Grundlage für das SLV Thor der USAF war die 1955-1957 entwickelte Mittelsteckenrakete (IRBM) PGM-17A (SM-75).
Die erste Version war eine dreistufige Thor-Able. Sie bestand aus der Thor DM-18 mit dem Triebwerk MB-3-I kombiniert mit der zweiten und dritten Stufe des SLV Vanguard der USNavy, das bereits 1957 getestet worden war.
Die zweite Stufe Able war mit der Treibstoffkombination UDMH/HNO4 gefüllt. Die Förderung erfolgte mittels Helium-Druckgas. Als dritte Stufe wurde ein Feststofftriebwerk Grand Central Rocket GCR X-242 verwendet.
Der erste Start dieser Thor-Able 1 mit der Raumsonde Pioneer am 17.08.1958 wurde zum Fiasko. Nach zwei Fehlversuchen und einem erfolgreichen Start wurde am 07.08.1959 eine verbesserte Thor-Able 2 eingesetzt. Die Able 2 hatte ein neues Einkammertriebwerk und die dritte Stufe war durch das Feststofftriebwerk ABL X-248 (Altair IA) ersetzt worden. Bis 1960 wurde diese Version 4 mal eingesetzt.
Da die Nutzlastkapazität der Thor-Able sehr beschränkt war, hatte die USAF rechtzeitig den Auftrag zur Weiterentwicklung der Thor-Able erteilt. Die 1960 präsentierte zweistufige Thor-Able Star bestand aus einer schubstärkeren DM-21 (MB-3-II) und einer Able Star, bei der gegenüber der Able das Tankvolumen vergrößert wurde. Durch Erweiterung des Durchmessers der Zelle konnten statt 1,575 t nun 4,862 t getankt werden. Das Triebwerk wurde weiter verbessert und war wiederzündbar. Diese Version wurde von 1960 bis 1965 insgesamt 19 mal von der USAF gestartet.
1960 wurde eine Variante Thor DM-19 (MB-3-I) mit einer zweiten Stufe Delta (entsprach der Able mit verbessertem Triebwerk) und der dritten Stufe Altair IA zum Start von 12 wissenschaftlichen Nutzlasten (Echo, Explorer, Telstar, Tiros, OSO, Ariel) entwickelt - genannt Thor-Delta. Die Thor-Able Star und Thor-Delta waren die Vorläufer der später so erfolgreichen Delta, die bis heute im Einsatz ist.

Thor mit Kick-Stufen (Feststoff)
Zum Start von Überwachungssatelliten (meist DMSP-I) startete die USAF von 1965 bis 1979 insgesamt 31 Thor mit aufgesetzten Kick-Stufen:

1965                Thor-MG-18            ( +Grand Central Rocket MG-18)
1965 - 1966     Thor-Burner I         ( +TE-M-640                                      = Star 20 )
1966 - 1971     Thor-Burner II        ( +TE-M-364-2 / TE-M-516             = Star 37B / Star 13 )
1971 - 1976     Thor-Burner IIA     ( +TE-M-364-2 / TE-M-442-1         = Star 37B / Star 26B )
1976 - 1980     Thor-ISS                  ( +TE-M-714-1 / TE-M-364-15       = Star 37XE / Star 37S )

Bei allen Varianten kamen Lagerbestände der Thor DM-18 mit dem Triebwerk MB-3-I zum Einsatz.

                                                                                                                                                                        

