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 such they had considerably more "pull" than non-airborne 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. The Sheridans' performance received mixed reviews. They were lauded by their operators and some commanders as providing firepower in situations which needed 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 fewer Shillelagh missiles were test fired at empty Iraqi bunkers. These few missiles, less than a half-dozen of the 88,000 produced, were the only Shillelaghs fired in a combat environment. Several attempts to up-gun or replace the Sheridan have been made, but none were successful. Several experimental versions of the Sheridan mounting a new turret carrying a 105 mm gun were made. The U.S. Army sought to replace the M8 Armored Gun System, but this was canceled in 1996. The Sheridan was retired without a replacement until the M1128 Mobile Gun System (MGS) came online. The MGS never reached its potential after a protracted procurement effort. It is being retired in 2022. The U.S. Army's current light tank procurement program is Mobile Protected Firepower. Main battle tanks M60A2 The M60A2, nicknamed the "Starship" due to its Space Age technology, featured an entirely new low-profile turret with a commander's machine-gun cupola on top, giving the commander a good view and field of fire while under armor but spoiling the low profile. It also featured a 152 mm cannon, which fired conventional rounds as well as guided missiles. The M60A2 proved a disappointment, though technical advancements would pave the way for future tanks. The Shillelagh/M60A2 system was phased out from active