NASA has officially confirmed its choice of United Launch Alliance's Centaur 5 as the new upper stage for the Space Launch System rocket, bypassing Boeing's troubled Exploration Upper Stage and the New Glenn Stage 2. Contract documents published Friday on SAM.gov locked in ULA as the sole-source provider, with the first Centaur 5-equipped SLS flight targeting no earlier than early 2028 on the Artemis 4 mission. The decision is the final piece of NASA's sweeping Artemis restructure announced by Administrator Jared Isaacman on Feb. 27. It closes the book on a Boeing upper stage program that consumed nearly $2.8 billion and never flew, while handing ULA a lucrative contract to adapt its flight-proven Vulcan upper stage for one of the most demanding missions in the American space program. AI-generated image The Centaur 5 uses a pair of RL10 engines burning liquid hydrogen and liquid oxygen, the same propellant combination used by the SLS core stage. Credit: AI illustration $2.8B EUS development cost (through 2028, canceled) 4 Successful Centaur 5 flights on Vulcan since Jan 2024 2028 Target year for first Centaur 5-powered Artemis mission 11 t Payload lost vs. EUS configuration to lunar surface $700M EUS cost overrun above 2023 baseline 2014 Year EUS development began (never flew) The Centaur 5 Comes to SLS The Centaur 5 was designed from the ground up as the upper stage for ULA's Vulcan Centaur rocket. Its first flight came in January 2024, and it has since completed four missions without a failure, most recently lifting the USSF-87 national security payload into orbit on Feb. 12, 2026. That track record is a central reason NASA chose it over any alternative. Physically, the stage is larger than the aging Interim Cryogenic Propulsion Stage (ICPS) that flew on Artemis 1 and 2. It burns liquid hydrogen and liquid oxygen , matching the propellant combination used by the SLS core stage, which simplifies ground support at Kennedy Space Center. NASA's Marshall Space Flight Center produced an eight-page justification document for the sole-source award noting that Centaur 5's compatibility with the existing Mobile Launcher 1 interfaces and propellant systems was a decisive factor. AI-generated image ULA's Vulcan rocket, which uses the Centaur 5 as its upper stage, has completed four successful flights since its January 2024 debut. Credit: AI illustration The agency was also explicit that time was a constraint. "The NASA Kennedy Space Center need date for processing is projected to be nine months prior to a launch," the procurement document states. "Award to another source would cause unacceptable delays to current launch schedules." That language effectively ruled out any competitor that would have required new hardware development or new personnel ramp-up. Why Not Blue Origin's New Glenn Stage 2? Blue Origin's GS2 was the only realistic alternative, but NASA cited two major obstacles. First, adopting it would require relocating the Mobile Launcher Crew Access Arm and modifying the upper stage umbilical retraction mechanism. Second, shortening the stage to fit inside the Vehicle Assembly Building height constraints would require "full scale development and testing to qualify the stage for the shorter configuration," which NASA said would cause "unacceptable schedule impacts and additional cost risk." What Happened to the Exploration Upper Stage Boeing's Exploration Upper Stage was supposed to be the cornerstone of SLS's evolutionary path. Designed with four RL10 engines instead of the ICPS's single engine, it would have delivered roughly 11 metric tons more mass to the lunar surface compared to the Block 1 configuration. That capability mattered for missions like Gateway module delivery and the heavier logistics required for sustained lunar surface operations. The EUS program started in 2014 and was originally supposed to fly on Artemis 3. By the time NASA pulled the plug, the first flight had slipped to 2028, the program had burned through an estimated $2.8 billion, and the NASA Inspector General was projecting costs at least $700 million above the 2023 baseline. The LOX tank for the structural test article had been welded as recently as late 2025, but the flight unit was not close to completion. EUS Program Timeline: A Decade of Delays • 2014: EUS development begins, targeting first flight on Artemis 3. • 2021: First delivery date slips from Feb. 2021 to later years, per OIG audit. • 2024: OIG projects EUS cost at $2.8B; delivery delayed to April 2027. • Feb. 27, 2026: NASA Administrator Isaacman announces move to "near-Block 1" standardization, effectively canceling EUS direction. • Mar. 6, 2026: Sole-source contract documents confirm Centaur 5 as EUS replacement. The question now is what happens to the EUS hardware already in fabrication at the Michoud Assembly Facility in Louisiana. NASA has not commented on the disposition of existing components, including the structural test article's LOX tank and the barrel sections being welded for the flight unit. That hardware represents sunk costs that won't be recovered. RL10 Heritage and the Human Rating Question AI-generated image The SLS and its mobile launcher will need modifications to accommodate the Centaur 5, though NASA says these are relatively minor compared to alternatives. Credit: AI illustration NASA's justification document leans heavily on the RL10's track record. The engine first flew in 1963 and has accumulated more than 4,000 successful firings across dozens of programs. Its heritage runs through Centaur 3, which was certified to carry humans as the upper stage for Boeing's Starliner on the Commercial Crew Program. NASA says that certification work provides a strong foundation for the Centaur 5's human rating process for Artemis missions carrying crew. The Centaur 5 uses two RL10C-1-1A engines, each producing about 22,890 pounds of thrust in vacuum. The ICPS used a single RL10B-2 producing roughly 24,750 pounds of thrust. The twin-engine configuration of the Centaur 5 provides some redundancy and performance advantages, though the overall mass delivered to translunar injection will fall short of what a fully developed EUS would have achieved. Upper Stage Engines Missions Status ICPS (Boeing/ULA) 1x RL10B-2 Artemis 1, 2 Active (retiring) Centaur 5 (ULA) 2x RL10C-1-1A Artemis 4+ (planned) Flight-proven (4 flights) EUS (Boeing) 4x RL10C-3 Canceled Development halted GS2 (Blue Origin) 1x BE-3U Ruled out Not selected Cislunar Implications: Payload to the Moon The shift to Centaur 5 carries a real cost in lift performance. The Block 1B configuration with EUS would have delivered roughly 11 metric tons more payload to the lunar surface per flight compared to the Block 1 rocket using ICPS or Centaur 5. Over multiple Artemis missions, that gap compounds. Gateway module deliveries, crew supplies, and scientific equipment all fit within a tighter mass budget. NASA's position is that the trade-off is worth it. The agency argues that a standardized, simplified SLS fleet will fly more often, reducing per-flight costs and compressing the schedule. Isaacman's stated goal is to "accelerate manufacturing, pull in the hardware, and increase launch rate." If that logic holds, frequent launches of a slightly less capable rocket could outperform infrequent launches of a more capable one in total payload delivered to cislunar space over a decade. AI-generated image Engineers at NASA's Marshall Space Flight Center in Huntsville produced the eight-page technical justification document for the sole-source Centaur 5 contract. Credit: AI illustration For commercial providers competing in the cislunar cargo market, the SLS payload reduction is less relevant than the signal it sends about NASA's willingness to restructure long-standing programs. The same Isaacman-era philosophy that killed EUS could accelerate decisions on Gateway, commercial lunar transport, and the CLPS program. 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