As NASA pushes out the schedule for Artemis II and III, development of the first major upgrade to the agency’s Space Launch System (SLS) rocket is moving into qualification. SLS Stages prime contractor Boeing is activating production areas for the new Exploration Upper Stage (EUS) at the Michoud Assembly Facility (MAF) in New Orleans in parallel with refining assembly and outfitting techniques for the stage.
EUS is the major piece of the new SLS Block 1B configuration, which NASA currently plans to debut on the Artemis IV mission at the end of 2028. Boeing was working to finish validating the final weld schedules on weld confidence articles once the last weld tools shared with core stage production becomes available; after that, work will proceed into assembly of the EUS structural test article (STA), with hopes of completing that next year.
Preparing to build EUS structural test article, qualify systems
“We’re in that preparation for qualification [phase], whether you’re talking components and subsystems or structures here at MAF,” James Savage, Boeing EUS Chief Engineer, said in an interview during a recent NSF visit to Michoud. “We’re building the structural test articles today, both in the supply base and here at MAF. In our subsystems and our components, all those kinds of things, we’ve had a few go through qual now, others are preparing for qual.”
“As far as what we actually here at MAF, a lot of this is STA hardware,” Eric Potter, Boeing MAF Deputy Site Lead and EUS Integrated Product Team Lead, added. “Over in the VWC, we have the flight aft adapter is getting ready to start there so we are working the flight article, as well.”
The VWC, or Vertical Weld Center, is the weld tool at Michoud; it uses friction-stir welding to assemble the 8.4-meter-wide barrels used on most SLS propellant tanks and “dry structures,” such as the core stage engine section and adapters that connect the Exploration Upper Stage and core stage.
“It’s actually exciting, because we’re to the point now where you [are working through] like a pre-flight checklist,” Erick Holsonback, SLS EUS subsystem manager for production and launch operations with Jacobs, noted. “Yes I’ve got the tooling, yes the facilities are there, I’ve got all my engineering released, I’ve got hardware coming in, I’ve got work instructions being released. So it’s in that right-before-execution phase of trying to make sure all this stuff is in place to go ‘we’re ready.’”
The major change with the SLS Block 1B vehicle is with EUS replacing the Block 1 Interim Cryogenic Propulsion Stage (ICPS). The new stage employs four RL10 engines versus the single RL10 engine on the ICPS; in addition to the four RL10 engines, the stage consists of the two liquid oxygen (LOX) and liquid hydrogen (LH2) propellant tanks, the mid-body assembly that connects the tanks, an equipment shelf that houses avionics and propulsion system elements, a thrust structure that the engines attach to, and a forward adapter.
(Photo Caption: An expanded view of the SLS Block 1B vehicle currently in development. The EUS upgrade includes new adapters to connect the bigger stage to the booster elements below and the payloads above.)
The bigger upper stage uses the same 8.4-meter diameter as the core stage, so Block 1B also has new connecting stage adapters; Boeing is also prime contractor for the interstage that connects the EUS to the core stage below it. Dynetics is the prime contractor for the Universal Stage Adapter that will connect EUS with Orion and also provides room for a large, “co-manifested,” secondary payload.
NASA and Boeing are currently working to complete the final tests of the weld tools they will use at MAF and at Marshall Space Flight Center (MSFC) in Huntsville, Alabama, where the SLS Program is based. Most of the friction-stir weld tools at MAF are used to weld multiple structures for the core stage and EUS, each with their own set of configuration parameters.
Full-scale weld confidence articles are used in the tools to verify they are ready to proceed into welding the structures for the STA and subsequent flight articles. “It’s the combination of the tool and the weld, so for example if we use the same weld on three different pieces of the vehicle, you only do one weld confidence article,” Savage explained.
As noted earlier, the VWC has already completed its development work and Boeing has started producing EUS barrels. The Segmented Ring Tool (SRT) and the Gore Weld Tool (GWT) have also completed their weld confidence article runs.
In addition, a few new welding tools for EUS are now being qualified. The EUS LOX tank is a smaller-diameter than the LH2 tank, two new tools are used to weld that, one at Marshall and one at Michoud.
