SLS Stages in assembly at MAF for future NASA Artemis launches

by Philip Sloss

Boeing, the prime Stages contractor for NASA’s Space Launch System (SLS) program, is working on the production and development of hardware for the third and fourth Artemis launches at the Michoud Assembly Facility (MAF) in New Orleans. Welded and bolted structures for the third and fourth Core Stage vehicles are being assembled in parallel with preparations to begin the initial production of the new Exploration Upper Stage (EUS).

Structural assembly of four of the five main Core Stage-3 elements is complete and Boeing has begun the long-lead engine section structure for Core Stage-4. Simultaneously, weld development for EUS continues before the assembly of first flight structures and the new production area in the middle of the factory is being staged for the upcoming arrival of tooling to build the upper stage.

Core Stage-3 structures nearing completion

Boeing is currently working on the assembly and production of the next three Core Stages at Michoud. In addition to the final assembly of Core Stage-2, which is targeted for delivery in March, the structural assembly of Core Stage-3 is almost complete and the long lead elements for Core Stage-4 are in production behind those two builds.

The liquid oxygen (LOX) tank for Core Stage-3, which is assigned to the Artemis 3 SLS vehicle, is the last structure that needs to be assembled. There are five main structural elements for a Core Stage, a forward skirt, LOX tank, intertank, liquid hydrogen (LH2) tank, and engine section.

Beginning with this third flight article build, procurement and production of the engine section was started first as the “long lead” element of the stage. The engine section, where the powerheads of the four RS-25 engines and all the supporting Main Propulsion System (MPS) equipment comes together, is the most complicated element of the SLS vehicle as a whole and the Core Stage specifically.

The structure of the element consists of a welded barrel and a bolted thrust structure, which are then bolted together in a floor assembly jig (FAJ). While thousands of bolts are added structurally to integrate the barrel and the thrust structure in the jig, some secondary structures like brackets to hold electrical wiring and other equipment are also added during this phase of assembly.

The Core Stage-3 engine section in the Floor Assembly Jig at MAF on July 20. Credit: Philip Sloss for NSF.

“They’re putting all of the brackets and some of the odds and ends that you want to do before you start putting all of the other hardware inside,” Eric Potter, Boeing’s Core Stage 4 Integrated Product Team Lead, said during interviews with NASASpaceflight at MAF on July 21. “[For] Core Stage-3 we didn’t have the tubes to put in, so we’re going to travel that work to later.”

Most of the orbital tube welding of MPS tubes for hydraulics, pneumatics, and propellant, will be done after structural assembly, but some “pre-integration” work is done earlier in the production process.  The engine section structure is expected to come off of the floor assembly jig in the next month or so and move into a separate work stand for the integration process.

On the opposite end of Building 103 at MAF, the other two “dry” structures, the intertank and forward skirt, are being worked on in the forward structures area. Structural assembly of the Core Stage-3 intertank is essentially complete, the work stands around the exterior of the element were pulled back at the time of the interview, and it will be moved sometime later in August to its next work center.

“They’re going to go into Cell G to do TPS (Thermal Protection System) [applications], which is about a six-month process; you have to individually do each one of those isogrids,” Potter said. The exterior of the intertank has pockets created by the intersection of circumferential and longitudinal ribs; each one of those has to be manually filled with spray-on foam insulation (SOFI).

After those isogrid “pockets” are all filled with the manual-spray foam, the outer circumference then receives a second layer of robotically sprayed SOFI. Following trimming of the foam, the intertank will be lifted out of Cell G, which is in Building 114 on the western end of the Building 103 complex, and placed on a Barrel Assembly Transportation Tool (BATT). The intertank will be returned to the forward structures area on the BATT, where integration work will begin to prepare it for the eventual “forward join” of the forward skirt, LOX tank, and intertank.

The largest element of the stage, the LH2 tank, was in Area 6 of Building 103 at MAF in late July. The unit is a holdover from early Core Stage production difficulties in the middle of the last decade, when initial circumferential welds of LH2 tank domes and barrels made in the Vertical Assembly Center (VAC) for the first Core Stage exhibited defects that took several months to recover from.

Credit: Philip Sloss for NSF.

