The four RS-25 engines for the first launch of NASA’s Space Launch System (SLS) have all arrived at the factory where their Core Stage is being completed. The engines were trucked one by one from Aerojet Rocketdyne’s final assembly facility at the Stennis Space Center in Mississippi to the Michoud Assembly Facility (MAF) in New Orleans, with the last engine arriving on June 27.



The engines are now back together next door to the final assembly area at MAF, where the elements of Core Stage-1 are being put together. Core Stage-1 is the first flight article in the SLS Program, and before the engines can be installed, prime contractor Boeing is working to finish functional testing of the engine section/boattail and then attach it to the rest of the stage.

Four of the five major stage elements were recently joined and outfitting of that section of the stage continues in parallel with testing and checkout of the engine section. NASA and Boeing are still targeting completion of the stage in December.

Engines next door to functional testing for engine section / boattail

The four engines were individually trucked from Aerojet Rocketdyne’s facility at Stennis to MAF over the last two weeks of June, with the fourth and final engine arriving on June 27. The four veteran Space Shuttle Main Engines, now called RS-25s and assigned to the first SLS launch have been ready for installation since October, 2017, but delivery to MAF was deferred until now due to delays in completing assembly of the first Core Stage.

(Photo Caption: The nozzles of the four RS-25 engines assigned to Artemis 1 lined up in a staging area at MAF next door to Core Stage-1 where they will be installed. From left to right (front to back): Engine 2045, Engine 2056, Engine 2058, and Engine 2060. Formerly called Space Shuttle Main Engines (SSME), all of them flew several Space Shuttle missions installed in the Orbiter fleet.)

Production and assembly work on Core Stage-1 is the critical path in the schedule for the first SLS launch on Artemis 1, which will send a NASA Orion spacecraft to the Moon. The engine section is the element of the stage that is taking the longest to complete due to its complexity.

It was largely completed as a standalone element in late March, and then the boattail was bolted to the bottom for the next round of integration and testing. Integration of the engine section with the boattail was completed in mid-June, allowing full functional testing of the assembly that houses most of the stage Main Propulsion System (MPS) to begin.

“We’re actually having our TRR, Test Readiness Review, right now,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said in an interview on June 11. “It will last all day to make sure that all the paper is closed and all the systems are ready to start test on Friday (June 14) which was on our plan.”

“We are marching right towards rotating the engine section and integrating it into the overall vehicle at the end of July per the plan that we’ve been working to. That gets us out of MAF in early December and we’re just staying right on our plan.”

“We learned a lot when we did the electrical systems testing and the avionics testing in the forward skirt and intertank,” he added. “We got a little bit balled up in the forward skirt and the intertank when we got to functional testing because we were finding issues.”

“Then when we started using the breakout box to kind of go through and do a sweep of all the wiring beforehand, [to] make sure the workmanship was exactly right and the channelization was exactly right, it makes going through functional test really quick.”

(Photo Caption: The aft end of the LH2 tank and the -Z side of the engine section in the Final Assembly area at MAF just prior to the first horizontal mate of stage elements in late May. The boattail attached to the engine section is hidden by the tooling the flight hardware is resting on.)

As the last engine was arriving, Boeing was reaching the point in testing for the engine section to be powered up.

“It’s going great, we’re kind of in the final steps of getting ready to turn power on to the first boxes,” Jason Grow, SLS Core Stage Lead Propulsion Engineer for Boeing, said during a media event at MAF on June 28. “After the TRR there are a lot of steps that need to happen to make sure the vehicle is in the right configuration and everybody is ‘thumbs up’ and a go for power on. So that’ll be happening here pretty soon, we’re excited.”

Grow outlined the functional testing Boeing will be doing for the next month. “You’ve got a number of different phases. One of the phases is just health and status of all of our flight computers and data boxes, that kind of thing. All of the instrumentation is included in that, as well.”

“Then we go on to some other subsystems like hydraulics and MPS where we’ll actually be exercising the system a little bit here.”

The engine section/boattail assembly is currently parked in the Final Assembly area, sitting in one of the tools built to support the new horizontal connection, surrounded by scaffolding. “While a lot of the system has been closed out we still have the ability to have a controlled work area in there,” Steven Ernst, Core Stage Engineering Support Manager for Boeing, also said on June 28. “We’ve got an access kit inside the engine section, so the employees actually crawl underneath to get inside.”

