Boeing starts final assembly for NASA’s first SLS Core Stage, work picks up for the second

by Philip Sloss

Aft join sequence into Cell A

The boattail is in part a heatshield at the base of the stage and an extension on the bottom of the engine section.  “It is ready and waiting,” Shannon noted.

“On the engine section we’re also doing a lot of work on the outside. They’ve started putting up the white Hypalon paint over the cork and they’ll do some of that work on the boattail as well, but otherwise the boattail is ready to go. But if you looked at it you’d see a lot of coiled wires ready to run up into the engine section after we do the mate.”

A graphic of the base of the SLS vehicle showing the location of different elements; the boattail (BT) assembly for the Core Stage includes that and the base heatshield (BHS). Engine-mounted heatshields (EMHS) are thermal blankets that will attach to the boattail and clips at the base of each RS-25 engine nozzle. Credit: NASA.


Once the boattail is physically mated to the bottom of the engine section and all of the tube and wiring connections are finished, functional testing of the engine section systems will begin. “When we get into the Cell A functional testing, which will take about thirty days, we’re actually sending signals down wires, receiving them in avionics boxes, checking output, checking calibration curves, all those types of things,” he said.

The LH2 tank for Core Stage-1 is almost complete, with one remaining installation to do. “The aft manifold is like this bubble that has the four lines that come out of it to the four hydrogen feed lines,” Shannon explained.

“That aft manifold has been outfitted with the anti-vortex baffles, so it’s been all built up. At noon today it should go over to that controlled work access area from its clean room area and we’ll plan on installing that aft manifold probably tomorrow late,” he said at the time of the interview on January 23.

Core Stage LH2 qualification tank in the Final Assembly area at MAF in August, 2018. The flight tank for Core Stage-1 currently takes up the same position after the qual tank was shipped to Marshall as the centerpiece of the structural test article (STA). The work stand in the foreground of the image encloses the forward end of the qual tank; for the flight tank, work on the forward end is complete and the stand has been moved to the aft end for installation of the aft manifold. Credit: Philip Sloss for NSF/L2.

The work flow for this first Core Stage build has been regularly adjusted to try to juggle blocks of work, Tetris-style, into different blocks of time. With additional time before the LH2 tank would be mated to the engine section, Boeing and NASA moved forward one of the final assembly jobs, installation of the LOX downcomers on opposite sides of the tank.

“We did some get ahead work — we realized it would be easier to put on the brackets for the downcomers, so we put on forty-four of these big aluminum brackets that run down the side of the LH2 tank that will support each of the downcomers and that work all went extremely well, also,” Shannon noted.

After the tank is stacked vertically with the engine section in Cell A, the sections of the two LOX downcomers will be attached both to the outside of the tank and joined to the sections already installed on the engine section.

Shannon said the get-ahead work on the brackets is almost complete: “We’ll do some finish-out TPS work over the next couple of weeks and then be ready to meet the engine section in Cell A.”

Shannon also noted that a lot of foam closeouts on both the LOX and LH2 tanks and other parts of the stage were recently completed: “We really had a push over the last two months for both the pour TPS and the spray TPS and we formed a special team under a guy named Tim Burroughs.”

“They did a great job over the holidays as well in doing the TPS closeouts and the inner and the outer pours on the engine section, so we’ve made a lot of progress on the TPS over the last two months.”

Core Stage-2 structures being assembled

In parallel with final assembly of Core Stage-1, the pace of work is increasing for Core Stage-2 (CS-2). “When you’re doing the first part of Core Stage-2, it’s structural elements and welding,” Shannon said. “When you’re doing final assembly, it’s a lot of TPS and wiring and checkouts so we’re effectively able to divide the workforce for that phase of the vehicle that is being completed at that time.”

Four out of the five major elements are welded structures. For CS-2, the aluminum alloy panels for the liquid oxygen tank, the forward skirt, and the engine section have already been assembled into barrels and domes, which then go into the Vertical Assembly Center (VAC) that is also located in the VAB, Building 110.

The Core Stage-2 forward skirt barrel in storage at MAF in August, 2018. The two L-rings that complete the primary structure have already been welded and are now being coated with corrosion-protecting primer along with the bare metal areas seen here between barrel panels. Credit: Philip Sloss for NSF/L2.

“We’ll get into the welding of the LOX tank next week, they were moving the dome over yesterday when I was in that aisleway in the way,” he said.

The LOX tank parts follow the CS-2 forward skirt into the VAC; welding of two rings to the top and bottom of the forward skirt was completed at the end of 2018. “We finished the welding of the flanges on the forward skirt for Core Stage-2 and it’s in priming now over in Cell G,” he noted.

The LH2 tank will be the final structure that will be welded in the VAC. Due to welding issues during development, the parts set for an additional LH2 tank had to be ordered.  All five barrels for the tank are complete and ready to be assembled.

The last two pieces to complete the domes for the LH2 tank are expected to arrive at MAF in March. Steven Ernst, Core Stage Engineering Support Manager for Boeing, said in an email that there would be a short gap between completion of LOX tank welding in the VAC and the start of LH2 tank there.

Completed Core Stage-2 barrels and domes in storage at MAF in August, 2018. The covered domes on the right are for the LOX tank; the forward dome is now loaded in the VAC and welding to the first barrel is set to begin this week. Credit: Philip Sloss for NSF/L2.

The intertank is the only element that isn’t welded, due to the thickness of its structural panels. The eight panels around its circumference are bolted together, with two thrust panels where the thrust beam attaches on the inside and the SRB forward skirt attaches on the outside.

“We bolt together the intertank out of eight panels; the thrust structure was put in the jig and seven of the panels have been loaded in,” Shannon explained. “We have what’s called the longerons which are used to keep them at the right distance from each other. We’re waiting on the final panel to be delivered the second week of February. Once that’s done we’ll start bolting up the intertank.”

“So structurally Core Stage-2 is well down the road.”

The engine section primary structure is part-welded, part-bolted. A ring is welded to the top of the barrel; for CS-2, that was the first element weld completed in the VAC back in late October, 2017. The barrel will eventually be bolted to the thrust structure.

The intertank structural assembly jig at MAF in August, 2018, with the thrust beam and some longerons loaded. Some of the eight overall panels can be seen in the area. Boeing is currently awaiting delivery of the eighth and final panel next month to begin assembly. Credit: Philip Sloss for NSF/L2.

Those two pieces are still being outfitted prior to being joined in the engine section’s structural assembly jig. The thrust structure is loaded into the jig first, and then the barrel is brought in over it for bolting.

One of the lessons learned during assembly of the qualification and first flight engine section structures was to have the vendor of the thrust structure pre-drill all the bolt holes before delivery to MAF. “Having the holes pre-drilled has worked out wonderfully, we’re just dropping in bolts where before we had all this very difficult drilling to do,” Shannon noted.

CS-2 will fly on the second SLS launch, which will either be Exploration Mission-2 (EM-2) or Science Mission-1 (SM-1); the former is the current front-runner and will be the first crewed Orion mission on a circumlunar flight, while the latter would launch the Europa Clipper spacecraft on a direct trajectory to Jupiter.

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