Officials from NASA along with prime contractor Boeing formally signed off on the first assembly of the most complicated element of the civilian space agency’s Space Launch System (SLS) rocket. After a review of data from two months of functional testing at the Michoud Assembly Facility (MAF) in New Orleans, the engine section element of the first SLS Core Stage is complete and is now cleared to be mated to the rest of the vehicle.



Reaching this engine section milestone took much longer than original estimates, which complicated the schedule for the first SLS launch on Artemis 1. Early in 2019, with the finish line for the engine compartment not appearing to get closer, the final assembly sequence was rewritten to do the remainder of it horizontally.

Work on the upper “four-fifths” of the stage was released from its dependency on the engine section in the Spring, those pieces were bolted together in late May, and standalone work is mostly complete. In parallel, the engine section/boattail assembly was also relocated to the same Final Assembly area at MAF in early April to complete outfitting, connections, and checkouts.

The next step is to move the engine section and boattail to another building, rotate them from vertical to horizontal, and then come back for bolting to the aft end of the stage in the last “major join” in its assembly. Boeing continues to aim to complete the full stage in December and barge it to the Stennis Space Center in Mississippi for a full, integrated checkout and acceptance firing as part of the Green Run test campaign.

Engine section functional testing completed

Functional testing of the engine section and boattail started in mid-June after all of their hardware was installed. The elements are parked in Area 47/48, the SLS Final Assembly area of Building 103 at MAF, in a tool designed to facilitate the revised final assembly sequence, which put together the remaining big pieces of the stage in a horizontal orientation rather than a vertical/horizontal mix.

The engine section is the compartment at the bottom of the stage where the Main Propulsion System (MPS) elements come together to meet up with the different propellant, hydraulic, pneumatic and electric interfaces of four RS-25 engines. The boattail assembly is a tapered, structural extension on the bottom of the engine section which includes a downward-facing base shield that provides protection from the heat generated during launch and ascent.

(Photo Caption: The integrated engine section/boattail assembly surrounded by scaffolding at MAF on June 28. Additional scaffolding is installed on the inside to provide hands-on access and support for technicians without having to step on or put excessive weight on internal flight hardware. The internal and external access platforms and the controlled work area inside need to be taken apart and removed to allow the assembly to rotated over to horizontal to mate to the rest of the Core Stage.)

As with the forward skirt and intertank elements before, a “break of configuration” review is a standard decision point at the end of production of each element. The functional tests systematically activate and operate all components from sensors to computers to power units to valves to pumps.

“We just finished up a dry run review of the break of configuration package, so we will finish up functional testing in the engine section and then have our quote-unquote, formal, ‘break of configuration review,'” Terry McGee, Engine Section Engineering Manager for Boeing, said during an August 14 interview.

“We’ll have all the stakeholders involved in that review to say ‘yup, we’ve collected all the data that we meant to collect, the data looks good’ or there may be some anomalies with it, but it’s anomalies that we can live with, it’s not an anomaly in the hardware, but more in the way that the test equipment was measuring the data and so forth, but that’s planned for later this week.”

Testing was completed late on Thursday, August 22 and the review was held the next day. Late on Friday, August 23, the ‘go’ was given by NASA to Boeing to breakdown the functional test configuration and proceed into final preparations to mate of the engine section to the rest of the stage.

The SLS team at MAF is hoping to bookend the unofficial summer in the U.S. with the final mates of the stage; the other sections were joined over the Memorial Day weekend in late May and they want to begin the sequence of moves to bolt the engine section to the liquid hydrogen (LH2) tank and fully join the stage over Labor Day weekend. Testing of components in the thrust vector control (TVC)/hydraulic system finished later than forecast, but other pre-mate preparations continued and the goal is still to pick up the engine section/boattail assembly and move it at the end of the week, which is also the end of the month.

“Because we have had a few small delays in the completing of the functional test, we have had the opportunity to pull forward some of the small items that were in that window to work now, so even though functional test/break of configuration has slipped a couple of days from the original plan, we’re still holding to the [schedule],” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, said on Thursday. “We still have work to do, and so there is some risk to that but that is still the plan and the goal for moving the engine section.”

(Photo Caption: The aft end of the LH2 tank and the -Z side of the engine section in the Final Assembly area at MAF in late May; these will be connected in the last “major join” for the stage. The engine section now has to essentially be tipped over on its side from the vertical orientation shown here, something it was not designed to do by itself. A series of tooling was built and recently delivered to MAF to allow cranes to break it over to horizontal in nearby Building 110.)

McGee said testing has gone pretty well: “We’ve had a few things that pop up,” he said. “You know, this is the first time we’re using this electrical ground support equipment (EGSE) and the first time we’re functionally testing out the wiring systems and the hydraulic and the pneumatic plumbing systems, so as you can imagine you get in and you have discovery and so forth.”

“That’s kind of the reason why you do your functional testing. [It’s] nothing that we haven’t been able to overcome. We had several test cases, functional tests, lined up.”

