Self-propelled anti-tank vehicles, generally known as tank destroyers, are a type of armoured fighting vehicle, used primarily in the defensive role in destroying enemy tanks. They may mount an ATGM launcher or a high-velocity dedicated anti-tank gun.
World War II
Anti-tank vehicles made their first major appearance in World War II. As combatants developed effective armoured vehicles and tactics to use them, there were a number of experiments at producing vehicles specifically designed to stop enemy tanks.
These tank desroyers fell broadly into two categories. Some were designed to be faster and cheaper than a medium tank while still able to destroy heavy armour at long range. The second design option was to create a heavily-armoured vehicle that was more effective in armoured combat than the enemy tanks.
In German and Soviet designs the traditional turret was removed from an existing tank design, and a larger gun was mounted with a limited traverse in the hull. The weight and space savings of removing the turret allowed a smaller chassis to carry a larger gun. This was used by both forces to keep older designs competitive in the rapidly up-armoring of all AFV's that took place during the war.
For instance, the German Panzer I was obsolete before the war even started, with thin armor and only machine guns for armament. Yet they were forced into battle during the invasion of Poland, where they were found to be deathtraps. Before the invasion of France, 202 were rebuilt with a Czech 47 mm gun, becoming the Panzerjäger I. Soon the same sort of thing happened to the Panzer II during Operation Barbarossa: crews took to using captured Soviet 76.2mm anti-tank guns on them in various makeshift mountings in order to allow them stand off from their opposition. In this form they were later modified at the factory, producing the Marder II. Whereas the Panzerjäger concept used the chassis of obsolescent tanks, dedicated Jagdpanzer versions of tanks were developed later in the war. The Jagdpanther version of the Panther tank is considered the best of the Jagdpanzers.
The US Army tank destroyer doctrine
United States designs, and British ones based on them, were very different in conception. In pre-WWII planning, US tanks were not to fight enemy tanks directly. Instead the tanks were dedicated solely to the role of infantry support in a fast moving battle. In order to deal with the enemy tanks they would instead rely on tank destroyers, organized into separate groups, dealing with armor at long range.
The resulting US designs retained the turret, but left it open on top for more working room with the larger gun. The larger guns required a weight to be added to the rear of the turret, which can be seen on designs like the M10. The open top made them particularly vunerable to even handguns, and the very idea of independent anti-tank groups was found unworkable. By 1944 the "basic" Sherman was being upgunned in the field to be a Sherman Firefly, and by the end of that year heavier guns were being mounted on production Shermans.
Post-World War II development
Post-war designs have focused almost entirely on weight. With the weight of combat capable tanks growing to the 50 to 70 tonne range, airborne forces were unable to deploy reasonable anti-tank forces. The result was a number of attempts to make a light vehicle with either recoilless or wire-guided missiles as armament, including the Hornet Malkara, the Ontos and Sheridan. However neither of these had the armor needed to protect them in a fight, so more modern designs simply place the same weapons on a normal APC chassis. For example, the US Army has vehicles (such as the M113 armored personnel carrier), which may mount anti-tank missiles for defense, but that is not their primary role, which is left to vehicles like the M901 "Improved TOW Vehicle," with its thermal imaging sights and TOW heavy wire-guided antitank missile launcher. Vehicles similar in concept have also been built in Russia, mostly built on the chassis of the old BRDM light armored car, and in Germany the Jagdpanzer Rakete series of tank destroyers built on the chassis of the Marder armored personnel carrier has been armed with a variety of heavy wire-guided antitank missiles.
With the onset of guided anti-tank missiles in the late 1950s and early 1960s, which enabled most vehciles to mount some sort of anti-tank weapon, the concept of the tank destroyer as a separate class of armored vehicle gradually disappeared.
Modern anti-tank vehicles
A modern anti-tank vehicle armed with anti-tank missiles is useful tactically principally on the defensive. Due to the fact that the gunner must continue to aim at the enemy tank until the missile strikes the target, in order to guide the missile, the anti-tank vehicle must not move, because this would disturb the gunner's aim. These missiles require a stable firing platform for accuracy. A helicopter gunship can sometimes get away with moving slightly while the missile is in flight because it moves smoothly, rather than bouncing violently cross-country and causing the gunner to get a black eye from being whacked in the face repeatedly by the eyepiece of his missile launcher's sight.
Part of the anti-tank vehicle's tactical role is filled in some armies by the infantry fighting vehicle which can be thought of as an armored personnel carrier with much heavier armament, usually (though not always) including a heavy 20mm to 30mm autocannon and/or a small 73mm to 100mm gun similar to those mounted on older tanks and/or an antitank missile launcher (examples include the Russian BMP series, the US M2 Bradley and LAV25, the British Warrior II, and the German Marder). The US Army retains both, and a US Army mechanized infantry battalion has both four mechanized infantry companies with infantry fighting vehicles but also a company of anti-tank vehicles which supplement the anti-tank guided missile teams that are part of every infantry platoon. Such a battalion can bring an astounding concentration of precise, accurate, and lethal fire to bear on an attacking enemy unit that uses AFVs, but is very expensive to equip, train, and maintain.