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The M551 "Sheridan" AR/AAV (
Armored Reconnaissance Armoured reconnaissance is the combination of terrestrial reconnaissance with armoured warfare by soldiers using tanks and wheeled or tracked armoured reconnaissance vehicles. While the mission of reconnaissance is to gather intelligence about ...
/Airborne Assault Vehicle) was a
light tank A light tank is a tank variant initially designed for rapid movements in and out of combat, to outmaneuver heavier tanks. It is smaller in size with thinner armor and a less powerful main gun, tailored for better tactical mobility and ease of ...
developed by the United States and named after
General A general officer is an Officer (armed forces), officer of highest military ranks, high rank in the army, armies, and in some nations' air forces, space forces, and marines or naval infantry. In some usages the term "general officer" refers t ...
Philip Sheridan General of the Army Philip Henry Sheridan (March 6, 1831 – August 5, 1888) was a career United States Army officer and a Union general in the American Civil War. His career was noted for his rapid rise to major general and his close as ...
, of
American Civil War The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States. It was fought between the Union ("the North") and the Confederacy ("the South"), the latter formed by states th ...
fame. 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 152 mm gun/launcher, which fired both conventional ammunition and the
MGM-51 Shillelagh The Ford MGM-51 ''Shillelagh'' (MGM meaning Mobile Guided Missile) was an American anti-tank guided missile designed to be launched from a conventional gun (cannon). It was originally intended to be the medium-range portion of a short, medium, and ...
guided anti-tank missile. The M551 Sheridan entered service with the
United States Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...
in 1967. At the urging of General
Creighton Abrams Creighton Williams Abrams Jr. (September 15, 1914 – September 4, 1974) was a United States Army general who commanded military operations in the Vietnam War from 1968 to 1972, which saw United States troop strength in South Vietnam reduced ...
, the U.S. Commander,
Military Assistance Command Vietnam U.S. Military Assistance Command, Vietnam (MACV) was a joint-service command of the United States Department of Defense. MACV was created on 8 February 1962, in response to the increase in United States military assistance to South Vietnam. MACV ...
, at the time, the M551 was rushed into combat service to
South Vietnam South Vietnam, officially the Republic of Vietnam ( vi, Việt Nam Cộng hòa), was a state in Southeast Asia that existed from 1955 to 1975, the period when the southern portion of Vietnam was a member of the Western Bloc during part of th ...
in January 1969. Later that year, M551s were deployed to units in Europe and South Korea. The Sheridan saw extensive combat in the
Vietnam War The Vietnam War (also known by #Names, other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vie ...
, where problems with the platform became evident, particularly its poor survivability and reliability. Based on its experiences in Vietnam, the Army realized the shortcomings of the Sheridan, and after the war in 1975 began to eliminate the vehicle from its units in 1979. A modest fleet of vehicles remained in the
82nd Airborne Division The 82nd Airborne Division is an Airborne forces, airborne infantry division (military), division of the United States Army specializing in Paratrooper, parachute assault operations into denied areasSof, Eric"82nd Airborne Division" ''Spec Ops ...
and the National Guard. Various improvement programs were successfully undertaken to improve the Sheridan's reliability. Problems persisted with the 152 mm gun/launcher, and various efforts explored to replace it with a more conventional model. The Sheridan went on to serve in the
invasion of Panama The United States invasion of Panama, codenamed Operation Just Cause, lasted over a month between mid-December 1989 and late January 1990. It occurred during the administration of President George H. W. Bush and ten years after the Torrijos– ...
and the
Gulf War The Gulf War was a 1990–1991 armed campaign waged by a 35-country military coalition in response to the Iraqi invasion of Kuwait. Spearheaded by the United States, the coalition's efforts against Iraq were carried out in two key phases: ...
. The Army sought to replace the Sheridan with the
M8 Armored Gun System The M8 Armored Gun System (AGS), sometimes known as the Buford, is an American light tank that was intended to replace the M551 Sheridan and TOW missile-armed Humvees in the 82nd Airborne Division and 2nd Armored Cavalry Regiment (2nd ACR) of ...
, but this was canceled in 1996, late in its development. The Sheridan was retired without a designated replacement in 1996. The Army acquired the
M1128 Mobile Gun System The M1128 Mobile Gun System (MGS) is an eight-wheeled armored car of the Stryker armored fighting vehicle family, mounting a 105 mm tank gun, based on the Canadian LAV III light-armored vehicle manufactured by General Dynamics Land Systems for ...
to fulfill a similar requirement, but this is being retired in 2022. The Army current light tank acquisition program is
Mobile Protected Firepower The Mobile Protected Firepower (MPF) system is a U.S. Army program to procure a light tank that is capable of providing mobile protected direct offensive fire capability. The program is part of the Next Generation Combat Vehicle program. A previo ...
