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Douglas SM-75/PGM-17 Thor

The Thor was the first Intermediate Range Ballistic Missile (IRBM) deployed by the U.S. armed forces. Although its military career was relatively short, its descendants are still in use as space launch vehicles.

Development began in 1954 with USAF studies about a 2400 km (1500 miles) range ballistic missile to complement the long-range ICBM. Soviet ballistic missile progress resulted in a decision in 1955 to develop an IRBM, to be named Thor, as quickly as possible. Using existing components (the Rocketdyne S-3D liquid-fuel rocket motor from the Army's SM-78/PGM-19 Jupiter IRBM, and the inertial guidance unit and Mk.2 reentry vehicle from the SM-65D/CGM-16D Atlas), and requiring the missile to be air-transportable by C-124 Globemaster transport aircraft, the basic design and overall dimensions of the Thor were quickly determined. Go-ahead for development was given in September 1955, and in December 1955, Douglas was selected as prime contractor for the SM-75 Thor IRBM.

Because of the many existing components development was extremely quick, and production of test missiles began as soon as the drawings were completed in August 1956. Testing of the XSM-75 missile began in December 1956, but the first launch attmepts all failed - sometimes in spectacular explosions - and the first successful flight finally occurred in September 1957. Other than the later production missiles, the first XSM-75s had small stabilizing fins at the base of the rocket. After the Soviet Sputnik launch in October 1957, the IRBM program was again accelerated, and Thor was ordered into full production in November 1957. In the next month, the first flight with a fully operational guidance system succeeded. All operational SM-75 missiles were stationed in Great Britain beginning in September 1958. Deployment was completed in June 1960 with 60 missiles at four bases. By then, the Royal Air Force had taken over the operation of the Thor bases and missiles.

Photo: USAF Photo: Boeing
XSM-75 (XPGM-17A)SM-75 (PGM-17A)

The SM-75 was a single-stage rocket, powered by a single Rocketdyne S-3D (designated LR79-NA by the USAF) engine fueled by kerosene and liquid oxygen. The complete main propulsion system was designated as MB-3. Two small Rocketdyne LR101 vernier engines were used for fine-tuning thrust and directional control. The Thor could carry a 1.45 MT W-49 thermonuclear warhead to a distance of 2400 km (1500 miles), and the all-inertial guidance unit achieved an accuracy of somewhere between 300 m (1000 ft) and 3200 m (2 miles) CEP (sources vary). To protect them from conventional attacks and the weather, the missiles were stored horizontally in soft shelters at the base. After the launch order, the missile would be raised into the vertical, for fueling and launch. This resulted in an overall reaction time of about 10 minutes. Unarmed training missiles were designated USM-75.

Photo: Rob Svirskas, CCAFS Virtual Tour
SM-75 (PGM-17A)

During 1962, the Thor was used in a series of exo-atmospheric nuclear tests (called "Starfish", "Bluegill", and "Kingfish"), including the explosion of a 1.4 MT device at an altitude of 450 km (280 miles). Also in 1962, the USAF already started to plan the retirement of the SM-75 IRBM. The intended replacements were the GAM-87/AGM-48 Skybolt ALBM (Air-Launched Ballistic Missile), which was later cancelled, and the Navy's UGM-27 Polaris SLBM (Submarine-Launched Ballistic Missile). The first Thor was removed from base in November 1962, and by September 1963, all Thors had been deactivated and moved back to the USA. Production of Thor IRBMs totaled about 225, with a peak deployment level of 60 missiles.

In June 1963, shortly before its retirement in the UK, all Thor missiles were redesignated in the PGM-17 series as follows:

Old Designation New Designation
SM-75 PGM-17A
USM-75 PTM-17A

The final chapter in the military career of the Thor was its use as an anti-satellite weapon. In February 1962 the USAF had started Program 437 to provide for a nuclear ASAT (anti-satellite) capability. Unarmed tests of Thors as ASAT missiles began in February 1964, and by September 1964 the ASAT Thor was declared operational. From that time until the retirement in December 1972, the ADC (Air Defense Command) always had two Thor ASAT launchers on 24h alert. The designation of the Thor in the ASAT role was apparently still PGM-17A, although a redesignation to PIM-17A would have been appropriate.

After retirement as an IRBM, disarmed PGM-17As, as well as new-built Thors, were used by the USAF as space launch vehicles under the basic designation of SLV-2. The Thor was developed by McDonnell Douglas (now Boeing) into the very successful Delta family of space launchers, still in use today. In 1990 the official designation of SB-3A was assigned to the USAF's Delta II rockets. For details on Thor/Delta launch vehicles, see page on SLV-2/SB-3.


Note: Data given by several sources show slight variations. Figures given below may therefore be inaccurate!

Data for PGM-17A:

Length19.8 m (65 ft)
Diameter2.44 m (8 ft)
Weight49800 kg (110000 lb)
Speed16100 km/h (10000 mph)
Ceiling480 km (300 miles)
Range2400 km (1500 miles)
PropulsionMain: Rocketdyne LR79-NA-9 (Model S-3D); 666 kN (150000 lb)
Vernier: 2x Rocketdyne LR101-NA; 4.5 kN (1000 lb) each
WarheadW-49 thermonuclear (1.45 MT) in Mk.2 RV

Main Sources

[1] James N. Gibson: "Nuclear Weapons of the United States", Schiffer Publishing Ltd, 1996
[2] Bill Gunston: "The Illustrated Encyclopedia of Rockets and Missiles", Salamander Books Ltd, 1979

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Last Updated: 27 July 2007