Encyclopedia Astronautica

Credit: © Thomas Kladiva - Thomas Kladiva
RAF Training Launch
Successful training launch by the Royal Air Force with the USAF 392nd Squadron acting as instructors. This launch was from the launch complex that consisted of launch pads 6, 7 and 8. During this time we were using all three launch pads for RAF training. This was the fourth RAF Thor training launch. Launch date was August 3, 1959.
Credit: Larry Rhoads
392nd Thor Squadron
Vandenberg 392nd Squadron Thor launch personnel, Vandenberg, 1962 or 1963.
Credit: Larry Rhoads
Credit: via Andreas Parsch
Thor Burner 2A
Thor Burner 2A - COSPAR 1975-043
Thor Burner
Thor-Burner 2 - COSPAR 1967-065
4300SS Thor Squadron
The USAF 392nd squadron trained the Royal Air Force with live launches. When the Thor missile was phased out in England, the 392nd become the 4300SS squadron with several 'all blue uniform' programs. This is a picture of the 4300SS squadron personnel taken sometime early 1964 or late 1963
Credit: Larry Rhoads
American liquid propellant intermediate range ballistic missile, developed by Douglas in 1956-1958. 60 deployed to Britain in 1958-1962. The basis for a family of Thor and Delta space launch vehicles, remaining in production into the 2010's.

In 1954 the USAF realized that the Soviet Union and perhaps more worryingly, the US Army was developing intermediate range ballistic missiles which would be deployed several years before the Air Force's Atlas ICBM. The result was Thor, a crash December 1955 program to produce an Air Force intermediate range ballistic missile with the same range as the Army's Jupiter. USAF Missile Czar General Bernard Schriever dictated that the missile would be air-transportable in a C-124 Globemaster, use the inertial guidance, re-entry vehicle, and nuclear warhead being developed for Atlas. A single Rocketdyne 150,000 lb (68-metric-ton) thrust lox/kerosene engine would power the missile. This was to have been used as the sustainer engine for Atlas when the requirement was to launch a 2700 kg thermonuclear warhead over an intercontinental range. When it became apparent that the warhead could be reduced to less than half that weight, Atlas was downsized, and the large sustainer motor was no longer required. But the design was a perfect fit for Thor (related designs from Rocketdyne with the same thrust were under development for the Navaho cruise missile and the Jupiter).

Douglas was selected as prime contractor in December 1955 for one of the most accelerated crash programs in history. 'Chief Designer' of the Thor was Jack Bromberg, hired by Donald Douglas in the 1930's. Although not educated formally as an engineer, he was smart and dynamic, and a major influence in Douglas' winning bid to the USAF. Bromberg managed to fly a first prototype only thirteen months after go-ahead. In September 1958 the first of sixty Thors were deployed to missile sites in Great Britain under Project Emily. Deployment to four bases was completed in 1960, but at the end of 1962 the Thors were withdrawn from Britain as part of the secret codicil of Kennedy's deal to end the Cuban Missile Crisis. A few continued in an offensive military role as a nuclear-tipped anti-satellite system, based on Johnson Atoll in the Pacific, until the 1970's. Surplus Thors were used for a variety of suborbital re-entry vehicle tests. However Thor remained in production as a space launch vehicle, with a variety of upper stages. Dubbed the Delta by NASA, with solid rocket boosters, upgrades to the main engine, stretches to the first stage, and improvements to the upper stage, it became a workhorse of the American space program. Still in production in the 2010's, Jack Bromberg's stop-gap design became America's most reliable, most economical, and longest-lived launch vehicle.

Development Cost $: 500.000 million. Recurring Price $: 6.250 million in 1958 dollars. Flyaway Unit Cost $: 0.750 million in 1958 dollars. Standard warhead: 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. Warhead yield: 1,440 KT. CEP: 0.97 km (0.60 mi). Boost Propulsion: Liquid rocket, Lox/Kerosene. Maximum speed: 17,740 kph (11,020 mph). Initial Operational Capability: 1958. Total Number Built: 224. Total Development Built: 64. Total Production Built: 160.

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:

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.


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

Data for PGM-17A:

Length 19.8 m (65 ft)
Diameter 2.44 m (8 ft)
Weight 49800 kg (110000 lb)
Speed 16100 km-h (10000 mph)
Ceiling 480 km (300 miles)
Range 2400 km (1500 miles)
Propulsion Main: Rocketdyne LR79-NA-9 (Model S-3D); 666 kN (150000 lb)
Vernier: 2x Rocketdyne LR101-NA; 4.5 kN (1000 lb) each
Warhead W-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

AKA: B-75; PGM-17A; Thor; SM-75.
Gross mass: 50,000 kg (110,000 lb).
Payload: 1,000 kg (2,200 lb).
Height: 19.82 m (65.02 ft).
Diameter: 2.44 m (8.00 ft).
Span: 2.74 m (8.98 ft).
Thrust: 666.00 kN (149,722 lbf).

More... - Chronology...

Associated Countries
Associated Engines
  • LR79-7 Rocketdyne Lox/Kerosene rocket engine. 758.7 kN. Out of production. Designed for booster applications. Gas generator, pump-fed. Isp=282s. First flight 1957. More...

See also
  • Delta The Delta launch vehicle was America's longest-lived, most reliable, and lowest-cost space launch vehicle. Development began in 1955 and it continued in service in the 21st Century despite numerous candidate replacements. More...
  • missile Guided self-propelled military weapon (as opposed to rocket, an unguided self-propelled weapon). More...

Associated Manufacturers and Agencies
  • Douglas American manufacturer of rockets, spacecraft, and rocket engines. Boeing Huntington Beach, Huntington Beach, CA, USA. More...

Thor Chronology

1955 December 1 - . LV Family: Delta. Launch Vehicle: Thor.
  • Missile programs get highest priority. - . Nation: USA. Related Persons: Eisenhower. Summary: President Eisenhower assigned highest priority to ICBM and Thor and Jupiter IRBM programs..

1956 August 27 - . LV Family: Delta. Launch Vehicle: Thor.
  • Thor engine first static test. - . Nation: USA. Summary: First static firing of Thor rocket engine at AFFTC, Edwards AFB..

1957 November 27 - . LV Family: Delta. Launch Vehicle: Thor.
  • Thor and Jupiter IRBM's ordered into production. - . Nation: USA. Summary: Thor and Jupiter IRBM's ordered into production for ultimate deployment by the USAF..

1958 January 1 - . LV Family: Delta. Launch Vehicle: Thor.
  • Thor put into operational service. - . Nation: USA. Summary: 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..

1961 October 13 - . LV Family: Delta. Launch Vehicle: Thor.
  • Thor 100th launch. - . Nation: USA. Summary: 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..

1962 June 8 - . Launch Site: Vandenberg. LV Family: Delta. Launch Vehicle: Thor.
  • Last launch of a Thor IRBM from Vandenberg - . Nation: USA. Summary: Last launch of a Thor IRBM from Vandenberg. (First launch and first missile fired from Vandenberg AFB on 16 December 1958.).

1964 June - . LV Family: Delta. Launch Vehicle: Thor. LV Configuration: Thor LV-2D.
  • Program 437 ASAT declared operational. - . Nation: USA. Spacecraft: Program 437. Summary: 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..

1966 March 30 - . Launch Site: Vandenberg. LV Family: Delta. Launch Vehicle: Thor.
  • Final mission of the Thor/Altair from Vandenberg - . Nation: USA. Summary: Final mission of the Thor/Altair from Vandenberg AFB (first launch on 18 January 1965)..

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