In addition to supporting the upcoming first launch of NASA’s new Space Launch System (SLS) rocket on the uncrewed Artemis 1 lunar test flight, Stages prime contractor Boeing is lining up the rockets that will support following missions. That work, taking place at the Michoud Assembly Facility (MAF) in New Orleans, is now expanding to include structures for the third and fourth Core Stages as well as the first Exploration Upper Stage (EUS).
The lingering COVID-19 pandemic is still affecting internal SLS schedules; recently, a supplier for internal parts critical to finishing the engine section for Core Stage-2 was closed for months. Boeing still has several months of margin remaining to meet NASA’s Artemis 2 need date, and they have reorganized the production sequence to make better use of time, freeing up parts of the workforce to start EUS manufacturing and assembly of the Core Stage-3 engine section structures.
Core Stage-2 production sequence adjusted for COVID supply chain snags
A lot of attention is currently concentrated on the first integrated SLS vehicle mated at the Kennedy Space Center for Artemis 1, but NASA’s SLS Program is simultaneously working to complete the second vehicle and build the third and fourth. The Core Stages for Artemis mission 2 through 4, and the EUS for mission four, are being manufactured and assembled at MAF.
The Artemis 2 mission is planned as a crewed test flight, and the first to bring a non-American to the vicinity of the Moon. It will be the second of three SLS Block 1 vehicles that will launch Orion spacecraft. NASA’s FY 2022 budget request released on May 28 projected the launch readiness date for Artemis 2 as no earlier than September 2023 due in part to Orion computer hardware dependencies between Artemis 1 and Artemis 2 spacecraft. NASA would need the Core Stage in Florida to begin launch preparations about six months before that.
“Our need date for the Core for [Artemis 2] was in March of ’23,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said in a July 13 interview. “If Artemis 1 moves off of November , I think that [Artemis 2 need] date might move. But we haven’t been notified of anything like that, and we’re well ahead of that March ’23 need for Core Stage-2. So we’re trying to stay on top of it.”
(Photo Caption: The boattail assembly for Core Stage-2 is prepositioned in Cell A of Building 110 on June 9 with the forward join in the background in Cell D. The boattail is a fairing and base heatshield structure on the bottom of the engine section barrel; the four large engine holes indicate where the powerheads of the RS-25 engines are located when installed. The boattail was moved to Cell A to be mated to the Core Stage-2 engine section, an operation that has since been completed.)
Prior to the COVID-19 outbreak early last year, Boeing had even more schedule margin, but some was lost due to the lingering pandemic. “It is remarkable to me with all the COVID impacts we’ve had, especially at the suppliers and especially, especially at the suppliers in California, that we’ve been able to reset the production sequence to account for those parts coming in later than expected and still be able to make the March of ’23 need date for the Core,” Shannon said.
He added that deliveries of a few hydraulic system parts were delayed by COVID, but “the biggest impact to us has been the big LOX (liquid oxygen) feedlines in middle of the engine section.” Two large diameter feedlines run hundreds of feet in length from the LOX tank at the top of the stage down to the bottom where the engines are mounted in the engine section.
The feedlines are assembled from several sections of tubing, including pieces connected together within the engine compartment where the two large lines that come from the outside fork to four feedlines on the inside to supply the cryogenic oxidizer to all four RS-25 engines. “The team that makes those out in California was out for quite a while,” Shannon said.
“They’ve come back. We’ll have the last of them in by the end of this month, and that’s primarily the biggest thing that has slowed us down on the engine section.” The sections of tubing are needed to complete outfitting of the engine section, and they require a lot of lead time at MAF to prepare them for installation.
The RS-25 engines are Space Shuttle Main Engines (SSME) adapted and repurposed for SLS; the Core Stage more efficiently delivers LOX from the propellant tank to the engines, which requires heaters on the feedline sections inside the engine compartment to condition the colder LOX to within the temperature range of each engine’s “start box.” All the feedlines also need thermal protection system (TPS) foam as insulation from the hundreds of degrees of temperature difference between the ambient temperatures inside the engine section and the cryogenic temperatures of the propellant.