Those two tools, the LOX Dome Weld Jig (LDWJ) at Marshall, and the LOX Tank Assembly Center (LTAC) at MAF, have completed their welds for that one weld confidence article. “On the LDWJ, which is currently sitting up at Marshall, that’s where we do the LOX dome to ring welds, and then we bring it here for the LTAC, which is in [Building] 115 on the back left side, that’s where we do the two rings together,” Potter explained.
“And that’s a single article, that’s where it gets hard to count,” Savage added. “You have two different welds, but a single article.”
“We actually just finished the weld confidence article on the LOX [tank] and if you go down [to Building] 115 you can actually see that,” Potter said. “They machined the ring and so there’s only three welds on that entire tank, the two rings to the domes and then we do one weld of the two rings together and that’s all the welding that’s done on that entire tank.”
“That’s a big plus because you don’t have all the gore panels and everything else that’s a challenge,” he added.
(Photo Caption: The EUS Gray Box Assembly Area at MAF. Hardware for the structural test article can be seen in the lower right foreground, with the tooling in the background to build the elements of the stage. From left to right, tooling for the LH2 tank, LOX tank, and mid-body assembly. Additional tooling for the forward adapter is out of view behind the mid-body assembly tool.)
A second circumferential dome welding tool (CDWT), also called the Universal Weld Station (UWS), was also built at MAF to weld the domes for the LH2 tank. One of the reasons for the second LH2 dome welding tool at MAF is that EUS is using an aluminum-lithium alloy for its structure, versus the aluminum alloy used for most of the core stage.
“Most of the structure of EUS is aluminum-lithium 2050,” Savage noted. “Obviously we did that for weight savings because it’s higher strength and lower density, so you get a great one-two punch for mass savings and performance for the upper stage.”
“That has been an exciting change, but it also means that a lot of things, you’re not just straight reusing core stage effort, we’re having to do additional development to prove out those processes, the welding, the machining, all of those things on the 2050. So far it’s been exciting, but it’s been very successful in saving a lot of mass and getting us a good lightweight, high-performing stage.”
Summarizing the status of the weld confidence welds, Potter said: “We’ve already been through the Gore Weld Tool, that’s already been through all of its testing, so once we get to that point, we’re ready to go into production there. We’re in the process of doing it on the UWS, we did it in the VWC already, and then the last one we’ll have to do will be in the VAC. And we did on the SRT, as well.”
“The [LH2 tank] dome welding and the VAC (Vertical Assembly Center) is what’s left and we’ve got a few plug welds that we’re still testing out,” Potter added. The weld confidence article is testing the parameters in the UWS to weld LH2 tank domes.
“We’ve already done the ring to gores and now they’re in the process of doing the trimming to do the cap to the gore weld,” Potter noted. Then that dome will be loaded into the VAC with a barrel already completed in the VWC and an L-ring from the SRT to test the two types of welds in the VAC that will be used for multiple elements.
“We’re going to do a dome to barrel weld and then we’re going to also do an L-ring to barrel, so then we’re going to get both of those [VAC welds] out of the same [confidence article],” he added. The VAC will weld L-rings to the forward and aft adapters and to the interstage for EUS.
Beginning work on other STA structures while waiting for VAC availability
After the VAC welds are completed, coupons will be cut out of each of the welds for an extensive verification process. “It’s about a three-to-four-month process,” Potter explained. “We do a lot of cutups [for verification], we do tons of cryo testing and stress testing to make sure that those welds are going to be sufficient before we actually start welding a STA vehicle.”
That process had started for the LOX tank weld confidence article, with several large coupons cut out of the article at the time of the NSF visit to MAF in October. What appears to be a fully welded dome can be seen in the background of a recent picture taken of a core stage production milestone at Michoud in early December, which could indicate its readiness to move on to the VAC weld tests.
For the time being, those EUS weld confidence tests in the VAC were waiting for the tool to become available. A partially completed Core Stage-3 LOX tank has been hanging in the tool for over a year waiting for an aft dome.
NASA and Boeing have been working through the issues on other weld tools with completing the dome, but in the meantime the partial structure had to remain in the tool, which also meant that it was unavailable for welding other structures. A core stage LOX tank aft dome was successfully welded near the end of 2023 and at year-end was being prepared to make the last weld for that tank and open the tool up for other work.
