Exploration Upper Stage

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Exploration Upper Stage
Exploration Upper Stage 2019.jpg
Diagram of the Exploration Upper Stage
Country of originUnited States
Used onSLS Block 1B[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
Engines4 RL10-C3[3][4]
Thrust440 kN (99,000 lbf)
PropellantLOX / LH2

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 RL10-C3[4] engines burning liquid oxygen and liquid hydrogen to produce a total of 440 kN (99,000 lbf) thrust. As of February 2015, the SLS Block 1B will have a payload capacity, to low Earth orbit, of 105 t (103 long tons; 116 short tons).[5] The EUS is expected to first fly on Artemis IV in March 2026.[6][7]


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 (ICPS) upper stage.[8][9]

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")[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 EM-2.[2] 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.[4]

In 2018, it was decided to optimise EUS for payload to lunar missions, by using smaller tanks.[13]

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.[14]

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.[15]

Role and comparable stages[edit]

A comparison of the EUS with the S-IVB stage of the Saturn V

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.

Cost concerns and alternatives[edit]

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.[16] NASA ordered eight EUSs from Boeing.[16]

See also[edit]


  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 "NASA confirms EUS for SLS Block IB design and EM-2 flight". NASASpaceflight.com. 6 June 2014. Retrieved 25 July 2014.
  3. ^ a b Bergin, Chris (7 April 2016). "MSFC propose Aerojet Rocketdyne supply EUS engines". NASASpaceflight.com. Retrieved 8 April 2016.
  4. ^ a b c Proven Engine Packs Big, In-Space Punch for NASA's SLS Rocket. October 2016 Public Domain This article incorporates text from this source, which is in the public domain.
  5. ^ "Advanced Boosters progress towards a solid future for SLS". NasaSpaceflight.com. 20 February 2015. Retrieved 12 March 2015.
  6. ^ "NASA's Management of the Gateway Program for Artemis Missions" (PDF). OIG. NASA. 10 November 2020. p. 3. Retrieved 4 January 2021. Artemis IV is scheduled to launch in March 2026 (as of August 2020).
  7. ^ Loff, Sarah (16 October 2019). "NASA Commits to Future Artemis Missions With More SLS Rocket Stages". NASA. Retrieved 4 January 2021. NASA aims to use the first EUS on the Artemis IV mission.
  8. ^ "SLS". Gunter's Space Page. Retrieved 25 July 2014.
  9. ^ "Space Launch System Data Sheet". SpaceLaunchReport.com. Retrieved 25 July 2014.
  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 completes Exploration Upper Stage CDR, focuses new office on SLS Block 1B development. Feb 2021.
  14. ^ Upper Stage RL10s arrive at Stennis for upcoming SLS launches. February 2020.
  15. ^ "SLS Exploration Upper Stage passes review". SpaceNews. 22 December 2020. Retrieved 22 December 2020.
  16. ^ a b NASA rejects Blue Origin's offer of a cheaper upper stage for the SLS rocket May 2019