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<\/p>\nWhen United Launch Alliance\u2019s final Delta II rocket deploys NASA\u2019s ICESat-II satellite on Saturday, the launch will bring down the curtain not just on the Delta II\u2019s own illustrious career, but also on a series of rockets dating back to the early days of the space race. Delta II is the ultimate evolution of the US Air Force\u2019s Thor missile, which first flew in 1957 and stood watch in Britain against Soviet aggression from 1959 to 1963.\n<\/p>\n
The Thor missile grew out of a 1955 study that called for a new type of missile, capable of delivering a nuclear warhead to targets 1,500 nautical miles (2,800 kilometers, 1,700 miles) from its launch site.<\/p>\n
At the time both the United States and the Soviet Union were developing their first intercontinental ballistic missiles (ICBMs), but it was thought that a shorter-range intermediate-range ballistic missile (IRBM) could be brought into service more quickly to gain the upper hand in the Cold War.<\/p>\n
This came at a time when US intelligence agencies believed that the Soviet Union could have missiles ready sooner than the United States, capable of striking targets in Western Europe and the United Kingdom.<\/p>\n
From the outset, Thor was intended to be developed and deployed quickly \u2013 when the Pentagon approved the program the Air Force\u2019s Western Development Division was directed to \u201cproceed at maximum pace\u201d.<\/p>\n
In late 1955 the US Air Force awarded the Douglas Aircraft Company a contract to develop the new missile, designated the XSM-75. The X, signifying experimental, would be dropped when the missile entered service, while Thor would later be redesignated PGM-17 after the Air Force changed its designation system.<\/p>\n
The new missile incorporated a Rocketdyne LR-79 engine, evolved from a powerplant originally developed for the boost stage of the Navaho missile and also known as the MB-3. This was a closely-related development to the MA-3 engines that would be used on early versions of Atlas, the primary intercontinental ballistic missile then under development in the United States, as well as the Jupiter IRBM that was being produced by the Army. A pair of vernier engines helped to steer the missile, providing roll control.<\/p>\n
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Thor missiles delivered to Johnston Island by C-124 \u201cGlobemaster II\u201d air cargo transports \u2013 via L2 Historical, uploaded by Ed Kyle<\/p>\n
The Thor missile stood about 19.8 meters (65 feet) tall with a diameter of 2.4 meters (8 feet) \u2013 dimensions that were driven by a requirement for the missile to be air-transportable via the C-124 Globemaster II cargo planes then in service with the Air Force.<\/p>\n
Astronomy (adsbygoogle = window.adsbygoogle || []).push({});<\/p>\n Thor had a sleek, tapered shape designed to improve its aerodynamic qualities, while the General Electric Mk.II Re-entry vehicle that would house the missile\u2019s W49 thermonuclear warhead gave it a blunt nose.<\/p>\n In January 1957, barely a year after Douglas were contracted to build it, the first Thor stood ready at Cape Canaveral\u2019s Launch Complex 17B (LC-17B). The maiden flight lifted off at about quarter to eight Eastern Time on the evening of 25 January (00:45 UTC on 26 January).<\/p>\n Thor 101 reached an altitude of about 15 centimeters (6 inches) before its engine lost thrust, sending the rocket crashing back to Earth. Thor exploded on impact with its launch pad.<\/p>\n Thor 101 fails \u2013 via L2 Historical \u2013 uploaded by Ed Kyle<\/p>\n Three months later, Pad 17B had been repaired and another Thor was in position to launch. This time the missile performed flawlessly. However, the range safety officer\u2019s console had been wired up incorrectly which led him to believe incorrectly that the rocket was off course \u2013 and he transmitted a self-destruct command about half a minute after liftoff.<\/p>\n These early setbacks were the start of a difficult period of testing for Thor \u2013 although many other early missiles were similarly affected by teething problems as the technology of the day was less refined and trial-and-error was part of the learning process. Thor was developed in a time before complex computer simulations when the only real way to test a design was to put it on the launch pad and light the blue touch paper.<\/p>\n Thor\u2019s third flight didn\u2019t make it as far as liftoff \u2013 about four minutes before the planned launch, during the final stages of fuelling, a valve failure caused too much pressure to build up in the rocket and the vehicle exploded. The fourth launch \u2013 the first to used Pad 17A \u2013 launched at the end of August 1957 but also failed.