cavalry in South Vietnam. When Abrams mentioned the cavalry's concerns over the new vehicle, Patton recommended that the Sheridans be combat tested by a divisional cavalry squadron as well as a squadron from his own regiment; as the squadrons had completely different missions. First deployment The first Sheridans to arrive in South Vietnam did so in January 1969 and were accompanied by their factory representatives, instructors and evaluators as the new vehicles were issued to the 3rd Squadron, 4th Cavalry Regiment, and the 1st Squadron of the 11th ACR. By the end of 1970, there were more than 200 Sheridans in South Vietnam, and they stayed in the field until the last U.S. armored cavalry unit, the 1st Squadron, 1st Cavalry Regiment prepared for re-deployment back to the United States in April 1972. By the end of its combat debut in 1972, the Sheridan had seen extensive action in the Vietnam War, being assigned to nearly all armored cavalry squadrons involved in that conflict. In 1969, armored cavalry units (minus the 11th ACR, which retained its M48 tank companies) began replacing their M48 Patton tanks, which in turn were normally transferred to the Army of South Vietnam. The opinions of crews on Vietnam-era M551s were mixed at best and assessments from senior commanders were often negative. This was due largely to the high loss rate of Sheridans and casualty rates among crews; landmines and RPGs that would damage an M48 Patton tank, tended to destroy a Sheridan and either kill or wound most of its crew. A 1969 evaluation of the vehicles found that the M551 was employed in reconnaissance, night patrol and road clearing, accumulating 39,455 road miles and 520 combat missions, with a ready rate of 81.3 percent. Despite vulnerability to rockets and mines, it was judged worthy of applying modifications and equipping all cavalry squadrons with the Sheridan. First combat/first losses In addition to the problems presented by aluminum construction, the Sheridan had a defect that no other common armored vehicle possessed: it fired caseless 152 mm main gun rounds. These rounds were "fixed", meaning that, unlike the artillery, the warhead was factory attached to the propellant, and if the warhead separated from the propellant during loading, which was not uncommon, the crewmen were instructed not to load the round. Sometimes, these unspent propellant charges remained on the turret floor due to the emergencies at the time,