Tank destroyers are used primarily to provide anti-tank support in combat operations but do not fit all the criteria of a tank. They may mount a high-velocity anti-tank gun but have an open turret, no turret at all or run on wheels instead of tracks. Vehicles which carry an anti-tank guided missile launcher are referred to as ATGM carriers.
Tank destroyers are smaller, faster and cheaper than main battle tanks. While they have the necessary weapons to destroy MBT's, tank destroyers are too lightly armored to withstand direct hits from MBT cannons, and can be destroyed by other weapons (like explosives or heavy machine guns) that would not destroy an MBT. Tank destroyer crews are always trained to primarily hunt down enemy tanks.
Gun-armed tank destroyers have been largely replaced by the more general-purpose main battle tanks since World War II, but lightly-armoured ATGM (anti-tank guided missile) carriers are used for supplementary long-range antitank capabilities. Modern tank destroyers primarily rely on missiles instead of guns to kill tanks.
But tank destroyers cannot fulfill the many roles of tanks; they are much less flexible, and usually lack a strong anti-infantry capability. A common feature of a tank destroyer is the absence of a turret, and compared to tanks, an even stronger disposition for heavy frontal armor (compared to side and rear armor). Tank destroyers, as they were made, were therefore even more vulnerable to non-frontal attacks than tanks. Common use of open-topped hulls was also problematic since it afforded the tank destroyer crew less protection, both from armor piercing weapons and NBC threats. Aiming the main guns of tank destroyers is also a much more tedious task due to the lack of a rotating turret. In practice turretless tank destroyers are most often used in concealed ambush positions where they can wait for a target to enter their line of fire. They are thus better suited to defensive rather than offensive operations.
Tank destroyers of WWII-vintage quickly became obsolete after the war as main battle tanks (MBTs) improved and outdistanced the former in capabilities and performance. Modern tank destroyers are still used by first-rate armies, though these vehicles often have rotating turrets, do not expose their crews to the outside, and often have a low pressure gun firing high explosive anti-tank (HEAT) rounds in place of a high velocity gun firing armor piercing rounds, because the tank destroyer's light chassis does not allow it to mount a high velocity gun capable of penetrating a modern MBT's armor protection. In addition to much lower cost, one advantage of modern tank destroyers is that they weigh much less than main battle tanks and thus are much more readily air portable, as well as being more suited to certain tactical situations (e.g. areas in which bridges cannot accommodate the 50-70 ton weight of modern MBTs).
Dedicated antitank vehicles made their first major appearance in the Second World War, as combatants developed effective armoured vehicles and tactics.
These tank destroyers fell broadly into two categories. Some were designed to be faster and cheaper than medium tanks while still able to destroy heavy armour at long range. Some of these designs were clearly experiments rushed into production. The second design strategy was to create heavily-armoured vehicles that were more effective in tank-versus-tank combat than enemy tanks.
Although the Panzerjager carried effective weapons they were generally lacking in protection for the crew having thinly armoured open-topped superstructures and used older designs of chassis. They were to be followed by the Jagdpanzer '("hunting tanks") which mounted the gun in better superstructures – sacrificing the wider traverse capability of the anti-gun carriage for good armour protection. The best of the designs is considered to be the Jagdpanther, which put an gun in one of the later tank chassis, that of the Panther tank. The Germans became side-tracked into production of large numbers of tank-destroyers because they could be produced more cheaply than full tanks. The Jagdpanzers were better suited to defense than attack because of their limited traverse and the later designs such as the Jagdtiger (of which just under 50 were built by May 1945) were heavy and consequently less manoeuvrable albeit extremely hard to defeat.
The most successful German tank destroyer wasn't supposed to be a tank destroyer at all. It was the Sturmgeschütz III (assault gun), a self propelled gun for infantry support with a design similar to the Jagdpanzer.
As with the Germans, the Soviet designs mounted anti-tank guns, with limited traverse in turretless hulls. The results were smaller, lighter, and simpler to build than tanks, but could carry larger guns. This design methodology was used by both the USSR and the Germans to provide heavier anti-tank capability at lower cost, during the rapid up-armoring of all AFVs that took place during the war. The Soviets produced the SU-85 and SU-100 self-propelled guns based on the same chassis as the T-34 medium tank, as well as the ISU-122 and ISU-152 which shared components with the IS-2 heavy tank and was nicknamed Zveroboy ("beast killer") for its ability to destroy German Tigers, Panthers and Elephants. In 1943, the Soviets also shifted all production of light tanks like the T-70 to much simpler and better-armed SU-76 self-propelled guns, which used the same drive train.
