Boeing completes first NASA SLS engine section, getting ready for final Core Stage mate

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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.

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.”

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.”

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.

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.

Final pre-mate work on ‘four-fifths’ of the stage

The changes to the final assembly plan applied earlier this year allowed work on the rest of the stage to continue in parallel. The forward join that includes the upper three elements of the forward skirt, liquid oxygen (LOX) tank, and the intertank, was attached to the liquid hydrogen (LH2) tank over the Memorial Day weekend in late May.

Since then work on the mated, upper “four-fifths” of the stage has progressed to point where it is basically ready for the engine section mate.

“We’ve maxed out our work content for the feedlines and the press lines,” Craig Williams, Boeing’s Core Stage Integrated Product Team Director, said. “The passive roller stands and the rotation equipment that we sit the vehicle on is maxed out in terms of weight and how much work we can do and still hold that article from a weight standpoint and a center of gravity standpoint.”

The LOX tank rides up front in the Core Stage, around one-hundred, fifty feet above the engines, so two, large-diameter feedlines run from the bottom of the tank that sits inside the intertank, down opposite sides on the exterior of the LH2 tank, and then into the engine section. Those long lines are connected in sections, attaching both to each other and to fixtures on the LH2 tank that hold them in place.

As Williams noted, work is complete on the sections of the LOX feedline that could be installed prior to putting the whole stage together. The remaining pieces will be connected from the bottom of the stage towards the top.

“[It’s] critical that we have the engine section installed,” Williams said. “That first feedline, the stub out from the engine section to that feedline number two we call it, it’s critical that we install that one first and then connect to feedline two through five.”

“Then that’ll leave six and seven which go up into the intertank as the last to install and that gives us the max flexibility of the BSTRAs (ball-strut tie-rod assemblies) in the feedline assembly to maneuver it into place,” he added. “It’s a much riskier operation to install the engine section one last, so the preferred production method is to do that engine section feedline first.”

The current segment of work inside the now-cramped volume of the intertank is also complete. After the LH2 tank was mated to the forward join, the volume in the intertank was largely consumed; however, an access kit was installed inside to complete several internal connections.

“For the intertank work volume that access kit was completely installed and the required work is all complete,” Williams noted. “We’re in the process right now of removing that access kit to allow us to go to the next stage of operations and [by] that I mean [we] have to have that kit out to rotate the vehicle.”

The front of the stage needs to be rolled ninety degrees from its current orientation for the engine section mate. It was rotated most of the way for one more major installation first.

“We’re taking out the intertank platform right now and this weekend we will be rotating that four-fifths of the rocket to what we call fourteen degrees up,” Williams explained on August 14. In that orientation, the footprint of the systems tunnel is pointed straight down at the floor.

“That will allow us to roll in the aft systems tunnel assembly that sits on that independent tool,” he noted. “We’ll air pallet that over and we’ll install the aft system tunnel on the hydrogen tank starting Saturday and that’s the last amount of weight we can put on the system with those passive roller stands and the T-RATT the rotation and transportation tool.”

Core Stage subassemblies are often held in Boeing’s Rotational Assembly and Transportation Tools (RATT); the T-RATT is a Transitional RATT that they modified for more flexibility in handling different subassemblies in different phases of the assembly/production process.

Williams subsequently noted that the long section of the systems tunnel on the LH2 tank was attached as planned on August 17 and since then the team is completing the full mechanical installation. The sections are staged with wire harness runs and tubing on a base plate that is fastened to the spray-on foam insulation (SOFI) on the outside of the stage. Cover plates will subsequently fit over the top of the tunnel.

Williams added that work connecting the installed systems tunnel sections will continue while the top of the stage waits for the engine section mate. “There was an interim rotation in there, to go from the fourteen degrees, if you will, to ninety, to allow for further system tunnel work to be done,” he said on Thursday.

The ninety-degrees up orientation is basically back to the orientation the stage was in during the Summer. “That rotation in there just allows extra work to happen during this time until the engine section is ready — wire harness routings, small-diameter tube installations, and then some TPS or foam ramps that we’re actually looking for the possibility to eliminate based on requirements,” he said.