“You have to condition that LOX to a fairly specific temperature range, and so we end up bonding on quite a few heaters on those LOX feedlines before they’re covered with the TPS,” Shannon explained. “It’s a fairly extensive process, and they’re right in the middle of the engine section.”
With that late start to prepping the feedlines for installation inside the engine section, Boeing and NASA reviewed the remainder of the production sequence for the Core Stage-2 build and reorganized some of the work to make better use of the time. “With all the COVID stuff going on, we’ve done a lot of what we did on Core Stage-1, and that’s resetting our production sequence consistent with when the parts become available,” Shannon said.
One of the major changes to the engine section build sequence was to mate the boattail to it early, which was just completed. The Engine Section was moved from its integration area in Building 103 to Cell A in Building 110 for stacking with the boattail. “We’ve [already] removed it from Cell A and taken it back to the integration area,” Shannon said. “It went together really well. No issues at all.”
In the original Core Stage final assembly build plan, when standalone engine section integration was complete, it would be moved to Cell A in Building 110 where it would first be joined with the boattail and then the huge liquid hydrogen (LH2) tank would be brought in and stacked vertically on top. When the final assembly plan for the first Core Stage was changed in early 2019 to mate the engine section to the LH2 tank horizontally, one of the new tools built by Futuramic for Boeing and NASA was an engine section boattail transportation tool.
For Core Stage-1, engine section integration was completed in March 2019, the boattail was stacked vertically at the end of that month in Cell A, and the mated duo was then lifted into the new tool for a few months of integration work before they were rotated from vertical to horizontal in September 2019 and mated to the rest of the rocket.
For Core Stage-2, Boeing and NASA advanced the engine section to boattail mate work, and they are now sitting on the transportation tool, which allowed that integration work to start early.
“We found a really good time to go put the boattail on, and then we’ve got a boattail access kit and it just gives you an opportunity to do the lower part of the engine section wiring integration and avionics and sensors and all that stuff while we’re waiting on the LOX feedlines to show up,” Shannon said.
“We looked at it and said ‘why don’t we put the boattail on early and open up this work while we’re waiting?’ That will put us ahead of the game as we go, and after doing it this time I think we’re going to make it a permanent change in how we build the engine sections — to have that early boattail integration just to allow the team to work on multiple levels inside the engine section.”
(Photo Caption: The forward skirt for Core Stage-2 is lifted into place on top of the liquid oxygen (LOX) tank in late May to complete forward join structural mating. The forward join assembly remains in Cell D in Building 110 at MAF while outfitting goes on both inside and outside.)
Shannon said the current schedule has the engine section work being complete in June 2022, but now instead of moving to Cell A for boattail stacking, the engine section and boattail integration will already be completed, which will allow them to be rotated from vertical to horizontal and mated to the rest of the stage. “Once those [LOX feedlines] come in [at the] end of this month, then we’ll be able to prep them for installation. And that’ll have us delivering the rocket some time in probably the third quarter of next year.”
Work on the rest of the stage is ahead of the engine section. “Core Stage-2 is in three big parts now,” Shannon said. “[The] forward join — the forward skirt, LOX tank, [and] intertank — are all together in Cell D.”
“We just took the engine section with the boattail out of Cell A, and it’s back in [the integration area]. And then you’ve got the LH2 tank, which is over in Cell N where we spray the TPS [foam] and it’s got its domes sprayed and it’s undergoing trims right now. So it’s just in three big parts.”
Following friction-stir welding, the aluminum-alloy metal exteriors of the liquid oxygen and liquid hydrogen tanks are sprayed with a coat of corrosion-protection primer and spray-on foam insulation (SOFI) in two cells of Building 131 at MAF, which adjoins the main industrial complex of buildings with Building 103 in the center. A custom cryogenic propellant rocket tank primer is applied first in Cell P. Then, following interim work, the SOFI is sprayed on the tanks in Cell N.
