Thor IRBM Credit - © Thom Media Gallery

Also PTM-17A, USM-75

Historical Essay © Andreas Parsch

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.

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 300+ m (1000+ ft) CEP. 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.

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:

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.

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

Data for PGM-17A:

[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

Manufacturer: Douglas. Liftoff Thrust: 666.000 kN (149,722 lbf). Total Mass: 50,000 kg (110,000 lb). Core Diameter: 2.44 m (8.00 ft). Total Length: 19.82 m (65.02 ft). Span: 2.74 m (8.98 ft). Standard warhead mass: 1,000 kg (2,200 lb). Maximum range: 2,400 km (1,400 mi). Number Standard Warheads: 1. Standard RV: Mk. 2. Standard warhead: W49. Standard warhead yield: 1,440 KT. Standard warhead CEP: 0.97 km (0.60 mi). Boost Propulsion: Liquid rocket, Lox/Kerosene. Guidance: Inertial. Maximum speed: 17,740 kph (11,020 mph). Development Cost $: 500.000 million. in: 1958 average dollars. Recurring Price $: 6.250 million. Total Number Built: 224. Total Development Built: 64. Total Production Built: 160. Flyaway Unit Cost $: 0.750 million. in: 1958 unit dollars.

1955 December 1 - Launch Vehicle: Thor.

• Missile programs get highest priority. Nation: USA. President Eisenhower assigned highest priority to ICBM and Thor and Jupiter IRBM programs. References: 17.

• Thor engine first static test. Nation: USA. First static firing of Thor rocket engine at AFFTC, Edwards AFB. References: 17.

• Thor and Jupiter IRBM's ordered into production. Nation: USA. Thor and Jupiter IRBM's ordered into production for ultimate deployment by the USAF. References: 17.

• Thor put into operational service. Nation: USA. Strategic Air Command assigned responsibility for U.S. operational ICBM capability; while the 672nd Strategic Missile Squadron, first to be equipped with USAF Douglas Thor IRBM, was activated. References: 17.

• Thor 100th launch. Nation: USA. Discoverer XXXII was placed into polar orbit; its capsule contained components of USAF satellite systems. This marked the 100th successful firing of the Thor booster rocket. References: 18.

• Last launch of a Thor IRBM from Vandenberg Nation: USA. Last launch of a Thor IRBM from Vandenberg. (First launch and first missile fired from Vandenberg AFB on 16 December 1958.) References: 88.

• Program 437 ASAT declared operational. Nation: USA. Spacecraft: Program 437. The system had a two week reaction time - the missiles and nuclear warheads were stored in kits at Vandenberg and had to be deployed to the Johnson Atoll launch site.

• Final mission of the Thor/Altair from Vandenberg Nation: USA. Final mission of the Thor/Altair from Vandenberg AFB (first launch on 18 January 1965). References: 88.