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2 The Thor-Delta Family of Space-Launch Vehicles, 1958–1990 The Thor missiles did not remain in operational use very long,1 but even before the Air Force retired them in 1963, it had begun to use Thor’s airframe and propulsive elements, including its vernier engines, as the first stages of various launch vehicles. With a series of upratings and modifications, Thor remained in use with such upper stages as Able, Able-Star, Agena, Burner I, Burner II, and Burner IIA until 1980. In addition, NASA quickly chose Thor as the first stage of what became its Thor-Delta (later called just Delta) launch vehicle family, which has had an even longer history than the Air Force’s Thor series of launch vehicles did. The Delta launch vehicles initially drew upon Vanguard upper stages, as did the Thor-Able used by the Air Force. After the initial launches of Thor-Able in 1958 to test reentry vehicles, followed by flights to test the radio-inertial guidance system for the Titan ICBM in 1959, Thor with its various upper stages went on to launch a great variety of spacecraft, including reconnaissance satellites and other classified payloads for the Air Force. Also included were many satellites to gather data about Earth, and an array of communications, navigation, and meteorological satellites, plus some payloads consisting of scientific experiments for DoD. Beginning in 1960, Delta with an even greater variety of upper stages launched planetary exploration probes as well as weather, communications, and scientific satellites. Starting with the passive Echo communications satellite, ranging through Telstar, Pioneer, Nimbus, and Landsat to the Cosmic Background Explorer, and including the early Intelsat communications satellites, Delta provided a reliable and comparatively inexpensive launch platform for a large number of missions and a variety of customers.2 Throughout its history, Delta evolved by uprating existing components or adopting new ones that had proved themselves. It used this low-risk strategy The Thor-Delta Family of Space-Launch Vehicles 41 to improve its payload capacity through the Delta II at the end of the period covered by this history. But it did not stop there, evolving through a Delta III, first launched (unsuccessfully) in 1998, and a Delta IV that finally had its (successful) initial launch on November 20, 2002. The unsuccessful first (and second) launch of the Delta III and numerous delays in the launch of the Delta IV because of both software and hardware problems suggested, however, that the design of new launch vehicles was still not something engineers had “down to a science” even in the twenty-first century.3 Thor-Able The initial Thor engine used for space ballistic-missile tests had 150,000 pounds of thrust, with the two vernier engines adding 1,000 pounds each. The main engine for this version was the Rocketdyne MB-3 Basic system, followed by a Block I. The Air Force used it with Thor-Able and some of the Thor–Able-Star launches. For some Able-Star missions, the Thor used an uprated MB-3 Block II or Block III engine with 170,000 pounds of thrust. The newer system had slightly higher chamber pressure and specific impulse with a hypergolic ignition system replacing the earlier pyrotechnic one. The vernier engine used with the Block II and III main Thor engines also used hypergolic ignition. The pumps for the main engine fed the verniers until engine cutoff, following which tank pressure fed them for nine seconds after the main engines ceased to fire.4 The use of the Able upper stage grew out of the Air Force’s desire to test more advanced reentry concepts than the blunt heat-sink type without interfering with ICBM development. In November 1957 the nonprofit Rand Corporation, an Air Force think tank, suggested in a briefing to Lt. Gen. Donald L. Putt, the Air Force’s deputy chief of staff for development, that a Thor first stage and a Vanguard second stage would provide a vehicle with numerous applications. Louis Dunn of Space Technology Laboratories (STL) urged the same combination after an analysis of available components for a two-stage test vehicle. The Air Force was able to award direction of the project to STL despite restrictions on the lab’s activities, as this was not a production project, with only three flights then scheduled. STL developed and produced the autopilot, guidance, and telemetry systems, with Aerojet providing the second-stage engines, evidently under subcontract to Douglas , the Thor contractor...

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