The
M551 Sheridan was a
light tankA light tank is a tank variant initially designed for rapid movement, and now primarily employed in low-intensity conflict. Early light tanks were generally armed and armored similar to an armored car, but used tracks in order to provide better cross-country mobility.The light tank was a major...
developed by the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
and named after
Civil WarThe American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...
General
Philip SheridanPhilip Henry Sheridan 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 association with Lt. Gen. Ulysses S...
. 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 ShillelaghThe Ford MGM-51 Shillelagh was an American anti-tank guided missile designed to be launched from a conventional gun . It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an...
guided anti-tank missile.
It entered U.S. Army service in 1967. Under the urging of General
Creighton AbramsCreighton Williams Abrams Jr. was a general in the United States Army who commanded military operations in the Vietnam War from 1968–72 which saw U.S. troop strength in Vietnam fall from a peak of 543,000 to 49,000. He served as Chief of Staff of the United States Army from 1972 until shortly...
, the US Commander of Military Forces in
VietnamThe Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
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
VietnamVietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
, and limited service in Operation Just Cause (
PanamaPanama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
), and the
Gulf WarThe Persian Gulf War , commonly referred to as simply the Gulf War, was a war waged by a U.N.-authorized coalition force from 34 nations led by the United States, against Iraq in response to Iraq's invasion and annexation of Kuwait.The war is also known under other names, such as the First Gulf...
(
KuwaitThe State of Kuwait is a sovereign Arab state situated in the north-east of the Arabian Peninsula in Western Asia. It is bordered by Saudi Arabia to the south at Khafji, and Iraq to the north at Basra. It lies on the north-western shore of the Persian Gulf. The name Kuwait is derived from the...
).
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 (
M103The M103 heavy tank served the United States Army and the US Marines during the Cold War. Until the development of the M1A1 in the mid 1980s, it was the heaviest and most heavily armed tank in US service...
) heavy tank battalion, and the fielding of the new
M60The 105 mm Gun Full Tracked Combat Tank, M60, also known unofficially as the M60 Patton, is a first-generation main battle tank introduced in December 1960. It was widely used by the U.S. and its Cold War allies, especially those in NATO, and remains in service throughout the world today...
series tank, the U.S. Army had adopted a
main battle tankA main battle tank , also known as a battle tank or universal tank, is a tank that fills the heavy direct fire role of many modern armies. They were originally conceived to replace the light, medium, heavy and super-heavy tanks. Development was spurred onwards in the Cold War with the development...
(MBT) doctrine; a single tank filling all combat roles. The U.S. Army still retained the
M41 Walker BulldogThe M41 Walker Bulldog was a U.S. light tank developed to replace the M24 Chaffee. It was named for General Walton Walker who died in a jeep accident in Korea...
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 TOWThe BGM-71 TOW is an anti-tank missile. "BGM" is a weapon classification that stands for "Multiple Environment , Surface-Attack , Missile ". "TOW" is an acronym that stands for "Tube-launched, Optically-tracked, Wire command data link, guided missile"...
would give infantry fighting vehicles like the
M2 BradleyThe Bradley Fighting Vehicle is an American fighting vehicle platform manufactured by BAE Systems Land and Armaments, formerly United Defense.As with other infantry fighting vehicles, the Bradley is designed to transport infantry with armor protection while providing covering fire to suppress enemy...
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.
Development
In the immediate post-
World War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
era the
US ArmyThe United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
introduced the
M41 Walker BulldogThe M41 Walker Bulldog was a U.S. light tank developed to replace the M24 Chaffee. It was named for General Walton Walker who died in a jeep accident in Korea...
into service to fill the role of a
light tankA light tank is a tank variant initially designed for rapid movement, and now primarily employed in low-intensity conflict. Early light tanks were generally armed and armored similar to an armored car, but used tracks in order to provide better cross-country mobility.The light tank was a major...
. 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-92T92 Light Tank was an innovative American light tank developed in 1950s by Aircraft Armaments. At 18.5 tonnes, 5m length, it was designed as an airborne/airdropped replacement for the 5 tonnes heavier M41 Walker Bulldog. The T92 was never accepted into service.The main gun was a conventional...
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-76The PT-76 is a Soviet amphibious light tank which 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 India, Iraq, North Korea and North Vietnam. Overall,...
