Armored Warfare K1a1

[Baldomero Prado]

Inthe Korean War M24 Chaffees were the first U.S. tanks to fight the North Korean T-34-85s. The M24 fared poorly against these much better-armed and armored medium tanks. When the war began in June 1950, the four American infantry divisions on occupation duty in Japan had no medium tanks at all, having only one active tank company (equipped with M24 Chaffee light tanks) each. When these divisions were sent to Korea at the end of June 1950, they soon found that the 75 mm gun on the M24 could not penetrate the armor of North Korean T-34 tanks, which had no difficulty penetrating the M24's thin armor. M24s were more successful later in the war in their reconnaissance role, supported by heavier tanks such as the M4, M26, and M46.

The development of the T37 began in 1947 to replace the M24 Chaffee. The vehicle was designed to be air-transportable, and the heavier firepower, provided by an advanced 76 mm gun. In 1949, with the adoption of a less ambitious rangefinder, the project's designation was changed to M41. Production started in 1951 at Cadillac's Cleveland Tank Plant, and by 1953 the new tank completely replaced the M24 in the United States Army. It was later designated the M41 Walker Bulldog.

The Walker Bulldog saw limited combat with the U.S. Army during the Korean War, but for the most part, the conflict served as a testing ground to work out the tank's deficiencies, especially with its rangefinder. At the time, it was designated as the T-41, and was rushed to the battlefield even before its first test run.[citation needed] This was due to the fact that the North Koreans were supplied with Soviet T-34 tanks, which were superior to the M-24. By 1961, one hundred fifty were delivered to the Japan Ground Self-Defense Force to supplement their Type 61 medium tanks.

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 calibers. Gun weight is typically the product of caliber and muzzle velocity, so in the case of the XM551 they sacrificed the muzzle velocity, producing the low-velocity but relatively large-caliber 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-caliber 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 caliber 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.

The vehicle designed to mount the gun had a steel turret and aluminum hull. It was powered by a large diesel engine. The M551 thus had excellent mobility, able to run at speeds up to 45 mph, which at that time was unheard of for a tracked vehicle. Swimming capability was provided by a flotation screen. Production started on late July 1966, and entered service in June 1967. More than 1,600 M551s were built between 1966 and 1970. Total cost of the M551 program was $1.3 billion.

The vehicle proved to be very noisy and unreliable under combat conditions. The armor was thin enough that it could be penetrated even by heavy machine gun rounds as well as being highly vulnerable to mines.

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 danger, despite the production of 88,000 of the expensive missiles. The gun had problems with cracks developing near the breech after repeated firing. Most field units were modified to help address the problem and 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.

The battle reports from the troops were sometimes glowing, while the reports higher up the chain of command were often negative. 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]

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).

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.

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.[2] 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.

51 Sheridans were deployed in the Gulf War as some of the first tanks sent. They would not be very effective against the Russian-built T-72s. Their role was limited by age and light armor to reconnaissance, possibly 6 or less Shillelagh missiles were fired[3] 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 105 mm 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 Stryker Mobile Gun System has filled this role United States.

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