The Thor-Ablestar, or Thor Able-Star, also known as Thor-Epsilon[1] was an early American expendable launch system consisting of a PGM-17 Thor missile, with an Ablestar upper stage. It was a member of the Thor family of rockets, and was derived from the Thor-Able.

Launch of a Thor-Ablestar 2 with a Transit satellite
FunctionOrbital carrier rocket
Country of originUnited States
Height29 metres (95 ft)
Diameter2.44 metres (8 ft 0 in)
Mass53,000 kilograms (117,000 lb)
Payload to 1100km LEO150 kilograms (330 lb)
Associated rockets
Launch history
Launch sitesLC-17, Canaveral
LC-75-1, Arguello
Total launches19
Partial failures2
First flight13 April 1960
Last flight13 August 1965
First stage – Thor
Engines1 MB-3-1
Thrust760.64 kilonewtons (171,000 lbf)
Specific impulse285 sec
Burn time164 seconds
Second stage – Ablestar
Engines1 AJ-10
Thrust36.02 kilonewtons (8,100 lbf)
Specific impulse280 sec
Burn time296 seconds

The Ablestar second stage was an enlarged version of the Able, which gave the Thor-Ablestar a greater payload capacity compared to the Thor-Able. It also incorporated restart capabilities, allowing a multiple-burn trajectory to be flown, further increasing payload, or allowing the rocket to reach different orbits. It was the first rocket to be developed with such a capability and development of the stage took a mere eight months.[2]

Nineteen Thor-Ablestars were launched between 1960–65, of which four failed, and a fifth resulted in a partial failure, as only one of two payloads separated from the upper stage.[2]

The first failure was the launch of Courier 1A, an experimental communications satellite, on 19 August 1960 when the first stage shut down 30 seconds earlier than planned and was destroyed by the Range Safety officer. On 30 November, another launch involving a Transit satellite failed in practically identical fashion. This episode nearly created an international incident as parts of the Thor landed in Cuba. Cuban leader Fidel Castro subsequently sold off the Thor's engine to the Soviets and the Chinese received its thrust vectors, which ended up proving valuable to the latter's development of a ballistic missile capability. To prevent this from happening again, future Thor Ablestar launches had their flight paths modified to avoid passing over Cuba.

The launch of a Transit satellite on 22 February 1961 was successful, but its companion Lofti satellite failed to separate from the second stage.

The third failure was the launch of several piggybacked satellites on 24 January 1962 when the second stage produced insufficient thrust to achieve orbital velocity.[3] The fourth and final failure was the launch of an Anna geodetic satellite on 10 May 1962 when the second stage completely failed to ignite.

Two versions were built; the Thor-Ablestar 1, with a DM-21 Thor, and an AJ-10-104 second stage engine, and the Thor-Ablestar 2, which had a DSV-2A Thor first stage, and an uprated AJ-10-104D engine on the second stage.[1] Thor-Ablestar 1 launches occurred from LC-17 at Cape Canaveral, and Thor-Ablestar 2 rockets were launched from LC-75-1 at Point Arguello, which has since become part of Vandenberg Air Force Base and is now designated SLC-2.

See also

  • List of Thor-Ablestar launches


  1. Krebs, Gunter. "Thor Able-Star". Gunter's Space Page. Retrieved 30 November 2008.
  2. Wade, Mark. "Delta". Encyclopedia Astronautica. Archived from the original on 22 May 2013. Retrieved 30 November 2008.
  3. "A-Okay So Far: Glenn, Atlas Ready". The Gazette. Montreal, QC. Associated Press (AP). 25 January 1962. p. 1. Retrieved 29 December 2015.
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