NASA’s Space Launch System (SLS) Program and prime contractor Boeing have a new framework in place to finish development of the next version of the new launch vehicle, called Block 1B. The biggest addition to the Block 1B SLS vehicle will be the Exploration Upper Stage (EUS), which completed its Critical Design Review (CDR) on December 18.
The CDR milestone indicates the design of the larger, higher performance upper stage is mature enough to begin construction and assembly of the test and flight articles needed for first flight certification while incorporating lessons learned while starting Core Stage production, including the 2020 creation of a separate Block 1B/EUS Development Office within the program to advance Block 1B to its first flight while much of the rest of the workforce focuses on efforts to get the initial SLS Block 1 vehicle off the ground for the first time on Artemis 1.
New upper stage Critical Design Review completed in December
The Critical Design Review milestone for the new, in-house upper stage for SLS was a series of reviews that culminated in a final board on December 18. “CDR is a big deal, so it’s not just a single meeting,” Steven Wofford, manager of NASA’s new Block 1B/Exploration Upper Stage Development Office, said. “CDR is really more kind of a campaign, if you want to look at it that way.”
The EUS is the major upgrade from the SLS Block 1 vehicle that will fly the program’s first three launches. On the SLS Block 1B vehicle, the EUS will replace the Interim Cryogenic Propulsion Stage (ICPS), which is closely derived from the 5-meter diameter upper stage of United Launch Alliance’s (ULA) Delta IV launch vehicle.
At its widest, the larger EUS is the same 8.4-meter diameter as the SLS Core Stage, uses the same hydrogen-oxygen commodities for its four Aerojet Rocketdyne RL10 engines as the Core Stage uses for its four RS-25 engines, and will increase the SLS’s overall payload performance from approximately 27 metric tons to 38 metric tons when inserting the Orion spacecraft on a trans-lunar trajectory.
For the EUS’s CDR, “The preparation and the review [took] place over a series of several months,” Wofford explained. “So we really worked most of calendar year 2020 getting ready for that December 18th culmination of the CDR.”
After a redesign that started in 2018 and ran through 2019, the stage and its related hardware were refined ahead of CDR by the SLS Program and EUS prime contractor Boeing.
“What goes into that is maturing the design, so when you mature the design you document it in various drawings and products and specs and procedures and all of that,” Wofford said. “Boeing uses what they call a ‘layout process’ to define the design and its proprietary process. Think of a layout as a superset of a drawing.”
“So a layout encompasses a part of the stage design, and it has everything you need to flesh out that part of the design in terms of drawings and manufacturing capabilities and tooling and supplier relationships and analyses and all that sort of stuff.”
The CDR campaign began several weeks before the final sign off. “Some weeks before the December review, we [had] what we [called] a ‘data drop,’ and that [was] where Boeing put the completed work out there in a repository open and available for NASA to review and critique,” Wofford said.
“Understand that the NASA people had been involved, hand in glove, with the Boeing folks all along towards this, so it wasn’t a big surprise that they’d been waiting for this thing to get dropped in this data drop. We’d been working really closely with them all throughout the year.”
(Photo Caption: Configurations of the SLS launch vehicle under development or under recent development. The SLS Block 1 Crew configuration (far left), labeled “SLS Block 1,” is now planned to fly the first three SLS launches of the Artemis 1, 2, and 3 missions. The SLS Block 1B Crew and Cargo configurations (third and fourth from left) will fly with the in-house Exploration Upper Stage beginning with Artemis 4. The SLS Block 1 Cargo configuration was in development to fly the Europa Clipper, but those plans were recently cancelled by NASA in favor of a commercial launch vehicle.)
“But there was an official data drop some weeks before the CDR, and the reviewers both from my team and outside reviewers that we invited in [went] through all of those products. And then they generated a series of questions and comments and review item discrepancies, and all of that was brought to the board on December 18th,” Wofford added.
With completion of the CDR, NASA and Boeing will now work to begin building the hardware that will support the first EUS flight. In addition to the working flight article, Boeing will construct and assemble a structural test article (STA) at the Michoud Assembly Facility (MAF) in New Orleans to support design certification for first flight.
Production of EUS units will begin to join the traffic of Core Stage and interstage hardware moving through MAF; current plans are to begin fabrication of the STA in mid-2021. The completed STA will then take a ride on the agency’s Pegasus barge up to the Marshall Space Flight Center in Huntsville, Alabama, where a structural testing campaign is currently forecast to begin in early-2023.
The STA will outwardly resemble the more complicated flight unit, but work to fully assemble, outfit, and checkout the first working stage is expected to last until late-2023.
The first flight article will then be barged to the relatively nearby Stennis Space Center in southern Mississippi for a “Green Run” test campaign similar to the one the first SLS Core Stage is undergoing in the B-2 Test Stand.
The first EUS will be installed in a modified B-2 Test Stand before its test campaign follows a similar process currently being employed for the Core Stage’s Green Run: systematic checkouts, integrated testing, and a test-firing of the stage’s four RL10 engines in the stand.
The current forecast calls for completion of the EUS Green Run in July 2024; it will then be returned to MAF to replace the set of RL10 Green Run test engines with a set of flight engines. Following the engine change-out, the EUS will be barged from MAF to the Kennedy Space Center in Florida for the first Block 1B launch campaign that is currently scheduled for the vaguely-detailed Artemis 4 mission.
Redesign optimized for lunar missions
The Exploration Upper Stage passed its Preliminary Design Review (PDR) in January 2017 and was tracking towards a CDR in 2018 when a series of programmatic and policy changes brought work to a stop. The EUS was planned to debut on the second SLS launch at that time, but in March 2018, Congress directed NASA to fly at least three missions using the Block 1 vehicle with ICPS along with building a second Mobile Launcher.
As the first SLS launch date was delayed by Core Stage issues, it was decided to focus program resources on that and re-evaluate EUS plans. “The program paused development in 2018, primarily to give the team the ability to focus on getting the Core Stage complete and successful,” Todd Holloway, the Contracting Officer Representative for NASA’s Block 1B/EUS Development Office, said.
Later in 2018, NASA decided to optimize the EUS design for missions to the Moon. “[The] choice to optimize [for the Moon] was made in order to be able to optimize the co-manifested payload mass, or the cargo mass if you will, to the Moon as the Artemis architecture evolved,” Holloway noted.
He explained that prior to the change in plans the EUS was designed to satisfy different types of design reference missions. “Through PDR and [approaching CDR], they had four different design reference missions they were designing to, and they basically break into two categories. One was lunar-optimized missions; the other was interplanetary-type missions,” Holloway said.
“If you’re going to a deep space destination, you’ll want large tanks, more fuel, longer burn time. It’s a very light payload, but you have a lot of thrust to go with that [which was] there to optimize that trajectory. But you don’t need those large tanks going to the Moon.”
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