M103 (heavy tank)

The M103 Heavy Tank (officially designated 120mm Gun Combat Tank M103) served the United States Army and the United States Marine Corps during the Cold War. The last M103s were withdrawn from service in 1974.

Design and development
Like the contemporary British Conqueror tank, the M103 was designed to counter Soviet heavy tanks, such as the later IS-series tanks or the  T-10 if a conventional World War III broke out. Its long-ranged 120 mm cannon was designed to hit enemy tanks at extreme distances. In 1953–54 a series of 300 tanks, initially designated T43E1, were built by Chrysler at the Newark plant. Testing was unsatisfactory; the tanks failing to meet Continental Army Command's standards and the tanks were put into storage in August 1955. After 98 improvement modifications were approved, on 26 April, 1956 the tank was designated the M103 Heavy Tank. Of the 300 T43E1s built, 80 went to the US Army (74 of which were rebuilt to M103 standard), and 220 were accepted by the US Marine Corps, to be used as infantry support, rebuilt to improved M103A1, then M103A2 standards.

Following contemporary American design philosophy, the M103 was built with a two-piece, cast elliptic armor scheme, similar to the M48’s design. It featured seven road wheels per side, mounted with long-arm independent torsion bars. The 28” track was shoed in steel backed rubber chevron tracks, allowing for a ground pressure of 12.9 psi. The Continental AV-1790 engine was placed at the rear of the tank, and produced a maximum output of 810 horsepower and 1600 foot-pound of torque, fed through a General Motors CD-850-4 3-speed transmission. This allowed the 60-ton heavy tank to achieve a maximum road speed of 34 km/h and a maximum climbing gradient of 60%.

Initial production versions suffered a host of drivetrain mechanical problems. The Continental powerpack, shared by the much lighter M48/M60 tanks, was insufficient to drive the much heavier M103. The resulting performance of the tank was dismal; being severely underpowered and very fuel intensive. This presented a host of logistical problems for the vehicle, most prominently the extremely limited range of just 80 miles. Though this was partially corrected with the introduction of the AV-1790-2 diesel unit, it would remain cumbersome and gas-thirsty for the majority of its production life.

For ease of production, many of the large components of the tank were made from cast plate. This design scheme was also much more mass efficient than traditional uniform plate armor. Despite being similarly (or even better) protected than the T29-series of prototypes which preceded it, the M103 was nearly 10 tons lighter, making it competitive with the Soviet T-10/IS-8 tank. The frontal hull glacis was a compound pike, welded at the center seam, with up to 10 inches thick armor at the direct front. Larger than that of the M48 and M60, the turret was a massive single-piece cast design, fitted with heavily sloped 10 inch rolled-homogenous plating.

The M103 was designed specifically to mount the 120mm M58 gun, fitted in the M89 turret mount. Using standard Armor-Piercing Ballistic Cap Tracer Rounds, it was capable of penetrating 221mm of 30-degree sloped rolled-homogenous armor at 1000 yards and 196mm at 2000 yards. It could also penetrate 124mm 60-degree sloped rolled-homogenous armour at 1000 yards and 114mm at 2000 yards. The commander could select from 34 rounds of either M358 Armor-Piercing Ballistic Cap Tracer Rounds or the M469 HEAT shells, mounted at the rear of the turret and in the hull. With both loaders, the maximum static firing rate of the gun was five rounds per minute, owing to the design of the two-piece ammunition. Using the electrohydraulic turret traverse, the gunner could turn the turret at 18 degrees per second, with 15 degrees of elevation and 8 degrees of gun depression.

Armor
The armor was made from welded rolled and cast homogeneous steel of varying thickness.

Service
In Europe, the US Army fielded only one battalion of heavy tanks, from January 1958, originally assigned to the 899th Tank Battalion, later re-designated the 2d Battalion, 33d Armor. The US Army heavy armor battalion, in contrast to other armor units, was organized into four tank companies, composed of six platoons each, of which each platoon contained three M103s, for a total of 18 tanks per company. Standard US Army armor battalions at the time had three companies per battalion, each with three five-tank platoons, with 17 tanks per company (two tanks were in headquarters platoon). The US Marine Corps assigned one M103 company to each of its three Marine tank battalions, including its Marine reserve units. The M103 was never used in combat.

While the US Army deactivated its heavy armor units with the reception of the new M60 series main battle tanks in 1963, the remaining M103s stayed within the US Marine Corps inventory until they began receiving the M60 series main battle tank. With the disappearance of the heavy tank from US forces came the full acceptance of the main battle tank in 1960 for the US Army, and 1973 for the US Marine Corps. Although the 21st century's M1 Abrams main battle tank utilizes the same caliber of main gun, 120 mm, the M103's cannon was a rifled gun firing a separate-loading round, in which the projectile was loaded into the breech, followed by a cartridge case consisting of a brass case, primer, and propellant in a fixed unit. This separate-loading system necessitated the use of two loaders. The only part of the cartridge case consumed during firing was the propellant and a plastic cap on the end of the brass cartridge case. The spent brass cartridge case was ejected after firing. The M1 tank's 120 mm main gun is a smooth bore firing a semi-caseless round, ejecting only a back cap of the original loaded round; the bulk of the M1's 120 mm shell casing is consumed during firing.

The ammunition fired by the M103's M58 cannon included: the APBC-T M358 shot, the HEAT-T M469 shell, the HE-T M356 shell, the TP-T M359E2 shot, and the WP-T M357 shot.

Variants

 * T43 – Six pilot vehicles produced in 1951.
 * T43E1 – 300 built in 1953.
 * T43E2 – Two vehicles produced from 1955–56. Turret basket and gunner moved to front of turret. New targeting system (T52 rangefinder, T33 computer, T44 gunners periscopic sight) and hydraulic turret traverse replaced with electric
 * M103 – Produced in 1957. 74 converted into other models.
 * M103A1 Produced in 1959. 219 converted or rebuilt. New sight (Stereoscopic T52) and M14 ballistic computer. Removed one coaxial machine gun. New turret electric amplidyne system traverse. Turret basket.
 * M103A2 Produced 1964. 153 converted or rebuilt. New 750 hp (559 kW) diesel engine from the M60 tank, increasing the road range to 295 mi (480 km) and maximum speed to 23 mph (37 km/h). The M15 Stereoscopic Rangefinder is replaced to M24 Coincidence Rangefinder.
 * Heavy Recovery Vehicle M51 Initially built 1954–1955 and modified 1956–58 to bring up to standard. Tank recovery version of the M103 heavy tank. 187 built by Chrysler.
 * Manned Evasive Target Tank M103A2s modified in 1977 for use as targets in training TOW missile crews (firing dummy warheads).

Operators

 * United States
 * The U.S. Army operated 80 T43E1 tanks, 74 of them were later converted to the M103 standard.
 * The U.S. Marines operated 220 T43E1 tanks, 219 of them which were later converted to the M103A1 then 154 were rebuilt to the M103A2 standard.

Surviving examples
There are several M103s in existence, including the late M103A2 version.