NASA lines up Exploration Upper Stage workhorse for SLS

by Chris Bergin

The powerful Exploration Upper Stage (EUS) has taken another step forward, following NASA’s request to industry to provide evaluations and costings into engine options. Four engines are expected to power the large Upper Stage of the Space Launch System (SLS) – a stage that is expected to become the workhorse of NASA’s deep space human missions.


SLS will initially launch as a 70mT capable rocket, utilizing a five-meter Delta Cryogenic Second Stage (DCSS), powered by an RL10-B-2 engine.

This stage is known as the ICPS (Interim Cryogenic Propulsion Stage) and will loft an uncrewed Orion on both Exploration Flight Test -1 (EFT-1) and Exploration Mission -1 (EM-1).

2014-10-12 15_40_26-https___prod.nais.nasa.gov_eps_eps_data_162784-OTHER-001-001.pdfEM-1, set for launch in 2018, will debut the Block 1 version of the Space Launch System (SLS). However, it may be the only launch of that configuration, as NASA managers look into an expedited move to the Block IB version of the rocket.

For this upgraded configuration, the transition would be relatively simple, given the vehicle’s core stage, the core stage RS-25 engines and Solid Rocket Boosters (SRBs) remain unchanged.

“The Block 1B vehicle would replace the Interim Cryogenic Propulsion Stage (ICPS) with a more powerful upper stage called the Exploration Upper Stage (EUS), which would provide for fulfillment of more ambitious missions beyond the capabilities of the Block 1 vehicle and provide greater mission margin at comparable vehicle costs,” noted a NASA overview associated with the EUS.

Z7Despite official denials in the media – countering the SLS team’s own internal documentation – this new stage could debut as soon as the second flight of SLS, as confirmed in the Request for Information release.

“NASA is interested in a first launch of the EUS as part of the SLS Program in 2021,” noted the RFI documentation.

2021 is currently the estimated date for the second SLS mission, although evaluations are continuing into the potential need for an uncrewed flight of the debut EUS mission, as part of its human rating requirements. A SLS cargo mission inbetween EM-1 and EM-2 is one such option being discussed.

Evaluations into the EUS have already reached the Point of Depature (POD) stage, baselining the SLS Block IB with four RL-10-C1 engine stage – a switch away from the now defunct J-2X engines – with a maximum propellant load of 285,000 lbm.

Z3Per the POD overview (L2), the EUS will carry a length NTE (Not To Exceed) 60 ft, an LH2 tank diameter of 8.4 m, and a LOX tank diameter of 5.5 m.

The NTE 60 ft length requirement does not relate to performance needs of the EUS but rather to the already baselined launch tower service arms.

Particularly, exceeding a 60ft length of EUS would impede the crew access arm’s adjustability limits to provide safe and necessary access to Orion.

The NTE length requirement also appears to have formed the now-understood LH2 and LOX tank sizes – as larger propellant tanks would have necessitated an increased length for EUS.

“The EUS design concept is for a high-performance, liquid oxygen/liquid hydrogen stage configured to use a cluster of four rocket engines. The initial planning for conceptual design considerations is for the EUS to accommodate the use of the RL10 engine,” confirmed the RFI overview – which will now consider potential alternatives to the RL-10.

Suitors of the EUS engine (EUSE) contract are required to propose an alternative that is consistent with the stage-to-engine interface of the RL10 engine, minimize the development cost impacts to NASA and to achieve cost-efficient production and operations in the long term.

2014-10-12 16_13_09-L2 Level_ Space Launch System Update Notes And Discussion - Sept, 2014 onwards“Responses to this RFI are intended to provide input for an examination into the potential for developing and using alternate engines that meet stage interfaces. NASA desires to minimize development time for an engine and to reduce manufacturing/production costs while still meeting NASA programmatic, technical, design, construction, and workmanship approaches and standards for human rating.”

The RFI did add some additional technical details into what NASA is looking for with the EUSE, which may foster a stage that could provide just enough mass to orbit capability that the SLS Block 2 – seen as a very expensive rocket that won’t even be ready until 2031 – as surplus to requirements.

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The documentation notes the EUSE should have a nominal vacuum thrust of between 24,000 and 35,000 lbs, with a threshold minimum vacuum specific impulse of 454 seconds when operating at corrected nominal thrust, mixture ratio, and engine inlet conditions – with an objective (goal) minimum vacuum specific impulse of 462 seconds.

Z8The EUSE should also be designed to provide for as many as three engine start, mainstage, and shutdown sequences within a single mission with as much as 5 days of in-space loiter between starts – pointing to the wide range of missions NASA wish to utilize with this new stage.

The documentation also notes the EUSE should provide hardware life without component refurbishment for at least five engine starts after engine delivery, pointing to its life during testing through to mission use.

2014-10-12 16_19_11-Hot fire at Stennis - Google SearchThe EUSE shall be tasked with providing hardware life without component refurbishment for at least 2,000 seconds after engine delivery and provide at least 800 seconds of continuous hot-fire operation.

For LOM (Loss Of Mission) ratings, the single-engine mean risk of 1 in 500 for an engine root-cause failure – leading to loss of mission across the complete mission profile – is required as the benchmark. The engine must also comply with NASA’s human rating requirements, although this is not a major part of the RFI outline at this time.

Once interested industry partners have presented their EUSE proposals, the information will be used by NASA for planning and acquisition strategy development and to allow industry the opportunity to provide beneficial cost solutions for a planned evolution to the Space Launch System, verify reasonableness and feasibility of the requirement, as well as promote competition.

Proposals need to be filed with the Marshall Space Flight Center (MSFC) by October 24.

(Images: Via L2 content (EUS lead image from L2 member Nathan Koga) and from L2’s SLS sections, which includes, presentations, videos, graphics and internal – interactive with actual SLS engineers – updates on the SLS and HLV, available on no other site.)

(L2 is – as it has been for the past several years – providing full exclusive SLS and Exploration Planning coverage. To join L2, click here:

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