Boeing working toward first SLS Core Stage final assembly at KSC

by Philip Sloss

As Boeing nears completing final assembly of NASA’s second Space Launch System (SLS) core stage at the Michoud Assembly Facility (MAF) in New Orleans, the space agency’s prime contractor for SLS stages is continuing production of hardware for the next two units. The core stage for Artemis II is the last one that will be completed at MAF, with future builds now planned to undergo final construction at their launch site, the Kennedy Space Center (KSC) in Florida.

The first core stage to be completed at KSC will be the unit for Artemis III; Boeing has already transported the engine section to Florida to complete its outfitting and is hoping to have its new facilities in the Vehicle Assembly Building (VAB) ready by the end of 2024 — the same time as it intends the stage hardware for that launch to be ready for final assembly. Structures for the Artemis IV core stage are also in production, with delivery of the engine section structure to Florida expected in the first part of next year.

Artemis III core stage will debut new final assembly methods

The core stage for Artemis II is in its final integrated testing ahead of completion in the next several weeks, and while that is the focus of Boeing’s production work at MAF, NASA is still pressing toward a scheduled launch of the next mission – Artemis III – only a year after the target date for Artemis II at the end of 2024. Work on the third core stage is aiming toward completion in 2025, with the final phase of production moving from MAF in New Orleans to facilities at KSC.

The structure of the engine section for the Artemis III core stage was transported to KSC last December for outfitting with all of its working equipment, which will be done from now on in the Space Station Processing Facility (SSPF). The rest of the stage will continue to be assembled at MAF and then that “subassembly” of the other four of the five major stage elements will be transported by barge to KSC for final assembly in High Bay 2 of the VAB.

“The current forecast is the end of next year to move into the high bay,” Tim Burroughs, Boeing’s Integrated Product Team Lead for both Core Stage-2 and Core Stage-3, said in a recent interview at MAF with NSF. “Obviously that depends on the readiness of the bay and the readiness of the engine section, but right now it’s postured to be at the end of next year.”

Two views of the intertank for Core Stage-3 in its integration area at Michoud in October. (Credit: Philip Sloss for NSF)

Outfitting or “integration” of the engine section typically takes the most amount of work and time to complete and is therefore often the primary schedule driver or “critical path,” but issues with completing the liquid oxygen (LOX) tank structure have pushed its production timelines and those related to it out into a similar timeframe.

“For Core Stage-3 (CS-3), the LOX tank is critical path to ‘4/5ths’ join,” Burroughs said. “It’s pretty much neck-and-neck with where engine section is forecast to be complete.” The “4/5ths” join refers to the other four major elements of the stage besides the engine section — the forward skirt, LOX tank, intertank, and liquid hydrogen (LH2) tank.

The top three elements of the stage, the forward skirt, LOX tank, and intertank, are the first to be connected, with “major join 1” also frequently referred to as the “forward join.” Those three elements are stacked vertically in Michoud’s VAB, which stands for Vertical Assembly Building at the New Orleans factory.

After those three sections are joined together structurally and functionally, the forward join is rotated to horizontal and moved into the final assembly area at MAF, where the bottom of the intertank is mated to the top of the LH2 tank to begin integration of the 4/5ths assembly.

The LOX tank had been progressing toward structural completion over a year ago in 2022, but that milestone is being held up by a problem with welding the aft dome of the tank. “We don’t believe there’s a specific issue with the weld in and of itself,” Matthew Stites, Boeing SLS Chief Engineer, said in the recent NSF interviews at MAF.

“It’s really the as-built condition of the gore body coming off the prior tool, the GWT (Gore Weld Tool), that we think is primarily the contributing cause to what we’re seeing when that part moves to the CDWT, the circumferential dome weld tool. There’s nothing specific about that weld that we think is problematic, it’s really about the as-built condition of the dome, it’s a little bit bigger than we were expecting it to be.”

Credit: Philip Sloss for NSF.

(Photo Caption: The elements of the LOX tank aft dome for Core Stage-3 are seen in the dome welding tool at MAF during welding preps in October. The dome tool is used to perform circumferential, friction-stir welds to the end cap, gore body, and Y-ring.)

