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.
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]
