M551 Sheridan
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M551 Sheridan
M551 Sheridan AR/AAV
Type Amphibious light tank[1]
Place of origin United States
Service history
In service 1969–1997
Vietnam War
Operation Just Cause
Persian Gulf War
Wars
Operation Desert Shield
Operation Desert Storm
Production history
General Motors, Cadillac Motor
Designer
Company
Designed 1959–1965
Cadillac Motor Company, Cleveland,
Manufacturer
Ohio[2]
Produced 1966–1970
No. built 1,662
Specifications (M551A1 Sheridan (TTS))
Mass 33,600 lb (15,240 kg)[3]
Length 248 in (6.3 m)[3]
Width 110 in (2.8 m)[3]
Height 116 in (2.9 m)[3]
Crew 4 (Commander, gunner, loader, driver)
Elevation +19.5° / -8°[3]
7039 aluminium alloy hull with Rolled
Armor
homogeneous steel turret[3]
M81E1 Rifled 152 mm Gun/Launcher
Main
20 rounds
armament 9 MGM-51 Shillelagh missiles[3]
1× .50 cal (12.7 mm) M2 Browning
machine gun with 1,000 rounds
Secondary
1× .30 cal (7.62 mm) M73/M219 co-
armament axial machine gun (later replaced by the
M240C) with 3,000 rounds
Detroit Diesel (General Motors) 6V53T,
Engine 6 cylinder, turbocharged diesel[3]
300 hp (220 kW) at 2800 rpm[3]
Power/weight 17.9 hp/ST (14.7 kW/t)[3]
Transmission XTG-250-1A
Suspension flat track, Torsion bar suspension
Ground clearance 19 in (48.3 cm)[3]
Operational
350 mi (560 km)
range
Road: 43 mph (69 km/h)
Maximum speed
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 General Philip Sheridan, of American
Civil War 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 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, Military Assistance Command Vietnam, at the
time, the M551 was rushed into combat service to South Vietnam in January 1969. Later that
year, M551s were deployed to units in Europe and South Korea.[4] The Sheridan saw extensive
combat in the Vietnam War, 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 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 and the Gulf War.
The Army sought to replace the Sheridan with the M8 Armored Gun System, 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 to fulfill a similar
requirement, but this is being retired in 2022. The Army current light tank acquisition program is
Mobile Protected Firepower.
A large bulk of Sheridans were retained in service at the National Training Center (NTC) at Fort
Irwin, California and as Armor Officer Basic training at Armor Training Center, then located at
Fort Knox, Kentucky. They worked as simulated Soviet armored opposition force (OPFOR) 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.[5]
Development
XM551, pilot #1
XM551, pilot #12
A 76 mm gun is installed on an M551 at Rock Island Arsenal
In the immediate post-World War II era, the U.S. 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.[6] 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 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.[7]
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 self-propelled gun.[8][9] 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 and Allis-Chalmers
Manufacturing Company. Mockups of both proposals were evaluated in May 1960.[8]
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.[10] 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.[11]
A test bed underwent operations at Cleveland Tank Plant in December 1961.[9] 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.[9]
By 1962, the Army realized that the MGM-51 Shillelagh 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
and TOW. The Army ultimately decided to arm the Sheridan with just 152 mm conventional
rounds until the Shillelagh was ready.[12]
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.[13]
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.[14] The third generation of pilots, starting with
pilot 7, eliminated the water jet propulsion.[15] Pilots 9–11 were delivered in 1964,[16] and pilot 12
was delivered in February 1965.[17]
In the 1960s the Army was also developing the MBT-70 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 (MBT) doctrine; a single tank filling all
combat roles.[18][19] 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.[20] 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."[21]
