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The Ford MGM-51 Shillelagh was an American anti-tank guided missile designed to be launched from a conventional gun (cannon). It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an excessively large gun. Developing a system that could fire both shells and missiles reliably would prove complex and largely unworkable. It served most notably as a primary weapon of the M551 Sheridan light tank, but the missile system was deleted from units serving in Vietnam. Ultimately very few of the 88,000 rounds produced were ever fired in combat.

The Shillelagh was a disappointment compared to the later BGM-71 TOW anti-tank guided missile first produced in 1970 by the U.S. The TOW system, which could not fire gun rounds, and was guided by a wire which directly sent commands to the missile, would prove simpler and more versatile. TOW would become the most widely used anti-tank guided missile in the world based on a range of light, armoured and flying vehicles. Main battle tanks of the late 20th century such as the successful M-1 Abrams tank would field improved conventional 105mm and 120mm guns which proved effective against enemy armor threats. Since then, the Soviets and other nations have developed gun launched missiles, and the US is developing guided tank rounds.

The name comes from a type of wooden club associated with Irelandmarker.

Background

With the rapid increase in armor during World War II, tanks were becoming increasingly able to survive rounds fired from even the largest of WWII-era anti-tank guns. A new generation of guns, notably the Britishmarker 105 mm Royal Ordnance L7, were able to cope with newer tanks, but it appeared that in another generation the guns needed would be too large to be practical.

Instead the US Army started concentrating on HEAT rounds in the 1950s. HEAT penetration is not dependent on the speed of the round, allowing it to be fired at much lower velocities, and thus from a much lighter gun. They also work better at larger diameters, and a large-diameter low-velocity gun makes for an excellent assault gun vehicle. On the downside, the slow speed also means that they become increasing hard to aim over longer distances. The Army looked to address this problem with the use of HEAT-equipped guided missiles for anything beyond a few hundred yards.

Development

In 1958 they felt the state of the art had progressed enough to start work on such designs, and in June 1959 Sperry and Ford Aeronutronic were asked for designs to fill the shorter range role. Ford won the contract and started work on the XM13. The first test shots occurred in 1960, and limited production started in 1964, now known as the MGM-51A.

The basic system was quite advanced for its day. The missile body consisted of a long tube with fold-out fins at the extreme rear, which was propelled from the new M81 gun with a small charge strapped on the rear. Once clear of the gun the fins popped open and the engine ignited. In order to keep it from spinning while in the gun due to the rifling, a small "key" fit into a straight groove in the rifled gun. Aiming the missile was simple; the gunner simply kept his gunsight on the target, while electronics in the sighting system tracked the missile optically and sent corrections through an IR link (similar to a TV remote control). In general the gunners were able to achieve excellent hit ratios with the system.

Because the system was so advanced, the development of the Shillelagh was fraught with problems. Ford Aeronutronic underestimated the complexity of the task of designing a missile this advanced, and there were major problems with the propellant, igniter, tracker and infrared command link of the missile.

The Sheridan

The M81/MGM-51 was first deployed on the M551 Sheridan, which appeared at first to prove the value of the system. The Sheridan was a light aluminum-armored AFV designed to be air transportable and provide fire support for airborne forces. The M81 armed with conventional warheads proved excellent in this role, but would normally be next to useless in the anti-tank role the airborne forces drastically needed. Here the Shillelagh took over, providing the small vehicle with the same hitting power as a main battle tank.

In the field the system proved to be less practical. The caseless ammunition large-caliber rounds were themselves troublesome because the casings did not burn completely, requiring a complicated and slow gas-driven scavenging system. They were also liable to cook off if the vehicle were hit. Firing the gun would cause such a large recoil as to cause failures in the delicate missile firing electronics on the tank. Such problems, and the lack of enemy tanks caused the Sheridan to be deployed to Vietnam without the complex missile system.

The Shillelagh was considerably larger than a conventional round, so only a small number could be carried. Typical loads consisted of only 8 missiles and twenty M409 HEAT rounds for short-range use. In addition the missile proved to have a very long minimum range due to the layout of the vehicle, the missile didn't come into the sight of the gun/tracker system until it was from the vehicle, at which point it could start to be guided. Because of its maximum range of about , the system was left with a fairly large "dead zone".

While the maximum range of was usable, the Army felt it could use improvement. Ford received a contract to study a longer range version in 1963, and returned a slightly larger design the next year. Test shots of the new MGM-51B started the next May, and production in October 1966. Only one other change was made to the system. In testing it was found that the key slot in the gun led to cracking after firing only a few shells. After further study a version with a shallower slot and new barrel was selected, creating the M81E1/MGM-51C. The missile was about long, about in diameter, and weighed . It remained in production until 1971, by which time 88,000 had been produced, probably in anticipation of use by main battle tanks (below). Two missiles fired at bunkers by Sheridans in the Gulf War are among the few times the missile has ever been fired in combat.

M60 "Starship"

Even with these problems the system clearly proved it could be used by an airborne tank to destroy a main battle tank. The question of whether or not it could fill its original role as the main armament of all tanks was still open. The Army had originally started development of a low-profile turret with a short barrel for their existing M60 tanks in the 1960s, but didn't contract for actual delivery until 1971 once the bugs had been worked out. They entered service in 1974, but were hampered by reliability problems, and soon phased out in 1980. The final revision of the M60A3 would use the same gun and turret as the M60A1.
M60A2 at the American Armored Foundation Museum in Danville, Virginia, July 2006.


MBT-70

MBT-70 prototype test firing an MGM-51
The most ambitious project based on the system was the MBT-70, an advanced US-Germanmarker tank design that started in 1963. It mounted a huge auto-loader turret on top of a very short chassis, so short that there was no room for a driver. Instead he was relocated into the turret with everyone else, in a rotating cupola that kept him facing forward. The gun was a new longer-barreled design, the XM-150, which extended range and performance to the point where it was useful for sabot type rounds as well. However the project dragged on, and in 1969 the estimated unit cost had risen 5 times, and Germany pulled out of the effort. The Army proposed a "cut-down" version of the system, but Congress cancelled it in November 1971. It started the M1 Abrams project with its funds the next month, which would use a conventional gun.

The Soviet KBP Instrument Design Bureau developed the somewhat similar AT-11 Sniper, launched by a 125 mm gun system. It had a Laser Beam Rider guidance system, tandem warhead to defeat vehicles fitted with Explosive Reactive Armour, and used on the T80/72M Tank.

References

  1. Technology and the American way of war. Thomas G. Mahnken. Columbia University Press, 2008. ISBN 9780231123365


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