Diagram of the Exploration Upper Stage
|Country of origin||United States|
|Used on||SLS Block 1B|
|Height||not to exceed 18 m (60 ft)|
|Diameter||8.4 m (28 ft)|
|Propellant mass||up to 129,000 kg (285,000 lb)|
|Thrust||440 kN (99,000 lbf)|
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 RL10C-3 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 provide thrust of 105 metric tons (231,000 lb). The EUS is expected to first fly on Artemis IV.
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 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.
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.
The EUS is to complete the SLS's ascent phase and then re-ignite to send its payload to destinations beyond low Earth orbit. This is a similar function to the S-IVB stage of the old Saturn V rocket, but the S-IV is comparable in also incorporating a cluster of RL-10 engines. The four RL10 engines on the EUS will produce slightly more thrust than the six RL10 engines of the S-IV.