The evolution of Thor – Delta II prepares for swansong

by William Graham

The Delta-C rocket was the first member of the Delta series to incorporate the X258 motor. It had first flown a few months before the OSO-B accident in November 1963 and was used to deploy four Explorer missions – the first three Interplanetary Monitoring Platforms (IMP) and the final EPE – four Orbiting Solar Observatories and three weather satellites: Tiros 9, Tiros 10 and ESSA-1.

To further increase its payload capacity, the Delta-C1 configuration was devised with an FW-4D third stage instead of the X258: this flew twice to deploy Explorer 32 in 1966 and OSO-5 in 1969.

Delta 26, a Delta-C, October 4, 1964, LC 17A, Explorer 21 – Via L2 Historical, uploaded by Ed Kyle

The pattern of incremental upgrades to Delta continued through the 1960s. In 1964, needing more performance, the Delta-D introduced the Thrust-Augmented Thor that was being used for Thor-Agena missions, with an MB-3-3 engine and three Castor-1 boosters.

Delta-D flew only twice, both times to geostationary transfer orbit. Its first mission carried Syncom 3, which would become the first satellite to be placed in an equatorial geostationary orbit, while its second would deploy Intelsat’s first satellite, Intelsat I F-1 – also known as Early Bird.

The next upgrade was the “Thrust-Augmented Improved Delta”. This used a new version of the Delta stage, abandoning the slender form factor inherited from Able in favor of tanks based on the wider Ablestar.

Thor 457 (64-17731)/Delta 34 – via L2 Historical, uploaded by Ed Kyle

An AJ10-118E powered the second stage and either a X258 or FW-4D motor was used as the third stage – with the rocket designated Delta-E or Delta-E1 respectively according to which one was flown. Six Delta E rockets deployed four ESSA weather satellites, the first Geodetic Earth Orbiting Satellite (GEOS) and the Pioneer 6 spacecraft.

In October 1966, Delta made its first launch from Vandenberg Air Force Base with a Delta-E carrying ESSA-3 to orbit.

Seventeen Delta-E1 rockets orbited four more IMP Explorer missions, Intelsat’s four second-generation communications satellites, two ESSA spacecraft, the Pioneer 7, 8 and 9 probes, a second GEOS and two international scientific missions: the European Space Research Organisation’s (ESRO’s) Highly Eccentric Orbit Satellite (HEOS) and Canada’s International Satellite for Ionosphere Studies (ISIS).

The Pioneer 6, 7, 8 and 9 spacecraft were placed into orbit around the Sun, slightly inside that of the Earth, to make the first dedicated in-situ study of interplanetary space.

To launch NASA’s first two Biosatellite spacecraft, a two-stage version of the Delta-E was used. The designation Delta-G was applied to this vehicle, which omitted the solid-fuelled third stage. The Delta-J, also based on the Delta-E but with a Star-37D third stage, was used for only a single flight with the Explorer 38 radio astronomy satellite. In 1967, Thor’s original manufacturer Douglas merged with McDonnell Aircraft to form McDonnell Douglas, who would take over production of Thor and Delta rockets.

By the end of the 1960s, Delta needed to grow once again to accommodate ever-larger payloads. Adopting the same stretched first stage and Castor-2 boosters as the Thorad-Agena, the first Long Tank Thor-Delta variants – Delta-L, M and N – began flying in 1968 and were the first members of the Delta family capable of putting a metric tonne (2,200 lb) of payload into orbit.

Delta 87, a Long Tank Thrust Augmented Delta, prepares to launch HEOS 2 from VAFB 2E on January 31, 1972 – via L2 Historical, uploaded by Ed Kyle

Delta-L used an FW-4D upper stage, Delta-M a Star-37D and Delta-N was a two-stage version of the rocket. Two more versions, the Delta-M6 and Delta-N6, increased the number of boosters from three to six to further increase capacity.

The Delta-L flew only twice. Its first launch in early 1969 carried an additional Pioneer spacecraft, Pioneer-E, which was to have joined Pioneers 6 to 9. However, the rocket failed to make orbit: a faulty valve led to a hydraulic leak on the first stage of the vehicle, followed by a loss of attitude control. Delta-L’s second flight came in 1972, successfully deploying ESRO’s HEOS-2 satellite.

