NASA selects Centaur for new SLS upper stage
NASA has selected the Centaur upper stage currently used on United Launch Alliance's Vulcan rocket for future flights of the Space Launch System.
In a procurement filing published March 6, NASA determined that the Vulcan Centaur 5 upper stage is the only option to replace the Exploration Upper Stage, or EUS, that was to be used on SLS launches after Artemis 3.
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 upper stages that will be used on the Artemis 4 and 5 missions, along with a flight spare. The filing redacted the estimated cost to NASA to procure the stages.
The agency announced Feb. 27 it would not pursue development of the Block 1B version of the SLS, which was to start flying on Artemis 3 and use the EUS in place of the Interim Cryogenic Propulsion Stage, or ICPS, used on the Block 1 version. The ICPS was used on Artemis 1 in 2022 and will be used on Artemis 2 and 3.
At the time of the announcement, NASA said it would standardize the SLS on a "near Block 1" version for missions beyond Artemis 3. Because the ICPS, also built by ULA, is based on the second stage of the now-retired Delta 4, the agency cannot simply procure additional ICPS units because the production line has been shut down.
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When NASA announced the decision not to continue work on the Block 1B, the agency did not disclose what stage it planned to use in place of the EUS. Agency officials deflected questions about alternative stages and even the contracting process for selecting the new stage.
It was clear at the time, though, that there were few alternatives for NASA other than Centaur. The stage has some design similarities to the ICPS, including the use of RL10 engines powered by liquid hydrogen and liquid oxygen. Few other upper stages have similar performance and use the same propellants as Centaur.
"This approach leverages current support infrastructure and will use, with relatively minor modifications, an existing ULA upper stage," the NASA filing 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 the only other stage NASA considered was the upper stage for Blue Origin's New Glenn rocket, powered by BE-3U engines that use liquid hydrogen and liquid oxygen.
That analysis concluded "only the Centaur is capable (with relatively minor modifications) of meeting the requirements of the Government," the document states. The New Glenn upper stage, by contrast, "would require significant modifications."
NASA concluded that the New Glenn stage, having flown only twice, "is still considered to be in early phases of development," whereas versions of Centaur have been in use for more than a half-century. Adapting the New Glenn stage to SLS would also require modifications to both the stage and ground infrastructure, including potentially shortening the stage so the full SLS could pass through the main door of the Vehicle Assembly Building.
"Full-scale testing and requalification would result in unacceptable schedule impacts and additional cost risk to the SLS Program," the document concluded about using the New Glenn stage.
The document did not disclose what "minor modifications" would be needed to Centaur to adapt it for use on SLS, but it emphasized ULA's familiarity with the SLS program and the common hardware between ICPS and Centaur.
"ULA's established infrastructure, resources, flight history, existing cross-program integration, and human-rating familiarity with the Centaur upper stage represents the only currently viable opportunity for the Government to accomplish Artemis mission objectives and requirements while also maintaining the agency's programmatic goals," NASA concluded in the filing.
Under current law, NASA is still required to develop the Block 1B version of SLS with the EUS. However, a NASA authorization bill approved by the Senate Commerce Committee March 4 would allow NASA to replace the EUS with an alternative upper stage if NASA determines the EUS is "unlikely to achieve the mission goals of the Artemis campaign."
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