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Thor IRBM

A Thor intermediate-range ballistic missile.

Thor was the first operational ballistic missile deployed by 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 Thor and Delta rockets—were derived from the Thor design. The Delta II is still in active service as of 2017 and with the retirement of Atlas and Titan in the mid-2000s is the last surviving "heritage" launch vehicle in the US fleet, being derived from a Cold War-era missile system.

Design and developmentEdit

Fearful that the Soviet Union would deploy a long-range ballistic missile before the United States, in January 1956 the Air Force began developing the Thor, a 1,500 miles (2,400 kilometres) intermediate-range ballistic missile (IRBM). The Thor program unfolded with amazing speed, and within 3 years of the program’s inception the first of twenty 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 United States became operational, existing 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 The last of the missiles was withdrawn from operational alert in 1963.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 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.

Initial development as an IRBMEdit

Development of the Thor was initiated by the US Air Force 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 kilometres) with a CEP of 2 miles (3.2 kilometres). 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:

  • A 1,750 miles (2,820 kilometres) range
  • 8 ft (2.4 m) diameter, 65 ft (20 m) long (so it could be carried by Douglas C-124 Globemaster)
  • A gross takeoff weight of 110,000 lb (50,000 kg)
  • Propulsion provided by half of the Navaho-derived Atlas booster engine (due, largely, to the lack of any alternatives at this early date)
  • 10,000 mph (4.5 km/s) maximum speed during warhead reentry
  • AC Spark Plug inertial guidance system with radio backup (for low susceptibility to enemy disruption)

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 Aircraft Corporation 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.

First launchesEdit

Thor test launches were to be from LC17 at Cape Canaveral. 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 LC17-B was just ready for the first test flight.

The first flight of the Thor IRBM was on 25 January 1957. The first airframe, number 101, was delivered in October of the previous year. The vehicle reached an apogee of 6 inches (150 millimetres) 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 overpressurization 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 kilometres) downrange. Estimated range without the extra load of the R and D instrumentation was 1,500 miles (2,400 kilometres).

1957 saw five more flights, the longest of which covered 2,700 miles (4,300 kilometres).

DeploymentEdit

Thor was deployed to the UK starting in August 1958, operated by twenty 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 sixty of the Thor missiles deployed in the United Kingdom 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 kilometres), 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.

Noteworthy Thor IRBM flightsEdit

  • 4 June 1962, failed Starfish flight, Thor destroyed, nuclear device lost.
  • 20 June 1962, failed Bluegill Prime flight, Thor destroyed, nuclear device lost.
  • 9 July 1962, Thor missile 195 launched a Mk4 reentry vehicle containing a W49 thermonuclear warhead to an altitude of 250 miles (400 kilometres). The warhead detonated with a yield of 1.45 Mt of TNT (6.07 PJ). This was the Starfish Prime event of nuclear test operation Dominic-Fishbowl.

Thor becomes a launch vehicleEdit

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.

