Just prior to expiration of U.S. federal government funding for NASA on December 21, the third of four structural test articles (STA) for NASA’s Space Launch System (SLS) left the Michoud Assembly Facility (MAF) in New Orleans on its way by barge to the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, for testing in 2019. The space agency is one of the federal agencies currently unfunded and almost all of the civil servant workforce must avoid any work activities.
Prior to NASA’s shutdown, SLS Stages prime contractor Boeing was also continuing to make progress completing production of the first SLS Core Stage, with the critical path work on engine section integration estimated to be about 85 percent complete. Some assembly, integration, and testing of critical path hardware for Exploration Mission-1 (EM-1) like the engine section, is continuing during the lapse in funding.
LH2 STA enroute to Marshall during NASA shutdown
The liquid hydrogen (LH2) tank STA was moved from MAF’s Vertical Assembly Building (VAB), Building 110, down to the dock where the Pegasus barge was berthed early on December 14. The STA consists of the LH2 tank qualification article and two simulators attached on both ends and stretches nearly 150 feet or about as long as the whole 154-foot long Space Shuttle External Tank.
Assembling the STA began with the LH2 tank qualification article being moved into Cell A of the VAB at MAF in late October. The qualification tank has an identical structure to a flight tank, but omits some flight elements not necessary for structural testing on the ground, such as parts of the propellant feedlines, operational instrumentation, and a full application of spray-on foam insulation (SOFI).
The cylindrical simulators mimic the strength and stiffness of the structures attached to a flight tank, with the forward simulator standing in for the intertank and the aft simulator replacing the engine section. Both are bolted to the tank with 360 bolts on rings around the circumference of the tank that sit between the two domes and the ends of the barrel.
Within the uncertainty about a lapse in NASA funding was the timing of a possible shutdown, which was deferred by a two-week long Continuing Resolution passed to extend funds for Fiscal Year 2019 through December 21. Although work to stack the STA and send it on its way to MSFC was completed early, it was not accelerated due to the looming loss of funding.
“We laid out a plan once we went to Cell A and we just beat it,” Steven Ernst, Boeing’s Core Stage Engineering Support Manager, explained in an interview on December 19. “We were able to do a little bit of the instrumentation work in parallel with the other mechanical mate activities, so that shaved some time off. It was good to get that out of here.”
The LH2 tank STA is the third of four test articles completed by Boeing at MAF and sent to MSFC. Previously the engine section STA left Michoud on Pegasus in April, 2017 and the intertank STA made a similar trip to Marshall in February. A structural test article for the forward skirt was not necessary and the fourth and last STA will be for the LOX tank.
The barge trip was one of the exceptions of work allowed to continue while NASA is closed. It was uncertain what work might continue when Pegasus arrives in Huntsville.
Flight engine section integration at 85 percent complete
The critical path of work in the schedule to finish assembly of Core Stage-1 (CS-1) remains outfitting or integration of the engine section. “[For] engine section integration we’re at 85 percent,” Jason Grow, SLS Core Stage Lead Propulsion Engineer for Boeing, said in an interview. “We’ve got some major feed system components to put in, we have some small tube assemblies to still put in, all of our wire harnesses still have to be final routed and checked out.”
The engine section structure was moved into the work stand for integration in late August of 2017, where equipment such as cryogenic propellant feedlines, helium pressure vessels, thrust vector control (TVC) equipment, avionics, tubing, wiring, and sensors were installed inside. A lot of the space inside the engine section will be filled by the bottom of the LH2 tank when they are mated, so pretty much all the internal engine section equipment is installed towards the bottom.
Subsystem installations and orbital tube welding almost complete. “We have about 95 percent of the orbital tube welds done for Core Stage-1,” Grow said.
“We have two more pieces for the LOX feed system, major pieces and that will finish that system out,” he added. “We’ve got four pieces of the hydrogen system to go in and then we’ve got all of the actuators for the hydraulic system.”
Except for the actuators, all the major equipment for the four hydraulic systems in the engine section (one for each of the RS-25 engines) are installed. The Core Stage Auxiliary Power Units (CAPU) and the hydraulic pumps are more recent additions following the installation of the four TVC platforms in late June.
Grow estimated that 98 percent of the over two-hundred wire harnesses for the engine section are now installed. Once final routing and checkout of the wiring is complete the avionics boxes can be installed, replacing the 3-D printed shells that mimic their form and fit and are currently holding their place.
“We’re coming up on installing those very soon,” he said. “We won’t be doing functional testing until we move into another area but we’re going to get them put in over here.”
The outside of the engine section is predominantly covered in cork. While closeouts of the cork are going on, work has also begun to apply the coat of white paint over cork.
As the remaining component installations, tube welding, and connections are made, functional checkouts to verify the working condition of the assembly will be performed. For example, the five large gaseous helium composite-overwrapped pressure vessels (COPV) in the engine section will soon be loaded and checked out.
“That’s coming up here in another three, four weeks or so,” Grow noted at the time.
“We’ll do a full system pressure checkout of them over here and then we keep them at a pad pressure for the rest of the time at MAF. We’ll still be using some helium out of those tanks during different functional tests, but we’re always keeping them topped off.”
Most of the functional testing will be done in place before the element moves to Cell A in Building 110, where the hydraulics will be fully checked out after the systems are loaded with fluid.