The domes of the propellant tanks are welded together in the CDWT from a ring, a gore body, and an end cap. The GWT uses friction-stir welding to assemble a gore body from 12 gore panels and what NASA and Boeing saw was the resulting gore body was slightly out of its allowable dimensions.

“It’s the overall, correct, assembled dimensions,” Terry Prickett, NASA SLS Stages Office Chief Engineer, said. “When you weld these gore panels together and produce this gore body, you had a stack-up that went around, due to the tooling having some issues with it, when it produced a gore body that’s dimensionally slightly out of tolerance.”

The problems were discovered with the first aft dome welded in the CDWT; the teams elected to set that article aside for now. “We’re working the path forward on whether we can use that dome,” Prickett said. “For schedule purposes, to use that dome was probably [going to require] a fair amount of testing to be done offline, and for schedule purposes we just set it aside for now and elected to build a new replacement dome.”

Both the Boeing and NASA teams have been going through the “root cause corrective action” (RCCA) process for the issues. “We went in and performed RCCA and that uncovered several things, both production-related and tooling-related, that we have corrected on that tool,” Prickett noted. “The reason it’s most important on this aft dome is because that’s the one that’s got the most critical weld on it, which is that aft dome cap, which has got the lowest margin weld that we have.”

Once the aft dome is completed in the dome welding tool, it will be prepared to be welded to the rest of the LOX tank, and then the tank will be prepared for the forward join (major join 1) next year. “[There’s] about 11 months from the time that the dome is complete [until] we should be able to go to a major join 1,” Burroughs noted. “[That would be] followed immediately by 4/5ths, and then do some outfitting here and then we’ll be able to ship it down, ready for the VAB about the end of the year, that’s where they’re kind of lining up with the engine section and being ready in the VAB.”

In a mid-November statement provided to NSF by public affairs, NASA’s SLS program said, “NASA and Boeing, the SLS core stage lead contractor, are undergoing final weld preparations and fit up operations at NASA’s Michoud Assembly Facility in New Orleans for the final friction-stir weld for the aft dome of the liquid oxygen tank for the Space Launch System core stage that will support the Artemis III mission.”

Credit: Philip Sloss for NSF.

(Photo Caption: The top three elements of the Core Stage-3 LOX tank are seen in the Vertical Assembly Center (VAC) in October. The forward dome and two barrels were welded together in the VAC over a year ago and are waiting for the aft dome to be completed to allow the full tank structure to be welded.)

Work to outfit the engine section is progressing in the SSPF at Kennedy, with the goal for that to be complete in the end-of-2024 timeframe when the rest of the stage would be ready for final assembly, with the new core stage facilities in the VAB in place for the process to take place at KSC for the first time. Orbital tube welding is the focus of the work inside the engine section, while large subassemblies and equipment are being prepared for installation next year.

“That’s kind of the build sequence, get all of the [orbital] tube runs completed and tested, then [installing] wire harnesses is the next big scope of work,” Burroughs said. “TVC platforms are being built up now, so once we get through our lower thrust structure tubing, the next step will be TVC platforms, and then helium tanks are a few months away but that’s after the platforms,”

There are four thrust vector control (TVC) platforms that hold the equipment for each of the main propulsion system (MPS) hydraulic systems and those subassemblies are first integrated outside the engine compartment before being “flown” inside by a crane lift. Similarly, the five large composite overwrapped pressure vessels (COPVs) that hold the in-flight supply of gaseous helium for the stage’s MPS and the four RS-25 engines will be staged prior to lifting into the engine section next year.

“[We do] all the tube runs and then we do a bunch of [wire] harnesses, we fly in our Y-ducts, [and then] helium tanks come in after the Y-ducts, so it’s probably sometime mid next year before the helium tanks go in,” Burroughs said.

Another milestone for engine section integration is mating the boattail, which Boeing plans to transport from Michoud to KSC next year after NASA’s Pegasus barge first makes the round trip with the Artemis II core stage. “Once the barge comes back from [delivering] Core Stage-2 (CS-2), we’re going to ship the Core Stage-4 (CS-4) engine section and the CS-3 boattail down,” Burroughs said.

That would be followed soon after by bolting the boattail to the bottom of the engine section. “We learned on Core Stage-2 we can mate [the engine section and boattail] very quickly and the advantage of the SSPF is that it’s got a higher hook height [than Michoud],” Burroughs noted.

