The PGM-17A Thor was the first operative ballistic missile of the United States Air Force (USAF). It was named after the Norse god of thunder. It was deployed in the United Kingdom with the Royal Air Force between 1959 and September 1963 as an intermediate-range ballistic missile (IRBM) with thermonuclear warheads. Thor was 65 feet (20 m) in height and 8 feet (2.4 m) in diameter. The first generation of Thor missiles were rushed into service, and design mistakes resulted in a 24% launch failure rate. The competing PGM-19 Jupiter missile saw more use, but both were quickly eclipsed by the Air Force's long range ICBM program, which could be fired from U.S. soil. By 1959, with the Atlas missile well on its way to operational status, both Thor and Jupiter programs became obsolete as delivery vehicles, yet continued to be built and deployed until 1963 for political reasons and to maintain aerospace industry employment. The missile's lasting legacy continued as the Thor and later Delta families of space launch vehicles used boosters derived from the initial Thor missile, and continued on into the 21st century.
Development Fearful that the Soviet Union would deploy a long-range ballistic missile before the U.S., in January 1956 the USAF began developing the Thor, a 1,500-mile (2,400 km) intermediate-range ballistic missile. The program proceeded quickly as a stop-gap measure, and within three years of inception the first of 20 Royal Air Force Thor squadrons became operational in the UK. The UK deployment carried the codename 'Project Emily'. One of the advantages of the design was that, unlike the Jupiter MRBM, the Thor could be carried by the USAF's cargo aircraft of the time, which made its deployment more rapid. The launch facilities were not transportable and had to be built on site. Once the first generation of ICBMs based in the U.S. became operational, Thor missiles were quickly retired. The last of the missiles was withdrawn from operational alert in 1963. A small number of Thors with "Thrust Augmented Delta" boosters and W-49 Mod 6 warheads remained operational in the anti-satellite missile role as Program 437 until April 1975. These missiles were based on Johnston Island in the Pacific Ocean and had the ability to destroy satellites in low Earth orbit. With prior warning of an impending launch, they could destroy a Soviet spy satellite soon after orbital insertion.
Initial development
Development of the Thor was initiated by the USAF in 1954. The goal was a missile system that could deliver a nuclear warhead over a distance of 1,150 to 2,300 miles (1,850 to 3,700 km) with a Circular Error Probability (CEP) of 2 miles (3.2 km). This range would allow Moscow to be attacked from a launch site in the UK. The initial design studies were headed by Commander Robert Truax (U.S. Navy) and Dr. Adolph K. Thiel (Ramo-Wooldridge Corporation, formerly of Redstone Arsenal and previously of Nazi Germany). They refined the specifications to an IRBM with:
A 1,750-mile (2,820 km) range 8 ft (2.4 m) diameter, 65 ft (20 m) long (so it could be carried by the C-124 Globemaster) A gross takeoff weight of 110,000 lb (50,000 kg) Propulsion provided by half of the existing SM-64 Navaho-derived Atlas booster engine 10,000 mph (4.5 km/s) maximum speed during warhead reentry Inertial guidance system with radio backup (for low susceptibility to enemy disruption) Thor had vernier engines for roll control flanking the main engine, similar to the Atlas vernier engines on the sides of the propellant tanks. On 30 November 1955, three companies were given one week to bid on the project: Douglas, Lockheed, and North American Aviation. The missile was to use existing technology, skills, abilities, and techniques to speed entry into service. On 27 December 1955, Douglas was awarded the prime contract for the airframe and integration. The Rocketdyne division of North American Aviation was awarded the engine contract, AC Spark Plug the primary inertial guidance system, Bell Labs the backup radio guidance system, and General Electric the nose cone/reentry vehicle. Douglas' proposal included choosing bolted tank bulkheads (as opposed to the initially suggested welded ones) and a tapered fuel tank for improved aerodynamics. The engine was a direct descendant of the Atlas MA-3 booster engine, with removal of one thrust chamber and a rerouting of the plumbing to allow the engine to fit within the smaller Thor thrust section. Engine component tests began in March 1956. The first engineering model engine was available in June, followed by the first flight engine in September. Early Thor engines suffered from foaming turbopump lubricating oil at high altitudes and bearing retention issues, resulting in several launch failures. The initial Thor tests in 1957 used an early version of the Rocketdyne LR-79 engine with a conical nozzle and 135,000 lbf (600 kN) of thrust. By early 1958, this had been replaced by an improved model with a bell-shaped nozzle see and 150,000 lbf (670 kN) of thrust. The fully developed Thor IRBM had 162,000 lbf (720 kN) of thrust.
Phase I test launches
… excerpt ends here. Continue reading the full article.
