The M1 Abrams was designed by Chrysler Defense (in 1979, General Dynamics Land Systems Division purchased Chrysler Defense Division) and is currently produced by General Dynamics Corporation in Lima, Ohio, and first entered US Army service in 1980. An improved version of the M1, the M1A1, was introduced in 1985. The M1A1 has the M256 120 mm smoothbore cannon developed by Rheinmetall AG of Germany for the Leopard 2, improved armor, and an NBC protection system. The M1A2 is a further improvement of the M1A1 with a commander's thermal viewer and weapon station, position navigation equipment, digital data bus and a radio interface unit.

Further upgrades include depleted uranium armor for all variants, a system overhaul that returns all A1's to zero hours (M1A1 AIM), a digital enhancement package for the A1 (M1A1D), a commonality program to standardize parts between the US Army and the Marine Corps (M1A1HC), and an electronic upgrade for the A2 (M1A2 SEP).

In this article, "Abrams" is used to refer to all variants of the tank, while the specific variants are referred to as the M1, M1IP, M1A1, and M1A2.

During Operations Desert Shield and Desert Storm and for Bosnia, some M1A1s were modified with armor upgrades. The M1 can be equipped with mine plow and mine roller attachments if needed. The M1 chassis also serves as a basis for the Grizzly combat engineering vehicle and the M104 Wolverine heavy assault bridge.

Over 8,800 M1 and M1A1 tanks have been produced.

Armor

The Abrams is protected by a type of composite armor (derived from British Chobham armour) formed by multiple layers of steel and ceramics. It may also be fitted with reactive armor if needed (as in the Urban Survival Kit); however, this modification has never actually been performed. Fuel and ammunition are in armored compartments with blow-off covers to reduce the risk of and protect the crew from cooking off if the tank is damaged. Protection against spalling is provided by a Kevlar liner. Beginning in 1988, M1A1 tanks received improved armor packages that incorporated depleted uranium (DU) mesh in their armor at the front of the turret and the front of the hull. Armor thus reinforced offers significantly increased resistance towards all types of anti-tank weaponry, but at the expense of adding considerable weight to the tank. The first M1A1 tanks to receive this upgrade were tanks stationed in Germany, since they were the first line of defense against the Soviet Union. US tankers participating in Operation Desert Storm received an emergency program to upgrade their tanks with depleted uranium armor immediately before the onset of the campaign. M1A2 tanks uniformly incorporate depleted uranium armor, and all M1A1 tanks in active service have been upgraded to this standard as well. The strength of the armor is estimated to be about the same as similar Western, contemporary main battle tanks such as the Leopard 2. The M1A2/M1A1 can survive multiple hits from the most powerful tank munitions (including 120 mm depleted uranium APFSDS) and anti-tank missiles. This was first shown in the Persian Gulf War, when Abrams tanks survived multiple hits at relatively close ranges from Iraqi T-72's and ATGM's.

Main armament

M68A1 rifled gun

The main armament of the original model M1 was the M68A1 105 mm rifled tank gun firing a variety of APFSDS, HEAT, high explosive, white phosphorus (smoke), and a highly efficient and lethal anti-personnel (multiple flechette) round. This gun is a license-built version of the British Royal Ordnance L7 gun. While a reliable weapon, the 105 mm was becoming obsolete in the face of advances in armor technology, which meant that another tank gun was needed for the M1.

M256 smoothbore gun

The main armament of the M1A1 and M1A2 is the M256 120 mm smoothbore gun, designed by Rheinmetall AG of Germany and manufactured under license in the US by General Dynamics Land Systems Division in their plant in Lima, Ohio. It fires depleted uranium armor-piercing, fin-stabilized, discarding-sabot long-rod penetrator (APFSDS) rounds like the M829A2 and high explosive anti-tank (HEAT) shaped charge rounds such as the M830, the latest version of which (M830A2) incorporates a sophisticated multi-mode electronic sensing fuse which allows it to be used effectively against both armored vehicles and personnel, or even (at least in theory) low-flying aircraft.

