculate HEAT jet penetration, since these models provide a fairly accurate approximation of the observed result. Indeed, it is often useful to imagine HEAT jets as a flowing liquid, since this helps to explain many of their behaviours, even though it does not exactly match material reality.
Another unhelpful factor which has muddied the waters around the true nature of HEAT jets is that in English-speaking nomenclature, these weapons have often been categorised as ‘Chemical Energy’ (CE) weapons, in contrast to Kinetic Energy (KE) weapons. This distinction is not a particularly helpful one, nor is it particularly accurate. For HEAT rounds it holds true only insofar as the initial energy imparted onto the liner is chemical energy, however the actual armour defeat mechanism is through the kinetic energy of a solid penetrator. Yet the exact same is true of traditional KE weapons, such as armour-piercing fin-stabilised discarding sabot (APFSDS) rounds; the main difference being that APFSDS rounds have chemical energy imparted onto them while the penetrator is still inside the barrel, while HEAT rounds impart chemical energy onto the penetrator in the moment of the warhead’s initiation. In both cases, chemical energy is being converted to kinetic energy, yet by a quirk of convention, HEAT warheads have retained the ‘CE’ label.
Extreme penetration potential
Penetration is typically measured in millimetres of ‘Rolled Homogenous Armor equivalent’ (RHAe) – the ‘equivalent’ portion refers to the fact that no modern tanks or vehicles will purely use Rolled Homogenous Armour (RHA), but often use composite components, including ceramics, resins, steel plates of varying hardness, plastics, aramids, and various other materials. All of these materials have different levels of resistance to various forms of penetrator. So, to simplify our approximations of the true capabilities of a vehicle’s armour, analysts and engineers often take the convention of approximating what all these layers of modern armour would be equivalent to in terms of protection if they were a single block of RHA, hence ‘RHA equivalent’. This helps to provide a standard, universal system of measuring both armour effectiveness and weapon penetrative capability, allowing us to conveniently compare different vehicles and weapons to one another.
HEAT warheads have been used as anti-armour warheads for many decades precisely because they offer frighteningly high armour penetration potential, typically comfortably exceeding that of APFSDS projectiles. By way of comparison, a reasonably modern tandem-HEAT warhead such as those used in Russia’s Kornet and Khrizantema families of anti-tank guided missiles (ATGMs) can penetrate over 1,300 mm of RHAe. This can be contrasted with the US M829A3 APFSDS round, which is estimated to be capable of penetrating around 400 mm of RHAe sloped at 60° from vertical at 2 km – this is materially equivalent to penetrating 800 mm of RHAe at 0° (vertical) – which is significantly less than the Kornet or Khrizantema.
Of course, APFSDS rounds have many advantages over HEAT rounds, not least of which is their much shorter flight time to target, and often better post-penetration effects. On the other hand, APFSDS projectiles lose penetration potential to aerodynamic drag as the projectile gets further from the barrel, but HEAT projectiles do not, owing to their chemical energy being imparted onto the penetrator at the point of impact, making them better-suited for use on long-range anti-armour weapons. Additionally, HEAT projectiles allow for the design of more compact and portable (and often guided) anti-armour weapons than APFSDS rounds, which usually require a long gun and are almost always unguided. This capability allows HEAT warheads to be far more universally employed in the anti-armour role, suited for use on UAVs and loitering munitions, helicopters and fixed-wing aircraft, artillery munitions and submunitions, dismounted infantry, light utility vehicles, APCs, IFVs, and tanks. Large-calibre APFSDS rounds on the other hand are largely limited to vehicles with a large-calibre gun, such as tanks and direct fire support vehicles.
The myth of standoff
Perhaps the most common myth which has arisen around HEAT warheads is that they can be easily defeated through the use of small amounts of standoff (empty space), with popular opinion frequently considering a gap of 0.3 m to 1 m to be sufficient to nullify the penetrative power of the jet. Once again, this is mistaken, as can be readily seen from footage of overfly top attack (OTA) weapons such as NLAW, which often activate their warheads at more than a metre over their target.[15] Nonetheless, common arguments that standoff defeats HEAT warheads have pointed to the adoption of spaced armour, including side skirts, and statistical armour protection such as bar armour (also known as slat armour), and RPG netting (such as Tarian) as proof that standoff is somehow effective against HEAT warheads. Again, these are largely inaccurate, and have arisen from a fundamental misunderstanding of how such forms of protection actually work.
To understand why, the first thing to note is that HEAT warheads actually require a relatively high degree of standoff distance just to reach their maximum penetration potential. The exact optimal distance will vary depending on various factors in the warhead design, such as liner material, liner angle, liner thickness, liner grain size, defect tolerance in liner manufacturing, explosive filler type, detonation wave shaping, along with various other factors, and will vary among different warheads. However, as a general rule, it is not possible to get the maximum level of performance out of a jet without a degree of standoff – simply put, some physical space is needed for the jet to focus itself into the optimal shape.
