M-551

M551 Sheridan
XM551 "Sheridan" Pilot #12
Type	Light tank
Place of origin	United States
Service history
In service	1969–1996
Wars	Vietnam War Operation Just Cause Operation Desert Shield/Desert Storm
Production history
Manufacturer	General Motors Corp., Cadillac Motor Car Div., Cleveland, Ohio
Specifications
Weight	15.2 tonnes (34,000 lb)
Length	Overall: 20.6 ft (6.3 m) (6.3 m)
Width	9.1 ft (2.8 m) (2.8 m)
Height	7.5 ft (2.3 m) (2.3 m)
Crew	4 (Commander, gunner, loader, driver)
Main armament	M81E1 Rifled 152 mm Gun/Launcher 20 rounds 9 MGM-51 Shillelagh missiles
Secondary armament	1× .50 cal (12.7 mm) M2 Browning machine gun with 1,000 rounds 1× .30 cal (7.62 mm) M73/M219 co-axial machine gun (later replaced by the M240C) with 3,000 rounds
Engine	Detroit Diesel (General Motors) 6V53T, 6 cylinder, turbocharged diesel 300 hp (220 kW)
Power/weight	19.7 hp (14.5 kW) / tonne
Suspension	Torsion bar suspension
Operational range	348 mi (560 km)
Speed	Road: 70 km/h (43 mph) Swimming: 5.8 km/h (3.6 mph)

The M551 "Sheridan" AR/AAV (Armored Reconnaissance/Airborne Assault Vehicle) was a light tank developed by the United States and named after Civil War General Philip Sheridan. It was designed to be landed by parachute and to swim across rivers. It was armed with the technically advanced but troublesome M81/M81 Modified/M81E1 152mm gun/launcher, which fired conventional ammunition and the MGM-51 Shillelagh guided anti-tank missile.

The M551 Sheridan entered service with the United States Army in 1967. At the urging of General Creighton Abrams, the U.S. Commander of Military Forces in Vietnam at the time, the M551 was rushed into combat service in Vietnam in January 1969. In April and August 1969, M551s were deployed to units in Europe and Korea, respectively. Now retired from service, it saw extensive combat in Vietnam, and limited service in Operation Just Cause (Panama), and the Gulf War (Kuwait). The Australian Army also trialled two Sheridans during 1967 and 1968, but judged that the type did not meet its requirements.

At the time of the M551's acceptance into service production in 1966, the United States Army no longer used the heavy, medium, and light tank classifications. In 1960, with the deactivation of its last (M103) heavy tank battalion, and the fielding of the new M60 series tank, the U.S. Army had adopted a main battle tank (MBT) doctrine; a single tank filling all combat roles. The U.S. Army still retained the M41 Walker Bulldog light tank in the Army National Guard, but other than the units undergoing the transitional process, the regular army consisted of MBTs. Partly because of this policy, the new M551 could not be classified as a light tank, and was officially classified as an "Armored Reconnaissance/Airborne Assault Vehicle".