 The United States Air Force developed the Thor launch vehicle from an intermediate-range ballistic missile. During the 1960s and early 1970s, Thor was a workhorse, transporting numerous classified payloads into orbit from Vandenberg Air Force Base in California. (credit: Douglas Aircraft Company brochure) |
By Dwayne A. Day
Monday, June 24, 2024
In the mid-1950s, when the United States Air Force began considering how it would put satellites into orbit, the obvious choice was the Atlas intercontinental ballistic missile then under development. Atlas was expected to have the performance necessary to place a good-sized payload (several thousand kilograms) into low-Earth orbit. But Atlas was relatively expensive and difficult to use, and larger than many missions required. Fortunately, the Air Force had a smaller missile in development that could also launch a payload into orbit, the Thor intermediate-range ballistic missile. The Thor's lower cost and easier handling made it a more useful rocket for the Air Force, and in the late 1950s Thor was tasked with carrying an increasing number of satellites into orbit, including CORONA reconnaissance satellites and the growing families of military and civil satellites. When Thor was retired from its missile role, many vehicles were freed up for conversion to launch satellites. Thor evolved over the next few decades into Thor-Delta and eventually the Delta II rocket, and was referred to by some in the space program as the workhorse rocket of the early American space programs.
 The Thor platforms at Vandenberg were miles from storage and processing facilities. They were towed to launch sites on trailers. (credit: Douglas Aircraft Company brochure) |
Although Thor launched unclassified payloads from Florida, most Vandenberg Thor launches were classified and were only officially declassified beginning in the 1990s. Due to its missions, the Air Force released very few photographs of the launches. There were unclassified photographs of most of the launches in military and corporate archives, but it was not until dedicated researcher Peter Hunter began painstakingly sorting and scanning hundreds of photographs in a corporate collection in the late 1990s that it was possible to document many of the releases.
 The early Thor pads at Vandenberg were relatively minimalist. Most of the launch preparation was done with the vehicle in a horizontal position, and it was assembled into a vertical position shortly before launch. (credit: Douglas Aircraft Company brochure) |
Recently, even more photographs of Thor's early operations at Vandenberg have become available, such as launch preparation and the ground support equipment used on several of Thor's early platforms. The photographs and illustrations give a good idea of the size of the vehicle, as well as the relative isolation of the early launch complexes, some of which were located very close to the Pacific Ocean. Thor burned kerosene and liquid oxygen, and these were transported by truck to remote platforms.
 As payload mass increased, Douglas Aircraft Company added solid rocket motors to the Thor. The solid rocket motors were carried to the pad on trailers and the entire trailer tilted to attach the rocket to the main vehicle. (credit: Douglas Aircraft Company brochure) |
Most of the military payloads launched into orbit by Thor in the early 1960s were launched from a few pads at Vandenberg Air Force Base in California. Although originally named differently, they were eventually referred to as Space Launch Complex 1 with its East and West pads, Space Launch Complex 2, also with East and West pads, and Space Launch Complex 10. There was also a pad for training Royal Air Force crews in launching Thor missiles, which the RAF operated in the United Kingdom, although not all RAF launches were from that pad.
 Three SRMs were fitted to early Thors to increase performance. The first of these launches took place in March 1963 with a LANYARD reconnaissance satellite, but it failed to reach orbit. (credit: Douglas Aircraft Company brochure) |
Finally, in the mid-1960s, Thor rockets were also launched from a converted Atlas pad, Space Launch Complex 3 West, a location that was initially the Navy's Point Arguello Launch Complex (PALC) before it was incorporated into Vandenberg. Some early Atlas and Thor rockets were officially launched from PALC pads before those pads were redesignated Space Launch Complex 3. At Vandenberg, SLC is pronounced “slick.” Slicks 1 and 10 have since been retired. Nearby, Slick-10 has been preserved as a national historic site.
 At Vandenberg during the 1960s, Thor rockets were launched from several launch pads. SLC-2W was later adapted to handle Delta and then Delta II rockets; the last Delta II was launched from that pad in 2018. (credit: USAF) |
Many of the newly discovered photos lack labels, but almost all are associated with Slick-2 at Vandenberg, which eventually became NASA's Delta II platform until Delta II's last launch from there in 2018. Slick-2 now has been converted to launch Firefly Alpha rockets, such as one scheduled to lift off later this week with several NASA-sponsored small satellites.
 The three Castor solid rocket motors were transported to the platform on trailers and the entire trailer was rotated to attach the motor to the central vehicle. (credit: USAF) |
The Thor underwent many upgrades over the years, including improvements to the core vehicle, the addition of solid rocket motors, and the addition of multiple upper stages. The Air Force still had the Atlas for larger payloads, but even the improved Thor was significantly cheaper, so for many years the goal was to squeeze more performance out of the Thor to carry payloads that were becoming heavier, thus avoiding a costly switch to the Atlas.
 A Thor on the platform in the early 1960s. Note the cover of the rocket nozzles. (credit: USAF) |
 Castor solid fuel engines increased the Thor's performance, allowing it to carry heavier payloads. Although the Atlas was also available, it was more expensive. (credit: USAF) |
Because it was originally a mobile rocket that traveled around Europe, operations at the Vandenberg pad for Thor were a bit more basic than you might expect, and photographs often reflect this. The rocket was transported to the launch pad on a trailer pulled by a truck. The trailer was then unhitched and covered by a long, wheeled shed to protect the rocket and later the upper stage and payload as they were moved to the launch site.
 In the early and mid-1960s, the Air Force was launching several Thors per month. Many of them carried reconnaissance satellites to provide intelligence on the Soviet Union. (credit: USAF) |
 Space Launch Complex 1 West in Vandenberg was located very close to the water. The large shed covered Thor and its payload for launch preparations and was removed so the rocket could be erected. (credit: USAF) |
To prepare for launch, the entire trailer, wheels and all, was lifted vertically to place the rocket on its stand. The trailer was then lowered and removed. Other trailers were used to carry the solid rocket motors to the pad so they could be attached to the core stage. There was no launch tower, and until the switch to Slick-3, workers accessed the top of the rocket and payload using long lifting cranes. As Thor had been designed as a missile, it was also designed to be fueled and fired relatively quickly, but the payloads were significantly more complicated and required greater care and attention on the platform.
 A Thor equipped with three Castor solid rocket motors and an Agena upper stage. Note the workers on the lifting platform accessing the Agena upper stage. Most likely this was a CORONA reconnaissance satellite mission, and the top of the rocket, with the payload, was covered with an air-conditioned cooling blanket. (credit: USAF) |
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