Thor
The U.S. Air Force defined the Thor IRBM in July-August of 1955. 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.
Douglas Aircraft Company won the SM-75 Thor contract in December 1955. 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. 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-Able
Thor-Able was a two-stage rocket created by stacking the Vanguard space launch vehicle's second stage atop Thor. The second stage, dubbed "Able" for this application, was a pressure-fed design built by Space Technology Laboratories. It burned nitric acid and unsymmetrical dimethyl hydrazine (UDMH) in a single 3.54 tonne thrust Aerojet AJ10 engine. Able was 5.67 meters long, 0.81 meters in diameter, and weighed about 1.88 tonnes, a substantial increase from Thor's standard one tonne warhead/reentry vehicle mass.
To convert Thor from a single-stage IRBM into the first stage of a multistage vehicle, the missile's inertial guidance system was removed, replaced by a guidance system located in the Able stage. The warhead was replaced by an interstage structure.
Two-stage Thor-Able flew nine times from Cape Canaveral in 1958-59. The first three flights were part of the Advanced ICBM Reentry. The final six two-stage Thor-Able flights were part of the Precision Guided Reentry Test Vehicle program. These flights used a "Thor-Able 2" configuration that tested the Titan ICBM radio-inertial guidance system.
Seven three-stage Thor-Able orbital launch attempts were also performed during 1958-1960. These flights added an Allegheny Ballistics Laborary X-248 "Altair" solid rocket motor as a third stage to create the "Thor-Able 1" variant. The spin-stabilized motor weighed 238 kg and was 1.83 meters long and 0.46 meters in diameter. It provided 1.27 tonnes of thrust for about 38 seconds.
The first orbital attempts were part of ARPA's "Operation Mona", better known as "Pioneer", the name assigned by NASA when it assumed control from ARPA after the first launch. Pioneer was the first U.S. attempt to reach the Moon. Pioneer only weighed 38 kg but still maxed out Thor-Able's payload capacity. Pioneer also was fitted with a small solid motor intended to perform a lunar orbit insertion burn.
The final Thor-Able flights, performed for NASA between July 1959 and January 1960, orbited Explorer 6, Pioneer 5, and Tiros 1, the first weather satellite. The Transit 1A navigation satellite launch failed to reach orbit when the third stage failed.
Thor-Able-Star
In 1960, ARPA and the U.S. Air Force began flying Thor-Able-Star, originally named "Thor-Epsilon" which used the Aerojet General "Able-Star" second stage.
Able-Star, the world’s first restartable stage, was a "wide-body" compared to the Able stage that carried more than twice as much propellant. Able-Star was 1.4 meters in diameter and 4.52 meters tall. It weighed 4.497 tonnes loaded and 0.599 tonnes empty. It was a helium pressure fed stage that used the same propellants as Able. It also used a similar engine, the AJ10-104, which produced 3.674 tonnes of thrust. The engine could burn for up to 262 seconds.
Thor-Able-Star flew 19 times during 1960-65, including 11 launches from the Cape and 8 from Vandenberg AFB. Using its second stage restart capabilty, it could lift more than 230 kg to a 1,000 km x 28.5 deg circular orbit. Able-Star performed the first in-space restart on April 13, 1960. After the stage completed its initial 258 second burn, it and its Transit 1B payload coasted for 19 minutes before the stage performed a second, 13 second long burn to finalize the 373 x 748 km x 51.28 deg orbit.
Early launches from the Cape orbited the U.S. Navy's initial Transit (navigation) and U.S. Army's Courier (communications) satellites, along with Solrad/GRAB electronic intelligence radar signal "spy" satellites flown piggyback with Transit. The final two of six Cape-launched Transit satellites were powered by SNAP 3B nuclear power sources (radio-isotope thermoelectric generators or "RTGs") - the first time that RTGs were launched into orbit. Thor-Able-Star also orbited Anna 1B (Army, Navy, NASA, Air Force) a satellite that carried beacons for use in ground surveying.
In 1963, Thor-Able-Star made its first flight from Vandenberg AFB. All eight Vandenberg launches carried U.S. Navy Transit navigation satellites, aimed toward near-polar orbits. The first three Vandenberg-launched Transits used SNAP 9A RTG nuclear power sources, which were loaded with more Plutonium 238 metal fuel than the SNAP 3B RTGs. The SNAP 9A design was discontinued after the Transit 5-BN-3 flight failed. The final five Transit-O ("Oscar") missions, all successful, used solar powered satellites.
Thor Burner
Thor Burner was created to orbit top-secret polar orbiting weather satellites for the Defense Meteorological Satellite Program (DMSP). DMSP was secret because one of its missions was to help Corona/Keyhole spy satellites avoid targeting cloud covered regions. Planners did not want to waste valuable orbited film on cloud photos.
The Thor Burner first stage consisted of a modified SM-75 Thor ballistic missile. Modifications included the installation of a BTL radio guidance system, a telemetry system, and a flight termination system. The warhead was replaced by a tapered interstage section.
The first Thor Burner type was originally called "Thor Altair", though none of its six flights actually used the original Allegheny Ballistics Laboratory (ABL) X-248 "Altair" solid motor as a second stage. Altair "clones" actually flew the Thor Altair/Burner 1 missions. These included MG-18 by Grand Central Rocket, used on the first two missions, and FW-4S by UTC, for the final four flights. FW-4S, sometimes called "Altair 2", weighed about 300 kg loaded and could produce an average of nearly 2.5 tonnes of thrust for up to 27 seconds.