NASA and Boeing have now automated both parts of the large-scale SOFI application; automation of the long tank barrel sprays was completed for the first flight articles, and development of the robotic sprays of the two domes was completed for the second set. The LOX tank completed its SOFI applications in November 2020, and the LH2 tank has now received all of its automated SOFI sprays in Cell N.
Before the tanks leave Cell N, another automated procedure machines a section of foam along the length of the barrel where the systems tunnel base plates are attached. Trimming is also done at this point. Examples of areas where the foam needs to be trimmed include around external protuberances such as development flight instrumentation (DFI) sensor islands and brackets for the LOX feedlines and propellant tank repressurization lines.
On Wednesday, @NASAStennis operators conducted an RS-25 engine test for future SLS flight and #Artemis missions. The engine fired at 111% of its original power level for a set time — and at 113%, which allowed operators to test a margin of safety. MORE >> https://t.co/bzbfFgwYzx
— NASA_SLS (@NASA_SLS) July 15, 2021
The LH2 tank is currently finishing up this work.
Several locations on the tanks where external equipment and structures will eventually be installed are masked off prior to the sprays to keep foam out.
Looking ahead, “We’ll have the four-fifths of the rocket as we say, which is the LH2 tank and the forward join all together, we’ll start that in October and be finished really with the top four-fifths of the rocket by early next year in the January/February time-frame,” Shannon noted.
Workforce moving between different stage builds
Given the changes to Core Stage-2 parts deliveries and schedules, some of the workforce at Michoud is focusing on the other production builds in work. “We will surge onto the [Core Stage-2] engine section whenever the parts are there to be able to do that, but while we’re waiting there’s [work] in putting Core Stage-3 together. The engine section integration has started, and then we’ve got work on other pieces for [Core Stages] three, four, and five,” Shannon said.
“But also EUS is really coming together, so we’ve started taking a significant number of our design engineers, manufacturing engineers working to prep and feed those lessons that we learned in the first Core Stage build to getting started right on the EUS. We’re going through [an EUS] manufacturing readiness review, and we’ll begin production of the structural [qualification] unit for the EUS this October.”
The Core Stage engine section structure is part welded, part bolted. The barrel is put together by friction-stir welding of eight panels and a ring with an L-shaped cross-section on top.
The thrust structure, which is where the RS-25 engines are mounted near the end of the build, is bolted together as a subassembly before the barrel is lowered over it. Over two-thousand bolts are then used to fully connect and reinforce the barrel and thrust structure, which is also where the aft Solid Rocket Booster attach struts are located and tied into the overall mated SLS structure.
(Photo Caption: One part of the Core Stage-3 engine section structure, the barrel, is lifted out of Cell G in Building 114 at MAF on April 1. Not seen here, the thrust structure where the engines are mounted arrived at Michoud, has been assembled, and mating of it to the barrel was expected to be started in the next few weeks.)
While the Core Stage-2 production sequence was being reset, the thrust structure elements for Core Stage-3 arrived at MAF early in 2021 and were bolted together. Integrating the barrel with the thrust structure is expected to start in the next few weeks.
Core Stage-3 is the first build under the new “Stages Production and Evolution Contract” that was initiated in 2019; the contract is not yet completely finalized, with the latest estimate for definitization being early in Fiscal Year 2022 (which begins on October 1st, 2021).
Beginning with the third build, major production is starting with the engine section. The two, large propellant tanks dominate the real estate of the Core Stage, but the three “dry” sections of the stage contain all of the equipment, from the rocket engines to the other high-energy moving parts to the computers and networking.
The engine section is the most complicated element, and it’s starting ahead of the rest of the pieces in the builds getting underway. “The engine section is by far the most complex. The next certainly would be the intertank, and then the forward skirt if you just look at avionics and wiring and [hazardous] gas sensors and all those kind of things,” Shannon said.
“The tanks are fairly simple. They have mass in them to do the gauging, baffle plates, [and] things like that, but we have pretty high confidence in all of that.”
