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|Place of origin||United States|
Operation Just Cause
Operation Desert Shield/Desert Storm
|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)|
|M81E1 Rifled 152 mm Gun/Launcher
9 MGM-51 Shillelagh missiles
|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)
|Suspension||Torsion bar suspension|
|348 mi (560 km)|
|Speed||Road: 70 km/h (43 mph)
Swimming: 5.8 km/h (3.6 mph)
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.
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).
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. While missiles fired out of guns would prove a disappointment, 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.
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 started to build an even lighter replacement mounting the same gun, resulting in the T-71 and T-92 test designs. Two prototypes of the 19 ton T-92 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 T-92 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. Heavy machine gun rounds could penetrate this hull, and a rocket propelled grenade 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.
Production started on July 29, 1966, and it entered service in June 1967 with 1st Battalion, 63rd Armor Regiment at Fort Riley. In the end 1,662 M551s were built between 1966 and November 2, 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. Some were experimentally fitted with conventional 76mm guns, but these never entered service.
Service history 
Vietnam War 
The US Army staff in Washington had been recommending since 1966 to the commander of US Forces in South Vietnam, General Westmoreland, that the Sheridan should be used in Vietnam. However, since the main gun ammunition was not available, he argued it was simply a $300,000 machine gun platform. By 1968, the new, or soon to be, US commander in South Vietnam, General Creighton Abrams had been notified that the 152mm shells were now available for the Sheridan. However, as General Abrams began to make preparations for the equipping of US Cavalry squadrons for the vehicle, the affected squadrons expressed their concerns that the new aluminum tanks were not only highly vulnerable to mines and anti-tank rocket fire, but they would not be as capable of "jungle busting" as the M48A3 medium tanks.
In late 1968, General Abrams met with Colonel George S. Patton IV - the son of World War II General Patton - who was the regimental commander of the 11th Armored Cavalry Regiment (11th ACR Blackhorse), the only full regiment of cavalry in Vietnam. When General Abrams mentioned the cavalry's concerns over the new vehicle, Patton recommended that the Sheridans be combat tested by a divisional cavalry squadron and a squadron from his own regiment; both of which had completely different missions.
First deployment 
The first Sheridans to arrive in Vietnam (January 1969) were accompanied by their factory representatives, instructors, and evaluators as the new vehicles were issued to the 3rd Squadron, 4th Armored Cavalry, and the 1st Squadron, 11th ACR. By the end of 1970 there were more than 200 Sheridans in Vietnam, and they stayed in the field until the last US Armored Cavalry unit, the 1st Squadron, 1st Armored Cavalry prepared for re-deployment back to the United States on 10 April 1972. At the end of its combat debut in 1972, the Sheridan saw extensive action in the Vietnam War, being assigned to nearly all armored cavalry squadrons involved in that conflict. In 1969, armored cavalry units (minus the 11th ACR which retained its M48 Patton tank companies) began replacing their M48 Patton tanks, which in turn were normally transferred to the South Vietnamese military. Like the M50 Ontos anti-tank vehicle, the battle reports from the troops were sometimes glowing, while the reports higher up the chain of command were often negative. This was largely due to the high casualty rate of both Sheridans and their crews as mines and rocket-propelled grenades (RPGs) that would only damage an M48 Patton tank, would destroy the Sheridan and kill or wound most, if not all, of its crew.
A 1969 evaluation of the vehicles found that the M551 was employed in reconnaissance, night patrol and road clearing, accumulating 39,455 road miles and 520 combat missions, with a ready rate of 81.3 percent. Despite vulnerability to rockets and mines, it was judged worthy of applying modifications and equipping all cavalry squadrons with the Sheridan.
