The Delta II space launch vehicle, now approaching the end of its remarkable life, is the final, most successful successor of the Thor intermediate range ballistic missile (IRBM). Since all of its contemporaries have retired, Delta II represents the final trace of the original U.S. “missile race” and “space race” launch systems.
In the United States, Atlas and Titan, the original Cold War intercontinental range ballistic missiles (ICBMs), won most of the space race glory. They moved memorably into public view by repeatedly launching NASA astronauts on widely-broadcast Mercury and Gemini missions. Atlas, with its shiny silver tanks and fire-breathing engines, and brutish, robotic Titan, with its exotic almost invisible exhaust, were both recognizable and unforgettable. Even Jupiter, the von Braun group’s less-successful Thor IRBM competitor, shared some glory by serving as the starting point for the Saturn launch vehicle family.
Thor, by comparison, gained relatively little public acclaim, but it thrived in its anonymity. It became the real U.S. space age workhorse, the backbone of U.S. efforts in space. Thor was the most dependable, most versatile, and most affordable orbital launcher in the U.S. arsenal. Thor was the first deployed U.S. IRBM, but it performed many other jobs. It launched live thermonuclear warheads into space (several tries failed, with nasty contaminating results.) Thor stood active anti-satellite duty for more years than it served as an IRBM. It flew suborbital satellite inspection intercepts, carrying a camera rather than a warhead.
Thor launched the first, tentative U.S. attempts to reach the Moon, sent U.S. satellites into deep space to sound the radiation belts and solar wind, launched the first weather, communications, navigation, and spy satellites, sent spacecraft to Mars, to asteroids, and to a comet.
Before Delta II retires, it will perform the 600th Thor-family orbital space launch attempt, most of any U.S. launcher and second in the world only to Russia’s R-7. Famed Atlas only attempted 326 orbital launches. Titan only lifted off intent on reaching orbit 219 times. The Thor family also surpassed its contemporaries in total launch numbers, including suborbital flights, having already flown more than 710 times.
Though ultimately proved to be very reliable, for the better part of 1957 it seemed that Thor would never fly true.
Thor
In 1955, with Atlas ICBM development underway, the U.S. initiated the Thor IRBM program. Thor (named after the Norse god of thunder, a god who protected humans against the forces of evil) was created as a stop-gap measure, to provide a ballistic missile deterrent for Western Europe until Atlas could enter service.
The U.S. Air Force Western Development Division defined Thor in July-August of 1955. Navy Commander Robert Truax and Dr. Adolph K. Thiel, a von Braun team member, prepared an early design study. Thor would leverage existing hardware. The missile would be powered by a single Rocketdyne LR79, 68 tonne thrust booster engine also used by Atlas and Jupiter. It would use Atlas vernier thrusters for roll control, a Mk 2 copper heat-sink reentry vehicle developed for Atlas, and an existing A.C. Spark Plug inertial guidance system. It would have 2,400 km range to target the Soviet Union from fixed launch sites in Great Britain. It would be small enough to be transported in Douglas C-124 aircraft.
Douglas Aircraft Company won the SM-75 Thor contract in December 1955, and sprinted forward on a program that was given top national priority. The company’s engineering team completed its detailed Thor design by July 1956, modifying the Air Force design by tapering the upper half of the missile. Douglas used 2014 alloy aluminum with 7.5 cm squares milled into a “waffle pattern” in the tank walls to shave weight. The missile’s aft LOX tank was cylindrical. Its forward, kerosene tank was tapered, as was the guidance section above the tank. A blunt Mk 2 heat-sink reentry vehicle, designed to carry a W-49 1.44 megaton thermonuclear warhead, topped the rocket. The resulting 19.82 x 2.44 meter missile weighed only 3.125 tonnes dry and 49.59 tonnes fully fueled. It carried a warhead “payload” that weighed roughly 1 tonne.
Thor was the poster child for “concurrency”, the parallel development of missile design, production capacity, launch sites, and crew training.
thorline2.jpg (31334 bytes)Douglas Aircraft’s formidable mass-production capacity was soon on display at Santa Monica, California, where the company ultimately manufactured hundreds of Thors. Rocketdyne moved swiftly as well, firing up a first battleship LR79 at Edwards Rocket Base during March 1956, and delivering the first engine to Douglas six months later.
Douglas delivered the first Thor to Cape Canaveral’s Hanger M on October 26, 1956. Three launch pads were prepared for Thor at the Cape. Seven pads were constructed at Vandenberg AFB (an eighth was added later for space launches), and sixty operational pads were built in Great Britain. Two additional Thor pads were ultimately constructed on tiny Johnston Island in the mid-Pacific.
The first launch, on January 25, 1957, failed when Thor 101 rose six inches, lost thrust, fell back on its Cape Canaveral Launch Complex 17B pad, and exploded. A failed liquid oxygen valve was to blame. Thor 102 flew for 35 seconds in April 1957 before being destroyed by range safety. The destruct command, it turned out, was triggered by incorrect wiring in range safety ground equipment. Thor 103 blew up on its pad when loaded with propellant awaiting launch, with several minutes still on the countdown clock, in May. A faulty pressure relief valve was the cause. Thor 104 flew for 92 seconds in August before suffering a flight control failure.
thorvafb1.jpg (9124 bytes)Finally, on September 29, 1957, less than two years after the program began, Thor 105 flew a successful long-range flight from the Cape. One month later, Thor 109 flew the first full-range flight down the Atlantic Missile Range.
Also read: Thor: Flown Variants
All-inertial guidance system test flights began in December, 1957, with the unsuccessful Thor 112 flight. Thor 113 finally proved the guidance system several weeks later. Reentry vehicle test flights began in February 1958 with the Thor 120 launch. The first launch from a tactical launch mount set up at Launch Complex 18B was performed by Thor 115 on June 4, 1958.
The first crew training launch from Vandenberg, which was also the first long-range launch from that West Coast base, took place on December 16, 1958, when Thor 151 flew successfully over the Pacific Ocean.
Thor became operational in Great Britain during December 1959. There, 60 Thors were deployed along 200 miles of eastern England coast. Missiles were sited at and around four former airfield bases: Driffield, Hemswell, Feltwell, and North Luffenham. Each base had five “squadrons”, each with three closely-sited missiles, for a total of 15 Thors per base. RAF crews manned the missiles while U.S. personnel maintained the warheads. A total of 1,000 personnel manned each base.
thorpaduk.jpg (16518 bytes)Thor missiles were stored horizontally in retractable steel shelters on an erector-launcher mount. Missiles could be launched within 15 minutes of an order. Readiness was provided by keeping the guidance system continously powered up. In the event of a firing order, the shelter would be retracted, the missile would be erected, LOX and kerosene would be loaded from tanks positioned on opposite sides of the pad, and the guidance platformed would be aligned.
Thor remained in service in Britain until August 1963. U.S. Atlas ICBMs were entering service by then. The Cuban missile crises may have provided extra incentive to speed up the planned replacement of the hair-trigger IRBMs, but the historical record has so far been silent about a possible link.
The formerly on-duty Thor missiles and warheads were returned to the United States. There, the missiles were soon reassigned to other duties. Although its IRBM service had ended, Thor was only beginning its program life.
Author:
by Ed Kyle, Updated 5/23/2009
