NASA Selects ULA Centaur as the Only Option for Future SLS Flights | Taha Abbasi

NASA has officially selected the Centaur upper stage from United Launch Alliance’s Vulcan rocket to replace the cancelled Exploration Upper Stage on future Space Launch System flights. Taha Abbasi reports on this consequential decision that reshapes the Artemis program’s hardware architecture and answers a question that NASA leadership had been conspicuously avoiding for weeks.

In a procurement filing published March 6, NASA determined that the Vulcan Centaur 5 upper stage is the only viable option to fly on SLS missions starting with Artemis 4. The filing, formally known as a Justification for Other than Full and Open Competition, allows NASA to proceed with a sole-source contract to ULA for Centaur stages that will be used on Artemis 4, Artemis 5, and a flight spare.

Why NASA Had No Other Choice

The backstory here is critical. On February 27, NASA announced it would not pursue development of the Block 1B version of the SLS, which was to use the Exploration Upper Stage beginning with Artemis 3. The agency said it would standardize on a “near Block 1” configuration for future missions, but conspicuously declined to specify what stage would replace the EUS.

The problem was straightforward: the Interim Cryogenic Propulsion Stage (ICPS) used on SLS Block 1 is based on the second stage of the now-retired Delta 4 rocket. ULA has shut down the Delta 4 production line, meaning NASA cannot simply order more ICPS units. A new upper stage solution was essential.

The Centaur emerged as the logical, and ultimately the only, answer. It shares design similarities with the ICPS, including the use of RL10 engines powered by liquid hydrogen and liquid oxygen. Few other upper stages in existence have comparable performance characteristics and use the same propellant combination.

NASA’s filing explicitly states: “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.” The document reveals that NASA considered only one other option, the upper stage for Blue Origin’s New Glenn rocket, but determined it was not suitable for the SLS application.

What This Means for Artemis

The selection of Centaur has significant implications for the Artemis program’s timeline and capabilities. The ICPS used on Artemis 1 in 2022 and slated for Artemis 2 and 3 has known performance limitations. It was always intended as a temporary solution until the more capable EUS could be brought online. With the EUS cancelled, the Centaur must bridge that capability gap.

The Centaur 5 is a proven stage with an established track record on Vulcan missions, which provides a level of reliability confidence that a brand-new development program would not. However, integrating it with the SLS will require what NASA describes as “relatively minor modifications.” These modifications, while less extensive than developing an entirely new stage, still require engineering work, testing, and certification.

Taha Abbasi notes that this decision reflects a broader pattern in NASA’s current approach to Artemis. “The agency is pivoting from bespoke, purpose-built hardware to leveraging commercial solutions wherever possible. Selecting an existing commercial upper stage rather than developing a new one saves time and money, even if it means accepting some performance trade-offs compared to what the EUS would have provided.”

The SpaceX Factor

The elephant in the room for any SLS discussion is SpaceX’s Starship, which promises far greater payload capacity at dramatically lower cost per kilogram. As NASA restructures the Artemis program, the role of SLS relative to commercial vehicles continues to evolve.

The Centaur selection ensures SLS can continue flying while Starship matures, but it also implicitly acknowledges that the massive investment originally planned for the EUS is no longer justifiable. Every dollar saved by using an existing commercial stage is a dollar that can be redirected toward Artemis mission hardware, lunar surface systems, or commercial launch services.

SpaceX’s Starship V3 has already left the build site for prelaunch testing, and its eventual operational status could further reshape how NASA approaches deep space missions. The Centaur on SLS may ultimately be remembered as a transitional solution that kept Artemis moving while the industry evolved around it.

ULA’s Position Strengthens

For United Launch Alliance, the sole-source contract represents a significant win. The company now has its Centaur stage flying on two distinct vehicle platforms: Vulcan for commercial and military missions and SLS for deep space exploration. This dual demand helps justify continued investment in Centaur production and potentially drives down per-unit costs through higher production volumes.

The contract also demonstrates the enduring value of ULA’s hydrogen propulsion expertise. While the launch industry has broadly moved toward methane-fueled engines, the specific requirements of the SLS mission profile demand the higher specific impulse that hydrogen propulsion provides. Centaur’s RL10 engines, with their decades of flight heritage, offer a combination of performance and reliability that no methane-fueled stage can currently match for this application.

Looking Ahead

NASA has redacted the estimated cost of procuring the Centaur stages, but the expense is likely substantial given the integration work required. The agency will need to certify the Centaur for human-rated spaceflight on SLS, a process that involves extensive testing and documentation. Taha Abbasi will continue following the Artemis hardware evolution as NASA works to balance cost, schedule, and capability in its return to the Moon.

---

About the Author: Taha Abbasi is a technology executive, CTO, and applied frontier tech builder. Read more on Grokpedia | YouTube: The Brown Cowboy | tahaabbasi.com