|Country of origin||United States|
|Used on||SLS Block 1B|
|Height||not to exceed 18 m (59 ft)|
|Diameter||8.4 m (28 ft)|
|Propellant mass||up to 129,000 kg (284,000 lb)|
|Thrust||440 kN (99,000 lbf)|
|Propellant||LOX / LH2|
The Exploration Upper Stage (EUS) is being developed as a large second stage for Block 1B of the Space Launch System (SLS), succeeding Block 1's Interim Cryogenic Propulsion Stage. It will be powered by four RL10-C3 engines burning liquid oxygen and liquid hydrogen to produce a total of 440 kN (99,000 lbf) thrust. As of February 2015[update], the SLS Block 1B will have a payload capacity, to low Earth orbit, of 105 t (103 long tons; 116 short tons). The EUS is expected to first fly on Artemis IV in March 2026.
The Block 1 configuration of SLS will have 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 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 EM-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 optimise 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 SLS.
Boeing announced on 21 December 2020 that the Exploration Upper Stage (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.
Artemis IV is scheduled to launch in March 2026 (as of August 2020).
NASA aims to use the first EUS on the Artemis IV mission.