Delta IV Heavy launches on final mission

by William Graham

United Launch Alliance’s Delta IV rocket has successfully launched Tuesday on its final flight, the NROL-70 mission for the National Reconnaissance Office. Liftoff from SLC-37B at the Cape Canaveral Space Force Station has happened at 12:53 PM EDT (17:53 UTC) following a scrub on the initial attempt over a week ago.

The launch marks the 45th and last flight of the Delta IV, which first flew on Nov. 20, 2002. Along with the Atlas V, it was one of two rockets developed under the original phase of the Evolved Expendable Launch Vehicle (EELV — now National Security Space Launch, or NSSL) program. Delta IV was initially developed by McDonnell Douglas, which merged with Boeing in 1997.

While Delta IV and Atlas V are now stablemates at United Launch Alliance (ULA), during their early years the two rockets were direct competitors with Lockheed Martin developing and operating the Atlas. ULA was formed in December 2006, consolidating the two companies’ satellite launch operations into a single organization that would also market their launches to the US government.

Delta IV was designed as a modular system, able to fly in several different configurations depending on mission requirements, all based around the common booster core (CBC) which forms the rocket’s first stage. The smallest configuration to fly, Delta IV Medium, consisted of a CBC and the four-meter diameter version of the Delta cryogenic second stage (DCSS), while the most powerful version of the rocket — Delta IV Heavy, which will be used for this mission — uses three CBCs and a five-meter DCSS.

The intermediate Medium+(4,2), (5,2), and (5,4) configurations were also used to carry payloads too heavy for the Medium, but which did not require the performance of the Heavy. The two numbers at the end of their designation indicated the diameter of the upper stage (four or five meters), and the number of GEM-60 solid rocket motors attached to provide additional thrust during the early stages of flight (two or four).

The Delta IV Medium last flew in 2006, having only been used for three launches. In 2015, ULA announced plans to phase out the remaining members of the Delta IV family as the company began to ramp up development of its next-generation Vulcan rocket, with the Medium+ configurations targeting an out-of-service date in 2018 and the Heavy remaining in service until the US Air Force was satisfied that another rocket was available to fly the missions that it was being used for. The final Delta IV Medium+ flew on Aug. 22, 2019, carrying a Global Positioning System (GPS) satellite.

The mission was flown for the National Reconnaissance Office (NRO) and is designated NRO Launch 70 (NROL-70). Like most NRO missions, details of the payload are classified; however, information that has been made public leaves little doubt that it is an electronic signals intelligence (ELINT) satellite bound for geostationary orbit. From this high perch, the satellite will intercept radio signals from terrestrial sources and relay them back to the NRO for analysis.

The NRO’s geostationary ELINT satellites are part of a series known as Orion, which began with the deployment of the USA-8 spacecraft from the Space Shuttle Discovery during 1985’s STS-51C mission. The first two satellites were launched aboard the Space Shuttle, the next three by Titan IV rockets, with the Delta IV Heavy having been used since 2009. The NROL-70 mission will be the 17th Delta IV launch for the NRO — 12 of which have used the Delta IV Heavy — and the seventh time an Orion satellite has launched aboard a Delta IV.

The last Delta IV Medium+(4,2) launch in 2019, with the GPS III SV-2 spacecraft. (Credit: ULA)

The majority of Delta IV’s missions have been carried out for US military customers: the Space Force (and previously the Air Force) and the NRO. It only launched a single commercial satellite, Eutelsat W5, which was aboard the maiden flight. Three more early launches were made under commercial contracts, but carrying government payloads: the Geostationary Operational Environmental Satellite (GOES) 13, 14, and 15 weather satellites for the National Oceanic and Atmospheric Administration (NOAA). Delta IV Heavy also flew two high-profile missions for NASA: Exploration Flight Test 1 (EFT-1) in 2014 tested the agency’s Orion capsule in low Earth orbit ahead of its role in future crewed Lunar missions. In August 2018 the Parker Solar Probe was deployed to study the Sun.

The EFT-1 mission was flown under an FAA commercial launch license: the last Delta IV mission, and the only Heavy, to receive such a license.

