Thor missiles, repatriated from their “Project Emily” UK service by the end of 1963, were not only allocated for Program 437 launches from Johnston Island. They were also fitted with solid upper stages to fly orbital missions from Vandenberg AFB. The resulting launch vehicles made up the often-overlooked “Thor Burner” series.
There were 31 Thor Burner orbital launch attempts from 1965 to 1980. All but three of the launches were successful, and none of the shortfalls were Thor hardware failures, a fairly remarkable achievement for “re-tread” first-generation ballistic missiles manufactured during the 1950s.
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.
Scout launch vehicles performed the first DMSP 1A and 2A launches from Vandenberg in 1962, but only two of the five launches achieved orbit, and payload weight was limited. Four DMSP 3A satellites were orbited, two at a time by two Thor Agena D vehicles, from Vandenberg in 1964, but Thor Agena D was overkill for lightweight DMSP. Thor Burner, which was studied in late 1963, approved in March 1964, and began flying in 1965, was “just right” for the DMSP mission.
RCA-Astro built the early DMSP satellites. The spin-stabilized satellites were powered by batteries charged by solar cells. They used television type cameras. Later models employed infrared imagers. A typical DMSP constellation included two operating satellites and two spares. One of the operational satellites crossed the equator northbound early in the morning. The other crossed at noon.
DMSP satellites launched by Thor-Burner survived in orbit from three months to more than three years. The rocket flew as many as 4 times per year to establish the constellation, but only 1-2 launches per year were typically needed to maintain it once established.
The Thor Burner first stage consisted of a modified SM-75 Thor ballistic missile, recently returned from U.K. service. 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.
10th Aerospace Defense Squadron crews launched Thor Burner from Vandenberg AFB Pad 75-2-6, renamed Space Launch Complex 10 West in 1966. Launches took place from modified tactical launch equipment that had been returned from the UK. The Squadron was also responsible for the Program 437 launches at Johnston Island and crew training at Vandenberg.
Thor Burner 1
Thor 282 with Altair 2 (FW-4S) Upper Stage, Launched 5/20/1965
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 “slimline” 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.
The first Thor Burner 1 reached a good orbit, but the DMSP satellite failed to separate from the MG-18 second stage motor. The satellite, which was a Block 1 flight test vehicle, nonetheless operated for six months. One outright Thor Burner 1 launch failure occurred, on January 6, 1966, when the FW-4S second stage motor failed to ignite. The other launches boosted their DMSP payloads into sun synchronous orbits with perigees ranging from 470 to 650 km and apogees from 760 to 1050 km.
Also read: Thunder God Suborbital History
Thor Burner 2
Thor 210 and the Final Thor Burner 2, Launched June 8, 1971
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 37, from which Star 37B was derived, had originally been developed to serve as the Surveyor lunar lander retro-rocket. Star 37B weighed 718 kg loaded and produced about 4.59 tonnes of thrust for 42 seconds. The Burner 2 stage was fitted with 3-axis control by Boeing, allowing it to coast without spin stabilization after separating from Thor.
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.
Burner 2 Stage Position in Payload Fairing
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 and performing a pair of U.S. Air Force Space Test Program (STP) missions. 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 3,792 x 3,947 km x 90.1 deg polar orbit.
Thor Burner 2A
Boeing Burner 2A Third Stage, Star 26B Nozzle Visible in Center
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.
One of the Final Five Thor Star37/Star 37 Launch Vehicles
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, were flawless, but the flight record began to decline after 1975, when two of the final six launches failed. Both failures involved prelaunch planning/processing issues rather than faulty Thor or Burner hardware. [1] It may not be a coincidence that both failures occurred after Program 437, the other 10th Aerospace Defense Squadron launch mission, ended in 1975.
DMSP 5(A-C) Series Prelaunch Processing in Rollback Hanger
The first failure, of the final Thor Burner 2A with a Star 26B third stage on February 19, 1976, occurred because the Thor was not loaded with enough kerosene fuel. As a result, the first stage burned out a few seconds earlier than it should have. The upper stages performed their burns, but the end result was insufficient velocity to maintain a stable orbit. The DMSP 5C satellite reentered after one very low orbit. A faulty LR79 main engine data sheet entry was deemed responsible for the improper fuel loading.
The final failure occurred, dishearteningly, during the last Thor Burner 2A mission on July 14, 1980. That Thor Burner 2A, topped by a DMSP 5D1 series satellite, flew correctly through the second stage burn, its 21 year old Thor 301 missile first stage flying true, but then all telemetry was lost. It was subsequently determined that that connectors between the second and third stages had not disconnected due to a misalignment. When the Star 37S-ISS third stage ignited, the wiring harness was jerked out of the third stage and payload to remain with the second stage. Stage 3 and payload flew on out of control with a dead flight control system. The stage pitched down and failed to generate sufficient forward velocity to achieve orbit. It turned out that an incident during launch vehicle erection – a broken pin that caused the rocket to suddenly drop a few centimeters – had most likely caused the connector misalignment.
Thor Ends, Delta Continues
Ever-evolving DMSP finally outgrew Thor. After 1980, DMSP satellites would fly on retired Atlas and Titan II ICBMs. At Space Launch Complex 10 West, the doors were finally closed after 22 years of activity. The site is preserved today as a museum and National Historic Site.
Despite the late failures, Thor Burner provided a relatively low-cost, reliable ride to orbit for most of its 16-year life. The program outlived the more oft-flown Thor Agena by eight years. It outlasted the Johnston Island suborbital Thors by five years. By the end, only five to eight original Thor ballistic missiles remained, most stored at Norton AFB after having been assigned to Program 437.
1980 wasn’t the end for Thor. Far from it. Thor’s legacy continued at Huntington Beach, where McDonnell Douglas continued to build Extended Long Tank Thor first stages for its Delta vehicles. The Thor family would yet see three more decades. It would outlive a production line shutdown during the 1980s and fly at least 200 more times.
