In 1959 there followed six further suborbital missions using a new version of the Thor-Able, Thor DM-18 Able II. This used an upgraded version of Able – with an AJ10-42 engine – to demonstrate the precision guidance system and reentry vehicles intended for the Titan missile.
A three-stage version flew twice, failing to place the Transit 1A navigation satellite into orbit in 1959, but successfully deploying the first weather satellite, Tiros 1, the following year. Two further three-stage versions, Thor DM-18 Able III and Able IV, flew once each with Explorer 6 and Pioneer 5 respectively.
After Thor-Able came Thor-Ablestar, a more advanced rocket with a restartable upper stage. Ablestar – known as Epsilon at an early stage of its development – was wider than the Able and used an AJ10-104 engine. This could be shut down and then restarted in space after a coast of up to 20 minutes, allowing the rocket to achieve a more precise orbit.
Eleven Thor DM-21A Ablestar rockets were flown between 1960 and 1962 – carrying Transit navigation satellites, Courier communications spacecraft, a cluster of small satellites and on its final two launches ANNA geodetic spacecraft.
A second version of Thor-Ablestar was used between 1963 and 1965: The Thor DSV-2A Ablestar II – identified in some documents as the DSV-6 – made eight launches in support of the Transit program. Its first three launches carried Transit-5BN satellites, equipped with radioisotope thermoelectric generators (RTGs).
At the time, nuclear power was seen as a viable option for general use on satellites – and two Transit 4 satellites launched by earlier Thor-Ablestar vehicles had proven the operation of RTGs in space.
The third Transit-5BN launch demonstrated the danger of combining radioactive materials with 1960s rocketry. An upper stage guidance issue left Ablestar with insufficient velocity to make orbit. The upper stage and its payload were destroyed on reentry near Madagascar, spreading about a kilogram (2.2 lb) of plutonium-238 into the upper atmosphere.
1959 saw the introduction of another series of two-stage carrier rockets based on Thor: Thor-Agena. This incorporated Lockheed’s RM-81 Agena vehicle – also known early in its development as Hustler – which was designed to serve both as part of the rocket and a general-purpose platform upon which satellites could be constructed. Such a concept could be seen with the Corona reconnaissance satellites that Thor-Agena was tasked with placing into orbit, where the payload was integrated into Agena. Later models of Agena featured an engine that could be restarted in orbit, allowing the satellite to maneuver after launch.
The first Thor DM-18A Agena-A vehicle – although at this point the name Agena had not been adopted – didn’t get off the ground.
During final preparations for launch in January 1959, with technicians at the launch pad readying the vehicle for its mission, Agena’s onboard timing sequence was inadvertently activated. This caused the rocket to begin its stage separation sequence while still sitting on the launch pad – with pyros firing to separate Agena from the Thor vehicle and eject covers from parts of the spacecraft and small ullage motors on the Agena stage igniting.
While the rocket did not explode – indeed the Thor booster was refurbished and used for another launch eighteen months later – and nobody on the ground was injured, the Agena was damaged beyond repair.
The first Thor-Agena launch was made a month after the mishap – with the rocket carrying the first experimental Corona satellite – Discoverer I – towards polar orbit. Thor performed as expected and Agena ignited to inject its payload into orbit. As the vehicle crossed the horizon and passed out of range of tracking, the mission was proceeding to plan, and the US Air Force announced the successful launch. Only when the spacecraft failed to check in did it become apparent that it had not reached orbit: something had gone wrong while the rocket was out of communications, although the cause of the failure was never established. Discoverer II was launched successfully by the next Thor-Agena in April.
The Thor DM-18A Agena-A made a total of fifteen flights, all in support of Corona. These included both test payloads and early KH-1 reconnaissance satellites. On 10 August 1960, Thor-Agena boosted Discoverer XIII into orbit: a day later one of its prototype film capsules became the first spacecraft to be returned to Earth from orbit, with a splashdown in the Pacific Ocean. Discoverer XIV followed eight days behind Discoverer XIII, becoming the first satellite to return photographic film from orbit and completing the first successful aerial recovery of a film capsule via a C-119 Flying Boxcar aircraft.
Less than two minutes after Discoverer XIV lifted off from Vandenberg Air Force Base a Thor-Ablestar launched from Cape Canaveral in an unsuccessful attempt to deploy the Courier 1A experimental communications satellite. The exact gap between the two launches is not known as the Ablestar’s launch time was only recorded to the minute, so there were between 52 and 122 seconds between the two liftoffs.