The spin-stabilized upper stages and payloads were housed within payload fairings similar to those used for the Scout-launched DMSP missions. The first two flights used Scout fairings. The latter four used wider bulbous fairings.
Thor Altair/Burner 1 could lift about 250 kg into the DMSP orbit. The rocket performed six launches between January 19, 1965 and March 31, 1966. The first three launches boosted "Block 1" DMSP satellites. The final three flights launched similar "Block 2" satellites.
In March 1964, the DMSP program office approved plans to develop a more powerful Thor Burner 2 launch vehicle. Burner 2 used a Thiokol Star 37B motor (TE-M-364-2). Star 37B weighed 718 kg loaded and produced about 4.59 tonnes of thrust for 42 seconds.
Boeing's Burner 2 stage was built around, and included, the Star 37B motor. The stage had its own guidance and control system. It used both hot-gas and cold-gas reaction control engines. Four 10 kg force hot-gas hydrogen peroxide thrusters performed stage separation, provided pitch and yaw reaction control thrust during the Star 37B motor firing, and completed a vernier maneuver immediately after the Star 37B burn. Eight 1 kg force gaseous nitrogen cold-gas thrusters on the stage provided pitch-yaw-roll attitude control during coast and performed spacecraft spinup and post-spacecraft separation maneuvers.
Thor Burner 2 was topped by a new truncated conical fairing that enclosed the upper stage and payload. The unpainted fairing was distinctively vermilion (orange-red) in color. A modified adapter connected the Thor to the fairing. The two-stage rocket could lift 420 kg to the DMSP orbit.
Thor Burner 2 flew 12 times from September 16, 1966 to June 8, 1971, carrying 10 DMSP Block 4 and 5A satellites. All 12 launches were successful. One of the STP missions, flown on June 29, 1967, used a 0.64 tonne thrust Star 13A third stage motor to lift Aurora 1 and SECOR 9 into a polar orbit.
Thor Burner 2A added a third stage and a modified fairing to the Thor Burner 2 design. A Star 26B motor served as the third stage during the the first eight Thor Burner 2A missions, flown between October 14, 1971 and February 19, 1976. The fairing was extended by the addition of a cylindrical section between the Thor adapter and the truncated conical "nose cone" (see Thor Burner 2A photo at beginning of article). The launches orbited DMSP Block 5B and 5C satellites.
The final five flights, launched between September 11, 1976 and July 14, 1980, used tailored Star 37 motors for both second and third stages. These launched three-axis controlled DMSP Block 5D-1 satellites. Burner 2A upper stages were not actually used - the advanced DMSP satellite provided upper stage flight control - but the flights are still commonly considered part of the Thor Burner program. A Star 37XE motor served as the second stage while a Star 37S-ISS acted as the third stage. A longer payload fairing with a blunter nose housed both stages and the payload.
The upgraded launch vehicle could lift at least 513 kg to the DMSP orbit. Payloads included Block 5B, 5C, and 5D-1 DMSP satellites. The first seven "Thor Burner 2A" missions, flown during 1971-1975.
Thor-Delta
For the Thor-Delta were used the upper stages from the Vanguard rocket, but with modifications. Thor-Delta could lift 295 kg to low earth orbit, or 45 kg to geosynchronous transfer orbit. The rocket was 31 meters tall and weighed 54 tonnes at liftoff.
The 49.34 tonne Thor DM-18A first stage was powered by a 68 tonne thrust turbopump-fed Rocketdyne MB-3 Block 1 main engine augmented by two 0.45 tonne thrust roll control verniers, all burning kerosene/LOX. The mostly-aluminum stage was 2.44 meters in diameter and 18.42 meters long. The first stage was manfactured by Douglas Aircraft.
The sonamed "Delta"second stage used common-bulkhead tanks, with a cylindrical helium pressurant tank positioned between a forward UDMH fuel tank and the aft IRFNA oxidizer tank. The tanks were made from stainless steel to combat the corrosive effects of the propellants. A pressure-fed Aerojet AJ-10-118 3.49 tonne thrust engine powered the 4.472 tonne second stage. The slender stage, also built by Aerojet, was 5.88 meters long and only 0.813 meters in diameter.
The third stage, mounted to a spin table on top of the second stage, was an Allegheny Ballistics Laboratory (ABL) X-248 spin-stabilized “Altair” solid motor. Altair, a 1.83 meter long, 0.46 meter diameter fiberglass-case motor that only weighed 227 kg, could provide 1.27 tonnes of thrust for 38 seconds.
The third stage motor and payload were both housed within a payload fairing. There were two fairing types. One was a low-drag type with a constant 0.813 meter diameter and a conical nose. The other was a “bulbous” fairing used for wider diameter payloads.
A cold gas-jet attitude control system was added to the previous Able second stage to create Delta. With this system, Delta could coast and reorient itself in space after its AJ-10-118 engine had performed its burn. This improved the accuracy of third stage spin-up insertions.
After second stage burnout, and after the stage coasted to an appropriate point in space, small solid motors would fire to spin up the table, giving the third stage gyroscopic stabilization. The third stage could, as a result, fly “dumb”, without need for a guidance or flight control system, during its burn. After spin-up, the stage would be released and, after a delay of 15.5 seconds to allow the stages to separate from each other, would ignite.
NASA’s Goddard Space Flight Center ordered 12 Thor-Deltas from Douglas Aircraft. The first, carrying Echo 1, was launched from Cape Canaveral on May 13, 1960.

Source: Ed Kyle, Space Launch Report, 2009