Delta IV Heavy embarks on its last mission, set for launching

United Launch Alliance’s Delta IV rocket has concluded its final flight with the successful launch of the NROL-70 mission for the National Reconnaissance Office. The launch took place at 12:53 PM EDT from SLC-37B at the Cape Canaveral Space Force Station. This marks the 45th and last flight of the Delta IV, which first flew in 2002 and was developed under the Evolved Expendable Launch Vehicle (EELV) program. Delta IV was designed as a modular system, capable of flying in various configurations depending on mission requirements.

Initially developed by McDonnell Douglas and later merged with Boeing, Delta IV competed directly with Lockheed Martin’s Atlas rocket. In 2006, United Launch Alliance was formed to consolidate the satellite launch operations of both companies and market their launches to the US government. The Delta IV rocket family consisted of different configurations, with Delta IV Heavy being the most powerful version used for the NROL-70 mission. It utilized three common booster cores (CBCs) and a five-meter Delta cryogenic second stage (DCSS).

Delta IV had primarily served US military customers, including the Space Force, Air Force, and the National Reconnaissance Office. It also launched a single commercial satellite, Eutelsat W5. The rocket carried out various missions, including deploying weather satellites, GPS navigation satellites, and defense and surveillance satellites. Notably, it flew two high-profile missions for NASA: testing the agency’s Orion capsule in low Earth orbit and deploying the Parker Solar Probe to study the Sun.

The NROL-70 mission, flown for the National Reconnaissance Office, is classified, but it’s believed to be an electronic signals intelligence (ELINT) satellite destined for geostationary orbit. Delta IV has a history of launching geostationary ELINT satellites known as the Orion series. These satellites intercept radio signals and transmit them back to the NRO for analysis. The NROL-70 mission will be the 17th Delta IV launch for the NRO and the seventh time an Orion satellite has been launched aboard a Delta IV.

Delta IV’s final launch took place at Space Launch Complex 37B, which was originally built for the Saturn I rocket during the Apollo program. The launch complex has been the primary site for Delta IV launches, with 35 out of 45 missions taking place there. The remaining 10 launches occurred at Vandenberg Space Force Base’s Space Launch Complex 6. With the retirement of Delta IV, SpaceX has secured a lease to use SLC-6 as a launch site for its Falcon rockets.

The Delta IV rocket used for the NROL-70 mission was assembled in the Horizontal Integration Facility (HIF) at SLC-37 and raised to the vertical position on December 20, 2023. The NROL-70 payload was encapsulated in its fairing and installed atop the rocket on February 26, 2024. Delta IV Heavy is an all-cryogenic rocket, with its engines burning liquid hydrogen and liquid oxygen. The rocket experienced a staggered engine start to mitigate the effects of hydrogen buildup. Despite expected fires on the aft insulation during liftoff, they do not affect the mission.

During its ascent, Delta 389 reached Mach 1 and passed through the maximum dynamic pressure (Max-Q) zone. The boosters throttled down as they approached propellant depletion, and two outboard CBCs shut down their engines, separating from the center core. The center core continued to burn until booster engine cutoff (BECO), followed by stage separation and ignition of the second stage engine. The payload fairing protecting the NROL-70 satellite was jettisoned, and subsequent flight events are classified. The second stage will perform multiple burns to reach geostationary orbit before deploying the payload.

The retirement of Delta IV concludes its 22 years of service and signifies the end of the Delta rocket family’s long history. United Launch Alliance will now focus on its remaining Atlas V launches and the next-generation Vulcan rocket. Vulcan has been selected, alongside SpaceX’s Falcon rockets, to fulfill US military launch requirements in the future. These rockets will take over the responsibility of launching oversized payloads previously assigned to Delta IV Heavy.