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.

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 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 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.

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 divisional 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 it's 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 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 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 flechettes 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 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 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-72s 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.

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 missiles. 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 rifles 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.

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.

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 'vismods', short for VISually MODified. They have since been retired in favor of similarly converted M113s and M1 Abrams.

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.

Operators

 * 🇺🇸 (in Reserve Storage after the 1990s)

Survivors

 * Carolinas Aviation Museum