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. Unfortunately, the armor was thin enough that it could be penetrated even by heavy machine gun rounds and when hit by a rocket propelled grenade the vehicle would "brew up" due to the main gun propellant being stored in cardboard tubes. Like the M113, it was also highly vulnerable to mines.
Swimming capability was provided by a flotation screen, similar to that used by the World War II, amphibious
DD TankDD tanks , were a type of amphibious swimming tank developed by the British during the Second World War...
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 this useless, and the driver instead had to take steering directions from the vehicle commander. The
M2 BradleyThe Bradley Fighting Vehicle is an American fighting vehicle platform manufactured by BAE Systems Land and Armaments, formerly United Defense.As with other infantry fighting vehicles, the Bradley is designed to transport infantry with armor protection while providing covering fire to suppress enemy...
would adopt a similar solution, but dropped it with upgraded armor.
In the
Vietnam WarThe Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
, firing the gun would often adversely affect 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 would end up almost never being fired in anger, despite a production run of 88,000 units.
Production
Production started on July 29, 1966, and it entered service in June 1967 with 1st Battalion, 63rd Armor Regiment at
Fort RileyFort Riley is a United States Army installation located in Northeast Kansas, on the Kansas River, between Junction City and Manhattan. The Fort Riley Military Reservation covers 100,656 acres in Geary and Riley counties and includes two census-designated places: Fort Riley North and Fort...
. 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.
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
M48A3The M48 Patton is a medium tank that was designed in the United States. It was the third and final tank to be officially named after General George S. Patton, commander of the U.S. Third Army during World War II and one of the earliest American advocates for the use of tanks in battle It was a...
medium tanks.
In late 1968, General Abrams met with
ColonelColonel , abbreviated Col or COL, is a military rank of a senior commissioned officer. It or a corresponding rank exists in most armies and in many air forces; the naval equivalent rank is generally "Captain". It is also used in some police forces and other paramilitary rank structures...
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
divisionalA division is a large military unit or formation usually consisting of between 10,000 and 20,000 soldiers. In most armies, a division is composed of several regiments or brigades, and in turn several divisions typically make up a corps...
cavalry squadron and a squadron from his own
regimentA regiment is a major tactical military unit, composed of variable numbers of batteries, squadrons or battalions, commanded by a colonel or lieutenant colonel...
; both of which had completely different missions.
First Deployment
The first Sheridans to arrive in country (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 would be more than 200 Sheridans in Vietnam, and they would stay 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 would see extensive action in the
Vietnam WarThe Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
, being assigned to nearly all armored cavalry squadrons in country. In 1969, armored cavalry units (minus the 11th ACR which retained its
M48 PattonThe M48 Patton is a medium tank that was designed in the United States. It was the third and final tank to be officially named after General George S. Patton, commander of the U.S. Third Army during World War II and one of the earliest American advocates for the use of tanks in battle It was a...
tank companies) began replacing their M48 Patton tanks, which in turn were normally transferred to the South Vietnamese military. Like the
M50 OntosThe Ontos, officially the Rifle, Multiple 106 mm, Self-propelled, M50, was an American light armored tracked anti-tank vehicle developed in the 1950s...
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 one glaring defect that no other 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 ProcedureThe terms standard operating procedure or SOP, is used in a variety of different contexts, such as healthcare, education, industry or the military. The military uses the term Standing Operating Procedure- rather than Standard- because an SOP refers to an organization's unique procedures, which are...
to abandon the tank immediately. On 15 February 1969, just one month after the Sheridan's arrival to South Vietnam, an M551 from the 3rd Squadron 4th Armored Cavalry detonated a 25 pound pressure-triggered
land mineA land mine is usually a weight-triggered explosive device which is intended to damage a target—either human or inanimate—by means of a blast and/or fragment impact....
which ruptured its hull, and ignited the 152mm shells, which resulted in a secondary explosion, destroying 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. In 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 PattonThe M48 Patton is a medium tank that was designed in the United States. It was the third and final tank to be officially named after General George S. Patton, commander of the U.S. Third Army during World War II and one of the earliest American advocates for the use of tanks in battle It was a...
