The engine section of the second NASA Space Launch System (SLS) Core Stage will look a lot like the first one, but prime contractor Boeing redesigned the way they are putting it together. The engine compartment, which is the bottom of the five major pieces that compose a full stage, is the most complicated piece of the launch vehicle.



Boeing just completed standalone assembly and checkout of the first engine section and its boattail fairing extension, and while most of the attention is focused on joining it to the rest of Core Stage-1 and completing final assembly, engineers took lessons learned from the first-time build and rewrote the script for the second one.

Although the basic aspects of assembly and production of the Core Stage-2 engine section remain the same, the better understanding gained from going through the whole assembly process has already resulted in significant changes to the arrangement of work and improvements to the quality of the output.

The main integration of internal equipment such as tubing, wiring, and electronics will follow assembly of the primary structures, but some of that work has already started; efforts will continue into successive builds to refine the production process for the Core Stage from work on the individual elements to final assembly.

Reconstructing the construction plan

The hardware for Core Stage-2 is well into production at NASA’s Michoud Assembly Facility (MAF) in New Orleans, and Boeing and NASA are applying lessons learned from the first build to the second. The Core Stage is the new piece of SLS; although the engines and boosters for the government launch vehicle pre-dated it, the stage was developed at the beginning of the decade in the wake of the end of the Space Shuttle Program and the cancellation of the Constellation Program.

The stage acts as a ground-started sustainer during launch, playing the same role Orbiters did for the Space Shuttle. The multi-purpose Shuttle orbiters performed several functions beyond a launch vehicle rocket stage, but that’s the only purpose for the Core Stage — there are no in-space, upper stage elements like Orbital Maneuvering System (OMS) pods, there is no payload container like a cargo bay, there are no re-entry and landing elements like delta wings, and there is no spacecraft crew module.

Although redrawn around the Shuttle’s liquid propellant tanks, Main Propulsion System, and RS-25 rocket engines, the stage was also reset into a bigger, inline launcher; development of new construction techniques and then putting those into practice on qualification articles and Core Stage-1 made for a bumpy first-time learning curve.

Assembly of the first working engine section ended up being the biggest learning curve of the build and Boeing has deconstructed and reconstructed the work for the second one to improve the speed and the quality. “We have a new workforce here, we hired people into this facility, a lot of people transferred into this facility,” Todd Nicholson, Boeing’s Engine Section Integrated Product Team Lead, said, “but if you think of it as a new workforce, I call it ‘the factory has learned.’

“It has matured and the non-conforming conditions are down compared to Core Stage-1, which says you have less churn of doing red-lines to manufacturing engineering. The shop has to say ‘engineering, we tried to build it, we encountered a problem, what do you want us to do with this problem?’ So that’s what I mean by less churn on that cycle of non-conforming conditions.”

(Photo Caption: The Core Stage-2 engine section in mid-July in its structural assembly jig at MAF. Following completion of structural assembly, technicians will work on the bulk of the integration phase the element’s construction.)

The engine section structure is a combination of welded and bolted structures, and a milestone in structural assembly was mating the welded barrel with the thrust structure the engines are mounted to. The barrel was welded together from eight panels in the Vertical Weld Center (VWC) at MAF and then an L-ring was welded to the top in the Vertical Assembly Center (VAC); that work was completed in 2017, with the VAC weld finished at the end of October, 2017.

The thrust structure is bolted to the inside the barrel; the engines will be mounted to this structure on the bottom and thrust vector control (TVC) platforms (also referred to as MPS platforms) with much of the hydraulic system equipment will eventually mounted on top. The two pieces were brought together at the end of May, but several tasks performed downstream of that work sequence for the first article were brought forward for the second.

“So Core Stage-2, first and foremost, we tackled more efficient ways to do the work statement,” Nicholson explained.

“For example, the thrust structure has an enormous amount of tubing and cabling that comes to it because that’s where the engines react loads up into the vehicle. So we build the thrust structure first and then we put the barrel on top of it, right?”

“One of the things that we wanted to do on Core Stage-1 but we just couldn’t pull it off due to timing, it was pretty much before I got on the program, was to pre-populate to the maximum extent that we could all the equipment and sub-assemblies on to that thrust structure,” he continued.

Vito Neal, a welding engineering with Boeing, noted that orbital tube welding started much earlier in the sequence than during integration for the first engine section.

“Vito’s right, we did some welding,” Nicholson said. “We called all that ‘pre-integrate’ [work], so we pre-integrated the thrust structure prior to putting the thrust structure and the barrel together. We started putting tubing and all kinds of bracketry on it.”

“The cool part about that is, the most efficient part about that is, you have so much more access [when] you don’t have that barrel on you,” he explained. “A technician doing work and needing help or to ask somebody a question, he doesn’t have to come all the way out of the footprint or the barrel to go talk to the manufacturing or quality engineering.”

“He can pretty much just holler over the structure and say ‘hey come here for a minute.’ So [that is] just the human element of efficiency, number one.”

Even prior to the early integration work on thrust structure at MAF, Boeing moved up some of the structural assembly tasks for the hardware, sending the match drilling work back to the vendor.

“For Core Stage-2 and on we established an initiative, we refer to it as ‘pre-drilling’ our holes,” Nicholson said. “We exercised a change in the program to say why don’t we drive that all the way to the fabricator of the raw part, give enough tolerance on those holes, and account for it here during the assembly process so that we’re not putting a match drill operation on the mechanic and running the risk of mis-drilling it, because that’s all by hand.”

