|Country of origin||United States|
|Used on||SLS Block 1B and Block 2|
|Height||not to exceed 18 m (59 ft)|
|Diameter||8.4 m (28 ft)|
|Propellant mass||up to 129,000 kg (284,000 lb)|
|Powered by||4 RL10C-3|
|Maximum thrust||433.1 kN (97,360 lbf)|
|Propellant||LOX / LH2|
The Exploration Upper Stage (EUS) is being developed as a large second stage for Block 1B and Block 2 of the Space Launch System (SLS), succeeding Block 1's Interim Cryogenic Propulsion Stage. It will be powered by four RL10C-3 engines burning liquid oxygen and liquid hydrogen to produce a total thrust of 433.1 kN (97,360 lbf). As of October 2017[update], the SLS Block 1B will have a payload capacity to low Earth orbit of 105 t (116 short tons; 231,000 lb) and Block 2 will have a payload capacity of 130 t (140 short tons; 290,000 lb). The EUS is expected to first fly on Artemis IV in March 2026.
The Block 1 configuration of SLS 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 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.
Due to the possible cost of EUS (about US$800 million each), NASA invited proposals for alternatives, but in May 2019 rejected Blue Origin's proposal. NASA ordered eight EUSs from Boeing.
The recent GAO-21-105 audit noted that the Artemis 4 mission that would debut Block 1B and EUS is forecast to launch in March 2026.
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