This basic principle was understood on early HEAT warhead designs, but the degree to which standoff could improve performance was underestimated compared to today, which is in large part why so many early HEAT weapons positioned the warhead very close to the tip of the munition – as seen on PG-7V, or the 9M14 Malyutka, and BGM-71A TOW. The small amount of built-in standoff in such early warheads was typically considered ‘optimal’ for their warhead design. However, it is also fair to say that many early warhead designs of this period were also pretty crude by modern standards, and were not manufactured to the same high design tolerances they are today. Consequently, the jets produced had lower performance and were more susceptible to standoff-based degradation. As a general rule, flaws in the warhead liner are to be avoided as much as possible, as even relatively small flaws (such as dents, bumps, or minor asymmetries) become multiplied by orders of magnitude by the extreme forces involved during the process of liner collapse. Modern HEAT warheads typically employ liners built to extremely high manufacturing tolerances for precisely this reason.
During the mid-Cold War, as the science of shaped charges became better-understood, anti-tank weapons started to adopt measures to increase standoff. Perhaps the most famous example are ‘standoff probes’ (also known as ‘standoff spikes’) – with a good example being the M456 HEAT-T round for 105 mm tank guns. These were effectively a long hollow probe housing an impact-activated switch in the tip, to trigger the fuze located behind the warhead, and later some designs used the probe as the housing for a smaller precursor warhead. These designs allowed for slightly more distance for the jet to form, which improved penetrative performance. However, the standoff probe solution was not ideal, there was more performance to be eked out by increasing standoff further. This may have not mattered as much during the mid-Cold War when such rounds were generally sufficient to defeat the tank armour of their day, and where extremely high manufacturing tolerances for the liner were more difficult to achieve, but it began to matter more as composite armour and explosive reactive armour (ERA)-equipped tanks began to be fielded.
According to data gathered by Dr Manfred Held while working for what is now MBDA Deutschland, during tests conducted in 1985 using several common ATGMs of the day, the impact of standoff was typically an increase in penetration performance up to a point, after which it would begin to decrease.[16] However, the rate of this decrease could vary quite wildly among warheads. At this point it is important to explain that both standoff and penetration for HEAT warheads are commonly measured in ‘charge diameters’ (CD), which are multiples of the warhead diameter. This is because the latter plays a decisive role in predicting the penetrative performance of the warhead; generally speaking, the wider the better.
According to Dr Held’s data, the 150 mm warhead used by HOT 2 reached its peak penetration of 1,150 mm of RHAe at around 1,200 mm of standoff (8 CD), and by 3,000 mm standoff (20 CD), penetrative performance had decreased to around 750 mm of RHAe (a 35% decrease). By contrast, the BGM-71D TOW 2, fitted with a slightly different but same 150 mm diameter warhead achieved its peak penetration of 1,000 mm of RHAe at only around 850 mm of standoff (5.6 CD), though its performance degraded much more quickly than that of HOT 2 – at approximately 2,850 mm standoff (19 CD), performance had dropped to around 500 mm of RHAe (a 50% decrease). Broadly speaking though, a trend is evident in Held’s graph – that newer warheads typically underwent less degradation due to standoff than older designs.
It should also be noted that while these performance degradations are noteworthy in engineering terms, they do not necessarily equate to meaningful loss of capacity to defeat an enemy vehicle on a real-world battlefield. To take an example, even the worst-performing warhead on Held’s graph, the ‘TOW 1’ (presumably referring to the BGM-71C ITOW), underwent a roughly 53% decrease in penetrating power at a standoff of 2,500 mm. Yet even at this distance, the jet was still capable of penetrating around 380 mm of RHAe – which is roughly equivalent to the total penetrating power of an early model APFSDS round for a 125 mm tank gun. This is more than sufficient to penetrate most vehicles with sub-MBT levels of protection, and could even pose a threat to MBTs if directed against a less-armoured area of the vehicle.
A few years later, in their 1989 book ‘Fundamentals of Shaped Charges’, William Walters and Jonas Zukas provided a model for approximate jet performance at various levels of standoff.[17] According to their graph of jet performance and standoff, under ideal conditions, jet performance degrades quite slowly through air, with penetration into armour decreasing from about 7.5 CD to around 7 CD, after travelling a standoff distance of 24 CD. To put this into context, assuming a 150 mm HEAT warhead, the penetration potential would decrease from 1,125 mm of RHAe to about 1,050 mm of RHAe after the jet has travelled 3.6 m through the air – a reduction of only around 7% at a distance far greater than any standoff likely to be encountered. This was significantly less than the observed standoff degradation in real-world testing presented by Held, but Held used a range of warheads which varied greatly in quality, and Walters and Zukas’ ‘ideal’ jet performance curve does appear to be broadly accurate with respect to penetration by some of the more capable warheads of the time. For instance, around that time large unitary HEAT warheads used by missiles such as the Milan 2 (introduced in 1984) and HOT 2 (introduced in 1985) were respectively reported to be capable of penetrating 880 mm (7.6 CD for the 115 mm warhead) and 1,100 mm (7.3 CD for the 150 mm warhead) of RHAe, broadly in line with Walters and Zukas’ graph curve for ‘ideal’ jet performance.
However, warhead design has come a long way since the mid-late Cold War ATGMs seen in Held’s graph, and penetration loss rated in air for modern warheads are today probably closer to Walters and Zukas’ model for an ‘ideal’ jet in air, than they are to even high-performing examples in Held’s graph, such as HOT 2. Modern warheads typically include several times more built-in standoff than their predecessors, and have broadly exceeded Walters and Zukas’ model of ‘ideal’ jet penetrative performance by quite some margin.
