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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 Deployment to four bases was completed in 1960, but at the end of 1962 the Thors were withdrawn from Britain as part of the secret codicil of Kennedy's deal to end the Cuban Missile Crisis.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 |
| XSM-75 | XPGM-17A |
| 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.
SpecificationsNote: 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 |
[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).