Exploration Upper Stage

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, though Congress in July 2025 provided funding for Artemis IV anyway in the One Big Beautiful Bill Act.^[8]

Exploration Upper Stage

2020 design of the Exploration Upper Stage
Manufacturer	Boeing[1]
Country of origin	United States
Used on	Space Launch System (Block 1B/2)[2]
General characteristics
Height	17.3 m (57 ft)[3]
Diameter	LH2 tank: 8.4 m (28 ft) LOX tank: 5.5 m (18 ft)[4]
Propellant mass	129,000 kg (284,000 lb)
Empty mass	14,110 kg (31,110 lb)[5]
Engine details
Powered by	4 × RL10C-3[3]
Maximum thrust	433.1 kN (97,360 lbf)
Specific impulse	460.1 s (4.512 km/s)[6]
Burn time	1,275 seconds
Propellant	LH2 / LOX

Development

The Block 1 configuration of the SLS, which first flew the Artemis I 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.^[9][10]

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"),^[11] but was later renamed the Exploration Upper Stage (EUS) due to concerns that DUUS sounded like a profanity in Japanese.^[12]

In 2014, NASA announced it would proceed with development of the Block 1B configuration using the EUS,^[13] initially planning to use it on Exploration Mission 2, now referred to as Artemis II.^[14] 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.^[15]

In 2018, NASA decided to optimize the EUS for lunar missions by using smaller tanks.^[16] By February 2020, delays in the development contract led NASA to plan on using the ICPS for the first three SLS launches.^[17]

On 21 December 2020, Boeing announced that the EUS had completed a critical design review (CDR) with NASA, allowing hardware development to proceed.^[18]

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%.^[19]

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.^[20][21] The proposal described the SLS program as "grossly expensive" and exceeding its budget, and allocated funding to transition to "more cost-effective commercial systems".^[22] However, Congress, through the One Big Beautiful Bill Act which Trump signed into law in July 2025, provided $4.1 billion in funding for the SLS units for Artemis IV and V, though it did direct NASA to investigate adopting commercial alternatives to EUS. ^[8]

Funding history

Fiscal year	Nominal (in millions)	Inflation adjusted (FY24, in millions)[23]
2016	$77	$98.9
2017	$300[24]	$377.4
2018	$300[25]	$368.1
2019	$150[26]	$180.6
2020	$300[27]	$353.4
2021	$400[28][a]	$454
2022	$636.7[29]	$684.4
2023	$648.3[30]	$669
2024	$600[31]	$600
Total	$3,412	$3,785.9

See also

• Delta Cryogenic Second Stage
• Earth Departure Stage
• S-IVB

Notes

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