Air Force and Space Force payloads carried by Delta IV include two Defense Satellite Communications System (DSCS) communications satellites and eight spacecraft for the successor Wideband Global Satcom (WGS) program; seven GPS navigation satellites; two pairs of Geosynchronous Space Situational Awareness Program (GSSAP) space surveillance satellites and a Defense Meteorological Satellite Program (DMSP) weather satellite. A single Defense Support Program (DSP) missile detection satellite was launched aboard the second Delta IV Heavy mission after the configuration’s maiden flight had carried a mass simulator.

Delta IV Heavy launches from Vandenberg Space Force Base during 2022’s NROL-91 mission. (Credit: ULA)

As well as its seven Orion missions, Delta IV has deployed five KH-11 Crystal optical reconnaissance satellites, three Topaz radar-imaging satellites, one Quasar communications satellite, and one elliptical-orbit ELINT satellite for the NRO. Like other aspects of the missions, most of the names of the NRO’s satellites are classified but many have found their way into the public domain through declassified or leaked documents over the years. The elliptical-orbit ELINT satellites do not have a commonly accepted name but are believed to be successors to an earlier program known as Trumpet.

The launch is the 16th flight of the Delta IV Heavy, which overtakes the Medium+(4,2) to become the most-flown Delta IV configuration. Fifteen Medium+(4,2) launches were made, with the Medium+(5,4) making eight flights — all with WGS satellites. The Delta IV Medium and Medium+(5,2) made three flights apiece.

Of the 44 missions it has flown before the swansong, Delta IV has completed 43 successfully, with the only blemish on its record being the maiden flight of the Heavy configuration — and the type’s fourth launch overall — in December 2004.

The launch was to have placed a mass simulator into a near-geosynchronous orbit, deploying two small satellites along the way. Cavitation — the formation of small bubbles or voids due to a pressure differential — occurred within the oxidizer feed lines in all three CBCs. The presence of these bubbles caused cutoff sensors to incorrectly determine that the oxidizer supply had been depleted, resulting in all three boosters shutting down prematurely. The rocket achieved a lower orbit than had been planned.

Delta IV Heavy at SLC-37B ahead of the NROL-44 mission in 2020. (Credit: ULA)

The launch will take place from Space Launch Complex 37B (SLC-37B) at the Cape Canaveral Space Force Station. This pad was originally built for the Saturn I rocket during the early days of the Apollo program in the 1960s. It was used for six Saturn I and two Saturn IB launches. The first launch from the pad was SA-5, the first orbital launch of the Saturn I, while the pad’s last Saturn launch was the Apollo 5 mission which carried out an uncrewed test of the Lunar Module in low-Earth orbit (LEO).

Launch Complex 37, as it was then designated, consisted of two pads with a shared mobile service tower, although pad 37A was never used for a launch. After Apollo 5 the complex was mothballed in anticipation that it would be needed when LEO missions resumed after Apollo had completed its objective of landing astronauts on the Moon.

Instead, the follow-on Apollo Applications Program was cut back to just three crewed missions to the Skylab space station and the Apollo-Soyuz Test Program, and NASA opted to modify a Mobile Launcher to permit Saturn IB launches from Launch Complex 39 instead of reactivating the other pads. Complex 37’s towers would later be demolished and the pad would remain derelict until the end of the 1990s when SLC-37B was selected as the East Coast home of the Delta IV.

Thirty-five of Delta IV’s 45 missions have taken place from SLC-37B, with Vandenberg Space Force Base’s Space Launch Complex 6 (SLC-6) serving as the venue for the remaining 10. The last Delta IV launch from Vandenberg was NROL-91, which took place in September 2022. With Delta’s retirement, SpaceX has already secured a lease to take over SLC-6 as a launch site for its Falcon 9 and Falcon Heavy rockets, while plans have also been announced to convert SLC-37 into a launch pad for Starship and Super Heavy.

The rocket that will fly this mission has flight number, or Delta number, D389. Like all Delta IV Heavy rockets, this consists of three CBCs making up the first stage, with a five-meter DCSS second stage. The Delta IV Heavy has used two different types of payload fairing during its career: a purpose-built bisector (two-part) fairing made of composite materials, and a tri-sector (three-part) aluminum fairing derived from that used by the Titan IV. All previous Orion launches have used the tri-sector fairing, and Delta 389 is no exception.