“If we came across a stumbling block on one, where we had to go scratch our heads and think for a while, we were easily able to slew our sights over to one of the other functional tests and keep testing,” he added. “So no real downtime during the functional testing, but we do have what I would call minor discoveries come up along the way.”

With all the working equipment inside the engine section configured and connected, the EGSE hardware and software emulators plug into channels that the real vehicle avionics will ultimately use.

“From the EGSE we run all the big cables over to the systems tunnel interface on the engine section,” McGee explained. “The systems tunnel runs from the forward skirt all the way down to the engine section and all those cables come into the engine section volume, so we stimulate, power up, [and] energize all those avionics systems through that systems tunnel interface.”

“There’s also a few electrical and pressure interfaces that go through the umbilical panels and then throughout the design of the helium system and the TVC (thrust vector control) system there are test ports that we use to go tap into the pressurization panels of the test equipment. So most of that test equipment is all standing outside of the engine section volume and what I would call electrical and pressurization umbilicals run into the engine section volume and tie into test ports or the systems tunnel.”

(Photo Caption: A shot of hydraulic system servicing ports on the boattail. The boattail is a set of structural pieces that are first bolted to the bottom of the engine section and then utilities that extended out of the element to the boattail were connected up. The engines themselves extend out through the boattail, along with servicing ports like this, and other openings such as for exhaust gas from the four auxiliary power units.)

As with the other parts of the stage outfitted with avionics, the functional tests were the first real opportunity to verify that the computer equipment was talking to the rocket equipment correctly. All the functions of the rocket come together in the engine section, and there’s a lot more of everything there to test.

“For this engine section functional testing we’re doing the isolated subsystems,” McGee said. “We’ll do a more integrated kind of test when we get to Final Integration Functional Test (FIFT), but for engine section, we’re really testing out the avionics systems that are in the engine section. (The FIFT will be performed when the stage is fully assembled with its engines installed.)

“Anything that that system would need to talk to like a flight computer up in the forward skirt would be simulated by the electrical ground support equipment, so it would be kind of a simulated portion of that test. We test the avionics, there’s hundreds of sensors in the engine section, there are solenoid valves that have to be actuated and so forth through the avionics systems.”



“We do avionics, then we do Main Propulsion System (MPS) testing, which is mostly helium-activated systems that operate like the big pre-valves,” he added. “We do a check on check valves and solenoids and so forth that are run through the pressurization systems, and then the last one is [where] we activate the thrust vector control system, the hydraulic systems that run the actuators that give the gimbal control of the engines.”

The last of the tests of individual parts was with those TVC/hydraulic system pieces, which have a lot of moving parts. “We completed the portion of the TVC test with the CAPUs (Core Auxiliary Power Units) yesterday and today we are working to complete the functional test of the circulation pumps,” Looser said on Thursday.

“We’ll go through engine 1, engine 2, engine 3, engine 4, one at a time testing them. Each [RS-25] engine has its own TVC system, right? So we’re kind of numbering them by the engine that they’re supporting, they are tested as individual components.”

More broadly, Looser described the purpose of the testing. “If they’re electrical components or sensors [the purpose is] to check that you have continuity and those things are performing like they’re intended to,” he said.

“If it’s a valve or a pneumatic or a hydraulic system that they are up and functioning as they’re designed to. It’s not a full system test to put the component through the paces, it’s a functional check.”

The Core Stage is the ground-started sustainer in the “stage and a half” basic SLS configuration. The first flights will fly with a derivative of the Delta 4 upper stage and NASA and Boeing are developing a larger upper stage for later flights.

For the first Block 1 flights the Core Stage finishes the eight-plus minute orbit insertion, starting from just before liftoff and carried off the pad by the two, large Solid Rocket Boosters (SRB). The SRBs burn out after two minutes, leaving the Core Stage to continue to insertion.

(Photo Caption: Graphic of the Core Stage divided into its elements and highlighted within the context of the overall launch vehicle. The engine section is shown here with the boattail attached and engines installed, although those are separate parts. The boattail was not attached to the engine section until a year and a half after integration of this first build started. The engines won’t be installed until after the engine section and boattail are attached to the rest of the stage.)

The stage is designed around the RS-25 engines that are modified Space Shuttle Main Engines (SSME), with the large propellant tanks and MPS sized to feed four engines instead of three. It houses the flight computers at the top that choreograph final countdown and launch operations, assess and manage vehicle health, and navigate the rocket to a planned targeted speed and point in space when its engines are done firing.

The stage MPS also provides the hydraulic power for pointing the RS-25 engines to steer the vehicle during powered flight to converge on the planned Main Engine Cut Off (MECO) speed and location targets.

The Core Stage is the major new development for the SLS Program. The RS-25 engines were already built and flying Space Shuttle missions and the evolved Solid Rocket Booster design was already in development testing when the predecessor to SLS, the Constellation program, was canceled in 2010-2011.

The engine section is the most complicated part of the stage and the time it would take to assemble all the equipment for the first time in the cramped interior was underestimated; issues with parts contributed to some of the delays, but the first-time learning curve was also a major factor.