Toward the end of the Cold War, as our collective understanding of the role of standoff in optimal jet formation improved, weapon designs began to emerge in the 1990s and 2000s featuring far more built-in standoff than previous-generation models. This is precisely why Kornet, Javelin, Spike, Akeron, and many other modern ATGM families position the main HEAT warhead towards the rear of the missile, and feature a hollow channel in front of the warhead central axis to provide more space for the jet to form optimally. Typically, modern ATGMs will also feature an arrangement known as a ‘tandem-HEAT’ warhead which include a smaller precursor warhead close to the front of the missile to deal with ERA, clearing the path for the main charge. Typically, these precursors are a small HEAT warhead, but small EFPs have also been used as precursors in some designs, to avoid activating the ERA altogether, as on the DND Panzerfaust 3-T. These design trends have largely held true for most ATGMs developed since the 2000s, with few outliers.
Overall, post-Cold War HEAT warheads are significantly more capable than their predecessors, with the additional standoff giving more space for optimal jet formation and with modern liners capable of being manufactured to sub-micron design tolerances. In testing, they have regularly exceeded even the nearly 8 CD penetrative parameters of ‘ideal’ jet performance in Walters and Zukas’ model. By way of example, the 1,300 mm of RHAe stated penetration capability of the 9M133M-2 Kornet-M is equivalent to around 8.7 CD for the 152 mm warhead. While this is not a perfect example since 9M133M-2 uses a tandem-HEAT warhead with the precursor and main charge axially aligned, it nonetheless shows what is possible with modern warhead designs.[18]
Yet, even these figures of 8.7 CD penetration are relatively modest by the most recent standards. Anecdotally, in a conversation with one individual who previously used to test HEAT warheads, this author was told that they have observed penetration equal to 10 CD be achieved, and this was used as a rule of thumb for calculating penetration for a modern HEAT warhead. Yet even 10 CD, previously seen as a limit, has been exceeded. According to statements made by two Saab Bofors Dynamics Switzerland representatives in 2022, the company has developed warheads, such as that used on the Milan ER developed in the early-2000s, and more recently on MBDA’s Akeron MP, which have proven capable of 11-12 CD penetration, with one representative noting that trend was to aim for 15 CD.[19] To put this into perspective, for the 115 mm K-charge warhead on Akeron MP, one would expect a penetration of around 1,265 mm to 1,380 mm of RHAe to be possible, significantly more than the boilerplate “more than 1,000 mm” figure (equivalent to a mere 8.7 CD) listed in MBDA marketing material would lead one to assume.[20] This goes to show just how shockingly powerful modern HEAT warheads have become. So while penetrative performance has increased significantly in the past several decades, where does this leave standoff?
According to a model using analytical experimental data presented by William Walters in his 2007 presentation ‘Introduction to Shaped Charges’, an observed trend for a representative Copper-lined HEAT warhead is that it required require standoff in the region of approximately 6-8 CD to reach maximum penetrative performance.[21] To put this into context, a reasonably modern ATGM such as the 9M133M-2 Kornet-M, with a 152 mm warhead, would require roughly 0.9–1.2 m of standoff just for the warhead to reach its full penetration potential. This is precisely why a large degree of standoff is built-in on most modern ATGM designs.
Assessing performance for modern warheads operating far beyond optimal standoffs is not straightforward, for instance obtaining reliable test data on standoff curves for modern warheads is difficult at best. However, using the trend of diminishing standoff-based degradation on more recent warheads observed in Held’s graph as a benchmark, and considering the extent to which both average built-in standoffs and penetrative performance have both increased greatly since then, all suggests that modern warheads are even less sensitive to standoff-based degradation than their Cold War predecessors. As Held’s graph showed, even warheads considered relatively old by today’s standards broadly retained high penetration potential even at standoffs of several metres – far more standoff than one is ever likely to realistically encounter on any battlefield vehicle.
Furthermore, while standoff-based protection may be especially ineffective against modern warheads, even some sources writing as far back as the early Cold War have stated that in testing, standoff was not found to be an effective form of protection against HEAT warheads.[22] In a 1950 paper titled ‘Spaced Armor’ presented at the Second Tank Conference, Ballistic Research Laboratories, Aberdeen Proving Ground, author A. Hurlich noted the following with regard to the effectiveness of standoff-based protection against HEAT warheads of the time:
“More recent information on newly designed shaped-charge shell indicate that they are not degraded by spaced armor combinations unless the skirting plates are placed at distances from the main armor which are impossible from a practical engineering viewpoint to use in actual vehicle designs.”[23]
Further out, even at extremely large standoffs, the remnants of a jet can remain dangerous, in particular the rear portion, known as the ‘slug’, comprising the majority of the liner mass. A 2012 paper by Fredrik Johnsson, Bengt Vretblad, and Åke Sivertun published in the Journal of Military Studies presented a model for estimation of the maximum hazardous area for shaped charge warheads. According to the authors’ model, a 150 mm jet remnant ‘slug’ aimed at an elevation angle of 30° could travel out to around 1,700 m if it began tumbling during its flight, or out to around 3,600 m if its trajectory remained stable.[24] While such a slug would be highly unlikely to penetrate heavily-armoured vehicles such as tanks at such ranges, it would still pose a risk to soft targets such as infantry and unarmoured vehicles.
