NASA has officially selected United Launch Alliance’s Centaur 5 as the upper stage for its Space Launch System rocket starting with the Artemis 4 mission, which is not planned to launch before early 2028.
Centaur 5 was developed as the upper stage of ULA’s Vulcan rocket. The launch vehicle has flown four times since its launch in January 2024 and the upper stage performed well in all flights.
The news, disclosed in contract documents published Friday, comes a week after NASA Administrator Jared Isaacman announced that the agency would “move toward standardization.” [Space Launch System rocket] The fleet is to be… near-Block 1 configuration.”
“The idea is that we want to reduce complexity as much as possible,” Isaacman said during a briefing at Kennedy Space Center on Feb. 27.
Originally, NASA planned to launch the first three missions for the Artemis program using ULA’s Interim Cryogenic Propulsion Stage (ICPS), a modified version of its Delta 4 cryogenic second stage, and then transition to the Boeing-built Exploration Upper Stage (EUS), beginning with the Artemis 4 mission.
NASA, under Isaacman’s leadership, decided to move away from those plans due to cost and schedule overruns.
Long before this decision, Tory Bruno, ULA’s president and CEO at the time, was asked during a reporter roundtable in December 2024 how the company would handle the principle change in architecture for the SLS rocket. The question came a month after President Donald Trump was elected to a second term, sparking discussion about whether SLS plans might change at that time.
“The exploration upper stage is a very, very large upper stage. It’s much larger than the interim cryogenic upper stage that we’re providing right now. It’s even larger than Centaur 5,” Bruno said. “If the government wants to make some changes to the architecture of SLS, they will let us know and we will tell them what we can do.”
That ‘what if?’ The scenario is now reality.
In its procurement statement, NASA said it intended to issue a sole source contract to ULA, meaning it is the only upper stage to be considered for this new iteration of the SLS rocket. An eight-page supporting document from NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Alabama, was published to document the reasoning for its decision.
Among the reasons cited are the decades-old legacy of the RL10 engine, which has matured over time; The ability of Centaur 5 to use liquid oxygen and liquid hydrogen propulsion items, as well as interfaces available on Mobile Launcher 1 (ML1); and the experience of ULA teams working with NASA’s Exploration Ground System (EGS) at Kennedy Space Center and elsewhere in the country.
He also noted that the Centaur 3 upper stage, which has achieved certification to launch humans as part of the Commercial Crew Program, has many common features with Centaur 5.
“This approach takes advantage of existing support infrastructure and will use the existing ULA upper stage with relatively minor modifications,” NASA said. “All other alternative solutions fail to meet performance requirements, would require significant modifications to hardware that is still under development, or would require the development of new hardware that does not currently exist.”
NASA also said that time constraints in this decision led them to choose ULA as their only choice.
“NASA Kennedy Space Center (KSC) is anticipated to require up to nine months prior to launch date for processing,” NASA said. “Awarding the award to another source would cause unacceptable delays to the current launch schedule.
“These delays will arise from the procurement process, on/off ramping of new contractor personnel, the potential need for reengineered activities, as well as the efforts required to satisfy SLS technical and programmatic drivers.”
The second upper stage that may have been in contention was from Blue Origin’s New Glenn rocket. In addition to not having the previously stated benefits from NASA’s perspective, the agency also expressed concern over the modifications required to adopt Glenn Stage 2 for ML1.
NASA said, “Using NGUS would require significant modifications to both the stage and EGS infrastructure. For example, using NGUS would require relocating the mobile launcher crew access arm and modifications to the upper stage umbilical retraction mechanism.”
“The stage can be shortened to meet VAB altitude constraints, but smaller configurations would require full-scale development and testing to qualify the stage. Full-scale testing/re-qualification would result in unacceptable schedule impacts and additional cost risk to the SLS program.”
Another GS2 ready to fly. We completed a 15 second hotfire with number 4 today. Incredible work by our New Glenn team accelerating GS2 production. pic.twitter.com/Ltjq3psFwK
– Dave Limp (@Devil) 12 December 2025
What happened to the upper stage of exploration?
The original plan to use an EUS-capable rocket would have led to what NASA called a “more ambitious mission” to the Moon, noting that it would allow delivery of 11 metric tons more mass to the lunar surface under the Block 1B configuration than an ICPS-powered Block 1 rocket.
However, a 2024 report by NASA’s Office of Inspector General found that, despite SLS Block 1B being in development since 2014 and carrying the first flights from Artemis 3 to Artemis 4, it is behind schedule due to what the OIG called “quality control issues” at the Michoud Assembly Facility (MAF) in Louisiana.
“We estimate that the cost of SLS Block 1B will reach approximately $5.7 billion before system launch in 2028,” the report said. “This is $700 million more than NASA’s 2023 agency baseline commitment, which had established a cost and schedule baseline of approximately $5 billion.
“EUS development accounts for more than half of this cost, which we estimate will grow from an initial cost of $962 million in 2017 to approximately $2.8 billion by 2028.”
The mid-2024 report also stated that at that time, delivery of the EUS to NASA was “delayed from February 2021 to April 2027”. This further delayed the Artemis 4 flight, which was projected for September 2028.
In late September 2025, Spaceflight Now spoke to Sharon Cobb, associate program manager for SLS at Boeing, about the Artemis 2 mission as well as progress on the EUS.
“We are working very diligently on the Exploration Upper Stage. I was at MAF just last week and was able to see the liquid oxygen tank being welded and tested,” Cobb said. “We’ve also got barrels in the works there that are going to be welded to the flight unit. The LOX tank is a structural test article. So, we’re making really good progress on developing that exploration upper stage.
Like the core stage that launched the Artemis 1 mission, the plan was to perform what was called a ‘green run’ with the EUS at NASA’s Stennis Space Center in Mississippi. It will also include full refueling of the upper stage and full duration static fire testing of four RL10 engines.
Presumably, with this new direction for the SLS rocket, this will no longer be the case, although NASA has not specifically commented on what will happen with the currently flown EUS hardware.
