| SM-75/PGM-17A Thor | |
|---|---|
A Thor intermediate-range ballistic missile. |
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| Type | Intermediate Range Ballistic Missile (IRBM) |
| Place of origin | United States |
| Service history | |
| Used by | United States Air Force (testing) Royal Air Force (operational deployment) |
| Production history | |
| Designed | 1957 |
| Manufacturer | Douglas Aircraft |
| Produced | 1959-1960 |
| Number built | About 225; peak deployment was 60 |
| Variants | Delta rockets |
| Specifications | |
| Weight | 109,330 lbs. at launch |
| Length | 64 ft. 10 in. |
| Diameter | 8 ft. 0 in. |
Thor was the first operational ballistic missile of the U.S. Air Force (USAF). Named after the Norse god of thunder, it was deployed in the United Kingdom between 1959 and September 1963 as an intermediate range ballistic missile (IRBM) with thermonuclear warheads. Thor was 65 feet (20 m) in height and 8 feet (2.4 m) in diameter. It was later augmented in the U.S. IRBM arsenal by the Jupiter.
A large family of space launch vehicles—the Delta rockets—were derived from the Thor design. A modified version was still in use in 2011 as the first stage of the Delta II.
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Fearful that the Soviet Union would deploy a long-range ballistic missile before the U.S., in January 1956 the USAF began developing the Thor, a 1,500 miles (2,400 km) intermediate-range ballistic missile. The Thor program unfolded quickly, and within three years of the program’s inception the first of 20 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 U.S. became operational, Thor missiles were quickly retired. The last of the missiles was withdrawn from operational alert in 1963.
A small number of Thors, converted to "Thrust Augumented Delta" 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 Ocean 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 remain in storage, and could be reactivated, though the W-49 Mod 6 warheads were all dismantled by June 1976.
Development of the Thor was initiated by the USAF in 1954 as a tactical ballistic missile. The goal was a missile system that could deliver a nuclear warhead over a distance of 1,150 to 2,300 miles (1,850 to 3,700 km) with a CEP of 2 miles (3.2 km). 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 Corporation, formerly of Redstone Arsenal). They refined the specs to an IRBM with:
On November 30, 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 27, 1955 Douglas 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 turbopump problems. Early Thor engines suffered from what was known as "bearing walking", whereby the turbopump bearings shift axially within their housing, causing rapid wear and the bearings to seize.
Thor test launches were to be from LC-17 at Cape Canaveral Missile Annex. 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 LC-17B was just ready for the first test flight.
The first flight of the Thor IRBM was on January 25, 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-pressurization 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 (2,400 km).
1957 saw five more flights, the longest of which covered 2,700 miles (4,300 km).
Thor was deployed to the UK starting in August 1958, operated by 20 squadrons of RAF 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 60 of the Thor missiles deployed in the UK 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 used an electric motor to roll back the missile shelter (essentially a long shed mounted on steel rails), then used a powerful hydraulic launcher-erector to lift the missile to an upright position for launch. Once it was standing on the launch mount, the missile was fueled 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 (450 km), 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 warhead with an explosive yield of 1.44 megatons.
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.
see Project Emily Stations and Squadrons
For RAF Squadrons list:
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