In sum, it is true that a HEAT jet does gradually lose coherence and begins to particulate, and hence lose penetration potential, as it travels through air. However, in order for this to make a meaningful difference on the battlefield, this would require such large standoffs that they would be completely impractical to implement (for instance through spaced armours) on any realistic military vehicle design. In addition, since many modern HEAT warheads already massively overmatch the passive protection of most vehicles, other than the frontal arc of a modern MBT (and with modern warheads, sometimes even that), so even if the jet does undergo some degree of degradation, it will usually not be enough to protect most vehicles.
It is therefore fair to say that, for the vast majority of HEAT warheads one is likely to encounter, standoff-based jet degradation is not a relevant factor in most real-world engagement scenarios involving a HEAT warhead. As such, standoff should not be considered a viable or practical form of protection against this threat. With the above in mind, if spaced armour, bar armour, ERA, and various other forms of protection are not about creating standoff, how do they really work?
Understanding protection – Spaced armour
Spaced armour has long been popularly misattributed as a standoff-based defence against HEAT rounds, with a particularly enduring myth being that the German Armed Forces used ‘Schürzen’ or ‘skirts’ on their tanks during WWII to defeat HEAT rounds.
In reality, these ‘Schürzen’ were primarily intended to strip the soft metal jackets off the armour-piercing projectiles used by anti-tank rifles and (depending on the thickness of the ‘skirt’) gun-fired armour-piercing capped projectiles. Without the soft metal jacket to moderate penetrator-armour interaction, the result was that the hard-but-brittle hardened steel or Tungsten carbide core of the projectile would often fracture or shatter when it impacted the target’s main armour. The spacing could also induce a degree of yaw onto the projectile, further reducing its effectiveness. As noted by Richard Marian Ogorkiewicz in his book ‘The Technology of Tanks’, regarding spaced armours seen during WWII:
“spaced armour offered the possibility of its outer plate stripping off the ballistic cap of armour piercing projectiles or of deflecting them and therefore making them less well able to penetrate the inner plate. However, spaced armour was not considered to offer sufficiently better protection against the contemporary kinetic energy projectiles to justify the complication in the construction of tanks which its use entailed.”[25]
Spaced armour is still used today, when anti-tank rifles are a comparatively uncommon sight, because it provides effective protection against a wide variety of threats – from common ballistic threats, to high explosive (HE) threats, and high explosive squash head (HESH) rounds.
Against conventional ballistic weapons such as bullets, aside from the aforementioned benefits of interaction with the penetrator jacket and introduction of yaw, the first plate in a spaced armour array does not pass on impact-related stresses to the second plate, meaning that the initial impact energy of the round is only entirely absorbed by the first plate, with base armour (or second and third plates if present) having to absorb diminishing amounts of impact energy. Additionally, spaced armour arrays using thinner plates also provide additional benefits against ballistic projectiles, due to their rather unique interaction with the projectile, as noted in the 2018 paper ‘A review of the integrity of metallic vehicle armour to projectile attack’ by Lenihan, Ronan, O’Donoghue, and Leen:
“Thick plates tend to fail by [brittle fracture, ductile hole growth, radial fracture, plugging, or fragmentation], whereas thinner plates tend to bend and stretch around the area of impact [petaling], thus absorbing much of the kinetic energy of the projectile. In this way, thinner plates can be more effective per unit mass at stopping projectiles than thicker ones. This means that spaced armour consisting of multiple layers of thinner plates offset from one another can be a mass-effective means of protection. However, this has the disadvantage of adding width and greater geometric complexity to a vehicle hull.”[26]
Against HE and HESH rounds, spaced armour arrays conduct minimal amounts of the explosion’s energy to the vehicle interior, since air is a poor conductor of energy. This prevents the shockwaves generated by high explosives from reaching and being transmitted through the vehicle main armour, significantly improving protection against such threats compared to a vehicle with monolithic armour.
The above merits do not really apply to HEAT warheads, unless the basic spaced armour is somehow augmented. While notionally, in certain cases, spaced armour could potentially (depending on the warhead design, plate thickness, slope angle, materials, and spacing) be expected to cause some slight reduction in the penetrating power of a HEAT jet, as outlined above, it is generally inadequate to provide any meaningful protection against this class of threat by itself. Added to which, there are far more effective solutions available. However, as will be explained later with reference to recent battlefield adaptations seen in Ukraine, some forms of spaced armour can provide other advantages.
Bar/slat armour
While commonly claimed to defeat HEAT warheads by introducing standoff, as noted above, a HEAT jet is an extremely powerful solid object with a tip travelling at hypersonic speeds of 8-10 km/s or more – it doesn’t really suffer any noteworthy performance degradation by having to travel through 30 cm or so of air. Indeed, if it did, one would expect all tanks to be fully covered in bar armour. As such, it is inaccurate to say that bar armour offers protection against shaped charge weapons generally.
To be precise, bar armour only offers protection against a specific fuze design used by PG-7V/PG-9V/PG-15V munitions, along with some variants and clones. However, given the sheer ubiquity of these munitions on battlefields the world over, bar armour and its textile-based equivalents remain a useful lightweight and low-cost form of protection against this common threat.
