Prior to workplace closures, SLS was making good progress ahead of opening missions

by Philip Sloss

Artemis 2 preparations

SLS is also working on the hardware for its second launch, Artemis 2. The mission will be the first crewed flight of both Orion and SLS, with the launch vehicle flying in largely the same configuration as Artemis 1. Prior to the COVID-19 stand-downs, the target time-frame for launch was trending towards the end of 2022 or early 2023.

Following lessons learned with the first full Core Stage production build, assembly and test of the second flight article is progressing better than the first; along with the decision by agency leadership to cancel a second Green Run test campaign, the stage is not currently the critical path element for Artemis 2 launch readiness like it is for Artemis 1. Before Core Stage-2 assembly, integration, and test work was suspended in March to protect the workforce from COVID-19, the stage was targeted for completion in March, 2022.

Beyond MAF, the virus is having less of an impact on parts suppliers around the U.S. and abroad. “The supplier base generally is continuing with limited impacts,” Shannon said. “We talk to them daily. We have had six suppliers that have notified Boeing of some potential impacts due to the virus.”

“Two of those impacted suppliers are in Europe, [the other] four are on the West Coast. None of those right now are hitting us on the critical path, so we’re not going to have any Core Stage-2 parts impacts when we come back.”

Credit: NASA/Eric Bordelon, Steven Seipel, Jared Lyons, Jude Guidry.

(Photo Caption: From a recent NASA video resource reel, the Core Stage-2 intertank is moved from Cell G following application and trimming of spray-on foam insulation (SOFI) to an integration area in Building 103 in early February. When the workforce can return to MAF, work to outfit the inside of the structure with wiring, shelves, tubing, and equipment will resume.)

By the time of the mid-March stand down at MAF, assembly of the primary structure of all five of the major Core Stage-2 was complete. The stage consists of two propellant tanks, one for liquid oxygen (LOX) and one for liquid hydrogen (LH2), and three “dry” structures, the forward skirt, intertank, and engine section.

The propellant tanks are filled with fuel on launch day, but the dry structures are filled during the integration phase of production with secondary structures like brackets and struts and rocket machinery from wiring and electronics to fluid tubing and pressure vessels for hydraulics and pneumatics to valves and manifolds and pumps.

All of the elements are also covered with thermal and corrosion protection; the forward skirt and the intertank have completed their large-scale, “acreage” treatments of primer and spray-on foam insulation (SOFI) before they started integration and the propellant tanks are next after being tested to make sure they were welded pressure-tight to specifications.

The most complicated element is the engine section, which is a structure that is both welded and bolted and which uses cork on the exterior for thermal protection and SOFI on interior cryogenic propellant lines. A boattail structure will eventually be bolted to the bottom of the engine section barrel, with additional orbital tube welding of fluid lines between the two.

“To give you the real quick rundown, the LH2 tank we’ve completed the proof testing and NDE (non-destructive evaluation) inspection and we’re readying for clean operations when we get back,” Shannon said. “The LOX [tank] completed all the priming operations in Cell P.”

The LH2 tank was moved to Area 6 in Building 103 at MAF in mid-February following proof testing in Building 451. With post-proof NDE complete, when work resumes the tank will be moved into Building 110 for the next major production steps.

Credit: NASA/Jude Guidry.

(Photo Caption: The 130-foot long Core Stage-2 LH2 tank is rolled into Area 6 of Building 103 at MAF on February 13. The tank was returning from proof testing in Building 451.)

The inside of the propellant tanks are cleaned with water by a washing machine in Cell E in Building 110 at MAF; the LH2 tank will follow in the steps of the LOX tank with the internal clean in Cell E and the application of primer rings at the junction where the two domes of the tank meet the ends of the long barrel. That allows the tanks to be placed in ground support equipment (GSE) called Rotational Assembly and Transportation Tools (RATT) for the full application of primer in Cell P in the adjacent Building 131.

The LOX tank had already completed the full primer work in Cell P and was moved into Area 47/48 in Building 103 in mid-March just before operations were suspended. When work resumes that tank will continue outfitting, eventually to be followed by a trip to Cell N for SOFI applications.

“The forward skirt is in integration activities focusing on all our wire harness routing,” Shannon added. “That is about to be finished up when we get back. The intertank is also in integration and starting its wire harness routing.”