“And we just wrapped up building up this scalffolding to largely support some of the interconnects and some of the work on the external part of the engine section.” Conditioned air is pumped in to maintain a positive pressure flow from the inside out through screens in the engine section vents.

“We do have some vent holes and they are covered to keep wildlife and things like that out,” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, noted at the event. “You maintain a positive pressure inside the volume as we’re working to keep a clean environment, that’s what you see with the plastic connected to them now.”

Major join 2

The engine section completion date was still uncertain earlier in the year, pushing the projected launch date into 2021, so a new final assembly sequence was developed for the stage to save time. At about 200 feet in length without the engines installed, the stage is too tall for any of the buildings at MAF to be assembled vertically in one piece so the original plan was to stack the stage first in two sections vertically and then make the final mate horizontally.

That plan had engine section completion in the overall critical path for other parts of the stage, holding up remaining work; the new plan allowed work on the other elements to run in parallel with finishing the engine section. The first major join of the top three elements, the forward skirt, the liquid oxygen (LOX) tank, and the intertank was completed earlier in the year on the original plan.

(Photo Caption: This view from forward end of the LH2 tank shows it suspended by jacks over the blue-colored Passive Roller System (PRS) tools that it now rests in. The yellow lift brackets were first attached to the flanges at the base of the domes on either end of the tank, to allow it to be lifted by the two sets of jacks on the side. The passive rollers allow the tank to be rolled without blocking the mating flanges on the tank.)

The new plan joined the liquid hydrogen (LH2) tank, the other major element of the stage, to the forward join first. When they were ready, the engine section and boattail would then be mated to the rest of the stage, with both of those connections being done horizontally.

“We had been thinking about it for quite a while,” Shannon said. “What it does is it takes the engine section out of the critical schedule path, because we can go do so much work on the rest of the vehicle once it’s mated together and all we had to do was figure out a way to rotate that engine section and attach it and we figured out some new tooling that would allow us to go do that.”

“Everything else was done for the most part, except for the engine section,” Shannon added. “So we’ve been thinking for a while here ‘hey since engine section is taking so long — it’s so complex for this first build — let’s go put the rest of the vehicle together.'”

“This is allowing us to get feedlines installed, you couldn’t see them but the press lines are largely installed, so [the new plan] allowed us to get a lot of work done,” Ernst said. “[We’re] getting all the interconnect work done on the intertank between the LOX and hydrogen tank, so we’re progressing on that volume while we’re working on [the engine section] and it will all come together at some point next month.”

Major join 2 was completed over the Memorial Day weekend in late May. Both the LH2 tank and the forward join were already in the Final Assembly area at MAF (Area 47/48), with the LH2 tank originally moved into Area 47 last November and forward join brought in Area 48 in mid-April; a series of hardware shuffles were made to do the join.

(Photo Caption: The forward join is rolled over from Area 47 to Area 48 for mating to the LH2 tank. The hardware is held in Rotational Assembly and Transportation Tools (RATT), a Core Stage RATT on the front around the forward skirt upper flange (left, background) and a Transitional RATT around the upper flange of the LOX tank (middle, foreground). The two RATTs will again be used to rotate the assembly of four-fifths of the stage for upcoming installations and the engine section mate.)

The two traded places first; the forward join moved to Area 47 and the LH2 tank was moved by Boeing’s “indoor” Manufacturing, Assembly, and Operations (MAO) Self-Propelled Modular Transporters (SPMT) into position in Area 48 on a pair of specialized Rotational Assembly and Transportation Tools (RATT) that have moved it through most of its post-weld phases of production.

The tank was then transferred from the RATTs to new Futuramic tooling built to facilitate the horizontal assembly plan.

“[We] put those lift brackets on the flanges at either end, jacked it up, moved the RATTs out, moved the passive roller stands (PRS) in, and lowered it down onto the passive roller stands,” Ernst said.

