After its debut was postponed and production plans suspended, NASA gave its OK in June to a set of design changes proposed by prime contractor Boeing for the Exploration Upper Stage (EUS). EUS was planned to become the upper stage for NASA’s Space Launch System (SLS) rocket on its second launch, but last year the space agency requested a study of possible changes after Congress decided to fly more SLS launches with its current interim upper stage.



Boeing provided NASA with choices to optimize the stage for lunar payloads at the end of 2018, but the future of the stage came into question at the beginning of this year. With the delay in its first launch, funding was first cut in half in the last fiscal year and then the White House proposed completely defunding it this year.

Shortly thereafter, the Artemis Program was started with the goal to land U.S. astronauts on the Moon by the end of 2024 and with its own funding needs, EUS is not a part of the White House’s lunar landing plans. Congress has been the main champion of development and application of the stage, including restoring funding to its previous level, and if the spending bill up for vote this week were to be enacted into law, it is possible that Critical Design Review (CDR) could occur during Fiscal Year 2020.

Upper stage redesigned to optimize payload for trans-lunar trajectories

The EUS budget was cut in Fiscal Year (FY) 2019 from $300 million to $150 million as NASA pivoted to additional Block 1 launches using United Launch Alliance’s (ULA) Interim Cryogenic Propulsion Stage (ICPS), a derivative of the Delta IV upper stage. This pushed the first EUS flight from the second SLS launch to the fourth, behind the first Orion crew mission and launch of the Europa Clipper planetary probe.

Plans to begin fabrication of major stage structures were suspended and NASA gave Boeing a “Trans Lunar Injection (TLI) Optimization” task to study how to increase the stage’s lunar payload performance. Initial launches of the SLS Block 1B configuration using EUS fly Orion and crew as their primary payload; EUS provides the ability to fly a “co-manifested” secondary payload in trunk space behind Orion and NASA wanted to increase the new upper stage’s performance to a minimum of ten metric tons of co-manifested payload.

Boeing completed the study last year. “We came up with ten different options using different types of tanks or different engines or different things and NASA received that study and they are thinking about what they would like to do on the path forward,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said in late January, 2019. “We were able to increase the co-manifested capability significantly.”

Although the White House proposed defunding EUS in March beginning with FY 2020, the agency settled on a redesign mid-year. “Design work for the TLI optimized EUS began in June 2019 after downselect and approval by agency leadership,” NASA said in a statement provided by Kathryn Hambleton, Public Affairs Officer at NASA Headquarters.

(Photo Caption: An expanded view of the SLS Block 1B Crew configuration. NASA asked Boeing for EUS design optimizations that would allow the Block 1B to deliver at least ten metric tons of payload to TLI above and beyond the twenty-six or so metric ton mass of Orion.)

“NASA had asked us to size the LOX (liquid oxygen) and hydrogen tanks so that there was an evolution path so that it would actually intercept the place for a larger engine and that left you with a compromise when you went with what we had,” Rick Bottomley, Boeing’s EUS Program Manager, explained in an interview during the Artemis Day media event on December 9 at Michoud Assembly Facility (MAF) in New Orleans. “So what we did is we resized the tanks to be optimized for the RL-10 configurations that we have now and it gave us much better performance.”

EUS is a liquid hydrogen, liquid oxygen stage using four Aerojet Rocketdyne RL-10C3 engines. “As a part of that we needed less volume, so the LOX tank that you saw before that had an extended barrel becomes more of a spheroid and by that shape, it got the right volume. Because of the shape, it was also able to take a bigger load added with a lighter weight.”

“The whole stage got lighter, more efficient so there wasn’t anything that we were carrying that’s not working on the mission,” he added. The LOX tank’s barrel isn’t completely eliminated: “It’s not quite dome to dome, it’s about the equivalent of three rings tall.”

Boeing was also able to take advantage of updated loads data from the program’s ongoing vehicle design analysis cycles to make changes to major elements of the stage design. “[The] hydrogen tank is shorter [now], I mean everything is optimized,” he said.

“We went in and we have more accurate loads now, so we were able to lighten things up. We resized it, we relooked at the load paths, we actually have some places where the optimization is more than just structure trying to make it higher performance from a payload capacity.”

