Thor

by Jean-Jacques Serra

Origins

Thor Able

Thor Ablestar

Thor Agena

Thor Delta

Thor with solid top stage (Altair, Burner)


Origin: first generation of American Intermediate Range Ballistic Missiles

Until 1953 the USAF had stressed on aerodynamic missiles (while the Army was developping the Jupiter) because they believed long distance ballistic missiles would not appear before ten years. The construction of Thor wasn't awarded to Douglas Aircraft before December 1955. The program was given a very high priority and the first missile was delivered to the USAF in October 1956.

The MB-1 propulsion system was made of a Rocketdyne LR79 (liquid oxygen and kerosene) engine with 670 kN derived from Jupiter and Atlas' booster. An LR101 vernier rocket on each side of the missile' base controled the roll and the final ajustement of the speed after the main engine shutdown. The missile used Atlas' Reentry Vehicle, measured 2.44 m in diameter and 19.8 m in height, and weighed over 46 tons.

The first four tests carried out from 25 January 1957 on were failures. The first successful launch occured on 20 September and was commissioned in 1959.

Thor's propulsion system quickly evolved to an MB-3 version used by IRBMs and space launchers, especially by USAF's Thor SLV-2 and NASA's Thor Delta launchers. In the seventies the MB-3 motor was used by the Japanese N-1, N-2 and H-1 launch vehicles. An improvement program conducted between 1957 and 1959 named X-1 leaded to an H-1 version designed for Saturn-1. Later on H-1 was modified for the propulsion of Thor Deltas (39xx series) and became RS-27. The latest version RS-27A intended to Delta-2 launchers is optimized for greater performance at high altitude.

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Thor propulsion system

Designation	Year of first launch	Main engine Thrust (kN)	Main engine SI (sec.)	Application
MB-1	1957	600	250	IRBM
MB-3	1958	670, 735, 755 (1)	252	IRBM, launchers
RS-27	1973	920-1030 (2)	263-295 (2)	Delta
RS-27A	1991	890-1055 (2)	255-305 (2)	Delta-2

Notes: (1) Blk 1, 2 and 3; (2) values given at sea level in vacuum

In 1963 Thor's thrust became insufficient to carry out its missions. The solution (innovative at that time) found was to enhance it with 3 solid fuel strap on Thiokol Castor boosters. This resulted in TAT: Thrust Augmented Thor. Those boosters were used grouped by 3 then 6 and up to 9. After 1975 when the only Thor first stage used was the Delta the Castor-2 boosters were replaced by the 1 m-diameter Castor-4.

The burn duration of Thor (160 seconds) was also enhanced by lenghtening its tanks. This gave birth to the LTT (Long Tank Thor) for which the burning time reached about 220 seconds, used from 1966 wih Agena stages and from 1968 with Delta stages. The first stage was streched again in early 70' giving ELTT (Extended Long Tank Thor); the burning time wasn't much modified but the thrust was increased. The Delta-2 version, used since 1989, was lengthened in order to obtain a 265 seconds burning time.

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