Originally designed as a commercial helicopter, the HTL Sioux was aquired by the Navy for training purposes in the late '40s. Until retired in April 1964, HTLs were used in the Apollo program to familiarize astronauts with its auto-rotation sink rate which was simliar to that of the Lunar Excursion Module. HTLs also became famous as the medical evacuation helicopter featured on M*A*S*H.