Logical evolution of earlier HO4S (H-19) reflecting services' increasing acceptance of helicopter and development of expanded operational roles. Korean War provided solid operational experience upon which to define technical and functional requirements as well as highlight some problems. These included inadequate center-of-gravity range and control power to accommodate it, management of control loads as helicopters got larger, stability characteristics adequate for instrument flight, and more uniform and durable rotor blades. S-58 had improved rotor and control system and, in general, significantly improved payload capacity, speed, and range which increased variety of operational missions it could satisfy. S- 58 designed specifically for Navy's anti-submarine warfare (ASW) mission; was able to carry increasingly sophisticated sonar equipment used by Navy. Became known as "push button" helicopter; could be placed on automatic pilot to maintain 80 kts airspeed 200 ft above water then automatically hover at 50- ft altitude over a preselected spot for lowering sonar equipment. Navy's success with S-58 in ASW role led to large orders from Army and Marines for other missions. 3 commercial versions also developed. Army was primary user as troop and supply transport. By 1958, CH-34s on duty in the US, Asia, and Europe. Marines used UH-34D mainly for assault missions. In Vietnam, UH-34s were primary means of troop transport and casualty evacuation. Also operated by Army and Marines as part of shared responsibility in operating Executive Flight Detachment for transporting President and other VIPs. 1961, Marine UH-34D was used in recovery of astronaut Alan Shepard following splashdown in Freedom 7. Final version, S-58T, used to test turbine engines. Latter offered marked reduction in weight; R-1820, lightest reciprocating engine weighed 1.31 lb/hp, while early T58 turbine weighed only 0.22 lb/hp. S-58 achieved world speed records of 141.9 mph over 100 km, 136 mph over 500 km, and 132.6 mph over 1000 km.