To understand how bar armour really works, it is important to first understand how PV-7V/PG-9V/PG-15V munitions work. All of them work on the principle of a piezoelectric fuze component at the tip of the nose, and an electrically-activated fuze component located behind the warhead. In order for the warhead to detonate, two things need to happen:
- The piezoelectric fuze component needs to impact the target. Doing so means that the piezoelectric crystals within are crushed, generating an electrical charge.
- This electrical current then needs to travel to the rear electrically-activated fuze component via the inner ‘skin’ of the nose, which is conductive.
Once the electrical signal has reached the rear fuze component, the warhead is able to detonate. Bar armour tries to circumvent this by forcing the inner ‘skin’ (forming the conductive path for the current from the fuze to the rest of the fuze train) and outer ‘skin’ (forming the aerodynamic shell of the warhead nose) together. The bars achieve this by being narrower than the warhead, and so as the warhead tip slips past the bars, the outer skin gets rapidly pushed into the inner skin. Since both the inner and outer skin are conductive, this means that even if the piezoelectric fuze activates, the resultant electrical signal is unlikely to reach the rear portion of the fuzing system, since the contact between the two skins forms a short circuit for the conductive path. Yet this describes an ideal scenario, and in practice, bar armour can induce other beneficial effects during interaction with the warhead including: impact-induced liner degradation, which prevents the formation of a coherent jet; impact-induced deflagration of the warhead, which prevents successful warhead detonation; and (in rare cases) the warhead getting stuck in the bars without detonating.
This form of protection does not always work as intended. The greatest risk is that the warhead tip impacts one of the bars, or the edge of the frame holding them, in which case the warhead would detonate normally. In some niche cases, the extra standoff provided by a detonation on the bars or frame may even slightly improve the warhead’s penetration, since the jet has more space to form optimally. Additionally, the angle at which the warhead impacts the bars plays a role in whether or not the armour works as intended. For instance, while the gap between bars (from the projectile’s point of view) seems large when looking directly perpendicular to the bars, this gap becomes much smaller when aiming down at a vehicle side from a higher elevation, as may occur with an RPG gunner sitting on a rooftop. As the gap between the bars becomes smaller from the standpoint of the projectile, the armour becomes less likely to work as intended, since the projectile is effectively facing a greater surface area where successful detonation is possible. For these reasons, bar armour and common equivalents such as Tarian RPG netting, or the Israeli ball-and-chain arrangement seen on the Merkava tank family are often referred to as ‘statistical protection’.
As such, bar armour provides no meaningful protection against typical modern ATGMs, which are nearly always much wider than the bars, use an impact fuzing system which is difficult to meaningfully interfere with (such as a crush switch), and on modern examples, have the warhead located far back inside the projectile. Yet even the munitions bar armour is designed to protect against can be modified to be unaffected by it. As a case in point, in the ongoing War in Ukraine, first-person view (FPV) drones have often been observed using a common HEAT warhead such as PV-7V with a jury-rigged fuzing system. The most basic and early versions of these involved using a pair of wires which are designed to make contact with one another as the drone impacts the vehicle, thereby completing the fuzing circuit and activating the warhead. Since these do not rely on the PG-7V’s normal fuzing system, bar armour is entirely ineffective against such modified fuze types.
Indeed, from an engineering standpoint, a PG-7V warhead would be trivial to proof against bar armour, even without modifying the fuzing system. At its most basic, all that would be required is to construct an ogival aerodynamic housing or fairing around the grenade warhead, and attached to the tip. As long as the fairing is wider than the bar armour gap, and the tip of the fairing is sufficiently sturdy to reliably ensure transfer of impact forces onto the piezoelectric element at the warhead tip, it should ensure reliable warhead activation against bar armour. The added weight and bulk of the fairing may reduce range and accuracy (somewhat problematic given PG-7Vs are not particularly accurate to begin with), but there are scenarios where these may be less important than reliable warhead activation.
Hedgehog armour
While ‘hedgehog’ armour is a relatively uncommon sight on armoured vehicles, it has been employed by the German Army, most notably on the Puma infantry fighting vehicle (IFV) and PzH 2000 self-propelled howitzer (SPH), it is still a relevant example of another often-misunderstood form of protection. Much like bar armour, this form of protection doesn’t provide protection against HEAT warheads at large, but rather against a single type of warhead design which is relatively common – namely ‘open-cup’ type HEAT bomblets, typically employed as submunitions from artillery or air-launched bomb.[27] Common examples in this category include the US M42, M77, and M80 DPICM, or the German DM-1385, or the Russian 3B30, among many.
The defeat mechanism of this form of protection is pretty straightforward – when an open-cup bomblet lands on a roof protected by hedgehog armour, some of the hedgehog ‘spines’ end up protruding into the warhead’s liner cavity. As such, when the warhead activates, and the liner begins to collapse, some portions of the liner will encounter these spines, slowing down relative to the rest of the liner, and ultimately resulting in an asymmetrical liner collapse. Since HEAT warheads rely on a smooth, symmetrical liner collapse in order to focus their energy into a coherent jet, this disruption of their basic operating process hinders successful jet formation, thereby radically decreasing the warhead’s penetration potential. Given that this armour entirely relies on being able to protrude into the warhead cavity to achieve the aforementioned effects, it is ineffective against shaped charge munitions with an enclosed warhead.
