M551 Sheridan The M551 Sheridan was an " Armored Reconnaissance / Airborne Assault Vehicle " developed by the United States , named after Civil War General Philip Sheridan . It was designed to have both an air drop and swimming capability . 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 . Production began in 1966 , entering the US Army inventory 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 Operation Desert Shield / Desert Storm ( 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 US Army had adopted the new doctrine of Main Battle Tank ( MBT ) ; a single tank filling all combat roles . The US 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 M - 2 Bradley the firepower to destroy armored targets along with the ability to carry troops . Though other light tanks were evaluated , the wheeled Stryker Mobile Gun System currently provides an armored 105mm gun platform that is lighter than a battle tank for fire support . History . Development . In the immediate post - World War II era the US Army introduced the M41 Walker Bulldog into service to fill their light tank role . The lifetime of this system was fairly short ; the 25 - ton tank 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 amphibious , and soon there were demands that any U . S . light tank 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 an RPG the vehicle would " brew up " , caused by the main gun propellant being stored in cardboard tubes , thus trapping the crew in an inferno they were unlikely to escape . Like the M113 , it was also highly vulnerable to mines , which were better sustained by heavier tanks like the M48 , though like all tanks it was also subject to considerable damage . Swimming capability was provided by a flotation screen , similar to that used by the World War 2 , amphibious DD Tank 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 even 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 basically useless , and the driver instead had to take steering directions from the vehicle commander . The M2 Bradley would adopt a similar solution , but dropped it with upgraded armour . In the Vietnam War , firing the gun would often adversely affect the delicate electronics , which were at the early stages of transitioning to solid state , so the missile and guidance system was omitted from vehicles deployed to Vietnam . Indeed , this missile would end up almost never being fired in anger , despite the production of 88 , 000 of the expensive missiles . Production history . Production started on July 29 , 1966 , and entered service in June 1967 with 1st Battalion , 63rd Armor Regiment at Fort Riley . 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 were not made operational . Vietnam War . The New Aluminum Tank . The US Army staff in Washington had been recommending to the commander of US Forces in South Vietnam , General Westmoreland , to utilize the Sheridan in Vietnam since 1966 . However , since no main gun rounds were available for the M551 , 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 " 152mm " tank 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 ( son of WWII 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 Sheridan ' s be combat tested by a division al cavalry squadron and a squadron from his own regiment ; both of which had completely different missions . First Deployment . The first Sheridans arriving in country in 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 War , being assigned to nearly all armored cavalry squadrons in country . 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 grenade s ( 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 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 worth applying modifications and equip all cavalry squadrons with the Sheridan . First Combat / First Losses . Like the M113 , which also was built of aluminum , and suffered from the same weapons and their effects , the Sheridan had one glaring negative side effect 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 Procedure ( SOP ) 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 mine which ruptured it ' s hull , 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 wading across a river near the DMZ , when three of their nine M551s detonated mines , completely destroying 3 Sheridans . 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 M - 48 Patton tank did , nor did it throw its track off as often as the Patton . This alone was enough to win the tank crewmen ' s favor . The light weight and high mobility proved their worth , and the gun proved an able anti - personnel weapon when used with either the M657 HE shell or the M625 canister round , which used thousands of flechette s as projectiles . Although an average M48 Patton tank 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 . This was because the M48 Patton ' s 90mm cannon fired fixed shells encased in metal , the same as with a standard rifle cartridge . Whereas , as stated above , the 152mm was caseless , requiring air vents to clear the gun tube and breech prior to loading another round , contrasting with the Patton ' s breech block , which opened instantly as the shell ejected and instantly closed as the new shell was shoved in . In effect , the stronger the loader , the faster the Patton ' s gun could be fired . For the Sheridan , a strong loader ( to lift and shove heavy shells ) would still be required to wait for the ready lights to come on inside the turret before the Tank Commander could fire . After firing , the loader , waiting with the 152mm shell in his hands , would have to wait , as he watched the breech slowly open rearward then turn downward . After another instrument indicated that all turret systems were still operational , the loader would gently push his 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 . But firepower is still firepower , and 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 M113 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 of only 20 rounds and 8 missiles ( though M551s in Vietnam service were not equipped with missiles or their guidance equipment , increasing 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 full might of 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 - 70 ' s was the addition of the " Cereal Bowl " commander ' s copula . 