Delta IV is raised into position at SLC-37B. (Credit: ULA)

The rocket’s CBCs arrived at Cape Canaveral aboard ULA’s R/S RocketShip vessel in May 2023, a few weeks before the previous Delta IV mission lifted off on June 22. The rocket was assembled, without its payload fairing, in the Horizontal Integration Facility (HIF) at SLC-37, before being rolled to the launch pad and raised to the vertical on Dec. 20 in a milestone designated Launch Vehicle on Stand. The NROL-70 payload, already encapsulated in its fairing, arrived at the pad on Feb. 26 for installation atop the rocket.

Delta IV Heavy is an all-cryogenic rocket, meaning its engines burn liquid hydrogen and liquid oxygen as fuel and oxidizer respectively. These propellants were loaded into the first and second stages of the vehicle as the countdown progressed. As the countdown approaches T0, radially outward firing initiators (ROFIs) ignited to burn off hydrogen gas around the base of the vehicle. At about T-7 seconds the RS-68A engine at the base of the starboard CBC ignited, with the port and center cores igniting their engines two seconds later.

NROL-70 launches on the Delta family’s last ever flight. (Credit: Max Evans for NSF)

Along with the ROFIs, this staggered engine start was intended to mitigate the effects of hydrogen building up around the rocket, which still usually ignites into a fireball as the engines are lit. Despite these measures, it was not unusual to see Delta IV Heavy lift off with its aft insulation on fire — this does not pose a significant risk to its mission and will be extinguished as the rocket climbs through the atmosphere.

Liftoff itself occurred at the T0 mark in the countdown. About 9.4 seconds later, after clearing the tower, Delta 389 made a pitch-and-yaw maneuver to put it on the planned track for its ascent. This launch saw Delta IV fly an easterly trajectory out over the Atlantic Ocean, targeting a geostationary orbit. The rocket reached Mach 1, the speed of sound, one minute and 18.5 seconds into flight, passing through the area of maximum dynamic pressure, or Max-Q, two seconds later.

As the port and starboard boosters approach propellant depletion, they throttled down to partial thrust mode, limiting acceleration. Three minutes, 55.8 seconds after liftoff the two outboard CBCs shut down their engines, separating from the center core 2.1 seconds later. The center core continued to burn until the five-minute, 35.6-second mark in the mission, which is designated Booster Engine Cutoff (BECO). Six and a half seconds after BECO the first and second stages separated and the DCSS began to deploy the extendible nozzle of its RL10C-2-1 engine.

Ignition of the second stage engine happened thirteen seconds after stage separation. The payload fairing, used to protect the NROL-70 payload during its ascent through the atmosphere, was jettisoned about 42 seconds into the burn. This is the point at which all official coverage of the mission concluded, with the timing of subsequent flight events being classified. An idea of likely timings can be inferred from the profile published for Delta IV Heavy’s maiden flight, which was intended to simulate a national security launch to near-geostationary orbit, although upgrades — such as replacing the original RS-68 main engines with RS-68As and changing the DCSS engine from an RL10B-2 to the RL10C-2-1 used today — have been made to the Delta IV since that mission took place.

The DCSS will make three burns in order to place its payload directly into geostationary orbit. The first of these will last about seven minutes, establishing the vehicle in an initial parking orbit. A second burn, lasting about eight minutes, will be performed after a short coast and will set up a geostationary transfer orbit. This will be followed by a much longer coast phase — around five hours — to reach the apogee, or highest point, of the trajectory. At apogee, the DCSS will restart for its third and final burn which will take about three and a quarter minutes to circularise the orbit.

Spacecraft separation is likely to follow a few minutes after the end of the third burn. Once the NROL-70 payload has been deployed, the DCSS will perform a collision avoidance maneuver, taking itself out of geostationary orbit to ensure it does not pose a threat of collision to operational satellites.

While Delta 389’s launch brings down the curtain on Delta IV’s 22 years of service, that flight number speaks to a much longer history which will also end with this launch. The name Delta dates back to the early days of the US space program and was originally given to an upgraded version of the earlier Able upper stage intended for use in conjunction with the Thor intermediate-range ballistic missile for launching satellites.