. A large bulk of Sheridans were retained in service at the National Training Center (NTC) at
Fort Irwin Fort Irwin National Training Center (Fort Irwin NTC) is a major training area for the United States military in the Mojave Desert in northern San Bernardino County, California. Fort Irwin is at an average elevation of . It is located northeast o ...
, California and as Armor Officer Basic training at Armor Training Center, then located at
Fort Knox Fort Knox is a United States Army installation in Kentucky, south of Louisville and north of Elizabethtown. It is adjacent to the United States Bullion Depository, which is used to house a large portion of the United States' official gold res ...
, Kentucky. They worked as simulated Soviet armored opposition force (
OPFOR An opposing force (alternatively enemy force, abbreviated OPFOR) is a military unit tasked with representing an enemy, usually for training purposes in war game scenarios. The related concept of aggressor squadron is used by some air forces. ...
) to train U.S. military units on simulated tank on tank armored combat to test on combat effectiveness in a desert environment. They were finally retired from the NTC in 2003.


Development

In the immediate post-World War II era, the
U.S. Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cl ...
introduced the
M41 Walker Bulldog The M41 Walker Bulldog, officially 76-mm Gun Tank, M41, was an American light tank developed for armed reconnaissance purposes. It was produced by Cadillac between 1951 and 1954 and marketed successfully to the United States Army as a replaceme ...
into service to fill the role of a
light tank A light tank is a tank variant initially designed for rapid movements in and out of combat, to outmaneuver heavier tanks. It is smaller in size with thinner armor and a less powerful main gun, tailored for better tactical mobility and ease of ...
. The lifespan of the M41 was fairly short; at 25 tons it was considered too heavy to be a true light tank, and had a rather short cruising range. Plans were started to build an even lighter replacement mounting the same gun, the T71 and T92. The T92 appeared to be the more promising of the two. However, as the prototypes were entering testing, information about the new Soviet
PT-76 The PT-76 is a Soviet amphibious light tank that was introduced in the early 1950s and soon became the standard reconnaissance tank of the Soviet Army and the other Warsaw Pact armed forces. It was widely exported to other friendly states, like ...
light tank became available. The PT-76 was amphibious, and soon there were demands that any U.S. light tank should be able to swim as well. The T92 could not be easily refitted for this role, so the Army canceled the program in 1958. In January 1959, the first concept studies were initiated for the armored reconnaissance/airborne assault vehicle that would replace both the M41 and
M56 Scorpion The M56 "Scorpion" Self-Propelled Gun is an American unarmored, airmobile self-propelled tank destroyer, which was armed with a 90mm M54 gun with a simple blast shield, and an unprotected crew compartment. History The M56 was manufactured from ...
self-propelled gun Self-propelled artillery (also called locomotive artillery) is artillery equipped with its own propulsion system to move toward its firing position. Within the terminology are the self-propelled gun, self-propelled howitzer, self-propelled mo ...
. By October 1959, 12 proposals had been received by the Ordnance Tank Automotive Command. Two proposals were downselected in December: One from Cadillac Motor Car Division of General Motors Corporation, and a joint venture of
AAI Corporation AAI Corporation is an aerospace and defense development and manufacturing firm, located in Hunt Valley, Maryland, US. Formerly a wholly owned subsidiary of United Industrial Corporation, AAI was acquired by Textron in 2007. It currently operates ...
and
Allis-Chalmers Manufacturing Company Allis-Chalmers was a U.S. manufacturer of machinery for various industries. Its business lines included agricultural equipment, construction equipment, power generation and power transmission equipment, and machinery for use in industrial set ...
. Mockups of both proposals were evaluated in May 1960. The AAI candidate had three crewmen, and weighed the closest to the 10-ton weight limit specified in the requirements. The Cadillac design was only slightly heavier, with four crew. The three-man turret of the Cadillac proposal was considered more effective than the two-man turret proposed by AAI. The weight limit was reset at 15 tons. In June 1960, Cadillac Motor Car Division signed a contract to develop their concept further, which was designated as the AR/AAV XM551. In August 1961, the Secretary of the Army approved the name "Sheridan," after Major General
Philip Sheridan General of the Army Philip Henry Sheridan (March 6, 1831 – August 5, 1888) was a career United States Army officer and a Union general in the American Civil War. His career was noted for his rapid rise to major general and his close as ...
