Thor

In 1946, as the US Army fired off the V-2 missiles that it had captured from Germany, the Naval Research Laboratory ordered a study of a high-performance sounding rocket to be its successor. This led to the issuing of a contract to the Glenn L. Martin Company to develop the Viking, utilising the XLR-10 rocket motor that (as in the case of the engine of the V-2) burned alcohol and oxygen.

On 1 August 1955 President Dwight Eisenhower told the Navy to manage the Vanguard project to place a satellite into orbit during the International Geophysical Year, which was to run from mid 1957 to the end of 1958. On 7 October the Navy gave Martin the contract to upgrade the Viking with General Electric's X-405 kerosene-burning motor, and to install two upper stages to enable it to function as a launch vehicle. For the second stage, Aerojet supplied an AJ-10 motor based on the Aerobee-Hi sounding rocket that used hypergolic propel-lants. The third stage was a 33-KS-2800 solid rocket by the Grand Central Rocket Company. In the event, the goal of being the first to place a satellite into orbit was lost to the Soviets on 4 October 1957. The attempt to even the score using the new Vanguard on 6 December was a fiasco, with the rocket losing thrust after a few seconds, falling back, and exploding. The next attempt failed after 57 seconds when the vehicle lost control, but the third launch on 17 March 1958 succeeded.3 As the second stage had the flight control apparatus, after its main engine shut off it controlled its attitude using helium jets while it coasted, until the rotator on its nose spun the third stage to ensure that this would remain stable following its release. After a repeat performance on 17 February 1959, the The launch of a second stage was upgraded with an AJ-10-142, and the third Semyorka rocket stage was superseded by a fibreglass-reinforced plastic X-248 on 12 April 1961 Altair I solid rocket that had been developed by the Hercules carrymg Vostok 1 Powder Company in collaboration with the Allegheny and Yuri Gagarin- Ballistics Laboratory.4

Meanwhile, in December 1955 the Air Force had issued a contract to the Douglas Aircraft Company to develop the Thor intermediate-range ballistic missile utilising the Rocketdyne MB-3-I kerosene and oxygen engine. On 20 September 1957, after a very rapid development, the missile successfully flew to its assigned rangeA6 After installing the Vanguard's upper stages on a Thor in a configuration called Thor-Able, on 17 August 1958 the Air Force tried to send a probe to the Moon. The Thor rose smoothly from Pad 17 at Canaveral but its turbopump seized at T+17 seconds and the vehicle exploded. On trying again on 11 October, the second stage shut down prematurely due to an

The attempt to launch Vanguard on 6 December 1957 failed when the vehicle lost thrust, toppled back onto the pad and exploded.

Thor-Delta models (left to right): Thor-Delta-B, 21 December 1963 with TIROS 8; Thor-Delta-Cl, 25 May 1966 with Explorer 32; Thor-Delta-D, 19 August 1964 with Syncom 3; Thor-Delta-E1, 11 January 1967 with Intelsat 2F2; Thor-Delta-G, 14 December 1966 with Biosat 1; Thor-Delta-M6, 13 March 1971 with Explorer 43.

Thor-Delta models (left to right): Thor-Delta-B, 21 December 1963 with TIROS 8; Thor-Delta-Cl, 25 May 1966 with Explorer 32; Thor-Delta-D, 19 August 1964 with Syncom 3; Thor-Delta-E1, 11 January 1967 with Intelsat 2F2; Thor-Delta-G, 14 December 1966 with Biosat 1; Thor-Delta-M6, 13 March 1971 with Explorer 43.

accelerometer programming error, the third stage was unable to make up the shortfall, and Pioneer 1 peaked at an altitude of 113,000 kilometres before falling back. The accelerometer system was improved to ensure that the next probe was given the correct velocity. On 8 November, however, the third stage failed to ignite. Given the state of the technology at that time, these were very ambitious missions.

It was fortunate that the Thor had matured so rapidly, because the Air Force planned to fit it with an Agena upper stage equipped for orbital reconnaissance. Built by the Lockheed Aircraft Company, the Agena used the XLR-81 hypergolic engine that had been developed for the rocket-powered pod that was to have been released by the B-58 Hustler strike aircraft to deliver a nuclear warhead. The first launch of the Thor-Agena in January 1959 for the Discoverer series failed, but on 28 February the Agena attained low polar orbit for the first of a series of flights to test a capsule for returning film from a spacecraft, and this was subsequently utilised operationally with successive generations of 'KH' cameras.7*8

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