(Photo Caption: The forward structures for Core Stage-3 are seen on the east end of Building 103 at MAF on July 20, the forward skirt in the left foreground and the intertank on the far right. In between from this vantage point some of the dome welding tools can be seen along with partial structures for the Core Stage-3 LOX tank.)

“Essentially when the welding occurred it left what’s called “low ductility, low topography” indications on there,” Andrew Rostron, Boeing’s Technical Lead Engineer for friction stir welding (FSW) at MAF, said during the July 21 interviews. “We utilized a gas-tungsten automated system to repair it — reheat the area up, consume that defect, and then return it to its proper tensile strength.”

The six-year-old tank passed an initial set of proof testing early in 2022 but will need to repeat those proof tests. “We went through a proof test on that tank after the repair and it passed the leak test but they found a few indications when they did the [non-destructive evaluations afterwards], so we fixed all those and we’re going to go and do another proof test on it,” Potter said.

“You would see indications in our X-ray radiography and that would indicate what’s called porosity; depending on the porosity we would either do a blending effort to remove it if it was just surface or we would do a manual repair, so we would apply filler material and do a manual fusion repair,” Rostron noted. Potter said that following the refurbishment of some of the equipment in the Building 451 proof test cell, the tank would roll out there in the next month or so, around late August or early September, to repeat the proof test series.

The tank is not a critical path item for the build and due to its unusual heritage is ahead of the forward join elements in the process rather than behind at this stage of production.

Not far from the intertank assembly and production area, the forward skirt for Core Stage-3 is being outfitted in its integration stand. The forward skirt is the least complicated dry structure and the last of the forward join elements that would be stacked, so it was started later in the Core Stage-3 production than the first two vehicles.

The structure has already been welded and coated with its corrosion-protecting primer; eventually, it will go to Cell G for its less-complicated foam spray before returning to the same work stand to complete its integration.

The LOX tank is the last element to be assembled for Core Stage-3; the tank structure consists of two barrels and a dome on each end. The barrels were already welded in the Vertical Weld Center (VWC) and are ready to go into the VAC when the time comes, but first, the welding of the two domes needs to be completed.

Credit: Philip Sloss for NSF.

(Photo Caption: One of the first two Core Stage LH2 tanks, originally assembled in 2016 and now assigned to Core Stage-3 is seen in Area 6 at MAF on July 20. Originally intended to support the first SLS launch, repairs were developed to fix weld defects in the tank and it is being proof tested to verify the welds meet requirements.)

The forward dome is currently in the circumferential dome weld tool (CDWT). “It’s basically done; we’ve got to do a plug weld on it,” Potter said. “[Then] that’ll come off and then we’ll do the Core Stage-3 aft [LOX] dome.”

The domes are welded from 12 gore segments, a Y-ring, and an end cap. Welding of the gore “body” for the aft dome is complete and it is ready to be welded with the ring and end cap.

In the meantime, the completed forward dome will be moved across the factory to the VAC to begin assembling the LOX tank. The forward dome is loaded into the VAC first and then the first barrel is welded to the dome, followed by the second LOX barrel, and eventually the aft dome.

Publicly, NASA is not saying what the internal working date is for delivery of Core Stage-3, only that completion is estimated to be some time in 2024. The timing of the Artemis 3 lunar landing mission that this unit is assigned to is uncertain, and NASA is also quiet about trends in the schedule for that mission. Publicly, the launch of the SLS for that mission is currently said to be no earlier than some time in 2025.

Exploration Upper Stage weld development continues

NASA and Boeing continue to work on the development of the first SLS upgrade, called Block 1B. The Block 1 vehicle, using a modified Delta 4 upper stage “off the shelf” from United Launch Alliance called the Interim Cryogenic Propulsion Stage (ICPS), is finally nearing its first launch on the Artemis 1 mission.

After a total of three Block 1 launches, the Block 1B SLS upgrade with EUS would first launch on Artemis 4; the main difference from Block 1 to Block 1B is a new, larger upper stage developed in-house by the SLS Program specifically for this launch vehicle called the Exploration Upper Stage (EUS). Currently, Boeing and NASA are working to verify the weld parameters for the structures of the EUS propellant tanks; like the Core Stage and ICPS, EUS will use cryogenic liquid oxygen and liquid hydrogen propellants.

Credit: Philip Sloss for NSF.

