Boeing, NASA look to finish first SLS Core Stage by end of year

by Philip Sloss

Final assembly activities

While engine section and boattail assembly are fully connected and checked out in the Final Assembly area, the other two pieces of the rocket will be connected next month for continued outfitting. Final work on the forward join in Cell D where it was stacked together in January was scheduled to be completed for its move to Final Assembly.

“We’re completing it all this week,” Williams said. “I think there was five or six closeouts on each duct and they were wrapping those up yesterday and today, so we’re all installed with the ducts and they’re leak tested, now they’re doing the component pours there. And we have a loose install of a vent line to put in and then we’re basically done with the mechanical and TPS work.”

The ducts are the liquid oxygen (LOX) propellant feedlines that start at the sump at the bottom of the LOX tank that sits inside the intertank. The S-ducts run in the forward join run from the sump to just outside the intertank.

“Wednesday we’ll start attaching our lift rings and we what we call our tic-tac-toe major toolset to allow us to lift it and break it over,” Williams added. “Our commitment point to the program was the 15th but we’re trying to stay ahead of that, and then we would break that over horizontally and then drive that into it’s parking place in [Area] 48 and then the next task there would be to use our system tunnel tool, which is air-bearing pallet moved.”

A sequence of images of the forward join for Core Stage-1 being assembled in Cell D in Building 110 at MAF in January. Outfitting and foam closeouts of the mated assembly there are being completed and it will be moved into the Final Assembly area next door in the next few days. Credit: NASA/Jude Guidry.

“We’ll drive that forward section of the system tunnel over to 48 and then install that, that’s one of the first tasks we’re going to do over there when we get to Final Assembly with the forward join.”

LH2 tank work continues ahead of joining it first to the three-piece forward join, adding the brackets on the outside of the tank that will hold the different commodity lines that run up and down the side of the rocket. “We’ll continue to do that work up until the 5/15 (May 15) target date to make that original mate that we talked about with the forward join,” Williams said.

The mid-May mate between the bottom flange of the intertank and the top flange of the LH2 tank will be done horizontally as planned, but as the second major join instead of the third and last. This will allow a significant amount of work to be done attaching most of the runs of the outer protuberances while waiting for the engine section functional testing to be completed.

“Once we get that join, then that’s when we’ll start running the feedline lengths on the rocket on both sides and then the press (pressurization) lines run, and then the system tunnel work and that work will go on until we’re ready to mate with the engine section in late July,” Williams noted. SLS has two LOX feedlines that run on opposite sides of the Core Stage on the +Z and -Z sides.

The pressurization lines run from the engine section up to the top of both propellant tanks on the -Z side, along with the system tunnel for the stage. In the original plan, a lot of the work to attach those lines on the outside of LH2 tank couldn’t be done until after the engine section was mated.

“That was another limiter with our original plan,” Williams said. “The hydrogen tank itself was limited about how much work we could do in advance of joining to the engine section, for a few reasons. One, CG (center of gravity) and offset of extra equipment on the LH2 tank and two, the platform system in Area 39.”

Most of the larger external protuberances on the Core Stage are on the +Z and especially -Z sides of the vehicle. Credit: NASA.

“We cut out those platforms several months ago after Christmas to enable our early feedline opportunity to be captured. With the cutouts we could physically put it in Area 39 with the original plan when we were going to vertically mate with the engine section.”

“What we couldn’t do was put on press lines or a system tunnel because then we have a very troubling time handling the tank and the cell itself wasn’t configured to handle it with that extra equipment on the tank and we got into a weight problem as well as the tooling that was out there,” he added. “So that was another factor trying to deploy this horizontal approach earlier, mating it to the forward join, and then being able to do all those three flavors of work ahead of the engine section.”

As Williams noted, the system tunnel will be attached in sections to the stage. “The forward join itself portion of the systems tunnel is completely built out at the level on the tool, so all the wire harnesses that are in there, a little bit of the pneumatic tubing is in there and a few other things but the original plan it’s built out to its capacity on the tool so that one is ready to go in,” he explained.

