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Exploration Upper Stage
Rocket stage in NASA's Space Launch System From Wikipedia, the free encyclopedia
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The Exploration Upper Stage (EUS) is a rocket stage under development for future flights of NASA’s Space Launch System (SLS). Designed for use on the SLS Block 1B and Block 2 configurations, it will replace the Interim Cryogenic Propulsion Stage used on the Block 1 variant. The EUS will be powered by four RL10C-3 engines burning liquid oxygen and liquid hydrogen, generating a total thrust of 433.1 kN (97,360 lbf). Its first flight is planned for Artemis IV in 2028.[7] However, the Trump administration has proposed terminating the SLS program after Artemis III, which would eliminate the need for the EUS.
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The Block 1 configuration of the SLS, which first flew the Artemis I 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.[8][9]
NASA is developing 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"),[10] but was later renamed the Exploration Upper Stage (EUS) due to concerns that DUUS sounded like a profanity in Japanese.[11]
In 2014, NASA announced it would proceed with development of the Block 1B configuration using the EUS,[12] initially planning to use it on Exploration Mission 2, now referred to as Artemis II.[13] In April 2016, NASA selected a design using four RL10-C3 engines for the EUS,[6] and in October 2016 confirmed an order for 10 of the engines.[14]
In 2018, NASA decided to optimize the EUS for lunar missions by using smaller tanks.[15] By February 2020, delays in the development contract led NASA to plan on using the ICPS for the first three SLS launches.[16]
On 21 December 2020, Boeing announced that the EUS had completed a critical design review (CDR) with NASA, allowing hardware development to proceed.[17]
In March 2022, Boeing explored the use of a carbon composite liquid oxygen tank on the EUS, which could reduce mass by up to 30%.[18]
On 2 May 2025, the Trump administration released its fiscal year 2026 budget proposal, which called for terminating the SLS and Orion spacecraft programs after Artemis III.[19][20] The proposal described the SLS program as "grossly expensive" and exceeding its budget, and allocated funding to transition to "more cost-effective commercial systems".[21]
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