1.2 Geometry
1.2.1 Slope and ricochet
The next factor in determining the effectiveness of a tank’s armor is slope. On the face of it, slope should not impact on armor design at all since themore you incline a plate to armor a volume or profile, the more material you need to cover that profile
. Where slope becomes a factor is in the effectit has on the attacking projectile. This means that whatever effects it has, it’stied to the projectile nose design as much as the armor slope.
Firstly, all projectiles will ricochet. The real question is at what angle and velocity do they ricochet. Ricochet occurs when a attacking projectileglances off the sloped armor of an AFV without digging in far enough topenetrate the plate. If it has no time to dig in before it ricochets, it can’t pene-trate even modest amounts of armor. A complex model has been developed to predict the angle at which a projectile is expected to ricochet, this is called the‘critical ricochet angle’.1
The longer the rod, the higher the ricochet angle and the faster the rod, the higher the critical ricochet angle. In addition, heavy metal rods of WHA ordU 2
ricochet at higher angles that steel. The critical ricochet angle is meas-ured from the vertical plane [i.e. 90° is horizontal]. A rod of 10:1 L/d [Length to rod Diameter ratio] @ 1.7km/s should ricochet at ~78° when made of steel,while its WHA /dU counterpart will ricochet @ 81°. Stretching the penetratorto 15:1 L/d increases the ricochet angle to 82—83°, and it’s likely that 30:1 rods will ricochet at >84—85°. Tate’s ricochet formula predicts a ±5° variation around these values, so 50% of the 10:1 steel rods should ricochet @~78°, while ricochet will occur as high as 83°and as low as 73°. The above cases apply to thin plate targets, but if the plate is over 4:1 T/d [plate Thickness / rod diameter ratio] the ricochet angles should go down a few degrees.
Since the time it takes a projectile to ‘turn’ is around 40—60 micro seconds, and since the entire penetration event takes 300—400 microseconds [large warhead], even shaped charge warheads [HEAT] will ricochet when the right combination of striking velocity and angle are reached.
Modern HEAT rounds will ricochet as well, the only question is whether this is before or after jet penetration. Modern shaped charges with standoff probes and base initiation will start the jet penetration process before the main round impacts the slope armor. Since this is a 400 micro second event [½ a millisecond], it is quite likely that the main body of the round will not even have reached the plate by then.The second aspect of slope is the asymmetrical force acting on the penetrator.When a projectile strikes a sloped plate, the side of the penetrator closest tothe plate will suffer more force, erosion, and damage than the opposing side.This puts an unbalanced force on the rod, turning it in towards the plate – and then into the opposite direction. The penetrator takes a longer overall route through the armor, resulting in less penetration of sloped armor. 3
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