As the Core Stage builds begin to overlap more with each other, there will eventually be several engine sections in build-up flow together. “Our vision is to have four or five engine sections all lined up in a row, all being worked on, in different stages of production completeness,” Shannon said.
“We have certainly found that while none of it is easy, it is much quicker to put the large-scale tanks together now that we have all of the tooling really dialed in and we’ve got the robotic TPS application [demonstrated]. The level of complexity in the engine section compared to the rest of the rocket is, it’s way more complex.”
“So lining those [engine sections] up, getting those completed, and then attaching the other four-fifths of the rocket is the way to do this build,” Shannon observed. Speaking of the Core Stage-3 engine section structural assembly, Shannon said: “I just thought it was really cool that [when that’s completed] we’d have an engine section with the boattail on it for Artemis 2, sitting there with an engine section with the thrust structure in it for Artemis 3.”
“I’d like to have two more of them ready to go,” he said of the future. “The good news is that the parts for [Core Stage] three and four, barring any additional pandemic impacts, look like they’re right on track to support our need dates and our production dates. Even though we got this pretty big upset about some of our major suppliers not being there for several months, it looks like things are smoothing out now.”
Boeing is also getting ready to begin initial production of the hardware needed for the first flight of the EUS, now targeted for Artemis 4. The combination of the EUS with the Core Stage and Solid Rocket Boosters forms a new Block 1B configuration which is also in development as an integrated launch vehicle.
Just like with the Core Stage hardware that’s been tested over the last four years ahead of Artemis 1, two identical structures will be produced for the EUS, a structural qualification article and the flight article. The qualification articles will be assembled into a structural test article (STA) that will be transported to the Marshall Space Flight Center in Huntsville, Alabama, for a structural test campaign.
The first flight article will be assembled into a working upper stage rocket, with an SLS flight control computer system, attitude control equipment, main propulsion system, and eventually four RL10 rocket engines furnished by Aerojet Rocketdyne. Before its flight, it will be transported to the Stennis Space Center for a Green Run design verification campaign like the Core Stage just completed.
Shannon noted that initial EUS production benefits from the years of groundwork established by the first Core Stage builds and all the lessons learned. “It’s way easier on EUS than it was on Core Stage-1 because the tooling is already in place and the teams are already familiar with it; the software is in place and the teams know how it works,” he said. “We’re starting with a modern manufacturing system.”
(Photo Caption: The liquid hydrogen (LH2) tank for Core Stage-2 is parked in the Building 110 Transfer Aisle in late April in the middle of forward join activities in Cell D. The tank was temporarily moved from the Core Stage final assembly area in Building 103 into Building 110 before entering Cell N in Building 131 for the application of spray-on foam insulation that has since been completed.)
“NASA has done a really nice job of upgrading MAF to support the EUS, so there is plenty of work going on at Michoud. As we’re waiting for the Core Stage-2 parts and have that gap before we can rotate the engine section and join it to the rest of the four-fifths of the rocket, the team will be extremely busy building follow-on parts of the rocket and preparing for the first EUS builds.”
In preparation for the start of EUS production work, design, manufacturing, and industrial engineers are reviewing the overall ability of the factory to integrate EUS manufacturing and assembly with Core Stage manufacturing and assembly.
“That’s a big part of the manufacturing readiness review for the EUS,” Shannon noted. “It’s not just ‘how are you going to build EUS?’ and ‘what’s your timeline?’ It’s ‘how are you going to build EUS consistent with all the other Core Stage work that is going on in the factory in order to keep the flow going?'”
“The team has good plans,” he observed. “We actually have more capacity than we need. We could ramp up to produce more SLSs given the tooling and floor space that we have, but you want to make sure, first time you go through EUS, that you’ve got some margin in the schedule in case you find something that you didn’t expect that won’t take down an area that you need for Core Stage later on.”
“The team is doing a really nice job of making sure that they have appropriate margin in their build timelines for the first EUSes but also to not impact the Core Stages.”
Lead image credit: NASA/Michael DeMocker.