First combat/first losses 
In addition to the problems presented by aluminum construction, the Sheridan had a defect that no other common armored vehicle possessed; it fired caseless 152mm main gun rounds. These rounds were "fixed" meaning that unlike the artillery, the warhead was factory attached to the propellant, and if the warhead separated from the propellant during loading, which was not uncommon, the crewmen were instructed not to load the round. Sometimes these unspent propellant charges remained on the turret floor due to the emergencies at the time, and in either case, all of the remaining serviceable 152mm shells still remained caseless, albeit attached to their warheads, and sleeved into a re-usable white nine-ply nylon bag which was form-fitted to hold the propellant portion of the shell. The white/silver-colored bag had a strap attached to the bottom which the loader would grab and pull off prior to gently inserting the shell into the breech. Once a mine or RPG type weapon created the spark, smoke and fire became imminent, and it became a matter of Standing Operating Procedure to abandon the tank immediately. On 15 February 1969, just one month after the Sheridan's arrival in South Vietnam, an M551 from the 3rd Squadron 4th Armored Cavalry detonated a 25 pound pressure-triggered land mine which ruptured its hull, and ignited the 152mm shells, which resulted in a secondary explosion that destroyed the tank. In late 1969, nine Sheridans from the 4th Squadron 12th Armored Cavalry were fording a river near the DMZ, when three of the M551s detonated mines, completely destroying them. On March 1971, five Sheridans from the 11th ACR were lost in one day to RPG fire, all five vehicles burst into flames and were totally destroyed. It became a common scene to observe melted Sheridan hulls with their sunken steel turrets sitting at odd angles with their gun tubes pointing towards the sky in various parts of the country, either awaiting final disposition, or simply forgotten.
However, the Sheridan did not get stuck in the mud as often as the 52 ton M48 Patton tank did, nor did it throw its track off as often as the Patton. This alone was enough to win the tank crews' favor. The light weight and high mobility proved their worth, and the gun proved an effective anti-personnel weapon when used with either the M657 HE shell or the M625 canister round, which used thousands of flechettes as projectiles.
Although an average M48 Patton crew could fire as many as seventeen 90mm shells during a "mad minute" (sixty seconds with all guns firing-on command), the Sheridan was known to put out only two 152mm shells during the same time frame. While the M48's 90mm cannon fired fixed metallic cased rounds, the 152mm was caseless. The caseless rounds needed air vents to clear the gun tube and breech prior to loading another round, while the M48 breech block opened as the used shell was ejected and closed as the new shell was shoved in. The faster the loader, the faster the Patton's gun could be fired. For the Sheridan, the loader had to wait for the mechanism. After firing, the loader would have to wait, as the breech slowly opened rearward then turned downward. After another instrument indicated that all turret systems were still operational, the loader would gently push the 152mm fixed round into the breech and watch the breech block slowly rotate upward, then forward into the breech, then again, wait for the lights.
The Sheridan was much appreciated by the infantry who were desperate for direct-fire support, which generally served in combination with ACAVs (M113s) as armored cavalry units consisted of both M113s and M551s as part of their TO&E. Armor units consisted solely of tanks (minus headquarters company) and Mechanized Infantry units consisted solely of M113s. In this role the real problem with the Sheridan was its limited ammunition load. Normally only 20 rounds and 8 missiles although as the M551s in Vietnam service were not equipped with missiles or their guidance equipment, this increased the basic load of conventional rounds. Sheridan losses were heavy during normal operations, largely due to land mines and anti-armor weapons, but were especially heavy after President Richard Nixon ordered US forces into Cambodia on May 1, 1970 in which, among other cavalry squadrons, the 11th Armored Cavalry Regiment (Blackhorse) was thrown into the fight. The second heaviest losses were during the U.S. Army's final offensive of the war, operation Dewey Canyon II (Dewey Canyon II was an operation in support of the ARVN Lam San 719 Operation, in which the code 719 meant the year 1971 along Route 9), when the cavalry's remaining Sheridan Squadrons met near disaster on the Lao border during the early months of 1971, in particular the 1st Squadron of the 1st Cavalry Regt.
Combat field modifications 
A common field-modification was to mount a large steel shield, known as an "ACAV set" (Armored Cavalry Assault Vehicle), around the commander's 50-cal. (12.7 mm) gun, allowing it to be fired with some level of protection. The driver has an unusual rotating hatch which has vision blocks when rotated forward. Included with the set was an extra layer of steel belly armor which was bolted onto the vehicle's bottom, although only covering from the front to half way to the end, possibly due to weight reasons.
A standard modification made during the mid-70s was the addition of the "Cereal Bowl" commander's cupola. This mod came about due to the broken rib effect that occurred when the Sheridan fired conventional rounds, the recoil would pitch the TC against the armor plating resulting in cracked ribs.
