Exploration Upper Stage
Exploration Upper Stage 2020 Design.png
2020 Design of the Exploration Upper Stage
Country of originUnited States
Used onSLS Block 1B and Block 2[2]
General characteristics
Heightnot to exceed 18 m (59 ft)
Diameter8.4 m (28 ft)
Propellant massup to 129,000 kg (284,000 lb)
Engine details
Powered by4 RL10C-3[3]
Maximum thrust433.1 kN (97,360 lbf)
PropellantLOX / 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, 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).[2] The EUS is expected to first fly on Artemis IV in March 2026.[4]


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.[5][6]

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")[7] but was later renamed the Exploration Upper Stage (EUS) due to DUUS sounding like a profanity in Japanese.[8]

In 2014, NASA announced that it would proceed with development of Block 1B with the EUS[9] and would use it on EM-2.[10] In April 2016, it was reported that NASA has chosen to use a design based on four RL10-C3 engines for the EUS,[3] and in October 2016 NASA confirmed they had ordered 10 of the engines.[11]

In 2018, it was decided to optimise EUS for payload to lunar missions, by using smaller tanks.[12]

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.[13]

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.[14]

Funding History

Fiscal year Nominal


In 2021[15](millions)
2016 $77.0 $84.8
2017 $300.0[16] $324.3
2018 $300.0[17] $316.5
2019 $150.0[18] $155.1
2020 $300.0 $303.9
2021 $400.0[note 1] $400.0
Total: 2016–2021 $1,527.0 $1,584.6

Cost concerns and alternatives

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.[19] NASA ordered eight EUSs from Boeing.[19]

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. ^ a b "SLS October 2017 Fact Sheet" (PDF). Archived (PDF) from the original on 1 November 2021. Retrieved 18 November 2021.
  3. ^ a b "RL10 Engine". Aerojet Rocketdyne. Archived from the original on 7 November 2021. Retrieved 18 November 2021.
  4. ^ Sloss, Philip (17 February 2021). "NASA completes Exploration Upper Stage CDR, focuses new office on SLS Block 1B development". NASASpaceFlight.com. Retrieved 17 November 2021. 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.
  5. ^ "SLS". Gunter's Space Page. Retrieved 25 July 2014.
  6. ^ "Space Launch System Data Sheet". SpaceLaunchReport.com. Retrieved 25 July 2014.
  7. ^ "SLS prepares for PDR – Evolution eyes Dual-Use Upper Stage". NASASpaceflight.com. June 2013. Retrieved 25 July 2014.
  8. ^ Bergin, Chris (28 March 2014). "SLS positioning for ARRM and Europa missions". NASASpaceflight.com. Retrieved 8 November 2014.
  9. ^ Bergin, Chris (30 July 2012). "Wind Tunnel testing conducted on SLS configurations, including Block 1B". NASASpaceFlight.com. Retrieved 12 March 2015.
  10. ^ "NASA confirms EUS for SLS Block IB design and EM-2 flight". NASASpaceflight.com. 6 June 2014. Retrieved 25 July 2014.
  11. ^ "Proven Engine Packs Big, In-Space Punch for NASA's SLS Rocket". NASA. 21 October 2016. Retrieved 18 November 2021.
  12. ^ NASA completes Exploration Upper Stage CDR, focuses new office on SLS Block 1B development. Feb 2021.
  13. ^ Upper Stage RL10s arrive at Stennis for upcoming SLS launches. February 2020.
  14. ^ "SLS Exploration Upper Stage passes review". SpaceNews. 22 December 2020. Retrieved 22 December 2020.
  15. ^ "NASA FY20 Inflation Tables – to be utilized in FY21". NASA. p. Inflation Table. Retrieved 11 October 2021.((cite web)): CS1 maint: url-status (link) Public Domain This article incorporates text from this source, which is in the public domain.
  16. ^ "NASA outlines plan for 2024 lunar landing". SpaceNews. 1 May 2019. Archived from the original on 30 September 2021. Retrieved 15 May 2019.
  17. ^ 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.
  18. ^ 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.
  19. ^ a b NASA rejects Blue Origin's offer of a cheaper upper stage for the SLS rocket May 2019