During one incident in Ukraine during late 2022 involving a hedgehog armour-equipped PzH 2000, a popular claim arose that hedgehog armour had successfully protected the vehicle from the HEAT warhead of a Lancet.[28] In fact, there was precious little evidence to substantiate this claim, with neither the weapon in question positively identified, nor the extent of the penetration achieved. Having said that, available evidence from the damage pattern would suggest that this was indeed some form of HEAT warhead, however, it appears to have struck the vehicle on the right-hand side of the hull roof, just next to the driver’s hatch. At this spot on PzH 2000, there is nothing for the HEAT jet to realistically hit which could cause catastrophic destruction to the vehicle; so while penetration was probably successful, the post-penetration effect was likely only superficial damage.
Hence, it would be more accurate to assess this as a case of the PzH 2000 crew getting lucky that their opponent hit the wrong spot on their vehicle, rather than evidence of hedgehog armour’s effectiveness against HEAT warheads at large. To cause real damage, the shooter would have been better served by aiming for the vehicle’s turret, which is loaded with explosives and propellant charges, providing ample opportunity for catastrophic post-penetration effects.
Explosive reactive armour
The standoff myth has also extended to explosive reactive armour (ERA). The most common version of the myth states that when the ERA activates, the outer plate’s movement effectively creates standoff space which decreases the jet’s penetration potential. Once again, this is incorrect. To understand how ERA functions, we will need to look at a few key snapshots in time during the jet-ERA interaction.
The most common ERA design comprises a sandwich of two metal plates separated by an explosive filler layer in the middle, and usually housed within a brick or tile-shaped block. When a HEAT warhead detonates against the block, the tip of the jet will penetrate the ERA block outer casing, then the front plate of the ERA cassette ‘sandwich’, and initiate the explosive filler, causing front and rear plates of the ERA to start moving. The tip of the jet is the fastest-moving, narrowest, and most penetrative portion of the jet, and typically travels too quickly to be caught by the moving plates. So it exits the rear plate of the ERA cassette as the detonation is in progress and strikes the main vehicle armour. This means that for protection to be effective, the vehicle requires a certain level of base passive armour to absorb at least the jet tip. Next, as the detonation progresses, the plates start moving apart from one another which results in two primary effects occurring simultaneously:
- The movement of the two plates and jet-penetrator interaction induces Kelvin-Helmoltz instabilities within the jet, effectively disrupting the state of continuous flow at various points along the jet.
- Since the plates are designed to be positioned at an angle to the incoming projectile, when they move apart, both plates continue to feed fresh material into the path of the jet, wasting some of its cutting power on the plates rather than the vehicle armour.
These processes occur extremely quickly, taking place roughly over the span of 50-120 µs (millionths of a second) from warhead detonation, and together can reduce the penetrating power of the HEAT jet by anywhere from 50-90%, depending on the model of ERA and the HEAT warhead in question.
Kelvin-Helmholtz instabilities are a kind of shear in a continuous flowing substance. The jet is a coherent, focussed stream of ductile metal in a state of high-speed plastic flow. To reach its full penetration potential, the jet relies on this coherence to focus as much energy as possible onto as narrow a point as possible. In essence, it must remain as tightly-focussed and straight as possible. Disruptions in this continuous flow create instabilities (these can be likened to waves) within the jet, leading it to lose coherence and dispersing its energy over a wider area, resulting in a major loss of penetrating power.[29]
While ERA has proven a highly effective and perhaps the most weight-efficient form of protection against HEAT warheads, its effectiveness is ultimately influenced by factors such as the impact angle of the jet and spacing from the passive armour, according to Ogorkiewicz: “the ERA sandwich needs to be at an angle to the shaped charge jet to be effective. In fact, ERA is relatively ineffective until it is inclined at more than about 25 degrees from the normal to the jet but its effectiveness then increases with the angle of inclination. The ERA sandwich must also be located some distance in front of the main armour to provide room for the movement of the back plate, which causes more perturbations to the jet than the front plate, even when the two are similar, as it moves in the same direction as the jet and this causes a greater mass of it to be involved with the jet”.[30]
New improvisations – ‘Cope cages’ and ‘turtle tanks’
A final protection technology to be examined concerns the proliferation of overhead protection seen in particular on the Russian side of the War in Ukraine, driven largely in response to the massive proliferation of small drones on the battlefield. Modifications in this class range from relatively modest forms, such as the so-called ‘cope cages’ at one end, to fairly extreme modifications at the other, such as the so-called ‘turtle tanks’ seen in recent months. The effectiveness of such forms of protection can vary wildly, depending on various factors, including size/coverage, shape, armour thickness, as well as the presence or absence of ERA.
At their most basic, as seen circa June 2021 and going into the first phase of Russia’s February 2022 invasion, were the ‘bar armour lookalike’ versions of ‘cope cages’.[31] These will have provided some minor protection against plunging high-explosive warheads, and hindered small drones from dropping grenades down the crew hatches, as well as potentially provided some level of protection against small-diameter artillery bomblets such as the M77 DPICM. The latter have a very small warhead diameter of 38 mm, and a small explosive yield, so the warhead is fairly weak in relative terms, reportedly capable of penetrating around 100 mm of RHAe (only 2.6 times the warhead diameter).[32] As such, these weaker types of warhead would be more susceptible to standoff-based jet degradation than a larger and more powerful example such as that of Javelin.