This remained the shortest time between two orbital launches worldwide until at least 23 December last year, when an H-IIA and a Falcon 9 lifted off from opposite sides of the Pacific sixty-two seconds apart.
An upgraded version of Agena, the Agena-B, was introduced in late 1960 with stretched propellant tanks and a restartable engine.
The new version of Thor-Agena, the Thor DM-21 Agena-B also incorporated the upgraded DM-21 first stage and flew 43 times between 1960 and 1965 with mostly military payloads. These included KH-2, KH-3 and early KH-4 Corona satellites, the first KH-5 Argon satellites, an area survey reconnaissance satellite designed to complement Corona and three signals intelligence (SIGINT) satellites known as “Heavy Ferrets”. Two of the Corona launches also deployed the first OSCAR amateur radio satellites.
Towards the end of its time in service the Thor DM-21 Agena-B, which had by this time been redesignated Thor SLV-2 Agena-B, was employed for a small number of civilian launches. These included carrying the first Canadian satellite, Alouette 1, in September 1962, NASA’s Echo 2 passive communications balloon in January 1964, the Nimbus 1 weather satellite in August 1964 and on its final flight in November 1965, Alouette 2 and NASA’s Explorer 31 ionosphere research satellite.
Two additional launches with the Agena-B were made using the Thrust-Augmented Thor SLV-2A Agena-B configuration. The Thrust-Augmented Thor (TAT) incorporated three Castor-1 solid rocket motors (SRMs) clustered around the base of the vehicle. These provided additional thrust at liftoff, increasing the rocket’s payload capacity. The SLV-2A Agena-B flew in June 1963 to deploy a Program 698 Heavy Ferret and May 1966 with the Nimbus 2 weather satellite.
The definitive version of Agena, the Agena-D, first flew in June 1962 as part of a Thor DM-21 Agena-D vehicle (later Thor SLV-2 Agena-D). Twenty-one were launched, a little over half of which carried KH-4 and 5 reconnaissance satellites.
Other payloads included four pairs of Poppy signals intelligence satellites which each flew with a number of secondary payloads and two launches with pairs of Defense Satellite Application Program (DSAP) weather satellites – a predecessor to the Defense Meteorological Satellite Program (DMSP).
Two one-off launches carried the Starfish Radiation (Star-Rad) payload in 1962 to study the effects of the Starfish Prime nuclear test and the Multi Research Payload Vehicle (MPRV), a USAF scientific satellite that it failed to put into orbit in 1965. The last SLV-2 flew in May 1967 – over two years after the type’s previous launch – with a Poppy payload.
Just as two Agena-B launches had used Thrust-Augmented Thor boosters, so too was an augmented version of Thor with the Agena-D flown. The Thrust-Augmented Thor SLV-2A Agena-D (TAT-Agena-D) made sixty-one launches between February 1963 and January 1968, the majority with KH-4 and later KH-4A satellites. KH-5s and the short-lived KH-6 Lanyard satellites were also flown aboard this version of Thor-Agena, as were several Poppy and “Heavy Ferret” SIGNIT missions.
In December 1964 a TAT-Agena-D deployed the National Reconnaissance Office’s first radar imaging satellite, Quill (OPS 3762). The images returned by the satellite were not found to be useful, and the mission was not repeated.
TAT-Agena-D vehicles were also responsible for deploying two of NASA’s Orbiting Geophysical Laboratory (OGO) satellites, OGO-2 and OGO-4, which studied Earth’s aurora and magnetic fields.
In 1966 Douglas introduced the “Thor Advanced” booster, or Thorad. The ultimate revision of the Thor-Agena concept, the Long Tank Thrust-Augmented Thor-Agena was a stretched version of Thor without the distinctive tapered fuselage of its predecessors and was used in conjunction with the Agena-D and three Castor-2 strapons.
Forty-three Thorads were launched: thirty in the Thorad SLV-2G Agena-D configuration and thirteen in the Thorad SLV-2H Agena-D configuration. These carried many of the same types of payload as Thor-Agena: KH-4 satellites, by now of the KH-4A and 4B variants, “Heavy Ferrets”, Poppy and occasional scientific missions.
The final Thorad-Agena lifted off from Vandenberg Air Force Base’s Space Launch Complex 3W on 25 May 1972.
From 1965, the US Air Force began flying surplus Thor missiles with solid-fuelled upper stages to carry small payloads into orbit. Known as Thor-Burner, this project saw thirty-one launches up until 1980, most of which carried DSAP – and later DMSP – weather satellites.
