The M1 Abrams main battle tank has been in service since 1980. Since then, it has gone through dozens of upgrades and been the baseline variant of several vehicles.
Development MBT-70
In 1963, the U.S. Army and the West German Bundeswehr began collaborating on a main battle tank (MBT) design that both nations would use, improving interoperability between the two NATO partners. The MBT-70, or Kampfpanzer 70 as it was known in Germany, incorporated many new unconventional technologies. Conventional tanks of the time had a crew of four, with the driver located in the hull. In the MBT-70, the loader crewmember would be replaced by a mechanical autoloader and the driver would be located inside the NBC-protected turret with the other two crewmembers. Like the M60A2 MBT and M551 Sheridan light tank then under development, the MBT-70 was armed with a 152 mm gun-launcher that, in addition to firing conventional ammunition, would also fire the Shillelagh missile. A hydropneumatic suspension provided improved cross-country ride quality and also allowed the entire tank to be raised or lowered by the driver.
The United States team was led by General Motors while the German team consisted of a consortium of firms. While it was intended both nations would split costs and receive an equal stake in decisions about the design, the collaboration between the two teams was rocky from the start, with many cultural differences and disagreements about the design hindering progress. Germany favored a tank optimized for the terrain of central Europe while the U.S. attached importance to operating anywhere in the world. The Germans had reservations about the Shillelagh missile and developed a 120 mm high-velocity gun as an alternative. Perhaps the most contentious disagreement, never fully resolved, concerned the measurement system to be used in drafting. Germany became concerned with the excessive weight of the tank. One solution proposed removing radiation hardening from the turret, but this called into question the wisdom of the driver-in-turret configuration, which was intended to protect the crew in the event of a nuclear blast. In light of growing costs, delays and overall uncertainty as to the soundness of the tank design, the United States and Germany ended their MBT-70 partnership in 1970. The U.S. Army began work on an austere version of the MBT-70, named XM803. Systems were simplified or eliminated altogether and the unreliable autoloader was improved. These changes were ultimately insufficient to allay concerns about the tank's cost. Congress canceled the XM803 in December 1971 but permitted the Army to reallocate remaining funds to develop a new main battle tank. Congress appropriated $20 million for the development of a new tank program.
The Army restarted its M60 successor program with Major General William R. Desobry leading the team formulating requirements in March 1972. Army officials told congressmen in April that there was little that could be salvaged from the past efforts, and that a new tank would take at least eight years to develop. A Pentagon task force submitted requirements for the tank in January 1973. By April the Pentagon approved the project, and in July Brigadier General Robert J. Baer was named as project manager. Baer's right-hand man was newly named Army Armor School commander Major General Donn A. Starry. Desobry told The New York Times, "We ought to be shot if it doesn't work."
Starting afresh The Army began the XM815 project in January 1972. The Main Battle Tank Task Force was established under Major General William R. Desobry. The task force prepared design studies with the technical support of Tank-automotive and Armaments Command (TACOM).
In early 1972, Lieutenant General William E. DePuy met with other generals to determine the Army's most important priorities. The resultant agreement established the new tank as the Army's top acquisition program, one of five major programs that were to be marketed as "The Big Five."
By July, Desobry's task force had completed its work. The task force named weight as one of the most important characteristics of the new tank. The new tank would weigh between . It was believed that more armor would be insufficient to stop new generations of projectiles. Instead, the new tank would use speed, low silhouette, and agility to avoid being hit in the first place. The task force ruled out technologies it believed amounted to gold plating that would increase costs. These included an autoloader, hydropneumatic suspension and a coffee maker for the crew. The 105 mm gun of the M60 tank was deemed an obvious choice.
The task force's push to reduce weight might have gained more acceptance if not for a chance encounter. In the spring of 1972, Desobry visited British Army's Fighting Vehicles Research and Development Establishment at Chobham Common to view a new tank gun. Noticing his disinterest in the gun, his British hosts took the opportunity to brief Desobry on their own highly sensitive "Burlington" armor. Desobry had heard about the new armor in passing from his peers, who told him to disregard it.
The armor fared moderately better against kinetic energy rounds, but where it excelled was its performance against shaped charges such as HEAT rounds. Chobham armor uses baffles and spaced layers to counteract shaped charges. Shaped charges create a focused metal jet to penetrate armor. The baffles disrupt the jet, and spaced layers cause premature detonation or deformation, reducing its effectiveness. Pound-for-pound it offers better protection than rolled homogeneous armor.