. A test bed underwent operations at Cleveland Tank Plant in December 1961. The decision to use the 152 mm caliber XM81 gun-launcher instead of a more conventional gun was driven by the desire to save weight. The XM81 weighed about half as much as the 105 mm caliber M68, and could fire both conventional and missile rounds. Testing of the XM81 began at Erie Army Depot in late 1961. By 1962, the Army realized that the
MGM-51 Shillelagh The Ford MGM-51 ''Shillelagh'' (MGM meaning Mobile Guided Missile) was an American anti-tank guided missile designed to be launched from a conventional gun (cannon). It was originally intended to be the medium-range portion of a short, medium, and ...
missile system would not immediately be ready for the Sheridan, and so considered several alternative gun systems. These including conventional 76 mm, 90 mm and 105 mm options, as well as missile systems such as
ENTAC ENTAC ("Engin Téléguidé Anti-Char") or MGM-32A was a French MCLOS wire-guided Anti-tank missile. Developed in the early 1950s, the weapon entered service with the French army in 1957. Production ended in 1974 after approximately 140,000 had b ...
and
TOW Towing is coupling two or more objects together so that they may be pulled by a designated power source or sources. The towing source may be a motorized land vehicle, vessel, animal, or human, and the load being anything that can be pulled. Th ...
. The Army ultimately decided to arm the Sheridan with just 152 mm conventional rounds until the Shillelagh was ready. The first of 12 pilots was delivered in June 1962. Pilots 1–3 comprised the first generation. With the second generation pilots 4–6, the band track was replaced with single-pin, link type tracks. An XM551 test bed turret with gun-launcher was mated to an M41 chassis, which began firing tests in August 1962 at Aberdeen Proving Ground. The third generation of pilots, starting with pilot 7, eliminated the water jet propulsion. Pilots 9–11 were delivered in 1964, and pilot 12 was delivered in February 1965. In the 1960s the Army was also developing the
MBT-70 The MBT-70 (German: ''KPz 70 or KpfPz 70'') was an United States, American–West German joint project to develop a new main battle tank during the 1960s. The MBT-70 was developed by the United States and West Germany in the context of the Cold ...
main battle tank with West Germany. The U.S. 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 A main battle tank (MBT), also known as a battle tank or universal tank, is a tank that fills the role of armor-protected direct fire and maneuver in many modern armies. Cold War-era development of more powerful engines, better suspension sys ...
(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. Fearing Congress would balk at funding two developmental tank programs, the Army chose to designate the Sheridan as an armored reconnaissance vehicle rather than a light tank. The Army also believed "tank" too much evoked the main battle tank, a different role altogether, so the new project was instead officially classified as an "Armored Reconnaissance/Airborne Assault Vehicle."


Production

In April 1965 the Army awarded a four-year $114.5 million contract to the
Cadillac Gage Textron Marine & Land Systems, formerly Cadillac Gage, is an American military contractor that manufactures armored vehicles, turrets, advanced marine craft, surface effects ships, and other weapon systems. It is owned by Textron, and was formed i ...
division of the
General Motors The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
(GM) for the production of the
M109 howitzer The M109 is an American 155 mm turreted self-propelled howitzer, first introduced in the early 1960s to replace the M44. It has been upgraded a number of times, most recently to the M109A7. The M109 family is the most common Western indirect-fi ...
and the XM551 General Sheridan. Limited production was approved in May, and then classified as "Standard A" in May 1966. Production took place at Cleveland Army Tank Automotive Plant. The first two production units were delivered to the Army in July. During development, the Marine Corps evaluated the Sheridan as a possible replacement for the
M50 Ontos Ontos, officially the Rifle, Multiple 106 mm, Self-propelled, M50, was a U.S. light armored tracked anti-tank vehicle developed in the 1950s. It mounted six 106 mm manually loaded M40 recoilless rifles as its main armament, which could b ...
. The Corps determined that the Sheridan would be too costly. The Sheridan entered service in June 1967 with 1st Battalion, 63rd Armor Regiment at
Fort Riley Fort Riley is a United States Army installation located in North Central Kansas, on the Kansas River, also known as the Kaw, between Junction City and Manhattan. The Fort Riley Military Reservation covers 101,733 acres (41,170 ha) in Gear ...