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
HEAn explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...
shell or the M625
canister roundCanister shot is a kind of anti-personnel ammunition used in cannons. It was similar to the naval grapeshot, but fired smaller and more numerous balls, which did not have to punch through the wooden hull of a ship...
, which used thousands of
flechetteA flechette is a pointed steel projectile, with a vaned tail for stable flight. The name comes from French , "little arrow" or "dart", and sometimes retains the acute accent in English: fléchette.-Bulk and artillery use:...
s as projectiles.
Although an average M48 Patton crew could fire as many as seventeen 90mm shells during a "
mad minuteMad minute was a pre-World War I term used by British riflemen during training to describe scoring 15 hits onto a 12" round target at 300 yd within one minute using a bolt-action rifle . It was not uncommon during the First World War for riflemen to greatly exceed this score...
" (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
M113The M113 is a fully tracked armored personnel carrier that has formed the backbone of the United States Army's mechanized infantry units from the time of its first fielding in Vietnam in April 1962. The M113 was the most widely used armored vehicle of the U.S...
s 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 NixonRichard Milhous Nixon was the 37th President of the United States, serving from 1969 to 1974. The only president to resign the office, Nixon had previously served as a US representative and senator from California and as the 36th Vice President of the United States from 1953 to 1961 under...
ordered US forces into
CambodiaCambodia , officially known as the Kingdom of Cambodia, is a country located in the southern portion of the Indochina Peninsula in Southeast Asia...
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 AirborneThe 82nd Airborne Division is an active airborne infantry division of the United States Army specializing in parachute landing operations. Based at Fort Bragg, North Carolina, the 82nd Airborne Division is the primary fighting arm of the XVIII Airborne Corps....
were able to keep them on until 1996. The Sheridan was the only 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.
The Sheridan's only air drop in combat occurred during the
United States invasion of PanamaThe United States Invasion of Panama, code-named Operation Just Cause, was the invasion of Panama by the United States in December 1989. It occurred during the administration of U.S. President George H. W...
(Operation Just Cause) in 1989, when fourteen M551s were deployed: four were transported by
C-5 GalaxyThe Lockheed C-5 Galaxy is a large military transport aircraft built by Lockheed. It provides the United States Air Force with a heavy intercontinental-range strategic airlift capability, one that can carry outsize and oversize cargos, including all air-certifiable cargo. The Galaxy has many...
s 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 AirportTocumen International Airport is an international airport located from Panama City, Panama. In 2006, it underwent a major expansion and renovation program in order to modernize and improve its facilities...
some hours after H-hour by Low-Velocity Airdrop (
LVADMilitary free-fall form of insertion.PurposeDelivering personnel, equipment, or supplies.HALOHigh-Altitude/Low-OpeningHAHOHigh-Altitude/High-OpeningOrigins...
) 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.
In the early 1980s the M551A1 was fitted with a visual modification kit to resemble
Warsaw PactThe Warsaw Treaty Organization of Friendship, Cooperation, and Mutual Assistance , or more commonly referred to as the Warsaw Pact, was a mutual defense treaty subscribed to by eight communist states in Eastern Europe...
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 reefAn artificial reef is a human-made underwater structure, typically built to promote marine life in areas with a generally featureless bottom, control erosion, block ship passage, or improve surfing....
s.
During Operations Desert Shield and Desert Storm, 51 Sheridans were deployed by the
82nd Airborne DivisionThe 82nd Airborne Division is an active airborne infantry division of the United States Army specializing in parachute landing operations. Based at Fort Bragg, North Carolina, the 82nd Airborne Division is the primary fighting arm of the XVIII Airborne Corps....
, 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 not have been very effective against the Russian-designed
T-72The T-72 is a Soviet-designed main battle tank that entered production in 1970. It is developed directly from Obyekt-172, and shares parallel features with the T-64A...
s 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.
Several attempts to improve or replace the Sheridan have been made over the years since it was introduced, but none have yet been successful. 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 SystemThe United Defense M8 Armored Gun System was a light tank that was intended to replace the M551 Sheridan in the 82nd Airborne Division, as well as being expected to replace TOW-equipped Humvees in the 2d Armored Cavalry Regiment . The M8 project was eventually canceled...
effort of the 1980s, but none of these entered service. The new Stryker Mobile Gun System, which has been controversial in its own right, is in line to fill a similar role in the current US Army force structure.
