152 mm Gun/Launcher M81
The 152 mm Gun/Launcher M81 is an American dual-purpose armament and was an innovative attempt at making a weapon system able to fire traditional tank shells and missiles at a target. Mounted on the M551 Sheridan light tank and the less-successful MBT-70 prototype, it had a very bad reliability record during its start-up and questions were raised on the utility of such weapon on the modern battlefield.
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History of creation and combat usage
US Army prioritized development on the HEAT warheads in the 1950s due to the belief that new Soviet main battle tanks may be able to resist the 105 mm L7 gun, and a cannon larger than that to fire kinetic rounds would be rather impractical and unwieldy. The HEAT charges were favorable because it focused more on the shell's impact onto target rather than its kinetic energy like a standard armor-piercing round. HEAT charges also work better with larger caliber, low velocity cannon and thus the gun can double as an infantry support. However, the problem comes at that a low velocity round would make it harder to fight targets at a longer range, so the plan was to develop guided missiles with HEAT charges for the gun to fire at a long distance.
The Army began development of said missile in 1958, assigning Sperry and Ford Aeronutronic in June 1959 to create the missile system. Ford's XM13 design won, with testing beginning in 1960 then jumping to limited production in 1964. The missile was designated the MGM-51A Shillelagh. The M81 gun/launcher system was developed for the missile to be launched and to also act as an infantry support howitzer. The use with HE shells use grooves to stabilize the shells, but a straight groove lines in the cannon for the missile to be fired via a "key" to prevent it from spinning. The missile was launched via the M81 gun via a small charge on the rear of the missile tube. Once launched, the missile engine ignites and follows the guidance system to its target via an IR link. The system was considered advanced at the time, but was also rife with technological problem, mainly in its propellant, ignited, tracking, and infrared command link, all vital to make the missile function properly. The missile system continually improved in design flaws and operation with the MGM-51B in 1966 which lead to improved effective ranges. A redesign in the gun barrel was made sometime after that to remove the straight groove for the missile, which caused the gun to crack after a few shells. The redesigned gun and missile was designated to M81E1 and MGM-51C respectively. The missile remain in production from 1964 to 1971, with about 88,000 missile units produced.
Gun Mounts and Testing
The Shillelagh missile system and the M81 saw its first vehicle mount on the M551 Sheridan light tank. As an air-transportable lightweight vehicle, it was to provide an anti-tank and infantry support role for airborne troops. Unfortunately, the vehicle was plagued with the same problems as the launcher and missile itself, the worst part was the extremely volatile combustible case that could detonate if in contact with heat. The first usage of the Sheridan and its M81 gun was in Vietnam, but the missile system was adapted out due to the unsuitable environment as per recommendation by Lt. Col Burton Boudinot. The M81 performed quite well with HEAT rounds and a newly developed fletchette round, able to fight fortifications and infantry effectively. The Sheridans are also the only vehicle known to fire the Shillelagh missile at a hostile target in the Gulf War during Operation Desert Storm.
The M81 was also equipped on the experimental MBT-70 and the M60A2 "Starship", these two served as the test bed on whether the gun system was suitable on a main battle tank. The MBT-70, starting in 1963, was equipped with a variant of the M81, the XM-150 that could fire the Shillelagh missiles, itself the reason why the Sheridan kept the M81 gun throughout its career. The MBT-70 itself was cancelled in 1971 for going over-budget. The M60A2 was made in the 1960s, but it wasn't until 1971 when the Army decided to acquire a few. Though the M60A2 entered service in 1974, complications in the computer system and reliability of the missile caused it to be phased out by 1980 for the M60A3 tank.