Credit: Philip Sloss for NSF.

(Photo Caption: The LH2 tank for Core Stage-3 is seen in Cell E at MAF in October. The interior, storage area of the tank is washed and cleaned in the processing cell to prepare it for installation of internal sensors. That will be preceded by the application of primer and foam to the outside of the tank.)

“One of the reasons we did not mate the boattail as early as we could have on CS-2 is we could not fly in some of our big structures [at Michoud] because we ran out of hook height and ceiling, so the plan is as soon as it gets down there, we’ll start staging that and immediately go to install it. That opens up additional electrical scope [that can be worked on], it prevents a lot of loose installs that [would otherwise] have to go into the boattail.”

“Once we go and install the boattail, then we can go [do] the full [wire] harness install.” Boeing is continuing to work on the boattail while it is at MAF waiting for a ride down to KSC: “We’re still parallel working boattail here, they’re doing hydraulic tubing,” Gregg Eldridge, NASA SLS Stages MAF Resident Management Office (RMO) Manager, said. “They’re in process [working on] leak testing right now actually.”

The other elements of the Artemis III core stage in production at Michoud: the forward skirt, the intertank, and the LH2 tank, are progressing while troubleshooting on the LOX tank build continues. “The intertank has all [its] TPS (thermal protection system) [applied], all the external structure is complete,” Burroughs said.

“They’re in the avionics bay installation process now, which is followed up by electrical, so we’ll be two or three months of hard-core electrical, and then should be in a functional test configuration by end of first quarter next year and then go into the functional test and it’ll be ready for major join 1 sometime [in the] mid next year timeframe. Forward skirt [is] clicking along, it’ll be ready well before the intertank and LOX tank for stacking.”

LH2 tank moving ahead and the return of the “aft join”

The other propellant tank for CS-3, the LH2 tank, is now being prepared for the application of its corrosion-protective primer, followed by the insulating spray-on foam insulation (SOFI). The tank article was set aside many years ago due to welding issues at the time the tanks for the first core stage were being welded in 2016 and 2017.

Following years of research, repairs of the “low-ductility, low topography” indications on the tank were completed in the 2021-2022 timeframe. The tank passed a second proof test earlier this year and in October was positioned in Cell E, the internal cleaning facility. Following the cleaning of its interior, the tank will move to the production cells in nearby Building 131, Cell P for primer application and Cell N for SOFI application.

Burroughs noted that the third core stage will look a little different than the first two, with the omission of “sensor islands” for development flight instrumentation (DFI) on the Artemis I and II cores. “Sensor islands are gone on CS-3 and beyond,” he said. “[Core Stage-3] has a lot fewer sensors, we took a lot of our DFI sensors away for Three.”

The reduction of DFI also eliminates some work required to affix wire harnesses to the skin of the tanks, and mask off the sensor island areas on the “acreage” of the tank for SOFI application. Since the LH2 tank is a little early in the production timeline and the LOX tank is delayed, the LH2 tank will go through the primer and SOFI cells first for this build.

After the LH2 tank receives its primer and SOFI, it will be prepared for two joins now that the engine section is being completed at KSC and final assembly is also moving there. Originally, Boeing had planned core stage assembly around two vertical joins of the top and bottom halves of the vehicle, a forward join and an aft join (also referred to as “major join 2”, followed by a final major join of the two halves in a horizontal orientation.

Credit: NASA/Eric Bordelon.

(Photo Caption: The core stage LH2 structural test article is mated in Cell A at MAF in November 2018 to the second of two simulators. The LH2 tank for the Artemis III core stage will be stacked on top of a simulator in Cell A in an “aft join” similar to this.)

The final assembly plan was modified in 2019 to speed up completion of the first core stage build, by rearranging the sequence of joins and allowing the 4/5ths part of the rocket to continue production in parallel with the trailing engine section. The aft join will return, with the LH2 tank being stacked on top of a simulator for the engine section that is being repurposed following structural testing that was completed before Artemis I.

Adding an engine section simulator to the 4/5ths “stack” allows it to be carried by the existing “roadway” transportation tools. “We’re going to go back to the original baseline, major join 1 and 2,” Burroughs said. “We’re going to stack the sim in Cell A on the hydrogen tank, and then we’ll have the bottom half of a vehicle.”