“The final rotation to mate the engine section is at zero degrees and we’ll place that in that position when the engine section is ready to come back from 110.”

Preps for final mate

Now that all parties have signed off on the functional test results, there’s a lot of preparation work to do before the front of the engine section can be rotated and connected to the back of the LH2 tank. Not only do the intricacies of the test equipment, controlled work area platforms and fixtures, and scaffolding have to be cleaned up and moved out of the way, but there’s final quality inspections to do.

“What we’re trying to do is make sure we have all of our mechanical assembly type work done on the inside of the volume,” McGee explained prior to the review. “So two things before we start pulling out stands, internal stands and the external stands. We have to finish the break of configuration, so we’d have to say ‘ok, we’re ready to turn off our equipment and move it all back, does everybody concur with that?'”

“Once we get that then we’d be free to start disconnecting, and then to have what we call ‘shakedowns,'” he noted, referring to a protocol of Defense Contract Management Agency (DCMA) final quality inspections of the inside of the element while there is still well-established access.

“So we have internal Boeing and also government DCMA shakedowns, which is really going into the volume and looking to say ‘ok, we’re done with this work statement,'” McGee explained. “This is your last chance to come in and really look at everything before we go horizontal and mate, so that latter part of that work has to take place after the break of configuration.”

While the results of the different functional tests were being reviewed, some configuration and TPS work also continues on the engine section and boattail.

“There’s four vents that go on the inside of the engine section, these are vents that keep rainwater out and bugs and all that kind of stuff and so those are kind of the last few things to put in,” McGee noted mid-month. “We may have already finished this.”

“Right in front of the vent is what we call the haz gas tube, which is the system inside the engine section to sniff for any hazardous gases that are building up in the engine section volume while you’re either at Stennis or on the launch pad, so we are just buttoning up a few things like that. Other work that we’re doing in there is installing Green Run hardware, there are some cameras that are in there and wires for Green Run sensors, we’re finishing up that work.”

“We’re also doing some RT-455 [applications], the ablative material on the outside, trying to get all of that work done while we’re finishing up with our functional testing,” McGee added. “It’s hand-applied stuff that you can mold and shape.”

“It cures and then you come back later and shave it down to profile,” he explained. “So it’s a material that goes over like fastener heads and structural joints where the cork can’t really adhere to a nice, smooth surface.”

In addition to the inspections and removal of access platforms and other non-flight equipment, there will be a few final tasks to complete. The temp-installed LH2 propellant tubing, the four individual engine feedlines and the fill and drain line, will also have to braced for all of the upcoming shipping and handling of the engine section — first by itself around the factory, then with the rest of the stage on the trip to Stennis, and finally being lifted up into the B-2 Test Stand.

“To go horizontal with the LH2 feedlines detached from the hydrogen tank, we’ve had to manufacture some tooling struts that tie the LH2 feedlines over to the primary structure and hold those in place during transportation,” McGee said. “So we’ve got to put that in and then we’ll be ready to start pulling out the internal access stands.”

McGee also noted one of the tasks is to connect lines to pressurize the propellant tanks for transportation.

“To transport the Core Stage we have to pressurize the LOX and the LH2 tanks, so one of the last things that we do is hook up those lines to the press systems and then run those over to the access door, where we have a pressure panel that is mounted to the access door,” he said.

“So just a handful of little things like that that we’ve got to do before we start pulling out the scaffolding. When you see the stands all coming out of there, you’ll know we’re really, really close to getting ready to roll over to Building 110 for the rotation.”

Breakover and mate

The engine section was designed for vertical integration; it was built in that orientation, and the original plan was for it to be stacked in that orientation. The stage was designed to lie horizontally during final assembly, including all its elements, but the first time the engine section would be physically tipped over in the old plan and rotated from vertical to horizontal was after the 130-foot long LH2 tank was bolted on top of it.

