NASA, Boeing approaching first major join of second SLS Core Stage

by Philip Sloss

Along with the wiring installations, hundreds of orbital tube welds are being done to connect the different MPS fluid systems, from propellant lines to hydraulics to pneumatics. A couple of the larger engine section subsystems that are first assembled and processed “offline” outside the volume — the thrust vector control (TVC) platforms and the large gaseous helium composite overwrapped pressure vessels (COPV) — are now being completed for subsequent installation.

“TVC platforms got flown in this week and last,” Birkenstock said. “Helium tanks have not [been installed] yet; we’re still working [preparations for] that.”

The TVC platforms, one for each of the vehicle’s four hydraulic systems, are installed on top of the thrust structure that is positioned near the bottom of the engine section barrel. “We’ve gotten three out the four over there,” he noted. “They finished their high-pressure helium leaks checks [on March 4th], and so that’ll go into the barrel.”

Each TVC platform holds most of the equipment for the hydraulic system, including the auxiliary power unit, hydraulic accumulators, reservoir, filter manifold, and circulation pump. When fully outfitted, the engine section will have five helium COPVs installed inside.

“We had to reconfigure the [work] platforms and the tooling in there to open up some more work, so that’s part of what they were doing was bringing online new, more robust [work] platforms inside [the engine section] so you could have more people [safely] working,” Birkenstock said.

Birkenstock also noted that the current forecast is for standalone engine section integration to be completed around the end Summer 2021. When complete, the element will be transported and installed in Cell A in Building 110 where it will be bolted together with the boattail assembly.

Credit: NASA/Steven Seipel.

(Photo Caption: In the foreground, the boattail assembly, and in the background the engine section for Core Stage-2 on March 5 at MAF. Standalone work on the boattail extension and base heatshield was recently completed and the structure was moved from across the aisle where the engine section is to this location. NASA and Boeing are reworking the floor plan at MAF to expand Core Stage production, particularly future engine section builds.)

The boattail is a short extension and fairing bolted to the bottom of the engine section that includes the locations of exhaust ports and access panels for some of the equipment inside. It also is the base heatshield for the stage where different thermal protection system (TPS) elements are attached, such as the engine mounted heat shield (EMHS) blankets that protect the powerheads of the four RS-25 engines.

“Boattail is done, that work scope is done,” Birkenstock said. “It’s ready to be integrated and we’ve got cork on the outside. They’ve actually finished out that work scope, so it’s ready and that’s just waiting for the integration.”

Boeing and NASA are incorporating lessons into assembly and integration of the second engine section build from the long learning curve of the first one. Birkenstock said the production work is going more efficiently this time and so far has not required extensive, around-the-clock shifts.

“It’s eased a little bit, but it’s going to pick up again,” he said. “When we’ve got delivery of critical parts, we will ramp up and surge accordingly. So if we got a TVC platform that we want to get in that opens up a bunch of job kits, we’ll work really hard to get that [done] and then that opens up a bunch more work scope that we can go work through at a regular pace. We haven’t gone 24/7 yet on anything, but at some point we’ve got that knob to turn if it comes to it.”

One of the watch items that a year of living with a pandemic increased attention on was the parts supply chain. “We’re still fighting parts being delivered on-time,” Birkenstock said.

“Part of that is the supply chain all has different, varying levels of COVID effects and how they’re dealing with it. We’ve gone from the extreme of companies shutting down and not letting anyone else in to somewhat making their marks and the whole gamut in between.”

Credit: NASA/Jared Lyons.

(Photo Caption: The Core Stage-2 forward skirt (foreground, right) is prepared for lifting out of Area 15 at MAF on February 19, with the recently-welded barrel of the Core Stage-3 engine section (middle) and the Core Stage-2 engine section in its integration area (left) looking on. Engine section integration will be taking over this area, and the forward skirt move was done to allow the blue pedestals and forward skirt tooling to be relocated to the east side of the building.)

“I’ll say the Core Stage-2 sparing strategy hasn’t in general changed; it’s more of a day-to-day how do we [adjust],” he added.

After standalone engine section integration is completed and the engine section and boattail are bolted together, an additional set of connections and orbital tube welds will be made in preparation for full functional testing — following which the engine section/boattail assembly will be ready to be mated horizontally to the rest of the rocket.

LH2 tank primer spray retry completed

In late-February, the LH2 tank was still in Cell P of Building 131 where a second round of primer sprays on the metal skin of the tank structure was being completed. The tank was in Cell P longer than expected because the first attempts to apply the corrosion-protecting primer did not meet requirements.

“The first couple attempts at getting the primer on this time around didn’t go so well, so we had to figure out ‘do we strip it all off or do we just take parts [off],'” Birkenstock said. “It was failing its checkout requirements after the priming operation.”

After the primer is sprayed on and dries, they test the robotic paint job by putting tape on top of the coat of primer paint and watching what happens when the tape is pulled off. “It’s got a post-prime lift-off tape test that they do, and so if you spray the paint on and it comes off [with the tape], that’s not good,” he said.

After stripping the primer off, reviewing what happened, and correcting issues found with the process, the LH2 tank team was ready to try again. Birkenstock said the issue was with the process of preparing the tank surface for the spray, not necessarily with the procedures to apply the primer.

Credit: NASA/Jared Lyons.

(Photo Caption: The Core Stage-2 LH2 tank is backed out of Cell P in Building 131 on March 3 following completion of primer sprays. The primer application was completed after initial attempts were unsuccessful; the tank was moved into the final assembly area in Building 103 nearby for additional outfitting and preparations to return to Building 131 for foam sprays in Cell N.)

“We made them take it all off from the first attempt. We did a robust RCCA, root cause corrective action, and had the output from that as a gate before we let them spray again,” he noted. “They were having issues with the surface prep before they sprayed and making sure that it would work with that process. So we worked through that, implemented some corrective actions, and we’re doing well the second time around.”

The final primer sprays were completed on February 25, and on March 3 the LH2 tank was moved out of Cell P and over to the final assembly area in Cell N in Building 103 for preparation for foam sprays of the barrel section and the domes on both ends.

“They’ve got some additional outfitting that they’ll do before it goes into [Cell] N,” Birkenstock said. “They’ve got to get some sensor runs on, those kind of things, and then it’ll go get its foam.”

Following the foam sprays and trimmings, the LH2 tank will return to the final assembly area of Building 103 for additional outfitting of hardware, such as the brackets and fittings for the two liquid oxygen feedlines that are attached to the exterior of the tank. The tank will subsequently be mated with the forward join to form the upper four-fifths of the rocket.

(Lead image credit: NASA/Eric Bordelon.)

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