“Those [jacks] are actually provided by a crane and rigging company so it’s really a standard piece of handling or field type of equipment that you use. The only unique parts of that were the brackets that we had to design and fabricate to allow us to pick the tank up. So those were again bolted directly to the flanges and that’s what we used to support the tank.”



“The passive roller stands that we came up with really enabled the change in sequence to what we call the ‘four-fifths’ (referring to four-fifths of the stage, the three forward join elements and the LH2 tank),” he added. “These are actually air-bearing, so when we had the entire LH2 tank lifted up with the jacks that you saw in the b-roll, six of us literally pushed these into the right position on the air bearings and lowered them down.”

The tank was lowered down onto the roller assemblies, which make contact in a similar location to where the RATTs held it on each end along an unfinished strip of the tank acreage that hasn’t been covered with Spray-On Foam Insulation (SOFI).

“We [laser] shot in advance the location of these and these guides positioned us in the side-to-side direction and then we actually used shims to ensure we maintain the proper spacing between the wheels and the flange because just for stress reasons we wanted to be as close to the flange as we possibly could be without contacting them,” Ernst explained. “They’re as close to the flange as we can be.”

“They’re (the PRS rollers) a little further away from the flange on the forward end, that’s of course so the heads from the nuts for the join bolts [are clear].”

The forward join section was then backed out of Area 47 by the SPMTs, moved over in front of Area 48, and then moved in to line up with the LH2 tank. The LH2 tank was already rotated to the correct orientation for mate, but the forward join still needed to be rotated using the RATT rings to line up with the LH2 tank’s orientation before pushing into fine alignment of the bolt holes.

(Photo Caption: Boeing technicians start lining up the forward join (left) with the LH2 tank (right) in late May. The forward join was subsequently rotated to align the correct bolt holes with the ones on the LH2 tank. Boeing uses the white-colored Self-Propelled Modular Transporters (SPMT) extensively at MAF to carry ground support equipment that holds Core Stage vehicle hardware elements. They will also be used for the upcoming RS-25 engine installations.)

Alignment of six of the three-hundred sixty bolt holes around the circumference of both the aft intertank flange and the forward LH2 tank flange was monitored in real-time as they were brought together. “All in all the join went pretty smoothly,” Ernst said.

“Once we got everything set there was a couple of days to work through getting all the fasteners in. Neither the hydrogen tank nor the [intertank] is [perfectly] flat or round, [so] we had to work through that to get the bolt holes lined up.”

“It was a wonderful sound as you would get a hole aligned and get a bolt dropped in there,” Ernst added. Shannon said that although the team started a little behind their milestone schedule, they had caught up by the time they were done.

“We had a vent line inside the intertank that looked like there was going to be a little bit of interference so we went and we reworked the supports for that,” Shannon noted. “So we were two days late, which is the first thing since we replanned the assembly sequence, that was the first thing we’ve been late on.”

“Even though we were two days late in getting the two pieces to mate, we were two days early in actually getting all the torquing done on the bolts, it just went together that well. The team was kind of teasing me that we picked the wrong milestone to measure ourselves against, it wasn’t the mate, it should have been the torque.”

“There were a lot of guesses,” he added. “You know [it’s] the first time you took this big hydrogen tank with the flange on it and all the holes drilled and you’ve got the bolted together intertank and you bring them together for the first time — is it going to go right? And it went unbelievable well.”

(Photo Caption: Boeing technician Paul Diaz is shown fitting a bolt into the mated LH2-to-Intertank flange. Three-hundred sixty bolts at one-degree intervals around the circumference are used to structurally attach the two flanges. The gap in the upper LH2 flange on the left is the result of the way the rings on the welded structures are manufactured. (The intertank is bolted and doesn’t have the gap.) A T-shaped “splice plate” was put in the space and secured with bolts through the two holes.)

Once the ‘four-fifths’ stage was bolted together for the structural mate, work then began to make other connections and continue outfitting to eventually complete all the connections. “We’ve done the majority of our external electrical connects, so the next step which won’t happen until a little later in the flow is to start the foam closeouts and that’ll just go right over the top of these wires,” Ernst said.

Technicians are now working on the internal connections inside the intertank. It is now a cramped compartment, with the LOX and LH2 tank domes that almost touch the large thrust beam running through it and two large-diameter, “S-duct” LOX feedline sections running through it.

