Saturday, June 6, 2009

Transportable SPATG

Any military vehicle is some compromise of armor, firepower, and agility. On the one end of the military land vehicle spectrum are super fast and agile but unarmored vehicles like the fast attack vehicle. On the other end of the spectrum are extremely armored vehicles like the German Sturmtiger, which was usually only able to be stopped by bombing or heavy artillery attack. Or running out of fuel.

One attempt to give U.S. forces a rapidly transportable vehicle capable of disabling enemy tanks was the M56 Scorpion. This small tracked self-propelled gun was meant to be a tank destroyer. The M56 is 2.57 meters wide, and 4.55 meters long. Fully loaded, it weighs a little under 8 tons.

The first M56s were made in 1953. Production stopped in 1959. The Scorpion was used by the 82nd and 101st Airborne Divisions, and saw use in Vietnam. Because of the scarcity of tanks confronting U.S. forces, the Scorpion was most frequently used as an assault gun, using its 90mm M54 cannon to provide fire support. Like many tank destroyers, insufficient armor was mounted on the Scorpion to make it an effective fighter against close-range threats- in fact, the only armor was on the shield mounted towards the base of the 90mm gun.

A novel tank destroyer built using the running gear of the Scorpion, the M50 Ontos was at one point considered for purchase in extremely large quantities. The Ontos had a light turret on which six M40 "106mm" recoilless rifles were mounted. Four of the rifles were fitted with .50 caliber spotting rifles with ammunition that mimicked the recoilless rounds' flight performance. The M40 has a maximum effective range of about 7300 meters. A .30 caliber M1919A4 was also mounted for close-range antipersonnel use.

The Ontos was approximately the same dimensions as the Scorpion, except for height. Its limited-traverse turret and light armor made the Ontos about a ton heavier than the Scorpion. Tank destroyers are typically much cheaper than tanks, usually due to simpler turrets and lighter armor. The possibility of buying an Ontos for about $30,000 instead of an M47 tank for $240,000 was an enticing idea, but the unconventionality of the Ontos approach (the six recoilless rifles could only be loaded from outside the vehicle) ultimately led to the Army's decision not to purchase the system. The Marines needed anti-tank vehicles, though, and so bought 297 Ontos, which were built between 1955 and 1957.

As with the Scorpion, the Ontos would be used primarily in a fire support role instead of as the tank destroyer originally envisioned. It did have several drawbacks in use, including vulnerability to mines and RPGs. However, its 105mm (M40 ammunition was only called 106 to avoid confusion with earlier M27 105mm ammunition) guns proved very effective on both structures and when used against personnel. The Ontos' light weight also enabled it to go places heavier U.S. vehicles could not.

The Scorpions were replaced by the M551 Sheridan "Armored Airborne Reconnaissance Assault Vehicle" (so called because the U.S. had gone to a "main battle tank" system and therefore could not admit to accepting a light tank). The Sheridan had a real turret and better armor, with a resulting weight increase, putting it slightly over 15 tons. This was still light enough to be airlifted by some platforms.

Well before the late 1960s, tank armor had increased to the point that mounting an effective main gun on a light tank was a frustrating exercise. Armor can be penetrated through one of two mechanisms: high velocity or explosive power. The Sheridan went with the latter, using a low velocity 152mm gun. To enable hits at longer ranges, an anti-tank missile was developed that launched from the gun bore. A 7.6x51mm coaxial and 12.7mm anti-aircraft machine gun was mounted.

Production of the Sheridan began in 1966, and continued until 1970. 1662 Sheridans were built, and Sheridans fought for about three years in Vietnam. Reviews were mixed.

Despite increased armor compared to the M50 and M56, the M551 still had light armor that was vulnerable to mines and RPGs. Recoil of the 152mm M81 gun in the light vehicle was vicious. Without tank targets, the expensive Shillelagh missiles were removed from Vietnam tanks. Eccentricities of the ammunition for the M81 made the Sheridan very vulnerable to fire, and gave it a very slow rate of fire compared to the M48 tank. Like the previous platforms, in Vietnam the Sheridan was mostly used to provide fire support.

Even without a real replacement, the Army began to phase out the Sheridan in 1978, though it was retained by the 82nd Airborne until 1996. The Sheridan played minor roles in Panama and Desert Storm, and was most recently used at the National Training Center to simulate Communist Block-built vehicles

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