Exploration Upper Stage
2020 design of the Exploration Upper Stage
Country of originUnited States
Used onSLS Block 1B and Block 2[2]
General characteristics
HeightNo more than 18 m (59 ft)
Diameter8.4 m (28 ft) (LH2 tank) 5.5 m (18 ft) (LOX tank)[3]
Propellant mass278,000 lb (126,000 kg)[4]
Empty mass31,110 lb (14,110 kg)[5]
Engine details
Powered by4 RL10C-3[6]
Maximum thrust433.1 kN (97,360 lbf)
Specific impulse460.1 seconds (vacuum)
PropellantLOX / LH2

The Exploration Upper Stage (EUS) is a rocket stage under development that will be used for future flights of NASA's Space Launch System (SLS). Used on SLS Block 1B and Block 2, it will replace the SLS Block 1's Interim Cryogenic Propulsion Stage. The stage 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). The EUS is expected to first fly on Artemis 4 in 2028.[7]


The Block 1 configuration of SLS, which first flew the Artemis 1 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 will develop 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 DUUS sounding like a profanity in Japanese.[11]

In 2014, NASA announced that it would proceed with development of Block 1B with the EUS[12] and would use it on the now cancelled Exploration Mission 2, now referred to as Artemis 2.[13] In April 2016, it was reported that NASA has chosen to use a design based on four RL10-C3 engines for the EUS,[6] and in October 2016 NASA confirmed they had ordered 10 of the engines.[14]

In 2018, it was decided to optimize EUS for payload to lunar missions, by using smaller tanks.[15]

By February 2020, the development contract for EUS had been delayed, and NASA was planning to use ICPS for the first three launches of the SLS.[16]

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.[17]

In March 2022, Boeing revealed plans for an all-composite variant of the EUS, as well as the successful results of pressure tests done on a prototype. The variant was projected to save enough mass to increase the SLS's trans-lunar injection performance by 30 percent.[18]

Funding history

This section needs to be updated. Please help update this article to reflect recent events or newly available information. (June 2023)
Fiscal year Nominal


In 2021[19](millions)
2016 $77.0 $84.8
2017 $300.0[20] $324.3
2018 $300.0[21] $316.5
2019 $150.0[22] $155.1
2020 $300.0 $303.9
2021 $400.0[note 1] $400.0
Total: 2016–2021 $1,527.0 $1,584.6

See also


  1. ^ The FY2021 spending plan indicates that this is for "Block 1B (non-add) (including EUS)"


  1. ^ "NASA, Boeing Finalize US$2.8 billion SLS Core Stage Contract". SpaceNews. 4 July 2014. Archived from the original on 27 July 2014. Retrieved 25 July 2014.
  2. ^ "Space Launch System Fact Sheet" (PDF). NASA. Retrieved 16 May 2023.
  3. ^ "1 year down, a galaxy to go". Boeing. Retrieved 12 April 2024.
  4. ^ |https://arc.aiaa.org/doi/pdf/10.2514/6.2018-4639
  5. ^ |https://arc.aiaa.org/doi/pdf/10.2514/6.2018-4639
  6. ^ a b "RL10 Engine". Aerojet Rocketdyne. Archived from the original on 7 November 2021. Retrieved 18 November 2021.
  7. ^ Foust, Jeff (30 October 2022). "Lunar landing restored for Artemis 4 mission". SpaceNews. Retrieved 31 October 2022.
  8. ^ "SLS". Gunter's Space Page. Retrieved 25 July 2014.
  9. ^ "Space Launch System Data Sheet". SpaceLaunchReport.com. Archived from the original on 11 April 2013. Retrieved 25 July 2014.((cite web)): CS1 maint: unfit URL (link)
  10. ^ "SLS prepares for PDR – Evolution eyes Dual-Use Upper Stage". NASASpaceflight.com. June 2013. Retrieved 25 July 2014.
  11. ^ Bergin, Chris (28 March 2014). "SLS positioning for ARRM and Europa missions". NASASpaceflight.com. Retrieved 8 November 2014.
  12. ^ Bergin, Chris (30 July 2012). "Wind Tunnel testing conducted on SLS configurations, including Block 1B". NASASpaceFlight.com. Retrieved 12 March 2015.
  13. ^ "NASA confirms EUS for SLS Block IB design and EM-2 flight". NASASpaceflight.com. 6 June 2014. Retrieved 25 July 2014.
  14. ^ "Proven Engine Packs Big, In-Space Punch for NASA's SLS Rocket". NASA. 21 October 2016. Retrieved 18 November 2021.
  15. ^ NASA completes Exploration Upper Stage CDR, focuses new office on SLS Block 1B development. Feb 2021.
  16. ^ Upper Stage RL10s arrive at Stennis for upcoming SLS launches. February 2020.
  17. ^ "SLS Exploration Upper Stage passes review". SpaceNews. 22 December 2020. Retrieved 22 December 2020.
  18. ^ Gebhardt, Chris (5 March 2022). "With all-composite cryogenic tank, Boeing eyes mass-reducing space, aviation applications". Archived from the original on 7 March 2022. Retrieved 18 May 2023.
  19. ^ "NASA FY20 Inflation Tables – to be utilized in FY21". NASA. p. Inflation Table. Retrieved 11 October 2021. Public Domain This article incorporates text from this source, which is in the public domain.
  20. ^ "NASA outlines plan for 2024 lunar landing". SpaceNews. 1 May 2019. Archived from the original on 30 September 2021. Retrieved 15 May 2019.
  21. ^ Berger, Eric (20 May 2019). "NASA's full Artemis plan revealed: 37 launches and a lunar outpost". Ars Technica. Archived from the original on 23 May 2019. Retrieved 20 May 2019.
  22. ^ Sloss, Philip (18 December 2019). "Amid competing priorities, Boeing redesigns NASA SLS Exploration Upper Stage". NASASpaceFlight.com. Archived from the original on 7 August 2020. Retrieved 25 July 2020.