Delta-M also failed on its maiden flight: the rocket lost pitch control early in its ascent and was destroyed by range safety after it had already begun to disintegrate. On board was the first of eight third-generation Intelsat communications satellites that would launch on the Delta-M. The rocket’s four other launches carried military communications satellites: two for NATO and the first two spacecraft for the British Skynet system. A single Delta-M6 was launched in March 1971 to deploy another Interplanetary Monitoring Platform, Explorer 43.

Six Delta-N vehicles were launched: two carried ESSA weather satellites, one carried the Biosatellite 3 biological research payload, two deployed Orbiting Solar Observatories and the final launch put up a satellite for ESRO named after its carrier rocket, Thor Delta 1A. The Delta N6 made three flights with Improved Tiros Operational System (ITOS) weather satellites, a new generation of spacecraft that succeeded ESSA.

The increasing options for number of boosters, number of stages and type of third stage meant that continuing to assign Delta sequential alphabetical designations had become confusing and impractical. From the next revision, which upgraded the second stage with an AJ10-118F engine, the letters were abandoned in favor of a numerical designator with each digit signifying a different aspect of the vehicle. The Delta 300 had three Castor-2 boosters, while the Delta 900 used nine.

Both of these were short-lived two-stage rockets, with the Delta 300 launching only three weather satellites and Delta 900 making two launches. One of these carried an experimental weather satellite – Nimbus-5 – while the other deployed the Earth Resources Technological Satellite (ERTS). Later renamed Landsat 1, ERTS was the first satellite in a series of global mapping and resource management spacecraft whose successors remain in service.

Delta’s first stage was stretched further to become the Extended Long Tank Thor (ELTT). To signify this, another digit was added to the front of the rocket’s designation, with “1” signifying the use of an ELTT first stage and Castor-2 boosters. The second digit denoted the number of boosters – with four, six or nine available for the 1000-series.

Delta 109, a 1410 model, orbited GEOS 3 from VAFB SLC 2W on April 9, 1975. The last white Delta. – via L2 Historical, uploaded by Ed Kyle

Most later versions up until Delta II would use three or nine boosters. The third digit gave the type of second stage: zero indicated the AJ10-118F-engined stage while a one meant that the rocket would be flying with a new upper stage, Delta-P. This was powered by a TR-201 engine, developed from the descent propulsion system of the Apollo Lunar Module, with a larger diameter of eight feet (2.44 meters) – to match the first stage. The use of this upper stage earned Delta the nickname “Straight Eight”.

The fourth digit of Delta’s designation identified the third stage: zero for no upper stage, one for a Star-24, two for an FW-4D, three for a Star-37D and four for a Star-37E. The Star-24 and FW-4D were never used on numbered vehicles.

In January 1974 Delta 100 was launched from Cape Canaveral. Officially the 100th Delta rocket to be launched – a number that does not include the three suborbital ASSET missions – Delta 100 used the Delta 2313 configuration. This was the first time Delta had flown with a re-engined ELTT first stage, a Rocketdyne RS-27 replacing the MB-3-3.

Delta 101 was a 2914 model that sent Westar 1 to GTO from LC 17B on April 13, 1974 – via L2 Historical, uploaded by Ed Kyle

The RS-27 was a modified version of the H-1 engine that had powered the Saturn I and IB rockets: with the end of the Apollo program a large surplus of these engines became available for other applications so they were modified for Delta. The Delta 100 mission itself ended in failure: an electrical fault on the second stage meant that the Skynet 2A communications satellite barely made orbit and reentered within days.

At the end of 1975, another new first digit was added to the designation system: the 3000-series used the same RS-27-powered first stage as the 2000-series, but with Castor-4 boosters instead of Castor-2s. The 2000- and 3000-series Deltas sustained Delta well into the 1980s, deploying many commercial communications satellites as well as weather and scientific payloads. Of note, the National Oceanic and Atmospheric Administration (NOAA) saw its first geostationary weather satellites – Synchronous Metrological Satellite (SMS) and Geostationary Operational Environmental Satellite (GOES) deployed by Delta 2914 and later Delta 3914 vehicles.

