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 The business end of a Thor equipped with three solid rocket motors. This rocket carried a MULTIGROUP signals intelligence satellite. (credit: USAF) |
Thor’s legacy
In the space program, it has long been popular to proclaim “firsts” for each new achievement, sometimes to the point of absurdity. Naturally, the earliest rockets accomplished many significant “firsts,” and the Thor racked up a lot of firsts:
- First operational American ballistic missile
- First missile to be launched from Vandenberg Air Force Base
- First booster to launch a spacecraft into polar orbit
- First booster to launch a payload recovered from orbit
- First booster to launch a reconnaissance satellite
- First booster to launch a signals intelligence satellite
- First booster to launch a communications satellite
- First booster to launch a meteorological satellite
- First booster to launch a navigational satellite
- First long-range vehicle to record, successively, 100, 200, 300, and 400 launchings
 Space Launch Complex-1 West at Vandenberg Air Force Base in the 1960s. People who worked there found it cold and windy year-round. (credit: USAF) |
Many of these missions launched into polar orbits from Vandenberg Air Force Base on the California coast, from relatively remote, and simple, launch sites such as Space Launch Complexes 1, 2, and 10. The simplicity was an evolution of Thor’s development: the earliest Thor launch facilities were built at Patrick Air Force Base in Florida for testing the new intermediate range ballistic missile. In part because testing often meant lengthy pre-launch preparations, not to mention post-launch explosions, the Florida facilities were relatively robust and heavy. In fact, during the very first test launch in January 1957, the Thor barely rose off the pad before exploding. Major General Bernard Schriever, who was then in charge of Air Force ballistic missile development, quipped that the missile’s accuracy was fine, they just needed to increase the range.
 Thors being prepared for launch, probably at Vandenberg. These are probably intermediate-range ballistic missiles for the Royal Air Force, which operated them in the United Kingdom, but conducted operational training launches at Vandenberg. (credit: USAF) |
Project Emily
When Vandenberg was established, its facilities were constructed to handle operational, not test, Thor missiles, requiring relatively less pre-launch work. Much of the equipment at the launch sites was identical to equipment designed to be transported by air to sites in Europe. The Thor rockets and other support equipment were mounted on trailers that could be driven into Air Force cargo transports.
 A Royal Air Force Thor being prepared for launch in the United Kingdom. The RAF did not launch any of the missiles from the UK, but trained regularly. This pad is nearly identical to the ones at Vandenberg. (credit: USAF) |
From 1959 to 1963, the Royal Air Force operated 60 Thor intermediate-range ballistic missiles from 20 RAF air stations within the United Kingdom. This was known as Project Emily. The Thors were a major part of the British nuclear deterrent, although in an unusual arrangement, the United States had operational control over the nuclear warheads. The RAF Thor crews practiced launching nearly two dozen missiles from several sites at Vandenberg. When the program ended in 1963 and Britain decided to purchase Polaris submarine-launched ballistic missiles, most of the RAF Thors were returned to the United States and converted into launch vehicles.
 The SLC-1 pad at Vandenberg Air Force Base in the early 1960s. These pads were relatively austere, and most rocket and payload final processing was done horizontally in a retractable shed. Most of the equipment at the pad was designed to be air transportable. (credit: USAF) |
Thor Standard Launch Vehicle
Even though it was designed as a weapon, Thor was soon adapted for space launch, which would only take place from a few sites at Vandenberg. (See “Things that almost go boom,” The Space Review, June 17, 2024.) SLCs-1, 2, and 10 served most of their operational lifetimes with relatively spartan space support facilities. Most launch prep was done horizontally in retractable sheds, just like the missiles, and there were only limited payload servicing capabilities at the launch sites. SLC-2E eventually acquired a larger support tower to replace the cherry pickers used early in the program. SLC-2W received a much larger tower to support NASA’s Thor-Delta program and its descendants, and to enable vertical stacking of satellite payloads.
 For early Thor launches, technicians reached the Agena upper stage and its payload using a cherry picker. (credit: USAF) |
In 1963, Thrust-Augmented-Thor Agena D rockets carrying CORONA reconnaissance satellites were launched from the converted Atlas pad, initially designated PALC-1-1, but later renamed SLC-3W. SLC-3W had a flame trench, a launch tower, and more extensive support facilities, and CORONA launches continued from that site until the end of the program in 1972. After that, SLC-3W was converted back to support Atlas launches.
 Although Thor launches started at SLC-1 and SLC-2, by the mid-1960s many of them were launching from the converted Atlas pad at Space Launch Complex-3 West. The pad had greater capability to support the payload. (credit: USAF) |
Thor rockets flew with a variety of upper stages over the years, including the Able I and II, Ablestar, Burner I and II, and Agenas A, B, and eventually D. The Agena D, like Thor, also earned a reputation as a versatile workhorse. It not only served as a second stage but also supported many payloads in orbit by providing stability, power, and communications. Agena upper stages on top of Thors carried more than a hundred CORONA reconnaissance satellites into orbit. They also carried over a dozen large signals intelligence satellites with names like MULTIGROUP and STRAWMAN into space from Vandenberg until the early 1970s. Thor-Agenas launched other classified payloads such as the QUILL radar satellite and Program 989 signals intelligence satellites. Agenas were also used on Atlas rockets on both coasts, and Titan rockets at Vandenberg, but Thors at Vandenberg carried the most Agenas to the edge of space, where the Agena took over.
 The Thor evolved into multiple variants, including multiple upper stages. For satellites that only needed a boost to orbit, not power, stabilization and communications, the Agena was unnecessary, leading to upper stages such as Able I and II and Burner I and II. (credit: USAF) |
Thor fades away
The recently discovered collection of Vandenberg Thor photos includes few payload photos, save for some photos of a 1963 Transit navigation satellite launch. (See “Nuclear Transit: nuclear-powered navigation satellites in the early 1960s,” The Space Review, February 12, 2024)
 One of the Transit5BN navigation satellites being prepared for launch. Three such satellites were launched between December 1963 and April 1964 from Vandenberg. The last one failed to reach orbit. (credit: USAF) |
Thor’s record in its early years was not great—that was true for all rockets in the early space age—but it quickly improved over time to better than 95% by the early 1970s. Still, there were notable launch accidents. Two of them involved nuclear power sources. In 1964 a Transit navigation satellite launched from Vandenberg spread its radioactive material in the upper atmosphere. In 1968, a Nimbus weather satellite fell into the Santa Barbara Channel just downrange from Vandenberg. It was located by a search team, then lost, then relocated and recovered. The full story of that search and recovery effort has not been fully reported. But the most spectacular failure in the Thor extended family was a Delta II that exploded just above its Florida launch pad in 1997, spraying debris over the surrounding scrub and destroying the cars of pad workers.
 A Transit 5BN satellite being prepared for launch at Vandenberg. The satellite was nuclear powered, using a small radioisotope thermoelectric generator (RTG) with approximately one kilogram of plutonium. (credit: USAF) |
Unflown Thor missiles and launch vehicles are on display in several American and British museums and there is a Thor inside a shed at the preserved SLC-10 launch site at Vandenberg Space Force Base. The last Delta II rocket to be built is on outdoor display at the Kennedy Space Center Visitor’s Center. The workhorse is long retired, fading from memory, but not yet forgotten.
 Thor rockets operated with various upper stages during their service, but the most common was the Agena, which also served as a spacecraft in low Earth orbit, supporting photo-reconnaissance and signals intelligence payloads. (credit: USAF) |
Next: Part 3, Thor gallery.
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