With the additional time to look at the stage design, Boeing also added functionality into the new baseline. “The other point is we added capability, so it can now receive uplink commands from Orion and from the ground so we have extra avionics,” Bottomley explained. “We have some additional navigation things that we can do now. So it was actually trying to tailor ourselves literally for an Orion lunar mission and also likewise have those same tools that we’ll need if we want to go on to Mars.”

(Photo Caption: A recently released NASA infographic provides a couple of the updated measurables for the redesigned EUS. After resizing the stage’s propellant tanks and other elements, the stage is less than a foot shorter and approximately a ton lighter when dry.)

The stage includes other structures and elements such as bottles for gaseous helium and Bottomley indicated that some of those were also optimized and its overall length is now shorter than the previous design. “There’s one thing where we actually go to a longer nozzle, but right now we’re actually shorter.”

The redesign retains original components within the resizing and repackaging. “Almost nothing stays the same from the standpoint of in the requirements space once we got the new loads we tried to lighten everything up,” Bottomley explained. “And then when we have the avionics we have to go repackage some of the thermal [protection].”

“So at a component level, almost all the components are the same so we bring that over. Construction techniques are largely the same, the tools that we use to produce it are the same. The specific design is refined as we move towards CDR.”

The thickness of the propellant tanks was also modified in some areas as a part of the redesign; the same material stock is used but is now machined thinner in some areas.

Assembly, production on hold during funding uncertainty

The timetable for finishing development and flying the single EUS flight unit currently under contract is caught between competing priorities advanced by the White House and Congress. When Vice President Pence announced the goal at the end of March of landing U.S. astronauts on the Moon by the end of 2024 that became NASA’s highest priority.

EUS plays no role in those plans, as SLS is only necessary for a single launch in support of the Artemis 3 mission that would accomplish the lunar landing goal within five years. Commercial launch vehicles will launch all the elements in the Artemis infrastructure including both required elements of the cislunar Gateway, all lunar Human Landing System (HLS) elements, and any required Gateway logistics supply spacecraft.

The final planned Artemis 3 launch would be an SLS Block 1 vehicle that uses an ICPS upper stage to send Orion and its lunar landing crew to dock with the Gateway where the Artemis HLS infrastructure will already be pre-assembled. Without a clear role for EUS in their plans for Gateway or Artemis, the White House proposed zeroing out funds for it beginning in the current fiscal year that began on October 1.

Boeing is late in its delivery of the first Core Stage, which was the primary new development for the SLS Block 1 vehicle, and the Administration wants to narrow the focus of the SLS Program to building the total of three cores it needs for 2024. “Recent performance issues and delays in SLS core stage manufacturing and design updates related to the Exploration Upper Stage requirements require that NASA concentrate all available resources in the near term on the successful completion of Artemis I, II and III, and supporting a reliable annual SLS and Orion flight cadence thereafter,” NASA’s Acting Administrator of the Human Exploration and Operations Mission Directorate (HEOMD) Ken Bowersox wrote in written testimony submitted in advance of a mid-September House of Representatives subcommittee hearing.

“As a result, we have proposed to defer SLS Block 1B development efforts to later exploration missions.”

In testimony during that September 18 hearing, Bowersox indicated that the agency had not found any specific mission for the new upper stage. “Right now we’re looking at where we can use EUS,” he said. “At this point, the earliest that we would probably be able to use it is around Artemis 4, but we need to work that internally with our budget estimates.”

“Right now our current plan would be to go ahead without EUS — that’s what’s in our official President’s Budget submission. However, Congress has been very helpful in providing funding for EUS and so the earliest we would be likely to use it is Artemis 4.”

As Bowersox noted, Congress provided funding for EUS in its FY 2020 appropriations bills, with both chambers proposing levels above the FY 2019 mark. The House appropriations committee bill language allocated a minimum of $200 million from within the SLS budget explicitly for EUS development while the Senate bill proposed no less than $300 million.



Boeing also made EUS and SLS Block 1B a centerpiece of their recent lunar lander proposal for the first two HLS missions. Their concept of operations would rely on completing EUS development and ramping up Core Stage production to field both EUS and an additional SLS Core (a fourth one) in the same 2024 time-frame.