Going a step further, we get to the more ‘premium’ industrial examples, such as the ‘cope cage’ seen on a T-72B3M at the Armiya 2023 defence exhibition.[33] This version featured a two-layer corrugated spaced armour array, along with RPG netting and the Triton jammer for added protection against drones. This represents a more serious form of top-attack protection, but with the exception of the jammer, would be expected to provide only marginal improvements over the most basic version, and broadly against the same threat types.
Going even further, some Russian tank crews have adopted the practice of adding ERA to their ‘cope cages’, which would be expected to provide more serious protection against top-attack ATGMs, but assessing their probable effectiveness is not straightforward for several reasons.[34] Firstly, many crews have mounted the ERA flat against the cage roof rather than at an angle. This is partially offset when using Kontakt-1 blocks, which have two 4S20 explosive cassettes inside, with the bottom cassette lying flat and the top cassette angled at 9°.[35] Recalling Ogorkiewicz’ point on the importance of angling ERA at >25° from normal, neither top nor bottom cassette would be expected to be sufficient against threats approaching at very steep angles. However, in practice, relatively few munitions possessed by Ukraine would be expected to come in at a near-vertical angle, and the typically shallower approach angle of some FPVs and loitering munitions may mean that flat cassettes are sufficient for the task.
Reaching the current extreme iteration (but probably not the final form) of the ‘cope cage’, we end up at the so-called ‘turtle tanks’, sometimes referred to as ‘mobile barns’ or ‘assault sheds’, comprising massive sheet metal shelters covering the entire vehicle from the sides and rear, leaving only the front exposed. While this modification has been widely panned on social media, the reality is that it isn’t quite as stupid as many seem to think.
While it is true that such modifications preclude the full use of the turret’s ability to traverse, and massively decrease the crew’s overall situational awareness, their use has been largely restricted to specific breakthrough operations where the main priority is for the vehicle to survive long enough to achieve a specific goal. In at least one notable case, the ‘turtle tank’ in question was fitted with a mine plough, and drone jammer, indicating that its main task was to clear the path through a mined area for follow-on forces.[36]
It is also true that the level of ballistic protection afforded by the sheet metal is negligible at best, however, its real purpose is probably not ballistic protection. The main benefit which comes to mind with such a large shelter covering the vehicle is that it becomes more difficult for FPV pilots to engage known weak points on the vehicle. While a HEAT jet will face no difficulty tearing right through a thin layer of metal, the pilot would need to accurately predict what the jet will end up hitting on the other side once it does penetrate. This is not an easy task given that the dimensions of such shelters are not standardised, and the targets in question are often moving.
Despite looking rather crude, ‘turtle tanks’ nonetheless have appeared to be quite effective for their intended purpose. Following a Russian assault on Krasnohorivka involving a turtle tank, the Kyiv Post reported in a 19 April 2024 article: “following Russian tanks and infantry fighting vehicles were hit and sometimes exploded during the attack, but the clumsy, box-like assault tank even drove into and out of the village apparently unscathed. Ukrainian drones appeared not to attack it.”[37] Elsewhere, a Ukrainian Telegram channel ‘Life from the Frontline’ reported on 29 April 2024 that during one attack featuring a ‘turtle tank’ in the direction of Chasiv Yar, “A lot of FPVs were spent on one tank. Everyone is laughing at their shed design, but in fact they fucking work.”[38]
Having said that, it is important to remember that ‘turtle tanks’ are a fairly new and disruptive presence on the battlefield in Ukraine, which means that relatively few soldiers have experience fighting against them. Most such new and disruptive equipment tends to enjoy a period of initial success, after which their opponent begins to adapt to them, and then their performance starts to rapidly decline. This was broadly true for the Russians facing many of the Western weapons brought into Ukraine, and it is highly likely that ‘turtle tanks’ will follow the same trajectory, especially as Ukraine’s FPV pilots gain more experience combating them. Indeed, Ukrainian forces already showed signs of adapting to them by mid-May 2024.[39]
Deeper understanding
Having gone through the basic operating principles of HEAT warheads, along with many of the common countermeasures fielded against several common warhead and fuze designs, readers should note that this article only scratches the surface of a vast field – the rabbit hole goes much deeper. This piece sought to dispel some of the more persistent myths surrounding shaped charges and several related countermeasures. While it serves as a primer, many fascinating technologies were left uncovered – from a closer look at EFPs, tandem-HEAT warheads and the different types of precursor charges, to active protection systems (APSs), to various ERA and NERA designs, and many others. At the very least it should now be clear that HEAT warheads and common vehicle protection technologies are far more complex and interesting than they are popularly depicted to be.
As indicated by some of the adaptations seen by both sides in Ukraine, the fields of firepower and vehicle protection can be quite fast-moving, and while the solutions opted for may appear strange or even ridiculous to the layman, there are usually very good reasons why engineers opt for certain solutions. However, it should also be borne in mind that many solutions, particularly ad-hoc forms of vehicle protection mounted in-theatre are often not developed by engineers, but frequently by proactive soldiers with a limited working understanding of the key physical principles at work. Their understanding is often not helped by many of the popular myths which have sprung up around the threats they face. As such, these forms of protection can be highly variable in their effectiveness, which often muddies the waters further, as any failures of a given protection package to work as expected seen may be used as evidence that the form of protection in question doesn’t work. However, as this article has illustrated, more often than not, the correct answer is that the given form of protection does work, just not against the threats many think it does.