Boeing completed the external SOFI applications and trimming on the intertank in Cell G in Building 114, which is located with Building 103 and Building 110 on the west end of the main complex at MAF. In early February, the element was moved out of Cell G and out to an integration area in Building 103.

“On the engine section integration work we were making good progress on orbital tube welding,” he also noted. “We’ll start right back up when we get back and the boattail structural assembly was just starting, so we’re going to be in good shape when we get back to pick right back up on those activities.”

Credit: NASA/Steven Seipel.

(Photo Caption: The Core Stage-2 engine section in its standalone integration area in Building 103 at MAF in late January. Orbital tube welding of lines for fluids like propellants, pneumatics, and hydraulics was in work at the time operations were suspended in mid-March.)

ULA production of the ICPS unit for Artemis 2 also continues. “No change to the ICPS-2 production status it’s continuing to be built at ULA in Decatur,” Shannon said. “They have not been greatly affected by any supply issues or by the team issues out there.”

“They’ve just finished the hydrogen tank and they’re starting the liquid oxygen tank and they’re well into their integration activities. Since it will be crew-rated there are some design changes on that vehicle, they’ve all been approved and are ready to go so really no production impacts to the ICPS-2 production schedule.”

The most recent planning date for ICPS-2 delivery was July, 2022.

Like the solid-fueled motor segments for Artemis 1, production of the Artemis 2 segments is also complete and those are stored at Northrop Grumman’s Promontory facility in Utah. Fabrication of subassemblies for the other launch vehicle elements has also started, the Launch Vehicle Stage Adapter (LVSA) that connects the ICPS to the Core Stage, and the Orion Stage Adapter (OSA) that connects Orion to the ICPS.

Core Stage-3 LH2 tank repair plans

Although work has started on long-lead materials and equipment for Core Stage-3, assembly and production are still off in the future except for a leftover LH2 tank article. It was one of two identical structures welded for first flight development in 2016; one became the main element of a structural test article that was tested at Marshall last year, eventually being tested to failure after completing test cases verifying the tank met human-rated factor of safety requirements.

The other tank was meant to be the Core Stage-1 LH2 flight article; however, issues with the quality of the welds done in 2016 performed in the Vertical Assembly Center were discovered after both LH2 tank articles assembled in the production tool. “It was what we call a low ductility, low topography anomaly that was found in testing at Marshall,” Shannon said.

“The root cause of that was eventually traced to the pin that we used to do the friction stir welding and that was modified for follow-on tanks.” (The two LOX tank articles for first launch development and flight had not been welded yet and both were employed as originally planned.)

Credit: NASA.

(Photo Caption: From the program’s February monthly highlights, LH2 tank serial number 1 waits for repairs at MAF. The tank article was set aside in 2017 due to anomalies in the welds that joined its barrels and domes together. NASA and Boeing have worked on research and development of repair techniques that they hope to apply to the tank sometime after work resumes. If the repairs are successful, plans are for the tank to go into production with Core Stage-3.)

The subassembly hardware for the Core Stage-2 LH2 flight article, barrel and gore panels and dome end caps, were already on hand at MAF and the decision was made in 2017 to use those instead for Core Stage-1 and order another set of hardware for Core Stage-2. In the meantime, the original flight tank with the weld anomaly was set aside.

“We preserved that tank with a goal towards finding a repair option because the flaw is almost a surface-level flaw but it reduces the effective thickness of the material so you don’t quite have the strength that you would want,” Shannon explained.

“The team came up with a gas tungsten arc weld where you basically take a router and you route out like an eighth of an inch of material all the way around the weld. You come back with this gas tungsten arc weld and we’ve done a lot of testing, really extensive testing on that. It shows by doing that we restore the mechanical properties to a really good level.”

“And so we purchased an automated system which is just basically a rail that attaches to that curvature of the tank on that weld,” he added. “It does the little routing, and then we use the same rail to go over it with the gas tungsten arc weld and when we get back we’ll be close to executing that repair on the tank. So we’re very excited that we’ll be able to use that for Core Stage-3 and have high confidence in it.”

After repairing the tank’s VAC welds, it would go through a similar sequence of steps as other LH2 tanks. The circumferential welds would need to be plugged to make the tank pressure-tight, followed by NDE of the welds. The tank would then be rolled out to Building 451 at MAF for proof testing.

LH2 tanks are proof tested pneumatically in Building 451 by pressurizing them with gaseous nitrogen, while loads are being simultaneously applied by a hydraulic test rig.

Lead image credit: NASA/SSC.

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