“That’s the first step of functional testing, fill the system up,” Grow noted. “We do pneumatic testing on our hydraulic system in this area, but we don’t do hydraulic testing until [we’re in Cell A] actually with hydraulics.”
As part of the hydraulics checkout the CAPUs will be spun up once the engine section is in Cell A. “We’ll spin them up like they would during flight,” Grow said. “We’re not anywhere near what we are in flight, but we’re actually spinning them, running like they are in flight [during] functional testing.”
While the engine section is in Cell A, it will be mated to the boattail first. Integration of the boattail and the engine section includes more than the structural bolting.
“We have some tube welding to do, we have harnesses and sensors that are pre-integrated, all that has to be mated up, so that integration between the boattail and the engine section is not a trivial item,” Ernst noted.
Later in the aft join sequence, the flight LH2 tank will be brought into the VAB, lifted into Cell A, and stacked on top of the engine section.
LH2 STA stacking moved ahead of forward join
SLS Stages prime contractor Boeing has been juggling the work schedules of the CS-1 forward join elements and the liquid hydrogen (LH2) tank structural test article (STA) for a few months. The top three elements of the Core Stage, the forward skirt, the liquid oxygen (LOX) tank, and the intertank are vertically mated or “stacked” first before subsequently being joined with the rest of the stage.
The CS-1 intertank completed its functional testing in early October and was moved into Cell D in Building 110 at MAF in mid-October, but completion of the LOX tank was delayed due to issues with some final parts to be installed. With the forward join of the flight vehicle delayed, LH2 tank STA buildup began in late October following foam applications, test equipment installations, and attachment of the aft manifold assembly.
The upper and lower “half-rocket” joins of the flight hardware have to be done from bottom to top, but some stacking of the less complex STAs has been done differently, in some cases upside-down.
For the LH2 tank STA, the qualification article tank was first brought into Building 110, rotated from horizontal to vertical, and set up in Cell A across the aisle from the CS-1 intertank in Cell D. The forward simulator, which mimics the strength and stiffness of the intertank structure, was then mated to the top of the tank with 360 bolts around the circumference of a ring that sits between the top dome and the barrel.
Stacking of the STA then temporarily bumped the CS-1 intertank out of Cell D.
“We delivered the intertank to Cell D for the beginning of forward join and created a bit of a dilemma for us because the plan for the simulator buildup is [the] tank goes into Cell A, forward simulator goes on,” Ernst explained. “The baseline plan was [then] to move it over to D, but there was an intertank in the way.”
“We worked with NASA on a plan to hang the forward sim (simulator) and LH2 tank in the aisle while we reconfigured Cell A to accept the aft simulator, but it turned out there was a lot of risk associated with that, so we actually removed the intertank [from Cell D] for a short period of time,” he noted. “Then [we] took the forward simulator / LH2 tank, moved that over [and] parked it in Cell D, reconfigured Cell A, loaded the aft sim, and then flew it back and started mating it up.”
After the STA was fully assembled, cranes lifted it out of the vertical integration cell, rotated it to horizontal, and placed it over NASA’s Multipurpose Transportation System (MPTS) parked in the transfer aisle of Building 110. The MPTS is support equipment that allows for careful ground transportation of the large Core Stage hardware.
Structural support hardware called hardware interface structures (HIS) connect the vehicle hardware to automated mobile transporters called self-propelled mobilized transporters (SPMT). In the case of the STAs, a common HIS attaches to each simulator; when the full Core Stage or mockup Pathfinder are moved, a forward HIS will be attached to each SRB attach fitting in the front, and one common HIS will be bolted to the engine section area in the back.
Each HIS is attached to a table-shaped multipurpose carrier (MPC). The MPCs also have pedestals attached to them that are used to secure the vehicle and MPTS when stationary, such as to the Pegasus barge.
After rolling the STA onto the barge, the SPMTs positioned the pedestals over mounts on the deck inside and lowered them down. Once the pedestals were secured to the barge deck, the SPMTs were repositioned in the barge for the trip.
Few exceptions to NASA closure
Even with the shutdown of NASA, support for critical mission operations are exempted activities such as for the International Space Station (ISS) or the recent New Horizons Kuiper Belt Object flyby. In addition to the Pegasus barge trip with the LH2 STA, contractor work on SLS Core Stage Production at MAF and EM-1 Orion assembly work in the Operations and Checkout (O&C) Building at the Kennedy Space Center (KSC) in Florida are also exempted activities.
NASA is trying to hold to its EM-1 schedule as much as possible and SLS Core Stage prime contractor Boeing and Orion prime contractor Lockheed Martin were already planning on working the holidays this year much more like the end of other months before the shutdown became a reality. Assembly, integration, and testing of Core Stage-1 at Michoud and the EM-1 Orion spacecraft are both critical paths in the EM-1 work schedule and work on some critical elements within those individual schedules goes on around the clock, such as with CS-1 engine section integration.
Following completion of the LH2 STA in mid-December, Boeing completed work to get the CS-1 LOX tank ready for forward join just before NASA was shutdown at the end of the day on December 21. The intertank was back in Cell D in the VAB at MAF waiting for the LOX tank to be brought in.
Standalone work on the third forward join element, the forward skirt, was completed in the Summer and it is waiting to be the last piece of the forward join some time in the new year. The LOX tank will be stacked first to the intertank, and it was moved from Area 6 in Building 103 to the VAB aisle on December 20 to start the work to lift it into place on top of the intertank.