Desobry was sufficiently impressed with the new armor, also known as Chobham. Upon his return to the U.S. Desobry learned that scientists at the Army's Ballistic Research Laboratory (BRL) at Aberdeen Proving Ground, Maryland, had already been at work improving Chobham. Desobry's enthusiasm for the new armor fell flat when he learned that to take full advantage of the new armor, the armor on the new tank would have to be around thick (for comparison, the armor on the M60 is around thick) and the tank weighs about . Against Desobry's wishes, incoming Army Chief of Staff General Creighton Abrams decided the issue and set the weight of the new tank at around . The original goal of keeping weight under was abandoned.
In July 1973, representatives from Chrysler and General Motors traveled to the United Kingdom, and were escorted by BRL personnel from the BRL and Baer to witness the progress of Chobham armor. They observed the manufacturing processes required for the production of Chobham armor and saw a proposed design for a new British vehicle utilizing it. The requirement to add Chobham armor led to major changes to both tanks, none more dramatic than the General Motors model, which received sloped armor on the turret cheeks.
The XM1 program was approved to begin in January 1973. TACOM began examining specific goals. After several rounds of input, the decision was made to provide armor to defeat the "heavy threat" posed by the T-62's 115 mm gun using projected improvements of their armor-piercing fin-stabilized discarding sabot (APFSDS) ammunition through the 1980s, and the new 125 mm gun of the T-64 and T-72 firing HEAT rounds. To this end, a new design basis emerged in February 1973. It had to defeat any hit from a Soviet gun within and 30 degrees to either side. By 1973, the Army had settled on buying 3,312 of the new tanks, with production beginning in 1980. The tank would be armed with the 105 mm M68 gun, a licensed version of the Royal Ordnance L7, and a 20 mm version of the M242 Bushmaster. The Army later deleted the latter from the design, seeing it as superfluous.
In May 1973, Chrysler Defense and General Motors submitted proposals. Both were armed with the 105 mm M68 gun, the licensed L7, and the 20 mm Bushmaster. Chrysler chose a 1,500 hp gas turbine Lycoming AGT1500. GM's model was powered by a 1,500 hp diesel similar to that used on the American MBT-70 and XM803.
At the time, the Pentagon's procurement system was beset with problems being caused by the desire to have the best possible design. This often resulted in programs being canceled due to cost overruns, leaving the forces with outdated systems, as was the case with the MBT-70. There was a strong movement within the Army to get a new design within budget to prevent the MBT-70 experience from repeating itself. For the new design, the Army stated the unit cost was to be no more than $507,000 in 1972 dollars ().
The Pentagon's approach to control of research and development was modified with the XM1. Previous acquisition strategy called for a significant amount of the design work to be done by the government. Under the new framework, contractors would competitively bid their own designs rather than compete solely for the right to manufacture the end product.
The price of the $3 billion program was assailed by Congressman Les Aspin in July 1973. The Pentagon had projected unit costs were to be less than US$507,000 in 1972 dollars. Aspin argued that were the research and development costs factored in, tanks would actually cost over $900,000 a piece (compared to $1.3 million for the canceled MBT-70). Noting that the M60 Patton cost only $500,000 each, Aspin said, "I'm sure that the Army's new tank is not twice as good as what we have today."
In June 1973, the Army awarded competitive three-year contracts – $68 million for Chrysler Corporation and $87 million to General Motors Corporation – for the production of prototypes. Ford had initially hoped to bid on the program but dropped out because the Department of Defense would not allow its Israeli designers to participate. In February 1976 the two prototypes were tested at Aberdeen Proving Ground. Chrysler chose a regenerative turbine engine made by Avco Lycoming while General Motors chose a Teledyne Continental diesel engine. They were armed with the license-built version of the 105 mm Royal Ordnance L7 gun.
In 1974 the United States signed a somewhat vague memorandum of understanding with West Germany; both countries affirmed the intention to make all reasonable efforts to standardize their tank programs. Neither nation was bound to select one of the tanks. However, the popular belief, reinforced from time to time by official statements of both Governments, was that both the XM1 and the Leopard 2 were bidding to become the Army's next main battle tank. West Germany was not committed to considering the XM1 for their forces.
More changes
Through the period while the initial prototypes were being built, a debate broke out between West Germany and U.S. about the use of the 105 mm gun. The Army was planning on introducing several new types of ammunition for the 105 that would greatly improve its performance, notably, the XM-774 using depleted uranium. These rounds would give it the performance needed to defeat any Soviet tank with ease. There was some concern that depleted uranium would not be allowed in Germany, perhaps just in peacetime, so improvements to the tungsten-cored M735 were also considered.
By 1977, the decision had been made to eventually move the new tank to a 120 mm gun. After head-to-head testing between the Royal Ordnance L11A5 and the Rh-120, the latter was chosen and later type-classified as the M256. The turret designs of the two prototypes were modified to allow either gun to be fitted. Although the L11/M256 120mm gun was chosen to be the main weapon of the Abrams in 1979, the improved ammunition for the gun still was not fully developed, thus delaying its fielding until 1984.