“Then we’ll mate it to major join 1, [the] top three sections, and essentially you’ll have a full core stage, less all the fun stuff inside the engine section.” The simulator also allows NASA’s Pegasus barge to transport the 4/5ths assembly without any modifications.

In addition to the standard production work on the core stage flight hardware, Boeing is working to construct a final assembly facility in VAB High Bay 2 at Kennedy. The final two elements of a core stage: the 4/5ths and the engine section/boattail, will be mated vertically in High Bay 2.

“This is where we get the benefits of vertical install and build,” Burroughs explained. “When we do it horizontally you’re kind of limited on any parallel work that you can do when you finish the engine section and final assembly at the same time.”

“The big advantage of going vertically is you do not have to finish your engine section prior to breaking over. You can go ahead and stack the top 4/5ths on the engine section and work final assembly in parallel with [the] engine section work scope, as well as the benefit of pulling in engine section functional testing and FIFT (final integrated functional testing) and combining that, so you’re looking at two to three months of overall savings from a schedule perspective.”

Credit: Julia Bergeron for NSF.

(Photo Caption: One of the doors for VAB High Bay 2 is seen open during a recent NSF flyover of Kennedy Space Center. Boeing is converting the high bay into a final assembly facility for SLS core stages beginning with the third unit that will fly with the Artemis III vehicle.)

The engine section was not originally designed to be mated while horizontal, so the new final assembly plan means that the CS-3 engine section can stay in a vertical orientation for the whole core stage build. When standalone integration work is completed in the SSPF, it will be moved to VAB High Bay 2, where it will be placed in one of the two production cells that will have been constructed there.

The two final assembly cells Boeing is building in High Bay 2 will have additional benefits from the original stacking cell at Michoud, Cell A. “It’s much more sophisticated than Cell A, so the bottom structure will be [elevated] so you can get engines installed, then there are multiple platforms around the entry points of the dry structures and then there’s other platforms,” Burroughs said. “It’s more aligned with a Cell A plus with the wrap-around platforms and an elevator going up and down, etcetera.”

In addition to the time savings from not having to rotate the engine section from vertical to horizontal and then back to vertical, Boeing can also leave some of the ground support equipment access kits inside the barrel, which will also save time breaking down all the GSE and then reassembling it. “The boattail kit, that can be used,” Burroughs noted.

“The internal access kit that we use here at MAF will not work with the LH2 tank you have to go to the access kit that we used at Stennis and on the ML for CS1. It’s a different kit because of the dome taking up a big portion of your volume [inside the engine section], but the boattail kit is essentially the same.”

Boeing is currently targeting completion of construction inside VAB High Bay 2 by the end of 2024, which would support delivery of the CS-3 vehicle to the facility in the same end-of-2024 timeframe. That would put completion of the Artemis III core stage somewhere in 2025. NASA is still targeting launch of Orion and SLS on the Artemis III mission for December 2025.

One of the future goals is for SLS production to reach a delivery rate of two vehicles annually. To facilitate this Boeing is looking to optimize the floor space at Michoud with the new two-site production plan, including the final assembly area in Building 103 at Michoud — also called Area 47/48.

“We’ve looked at that extensively, so 47/48 was always designed to have two stages in there,” Burroughs noted. “It gets very busy with ship-side support [personnel and cubicles] and pretty full, so the concept is when we get to the 4/5ths on Core Stage-3, for example, it’s not a full rocket so we can move some of our ship-side support around and we’ll still be able to get a LOX tank or an LH2 tank into 47/48 to free up other areas in the factory and eventually get to a two core stage a year type of a scenario and the footprint and all the modeling shows that’s feasible with the current footprint.”

Core Stages for Artemis IV and beyond will support Block 1B vehicle still in development

On CS-4, which will fly with the Artemis IV vehicle, the two lead elements of the previous core stage builds — the engine section and the intertank — are progressing toward structural completion. Shipment of the engine section structure to KSC is planned for early 2024, with the issues that have been encountered finishing the last welds for the Artemis III core stage having been factored into the overall production plan.

