Thor is a family of rocket motors that has included the Thrust Augmented Thor, or a Long Tank Thor, and a number of different booster configurations since May of 1960, which have provided the initial thrust for NASA and DOD space launch vehicle. The Delta launch vehicle family began development in 1959 when NASA's Goddard Space Flight Center awarded a contract to Douglas Aircraft Company (subsequently the McDonnell Douglas Corporation, which merged with Boeing to form Boeing Launch Services in 1997) to produce and integrate 12 space-launch vehicles. The Delta used components from the US Air Force's Thor Intermediate-Range Ballistic Missile program as its first stage and the US Navy's Vanguard launch-vehicle program as its second. The first Delta, with an Echo I communications satellite on board, was launched from Cape Canaveral Air Force Station on 13 May 1960.
Through the years, as missions became more complex and launch vehicle requirements more exacting, the Thor-Delta possessed the flexibility to accommodate the variety of missions assigned to it. The vehicle's long use stemmed from this tradition of adaptability through its "building block" configuration. The Thor-Delta evolved through a large number of configuration changes. This evolutionary process was a continuing one. The Thor-Delta launch vehicle experienced a consistent evolutionary growth since its initial launch in 1960. Through this growth, averaging one major change each year, the vehicle was able to keep pace with the various US space efforts, with payload requirements that were constantly changing.
When larger payloads called for a more powerful vehicle, the Air Force and McDonnell Douglas introduced the Improved Thor-Delta and the Thrust Augmented Delta. NASA contracted with the Air Force for the use of the Thor booster, while the Delta second stage vehicle was produced directly for NASA by McDonnell Douglas, which was awarded the initial contract for the vehicle in April of 1959. In both instances, utilizing previously developed improvements proven by Air Force configurations, three solid propellant motors were attached to the booster's base to provide increased thrust for the first part of flight.
Thor DM18/3 - The first stage of the two-stage Thor-Agena A launch vehicle was a Model DM-1812-3 Thor (essentially a Model DM-18A modified by the removal of the nose cone and guidance, which was incorporated in the second stage). The second stage was a Lockheed 2205 Agena A powered by a Bell Aircraft Hustler liquid-propellant engine. The Thor-Agena A was used for the first time on 28 February 1959, with the 1300-pound spacecraft Discoverer 1, carrying a payload twice as heavy as any previous one carried by an American launch vehicle. The performance of the initial Thor-Agena A was flawless throughout the flight, and Discoverer 1 was successfully placed into a polar orbit, the first such orbit ever effected by a United States spacecraft. The last Thor-Agena A launch occurred on 13 September 1960, with Discoverer 15 being successfully placed into orbit.
Thor DSV-2B - The two-stage Thor-Ablestar (Thor-Epsilon) launch vehicle employed two Thor model booster configurations: the DM-21A, and the DSV-6. Both configurations were essentially DM-21's with a new adapter section, forward of a transition section to accommodate the second stage. The Model DM-21A Thor-Ablestar combination was used for launching various earth-orbiting navigational, scientific, geodetic, and communications spacecraft, including those in the Transits Courier, and ANNA series. The added Ablestar stage was developed by Ramo-Wooldridge and Aerojet-General and later produced by Space-General. It featured the first liquid-propellant restartable engine which could stop and start once or twice in space. All previous upper stages in Thor-launched vehicles reached full burnout, having no capability of refiring under space conditions. By cutting off and restarting as much as 20 minutes after the launch sequence had ended, the Ablestar was able to "correct" its orbit, making it more circular or elliptical as the case might require.
Thor DSV-2S - The first stage of the two-stage Thor-Burner I launch vehicle was a Model DM-18A. The second stage was a Thiokol FW-4 TE 364-1 Burner I solid propellant rocket motor.
Thor DSV-6 - The two-stage Thor-Ablestar (Thor-Epsilon) launch vehicle employed two Thor model booster configurations: the DM-21A, and the DSV-6. Both configurations were essentially DM-21's with a new adapter section, forward of a transition section to accommodate the second stage. The Model DSV-6 Thor-Ablestar combination was used to place various Air Force and Navy classified payloads into orbit.
Thor PG-2A - The Discoverer satellite launches by no means came to an end with the last Thor-Agena A flight. The missions were simply continued with a new launch vehicle, the Thor-Agena B, which was an improved version of the Thor-Agena A. The booster for the vehicle was a Model DM-21 Thor, basically a Model DM-18C with a higher thrust engine, nose fairing removed, and guidance section replaced by a shorter, lighter transition section. The second stage was a Lockheed 8096 Agena B, similar to the Agena A, but with larger propellant tanks and an engine with restart capabilities. The new vehicle started its career with the Air Force on 26 October 1960, when it was used to launch Discoverer 16, but the Agena B vehicle failed to separate and the spacecraft, of course, never achieved orbit. On 12 November 1960, Discoverer 17 was successfully placed into orbit via Thor-Agena B.
Thor LV-2F - The first stage of the two-stage Thor-Burner II launch vehicle was a Model DM-18A. The second stage was a Thiokol TE 364-2 Burner II solid propellant rocket motor.
Thor SLV-2A - The Thor-Agena D was first used on 28 June 1962 to launch a classified Air Force payload from the Western test Range. Its immediate successor was the Thor-Agena B, which used the same first-stage Model DM-21 and DSV-2A Thor boosters but earlier models of the Agena stage. The lift capability of the vehicle was significantly increased when the new Model DSV-2C, or Thrust Augmented Thor, was introduced into the launch system, on 28 February 1963. That Thor configuration incorporated three solid-propellant motors which produced an additional 161,550 pounds of thrust and dropped off after burnout. Minor changes were made in the engine section structure and in the control circuitry to accommodate the solid motors.
Thor SLV-2G - The Model DSV-2L series booster, or Thrust Augmented Long Tank Thor, made its first appearance in the Thor-Agena D system on 9 August 1966, when it was used to launch a classified Air Force payload. The new booster was basically a DSV-2A with liquid propellant capacity increased 43 percent and augmented with three Thiokol TX354-5 solid propellant motors, each of which provided an additional 51,490 pounds of liftoff thrust. The launch vehicle was thus capable of placing a greater spacecraft weight in orbit than ever before. In the DSV-2L configuration, a transition section housing the flight control equipment, electrical power components, and an umbilical connection assembly was permanently attached to the forward end of the booster adapter section to provide mounting capability for the Agena D second stage, which carried the payload. The Thor-Agena D vehicle has been used primarily by the Air Force to launch classified payloads.
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