Boeing assembling structures for NASA’s second SLS Core Stage

At mid-year prime contractor Boeing was progressing through structural assembly of the second NASA Space Launch System (SLS) Core Stage at the Michoud Assembly Facility (MAF) in New Orleans. Completing final assembly of the first flight article remains the priority, but getting the next stage unit completed is likely to be a driver of the schedule for the Artemis 2 mission, which is planned to be the first crewed SLS launch in 2022 or 2023.

Assembly of the top two of the five major structural elements of Core Stage-2 is complete and those have continued into the following steps in their standalone production.

Tools spread out over the large factory floor at MAF help put the five sections together separately, with the propellant tanks being welded in different machines and the “dry” structures being bolted together. Boeing is making changes to work instructions and task sequences based on lessons learned during assembly of the first flight and working article.

NASA and Boeing are looking to improve schedule performance for the next build and optimizing the work plans long-term for subsequent stages.

Forward skirt, LOX tank through structural assembly

NASA’s initial SLS Stages contract with Boeing covers both Core Stage development and the production of the first two flight articles. Most of the structural hardware for a set of qualification structures and the two flight units was delivered to Boeing at MAF a few years ago.

Credit: NASA.

(Photo Caption: A diagram from NASA’s Core Stage fact sheet) showing the different structural elements (barrels, domes, rings) of the stage and the composition of each of the five major elements. The elements shaded in orange are welded. The intertank shown in gray is the only all-bolted element, but the welded engine section barrel is bolted to a thrust structure (not shown).

The structures are delivered to MAF as panels, where a set of tools is first used to weld the panels into the barrels and domes for the cryogenic propellant tanks, forward skirt, and engine section.  The qualification structures were assembled into individual structural test articles (STA) for each of the stage elements (minus the forward skirt) and the first flight structures were assembled into Core Stage-1.

Core Stage-1 will as pathfinder test article and then be assembled with the rest of the first SLS vehicle to launch the Artemis 1 mission. Core Stage-2 is scheduled for use to launch the Artemis 2 mission.  The panels for the Core Stage-2 structures were taken out of storage at MAF for buildup after the two sets of first-flight structures were completed.

“We’re in pretty good shape on Core Stage-2, the build is going extremely well,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said in a mid-June interview. “The number of non-conformances, the number of issues is like a tenth of what it was on the first build, which you kind of expect. The team is all trained up and has got good instructions and good parts and so the flow down at MAF for Core Stage-2 is just night and day difference from Core Stage-1.”

“It’s a lot of fun to be a part of it and to see things going together really easily, you can say ‘yeah I remember you know we were down a month on this’ while we went and figured out how to do this or how to do that.”

The Core Stage has five major structural elements; besides the two propellant tanks that store liquid oxygen (LOX) and liquid hydrogen (LH2), there are three “dry” structures — the forward skirt at the top, the intertank that sits between the LOX and LH2 tanks, and the engine section at the bottom.

The least complicated of the five elements is the forward skirt, which has progressed the farthest through its standalone workflow. “We’re just pressing ahead with integration,” Steven Ernst, Core Stage Engineering Support Manager for Boeing, said in late June. “Much like Core Stage-1 flow, the work content for forward skirt is smaller than the other volumes.”

Credit: NASA/Eric Bordelon.

(Photo Caption: The Core Stage-2 forward skirt is prepped for application of spray-on foam insulation (SOFI) in Cell G at MAF in late May. An automated process applies foam to the structure, which is rotated on a turntable in front of a spray gun. The intertank will go through a similar sequence in its SOFI application, except that the automated spray is the second part of a two-step process for that element. The forward skirt only receives the one automated spray, with several areas on the outside masked off with tape.  Additional foam “closeouts” are done to those areas later in the assembly process.)

The forward skirt structure consists of a short barrel and two L-rings. After the barrel was welded from eight panels in the Vertical Weld Center on the east end of Building 103, it was stored at MAF for most of 2018 while production work concentrated on Core Stage-1 and upgrades were made to the Vertical Assembly Center (VAC) in Building 110.

The L-rings were welded to the barrel in the VAC at the end of 2018. Following weld inspections, the element was moved to Cell G in Building 114 where primer to protect against corrosion was applied to the rest of the structure; the barrel panels were delivered to MAF with the primer already applied.