(Photo Caption: Some of the new tooling for assembly and production of Exploration Upper Stage units is seen on the right-hand side of the image surrounding the Vertical Weld Center (foreground left) in Building 115 at MAF on July 20. The tooling seen here is a part of the LOX Tank Assembly Center (LTAC).)

The LH2 tank for EUS will be the same 8.4-meter diameter as the Core Stage, sharing several of the same assembly tools at MAF. Boeing is currently working on weld confidence articles for the LH2 tank. “We’re building the LH2 forward dome,” Rostron said. “The LH2 barrel has already been built in the VWC, so that’s sitting out there in the factory.”

When the dome is completed, it will be welded to the barrel in the VAC. The EUS dome consists of the same basic elements, a ring, a gore, and an end cap, which are validating the EUS weld schedules for the tools at MAF.

“We’re in the process of welding the gore body and then when that’s complete, the ring, the gore, and the cap will be welded, so that’ll be your confidence [article],” Rostron said. “[We have] already completed [the barrel] confidence over here [in the VWC], we’ve already completed our confidence on the SRT (Segmented Ring Tool), the process is now to certify the GWT (Gore Weld Tool), and then it’ll go to the CDWT (Circumferential Dome Weld Tool).”

The gore body has twelve panels or segments that are welded in the Gore Weld Tool (GWT). Once those are complete, the EUS gore body confidence article will go to a newly installed tool.

“It’ll go to our new tool over there, which is our CDWT number two, which is pretty much a replica of [the first CDWT] but it’s an actual conventional weld,” Rostron said. The first CDWT performs self-reacting friction stir welds on Core Stage domes.

“It’ll be a conventional weld [for EUS] because of the different alloys and other kinds of material [from the Core Stage],” Rostron explained. The two welded subassemblies for the EUS LH2 tank confidence article will then be welded in the VAC on the other end of the factory.

“Those assemblies, forward dome and barrel, will then qualify the VAC. And then [for] all those welds we’ll excise those welds and [do] coupon testing of them to make sure the weld process itself is as seen in development in terms of tensile strength and elongation and everything.”

For initial EUS builds, welding of the 5.5-meter diameter EUS LOX tank is planned to take place at Marshall Space Flight Center in Huntsville, Alabama. Preparations and activities will begin after the welding of the third Launch Vehicle Stage Adapter (LVSA) for the Artemis 3 SLS vehicle are complete.

“They are finishing up the LVSA and then they’ll be installing the [EUS] tooling on there, so [we’re] looking at towards the end of this year for development and confidence [welds],” Rostron said. That would be in the same time frame that Boeing is looking to welding EUS structural qualification and flight articles.

“End of this year, we’ll start building the [EUS] qual and flight barrels,” Rostron said.

Credit: Philip Sloss for NSF.

(Photo Caption: The new Circumferential Dome Weld Tool, CDWT #2, is seen at MAF on July 20. The new tool is configured for conventional friction stir welding that will be done to fully assemble the LH2 tank domes for EUS. EUS uses a mixture of mostly different metal alloys for its propellant tanks when compared to the Core Stage. The first CDWT is configured for the self-reacting friction stir welds for Core Stage propellant tank domes.)

While weld development continues towards the assembly of the first flight EUS structures, Boeing is also preparing work areas at Michoud to outfit the welded structures, integrating all the other equipment needed to make a complete upper stage rocket. Some tooling to outfit and integrate the stage has arrived at MAF and more was expected soon.

“A lot of the EUS tooling is being delivered right now,” Gregg Eldridge, NASA SLS Stages Element Office Resident Management Office (RMO) Manager, noted. “Sixteen trucks are supposed to be here over the next few weeks,” Potter added.

The tooling will be used for subassembly build-up and integration of the EUS propellant tanks, mid-body, equipment shelf, adapters, and more in the “gray box” EUS integration area in the middle of Building 103. When the integration of the elements is completed, the final assembly of the stage and installation of its four RL10 engines will be done in Building 115, which stands on the east end of Building 103.

Lead Core Stage-4 structures in production

Production of the fourth Core Stage, pointed at the Artemis 4 mission sometime in the latter half of the decade, is also in progress behind Core Stage-2 and 3. Production of Core Stage-4 started with the assembly of the long-lead engine section structure.

