NASA is now planning for the Space Launch System (SLS) core stage assigned to the Artemis II lunar flyby mission to be completed in mid-December for subsequent delivery to its Kennedy Space Center (KSC) launch site. The four RS-25 core stage engines were installed in the stage in September at the Michoud Assembly Facility (MAF) in New Orleans, and work is moving towards testing and checkout of the stage to complete final assembly.
Completion and delivery of Core Stage-2 was delayed from early in 2023 due to supply chain issues and core stage prime contractor Boeing is also dealing with a new weld tool issue at MAF that has delayed completion of the liquid oxygen (LOX) tank for the subsequent unit, Core Stage-3. Despite the extra obstacles, the SLS program still sets the completion of Core Stage-2 for late this year and wants to have Core Stage-3 complete in late 2024 or early 2025.
Core Stage-2 completion aiming for mid-December
Boeing and RS-25 core stage engine prime contractor Aerojet Rocketdyne, an L3Harris Technologies Company, installed the fourth and final engine in the stage on Sept. 20, less than two weeks after gearing up to install all four engines. The first engine installed was engine 2059, which was loaded into the horizontal engine installer at MAF on Sept. 8.
That engine was the first to be “soft mated” to the stage in the number two position on Sept. 11, followed by engine 2047 in the number one position on Sept.15, engine 2062 in the number three position on Sept. 19, and finally engine 2063 in the number four position the next day on Sept. 20. “It’s been some time since we installed the engines for Artemis I, there are some new team members that are working the install for Artemis II that were not there for Artemis I,” Jonathan Looser, NASA SLS Core Stage Design Team Lead, said in a recent interview with NSF.
“The Boeing and Aerojet Rocketdyne team have worked extremely well in doing dry runs and trainings and meetings leading up to this install to make it goes as smooth as it’s going.” Core Stage-2 is assigned to the Artemis II lunar flyby mission, which will be the first to take a crew of four astronauts to the Moon in over 50 years.
The engines were shipped to MAF from Aerojet Rocketdyne’s engine facility at nearby Stennis Space Center in Mississippi over a year ago in September 2022 and were originally prepped for installation early in 2023, but delays in putting together the core stage pushed engine installation until September. One of the major hold-ups this year was due to a liquid oxygen feedline segment that had to be reworked at a supplier.
Two large-diameter LOX feedlines, also called downcomers, run from the LOX tank at the top of the stage down to the engine section at the bottom. The downcomers are assembled from several segments and completion of the final downcomer was delayed until recently. “That’s a component that has experienced some delays at one of the sub-tier suppliers and we’ve known about this for several months and that component has been reworked,” Looser said.
“The issue that we experienced at the sub-tier supplier was both a non-conformance and also just a supply chain prioritization, where there are some other government programs that out-prioritized our hardware,” he added. “That was part of the delay and not just the rework itself, so a lot of the challenges that we experienced at our sub-tier suppliers are very similar to some of the supply chain issues that we’re seeing across the rest of the industry.”
“Subcomponent availability is a little bit tougher and some that is post-COVID, but it is a little bit tougher than it was say, three, four, five years ago.” The part has arrived at MAF and is currently being processed for installation. “It’s on the 180-degree side is how we reference it,” Looser noted.
While the late downcomer segment was being reworked elsewhere, some of the time waiting was used to address a possible issue with the stage’s LOX prevalves. The prevalves isolate the liquid hydrogen (LH2) and liquid oxygen propellants in the large tanks until SLS is ready to start the engines during the launch sequence; they are located in the engine section upstream of the engine inlets and there was an issue with the LOX prevalves earlier in the year.
Looser noted that the production team took the opportunity to work on that issue at MAF rather than possibly needing to do it later after the stage was shipped to KSC. “Over the last several months we have completed some additional troubleshooting/testing that exonerated those valves that are going to remove that potential [work],” he said.
“It was never planned, but there was the potential for some work that would have to travel to KSC and we’ve been able to exonerate that hardware and remove that from our potential travel list. The downcomer has been one of the major drivers in a little bit of our schedule slip, but we did have some prevalve work that we did in the engine section.”
