Staff Writer (Updated: 9/16/2016):
The origins of the M1 Abrams can be traced back to a failed partnership between the United States and West Germany in producing a new generation main battle tank system with the latest capabilities to combat whatever the Soviets were brewing (this turned out to be the T-72). The US Army was then fielding the M60 Patton series (its own origins lay in the World War 2-era M26 Pershing heavy tank) as their main battle tank and it was largely believed that the T-72 would outclass the Patton as soon as it arrived - likewise for the West Germans and their post-war Leopard 1 main battle tanks. The joint design became known as the MBT-70.
The MBT-70 offered a low profile with a powerful main gun (152mm coupled to an XM-150 autoloader for the American model and an auto-loading 120mm Rheinmetall gun for the German model). The turret was sloped on all sides and set in the middle of the design and housed the entire crew (the remarkably low silhouette - at just over 6 feet tall - necessitated that the driver be placed in the turret with the tank commander and gunner). With the autoloader, operation of the MBT-70 required the use of only three personnel similar to Russian auto-loading MBTs. The hull was sloped towards the front, flat-faced at the rear engine compartment and showcased barely any surface exposure along the sides above the tracks. A 20mm cannon could be used in the air defense role, this springing up from behind the driver's cupola. A 7.62mm co-axial machine gun mount was standard on the MBT-70, and on any tank since World War 2 for that matter. The profile of the MBT-70 was characterized by its medium-sized road wheels with six fitted to a side. The American version of the MBT-70 would have fired a barrel-launched Shillelagh anti-tank missile at long ranges. Suspension was of an advanced hydropneumatic type that was fully adjustable on-the-fly by the driver. Despite it being a heavy 50+ ton tank design, the MBT-70 was reportedly able to reach an unheard of maximum speed of 52 miles-per-hour.
As the budget for the MBT-70 project rose widely out of control - both parties naturally developed different ideas as to the design direction of this new tank - the West Germans eventually dropped out of support for the program and instead poured their resources into producing a follow-up design to their successful Leopard 1 main battle tank, with the new design eventually becoming the equally potent Leopard 2. By 1971, the American Congress saw the ballooning MBT-70 project coming to naught and reinvested those funding dollars towards a program that would ultimately produce the M1 Abrams. The MBT-70 project, despite its revolutionary design implements, proved too ambitious and was effectively killed by the congressional decision.
The XM1 Abrams prototype was designed by Chrysler Defense. General Dynamics Land Systems Division subsequently purchased Chrysler Defense Division in 1979. The XM1 fitted the British Royal Ordnance L7 series rifled main gun of 105mm, a license-production copy built in the United States. The XM1 entered production as the M1 Abrams in 1979 and became operational the following year. The Abrams featured state-of-the-art armor protection for the crew complete with armored compartments for fuel and ammunition stores. Crew accommodations were for four personnel and consisted of the tank commander, gunner, loader and driver. The new tank weighed in at 67.5 short tons making it one of the heaviest such systems in the world.
After a short time in service, it was already becoming apparent that the 105mm rifled British armament would not stay up to the conventional tank standards being fielded in the East. As such, the Abrams was upgunned to incorporate the German-produced Rheinmetall AG 120mm smoothbore cannon known in the US inventory by the designation of M256. The Abrams was now produced under the M1A1 designation and appeared in 1986 with production running through 1992. Additional upgrades included improved armor allocation and the Chemical, Biological, Radiological and Nuclear (CBRN) protection system.
The M1A1 was soon followed by the improved M1A2. The A2 featured a revised weapon station for the tank commander incorporating the latest in digital systems, a separate thermal viewer system and improved navigation equipment. The M1A2 SEP (System Enhancement Package) brought the Abrams up to an even higher technological standard, incorporating digital maps, additional processing computers to handle the digital workflow and an improved cooling system. Depleted uranium armor was also part of follow-on upgrades while other subsequent upgrade programs have produced the M1A1 AIM, M1A1D, M1A1HC and the M1A2 SEP. Reactive armor and slat armor became optional and were offered as part of the Tank Urban Survival Kit (TUSK) developed for urban fighting - where conventional ranged tank engagement rules effectively fly out of the window.