<\/p>\n Thor\u2019s first successful launch came on 20 September 1957 \u2013 at the fifth time of asking. The rocket lifted off from Launch Complex 17B, flying out over the Atlantic Ocean for about 1,770 kilometers (1,100 miles, 960 nautical miles). Despite the success, Thor would suffer a further six failures over its next eight launches \u2013 including two more explosions on the launch pad.<\/p>\n Testing continued into 1958, with increasing success. The development version of Thor, known as the DM-18, made 21 launches \u2013 six successful \u2013 before giving way to the DM-18A: the operational version of the missile whose most noticeable difference was the absence of fins around its base.<\/p>\n While testing continued at Cape Canaveral, Thor\u2019s first combat training launch (CTL) was made from Vandenberg Air Force Base on the Western Range in December 1958. These training launches would continue throughout Thor\u2019s service, being performed by both US Air Force and Royal Air Force crews.<\/p>\n A modified version of Thor, with an improved MB-3-2 engine and a more pointed nose cone to reduce drag, was used for three missions to test and improve the Eastern Range: this DM-18C configuration would later form the basis of the Thor DM-21 used as the basis of many satellite launch vehicles.<\/p>\n Between June and November 1962, eight Thor DSV-2E missiles were launched from Johnston Island in the Pacific Ocean as part of Operation Fishbowl, a series of high-altitude nuclear tests. The first launch was a demonstration flight, with the other seven launches carrying live thermonuclear warheads.<\/p>\n The first two live launches \u2013 codenamed Bluegill and Starfish \u2013 both failed. However the third attempt \u2013 Starfish Prime \u2013 resulted in a successful detonation of the W49 warhead at an altitude of 400 kilometers (250 miles, 220 nautical miles) on 9 July. Radiation from the test damaged several satellites in orbit at the time.<\/p>\n Johnson Island \u2013 via L2 Historical, uploaded by Ed Kyle<\/p>\n After two more unsuccessful launches, the Bluegill Triple Prime test was completed on 26 October with the W50 warhead detonating at an altitude of 50 kilometers (30 miles, 27 nautical miles). The final Thor launch to carry a live warhead was Kingfish, on 1 November, which detonated at 97 kilometers (60 miles, 52 nautical miles)<\/p>\n Thor missiles were operated by the Royal Air Force under the Anglo-American Project Emily, whereby the missiles and their targeting were placed under British control \u2013 with British crews responsible for preparing and launching them \u2013 however the warheads remained under US jurisdiction. Officers from both countries would be responsible for launching the missiles should the need have arisen, with separate keys to enable launch and to arm the nuclear weapon.<\/p>\n The first of sixty Thors arrived in the United Kingdom in August 1958 and the missiles began to go on alert the following year.<\/p>\n An RAF Thor rocket \u2013 via Mercury Press<\/p>\n Also in 1962, NASA\u2019s Applications Vertical Test Program used two single-stage DSV-2D rockets for missions named Big Shot 1 and 2, demonstrating the deployment of a mylar balloon for the Echo project \u2013 which aimed to test the use of large balloons to facilitate communication.<\/p>\n The RAF\u2019s deployment of Thor continued until 1963 when the missiles were returned to the United States. By this time the advent of intercontinental ballistic missiles, such as Atlas and Titan, had reduced American reliance on missiles stationed in Europe. The missile\u2019s fuel mixture \u2013 kerosene and liquid oxygen \u2013 also proved impractical for a missile that was expected to launch at a moment\u2019s notice: liquid oxygen boils off at ambient temperatures so the missile had to be fuelled just before launch, reducing its reaction time. The introduction of storable, and later solid-fuelled, propellants rendered Thor obsolete.<\/p>\n Having also canceled its own Blue Streak missile, Britain had intended to replace Thor with the American-built air-launched Skybolt missile. After this project was canceled by the US Government, the UK eventually reached an agreement to purchase Polaris submarine-launched ballistic missiles instead.<\/p>\n<\/p>\n![\"\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2015\/09\/2018-09-12-205203.jpg\")
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