In Cell G, the weld lands where the panels were welded together in the VWC were coated with primer, along with the weld lands where the rings were welded to the barrel in the VAC and the rings themselves. In late February, the forward skirt was in its integration area in Building 103 for some pre-integration work on the inside, before heading back to Cell G which was reconfigured for application of spray-on foam insulation (SOFI).

Credit: NASA/Eric Bordelon.

(Photo Caption: A Boeing technician works on the outside of the Core Stage-2 forward skirt in mid-July following SOFI application to the outside of the barrel. The element is in its Final Assembly jig in the integration area at MAF. The integration areas for the forward skirt, engine section, and intertank are grouped near each other on the west end of Building 103 near Building 110 and the Final Assembly area for the full Core Stage.)

At the end of June, the forward skirt was back in its integration area to complete its standalone production work, where secondary structures like brackets and shelves will be attached to the inside. Those will support subsequent installations of air-conditioning ducts, fluid tubing, lots of wiring, and vehicle avionics boxes.

The other element through structural assembly is the LOX tank; at the end of June, the tank was set up horizontally in Area 6 in Building 103, where Boeing technicians were completing installation of slosh baffles in the aft end of the tank, and finishing non-destructive evaluation (NDE) inspections of plug welds.

In the middle of the spring, while final assembly activities for Core Stage-1 were going on around it in Building 110, the LOX tank was assembled with three welds in the VAC. The forward and aft domes and two barrels were connected together in the tool, which uses a self-reacting friction-stir welding process.

Credit: NASA/Eric Bordelon.

(Photo Caption: The Core Stage-2 LOX tank is prepared for breakover from vertical to horizontal in Building 110 at MAF in late May. The tank has been lifted out of Cell A (the vantage point of this image), where post-weld work on the tank was done after it was welded together in the VAC (blue structure to the left of the tank).)

“We shifted to a hundred and eight degrees on our self-reacting circumferential welds, hundred and eight degrees at a time, [so] we have two plugs per weld joint,” Ernst explained. The tanks are built from top to bottom; the forward dome is loaded first, followed by each of the barrels and then the aft dome.

With that semi-circular weld schedule, the pins used in the VAC to do the friction stir welding enter and exit through two holes on opposite sides. Those small holes are then plugged separately to make the welds pressure-tight using an automated tool after the tank was moved to Area 6.

“We just finished up the friction pull plug closeouts for the self-reacting circumferential welds, so that work is done and now doing some post-weld prep work on the final plug,” Ernst said at the end of June. “We’re finalizing what we call post-weld prep which is cleaning up on the IML (inner moldline) and the OML (outer moldline), the pull plugs.”

Inspections of the welds are done with a phased-array ultrasonic testing (PAUT) tool that performs the non-destructive evaluations.

Credit: NASA/Eric Bordelon.

(Photo Caption: The Core Stage-2 LOX tank in Area 6 at MAF in mid-July. The tank is sitting horizontally in two Rotational Assembly and Transportation Tools (RATT), with a dry, air-conditioned purge hose held in place and sealed up with tape into one of the manholes.)

In parallel with wrapping up the plug weld work, technicians are installing slosh baffles in the aft dome of the tank. Some initial work on the baffles is done while the tank is vertical following its VAC welds.

“We do all of the first ring that’s half way up the aft dome in the vertical for access reasons but just like with Core Stage-1 all the rest of the slosh baffle shelves are done in the horizontal,” Ernst said.

Boeing is incorporating lessons learned building all the structural test and flight Core Stage hardware needed for its first flight on Artemis 1. With only final assembly left to complete on the first flight article, Boeing is applying their previous structural assembly experience to current work on Core Stage-2.

“We understand how they behave a lot better, the variability of the shape of the tank or the slight amount of out-of-roundness,” Ernst explained. “We’ve also had some improvements [in the process], so we’re able to do some of the match drilling offline.”

Ernst noted that the slosh baffle tasks were moved in the work sequence for the second flight article build. “It’s being done a little bit differently now. We did them post proof test last time, now we’re doing this earlier,” he said.

Credit: NASA/Eric Bordelon.

(Photo Caption: A wider view of the Core Stage-2 LOX tank in Area 6 at MAF in mid-July. Area 6 sits between the Final Assembly area and Building 110 and has outside access, which allows large hardware to be moved between those areas in the complex and the nearby Building 131 where primer and SOFI are applied to the propellant tanks.)

The two propellant tanks in the stage are pressurized volumes; once the work in Area 6 on the plug welds and slosh baffles is completed, the tank welds will be proof tested.