. 2,426 Sheridans were planned. In the end, 1,662 Sheridans were built between 1966 and November 1970. Total cost of the M551 program was $1.3 billion. The M81 gun had problems with cracks developing near the breech after repeated firing, a problem that was later tracked to the "key" on the missiles that ran in a slot cut into the barrel. Most field units were modified to help address the problem, but later the modified M81E1 was introduced with a shallower slot, along with a matching modification to the missile, that cured the problem. The gun also has been criticized for having too much recoil for the vehicle weight, the second and even third road wheels coming clear off the ground when the main gun fired. In March 1967, a 105 mm howitzer XM103E7 and then a 76 mm gun was installed in the turret of an M551 at
Rock Island Arsenal The Rock Island Arsenal comprises , located on Arsenal Island, originally known as Rock Island, on the Mississippi River between the cities of Davenport, Iowa, and Rock Island, Illinois. It lies within the state of Illinois. Rock Island ...
. Neither configuration was adopted. In January 1969 two squadrons (54 Sheridans) were deployed to the
Republic of South Vietnam The Provisional Revolutionary Government of the Republic of South Vietnam (PRG, vi, Chính phủ Cách mạng Lâm thời Cộng hòa Miền Nam Việt Nam), was formed on June 8, 1969, by North Vietnam as a purportedly independent shadow gover ...
and assigned to the 3rd Squadron, 4th Cavalry and one squadron of the 11th Armored Cavalry Regiment for combat testing. In March 1969, after the Army invoked secrecy in declining to disclose program costs, a
Government Accountability Office The U.S. Government Accountability Office (GAO) is a legislative branch government agency that provides auditing, evaluative, and investigative services for the United States Congress. It is the supreme audit institution of the federal govern ...
(GAO) official said that development costs had reached $1.3 billion. Congressman Samuel S. Stratton criticized Army officials for the program's high costs, and accused officials of concealing cost figures to cover up for their own "bumbling ineptness." A GAO report leaked in May revealed the Army had fast-tracked the program to avoid budgetary scrutiny, despite indications by May 1966 that the tank's caseless ammunition was prone to
cooking off Cooking off (or thermally induced firing) is unfired weapon ammunition exploding prematurely due to heat in the surrounding environment. The term is used both for detonation of ammunition not loaded into a weapon, and unintended firing of a lo ...
. The problem had since then been resolved by a compressed-air system that forced hot ammunition residue from the breech, the Army told Congress. The Army said the Sheridan had performed well enough that it was planning to send hundreds more. A Congressional report in July identified $1.2 billion wasted on the M60 and Sheridan. The report attributed several Vietnam War casualties to Sheridan design faults, and said that the tank had been wholly unready for combat there "without extensive and costly retrofits." The vehicle designed to mount the gun had a steel turret and aluminum hull. Although the hull could deflect heavy machine gun fire of up to 12.7 mm AP, it was easily penetrated by
rocket-propelled grenade A rocket-propelled grenade (RPG) is a shoulder-fired missile weapon that launches rockets equipped with an explosive warhead. Most RPGs can be carried by an individual soldier, and are frequently used as anti-tank weapons. These warheads are a ...
s (RPGs), which could destroy the vehicle if the spalling contacted the caseless main gun rounds. Like the M113 armored personnel carrier, it was also vulnerable to mines. Swimming capability was provided by a flotation screen, similar to that used by the World War II, amphibious
DD Tank DD or Duplex Drive tanks, nicknamed "Donald Duck tanks", were a type of amphibious swimming tank developed by the British during the Second World War. The phrase is mostly used for the Duplex Drive variant of the M4 Sherman medium tank, that w ...
s. The front armor was overlain by a wooden "surfboard", actually three folded layers, hinged together. This could be opened up into a sloping vertical surface in front of the driver providing a bow of a boat hull, about level with the top of the turret. Fabric formed the rest of the water barrier, folding up from compartments lining the upper corner where the side met the top of the hull, and held up at the back with poles. The front of the "hull" was provided with a plastic window, but in practice it was found that water splashing onto it made it almost useless, and the driver instead usually had to take steering directions from the vehicle commander. The
M2 Bradley The M2 Bradley, or Bradley IFV, is an American infantry fighting vehicle that is a member of the Bradley Fighting Vehicle family. It is manufactured by BAE Systems Land & Armaments, which was formerly United Defense. The Bradley is designed for ...
adopted a similar solution, but dropped it with upgraded armor. In the
Vietnam War The Vietnam War (also known by #Names, other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vie ...
, firing the gun often adversely affected the delicate electronics, which were at the early stages of the transition to solid state devices, so the missile and its guidance system was omitted from vehicles deployed to South Vietnam. The expensive missile was
fired in anger {{Short pages monitor