“We’re in the process right now of building up our internal access kit so that we can do some of the interconnects between the hydrogen tank, LOX tank, and the intertank,” Ernst noted. There’s a floor level, there’s a second level and two more levels to go and that’ll give us access to every location we need to get to complete all the integration work between the two elements.”

“We can have up to six on the base deck but there’s not a lot of open space and there will be two people per platform after that, so yeah there’s not a lot of room in there,” he added. The work platforms for the kit will have to be broken down and removed before the four-fifths section of the stage is rotated again to support upcoming milestones.

(Photo Caption: Four-fifths of Core Stage-1 (right) during the mate of the forward join (foreground) to the LH2 tank (background) in late May. To the far left, the engine section/boattail assembly in its work stand tool.)

The forward section of the stage is currently oriented with the LOX feedlines (also called “downcomers”) on the sides at the nine o’clock and three o’clock positions when looking at it from one end or another. The feedlines are built up in sections and joined at bolted flanges; since they are carrying cryogenic propellants, they are also covered with SOFI.

Groundwork to attach the downcomer sections to the LH2 tank had started late last year on the tank by itself as get-ahead tasks under the original vertical assembly plan, with attach hardware installations and SOFI sprays. Ahead of the mate to the forward join, the team started attaching downcomer sections at the aft end of the tank.

Following the mate, they are working to connect up the remaining sections. “Segment five of the feedline goes here and then segment six interfaces to the S-duct, and that work will happen post engine section mate,” Ernst said, referring to the feedline at the zero-degrees location or +Z side of the stage. The two installed S-duct feedline sections run from the bottom of the LOX tank down and then out to just short of the two openings on the intertank.

A series of brackets flank the feedline mostly along the length of the LH2 tank, to attach and secure them to the stage structure. “There’s two attach points from two separate locations on each feedline segment and we have what we call a pivot fitting that attaches to that and then interfaces with the brackets,” Ernst explained.

A support tool that helps with the attachment also works with the pivot fittings; the connected sections of the feedline will need to be moved slightly in one direction to create the clearance needed to mate the last two flanges, such as the ones at the ends of the elbows already installed in the engine section. “This tool does a few things for us,” Ernst said.

“One as we’re putting the individual segments up here it controls the location of the end of the feedline. [It] also has the capability to shift the feedline back so when we do the engine section mate and then the integration of the final two segments that connect up with the S-ducts from the LOX tank we’ll have to shift the whole line back a little bit.”

(Photo Caption: The aft end of one of the cylindrical LOX feedline sections attached to outside of the LH2 tank (top middle) is seen just to the right of the work stand being put into place prior to the mate of the forward join and the tank. The two feedlines that run from the bottom of the LOX tank inside the intertank all the way down the +Z and -Z sides of the stage into the engine sections are assembled in sections; much of the work to attach the feedline and support hardware on the tank was done originally to get ahead of the baseline plan to vertically stack the LH2 tank with the engine section first. The horizontal final assembly plan allowed the feedline installation work to continue through the year without the engine section.)

Similarly, sections of the systems tunnel are being built up and attached along the length of the stage. “Two key sections that are built and integrated offline are what we call the LOX segment and then the LH2 and then you have a lot of manual work on the vehicle for the others,” Ernst said.

The segment that runs most of the length of the LOX tank was attached to the forward join prior to the mate with the LH2 tank. The next segment is much the longer one that runs most of the length of the LH2 tank.

The section of the systems tunnel base plates was already integrated on the intertank and another section will be attached to the bottom end of the systems tunnel on the engine section. Offline pre-integration of the base plate sections is done on a long tool; for attachment, the flight hardware is rotated so that the systems tunnel area is pointed straight down.

The tool is then moved in underneath the vehicle and the section raised up to attach to an array of studs that secure the base plates to the stage. “Most of those studs are in and we’ll mount the pre-integrated systems tunnel base plates, line it up, go against those [studs], and start installing,” Ernst explained.

The orientation of the four-fifths part of the stage for systems tunnel installations is close to what is necessary for the engine section mate; the stage would then be rotated slightly to put it in position for that major milestone.