Mark Cazalet
[1] [Note: Rather unhelpfully, some authors have tended to use the terms ‘HEAT’ and ‘Shaped Charges’ interchangeably. In the case of William Walters, in one presentation, he presented ‘Directed Energy’ as a parent category for ‘shaped charges’ and ‘EFPs’, while in a different paper, he described EFPs as “also a shaped charge”. This work follows the latter convention, treating ‘Shaped Charges’ as the parent category and using HEAT and EFP terms to avoid similar confusion.]
[2] https://apps.dtic.mil/sti/tr/pdf/ADA497450.pdf
[3] https://apps.dtic.mil/sti/pdfs/ADA226401.pdf
[4] Marsh, S. P.: Designing and testing a high-velocity self-forging fragment (1982); Los Alamos National Laboratory, p2
[5] https://apps.dtic.mil/dtic/tr/fulltext/u2/a220095.pdf
[6] https://apps.dtic.mil/sti/pdfs/ADA226401.pdf
[7] https://apps.dtic.mil/dtic/tr/fulltext/u2/a220095.pdf
[8] https://apps.dtic.mil/dtic/tr/fulltext/u2/a220095.pdf
[9] Walters, William: An overview of the shaped charge concept (undated), p3
[10] https://apps.dtic.mil/sti/tr/pdf/ADA240999.pdf, p19;
Walters, William: An overview of the shaped charge concept (undated), p1-2
[12] http://pen.ius.edu.ba/index.php/pen/article/viewFile/3500/1307
[13] https://apps.dtic.mil/sti/tr/pdf/ADA240999.pdf
[14] https://apps.dtic.mil/sti/tr/pdf/ADA240999.pdf
[15] https://www.youtube.com/watch?v=hupsUq-fzq8
[16] https://apps.dtic.mil/sti/pdfs/ADA599386.pdf
[17] Walters, William P & Zukas, Jonas A.: Fundamentals of Shaped Charges (1989)
[18] https://www.defensa.com/adjuntos/3(1638).jpg
[19] https://www.edrmagazine.eu/saab-further-develops-its-knowledge-in-warhead-technology
[20] https://www.mbda-systems.com/?action=force-download-attachment&attachment_id=26545
[NOTE: The original version of this article cited a figure of 140 mm for the warhead diameter, using MBDA’s figure for the missile diameter, and deriving from this a penetration figure of 1,540 mm to 1,680 mm of RHAe. However, since the article was published, the author has spoken to an individual with a deep technical familiarity with the missile’s design, and learned that these initial figures were incorrect. The individual noted that MBDA’s 140 mm diameter figure actually referred to the bulged missile nose portion housing the seeker, rather than the main missile body, which is slightly narrower at roughly 130 mm. Additionally, they noted that the warhead diameter had to be made slightly smaller than this due to the missile accommodating a frag sleeve around the warhead. A revised figure of 115 mm was therefore used for the warhead diameter, and consequently, a revised penetration estimate of 1,265 mm to 1,380 mm of RHAe. However, even accounting for such changes, the missile still vastly outperforms the boilerplate penetration figure given by MBDA.]
[21] https://apps.dtic.mil/sti/pdfs/ADA469696.pdf (fig. 183)
[22] However, it should be noted that there was some disagreement in various sources and experimental findings from the time, with some reporting positive results from spaced armour, and others little to no effect. For an overview, see: https://apps.dtic.mil/sti/pdfs/AD0309418.pdf (p3)
[23] https://apps.dtic.mil/sti/pdfs/ADA954865.pdf (p8)
[24] https://fhs.diva-portal.org/smash/get/diva2:643824/FULLTEXT01.pdf
[25] Ogorkiewicz, R.M: The Technology of Tanks (1991)
[26] https://journals.sagepub.com/doi/10.1177/1464420718759704?icid=int.sj-full-text.similar-articles.2
[27] https://twitter.com/2805662/status/1101154122730942465
[28] https://twitter.com/UAWeapons/status/1596599767672520704
[29] https://asmedigitalcollection.asme.org/appliedmechanics/article-pdf/doi/10.1115/1.4001738/5479456/051805_1.pdf
[30] Ogorkiewicz, R.M: The Technology of Tanks (1991)
[31] https://twitter.com/AndreiBtvt/status/1406581458114916362
[32] https://www.clusterconvention.org/files/publications/A-Guide-to-Cluster-Munitions.pdf
[33] https://www.RecoMonkey.com/Newsfeed/ARMY-2023-Static-displays-part-1/i-J7q8Qgq
[34] https://twitter.com/clashreport/status/1654810928276205569
[35] https://thesovietarmourblog.blogspot.com/p/kontakt-1.html
[36] https://www.kyivpost.com/post/31373
[37] https://www.kyivpost.com/post/31373
[38] https://t.me/The_life_of_Predova/2241
[39] https://twitter.com/RALee85/status/1792873604524257588;
https://twitter.com/kakkamax/status/1793237393165934871