The EUS weld confidence article is first in line. “As soon as the [Core Stage-3 LOX tank] comes out, we’re next,” Potter said. “There is a calibration schedule that it’s going to have to go through, but after that we’re next in line.”
(Photo Caption: What appears to be a completed LH2 tank dome is seen behind the American flag in this cropped image taken by NASA in December. The dome is a piece of one of the final EUS weld confidence articles to be welded; when the large VAC tool becomes available, the dome will be welded to a barrel and a ring. Following analysis of all of those welds, the EUS welding tools should all be ready to begin welding flight hardware.)
In February 2023, the EUS Gray Box Assembly Area was opened at MAF; located in the middle of Building 103 at the factory, Boeing will assemble and outfit the main elements of the upper stage before they are moved to the high bay in adjoining Building 115 for final assembly. Tooling and platforms for the propellant tanks, mid-body, and forward adapter are set up on the floor, waiting to begin processing of assembled structures.
Some of the structural elements are not welded and some work on those has started in the Gray Box in parallel with completion of the weld confidence work.
The equipment shelf and thrust structure will be assembled and outfitted in the EUS assembly area; assembly of the thrust structure unit for the STA was underway at the time NSF visited Michoud in mid-October.
“We’ve just started construction on our thrust structure, it’s the first article that we’ve actually been able to build,” Potter said. “This is the only structure we have that doesn’t depend on the weld centers. This is where the four engines will mount to.”
“We’re using our FSDA (Full-Sized Determinant Assembly) process, it’s basically pre-drilled holes that line up and we just went through this entire process and all four beams went in perfectly, they’re all pre-cut at the supplier, everything fit perfect, and it’s a fully built structure.”
“It’s a real success story because we at NASA were anxious how this FSDA process would work, because this the first time that this goes together and when you have big metal parts that get bolted together you often have to “ream” holes because things aren’t lined up exactly, but it went together wonderfully,” James Burnum, Deputy Manager of the NASA SLS Block 1B Development Office, said.
“We had to put in one shim, it was a three-thousandths shim that we had to put over in that one corner, everything else lined up perfect,” Potter added. “It was amazing how well it came together.”
(Photo Caption: A weld confidence article to validate welds for the EUS LOX tank sits in Building 115 at MAF during a recent visit. Large, rectangular coupons were cut out of the completed welds as part of the months-long analysis to verify they meet requirements and specifications.)
The mid-body includes the aft adapter and struts that structurally connect LOX tank below to the LH2 tank above; it also is where the helium bottles for the stage’s pneumatic system will be located. Boeing has received the struts for the STA and was planning to get started installing some the thousands of test instrumentation sensors that will eventually cover the test article.
“We’re getting ready to do some get-ahead work on our B-struts, that will eventually be built over on the mid-body stand,” Potter said. “The areas that aren’t in a pinch point or are going to have a lifting area, we’re going to go ahead and put those strain gauges and other instrumentation on there.”
“There’s over 3000 strain gauges and other bonded instruments, so where we have the opportunity we’re going to go ahead and start installing those to get ahead. As soon as they get all these brackets on within the next couple of weeks, we’ll start to put strain gauges on the thrust structure as well.”
Path to Green Run, first flight
Both the STA and flight articles will be assembled and tested in preparation for the first EUS/Block 1B flight on Artemis IV. In contrast to the four core stage separate structural test articles for the different elements, the STA for the shorter EUS will be a single, integrated structure.
The STA will be the first one completed, which is currently forecast for next year; it will be transported to Marshall and installed in Test Stand 4693 for that test campaign. The STA will have identical structures to flight articles, but it won’t include functional stage components like engines and avionics.
The first working EUS article will be the first flight stage, which will be shipped to the Stennis Space Center in nearby southern Mississippi for a Green Run design verification test campaign similar to the one that the first flight core stage completed at the beginning of the decade. Like the first core stage, the first EUS will be installed in the B-2 Test Stand at Stennis for months of testing and checkout, which will culminate in a hot-fire test of the stage.