Delta 2914 rockets were also used to launch three International Sun-Earth Explorer (ISEE) missions for NASA and the European Space Agency. ISEE-1 and 2 were launched together in October 1977, while ISEE-3 followed them into orbit the following year. The three spacecraft operated close to the L1 Lagrangian point between the Sun and Earth, studying the solar wind and interactions between the solar and terrestrial magnetospheres. ISEE-3 was later re-purposed as the International Cometary Explorer (ICE) and flown through the tails of comets 21P/Giacobini-Zinner and 1P/Halley.

In 1980, a Delta 3910 launched NASA’s Solar Maximum Mission (SolarMax) while the same configuration was used in 1983 to deploy the international Infrared Astronomical Satellite (IRAS). Also in 1983 a Delta 3914 launched ESA’s Exosat X-ray astronomy satellite.

From 1980 onwards, the Payload Assist Module – Delta (PAM-D) upper stage, based on a Star-48B motor, began to see use as the third stage on commercial Delta launches instead of the earlier Star-37D and E. This was not initially given a number in Delta’s designation system, so rockets incorporating this upper stage would be known as, for example, Delta 3910/PAM-D. A new second stage was introduced in 1982 – the Delta-K marked a return to the AJ10 engine with a new AJ10-118K model, although the stage retained the eight-foot diameter that had been pioneered with the Delta-P.

With the advent of the Space Shuttle, expendable rockets began to be seen as obsolete and both the US military and NASA started to wind down their rocket programs, including Delta. Only a handful of Delta launches remained on the books in January 1986, when Space Shuttle Challenger and her crew were lost seventy-three seconds after lifting off to begin the STS-51-L mission.

The tragedy forever changed the perspective in which the Space Shuttle would be seen – no longer was it a vehicle that could fly dozens of missions a year with rapid turnarounds and low running costs.

The US Air Force suddenly needed expendable rockets to launch its satellites, particularly its new Global Positioning System that would need a fleet of 24 spacecraft plus on-orbit spares to enable accurate worldwide navigation and location-finding. The Air Force commissioned new versions of the Atlas, Titan and Delta rockets, and Delta II was born.

The new rocket would incorporate a further-stretched first stage: Extra-Extended Long Tank Thor with an upgraded RS-27A engine, initially augmented by Castor-4A SRMs until more powerful Graphite Epoxy Motors (GEMs) became available. A Delta-K would be used as the second stage, while an optional PAM-D third stage – finally assigned the number five in Delta’s sequence of upper stages – would be available for spacecraft requiring higher orbits.

While development of the Delta II continued, McDonnell Douglas threw together two interim Delta configurations from spare parts. The Delta 4925 was used for two commercial communications satellite launches in 1989 and 1990 and used leftover MB-3-3-powered ELTT boosters, Castor-4A strapons and Delta-K and PAM-D upper stages. The Delta 5920 that launched NASA’s Cosmic Background Explorer (COBE) in November 1989 used an RS-27-engined core, Castor-4As and a Delta-K second stage.

Delta II made its first flight in February 1989, when Delta 184 deployed the first GPS Block II satellite, USA-35. The initial version of the Delta II was designated the 6000-series, with Delta 6920 rockets used for low Earth orbit missions and Delta 6925 configurations used to reach higher orbits. The 7000-series, incorporating GEM-40 SRMs first flew at the end of 1990.

Through the 1990s Delta II helped the US Air Force to set up its GPS constellation, while also deploying commercial communications satellites to geostationary transfer orbit. Towards the end of the decade geostationary satellites began to outgrow the Delta II. However, the rocket instead found a role boosting clusters of low orbit communications satellites for Iridium and Globalstar.

GPS launch in the 1990s – via USAF

In an effort to recapture some of the geostationary satellite market, McDonnell Douglas added a cryogenic second stage to Delta, making the Delta III or Delta 8930. The first stage propellant tank was widened and shortened to accommodate the new upper stage and GEM-46 boosters were swapped in for the Delta II’s GEM-40s.

McDonnell Douglas was bought by Boeing in 1997, and the Delta III first flew the following year with PanAmSat’s Galaxy 10 satellite aboard. Because of a flaw in the design, the rocket used up the hydraulic fluid used to thrust-vector its SRMs faster than expected and went out of control within about 75 seconds of liftoff.