The Final Assembly area in Building 103 at MAF on February 28. Credit: Philip Sloss for NSF/L2.

“The tool divides itself, you’ve got a forward section and a aft section, but the plan was always to put them on in separate positions if you will because the lead-in base plates need to go in by hand on the rocket, so I’ll give you an example. The forward section has seven base plates on it, we need to do three of the lead-in panels by hand just because of the nature of the assembly, so the aft is similar.”

“What we’re doing on the aft is the wire harnesses are obviously much, much longer and some of the input from the technicians, which has been great, is that it more desirable for some of those harnesses to be installed on the vehicle in their view so obviously we’re listening to them,” he added. “So there’s going to be a little different integration now of the aft portion just because of the length and the weight of those wire harnesses. And they have a good working level with our stand system on the vehicle as well so they’re working at desired height levels.”

New tools for horizontal mate

In addition to the new tools that allowed the engine section/boattail assembly integration and testing to be moved to the Final Assembly area, more tools are being built to support the pre-planned horizontal mate and the unplanned one. “We have eight additional tools that we’re fabricating, producing to support the horizontal assembly plan,” Williams said.

“One of them is a passive roller stand set and maybe you’re familiar with the Core Stage transportation RATTs (Rotational Assembly and Transportation Tool), well those RATTs always pick up at the flanges, right? So we’ve got to build a different toolset that comes inboard of those flanges so that the flanges are free when we mate, so that’s what these roller stands will do for us.”

“They’ll be in two pieces and kind of slide together underneath the tank and meet in the middle at both the forward and the aft end, so that’s one of the major toolsets we’re building. We’re tasking ourselves to have those here mid-May to make that hydrogen tank to the forward join mate.”

The boattail for Core Stage-1 is lifted out of its work stand (behind on the floor, background) in March in preparation for stacking with the engine section. Credit: NASA/Jude Guidry.

“Then there’s a couple of other tools that are important for us to physically handle the CG and the weight offset of the engine section itself when we turn it horizontal and we just call those [the] engine section lift and breakover fixture and those are like July time needs,” he added. “The team is finalizing exactly what that’s going to look like and then we have to put a ballast system on the forward flange of the engine section to set that CG so we can handle it correctly when we go vertical to horizontal.”

“The third piece that’s very critical for us is those MPS lines, those feedlines and the fill and drain line that we talked about, those are not permanently installed so we need to develop a stowage system that allows us to control those to one, break it over and mate it horizontally and then two, withstand the shipping loads it’ll see when it goes on the barge,” he continued. “And then there’s a small handful of accessory tools, but those are the three main focuses.”

After the horizontal mating of the forward flange of the engine section to the aft flange of the LH2 tank, the stage will be back to something like the original plan. “Once we mate the engine section, then the whole exterior of the vehicle is to the plan and we can do the tie-ins to the feedlines, the system tunnel, and the press lines into the engine section itself,” Williams said. “And then we’d install our engines horizontally as originally planned and then that would enable the rocket to be fully mated in a horizontal configuration.”

“What is not the old plan is those internal feedlines and the fill and drain line, they won’t be mated. We’re going to run eighty percent-ish of the FIFT, Final Integration Functional Test, and then there’s about twenty percent that’s required to be completed after those mates internally.”

Likely application for Core Stage-2

The Trump Administration’s push to accelerate launch schedules applies beyond the first Core Stage, so the Final Assembly changes being introduced now are likely to come into play with Core Stage-2. The structures for that stage are being assembled ahead of their integration and test cycles, but the critical schedule drivers look to be similar to the first build.

“I think it’ll be paced or informed by the engine section being our critical path,” Williams said. “We’ll have the ability obviously to do either plan.”

“This one obviously looks more favorable because we know the engine section will again be our challenge.”

Lead image credit: NASA/Jude Guidry.

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