America’s M551 Sheridan Light Tank: Where Did It Go?

 In Land, Air, Forces & Capabilities

Key point The tank has often for­got­ten but it served well in sev­er­al con­flicts. Here is how it man­aged to do its job despite many tech­ni­cal issues.

The M551 Sheridan light tank is large­ly remem­bered as a curios­i­ty, an inno­v­a­tive weapon system that proved an over­com­pli­cat­ed fail­ure in action. However, sev­er­al hun­dred Sheridans pro­vid­ed useful ser­vice in three wars, and left behind a small but notice­able gap in the force struc­ture since being with­drawn in the 1990s that the Army has strug­gled to fill. That’s because the Sheridan was easily trans­port­ed by air and could even be dropped by para­chute.

The Sheridan, named after the Union cav­al­ry gen­er­al of the Civil War, was intro­duced in a time when the U.S. Army was aban­don­ing the con­cept of light tanks in favor of the main battle tank: why bother with a tank that traded armor and arma­ment for speed when you could design one that bal­anced all three qual­i­ties?

This first appeared in 2017 and is being repost­ed due to reader inter­est.

But the Patton tanks of the era were still rel­a­tive­ly slow, with a max­i­mum speed of around thirty miles per hour, and the air­borne divi­sions lacked a tank light enough to be air­lift­ed to a drop zone. Experience in World War II had shown air­borne troops were vul­ner­a­ble to armored coun­ter­at­tacks after a para­chute drop, and could ben­e­fit from mobile anti­tank weapons to counter them. In fact, the U.S. Army had ear­li­er devel­oped the glider-borne M22 Locust tank, which were dropped in action with British para­troop­ers in the cross­ing of the Rhine. Furthermore, the Soviet Union began field­ing the amphibi­ous PT-76 light tank, and the U.S. Army felt com­pelled to match that capa­bil­i­ty.

In the end, the Army spent $1.3 bil­lion on the M551 Sheridan “Armored Reconaissance/Airborne Assault Vehicle” — which was def­i­nite­ly a tank, regard­less of the nomen­cla­ture — and pro­duced more than 1,562 of them between 1966 and 1970. Weighing in at fif­teen tons and capa­ble of rolling along at forty-three miles per hour, the Sheridan housed a crew of four in a thinly armored steel turret and alu­minum hull. A hinged flota­tion screen could be extend­ed from the top of the hull to permit river cross­ings at about three miles per hour.

The Sheridan’s most unusu­al fea­ture was its large M81 152-mil­lime­ter gun/missile system. Capable of firing enor­mous case­less shells more dev­as­tat­ing to infantry than the 90- or 105-mil­lime­ter shells fired by Patton tanks at the time, the short-bar­reled gun lacked the muzzle veloc­i­ty to be accu­rate at long ranges or pierce heavy tank armor through kinet­ic energy. Against armored threats at medium or long range, the gun could instead launch the newly devel­oped MGM-151 Shillelagh anti­tank mis­sile at tar­gets up to two or three kilo­me­ters dis­tant. It seemed like a bril­liant solu­tion to cram­ming heavy fire­pow­er into a light­weight vehi­cle. Each Sheridan car­ried nine Shillelaghs and twenty shells as stan­dard, as well as .50 cal­iber and 7.62-millimeter machine guns mount­ed on the turret and hull, respec­tive­ly.

The influ­en­tial Gen. Creighton Abrams was a believ­er in Sheridan, and once 152-mil­lime­ter ammu­ni­tion was avail­able, he dis­patched the first of a total of two hun­dred Sheridans to Vietnam in January 1969. The first served with the tank com­pa­nies of Third Squadron of the Fourth Cavalry Regiment and the Eleventh “Black Horse” Armored Cavalry Regiment (ACR), replac­ing heav­ier and slower M48 Patton tanks. Combat soon revealed some of the seri­ous defi­cien­cies of its uncon­ven­tion­al arma­ment.

The Sheridan’s light frame often leaped upwards into the air with each shot from its heavy main gun, poten­tial­ly caus­ing chest injuries to the com­man­der if he was peer­ing over the hatch, and also often scram­bling the mis­sile system’s elec­tron­ics. This, com­bined with a pon­der­ous par­tial­ly auto­mat­ed load­ing system, reduced the gun’s rate of fire to around two rounds per minute, in which time an expe­ri­enced M48 tank crew could poten­tial­ly fire a dozen or more ninety-mil­lime­ter shells. Furthermore, the case­less rounds had loose pro­pel­lant that posed a safety hazard under combat con­di­tions.

The Sheridan’s great­est short­com­ing in the field lay in sur­viv­abil­i­ty, as is usu­al­ly the case with light tanks. The M551’s armor was not intend­ed to repel any­thing heav­ier than a heavy machine gun round, so the tanks were highly vul­ner­a­ble to the land mines and rocket-pro­pelled grenades widely employed by Vietnamese adver­saries. Even worse, the Sheridan’s store of case­less 152-mil­lime­ter shells was prone to det­o­nat­ing cat­a­stroph­i­cal­ly when the vehi­cle was pen­e­trat­ed, which, com­pound­ed with the M551’s flam­ma­ble alu­minum hulls, led to a low crew sur­vival rate. The twelfth ACR lost three Sheridans to mines during a river cross­ing on one occa­sion, while five vehi­cles of the eleventh ACR were oblit­er­at­ed by Viet Cong rocket-pro­pelled grenades during the battle of Lam Son 719. Altogether, around one hun­dred M551s were lost in Vietnam, though some of these losses include break­downs in the field.

