The Exploration Upper Stage (EUS) is a rocket stage under development for future flights of

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

’s

Space Launch System The Space Launch System (SLS) is an American Super heavy-lift launch vehicle, super heavy-lift Expendable launch system, expendable launch vehicle used by NASA. As the primary launch vehicle of the Artemis program, Artemis Moon landing progra ...

(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

RL10 The RL10 is a liquid-fuel cryogenic rocket engine built in the United States by Aerojet Rocketdyne that burns cryogenic liquid hydrogen and liquid oxygen propellants. Modern versions produce up to of thrust per engine in vacuum. RL10 version ...

C-3 engines burning

liquid oxygen Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear cyan liquid form of dioxygen . It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which is ongoing. Physical ...

and

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

, generating a total thrust of . Its first flight is planned for Artemis IV in 2028. However, the Trump administration has proposed terminating the SLS program after

Artemis III Artemis III is planned to be the first crewed Moon landing mission of the Artemis program and the first crewed flight of the Starship HLS lander. Artemis III is planned to be the second crewed Artemis mission and the first crewed lunar landing ...

, which would eliminate the need for the EUS.

Development

The Block 1 configuration of the SLS, which first flew the

Artemis I Artemis I, formerly Exploration Mission-1 (EM-1), was an uncrewed Lunar orbit, Moon-orbiting mission that was launched in November 2022. As the first major spaceflight of NASA's Artemis program, Artemis I marked the agency's return to lunar e ...

mission, has a core stage powered by four

RS-25 The RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System. Designed and manufactured in the United States by Rocketd ...

engines, two

Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...

-derived five-segment solid rocket boosters, and an Interim Cryogenic Propulsion Stage (ICPS)

upper stage A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''stages'', each of which contains its own Rocket engine, engines and Rocket propellant, propellant. A ''tandem'' or ''serial'' stage is mounted on top of anoth ...

. NASA is developing the EUS to increase SLS performance for

trans-lunar injection A trans-lunar injection (TLI) is a propulsive maneuver, which is used to send a spacecraft to the Moon. Typical lunar transfer trajectories approximate Hohmann transfers, although low-energy transfers have also been used in some cases, as with ...

beyond Block 1 specifications. The improved upper stage was originally named the Dual Use Upper Stage (DUUS, pronounced "duce"), but was later renamed the Exploration Upper Stage (EUS) due to concerns that DUUS sounded like a profanity in Japanese. In 2014, NASA announced it would proceed with development of the Block 1B configuration using the EUS, initially planning to use it on Exploration Mission 2, now referred to as

Artemis II Artemis II is a planned mission under the NASA-led Artemis program, set to be the second launch of the Space Launch System (SLS) rocket and the first crewed flight of the Orion (spacecraft), Orion spacecraft. Scheduled to launch in early 2026, ...

. In April 2016, NASA selected a design using four

-C3 engines for the EUS, and in October 2016 confirmed an order for 10 of the engines. In 2018, NASA decided to optimize the EUS for lunar missions by using smaller tanks.''NASA completes Exploration Upper Stage CDR, focuses new office on SLS Block 1B development''
Feb 2021. By February 2020, delays in the development contract led NASA to plan on using the ICPS for the first three SLS launches.''Upper Stage RL10s arrive at Stennis for upcoming SLS launches''
February 2020. On 21 December 2020,

Boeing The Boeing Company, or simply Boeing (), is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, and missiles worldwide. The company also provides leasing and product support s ...

announced that the EUS had completed a critical design review (CDR) with NASA, allowing hardware development to proceed. 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%. 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

. The proposal described the SLS program as "grossly expensive" and exceeding its budget, and allocated funding to transition to "more cost-effective commercial systems".

Funding history

Notes

References

{{Upper stages Space Launch System Rocket stages

Development

Funding history

See also

Notes

References