Exploration Upper Stage

The Exploration Upper Stage (EUS) is a rocket stage under development that will be used for future flights of NASA's Space Launch System (SLS). Used on SLS Block 1B and Block 2, it will replace the SLS Block 1's Interim Cryogenic Propulsion Stage. The stage will be powered by four RL10C-3 engines burning liquid oxygen and liquid hydrogen to produce a total thrust of 97360 lbf. The EUS is expected to first fly on Artemis 4 in 2028.

Development
The Block 1 configuration of SLS, which first flew the Artemis 1 mission, has a core stage powered by four RS-25 engines, two Space Shuttle-derived five-segment solid rocket boosters, and an Interim Cryogenic Propulsion Stage (ICPS) upper stage.

NASA will develop the EUS to increase SLS performance for trans-lunar injection beyond Block 1 specifications. The improved upper stage was originally named the Dual Use Upper Stage (DUUS, pronounced "duce"), but was later renamed the Exploration Upper Stage (EUS) due to DUUS sounding like a profanity in Japanese.

In 2014, NASA announced that it would proceed with development of Block 1B with the EUS and would use it on Exploration Mission 2, now referred to as Artemis 2. In April 2016, it was reported that NASA has chosen to use a design based on four RL10-C3 engines for the EUS, and in October 2016 NASA confirmed they had ordered 10 of the engines.

In 2018, it was decided to optimize EUS for payload to lunar missions, by using smaller tanks.

By February 2020, the development contract for EUS had been delayed, and NASA was planning to use ICPS for the first three launches of the SLS.

Boeing announced on 21 December 2020 that the EUS of the SLS completed a critical design review (CDR) with NASA. That review confirmed the design of the EUS, allowing Boeing to proceed with development of the stage, including hardware fabrication.

In March 2022, Boeing discussed the possibility of using a carbon composite liquid oxygen tank on the EUS, offering mass savings of up to 30%.