The M551 Sheridan, in service from 1969 to 1996, was developed to meet the Army's need for an air droppable light tank that could cross waterways. It went into production in 1966 and used relatively lightweight aluminum armor and a newly designed 155 mm main gun that could fire conventional ammunition as well as the Shillelagh anti-tank missile. Crews initially favored it because it rarely was immobilized or threw track compared with the M48 Patton tank. Its light armor and unproven main gun, however, made it vulnerable to land mines and rocket propelled grenades, and its rate of fire was one fourth as fast as that of Patton tank crews. Despite technical problems, it remained useful by serving as an airborne tank and by posing as enemy tanks during training exercises at the NTC in California.