The U.S. tank destroyer designs were intended to be very mobile and heavily armed. Most retained a turret, but left it open on top both to save weight and to accommodate a larger gun. The earliest expedient design was an M3 Half-track mounting an M1897 gun in a limited-traverse mount, and called the 75-mm GMC M3. Another, considerably less successful, early design mounted a 37-mm antitank gun in the bed of a Dodge 3/4-ton truck - the 37-mm GMC M6. The larger guns required a counterweight at rear of the turret, which can be seen on designs like the 3-inch gun 3in Gun Motor Carriage M10 and the 90mm Gun Motor Carriage M36. The 76mm Gun Motor Carriage M18 came closer to the U.S. ideal; the vehicle was very fast, small, and mounted a gun in a turret. Of these, only the gun of the M36 proved to be effective against the Germans' larger armored vehicles at any significant range. The open top and light armour made these tank destroyers particularly vulnerable to anything greater than small-arms fire, which ensured that in a direct fight they were no match for tanks. As the number of German tanks encountered by American forces steadily decreased throughout the war, most battalions were split up and assigned to infantry units as support vehicles, fighting as assault guns or as local anti-tank reserves.
Anti-tank guns were the domain of the Royal Artillery rather than the Royal Armoured Corps and anti-tank gunned vehicles particularly anti-tank self-propelled guns such as the Deacon and Archer were their preserve.
The self-propelled guns that were built in the "Tank Destroyer" mould came about through the desire to field the formidable QF 17 pounder anti-tank gun and simultaneous lack of suitable tanks to carry it. As a result they were of a somewhat extemporized nature. Mounting the gun on the Valentine tank chassis gave the Marder-like Archer. The 17 pounder was also used to equip the US supplied M10 Wolverine to produce the Achilles. Another attempt to produce a specialist anti-tank vehicle was to fit the 17 pounder to the Cromwell chassis to give Tank, Cruiser, Challenger (A30) and its near open-topped variant Avenger. The latter delayed until post war before entering service.
The closest the British came to developing an armoured Tank Destroyer in the vein of the German Jagdpanzers or Russian ISU series was the Churchill 3 inch Gun Carrier - a Churchill tank chassis with a boxy superstructure in place of the turret. The design was rejected in favor of developing a 17 pounder armed Cromwell tank variant ultimately leading to the Comet tank
With the development of flexible Anti-tank guided missiles, which were capable of installation on almost any vehicle in the 1960s, the concept of the tank destroyer has waned. With the weight of main battle tanks growing to the forty to seventy-tonne range, airborne forces were unable to deploy reasonable antitank forces. The result was a number of attempts to make a light vehicle, including the conventional ASU-85, the recoilless rifle–armed Ontos, and missile-armed Hornet Malkara armoured car and Sheridan light tank.
There have also been dedicated antitank vehicles built on ordinary armoured personnel carrier or armoured car chassis. Examples include the U.S. M901 ITV (Improved TOW Vehicle) and the Norwegian NM142, both on an M113 chassis, several Soviet ATGM launchers based on the BRDM reconnaissance car, and the German Raketenjagdpanzer series built on the chassis of the HS 30 and Marder IFV.
A US Army mechanized infantry battalion has four infantry companies with TOW missile–armed Bradley IFVs and can bring a large concentration of accurate and lethal fire to bear on an attacking enemy unit that uses AFVs.
Missile carrying vehicles however are referred to as anti-tank guided missile (ATGM) carriers instead of tank destroyers. In modern times a tank destroyer is an armored fighting vehicle with a large caliber cannon which is not a tank because it does not fit into all the criteria defining a tank. A tank is an armored vehicle which runs on tracks and has a large caliber direct fire cannon in a 360 degrees rotating enclosed turret. This makes large caliber gun vehicles that have an open turret (M36 Jackson), do not have a turret (Strv 103, Kanonenjagdpanzer) or which run on wheels instead of tracks (Centauro, PTL02) tank destroyers.
Some gun-armed tank destroyers continue to be used. The German Army had specialized Kanonenjagdpanzer, similar in design to the WWII tank destroyers, from the mid-1960s until the 1980s. In the same time frame, Sweden developed armoured forces: the Strv 103, armed with a cannon (however its actual combat role is uncertain). The People's Republic of China has developed the tracked PTZ89 and the wheeled PTL02 tank destroyers. PTZ89 is armed with a smoothbore cannon while PTL02, developed by NORINCO for the PLA new light (rapid reaction) mechanised infantry divisions, carries a rifled gun. PTL02 is built on the 6×6 wheeled chassis of the WZ551 APC. Italy and Spain use the Italian-built Centauro, a wheeled tank destroyer with a cannon. The gun-armed tank destroyer may possibly see revival in the US Army through the introduction of the Stryker, more specifically, the M1128 Mobile Gun System, a Stryker variant armed with a cannon which has remote control and autoloading capabilities.
Ironically, modern tank destroyers, due to being more mobile than tanks, are more suitable for low-intensity and urban warfare that is being fought in the War on Terrorism. Some are considering the possibility of replacing tanks with lightweight, wheeled tank destroyers to deal with elusive insurgents encountered in both Iraq and Afghanistan. Originally, the Canadian Forces had considered replacing their aging Leopard 1 tanks with the Stryker Mobile Gun System, essentially creating a tank destroyers-only force but with the use of IED that are capable of destroying Strykers, had opted to buy Leopard 2 tank for now.