M551A1 Sheridan
M551 Sheridan light tank
Type	Light tank
Place of origin	United States
Specifications
Weight	15.2 tonnes
Length	20.6 ft (6.3 m)
Width	9.1 ft (2.8 m)
Height	7.5 ft (2.3 m)
Crew	4 (Commander, gunner, loader, driver)
Armor	mm
Primary armament	M81E1 152 mm Gun/Launcher 20 rounds 9 MGM-51 Shillelagh missiles
Secondary armament	7.62mm M73/M219 machine gun 3,000 rounds (early) 7.62 mm M240C machine gun 3,000 rounds (late) .50cal M2HB machine gun 1,000 rounds
Engine	General Motors 6V53T, 6 cylinder, supercharged diesel 300 hp (224 kW)
Power/weight	19.7 hp/tonne
Suspension	Torsion bar
Operational range	348 mi (560 km)
Speed	43 mph (69 km/h)

The M551 Sheridan was a light tank 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 MGM-51 Shillelagh gun-launched missile system. 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. Now retired from service, it saw extensive combat in Vietnam, and limited service in Operation Just Cause (Panama), and Operation Desert Shield/Desert Storm (Iraq).

At the time of the M551's entry into service in the 1960s, the United States Army no longer used the heavy, medium, and light tank classifications. 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 under going the transitional process, the regular army only consisted of MBTs. Partly because of this policy, the new M551 could not be classified as a light tank, and was referred to as a "armored reconnaissance/airborne assault vehicle".

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, armed with the World War II-era 76mm gun. However, the lifetime of this system was fairly short; the 25-ton tank was considered too heavy to be a true light tank (the same weight as WWII mediums), 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 US 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 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, and 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.

MGM-51 Shillelagh fired from a Sheridan light tank

The vehicle designed to mount the gun was based on an aluminum-armored multipurpose tracked vehicle, powered by a large 300-hp (224 kW) diesel engine. The XM551 thus had an excellent power-to-weight ratio and mobility, able to run at speeds up to 45mph, which at that time was unheard of for a tracked vehicle. However, the vehicle proved to be very noisy and unreliable under combat conditions. 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 in a unique and somewhat odd fashion. 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. Nevertheless, with the MBT-70 main battle tank cancellation, and the short service span of the M-60A2 Starship variant, the M551 would be the most successful application of the gun system, though the Russians also developed a missile, the AT-11 Sniper, fired out of the gun barrel.

Production history

Production started in 1966, and reached service in 1968 as the Sheridan. 1,562 M551s were built between 1966 and 1970. 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

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Beginning in January 1969 and ending with most of the U.S. ground troop withdrawals in 1972, the Sheridan saw extensive action in the Vietnam War being assigned to armored cavalry squadrons in country, by replacing their M48 Patton tanks, which in turn were 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.^[1]

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 asbestos bag which was form-fitted to hold the propellant portion of the shell. The 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.^[2] 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.^[3]

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.^[4]

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 M551's 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 decided to invade Cambodia beginning 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), when the cavalry's remaining Sheridan Squadrons met near disaster on the Lao border during the early months of 1971, in particular the lst Squadron of the lst Cavalry Regt.^[3]

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.

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.

Sheridan with late modifications and ACAV shields

Eight Sheridans were used in Operation Just Cause in Panama in 1989. They were delivered by Low-Velocity Airdrop (LVAD) technique from C-130 transports. They were again lauded by their operators as providing firepower in needed situations to destroy hard targets. 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, but several Shillelagh missiles were fired at Iraqi bunkers, the few ever fired in combat of the aforementioned 88,000 missiles produced.

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.

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.

Amphibious capabilities

Driver's hatch, front shield with window

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, and the M60's series) could not conduct 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.

Air drop

A C-130 delivering an M551 Sheridan tank using LAPES (low altitude parachute extraction system).

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).^[5] Many films exist showing the Sheridan being pulled out of a C-130 Hercules transport by brake chutes and skidding to a stop. The Low Altitude Parachute Extraction System (LAPES) is a somewhat risky maneuver which allows accurate delivery onto a field when landing is not possible. The tank is strapped down to a special pallet which absorbs most of the landing impact. The crew goes to the tank, releases the lines and drives 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 Service

The Sheridan was never used in Australian service. However it was trialled by the RAAC and appeared in several Manual of Land Warfare pamphlets (the equivalent of US military FMs) of late 60s 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, and the Sheridan did not meet Australian Army requirements. ^[6]

Operators

• United States (in Reserve Storage after the 1990s)

Combat history

• Vietnam
• Operation Just Cause in Panama
• Operation Desert Shield
• Operation Desert Storm

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.

Notes

References

The remainder of this article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (September 2007)

External links

Wikimedia Commons has media related to:

M551 Sheridan

• M551 SHERIDAN - LIGHT TANK – Walk around photos
• Vietnam-Germany-Fort Irwin: the Eaglehorse and the M551 Sheridan - an excellent and in-depth history of the M551 Sheridan
• AFV Database
• Global Security
• Tanques y Blindados: Historia del carro de combate