Initial launches used the Thor LV-2D Burner I, with a Thor DSV-2S first stage topped by an MG-18 upper stage for the first two launches and an Altair-3 for the next four.
Twelve Thor LV-2F Burner II rockets, consisting of a DSV-2U first stage and Star-37B second stage, were launched next – one of which incorporated a Star-13A third stage to place the Aurora satellite into a higher orbit. The Thor LF-2F Burner IIA made eight launches, adding a Star-26B third stage to lift heavier DMSP-5B and 5C satellites. The final version of this rocket was the Thor LV-2F Star-37XE Star-37S-ISS, which used a pair of Star-37 upper stages on top of a DSV-2U to deploy five DMSP-5D1 satellites.
At the end of the 1950s, NASA commissioned a new interim rocket based on the Thor-Able. Intended to serve as a stopgap for launching scientific missions until a more capable rocket based on the Atlas missile – such as the Atlas-Centaur – became available, the Thor DM-19 Delta – or DSV-3 – used a modified version of the Thor DM-18A – with fins re-added, a new interstage and the attitude determination and guidance systems removed as these functions would be delegated to the upper stage.
The upper stage itself, Delta, incorporated an AJ10-118 engine. An Altair 1A solid rocket motor, or X248-A5, provided the final kick to place payloads into orbit.
The original Thor-Delta made twelve flights over a twenty-eight month period from May 1960 to September 1962. In that time, it made a number of significant launches, including deploying the Echo 1A balloon communications satellite; the first satellite for a foreign nation, Britain’s Ariel 1 (making the United Kingdom the third country to have a satellite in orbit after the Soviet Union and the United States) and the first commercial satellite, AT&T’s Telstar 1.
Delta’s other early payloads included five Tiros weather satellites, placed into polar orbit via a dogleg ascent from Cape Canaveral, the first Orbiting Solar Observatory (OSO) satellite and two Explorer missions: Explorer 10 was placed into a very highly elliptical orbit to study Earth’s magnetic fields and Explorer 12 was the first of the Energetic Particle Explorers (EPE) designed to study cosmic rays.
The only launch failure for the original Thor-Delta came on the type’s maiden flight, which carried the Echo 1 communications balloon. This failed to achieve orbit after the second stage lost attitude control during the coast phase between its own burn and separation from the third stage.
Two more EPE satellites, Explorers 14 and 15, were launched in October 1962 using an upgraded Delta-A vehicle. This brought in the upgraded DM-21 version of Thor and an X245-A5D motor for the third stage.
A further upgrade in December 1962 was the Delta-B, with an AJ10-118D engine on its second stage. This increased the rocket’s payload capacity, allowing it to launch the first geosynchronous satellites, NASA’s Syncom 1 and 2. Other payloads deployed during its nine launches included the Relay 1 and 2 low Earth orbit communications satellites, Telstar 2, two further Tiros satellites and the Explorer 17 aeronomy satellite.
What would prove the final Delta-B launch took place in March 1964 with a Beacon Explorer geodesy satellite – part of NASA’s Explorer program. The third stage lost thrust midway through its burn, and the rocket was unable to reach orbit – the first time a Delta had failed during an orbital launch attempt since its maiden flight.
Despite the failure, preparations for the next Delta-B launch, which was to carry the second Orbiting Solar Observatory (OSO-B), continued. On 14 April 1964, the rocket’s third stage was being prepared in the Spin Test Facility at Cape Canaveral.
When a technician adjusted a plastic cover that had been placed over the vehicle, static electricity from the cover triggered the motor’s igniter. The solid motor burned for 42 seconds, killing three of the men working on it and injuring another eight. This accident led to NASA abandoning use of the X248 series of motors in favor of the X258, whose igniters could be stored separately and installed at the launch pad.
Although not considered true Delta vehicles, and not numbered as such, three Thor LV-2C or DSV-2G rockets flew suborbital missions with Delta-B second stages in 1964 and 1965 as part of the ASSET program. ASSET, which also incorporated three launches via the single-stage Thor DSV-2F, tested a lifting body vehicle to evaluate its performance at reentry.
The project was originally designed to help develop a heat shield for the canceled X-20 DynaSoar spacecraft. However, the flights gathered data that would prove valuable for other winged spacecraft projects including the Space Shuttle. Of the six ASSET missions, three were conducted with complete success – one of which was subsequently recovered, two completed their reentry demonstrations but their recovery equipment malfunctioned, and one was lost after the Delta stage failed to ignite properly.