PGM-17 Thor - Initial Development As An IRBM

Initial Development As An IRBM

Development of the Thor was initiated by the USAF in 1954 as a tactical ballistic missile. 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 CEP of 2 miles (3.2 km). This range would allow Moscow to be hit from a launch site in the UK.

The initial design studies were headed by Cmdr. Robert Truax (US Navy) and Dr. Adolph K. Thiel (Ramo-Wooldridge Corporation, formerly of Redstone Arsenal). They refined the specs to an IRBM with:

• A 1,750 miles (2,820 km) range
• 8 ft (2.4 m) diameter, 65 ft (20 m) long (so it could be carried by Douglas C-124 Globemaster)
• A gross takeoff weight of 110,000 lb (50,000 kg)
• Propulsion provided by half of the Navaho-derived Atlas booster engine (due, largely, to the lack of any alternatives at this early date)
• 10,000 mph (4.5 km/s) maximum speed during warhead reentry
• Inertial guidance system with radio backup (for low susceptibility to enemy disruption)

On November 30, 1955 three companies were given one week to bid on the project: Douglas, Lockheed, and North American Aviation. They were asked to create "a management team that could pull together existing technology, skills, abilities, and techniques in 'an unprecedented time.'" On December 27, 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 further refined the design by choosing bolted tank bulkheads (as opposed to the initially suggested welded ones) and a tapered fuel tank for improved aerodynamics. The engine was developed as a direct descendant of the Atlas MA-3 booster engine. Changes involved removal of one thrust chamber and a rerouting of the plumbing to allow the engine to fit within the smaller Thor boat-tail. Engine tests were being performed as of March 1956. The first engineering model engine was available in June, followed by the first flight engine in September. Engine development was complicated by serious turbopump problems. Early Thor engines suffered from what was known as "bearing walking", whereby the turbopump bearings shift axially within their housing, causing rapid wear and the bearings to seize.