The Thor-Delta, as the combined vehicle was designated, made its maiden flight on May 13, 1960, carrying the Echo 1 communications satellite, but failed to reach orbit after an upper stage attitude control problem. This launch was designated Delta 1, beginning the sequence which will end with Delta 389.

Three months after the failed Echo 1 launch, a replacement satellite was launched successfully. An upgraded version of the Thor-Delta, Delta A, was introduced in 1962 and quickly superseded by a further upgrade, Delta B. Improved versions would continue to be rolled out throughout the 1960s, with the designation reaching Delta N by 1968. While the first stage remained a derivative of the Thor missile, the name “Thor-Delta” was shortened to just “Delta” as the rocket became established in service.

From 1972, four-digit numeric designations were introduced for new versions of Delta, with the first digit indicating the type of first stage and boosters used, the second the number of solid-fuelled boosters attached to the first stage, the third denoting the type of second stage flown, and the fourth number indicating an optional third stage. The short-lived Delta 0100 and 1000 series gave way to the successful Delta 2000 and 3000. In the 1980s it was expected that the Space Shuttle would make expendable rockets like Delta obsolete, but following the loss of Challenger in 1986 a new generation of vehicles was conceived.

The Delta II first flew in February 1989 in its 6000-series form (the 4000 and 5000 designations having been applied to interim configurations using leftover hardware to fill the gap left after Delta 3000 retired), with the definitive 7000-series debuting the following year. Delta II remained in service until September 2018, making 155 flights that supported military programs — particularly GPS; carried NASA missions to study the Earth, explore the solar system, and improve our understanding of our universe; and deployed commercial communication and Earth-imaging satellites. Delta II was the last rocket to use the family’s traditional configuration of a Thor-derived first stage and a storable-propellant upper stage descended from the original Delta.

McDonnell Douglas developed the Delta III in the 1990s in an attempt to evolve the Delta family to keep up with the increasing size and mass of commercial communications satellites. This used a modified version of the Delta II’s first stage with a shorter and wider propellant tank, more powerful solid rocket boosters, and a new four-metre cryogenic-propellant second stage.

First flown in August 1998, Delta III made only three launches over the space of two years and failed on its first two flights. The third launch, carrying a demonstration satellite, reached a lower-than-planned orbit but Boeing — who had taken over the project after merging with McDonnell Douglas — deemed the launch a success within the parameters of its test. This could not save Delta III, and with customers unwilling to entrust their satellites to it, the program was abandoned.

Delta II lifts off for its final flight in September 2018. (Credit: Brady Kenniston for NSF/L2)

Delta III’s second stage led to the development of the DCSS stage used on Delta IV, which also replaced the Thor-derived first stage with its CBCs and in doing so severed the last physical link to the original Thor-Delta. DCSS will outlive Delta IV, continuing to fly as the Interim Cryogenic Propulsion Stage (ICPS), the second stage of the SLS Block 1 rocket, on the Artemis II and III missions. The final ICPS was delivered to Cape Canaveral last year in preparation for Artemis III.

Under the numeric designation system, Delta IV would have been the Delta 9000 series, however, these designations were dropped in favor of the simpler “Medium”, “Medium+” and “Heavy” appellations. While other traditions of the Delta program have continued — such as assigning a sequential Delta number to each launch — many independent sources tally Delta IV missions separately to earlier Delta flights. While the inclusion of Delta IV in statistics for the Delta family of rockets may be as much a question for the philosophers as that of the Ship of Theseus, what is certain is that after this launch, the name Delta will pass into history.

The retirement of Delta IV leaves United Launch Alliance with its workhorse Atlas V, which has 17 flights remaining before its own retirement, and its next-generation Vulcan rocket that made a successful maiden flight in January. Under the second phase of the NSSL program, Vulcan has been selected — alongside SpaceX’s Falcon 9 and Falcon Heavy vehicles — to satisfy US military launch requirements in the coming years. These rockets will now pick up the torch of carrying the oversized NRO payloads which have until now been the responsibility of Delta IV Heavy.

(Lead image: NROL-70 climbs through the sky on the Delta family’s final flight. Credit: Sawyer Rosenstein for NSF)

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