“Whereas at the fabricator you can set it up on a jig and use machinery to do those holes and they’re spot on. I don’t know the efficiency numbers but I’ve seen the charts and Core Stage-1 to 2, it’s at least two-fold better, it’s astronomically better, and we expect to reap that harvest going into the future with Core Stages once we’re put on contract with those.”

(Photo Caption: The barrel of the Core Stage-2 engine section is lowered down over the thrust structure in the structural assembly jig for engine sections at MAF in late May. The barrel and ring on top are welded, the thrust structure is bolted, and the two pieces are bolted together in the jig.)

For Core Stage-1, the thrust structure and barrel were brought together in the structural assembly jig at MAF towards the beginning of the build. At that point, the holes to hold tubing brackets were drilled.

“We match-drilled them in place,” Nicholson said. “That was the thought through the whole design phase of the Core Stage and you learn stuff with the first unit. We were like ‘this is too labor intensive’ which equates to time and we were having a lot of mismatched holes and so engineering came up with the clever idea that we sold to the program to drive this requirement to pre-drill these holes back at the fabricator of the piece part.”

“[The] program did its due diligence, engineering had to build a huge trade study to go convince the program that OK it’s an investment in time here but it saves you this much during assembly time and checkout,” he explained. “You won’t reap this harvest until months down the road.”

“The day we started, the day we started assembling all those pre-drilled parts everybody knew right then because our job count was exponentially higher just that one day because we all reflected back on when we did it with Core Stage-1, we said we would have a mismatched drilled hole and we would be sitting here doing a non-conformance right off the bat. [It was] an instant return on investment, an instant return on investment.”

“The engineers were sitting there saying ‘See? We told you so.'”

For future builds, Nicholson said the vendor will deliver the thrust structure to MAF after that work: “It’s part of the baseline design now, all the rest of them will be built that way.”

He also noted that not all the match-drilling will be done elsewhere. “There still is some match-drilling, but I’m talking about primarily brackets that retain tubes and wiring and spatial geometry on a substructure. We don’t see that being pre-drilled in the future, we don’t see the value in that because of the intricacies of putting those parts together, they’ve got to be spot-on the way you put them into position.”

Nicholson noted that Boeing is planning to incorporate lessons learned in future builds beyond Core Stage-2, for example with wiring. “That would be the next one we’d want to tackle in advance of Core Stage-3, is to get started on the electrical wiring earlier,” he said.

“We had a lot of unanticipated discoveries with Core Stage-1 with our electrical work statement. We’re incorporating those lessons learned within requisite engineering and that engineering is going through a release cycle right now to get those harnesses built incorporating those lessons learned.”

“Ideally I would want to have those harnesses a couple of months ago. If you go inside [the] barrel right now we’ve pre-populated those wire harness brackets to attach the wire harnesses to. I would want to in this build position go ahead and pre-install those wire harnesses to the maximum extent I could.”

For the second build, Nicholson said they are modifying some of the wiring to make it easier to handle: “We’re incorporating lessons learned from Core Stage-1 into Core Stage-2 and one of the lessons learned was production breaks in our electrical cables.”

(Photo Caption: The Core Stage-2 engine section barrel sitting in storage next to one of liquid oxygen (LOX) tank domes in February, 2018. The barrel is built from eight friction-stir welded panels and an L-ring on top that serves as the structural interface to the rest of the stage. The vertical weld lands for the panels can be seen here as breaks in the pre-welding primer treatment, along with the circumferential weld land for the ring at the top.)

He gave a hypothetical example: “We have an electrical cable that is say seventy-five feet that runs from one side of the barrel over and up and through and all the way around to the other side of the barrel. That’s a lot of labor to unfurl that cable around, put it in place, and get it just right.”

“Well, maybe it’s smarter to put a production break in there, so you have something smaller to deal with,” Nicholson explained. “You can just physically handle it faster and more readily and easily.”

“We’re implementing some of that, we’re biting that off in small chunks because the more you cut the cable up into shorter lengths you have what’s referred to as line loss, your current doesn’t hold all the way through. So we’re being very strategic about those kind of changes, but we are doing a little bit of that.”

In addition to efficiencies based on streamlining the work sequence, the quality of the work is improving. More work is being completed to its specification, which means less time is necessary to fix the output, fix the specifications, or both.

“Those three big things, incorporating total lessons learned, attacking the electrical system, and pre-integrating the thrust structure ahead of the way we did it in Core Stage-1 are gaining us leaps and bounds on efficiency,” Nicholson said. “Our non-conforming conditions, I don’t know the number but we’re light years down, ten-fold down from where they were in Core Stage-1.”

“And so our expectation is that Core Stage-2 and onward obviously our efficiency will go up a curve and then eventually it’ll plateau. What we don’t know yet is can we forecast that plateau.”

“Our industrial engineering team is off trying to analyze that with all the data from Core Stage-1 and current data on Core Stage-2 to try to help us understand when can we expect to say ‘ok this is as fast as we’re ever going to be able to build a rocket,'” he added. “History tells me — thirty-two years in this business, always building first-time builds, that’s all I’ve ever done — history tells me it’ll probably be the fourth one before we really understand that, but that’s just my opinion.”