M551 Sheridan

The M551 Sheridan 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/M81E1 152mm gun/launcher which fired conventional ammunition and the MGM-51 Shillelagh guided anti-tank missile.

It entered U.S. Army service in 1967. Under the urging of General Creighton Abrams, the US 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).

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".

The Sheridan was retired without replacement by an airborne tank that could swim. While missiles fired out of guns would prove a disappointment, the wire-guided BGM-71 TOW would 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 battle tank for fire support.

M551 Sheridan
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
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)
Armor	Aluminum armor
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, supercharged diesel 300 hp (220 kW)
Power/weight	19.7 hp/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)

Armament

Building a vehicle lighter than the T-92 required an innovative solution for the main armament. A gun firing kinetic energy penetrators to defeat modern tanks at reasonable range was too large for the XM551; gun weight was typically dependent on caliber and muzzle velocity. This was solved by arming the XM551 with a 152 mm. gun firing low velocity M81 HEAT rounds. The large caliber ensured a powerful shaped charge capable of killing tanks; the low velocity did not impact on HEAT effectiveness and kept gun weight down.

The gun was ideal for infantry support. The large, low velocity, gun could fire a large explosive shell or canister shot. In comparison, kinetic anti-tank guns over-penetrated soft targets, while smaller caliber weapons could not carry as great a payload.

Where the gun was not ideal was in medium- and long-range tank engagements. The low velocity produce extended flight times, and made it difficult to lead moving targets. In response, the gun was also designed to fire MGM-51 Shillelagh anti-tank missiles. The low launch velocity against longer-ranged targets was not an issue for the missile. However, the MGM-51 was considered a risky project.

A number of existing vehicles already mounted only ATGMs, or alternately recoilless rifles like the M50 Ontos, but these typically had limited utility in the infantry support role, or in the case of Ontos could not be reloaded from within the vehicle. The XM551 appeared to offer a superior balance between anti-tank and infantry support.

Mobility

Tactical mobility

The Sheridan was powered by a large 300-hp (224 kW) Detroit Diesel 6V53T diesel engine. The XM551 thus had an excellent power-to-weight ratio and mobility, able to run at speeds up to 45 mph, which at that time was unheard of for a tracked vehicle. However, the vehicle proved to be very noisy and unreliable under combat conditions.

The Sheridan could swim about a fifty-yard-wide river. Tanks in the Patton series (M46, M47, M48), as well as the M60 main battle tank could not perform these operations; they would have to crawl along the river bottoms using snorkels. Not by design, it was found that the swimming hardware acted to reduce the effectiveness of RPG rocket hits, as it was rarely used in Vietnam.

Strategic mobility

The Sheridan can be rigged for low-velocity airdrop from C-130 (42,000 lb max load) and C-141 aircraft (38,500 lb max load). Many films exist showing the Sheridan being pulled out of a C-130 Hercules transport by brake chutes and skidding to a stop. The Low Altitude Parachute Extraction System (LAPES) is a somewhat risky maneuver which allows accurate delivery onto a field when landing is not possible. The tank is strapped down to a special pallet which absorbs most of the landing impact. The crew does not ride in the tank during extraction, but parachutes from another plane. Upon landing, they go to their tank, release the lines, and drive it away.