A History of Research and a Review of Recent Developments

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Penetration and fragmentation
of energy and momentum. The piercing power is proportional to the length of
the jet and the square root of its density.
A shape charge performs at its best when the distance from the face of a
target to the point of detonation is precisely calculated (the stand-off). For this
reason many shaped charge missile warheads have an extended stand-off probe
to initiate detonation. Manufacturers of shaped charge warheads are now seeking
penetrative powers equal to six times the warhead diameter. However, there is
a problem because warheads with long probes tend to become unstable in flight.
The shaped charge liner is normally made of copper, and technology advances
have usually been improvements in the precision of the liner shaping and
centring. Efforts have also been made to produce detonation wave fronts
normal to the cone surface, so that the liner remains solid but changes shape
like a fluid. The high-velocity jet formed by the transformation of the metal
liner tends to break into separate particles, but the development of the
explosively formed projectile, or self-forming fragment, has meant the retention
of full effectiveness over long distances.
If the liner is made parabolic in shape, rather than conical, the slug, instead
of being needle shaped, becomes a chunky, U-shaped projectile. This type of
projectile does not burst into fragments or flatten against the target, but the
penetrating power is naturally reduced.
In a shaped charge, the initial velocity of the jet depends on the ratio ( )
between the mass of the liner and the mass of explosive located behind the
liner element. This velocity, V 1, is given by V 1=A( +k) –1/2 , where A is a constant
defining the specific energy of the explosive, and k is a charge configuration
constant. This formula was due to Gurney [7.70]. A less approximate analysis
of the relationship between V 1 and the charge configuration has been given
more recently by Chou et al. [7.71].
The defeat of shaped charge jets by armour material was a problem that
absorbed much military research and development in the years following the
Second World War. Eventually these studies turned towards the idea of ‘active’
or energetic armour elements, which would release energy when attacked that
would counteract the effect of the charge. The principle of operation was taken
an important step forward in the 1970s, by the invention of the ‘drive-plate’
explosive sandwich concertina of two metal plates with an explosive layer
between. When initiated by a high-speed jet, the explosive drives the metal
plates to one side, and the moving plates absorb the mass of the jet. Work began
in Israel in 1974 on prototype ‘add-on’ kits for tanks, which would cover the
turret and the forward part of the hull. It was important to show that the explosive
layer in the sandwich would not be initiated by any weapon other than the
high-velocity jet of a shaped charge. The kits finally took the form of ‘blocks’ or
‘tiles’ that could be fixed to the external surfaces, and which, in total, did not
add more than 100 kg to the weight of the tank. Reactive armour first appeared
on the battlefield in 1982, and one of the first unclassified accounts of its
development history was given by Vered [7.72] in 1987.