Following completion of the Green Run campaign, the flight article will be transported back to Michoud. Special sea-level RL10 engines are being provided by Aerojet Rocketdyne, an L3Harris Technologies Company, for the EUS Green Run; following those tests, the stage will be refurbished for flight and those engines will be removed and replaced with the four flight RL10s. Once that work is complete, the stage and interstage will be transported to Kennedy Space Center for launch preparations.
(Photo Caption: The barrel of the aft adapter flight article being assembled in the Vertical Weld Center tool in Building 115 at MAF. The VWC has already been qualified for EUS welding and now serves both EUS and core stage projects for NASA and Boeing.)
Validation of the last weld tools is still in front of the team, but a lot of the STA hardware is at Michoud, waiting for assembly. “Structure-wise we’ve got almost everything except for some of the dome caps,” Potter said. “The strain gauges are all here. I’d say it’s probably up in the 75-80 percent level.”
“You’ll see the simulators that go on either end of the structural test article are out here in work,” Savage added.
Building the flight article will include all the integration work of the different subsystems, including the avionics, main propulsion system (MPS) and reaction control system (RCS), and all the associated tube welding, wiring installation, and testing that go along with them. The equipment shelf will house the avionics for not only EUS, but the SLS flight computers and inertial navigation system for Block 1B.
It will also house a lot of MPS and RCS components, making the equipment shelf the most complicated element of the stage. A low-fidelity simulator will stand in for the equipment shelf on the STA, but Boeing has put a lot planning effort into its assembly and pre-launch maintenance.
A mockup of the equipment shelf was built to help familiarize and train personnel for hands-on work, which also allowed the detailed design and beyond to be revised with improvements. “One of the things we learned from core stage was that your big challenges come in tight packaging areas, high levels of integration in very small volumes,” Savage said.
“The equipment shelf, because it’s not part of our structural test article, is not one of the things we were going to build early and so we said [that] we don’t want to learn too late and on the flight article, so let’s go build a full-scale mockup and start learning. The first use of this was for the design integration, so we actually had the designers, design interns, design engineers actually building this thing as they’re designing the actual flight article and learning as they go.”
(Photo Caption: A Boeing graphic about the mid-body assembly tool, showing elements of EUS on the left. From top to bottom, the forward adapter, LH2 tank, mid-body, LOX tank, equipment shelf, thrust structure and RL10 engines, and the interstage.)
“We have a long list of lessons learned from having done this physically, things you just can’t catch in a CAD model, so that was the first learning,” he continued. “The second learning was we had our manufacturing engineers actually help assemble this thing and start working through the planning of doing the assembly and all the routing.”
“One of the things you don’t see on [the mockup] now is [the equipment shelf] is covered in wire harnesses for flight and so the first time we assembled this we [did] it at Marshall and the technicians were out there routing harnesses and making changes as we go to the design based on what they were learning during that process. [Changes were also made to] tooling and other things as we did this.”
“[After] that we said how else can we use this asset. And that’s when MAF said we would love to use this as a training aid, we would love to use it to test out some of our model-based instruction processes, so we disassembled it — learning there — [and] had them assemble it and then using it as a training aid and a workshop tool in the meantime.”
Boeing noted that personnel from Exploration Ground Systems (EGS) and Jacobs, their prime contractor for launch processing, had also started using the mockup for familiarization and training during visits to Marshall and Michoud.
Currently, Boeing is forecasting that the beginning of equipment shelf assembly could happen this year, but a lot of parts and machines that will eventually be outfitted on the element are still working their way through qualification. “All of the avionics parts are all going through a qual program, so the parts aren’t here to start installing them [yet],” Potter said.
“All the hardware is in flow in the supply base, so the individual pieces of structure, all the avionics boxes are in development right now,” Savage added. “Several of them are into qual and into acceptance testing ahead of qual, so all the pieces are out there in the supply base and working their way here.”
“We got a RCS system full qualification test for the whole system as well as the helium system that pressurizes it, and then we also have hot-fire tests for the thruster pack. There’s a whole cadre of building block tests for the RCS and then the individual components will all go through acceptance and checkout and demonstrate their capability before integration into the stage.”
(Lead image: Exploration Upper Stage flight and test hardware in Building 115 at MAF during NSF’s recent visit. Credit: Philip Sloss for NSF.)
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