Exploration Upper Stage

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 IV in 2028.^[7] However, the Trump administration has called for the termination of the SLS program after Artemis III, which would preclude the need for the EUS.

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	8.4 m (28 ft) (LH2 tank) 5.5 m (18 ft) (LOX tank)[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 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.^[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 Exploration Mission 2, now referred to as Artemis II.^[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 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 discussed the possibility of using a carbon composite liquid oxygen tank on the EUS, offering mass savings of up to 30%.^[18]

On 2 May 2025, the Trump administration released its fiscal year 2026 budget proposal for NASA, which calls for terminating the SLS and Orion spacecraft programs after Artemis III,^[19][20] which would preclude the need for the EUS. The budget proposal described the SLS program as "grossly expensive", noting that has exceeded its budget, and allocates funding for a program to transition to "more cost-effective commercial systems".^[21]

Funding history

Fiscal year	Nominal (in millions)	Inflation adjusted (FY24, in millions)[22]
2016	$77.0	$98.9
2017	$300.0[23]	$377.4
2018	$300.0[24]	$368.1
2019	$150.0[25]	$180.6
2020	$300.0[26]	$353.4
2021	$400.0[27][a]	$454.0
2022	$636.7[28]	$684.4
2023	$648.3[29]	$669.0
2024	$600.0[30]	$600.0
Total	$3,412.0	$3,785.9

See also

• Delta Cryogenic Second Stage
• Earth Departure Stage

Notes

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