There’s no building at MAF tall enough to enclose a two-hundred foot tall stage and the LH2 tank is now lying horizontally as a part of the rest of the core, so the engine section has to be rotated to horizontal by itself. For the new, all-horizontal plan for final assembly, Boeing worked with Futuramic to design and fabricate one set of tools to put the front of the stage together and a second toolset is necessary for the engine section rotation or “breakover” from vertical to horizontal.

“There were four remaining tools to enable the horizontal assembly approach and two of them came in the early part of this week, Monday,” Williams said on August 14. “The last two are due in today, so that’ll complete our suite of tools for the horizontal assembly process.”

“It has about four days of operations over there where we break it over, we install a ballast system on it to allow us to break it over and transport it in that horizontal position and then we come back to 47/48 for the structural mating operations.”

Building 110, also known as the Vertical Assembly Building (VAB) at MAF, is where most Core Stage breakover operations are performed; although it’s not tall enough to stand up a completed stage unit, it has the height and the heavy-lift cranes necessary for components. Prior to their connection in late May, the cranes were used to lay down both the forward join and the LH2 tank on ground support equipment in the VAB.

The first step in the breakover is to drive the engine section boattail back to the VAB. “That engine section boattail tool that the assembly sits on right now, that big, blue, base tool, we’ll drive that tool from where it is currently parked back to 110,” Williams said.

Without the attached LH2 tank that gave the old plan’s so-called “aft join” the weight and balance that the completed engine section was designed for, the new tooling is a temporary substitute structure about ten to fifteen high. “We basically have, it’s like a truss system if you would,” Williams explained.

“It’s somewhere in the neighborhood of 8000 pounds, but it provides the ballast to enable that center of gravity of that engine section to be in the appropriate spot for us to be able to handle and manipulate the engine section to go horizontal.”

Williams explained that breakovers are a two-crane operation and the engine section is too short by itself for the VAB cranes to rotate. “This truss system serves two purposes, really,” he said. “The crane bridges were too close together and the engine section is not long enough to be manipulated by two cranes, so this truss system allows the two cranes to be used, obviously it separates the distance they [are] apart, with that extra [length].”

Once the cranes have rotated the assembly to horizontal, it will be lowered down onto another set of modified tools. “We have a T-RATT system, the rotation and transportation tool, there’s a slight modification we’ve done to that for this extra weight and we’ll incorporate that new set of tooling into that T-RATT to allow it to hold this broken over the engine section,” Williams said.

The indoor Manufacturing, Assembly, and Operations (MAO) Self-Propelled Modular Transporters (SPMT) that Boeing uses to move tooling around MAF will then pick up and carry the horizontal engine section/boattail in its carrier back to Area 47/48, where the flange on the forward end of the engine section will be lined up with the aft flange on the LH2 tank.

“It’ll go on those T-RATTs and then traditionally how we transfer equipment, we’ll drive those SPMTs underneath that equipment and then drive it through the factory back from 110 inside the factory, we don’t have to go outside for these operations, and then come right into 47/48. [We’ll roll] from the same east part of 47/48, driving west to meet the aft end of the hydrogen tank.”

After the stage is fully joined, one of the tasks during the remainder of final assembly will be to reposition it in Area 47. “We’ll eventually move the mated portion because it’ll need those same test utilities and commodities that Terry has just used for the engine section so it’ll be on that side of the bay,” Williams explained.

After the stage is bolted together, preparations for installing the engines will begin. “We have to go to zero degrees for the physical mate, and once we’re mated we’ll go to what we call ‘ninety degrees up,’ and then that’s when we install the engine section horizontal platform,” Williams said.

“That’s a very complex set of tooling that goes up from the floor through the boattail, if you will, and allows the humans to access the volume to install the engines and we’ll stay in that rotation until done. Once we’re in that rotation we’ll want to be in our final spot and install this tool, and then we won’t move the vehicle again.”

Boeing is still targeting completion of the work at MAF in December. “Yup, still holding December 10th,” Williams said.

https://www.nasaspaceflight.com/2019/08/sls-engine-section-ready-final-core-mate/