Armour-piercing discarding sabot is a type of kinetic energy projectile fired
from a gun to attack armoured targets. APDS rounds are sabot rounds and were
commonly used in large calibre tank guns, but have now been superseded by
armour-piercing fin-stabilized discarding sabot projectiles in such
guns. However, APDS rounds are still commonly used in small or medium calibre
weapon systems. For a given calibre, this type of ammunition can effectively
double the armour penetration of a gun, compared to those firing
armour-piercing, AP - capped, or APC + ballistic cap projectiles. The Saboted
light armor penetrator concept applies this technology to small arms calibres.
History and development APDS was developed by engineers working
for the French Edgar Brandt company, and was fielded in two calibers just before
the French-German armistice of 1940. The Edgar Brandt engineers, having been
evacuated to the United Kingdom, joined ongoing APDS development efforts there,
culminating in significant improvements to the concept and its realization.The
APDS projectile type was further developed in the United Kingdom between
1941–1944 by Permutter and Coppock, two designers with the Armaments Research
Department. In mid-1944 the APDS projectile was first introduced into
service for the UK's QF 6 pdr anti-tank gun and later in September 1944 for the
17 pdr anti-tank gun. The reason for the development of the
APDS was the search for anti-tank projectiles with increased penetrating
performance. It was known that high impact velocity, or a larger diameter
projectile would be required to improve penetration. A larger projectile would
require a completely new weapon system and may have been too heavy to retrofit
onto existing armoured fighting vehicles. Increasing the velocity of the
current projectiles was also a problem due to the impact velocity limitations
of steel armour-piercing projectiles, which would shatter at velocities above
~850 m/s when uncapped. To allow increased impact velocity, a
stronger penetrator material was required. The chosen new penetrator
material was tungsten carbide, due to its greater hardness and its ability to
withstand the greater shock and pressure generated during a higher velocity
impact. As the density of WC is twice that of steel, such a shot was too heavy
at full bore to be accelerated to a sufficient muzzle velocity. To overcome
this, a lightweight full diameter carrier shell was developed to sheathe
the inner high density core. The name given to this projectile type was the
Armor-Piercing Composite Rigid. The APCR projectile was about half the weight of
a standard AP shot, but of the same diameter. Due to the large surface area
for the gases to impinge upon the lightweight APCR projectile, it
experienced a higher average acceleration in the gun barrel, in turn
imparting a higher muzzle velocity. Unfortunately the low sectional density
of the APCR resulted in poor carrying power, losing velocity and penetration
rapidly over distance. To overcome these limitations the
British devised a way for the outer sheath to be discarded after leaving the
bore. The name given to the discarded outer sheath was the sabot. For APDS
projectiles the sabot is also known as a pot, as the sabot resembles a flower pot
in shape. The APDS has the advantages of the lightweight projectile with regards
to bore acceleration and high muzzle velocity, but does not suffer from the
high drag of the APCR in flight. Sabot construction
The sabot of a large calibre APDS consists of a light high strength alloy
full diameter pot and base unit, which is screwed together. The front part of
the pot has 3-4 petals which are covered with a centering band. The rear half has
a rubber obturator and driving band held in place by the screw-in base unit. The
base unit, if a tracer element is attached to the sub-projectile, has a
hole located at the centre. Before firing, the sub-projectile and sabot are
locked together. Due to the high setback forces, friction between the pot and
sub-projectile allows spin to be transferred, so stabilising the
sub-projectile. Small/medium calibre APDS use a lightweight high strength
alloy base pot and three or more plastic petals. To transfer the spin to the core
in small/medium calibre weapons, the core tends to have a notch at its base.
Under bore acceleration, which can be higher than 100,000 g, the uneven base
is forced into the softer pot material, locking the sub-projectile to the pot
and imparting spin. Not all small/medium calibre APDS rely on this technique,
another method for spin coupling is by using the forward plastic petals. The
petals are of a slightly larger diameter than the lands in the rifled bore. This
forces the petals tightly against the core, increasing the friction between
them and allowing the spin to be transferred.
Sub-projectile construction The sub-projectile consists of a high
density core with a penetrating cap, enclosed within a high strength sheath
with a lightweight alloy ballistic cap. For modern small/medium calibre APDS
projectiles, the core is not sheathed and the ballistic and penetrating caps
are combined. A tracer element may be added to the APDS sub-projectile, for
large calibre weapons this is part of the outer sheath, for small/medium
calibre weapons it is contained within a hollow cavity in, or attached to, the
base of the core. Most modern APDS projectiles used high strength shock
resistant tungsten alloys. The main constituent is tungsten, alloyed or
sintered with/to cobalt, copper, iron or nickel. Very few APDS use depleted
uranium titanium alloy for the penetrator material, though the retired
20 mm MK149-2 Phalanx round did use DU. Sabot discard
When a large calibre APDS is fired and while still within the bore, the setback
forces shear the forward petals, partly unlocking the sub-projectile from the
sabot, but still holding it rigidly within the pot. Gas pressure is used to
delay the unlocking of the pins holding the rear part of the sub-projectile by
gyroscopic forces. Once outside the barrel, the pins, centering band and
forward petals are released or discarded by projectile spin, the aerodynamic drag
removes the pot/base unit. As an APDS sub-projectile does not require driving
bands and the core is supported at the base and ogive region, a far more
aerodynamic projectile shape can be chosen. This, in combination with the
sub-projectiles’ higher sectional density, gives the resulting
sub-projectile vastly reduced aerodynamic drag in comparison to the
APCR. Both the higher initial velocity and the reduced drag result in high
terminal velocity at impact. This also lowers flight time and improves
accuracy. Accuracy can suffer if there are unwanted sabot/sub-projectile
interactions during discard. Impact example
The sequence upon impact of the APDS projectile, for example the 120 mm L11,
as used on the Chieftain tank, fired L15 APDS , goes as follows: the lightweight
ballistic cap is crushed, the penetrating cap then strikes the armour,
distributing the shock across the whole surface of the core's nose, reducing the
initial shock experienced by the core. The steel sheath surrounding the core
peels away, and the core goes on to penetrate the armour. The penetration of
the L15 APDS is approximately 355 mm of rolled homogeneous armour at 1000 m.
FAPDS Many newer medium calibre APDS cores use
a frangible high density alloy, the resulting projectiles are called
Frangible Armour Piercing Discarding Sabot for APDS types, or FRAP for full
calibre projectiles. During penetration, a frangible projectile’s core fragments
into many high velocity pieces. The effect of a frangible projectile on a
lightly armoured target is much the same as a high explosive incendiary round,
but with a cloud of dense, high velocity fragments penetrating deeper into the
target's interior. Upon striking heavy armour the effect of FAPDS is more akin
to a standard APDS, albeit with higher fragmentation of the core, and hence
lethality if the armour is perforated. FAPDS is also known as a Penetrator with
Enhanced Lateral Effect. References