The LOX tank welds are proof tested hydrostatically in Cell F in Building 110; the tanks are filled with water and then pressurized to reach the test conditions. In contrast to the LH2 tank proof testing, no additional loads are applied to the tank.

Following the testing in Cell F, the tank will be moved back to Area 6 for post-proof NDE, where the state of the welds is compared from before and after the proof test to verify they are good. The tank will then start to be fitted with operational and development flight instrumentation and be cleaned inside and out to prepare it for application of primer and SOFI in Building 131 Cells P and N, respectively.


The bottom three elements are still in structural assembly, the intertank, LH2 tank, and engine section.

Credit: Philip Sloss for NSF.

(Photo Caption: The Core Stage-2 intertank structure in its structural assembly jig at MAF in late February. The white circle in the center of the image is an attachment fitting for the top of one of the Solid Rocket Boosters. The fittings sit on the ends of a thrust beam that runs through the middle of the intertank and is bolted into the rest of the element’s structure.)

The intertank is a bolted structure of eight, thick, sidewall panels and a thrust beam that runs through the middle that takes the vehicle thrust loads from the Solid Rocket Boosters (SRB). It’s the strongest structure of the stage and the panels are too thick to weld together.

The thrust beam and the panels were installed into their specialized structural assembly jig and bolting them together began earlier in the year. “They’re continuing to make good progress,” Ernst said in late June. “We’re seeing huge improvements, Core Stage-2 over Core Stage-1; we’ve adjusted the drilling processes, team is getting the work done a lot faster.”

Once all of the bolting is completed, the tank panels are already primed, so the next step will be to move it over to Cell G for its two-phase SOFI application.

Credits: NASA/Eric Bordelon.

(Photo Caption: Two Boeing technicians working on the umbilical plate of Core Stage-2 intertank in late June as it sat in the structural assembly jig at MAF.)

The SLS intertank has intersecting ribs that run vertically from top to bottom and in rings around the circumference; the intersections create rectangular pockets. The intertank panels also vary from one to another; the thrust panels on the side where the Solid Rocket Booster (SRB) attach hardware and the thrust beam connect have more crisscrossing ribs for more reinforcement, creating different pocket sizes than the other panels.

In the first part of the foam applications, the different rectangular pockets will be filled by technicians using a manually-sprayed SOFI formula. The intertank is set up on a turntable in Cell G which supports both types of foam applications.

The second coat of SOFI will be applied similarly to the single coat for the forward skirt, a robotically-applied auto-spray that uses another foam formula. The auto-spray foam has more strict temperature and humidity requirements, but the robot can apply the foam more precisely.

Following the SOFI applications and trimming, the intertank will then be moved to its integration area for the remainder of its outfitting. All of its internal brackets, shelves, and support structures will be added to support installation of a larger set of ducting, tubing, wire harnesses, and avionics boxes than the forward skirt.

Liquid hydrogen (LH2) tank

The last welded structure to be completed will be the LH2 tank. During the media event at MAF on June 28, the second of five barrels was being staged for loading into the VAC to be welded to the already-welded forward dome and first barrel.

This hydrogen tank was a late replacement; development issues with VAC weld pins meant that the weld strength for the original Core Stage-1 LH2 tank couldn’t be verified. That tank had to be set aside and the panels for the Core Stage-2 LH2 tank brought forward.

Credit: NASA/Eric Bordelon.

(Photo Caption: The first barrel of the LH2 tank for Core Stage-2 is positioned in the VAC in mid-June prior to the first weld to the forward dome. The barrel is sitting in an in-feeder that allows the hardware to be positioned on the floor and up into the VAC for the circumferential, self-reacting friction-stir welding in the tool.)

All the panels for the Core Stage-2 structures were at MAF at the time in 2017, but the weld development problem meant that materials for an additional LH2 tank had to be ordered for the second flight article. The problem was discovered prior to welding the thicker LOX tank barrels and domes, so none of those subassemblies had to be set aside.

The last pieces from the new order to be completed were the end caps for the tank domes. Welding of the LH2 barrels in the VWC was completed last year, but putting the full tank together couldn’t start until the first end cap arrived and welding of the LOX tank was completed.

Arrival of the first LH2 end cap allowed the forward dome to be assembled in the Circumferential Dome Welding Tool from a set of gore panels, the end cap, and Y-ring. Ernst noted in late June that the second end cap arrived at MAF in the late Spring and the welding of the second Y-ring was complete, allowing assembly of the aft dome for the LH2 tank to be completed.

