Shaped charges

A shaped charge, sometimes called a hollow charge, is the basic design for many cluster

bomblets. It is a warhead giving quite an extraordinary effect. It is impressive to see that a charge containing just a few tens of grams of explosive is able to perforate more than 100 mm of even the toughest steel armour. Larger charges used today are for man-portable systems that are able to penetrate more than one meter of steel. However, cluster bomblets will usually not exceed 250 mm penetration.

A shaped charge basically consists of 4 components:

- a cylindrical casing

- an inverted conical liner in the front of the charge - an explosive filling behind the liner

- an igniter opposite to the liner

The figure below shows the design of a M77 bomblet having a typical shaped charge design.

Upon detonation the liner, usually made of copper, collapses into a thin, high velocity jet. This jet, made of solid metal, has a tip velocity of 8 – 9 km/s. It is this jet that gives this type of charge its high penetrating capacity.

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Figure 6.7 The M77 bomblet[9].

The jet is often characterized as molten metal, or sometimes even as a hot gas. None of those claims are true. The jet is in fact solid metal, but the metal is in such an excessively stressed condition that it flows. However, it is a solid state flow; neither fluid nor molten.

Shaped charges have the peculiar effect that they need a certain stand-off to the target in order to work in an optimal way. This effect is due to the formation of the metallic jet. The penetrative capacity of such a jet increases with the length of the jet. The jet needs distance in order to stretch out. However, if the standoff becomes too large, the jet will overstretch and disintegrate. The penetration capacity of a shaped charge may therefore vary according to the figure below for different liner materials.

Figure 6.8 The stand-off vs. penetration curve for shaped charges [10]

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As the figure shows, a stand-off distance of 3 – 6 times the calibre of the bomblet is optimal.

Furthermore a shaped charge should not spin too violently at the time of impact. A high spin will tend to rupture the jet, with detrimental effects for the penetration capacity. These two factors are the reason why some bomblets are equipped with an expandable stand-off device like on Mk118, BLU-97 and BL-755, and why bomblets like M85 have spin braking winglets.

The shaped charge warhead is quite common among cluster bomblets. It is the main component in all DPICM, and in other bomblets dedicated for defeating armoured targets. Those known as SFW (see section 9.6) are also a kind of shaped charges, though working according to different principles than just described, in addition to having advanced sensor equipment.

When a shaped charge strikes an armoured target, the metallic jet perforates the protection, where the remaining high speed jet particles constitute the main injuring mechanism. In addition, particles from the armour itself are brought into the target. For large charges, high pressure detonation products will also contribute to the rising pressure inside the target. This pressure may be able to inflict ear drum rupture, and in rare cases even lung damage. However, for small DPICM of 60 mm calibre or less, this rise in pressure is not significant.

As the name indicates the DPICM should have an effect against both armour and soft targets. It is a common belief that a single hit with a DPICM on an armoured vehicle is equivalent to

neutralizing the vehicle. This is a claim that deviates somewhat from the realities.

• Firstly, DPICM bomblets are small – mostly from 31 to 42 mm calibre. The penetration capacity is in the range of 80 to 150 mm. Although this capacity is sufficient to perforate the roof armour of most vehicles, it is not the same as achieving a kill of the vehicle. The size of the explosive charge of these DPICMs is also inadequate to inflict the effects of pressure, heat or smoke required to put the target out of action.

• Secondly, most of today’s armoured vehicles are equipped with an interior lining in the crew compartment that reduces the effects of fragments that directly or indirectly enters the compartment. This liner is dimensioned to handle the effect of shaped charge warheads with a calibre of 100 mm or more. Such a liner works quite effectively against small DPICM charges. Therefore, in such targets only components situated in the direction of the impact may be damaged by the attack.

• Thirdly, in order to get a complete destruction of an armoured vehicle, vital or critical components must be hit. By vital components are meant components that are needed for maintaining firepower or mobility like the gun tube, fuel distribution system, parts of the transmission, the crew etc. Critical components may be explosives or propellant charges, where an ignition will completely damage the vehicle.

Based on the placing, size and distribution of such components, it may be stated that an armoured vehicle needs of the order of 10 hits with a DPICM in order to inflict a kill. For vehicles not containing large amounts of ammunition, the number will be even higher. Against modern tanks and modern howitzers, with adequate roof protection, the number may be still higher.

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This view can be supported by a report based on the experience of the Russian army in Chechnya in 1994[11]. This report presents the vulnerability of armoured vehicles. However, it can also be seen as an indication on how invulnerable such vehicles are, as three to six shots by shoulder fired RPGs were needed to inflict a lethal damage to the vehicles. The warheads of RPGs are at least three times better in terms of penetration capacity than a typical DPICM bomblet.

In order to achieve 10 hits, an armoured vehicle has to be inside the footprint of an M483A1 155 mm DPICM¹¹ around 200 times, which clearly shows the futility of defeating large armour formation with this kind of munition unless when there are a high number of targets within the footprint area. Other kinds of DPICM, containing a smaller number of bomblets are even less effective.

Figure 6.9 The distribution of vulnerable components in a modern tank[12].

6.5 Suppression

As mentioned earlier the purpose of suppressive fire is not primarily to inflict damage, but to avert the enemy from using his weapons. The suppressive effect of fire is more of the

psychological kind than of the physical kind. However, it is the physical effect and the presence of weapons that generates fear among the enemy.

11 Containing 64 M42 bomblets and 24 M46 bomblets

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Indirect suppressive fire is usually delivered by field artillery or mortars. Air bombing can in principle be of a suppressive kind, but there are logistical limitations on the use of aircraft in this role.

The suppressive effects are of three kinds [13]

• visual effects (flash, smoke, debris, wounds)

• aural effects (bang, whine, whiz, ricochet, screams)

• tactile effects (heat, pressure, debris, wind)

The effect will, different from other effects, have a duration; e g after being exposed, the soldier will hesitate to transform from a suppressive to an active state. This duration will increase with the strength of the suppressive effect.

The effect will also strongly be a function of discipline, motivation, training and the tactical situation. Not only are such effects dependent on qualitative factors, but may also vary strongly from person to person, and from unit to unit. In addition, one person’s reaction to suppressive fire may also affect the reaction of other persons. The different and very complex aspects of

suppression is also discussed in detail in[14;15].

US Army Field Artillery School [16] once put up some simple expressions quantifying the suppressive effect. The concept used is parallel to the lethal area concept, but now called suppressive area. Against personnel in the open, attacked by field artillery, this area seems to be around 100 times larger than the lethal areas. Another interesting property of the model is that the suppressive area is assumed as being in linear proportion to the effective explosive content of the ammunition. With respect to the use of cluster weapons, this implies that the suppression is better using a unitary warhead than a cluster weapon with the same dimensions, as the latter will contain far less explosive than the unitary one.

There does not seem to exist a good model giving the suppressive effect of multiple detonations taking place at the impact of a cluster weapons. We will here assume that the suppressive effect is cumulative in the sense that the overlapping effect between the suppressive areas of adjacent bomblets is not taken into account. The total suppressive area is thus the sum of all individual areas. This is a conservative approach as it may slightly favour cluster weapons in comparison to unitary warheads. However, as the duration of the suppressive effect is supposed to be much longer than the audible duration of a cluster impact, this assumption seems plausible.