Post-Vietnam service 
The Army began to phase out the Sheridan in 1978, although at the time there was no real replacement. Nevertheless the 82nd Airborne were able to keep them on until 1996. The Sheridan was the only "rapidly" air-deployable tank in the inventory. Their units were later upgraded to the M551A1 model, including a thermal sighting system for the commander and gunner.
In the early 1980s the M551A1 was fitted with a visual modification kit to resemble Warsaw Pact vehicles at the National Training Center at Fort Irwin, California. These modified vehicles no longer fulfill that role, having been retired at the end of 2003 and subsequently scrapped or made available as "hard targets" or, in a few cases, as museum pieces. Many were dumped to create artificial reefs.
During Operations Desert Shield and Desert Storm, 51 Sheridans were deployed by the 82nd Airborne Division, and were among the first tanks to be sent. Although photos that were published at the time showed rows of Sheridans ready to defend against Iraqi tanks, they would have a limited effect against the Soviet-designed T-72s which comprised the bulk of the Iraqi Republican Guard. Their role was limited to reconnaissance due to their age and light armor. It is likely that six or fewer Shillelagh missiles were fired at Iraqi bunkers; this appears to be the only occasion in which Shillelagh missiles were fired in a combat environment, from the inventory of the aforementioned 88,000 missiles produced.
The Sheridan's only air drop in combat occurred during the United States invasion of Panama (Operation Just Cause) in 1989, when fourteen M551s were deployed: four were transported by C-5 Galaxys and ten were dropped by air, though two Sheridans were destroyed upon landing. The four M551s transported by the C-5 were secretly deployed to Panama in November 1989, where they were attached to TF Bayonet (193rd SIB), and attached down further to TF Gator. These Sheridans took part in the attack on the Commandancia, initially supported by fire from Quarry Heights, and later displacing forward into the city. As part of Team Armor, these Sheridans later provided support to JSOC elements as they secured high value targets throughout Panama City. The remaining eight Sheridans were delivered to Torrijos-Tocumen Airport some hours after H-hour by Low-Velocity Airdrop (LVAD) technique from C-130 transports. The Sheridans' performance received mixed reviews. They were lauded by their operators and some commanders as providing firepower in needed situations to destroy hard targets. However, the Sheridans' employment of only HEAT rounds limited their effectiveness against reinforced concrete construction.
Several attempts to improve or replace the Sheridan have been made over the years since it was introduced. Several experimental versions of the Sheridan mounting a new turret carrying the NATO-standard 105mm gun were made, but the resulting recoil was so great as to make the vehicle almost unusable. Several possible replacements for the M551 were tested as a part of the XM8 Armored Gun System effort of the 1980s, but none of these entered service. The M1128 Mobile Gun System variation of the Stryker armoured fighting vehicle is the nearest analogue in the modern US Army force structure.
Australian trials 
The Sheridan was never used in Australian Army service, however it was trialled by the Royal Australian Armoured Corps beginning 1967 and appeared in several Manual of Land Warfare pamphlets (the equivalent of US military FMs) of late 1960s vintage. It was to have been issued to cavalry regiments. It was not purchased as there were concerns over the safety of the combustible case, the Sheridan did not meet Australian Army requirements.
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 destroying tanks; velocity has no impact on shaped charge penetration, and the short gun would reduce weight. However a low velocity reduces accuracy.
The gun was ideal for infantry support. The large, low velocity gun could fire a large explosive shell or canister shot. In comparison, high-velocity anti-tank guns over-penetrated soft targets, while smaller caliber weapons could not carry as great a payload.
The M81E1 was not ideal in medium- and long-range tank engagements lue to the fact that it fired it's projectiles at low velovities. The low velocity produced 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.
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 is about as fast, as soviet BT-5 from 1932. 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, but 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), C-141 aircraft (38,500 lb max load), and the C-5. 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, and the practice was stopped after a fatal crash was caused by one such failed operation. 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.
- XM551/M551 - The M551 was the basic production model, beginning production in 1967. The XM551 had been a limited run pre-production model produced in 1965.