OperatorsEdit

United States
United States Air Force
Royal Air Force

SpecificationsEdit

  • Popular Name: Thor.
  • Type: Intermediate range ballistic missile.
  • IOC: 1958. Year: 1958.
  • Family: Thor IRBM, Thor DM-18 (single stage LV); Thor DM-19 (rocket 1st stage), Thor DM-21 (rocket 1st stage), Thor DSV-2D,E,F,G (suborbital LV), Thor DSV-2J (anti-ballistic missile), Thor DSV-2U (orbital launch vehicle).
  • Country: USA. Department of Defense
  • Designation: PGM-17A.
  • Alternate Designation: SM-75, PTM-17A, USM-75
  • Manufacturer: Douglas Aircraft Company
  • Thrust (vac): 760 kN
  • Liftoff Thrust (sl): 670 kN (150,000 lbf)
  • Isp: 282 s (2.77 kN·s/kg)
  • Isp(sl): 248 s (2.43 kN·s/kg)
  • Burn time: 165 s
  • Core Diameter: 2.44 m
  • Total Length: 19.82 m (65.0 ft)
  • Span: 2.74 m (9.0 ft)
  • Weight: 49,800 kg (109,800 lb)
  • Empty Weight: 3,125 kg
  • Standard warhead mass: 1,000 kg
  • Maximum range: 2,400 km (1,500 mi)
  • Ceiling: 480 km (300 mi)
  • Number Standard Warheads: 1.
  • Standard RV: Mk. 2.
  • Standard warhead: W49.
  • Standard warhead yield: equivalent to 1,440 kilotons of TNT (6.02 PJ)
  • Standard warhead CEP: 1 km
  • Boost Propulsion: Liquid fuelled rocket, LOX and Kerosene.
  • Power plant:
    • Main: Rocketdyne LR79-NA-9 (Model S-3D); 666 kN (150000 lbf)
    • Vernier: 2x Rocketdyne LR101-NA; 4.5 kN (1000 lbf) each
    • Manufacturer Name: MB-1.
    • Government Designation: LR-79-7.
    • Designer: Rocketdyne.
    • Developed in: 1955.
    • Application:
    • Used on stages: Thor DM-19.
    • Used on launch vehicles: Delta, Thor, Thor Able, Thor Agena A, Thor Agena B, Thor Agena D, Thor Burner.
    • Propellants: LOX/Kerosene (Thor kerosene propellant was referred to as 'RP1' by RAF users)
    • Thrust (vac): 760 kN
    • Isp: 282 s (2.77 kN·s/kg)
    • Isp (sea level): 248 s (2.43 kN·s/kg)
    • Burn time: 165 s
    • Mass Engine: 643 kg
    • Diameter: 2.44 m
    • Chambers: 1
    • Chamber Pressure: 4.1 MPa
    • Area Ratio: 8.00
    • Thrust to Weight Ratio: 120.32
    • Country: USA
    • Status: Out of Production.
    • First Flight: 1958
    • Last Flight: 1980
    • Flown: 145.
    • Comments: Designed for booster applications. Gas generator, pump-fed
  • Guidance: Inertial
  • Maximum speed: 17,740 km/h (11,020 mph)
  • Development Cost US dollars: $500 million
  • Recurring Price US dollars: $6.25 million
  • Total Number Built: 224
  • Total Development Built: 64
  • Total Production Built: 160
  • Flyaway Unit Cost: US$750,000 in 1958 dollars
  • Launches: 59
  • Failures: 14
  • Success Rate: 76.27%
  • First Launch Date: 25 January 1957
  • Last Launch Date: 5 November 1975

See alsoEdit

ReferencesEdit

  • Boyes, John. Project Emily: The Thor IRBM and the Royal Air Force 1959–1963. Prospero, Journal of the British Rocketry Oral History Programme (BROHP) No 4, Spring 2007.
  • Boyes, John. Project Emily: Thor IRBM and the RAF. Tempus Publishing, 2008. ISBN 978-0-7524-4611-0.
  • Boyes, John. The Thor IRBM: The Cuban Missile Crisis and the subsequent run-down of the Thor Force. pub: Royal Air Force Historical Society. Journal 42, May 2008. ISSN 1361 4231.
  • Forsyth, Kevin S. Delta: The Ultimate Thor. In Roger Launius and Dennis Jenkins (Eds.), To Reach The High Frontier: A History of U.S. Launch Vehicles. Lexington: University Press of Kentucky, 2002. ISBN 0-8131-2245-7.
  • Hartt, Julian. The Mighty Thor: Missile in Readiness. New York: Duell, Sloan, and Pearce, 1961.

For RAF Squadrons list:

  • Jefford, Wing Commander C.G., MBE, BA, RAF(Retd.). RAF Squadrons, a Comprehensive record of the Movement and Equipment of all RAF Squadrons and their Antecedents since 1912. Shrewsbury, Shropshire, UK: Airlife Publishing, 1988 (second edition 2001). ISBN 1-85310-053-6. p. 178.
  • Wynn, Humphrey. RAF Strategic Nuclear Deterrent Forces, their Origins, Roles and Deployment 1946-69. London: HMSO, 1994. ISBN 0-11-772833-0. p. 449.
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External linksEdit


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