The four RS-25 engines were initially soft-mated to the stage in September and one of the next steps was to fully integrate them with the stage. The stage provides all of the commodities and services that the engines need to fire, not just propellant, but electrical power, computer networking, hydraulics, and pneumatics.
The stage also uses its hydraulics to point the engines for steering, and the thrust vector control (TVC) pitch and yaw actuators have to be secured to the mounting points on each of them. In a follow-up email on Oct. 5, NASA public affairs officer Jena Strawn noted that the hard-mate of the engines was expected to be completed by the end of that week.
The remainder of final assembly will be testing and checkout of the stage, which is called final integrated functional testing (FIFT). “A majority of the work that is remaining for Core Stage-2 revolves around FIFT,” Looser explained. “By [the] first week of October, we should be ready to get into that final integrated functional testing which will be the last major hurdle before we start buttoning everything up and preparing to ship.”
“I mentioned the FIFT starting in early October, that gets us to a completion date around the middle of December and then once we complete and prep to ship there’s about a week of time it takes between that and arriving at KSC. So, we’re on plan and pressing towards the second week of December timeframe for completing Core Stage-2 and getting it ready to ship to KSC.”
Strawn also confirmed in the Oct. 5 email that the test readiness review for the FIFT was completed on Oct. 2, and the first test case was underway.
Core Stage-3 production working to overcome weld issue
Production of Core Stage-3 is currently in progress at both MAF and KSC. Beginning with the third core stage, final assembly is moving to KSC; the engine section was already transported there almost a year ago to be outfitted with all its moving parts and working equipment. That core stage unit is assigned to the Artemis III mission, planned to be the first crewed lunar landing since Apollo 17 in late 1972.
The rest of the stage, the upper “four-fifths,” is still under construction and assembly at MAF. The only remaining structure that has yet to be completed is the LOX tank; three of the four pieces of the tank have essentially been stuck in the Vertical Assembly Center (VAC) throughout 2023 waiting for the final piece.
The forward dome and two barrels were welded in the VAC in the summer and fall of 2022, but an unspecified issue welding an aft LOX tank dome has stalled the overall completion of the tank. The propellant tank domes are welded together from a gore section, end cap, and ring in a Circumferential Dome Welding Tool (CDWT).
“We’ve been going through some weld issues on the LOX aft dome for the last several months,” Looser acknowledged. “I don’t want to get into too many of the technical details of that as Boeing is still working through and completing the root cause and corrective action, but I will say that we are nearing the end of that and on a path to resuming welding on the LOX aft dome.”
“The forward two-thirds of the LOX tank is in the VAC, waiting on the LOX aft dome, and that team is working as we speak to get back into welding to be able to complete that aft dome and complete the LOX tank.” The issues have also kept the VAC from being used for welding other Core Stage and Exploration Upper Stage structures.
The five main elements of the stage are, from top to bottom, the forward skirt, the LOX tank, the intertank, the LH2 tank, and the engine section. Besides the LOX tank that is waiting for the resolution of the aft dome/CDWT weld issues, the other three structures have already moved into integration.
The LH2 tank is actually 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 for the first Core Stage exhibited defects that took several months to recover from.
After the development of repair techniques, the tank passed its proof test after a few areas needing additional work were found in an initial proof test. “It’s also completed all the post-proof test X-rays and other NDE, non-destructive evaluation, of the tank,” Looser added.
“All of those results from those scans are positive and we are moving forward with the hydrogen tank in production. There’s been a lot of work that has gone into proving the flight worthiness of that tank and that is now in the flow as a flight article and moving forward with getting ready for the next step, which would be Cell E.”
The interior of the tank will be washed and cleaned in Cell E in preparation for the outfitting of equipment such as propellant level sensors. Even with the issues holding up the LOX tank, this LH2 tank was ahead in the production process and Looser confirmed that plans are for it to go through the exterior application of primer and spray-on foam insulation (SOFI) before the LOX tank, rather than after.
The other two elements of Core Stage-3 are being outfitted with their internal equipment, which includes wire harnesses and brackets for connecting and holding computer and spaceflight equipment. “Intertank and forward skirt are both progressing to plan and those are both moving towards a forward join of next summer, late summer time-frame,” Looser noted. “Those teams are both executing at a high level and no issues that we’re working there.”
