Thor Missile

Design and Development

Douglas PGM-17 Thor Missile

Fearful that the Soviet Union would deploy a long-range ballistic missile before the United States, in January 1956 the Air Force began developing the Thor, a 1,500 miles intermediate-range ballistic missile (IRBM). The Thor program unfolded with amazing speed, and within 3 years of the program’s inception the first of twenty 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 IRBM, the Thor could be carried by the USAF's cargo aircraft of the time, which made its deployment more rapid, although the launch facilities were not transportable, and had to be built on site. The Thor was a stop-gap measure, however, and once the first generation of ICBMs based in the United States became operational, existing Thor missiles were quickly retired. The last of the missiles was withdrawn from operational alert in 1963.

A small number of Thor's, converted to Thrust Augmented Delta, and launchers remained operational in the Anti-Satellite missile role as Program 437 until April 1975. These missiles were based on Johnston Island in the Pacific 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. These missiles remained in storage, but could be reactivated, though the W-49 Mod 6 warheads were all dismantled by June 1976.

Development of the Thor was initiated by the US Air Force in 1954 as a Tactical Ballistic Missile. The goal was a missile system that could deliver a nuclear warhead over a distance of 1150 to 2300 miles with a CEP of 2 miles. 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 Corp, formerly of Redstone Arsenal). They refined the specs to an IRBM with:

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

On November 30th, 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 27th, 1955 Douglas Aircraft Corporation 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 turbo pump problems. Early Thor engines suffered from what was known as "bearing walking", whereby the turbo pump bearings shift axially within their housing, causing rapid wear and the bearings to seize.

Thor test launches were to be from LC17 at Cape Canaveral. The development schedule was so compressed that plans for the Atlas bunker were used to allow the completion of the facility in time. Nevertheless pad LC17-B was just ready for the first test flight.

The first flight of the Thor IRBM was on 25th January 1957. The first airframe, number 101, was delivered in October of the previous year. The vehicle reached an apogee of 6 inches (150 mm) whereupon contamination destroyed a LOX supply valve causing the engine to lose thrust. The Thor slid backwards through the launch ring and exploded on contact with the thrust deflector. Serious pad damage occurred. The second Thor flight (102) lasted 35 seconds after an April 1957 launch. It was ended by a range safety officer who destroyed the missile after seeing faulty data on a readout which indicated that the missile was heading inland over Florida.

Thor vehicle 103 (May 1957) exploded on the pad during tanking due to a faulty main fuel valve resulting in tank over pressurisation leading to tank rupture. Thor vehicle 104 (August 1957) broke up after 92 seconds due to a loss of guidance. Thor vehicle 105 (20 September 1957), 21 months after the start of construction, flew 1,100 miles (1,800 km) downrange. Estimated range without the extra load of the R and D instrumentation was 1,500 miles. 1957 saw five more flights of the Thor missile, the longest of which covered 2,700 miles.

The Thor rocket was also used as a space launch vehicle. It was the first in a large family of space launch vehicles—the Delta rockets. Thor's descendants fly to this day as the Delta II and Delta IV.

Operational History

Thor was deployed to the UK starting in August 1958, operated by twenty squadrons of Royal Air Force Bomber Command. The first active unit was No.77 Squadron RAF at RAF Feltwell in 1958, with the remaining units becoming active in 1959. All were deactivated by September 1963.

All sixty of the Thor missiles deployed in the United Kingdom were based at above-ground launch sites. The missiles were stored horizontally on transporter-erector trailers and covered by a retractable missile shelter. To fire the weapon, the crew electronically rolled back the missile shelter and then, using a powerful hydraulic launcher-erector, lifted the missile to an upright position. Once it was standing on the launch mount, the missile was fuelled and fired. The entire launch sequence took about 15 minutes. Main engine burn time was almost 2.5 minutes, boosting the missile to a speed of 14,400 ft/s (4,400 m/s). Ten minutes into its flight the missile reached an altitude of 280 miles, close to the apogee of its elliptical flight path. At that point the reentry vehicle separated from the fuselage and began its descent down toward the target. Total flight time from launch to impact was about 18 minutes.

The Thor was initially deployed with a very blunt conical G.E. Mk 2 'heat sink' re-entry vehicle. They were later converted to the slender G.E. Mk 3 ablative RV. Both RVs contained a W-49 thermonuclear bomb of 1.44 megatons yield.


Length:19.82 m (65.0 ft)
Launch Weight:49,800 kg (110,000 lb)
Range:1500 miles
Maximum Speed:Mach 17.7
Propulsion: Liquid fuelled rocket, Liquid Oxygen and Kerosene