- "Two Box" M551 - With the obvious shortcomings of the Shillelagh missile, all but two of the guidance and fire control components of the missile system were removed (the power supply and rate sensor were retained. These were needed for stabilized turret operation.). The resulting additional space was filled with two separate boxes, one for 7.62x51mm NATO ammunition (coaxial machine gun), and one for 12.7×99mm BMG (.50 cal) ammunition, and the missile stowage was redesigned to accept conventional rounds.
- M551A1 - Upgraded M551 with AN/VVG-1 laser rangefinder.
- M551A1 TTS - Tank Thermal Sight, fitted with the AN/VSG-2B thermal sight unit, similar to the unit used on the M60A3 MBT. This later became standard to all M551A1s.
- M551 NTC - National Training Center. Using M551 hulls, the NTC created a number of mock vehicles for training exercises resembling common Soviet/Warsaw pact types. They were also known as 'vismods', short for VISually MODified. They have since been retired in favor of similarly converted M113s and M1 Abrams.
- Camp Joseph T. Robinson, Arkansas National Guard Museum
- Carolinas Aviation Museum
- First Division Museum at Cantigny, an example of the M551A1 is on display in the tank park outside the First Division Museum at Cantigny in Wheaton, Illinois
- Fort Polk, a "T-72" VISMOD is on display on Mississippi Avenue near the headquarters of 1st Battalion, 509th Parachute Infantry Regiment.
- Fort Richardson, Camp Denali on static display
- Hardwood Range, an example is on static display at the visitor center at Hardwood Air-to-Ground Weapons Range in Finley, Wisconsin.
- The Ontario Regiment Museum, in Oshawa has two Sheridan tanks on display. One is in not operating condition and the other restored with its original 11th Cavalry markings.
See also 
- American Society of Military History and Museums, South El Monte, Ca. 2 non running exhibits and a running, ex-vismod unit. 
- Fort Knox, KY. Wilson Road entrance
- List of "M" series military vehicles
- List of Vehicles of the U.S. Armed Forces
- M60A2 used a similar 152mm gun which also fired the Shillelagh missiles.
- M3 Bradley a modern AFV that sometimes covers the same roles.
- Tom Clancy (1994). Armored cav: a guided tour of an armored cavalry regiment. Penguin. ISBN 978-1-101-00226-1. Retrieved 20 January 2011.
- Hunnicutt, R. P. "Sheridan: A History of the American Light Tank." Volume 2, 1995, Presidio Press; ISBN 0-89141-570-X.
- 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.
- Starry p.143
- Starry p. 143
- Starry p. 144
- Starry p. 143-145
- Washer evaluation 1969
- Dunstan, Simon. Vietnam Tracks-Armor In Battle. 1982 edition; Osprey Publishing Limited. ISBN 0-89141-171-2.
- Starry (1989), p.144-145.
- Nolan (1986), p.
- Stanton (2003), p.277.
- Doyle, p. 4
- Doyle, p. 44, 46
- Doyle, p. 55
- Paul D Handel, Sheridan Tropical Trials in Australia
- "Sheridan Tank Weapons Demonstration (1969)". Special Operations History Foundation. Retrieved 30 April 2013.
- Hunnicutt, History of MBT, p. 149, 150, 174
- Gary's Combat Vehicle Reference Guide
- "First Division Museum at Cantigny". Firstdivisionmuseum.org. Retrieved 2012-10-25.
- "Wisconsin Air National Guard: Hardwood Air-toGround Weapons Range, Finley, Wisconsin". Volkfield.ang.af.mil. Retrieved 2012-10-25.
- Nolan, Keith W. Into Laos, Operation Lam Son 719 and Dewey Canyon II. Presidio Press: 1986.
- Sorley, Lewis. Thunderbolt: General Creighton Abrams and the Army of His Time. 1992. ISBN 0-671-70115-0.
- Stanton, Shelby, L. Vietnam Order of Battle. (1983–2003) ISBN 100883657090.
- Starry, Donn, GEN. "Mounted Combat In Vietnam." Department of the Army publication, 1989.
- Zaloga, Steven J. M551 Sheridan, US Airmobile Tanks 1941–2001. Osprey Publishing Ltd, 2009.
- Doyle, David. M551 Sheridan. (2008) Squadron Signal Publications. ISBN 978-0-89747-582-2.
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