Though not deployed in Vietnam, the Shillelagh mis­sile, too, proved to be a dis­ap­point­ment. It was plagued by numer­ous tech­ni­cal faults, and its infrared sensor could not lock onto tar­gets closer than eight hun­dred meters. The Army adapt­ed the M81/Shillelagh com­bi­na­tion for the M60A2 “Starship” Patton tank, but found the system so trou­ble­some that these were phased out after only a decade in ser­vice.

Nonetheless, the Sheridan was one of those unusu­al weapons that was report­ed­ly well liked by the troops despite it being found want­i­ng by senior offi­cers. The 152-mil­lime­ter shells made a pow­er­ful impres­sion in a fire­fight, and M625 can­is­ter rounds loaded with thou­sands of flechettes dev­as­tat­ed Viet Cong infantry in engage­ments in Tay Ninh and Bien Hoa. The Sheridans’ low ground pres­sure also made them more capa­ble of nego­ti­at­ing dif­fi­cult ter­rain than the heavy Patton tanks they replaced, though they were not immune to bog­ging down under the swampy con­di­tions in Vietnam. Grunts also field-mod­i­fied Sheridans with gun shields for the pintle-mount­ed .50 cal­iber machine guns, and addi­tion­al belly armor for pro­tec­tion against mines.

The Sheridan’s ser­vice in Vietnam ended with the with­draw­al of the last Armored Cavalry Regiments in 1972, and the Army began phas­ing the com­pli­cat­ed vehi­cles out of the cav­al­ry units by the late 1970s. However, they remained in air­borne for­ma­tions for lack of a replace­ment, and were upgrad­ed the M551A1 TTS model with an effec­tive ther­mal sight for night combat.

In 1989, eight to ten Sheridans of the Third Battalion of the Seventy-Third Armored were used in the first and only para­chute drop of U.S. tanks into combat by C‑130 trans­ports onto Torrijos/Tocumen Airfield. Four more M551s had already been secret­ly air landed into Panama before the com­mence­ment of hos­til­i­ties, where they were used to breach the La Comandancia strong­point with their heavy shells. Those dropped via the Low-Velocity Air Drop para­chute system did run into dif­fi­cul­ties, when two of the M551s landed a swamp where they could not be recov­ered. The remain­ing tanks were quick­ly brought into action to sup­press Noriega’s dug-in troops in urban combat.

A year later, fifty-one Sheridans were scram­bled into Saudi Arabia along with the rest of the Eighty-Second Airborne Division as part of Operation Desert Shield, rein­forc­ing a “thin line in the sand” of American light infantry. It was hoped these would be suf­fi­cient to dis­suade Saddam Hussein from invad­ing Saudi Arabia with his mas­sive armored for­ma­tions. Fortunately, such a sce­nario did not come to pass, and the United States had months to deploy heavy armored for­ma­tions to the the­ater.

Had the Sheridans been forced into battle, they would have been easy meat for Iraqi tank guns in the open desert. Still, some tank-killing armor was better than none, and the light tanks did see action in the Gulf War, firing around a half-dozen mis­siles at Iraqi bunkers and destroy­ing a single Type 59 tank. These were the only Shillelaghs ever used in combat out of more than eighty-eight thou­sand built.

The Sheridan lasted sev­er­al more years in U.S. ser­vice as part of the Eighty-Second Airborne’s rapid reac­tion force, but was ulti­mate­ly retired in 1996. Many served on as dummy OPFOR tanks at the National Training Center, but these too were with­drawn by 2003 due to their inten­sive main­te­nance require­ments, many ending their ser­vice as arti­fi­cial reefs.

The Airborne branch never received a sat­is­fac­to­ry replace­ment for the Sheridan tank. The Army first unsuc­cess­ful­ly attempt­ed to fit the M551 with a light 105-mil­lime­ter gun, then exper­i­ment­ed with a sim­i­lar­ly armed XM8, before final­ly deploy­ing the wheeled Stryker M1128 Mobile Gun System — but only in sup­port of Stryker motor­ized infantry units.

Today, air­borne troops are prob­a­bly better served by modern anti­tank mis­siles such as the Javelin for anti­tank defense, rather than a light tank which vir­tu­al­ly any form of antiar­mor return fire could anni­hi­late. On the other hand, light tanks would offer an effec­tive infantry sup­port weapon for blast­ing away at enemy strong­points, while remain­ing immune to return fire from small arms and light sup­port weapons. Modern active pro­tec­tion sys­tems may also offer rel­a­tive­ly light­weight pro­tec­tion from lighter antiar­mor weapons.

The expe­ri­ence with the Sheridan shows that even with its trou­ble­some arma­ment and inad­e­quate armor, troops in the field appre­ci­at­ed the Sheridan for being there where it was needed. The M1 Abrams tank is just as fast the Sheridan, vastly better armed and armored, and can even be car­ried by C‑5 cargo planes — but its bridge-col­laps­ing sev­en­ty-ton weight still severe­ly limits where and how quick­ly it can be deployed, and impos­es a for­mi­da­ble logis­ti­cal burden in terms of fuel and main­te­nance. And you cer­tain­ly can’t toss one out of a plane.

National Interest source|articles

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