M551 Sheridan


Country Of Origin:	United States
Designation:	Light Tank
Configuration:	Track
Manufacturer:
Crew:	4
Length:	20.6 ft (6.3 m)
Width:	9.2 ft (2.8 m)
Height:	7.6 ft (2.3 m)
Weight:	17.5 t
Clearance:	19 in (483 mm)
Fording:	mm
Obstacle:	33 in (838 mm)
Trench:	7 ft (2.13 m)
Speed:	42 mph (67 km/h) road
Range:	373 miles (600 km)
Primary armament:	152 mm gun
Secondary armament:	coaxial 7.62 mm MG, TC .50 cal M2HB machine gun
Armour:	mm
Power plant:	300 hp, diesel 6V53T engine
''The above data pertains to the M551A1 Sheridan model The M551 became the M551A1 when equipped with the AN/VVG-1 LRF and the Tank Thermal Sight (TTS) AN/VSG-2B. The M551 became the M551NTC when configured for use in training at the National Training Center (NTC) in Fort Irwin, California and the Joint Readiness Training Center (JRTC) in Fort Chaffee, Arkansas. The hull, suspension, and miscellaneous hull components of the M551A1 and M551NTC are identical.''

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. It is a typical example of space-age 1960s technology such as the M-16 and F-111 which were to result in mixed success as a result of space age over-innovation.

Development

In the immediate post-WWII era the US Army introduced the M41 Walker Bulldog into service to fill their light tank role, armed with the war-era British designed 76 mm 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 too far into the design to be refitted, so the design of an entirely new system started as the XM551, no longer known as a "light tank", but instead "armored reconnaissance/airborne assault vehicle".

The need for even lighter weight presented the design with a particularly 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. 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 a function of the calibre and muzzle velocity, so in the case of the XM551 they sacrificed the latter, producing the M81 152 mm gun with very low muzzle velocity.

The M81 would allow the XM551 to deal with most tanks, but only at short ranges due to the low accuracy of the "lofted" low-velocity rounds. At longer ranges the tank would be vulnerable, but it appeared there might be a solution to this problem. The solution was to equip the tank with gun-fired anti-tank missiles. 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 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. In the Vietnam War, firing the gun would often disable the delicate electronics, which were at the early stages of transitioning to solid state, so the missile was disabled. Indeed, this missile would end up never being fired in anger, with the MBT-70 cancellation, and the short service span of the M60A2 Starship variant, though the Russians also developed a missile fired out the gun barrel.

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. Unfortunately the armor was thin enough that it could be penetrated even by heavy machine gun rounds. Like the M-113, it was also vulnerable to mines, which did not bother heavier M-48 tanks deployed in Vietnam.

Swimming capability was provided in a unique and somewhat odd fashion. The front armor was actually three folded layers, hinged together. They 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 hull, folding up from hatches 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 stand up to see. The M2 Bradley would adopt a similar solution, but dropped it with upgraded armour.

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 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 gun also has been criticized for having too much recoil for the weight, with the tank nearly jumping in the air after being fired. Some were experimentally fitted with conventional 76mm guns, but these were not made operational.

The Sheridan saw limited action in the Vietnam War. Like the Ontos, the battle reports from the troops were glowing, while the reports higher up the chain of command were entirely negative. The light weight and high mobility proved their worth, and the gun proved an able anti-personnel weapon when used with the "beehive" flechette round, which used finned nail. It was generally loved by the infantry who were desperate for direct-fire support, and often appears in pictures with M-113 APCs. 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 loadout of conventional rounds).

A common field-modification was to mount a large steel shield 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.

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 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. Sheridans were used Operation Just Cause in Panama in 1989, and 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, though they would have been of dubious value in staving off an attack by main battle tanks, and there is not much record of their combat exploits there.

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 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 105 mm gun were made, but the recoil was so great as to make it almost unusable. Several new vehicles were tested as a part of the XM8 Armored Gun System effort of the 1980s, but none of these entered service. Today the less-mobile (and barely airmobile) Stryker is intended to replace the Sheridan at long last, but many have commented that the Stryker's wheeled configuration is even less capable than the Sheridan, especially off-road. Perhaps more annoyingly, the 105 is a dedicated anti-armor gun and therefore less useful in the anti-personnel role than the M81.

The most ironic part of the M551 story is that it was created in order to give the US forces an amphibious tank, causing the existing T-92 design to be abandoned. However, for all the trouble, it appears that the swimming system of the M551 was never used in combat.

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). The resulting additional space was filled with two separate boxes, one for 7.62x51mm NATO ammunition, and one for 12.7x99mm BMG ammunition, and the missile stowage was redesigned to accept conventional rounds.
M551A1 - Upgraded M551 with AN/VVG-1 laser rangefinder, used by the 82nd airborne only
- 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.

Operators

United States (in Reserve Storage after the 1990s)

Singapore (in Reserve Storage)

Combat History

Vietnam

Operation Just Cause in Panama

Operation Desert Shield

External links

Vietnam-Germany-Fort Irwin: the Eaglehorse and the M551 Sheridan - an excellent and in-depth history of the M551 Sheridan
AFV Database

Cold War American tanks | Light tanks | Reconnaissance vehicles