A second launch the following May left Orion Network Systems’ Orion 3 satellite stranded in low Earth orbit, a manufacturing fault in the second stage’s RL10B-2 engine caused its combustion chamber to rupture when the engine was restarted for its second burn.

The back-to-back failures shook confidence in the new rocket and no customers could be found for its third launch, which instead delivered a mass simulator to a slightly-lower-than-planned orbit – a result of the rocket intentionally being programmed to burn to propellant depletion instead of a precise trajectory. After the third flight, Boeing abandoned the Delta III.

Despite their setbacks with the Delta III, Boeing continued to offer the Delta II for US Government and commercial launches into the 21st Century, with production and launch operations passing to United Launch Alliance in 2006. Delta II was upgraded with GEM-46 boosters from the Delta III project, resulting in the more powerful Delta II Heavy vehicle.

Delta II has deployed spacecraft for NASA that have visited the Moon, Mars, Venus and Mercury, Comets, Asteroids and the Dwarf Planet Ceres. Other missions have deployed satellites to help with resource management, weather forecasting and climate research on Earth, studied the Sun, performed astronomy from above Earth’s atmosphere and hunted for extrasolar planets.

When Delta II begins its final flight, it will do so as the most reliable rocket in service – with a record of 152 successful launches from 154 flights. The type’s only failure cane in January 1997 when a crack in one of its solid rocket motors caused Delta 241 to disintegrate seconds after liftoff.

Two years earlier the rocket had suffered a partial failure while launching Mugunghwa 1 (later Koreasat-1), when one of the motors failed to separate. Mugunghwa 1 was placed into a lower orbit than had been planned but was still able to reach its planned geostationary orbit at the expense of propellant that would have been used for stationkeeping.

Once Delta II retires, the Delta name will be kept alive by United Launch Alliance’s Delta IV rocket.

Building on Boeing’s experience with the Delta III, the Delta IV is a fully-cryogenic rocket using a Common Booster Core first stage and a Delta Cryogenic Second Stage (DCSS) that evolved from Delta III’s upper stage.

Delta Family – Boeing Graphic.

Delta IV is itself being phased out in favor of Atlas V and ULA’s next-generation rocket, Vulcan, however the Delta IV Heavy configuration is expected to continue flying for the foreseeable future.

Thor’s legacy is not just its contribution to the US space program. On top of the multitude of satellites it has launched for international customers, partnerships and multinational research projects, in the 1970s Thor and Delta were exported to Japan where they served as a template for Japanese liquid-fuelled rocketry.

The Nippon I, or N-I rocket used a license-built Long Tank Thor first stage with Castor-2 boosters and a modified version of the Delta upper stage incorporating a Mitsubishi-built LE-3 engine. N-I flew seven times between 1975 and 1982 before giving way to the N-II.

The Nippon I, or N-I rocket used a license-built Long Tank Thor first stage – via JAXA

N-II used a license-built version of the Extended Long Tank Thor, with the MB-3-3 engine, as well as an AJ10-118F-powered Delta second stage, nine Castor-2 boosters and on seven of its eight flights a Star-37E third stage.

This would have been equivalent to a Delta 1904 under the US designation system – although such a configuration was never used outside of Japan. N-II was used from 1981 to 1987. The last of Japan’s Thor-derived rockets, H-I, made its debut in 1986 and was operated until 1992.

H-I kept the same Thor first stage and boosters as the N-II but replaced the license-built Delta second stage with a Japanese-built cryogenic stage powered by an LE-5 engine. H-I was launched nine times – three in a two-stage configuration and six with an UM-129A solid rocket motor as a third stage.

The experience Japan gained with the N-I, N-II and H-I led into the development of the fully-Japanese H-II family of rockets.

The H-I Launch Vehicle lifts off – via JAXA

Saturday’s launch will be the 724th flight of a Thor vehicle – including the versions of the rocket manufactured under license in Japan.

While through the process of incremental upgrades the Delta II rocket – Delta 381 – that will launch on Saturday bears little resemblance to the Thor 101 vehicle that exploded on Launch Complex 17B over sixty years ago, its retirement severs a link between the present and the past with the last liquid-fuelled rocket that can trace its heritage directly back to the United States’ early missile programs.

(Many thanks to Ed Kyle of – and the L2 Thor Section with hundreds of images from Thor’s service life – for the assistance in the creation of this article)

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