The forward join bolts the intertank, LOX tank, and forward skirt together and then begins the process of connecting the equipment between the three elements.
The engine section is the most complicated of the five elements and was the first one started for the third core stage build. Integration of the element began after it arrived at KSC and was set up in the Space Station Processing Facility (SSPF) last December.
Orbital tube welding and installation of wire harnesses will highlight the earlier phases, followed by installation of subassemblies like the five, large helium bottles and the four TVC platforms that hold most of the hydraulic systems equipment. Avionics and the large propellant feedlines will also be added internally, along with the addition of the thermal protection system on the exterior.
The boattail structure will also be shipped to Florida for attachment to the bottom of the engine section next year. “Engine section integration in the SSPF is going well, the boattail is scheduled to ship to KSC in spring of , then those will be mated together in the SSPF and that engine section/boattail assembly will then move over to the VAB (Vehicle Assembly Building) High Bay 2 in fall of next year, waiting on the top four-fifths to come from MAF around that same time and be mated.”
Under the new production plan, integration of the engine section and boattail will be completed in the SSPF, and the upper four elements will be attached and integrated at MAF. As Looser noted, final assembly of the third core stage will now be completed in the Vehicle Assembly Building at KSC in a vertical production cell rather than horizontally as it was done with the first two units at Michoud.
The Michoud assembly of the upper four elements will continue before that large subassembly is shipped to Florida. First, the forward join will be completed, followed by mating of the top of the LH2 tank to the bottom of the intertank. Given its delays, the progress of the LOX tank could be the pacing item for the forward join next year.
Before it is ready to be mated to the intertank and forward skirt, the LOX tank structure needs to complete welding, proof test, primer and SOFI applications, and internal outfitting of sensors and its sump assembly. Following the mating of the forward join and the LH2 tank, the equipment inside the four-fifths assembly will be interconnected, along with the installation of external equipment such as the systems tunnel and LOX feedline sections.
“That plan for the top four-fifths aligns with the completion of the engine section in late fall of next year and the engine section should be moving over a month prior to it and so that is on plan for late 2024,” Looser said. “Core Stage-3 completion is on track to be done at the end of 2024 is the contractual date. Boeing is on plan to that and that time-frame supports the KSC need for Artemis III and we’re still tracking to being able to deliver a fully completed Core Stage-3 prior to the stacking need date by KSC.”
Along with the contractual date of the end of 2024, the aspirational December 2025 launch date for Artemis III would put the need date somewhere in the first half of 2025; one NASA presentation earlier in the year also projected completion of the stage in “early 2025.” Vertical final assembly at KSC will also provide more flexibility for Boeing since major “joins” of the stage were originally planned to be in a vertical orientation.
“One of the benefits of doing engine section integration at KSC and then the vertical mate of the engine section to the hydrogen tank is the ability to have continued access up and down the vehicle, including inside the engine section,” Looser noted. “If there is a late part or rework that needs to be done, we do have access into the engine section while we are mating the hydrogen tank to the engine section and we’re able to do that work in parallel and not lose access when the vehicle is horizontal.”
“Those internal platforms can remain in the engine section and allow us to do work, if needed, but the plan is to complete all of the integration work prior to moving over the VAB.” After the upper four-fifths are stacked vertically on top of the engine section in the VAB, the four RS-25 core stage engines will also be installed vertically in the VAB.
After final installations and integration work across the stage is completed, the same integrated functional test and checkout will be performed on the stage, along with final exterior detailing work, before the unit is complete and ready to hand over to Exploration Ground Systems for Artemis III launch processing. This production will be the first time many of these operations are performed in a new environment with some new equipment and procedures, so those may be factors in how close completion of the stage is to the late 2024/early 2025 target.
Boeing and NASA are also working on the structures for the fourth core stage at MAF. Looser said that the engine section structure for Core Stage-4 is now expected to ship to KSC in the spring of 2024.
(Lead image: Core Stage-2 is seen at MAF on Sept. 21 after all four RS-25 engines were installed. Credit: NASA/Eric Bordelon.)