NASA has selected United Launch Alliance’s Centaur 5 upper stage for use on its Space Launch System (SLS) rocket beginning with the Artemis 4 mission, currently scheduled to launch no earlier than early 2028, according to contract documents released by the agency.
Centaur 5 was originally developed as the upper stage for United Launch Alliance’s Vulcan rocket, which has flown four times since its debut in January 2024. NASA said the upper stage demonstrated strong performance across those flights.
The decision follows comments by NASA Administrator Jared Isaacman outlining plans to standardize the SLS architecture closer to its Block 1 configuration. Speaking during a briefing at the Kennedy Space Center on Feb. 27, Isaacman said the agency aimed to simplify the system to improve production timelines and launch cadence.
“The idea is we want to reduce complexity to the greatest extent possible,” Isaacman said. “We want to accelerate manufacturing, pull in the hardware, and increase launch rate, which obviously has a direct safety consideration to it as well.”
NASA had previously planned to use United Launch Alliance’s Interim Cryogenic Propulsion Stage for the first three Artemis missions and then transition to the Exploration Upper Stage (EUS), developed by Boeing, starting with Artemis 4. However, the agency moved away from that plan due to cost and schedule overruns linked to EUS development.
In its procurement documentation, NASA said it intends to award a sole-source contract to United Launch Alliance for the Centaur 5 upper stage. An accompanying document from NASA’s Marshall Space Flight Center cited several reasons for the decision, including the long operational history of the RL10 engine used in the stage and compatibility with existing infrastructure such as Mobile Launcher 1 and the liquid oxygen and liquid hydrogen propulsion systems already in place.
“This approach leverages current support infrastructure and will use, with relatively minor modifications, an existing ULA upper stage,” NASA said in the document. “All other alternative solutions fail to meet the 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 noted that the agency faces time constraints tied to launch preparation timelines. The Kennedy Space Center requires the upper stage to be ready for processing about nine months before launch, and switching to another provider could lead to delays.
“Award to another source would cause unacceptable delays to current launch schedules,” NASA said. “These delays would derive from the procurement process, on/off ramping of new contractor personnel, the potential need for reworked activities, as well as efforts necessary to satisfy SLS technical and programmatic drivers.”
One alternative that could have been considered was the second stage from Blue Origin’s New Glenn rocket. However, NASA said adopting the New Glenn upper stage would require significant changes to the Mobile Launcher infrastructure and additional development work.
“Using the NGUS would require significant modifications to both the stage and the EGS infrastructure,” NASA said, adding that redesign and qualification testing would introduce additional schedule risks and costs.
Earlier plans to use the Exploration Upper Stage were intended to enable more ambitious lunar missions by allowing the SLS Block 1B configuration to deliver up to 11 metric tons more payload to the lunar surface compared with the Block 1 version. But a 2024 report from NASA’s Office of Inspector General found the EUS program faced delays and cost growth.
The report estimated that SLS Block 1B development could reach approximately $5.7 billion before its expected launch in 2028, about $700 million above NASA’s previous baseline estimate. It also noted that EUS development costs alone could rise from $962 million initially projected in 2017 to nearly $2.8 billion by 2028.
