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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.〔Hunnicutt, R. P. "Sheridan: A History of the American Light Tank." Volume 2, 1995, Presidio Press; ISBN 0-89141-570-X.〕 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.〔Hunnicutt, R. P. "Firepower: A History of the American Heavy tank." 1988, Presidio Press; ISBN 0-89141-304-9.〕〔Hunnicutt, R. P. "Patton: A History of the American Main Battle tank." 1984, Presidio Press; ISBN 0-89141-230-1.〕 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". The Sheridan was retired without replacement. While missiles fired out of guns would prove a disappointment for the U.S. military, the wire-guided BGM-71 TOW would later give infantry fighting vehicles like the M2 Bradley the firepower to destroy armored targets along with the ability to carry troops. Though other light tanks were evaluated, the wheeled M1128 Mobile Gun System currently provides an armored 105mm gun platform that is lighter than a main battle tank for fire support, though that vehicle is heavier than the tracked Sheridan, and it is neither amphibious nor parachutable. ==Development== In the immediate post-World War II era, the US Army introduced the M41 Walker Bulldog into service to fill the role of a light tank. 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, resulting in the T71 and T92 test designs. Two prototypes of the 19 ton T92 were later ordered. However, as the prototypes were entering testing, information about the new Soviet PT-76 tank became available. The PT-76 was an amphibious light tank, and soon there were demands that any U.S. light tank should be able to swim as well. The T92 was already in the prototype stage and could not be easily refitted for this role, so the design of an entirely new system started as the XM551. The vehicle designed to mount the gun had a steel turret and aluminum hull. Although the hull could defeat heavy machinegun fire of up to 12.7mm AP, it was easily defeated by rocket propelled grenades, 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 Tanks. 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 adopted a similar solution, but dropped it with upgraded armor. In the Vietnam War, 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 Vietnam. The expensive missile ended up almost never being fired in anger, despite a production run of 88,000 units. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「M551 Sheridan」の詳細全文を読む スポンサード リンク
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