“It’s all maintained in a factory model that we update basically monthly on where we’re at,” Burroughs noted. He also noted that they are looking at other work they can do on the CS-4 engine section structure while it waits for its ride to KSC.

Credit: NASA/Michael Democker.

(Photo Caption: Two of the core stage hardware elements that Boeing plans to transport to KSC soon are seen in the background of a recent picture taken in September while the RS-25 engines were being installed in the Artemis II core stage. The boattail that will attach to the bottom of the Core Stage-3 engine section is partially visible (left inset) along with the Core Stage-4 engine section structure (right inset))

“We’re going to try to do some external cork and other things here,” Burroughs said. “We’ll continue to work on it, because we have some time. Anything we can do here will buy us value in the future.”

“We did a fairly large scope of tooling around the thrust structure, based on sequence requirements certain components have to go in before you do your subsystem weld-ups, bleed ducts, and other things, which we’re going to go ahead and install when we get to Kennedy,” he added.

The intertank thrust beam and panels for CS-4 are being bolted together in the structural assembly jig adjacent to its counterpart for Core Stage-3. “Based on when intertank CS-3 completes [is] when you transfer the full force over to CS-4. Obviously, CS-4 has got to go get ‘TPS-ed,’ so it’ll be in the processing cells for a while getting that [work] completed before it comes back for integration.”

The need date for CS-4 will be driven by the development and construction of the new elements for Artemis IV. This will be the first flight of the SLS Block 1B vehicle with the new Exploration Upper Stage (EUS) currently in development and a new Mobile Launcher which is being constructed at KSC to support the longer, upgraded Block 1B rocket.

NASA’s current public schedule has Artemis IV launching in September 2028, which would be almost three years after the planning date for Artemis III, so completion of the core stage for Artemis IV may not need to follow as closely on the heels of the Artemis III unit.

Although the production of some structural rings has started at MAF, construction of the major elements for the fifth core stage that would fly on Artemis V — such as the long lead engine section and intertank — has not started. So far, NASA has contracted with Boeing to procure long-lead raw materials and parts for the fifth and sixth core stages, but the agency is planning to transition the SLS program to a commercial services contract.

Credit: Philip Sloss for NSF.

(Photo Caption: A mixture of core stage flight and ground support hardware is seen at MAF in October. The forward skirt for Core Stage-3 is seen in the foreground on the right being prepared for stacking with the top part of the rocket next year. A simulator that could be used for transporting core stage hardware is seen to the left. In the background, domes and barrels for Core Stage-4 are seen covered up in storage for future production work.)

The broad outline of that plan would be to begin with Artemis V, but NASA recently revised its plan to include an additional three-year interim contract before making a full transition to commercial services. Artemis V is also planned to be the first mission to fly on the annual cadence which has been a long-term goal for NASA’s Exploration divisions and directorate. Even with the uncertainty about the Artemis IV schedule, continuing production of Artemis V could at least keep some of those options open for longer.

“We’ve ordered long lead for [core stages] five and six, we’ve actually welded our first CS-5 ring in the past couple of weeks,” Burroughs said. “There’s discussion now of engine section and intertank thrust structure, so that’s probably the next activity.”

As with the production of Orion spacecraft for Artemis missions, SLS and Boeing are also monitoring the recovery of the aerospace supply chain following the disruptions that began with the onset of the COVID-19 pandemic in 2020. “We still have some COVID hangover, depending on where a supplier is, there’s different types of restrictions they had to deal with,” Burroughs remarked.

“We’ve got a fairly limited supply base with the cryogenic components and exotic materials, if you will. I think it’s recovered a lot, we’re not a hundred percent back to pre-COVID days, but we’re getting much closer to where we can manage it. Our dates that we receive are more achievable and more readily met, versus what they were right after COVID.”

“I think they’ve learned a lot,” Eldridge added. “The production system that Boeing has implemented, they’re very nimble, so it’s able to resequence and kind of keep the overall plan, rearranging different production lines, knowing that you have a part that may not [be] coming when you need it here, do another work, to keep the work going to not impact the overall [build].”

(Lead image: The propellant tanks for the Artemis III core stage are seen in the Vertical Assembly Building at MAF in October; LOX tank on the left, LH2 tank on the right. Credit: Philip Sloss for NSF.)

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