Welding of the barrel and ring and bolting of the thrust structure for the engine section are complete and mating of the two structural pieces is waiting for the one floor assembly jig (FAJ) tool in the factory, currently occupied by the Core Stage-3 engine section, to become vacant.

“We’re in the process of buying a new FAJ (Floor Assembly Jig) tool but we [don’t have it yet] so we kind of been creative on Core Stage-4 [engine section work],” Potter said. Most of the engine section work currently is on the thrust structure, which is initially put together upside down.

Credit: Philip Sloss for NSF.

(Photo Caption: Engine section structures for Core Stage-3 and Core Stage-4 are seen at MAF on July 20. Half of the engine section structure for Core Stage-4, the barrel, is seen in the foreground of this image with the fully assembled Core Stage-3 engine section structure in the background on the engine section Floor Assembly Jig.)

“You build [the thrust structure] upside down, you put your horse collars, your clevises, all the heavy stuff that goes underneath, all that gets bolted on there and then you flip it over, and [then you] should be moving into [the FAJ],” Potter said. Instead, the structure is housed in an environmentally controlled tent where work continues for now until the FAJ becomes available.

After the thrust structure elements are bolted together, secondary structures like bracketry can be installed and initial orbital tube welding can be done on it. In addition to the thrust mounts for the four RS-25 engines, the four platforms that hold the hydraulic systems equipment will eventually be attached to the thrust structure, so there is a lot of Main Propulsion System tubing on and around it.

“[For] Core Stage-4 we were able to get the tubes in time to support [this phase of production], so that’s what we’re working now,” Potter said. “That’s helping us save schedule while we’re waiting for the FAJ to become available, it gave us the opportunity to get that work done. There’s 45 jobs we can’t do there because when we pick it up with the crane and move it over those tubes would be in the way of the lifting fixture so we’re doing everything other than that.”

The barrel and ring completed their primer spray in Cell G recently and that subassembly is now stored on a dolly in the engine section integration area. Sometime after the thrust structure is loaded into the FAJ tool, the barrel will be lowered over it and then the two will be bolted together to complete the primary structure for the engine section.

Welding of the rest of the barrels for Core Stage-4 has also started. Both LOX tank barrels for the fourth build are already complete and the panels for the first of five barrels for the LH2 tank are being loaded and aligned in the VWC.

As with the engine section structural assembly tool, Core Stage-4 hardware is ready to move into other tools once the Core Stage-3 elements are completed. Once the completed Core Stage-3 intertank structure exits its structural assembly jig in the next few weeks, the panels and thrust beam for the Core Stage-4 intertank are ready to go in.

“We’re waiting for Core Stage-3 to get off and then Core Stage-4 will be ready to go right back on top as soon as that finishes,” Potter said. Similarly for the propellant tank domes, once the welding of the domes for the Core Stage-3 LOX tank is complete, the Core Stage-4 LOX domes will be next, followed by the LH2 tank domes.

Credit: Philip Sloss for NSF.

(Photo Caption: One of the two L-rings for the Core Stage-4 forward skirt is seen being machined in the Century Detroit tool at MAF on July 20. The segmented rings are first assembled by welding six forgings in the Segmented Ring Tool. The rings are eventually machined down to the dimensional requirements in the Century Detroit tool.)

The other element is the forward skirt; with plans for Core Stage-4 to be the first to fly in the Block 1B configuration, some modifications to the forward skirt structure are being made before the hardware is shipped to Michoud. “We had to go through a little bit of a development because it’s a Block 1B, that’s obviously the part that meets with the EUS,” Potter noted. “It’s got heavier loads, so we’ve had to make a few changes.”

In the Block 1 configuration, the forward skirt supports the LVSA, ICPS, and Orion payload on top. The Block 1B configuration will still have Orion at the top but now will have a Universal Stage Adapter that encapsulates an additional 10 metric ton payload, the larger EUS, and an interstage adapter.

As with Core Stage-3, NASA would only say that completion of Core Stage-4 is planned for somewhere in the 12-month period of the 2025 calendar year. Beyond Core Stage-4, NASA is procuring and receiving long lead materials for the fifth and sixth Core Stage builds; however, officials with Boeing said they are not yet contracted to begin production on those materials.

(Lead Image: Philip Sloss for NSF. Full collections of images and videos – and more – available in L2.)

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