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 air - deployable tank in the inventory , and as an elite force they had considerably more " pull " than general infantry and armor units who were forced to get rid of them . 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 Operation Just Cause in Panama in 1989 , when fourteen M551 ' s were deployed ; four were transported by C - 5 Galaxies and ten were dropped by air , but 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 . 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 . 51 Sheridans were deployed by the 82nd Airborne Division during Operations Desert Shield and Desert Storm as some of the first tanks sent . Though the pictures published appeared to show rows of Sheridans ready to defend against Iraqi tanks , they would not be very effective against the Russian - built T - 72 s which comprised the bulk of the Iraqi Republican Guard . Their role was limited by age and light armor to reconnaissance , possibly 6 or less Shillelagh missiles were fired at Iraqi bunkers , these fewer than a half - dozen missiles , were the only time that the Shillelagh had been fired in a combat environment , from the inventory of the aforementioned 88 , 000 missiles produced . Several attempts to upgun 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 System 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 . Design . Armament . The need for even lighter weight than the T - 92 presented the design with a particularly difficult problem ; guns capable of defeating modern tanks at reasonable ranges were so large that they demanded a large vehicle to carry them , so large that they couldn ' t really be used as a " light " tank . The use of HEAT rounds instead of conventional penetrating ammunition could address this , but HEAT rounds work better at larger calibres . Gun weight is typically the product of calibre and muzzle velocity , so in the case of the XM551 they sacrificed the muzzle velocity , producing the low - velocity but relatively large - calibre 152 mm M81 . HEAT rounds fired by the M81 could defeat any contemporary tank at shorter ranges , but its low velocity made it difficult to use at longer ranges , especially against moving targets . The large low - velocity gun was also ideal for infantry support , where higher performance anti - tank guns would often fire right through soft targets and their small - calibre guns left little room for explosive filler . The M81 would thus be ideal for both direct fire support as well as short - distance anti - tank engagements . The only niche where the M81 was not ideal was the medium and long - range anti - tank engagement . The muzzle velocity was so low that a HEAT round fired at longer ranges would have to be " lofted " , making aiming difficult , and the flight time would be so long that a moving target would be very difficult to hit . However , it appeared there was a solution to this problem by equipping the tank with gun - fired anti - tank missile s . For longer range engagements a missile would be fired instead of a HEAT round , and although its velocity would also be relatively slow , the guidance system would make a hit highly likely anyway . A number of vehicles mounting only ATGM ' s , or alternately recoilless rifle s like the US ' s own Ontos tank were already in service , but typically these vehicles had limited firepower in the infantry support role , or in the case of Ontos could not be reloaded from within the vehicle . The XM551 appeared to offer the best of both worlds ; for infantry support the large calibre gun allowed it to fire full - sized artillery rounds and canister shot , while also giving it reasonable short - range anti - tank performance from the same gun . Although the Shillelagh missile was considered a risky project , if it worked the XM551 would be able to deal with even the largest tanks at extreme ranges . 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 was equipped with cloth sides that were rolled up , or tucked into rubber tubes that were located on both sides of the tank ' s upper hull edges , and a combination wood / aluminum front shield with an acrylic glass window for the driver to look through while driving . This front shield was permanently attached to the vehicle and stored in the down position , the same way an M38 Jeep ' s windshield laid flat across the hood when not in use . Sheridans could swim about a fifty - yard - wide river . Upon crawling up onto the opposite bank and out of the river , the water would leak out of all areas of both the M113 and the Sheridan . Crewmen would dismount from their ACAVs to open the battle hatch ( located within the rear ramp door ) and let out the remaining water . As the cavalry continued to move forward the remaining water would be forced out by the forward motion of the armored vehicles . Tanks such at 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 . 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 .