Credit: Philip Sloss for NSF.

(Photo Caption: The second LH2 tank barrel is lifted off a transportation dolly on June 28 to prepare it for loading into the VAC and welding to the top of the tank. The lift operation coincided with a tour through the facility.)

The aft dome will be the last weld in the VAC to finish the major welds there. The tank will be moved from the VAC into one of the stacking cells in Building 110 (either Cell A or Cell D) for initial post-weld work and then it will be rotated to horizontal for its next steps.

“It’ll come over here to Area 6, we’ll do the plug welds and then we’ll get that prepped for proof test,” Ernst said. LH2 tanks are tested pneumatically in Building 451 by pressurizing them with gaseous nitrogen, while loads are being simultaneously applied by a hydraulic test rig.

“So later on this summer we’ll get that out to Building 451 and we just started reactivating that because it’s been a while since we’ve done a hydrogen tank proof test so we’re in the process of getting that cell all prepped,” Ernst noted.

Similarly to the LOX tank, after proof test, the LH2 tank will be moved from Building 451 back to Area 6 for post-proof NDE, where the state of the welds is compared from before and after the proof test to verify they are good. The tank will then start to be fitted with operational and development flight instrumentation and be cleaned inside and out to prepare it for application of primer and SOFI in Building 131 Cells P and N, respectively.

Engine section

The engine section structure is a combination of welded and bolted structures. The barrel is welded together in the VWC from eight panels and then an L-ring is welded in the VAC to the top; that work was completed back in the Fall of 2017.

A bolted thrust structure sits inside the barrel when finished; the engines will be mounted to this structure on the bottom and thrust vector control (TVC) platforms with much of the hydraulic system equipment will eventually mounted on top. One of the changes from structural assembly of the qualification article that was sent to MSFC for structural testing in 2017 and the Core Stage-1 flight article was to have the vendor that supplied the thrust structure do the work of drilling bolt holes in it.

More work was done on the thrust structure and barrel pieces separately before bringing them together in the structural assembly jig. “We had the barrel kind of staged off to the side while we were doing the thrust structure work,” Jason Grow, SLS Core Stage Lead Propulsion Engineer for Boeing, said during a media event at MAF on June 28. “We did some over in just the structural area and then we brought it over into that tool and we actually started integrating some tubing really early on.”

In late May, the two elements were loaded into the jig. The thrust structure is loaded first and then the barrel is brought in and lowered down over it so the two can be bolted together. “A lot of those holes are pre-drilled, not a lot of match drilling like we did before, they’re just flying,” Grow added.

Once the barrel and thrust structure bolting is completed, then integration of all the equipment that goes inside will pick up. “We’re going to get into integration this year,” Grow said.

Credit: NASA/Eric Bordelon.

(Photo Caption: The barrel of the Core Stage-2 engine section is lowered down over the thrust structure in the structural assembly jig for engine sections at MAF in late May. The barrel and ring on top is welded, the thrust structure is bolted, and the two pieces are bolted together in the jig.)

“We’ve got some started already, we’re in major structural stuff right now, and then we’ll really get into integration work, which is the balance of tubing, wire harnesses, components like the TVC hardware, we’ll be doing that at the same time. That requires brackets and additional structural pieces to go together, so they’re really being built simultaneously.”

“Once the major structure is out of the way it kind of opens the flood gates,” he added. The engine section is the critical path for Core Stage-1 and is expected to be the same for Core Stage-2, with integration being the phase that took the longest on the first flight article.

Some spare parts for Core Stage-2 are being ordered with long leads for the third Core Stage, but the second build won’t have as many spare parts to work with as the first. “We don’t really have that luxury for Core Stage-2,” Shannon noted.

“For Core Stage-1 of course with the first build, if we found issues especially in avionics we would go modify the Core Stage-2 part and then install it in the Core Stage-1 vehicle and then take the Core Stage-1 parts and then have them modified for the Core Stage-2 vehicle. We [didn’t] do that real frequently, but sometimes when you needed to have a quick turnaround for a part like that.”

“Hopefully we’ve learned all the things that we needed to learn to not have to do that and we have looked ahead,” he added. For things like avionics boxes, Shannon also said: “we’ve got other options, things like qual (qualification) units available to us, development units, we’ve got two different avionics laboratories that we can go and get hardware if we absolutely had to go and do that.”

Lead image credit: NASA/Eric Bordelon.

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