M551 Sheridan
The M551 Sheridan is a light tank and armoured reconnaissance vehicle developed by the United States. It is named after Civil War General Philip Sheridan.The Sheridan started development in the early 1960s, in order to provide the Army with a reconnaissance vehicle with enough firepower to be able to survive in a battlefield containing modern Soviet tanks. This presented the design with a particularily difficult problem, guns capable of defeating modern tanks at reasonable ranges were so large as to make the vehicles able to carry them far too large and heavy to be used in the light tank role. Smaller guns with very large calibre could be used to fire HEAT rounds able to deal with these tanks, but one small enough for the light tank role would have such a low muzzle velocity that it would be highly inaccurate at longer ranges.
The solution came in the form of the anti-tank missile. 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. The Sheridan combined the two needs with the use of a gun-fired missile, the Shillelagh, which provided long range anti-armor power from what would be considered a very small gun. For short range and anti-personelle use the large calibre 152mm M81 gun provided more than enough firepower, while at longer ranges where the gun would be too inaccurate, the missile took over.
The vehicle designed to mount the gun was based on an existing aluminum-armored multipurpose tracked vehicle, powered by a large 300hp diesel engine. The Sheridan thus has an excellent power-to-weight ratio and excellent mobility, able to run at speeds up to 45mph, which at that time was unheard of for a tracked vehicle. Unfrotunately the the armor was thin enough that it could be penetrated even by heavy machine gun rounds, and is apparently particularily vunerable to mines.
Production started in 1966, and reached service in 1968. 1,562 M551s were built between 1966 and 1970. The M81 gun had problems with cracks developing near the breach 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 similar modification to the missile, that cured the problem.
The Sheridan saw limited action in Vietnam. Like the Ontos, the battle reports from the troops were glowing, while the reports higher up the chain of command were entirely negative. Of course an anti-tank vehicle being fielded against an enemy that didn't use tanks was something of a waste, but the gun proved an able anti-personelle weapon, and was generally loved by the infantry who were desperate for direct-fire support. In this role the real problem with the Sheridan was it's limited ammunition load, of only 20 rounds and 8 missiles. A common field-modification was to mount a large steel shield around the commander's 50cal gun, allowing it to be fired with some level of protection.
The Army started 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, to the extent of 57 machines today, as it was the only air-deloyable 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. Sheridans were used Operation Just Cause in Panama in 1989, and were again lauded by their operators as providing firepower in needed situations. The Sheridan was also deployed in Operation Desert Shield in 1991.
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 recoil was so great as to make it almost unusable. Several vehicles were tested as a part of the Armored Gun System effort of the 1980s, but none of these entered service. Today the Stryker is intended to replace the Sheridan at long last, but many have commented that the Stryker's APC-based hull is even less capable than the Sheridan. Perhaps more annoyingly the 105 is a dedicated anti-armor gun and therefore less useful in the anti-personelle role than the M81.
General Characteristics
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