Australian trials
The Sheridan was never used in
Australian ArmyThe Australian Army is Australia's military land force. It is part of the Australian Defence Force along with the Royal Australian Navy and the Royal Australian Air Force. While the Chief of Defence commands the Australian Defence Force , the Army is commanded by the Chief of 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.
Armament
Building a vehicle lighter than the T-92 required an innovative solution for the main armament. A gun firing
kinetic energy penetratorA kinetic energy penetrator is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target....
s to defeat modern tanks at reasonable range was too large for the XM551; gun weight was typically dependent on caliber and
muzzle velocityMuzzle velocity is the speed a projectile has at the moment it leaves the muzzle of the gun. Muzzle velocities range from approximately to in black powder muskets , to more than in modern rifles with high-performance cartridges such as the .220 Swift and .204 Ruger, all the way to for tank guns...
. 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 chargeA shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, to initiate nuclear weapons, to penetrate armor, and in the oil and gas industry...
capable of destroying tanks; the low velocity had no apparent impact on HEAT effectiveness and kept overall 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 ShillelaghThe Ford MGM-51 Shillelagh was an American anti-tank guided missile designed to be launched from a conventional gun . It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an...
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 rifleA recoilless rifle or recoilless gun is a lightweight weapon that fires a heavier projectile than would be practical to fire from a recoiling weapon of comparable size. Technically, only devices that use a rifled barrel are recoilless rifles. Smoothbore variants are recoilless guns...
s 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 ratioPower-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
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
PattonPatton tank may refer to:*M46 Patton, a tank model operational during the Korean War*M47 Patton, a tank model in service from 1952 through 1959 with the U.S. Army, and through the mid 1990s in foreign service...
series (M46, M47, M48), as well as the M60
main battle tankA main battle tank , also known as a battle tank or universal tank, is a tank that fills the heavy direct fire role of many modern armies. They were originally conceived to replace the light, medium, heavy and super-heavy tanks. Development was spurred onwards in the Cold War with the development...
could not perform these operations; they would have to crawl along the river bottoms using
snorkelsA vehicle snorkel is the land-based equivalent of the submarine snorkel which allowed submarines to use diesel engines while submerged. Snorkels, when used by vehicles with air-breathing internal combustion engines, allow limited deep fording capability for river crossing or amphibious landing...
. 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 SystemLow Altitude Parachute Extraction System is a tactical military airlift delivery method where a fixed wing cargo aircraft can deposit supplies when landing is not an option in an area that is too small to accurately parachute supplies from a high altitude.This method was developed by the US...
(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.
Variants
- 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 'vismod
The term vismod is an abbreviation of the term visually modified, and is used by the elements of the United States Department of Defense to refer to any vehicle, aircraft, or other object that has been altered to simulate equipment used by an enemy for training purposes, often by specialized units...
s', short for VISually MODified. They have since been retired in favor of similarly converted M113The M113 is a fully tracked armored personnel carrier that has formed the backbone of the United States Army's mechanized infantry units from the time of its first fielding in Vietnam in April 1962. The M113 was the most widely used armored vehicle of the U.S...
s and M1 AbramsThe M1 Abrams is a third-generation main battle tank produced in the United States. It is named after General Creighton Abrams, former Army Chief of Staff and Commander of US military forces in Vietnam from 1968 to 1972. The M1 is a well armed, heavily armored, and highly mobile tank designed for...
.
Survivors
- Carolinas Aviation Museum
The Carolinas Aviation Museum is an aviation museum on the grounds of Charlotte/Douglas International Airport in Charlotte, North Carolina. The mission of the Museum is to educate the public about the importance of aviation to our society and inspire the next generation to excel academically in the...
- Fort Richardson, Camp Denali on static display
- Fort Polk
Fort Polk is a United States Army installation located in Vernon Parish, approximately 7 miles east of Leesville, Louisiana and 20 miles north of DeRidder, Louisiana....
, a "T-72" VISMOD is on display on Mississippi Avenue near the headquarters of 1st Battalion, 509th Parachute Infantry Regiment.
See also
- 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.
- American Society of Military History and Museums, South El Monte, Ca. 2 non running exhibits and a running, ex-vismod unit. http://tankland.com/
External links