Congressman Robert Aderholt of Alabama, Ranking Member of the Commerce, Justice, Science, and Related Agencies subcommittee of the House Appropriations Committee, reflected some of the thinking for this proposal in a question to Bowersox and NASA Administrator Jim Bridenstine at an October 16 hearing, asking: “I understand there is a growing confidence among the prime contractors for SLS to be able to produce two rockets a year starting in 2024 and they believe that they can deliver a Block 1B in 2024. What do you think?”

Both expressed strong doubts. “It depends what Boeing is willing to invest, quite frankly,” Bridenstine stated. “We don’t have the appropriations necessary to achieve that. If we were to do that we might need some more infrastructure that currently doesn’t exist.”

“What I’d say is we haven’t seen the performance yet that would indicate that we’re guaranteed the second Core we’d need for a Moon landing in 2024. We’re open to considering those types of options, we’re looking for that type of progress, but we just haven’t seen it yet,” Bowersox added.

“So to be clear I am confident that given our current rate of production we will have three SLS-es available and the third one would be for Artemis 3, that takes us to the Moon in 2024,” Bridenstine then said. “I think that is fully within the realm of possibility, but a lot of things have to go right to make that happen.”

“Adding an additional SLS into the mix? I’m not confident that that could happen.”

While the White House remains skeptical about Core Stage production rates, some in Congress are pushing to complete EUS development in time for it to be ready to fly by the third SLS launch of the Artemis 3 mission.

Introduced in November as a proposed NASA Authorization Act but not passed by either chamber of Congress, Senate bill S.2800 includes this directive: “To meet the capability requirements under section 302(c)(2) of the National Aeronautics and Space Administration Authorization Act of 2010 (42 U.S.C. 18322(c)(2)), the Administrator shall continue development of the Exploration Upper Stage for the Space Launch System with a scheduled availability sufficient for use on the third launch of the Space Launch System.”

NASA also expressed skepticism about that timeline.

(Photo Caption: Four RL-10s that Aerojet Rocketdyne has acceptance tested for future SLS use. An additional engine is already installed in the Artemis 1 ICPS unit. Some of these engines could be used in the first EUS flight article or could be assigned to ICPS articles for upcoming Artemis and planetary launches.)

Although there’s no compromise between these competing priorities, EUS development could continue depending on funding. “We got a hundred fifty million in ’19 so under a CR right now all we’ve got to work with is another hundred fifty, but I’m also pretty confident that in conference Senator Shelby (Richard Shelby of Alabama, Chair of the Senate Appropriations Committee) I’m sure will persuade the House to come forward with what we need next with EUS,” NASA Deputy Administrator Jim Morhard said on December 9.

Text for the conferenced, “minibus” bill that could come before both chambers this week was released on December 16 and uses the higher Senate mark of $300 million for EUS in FY 2020; in spite of the possible funding enactment, the back and forth between the White House and Congress is likely to continue with the President’s FY 2021 budget request coming in February.

“I don’t think that we can expect OMB (Office of Management and Budget) to all of a sudden say ‘Shazam! We support EUS.’ I just don’t see that,” Morhard said. “But also as a former appropriator if I’m the OMB guy I’m going to make the Congress put the money in for it. I don’t have to — I know they’re going to do it.”

“Part of this is the choreography of putting the President’s Budget Request together,” Morhard explained. “I can’t comment on what [FY] ’21 is going to look like. I’m hopeful that we’re going to have a robust budget.”

“But as far as EUS goes, [with] OMB whether they put in funds for EUS or not, historically they don’t sit there and capitulate. It’s usually more of the same and each year they know more and more that they don’t need to put it in the request because they know it’s going to get added.”

EUS would replace ICPS as the SLS upper stage; after the first three or four SLS launches, ICPS is likely no longer an option. NASA remains mandated to fly Europa Clipper on SLS; the current support plan would be to purchase another ICPS unit and fly the planetary mission after the higher priority Artemis 3 launch.

ULA produces ICPS units for NASA from the same Delta IV infrastructure in Decatur, Alabama, that is being retired. Production of Delta hardware will end after final orders for national security launches are fulfilled; the Decatur factory is being converted to production of ULA’s new Vulcan launcher family.

“NASA is in conversation with ULA about producing an additional ICPS if needed,” the agency’s statement from Hambleton said. “The agency does not anticipate needing ICPS units beyond the first three Artemis missions and possibly the Europa Clipper mission.”