The new M1028 120 mm anti-personnel canister cartridge has been brought into service early for use in the 2003 occupation of Iraq. It contains 1,150 ten-millimetre tungsten shot projectiles which spread from the muzzle to produce a shotgun effect lethal out to 500 m. The tungsten balls can be used to clear enemy dismounts, break up hasty ambush sites in urban areas, clear defiles, stop infantry attacks and counter-attacks, and support friendly infantry assaults by providing cover-by-fire.

In addition to this the new MRM-KE (Mid-Range-Munition Kinetic Energy), otherwise known as X-Rod, is also in development. Essentially a cannon-fired guided round, it has a range of roughly 12 km and uses a KE warhead which is rocket assisted in its final phase of flight.

Secondary armament

The Abrams tank has three machine guns:

A .50 cal. (12.7 mm) M2 machine gun in front of the commander's hatch. On the M1, M1IP and M1A1, this gun is on a powered mount and can be fired using a 3� magnification sight, while the vehicle is buttoned up. On the M1A2, M1A2SEP, the M2 is on a flex mount and now can be mounted with a similar system to the CROWS.

A 7.62 mm (.30 caliber) M240C machine gun in front of the loader's hatch on a skate mount.

A 7.62 mm M240C machine gun in a coaxial mount. The coaxial MG is aimed and fired with the computer fire control system.

The turret is fitted with two six-barreled smoke grenade launchers. These can create a thick smoke that blocks both vision and thermal imaging, and can also be armed with chaff. The engine is also equipped with a smoke generator that is triggered by the driver.

Aiming

The Abrams is equipped with a fire control computer that uses data from a variety of sources, including the Gunner's Primary Sight or "GPS" (thermal or daylight), a laser rangefinder, a wind sensor, a pendulum static cant sensor, and data on the ammunition type. The fire control system uses this data to compute a firing solution for the gunner. Either the commander or gunner can fire the main gun.

Mobility

The M1 Abrams is powered by a 1500 hp (1119 kW) Honeywell AGT1500 (originally made by Lycoming) gas turbine, and a 6 speed (4 forward, 2 reverse) Allison X-1100-3B Hydro-Kinetic Automatic transmission, giving it a governed top speed of 45 mph (72 km/h) on roads, 30 mph (48 km/h) cross-country. With the engine governor removed, speeds of around 60 mph (100 km/h) are possible on an improved surface; however, damage to the drive train (especially to the tracks) and an increased risk of injuries to the crew can occur at speeds above 45 mph. The tank can be fueled with diesel fuel, kerosene, any grade of MOGAS (motor gasoline), or JP-4 or JP-8 jet fuel; the U.S. Army uses JP-8 jet fuel in order to simplify logistics.

The gas turbine propulsion system has proven quite reliable in practice and combat, but its high fuel consumption is a serious logistic issue (starting up the turbine alone consumes 40 liters of fuel). The high speed, high temperature jet exhaust emitted from the rear of M1 Abrams tanks makes it difficult for the infantry to proceed shadowing the tank in urban combat. The turbine is noisy, comparable to a helicopter engine, although the noise character (pitch) is significantly different from a contemporary diesel tank engine. Future US tanks may return to reciprocating engines for propulsion, as 4-stroke diesel engines have proven quite successful in other modern heavy tanks, e.g. the Leopard 2, Challenger 2 and Merkava. The small size, simplicity, power-to-weight ratio, and easy removal/replacement of the turbine powerpack does, however, present significant advantages over any proposed reciprocating replacement.

The Abrams can be carried by the C-5 Galaxy and C-17 Globemaster III. The limited capacity (one combat-ready tank or two transport-ready tanks in a C-5, one combat-ready tank in a C-17) caused serious logistical problems when deploying the tanks for the First Gulf War, though there was enough time for 1,848 tanks to be transported by ship. Tanks shipped in the transport-ready configuration require depot-level maintenance to install a number of sections of armor, and need to be fueled and loaded with ammunition. Tanks shipped in the combat-ready configuration can enter combat immediately