Top-attack refers to a characteristic of anti-armor weapon systems. Since modern armored vehicles are most lightly armored on their top surfaces, it is from that aspect that they are most vulnerable. The armor is lighter here for reasons of pragmatism (you're less likely to take a tank round in the top of your turret than on your glacis plate, for example) and design constraints. Too much armor atop a tank raises its center of gravity, which decreases its stability.

In any case, due to this known vulnerability of tanks, weapon designers and tacticians have tried to find ways of attacking this weaker aspect. The first answer was (and is) to use aircraft, which in addition to being able to strike the top of the vehicle are less vulnerable due to their speed. It was for this exact task that the U.S. A-10 Thunderbolt II was built. It uses a 30mm rotary cannon loaded with depleted uranium shells, fired in a dive, to hit the top.

Another method is to use anti-tank missiles. Since the direction the warhead of an ATGM is traveling is unimportant due to the self-forging warhead producing the killing energy, the warhead can be placed at an angle in the missile. The missile is directed (or programmed) to 'pop up' at the last moment so as to fly directly over the target vehicle; when it is over the top, the charge fires straight down into the top armor.

Some weapons that have a 'top-attack' mode include the new variants of the TOW (2A, 2B) and the Javelin fire-and-forget ATGM, as well as the Hellfire Longbow.

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