After a week and a half long river trip on Pegasus, the second of four Core Stage structural test articles (STA) arrived at the Marshall Space Flight Center in Huntsville, Alabama, to start final preparations for a series of structural tests planned to begin this Summer.
The Space Launch System (SLS) Program completed construction of the STA at NASA’s Michoud Assembly Facility (MAF) in New Orleans.
Following assembly of the three-piece intertank STA, Core Stage prime contractor Boeing and facility operations and maintenance contractor Syncom Space Services (S3) placed the hardware on the new Multipurpose Transportation System (MPTS), which rolled the STA down to the Pegasus barge in its first official SLS duty on February 22.
Eventually, the MPTS will move finished Core Stages to and from Pegasus for trips between NASA assembly, test, and launch facilities in the region.
IT STA assembly completed at MAF
The intertank structural test article is the second of four completed, following the engine section STA that was shipped to Marshall last Spring and recently completed testing. It will be followed to Marshall by a liquid hydrogen (LH2) tank STA and a liquid oxygen (LOX) tank STA later this year.
The intertank is the only one of the Core Stage’s five major structural elements that is not welded. The other four structures are the forward skirt, the LOX tank, the LH2 tank, and the engine section. The barrel-shaped intertank is positioned between the LOX and LH2 tank on the stage, with a large crossbeam that runs through the middle where the forward end of the two SLS solid rocket boosters (SRB) attach.
The panels that make up an intertank barrel are bolted together because they are too thick to be welded. There are eight panels around its circumference, with two thrust panels at either end of the crossbeam. The outside of the panels is reinforced in places with longitudinal and circumferential ribs.
“That was a very challenging manufacturing process because it’s ribbed on the outside [but] not the inside,” SLS Stages Dry Structures Manager Heather Haney explained. “It took a long time to get those panels made.” The barrel was bolted together with the large crossbeam in a structural assembly jig at MAF over the last half of 2017. The crossbeam was loaded into the jig first and the eight panels were built up around it.
The STA consists of the main intertank qualification article, which is essentially a carbon copy of just the flight intertank’s structural elements, and simulators representing the part of the LOX and LH2 tanks that attach at the top and bottom of the intertank, respectively. The ends of the three hardware pieces have built-in rings that form an attachment flange at the interfaces between them. As will be the case for the real thing, three-hundred and sixty bolts, one for each degree around the circumference, mate each flange.
The areas around the flange are also outfitted with hardware to chill them down to cryogenic temperatures to better represent the launch environment the rocket will experience. “You can see in two places they’ve got the ‘Versa-Foam,'” SLS Stages Integration Manager Tim Flores noted.
“It’s not the type of stuff that we would do for the flight article, it’s just to create insulation so we can pump liquid nitrogen through there. The intertank is between the LOX tank and the hydrogen tank, so we’ve got to cool both of those two interfaces.”
The pieces were stacked vertically in Building 110 at MAF, the Vertical Assembly Building (VAB) that adjoins the main Building 103 on its west end. “We ended up having to use two cells which is pretty unique,” Flores said. Cell A in the VAB at MAF has been used for stacking launch vehicles since the Saturn days and Cell D was recently renovated into a second stacking or integration cell for SLS.
Flores explained why both cells were used: “It was access. First access to the two cells, and then access to where you needed to get to, to be able to do the Versa-Foam, because Cell D has a moving platform that actually can go up and down it, but it only goes so far. So using that one and the unique stacking sequence lended itself to being able to get to where we needed to do the Versa-Foam.”
In addition to the two stacking cells, Building 110 is also the home to the Vertical Assembly Center welding tool, a washing machine in Cell E that cleans the inside of the LOX and LH2 tanks, and next door in Cell F, a hydrostatic proof test facility for verifying the integrity of LOX tank welds. With SLS getting ready for its first flight, there are nearly two full sets of hardware currently being processed to ship out of MAF.
The intertank STA was surrounded by propellant tanks during stacking, creating some of the access realities that Flores noted. The inside of the flight LH2 tank was being washed in Cell E and the LOX tank STA was being proof tested in Cell F at the same time.
First SLS use of new ground transporters
After the STA was fully assembled, cranes lifted it out of the vertical integration cell, rotated it to horizontal, and placed it over the new Multipurpose Transportation System or 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 attaches from the vehicle hardware to automated mobile transporters called self-propelled mobilized transporters (SPMT).
The simulators on both ends of the intertank STA are attached to hardware interface structures (HIS). In the case of the STAs, a common HIS is used on both ends; 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.
As commanded by operators, four remotely-controlled SPMTs roll underneath the MPCs (two for each) and pick them up to move the whole assembly. “The SPMTs themselves are actually pretty amazing because they can crab, they can go any direction, they can turn on a dime,” Flores said. “Then they put them in group mode, whether it be two, whether it be four, but they all move together wirelessly. As close as they are right now, we just wired them.”
Once the STA was fully secured to the MPTS, its first move was backwards. “As soon as the IT structural test article was ready to go we moved it back east to get it out of the way,” Pat Whipps, the SLS Stages Resident Manager at MAF noted. The washing machine cycles on the flight LH2 tank were done and it needed to proceed directly to its next station. In fact, the LOX tank STA proof test was also complete but it would have to wait for the STA to leave and its next station was still occupied by the flight LOX tank.
On February 22, the SPMTs rolled the STA assembly over an approximately mile-long route from Building 110 to where the Pegasus barge was waiting at the dock. Although they have different speed settings, Flores said SPMT operators will tend to move at the slowest of their speed settings when carrying space hardware. The launch and ascent loads that the hardware is designed to take are not along load paths it will see on the MPTS, where it is being transported horizontally.
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. “On the way there, we’ll have them riding at the same time, but they’ll be disconnected,” Flores said before the trip.
“They’ll be moved up to the front of the barge, locked and chained down. The same thing we’ll do with all those D-rings that you see around on the MPCs. Everything will be locked down on the barge, but they’ll be separated.” Flores also noted that the SPMTs are needed back at MAF before the barge can return, so they will be returned by ground transportation once they have finished their work at Marshall.
Pegasus took a different river route from MAF to Marshall this time. When the engine section STA was delivered last Spring, the barge was towed by tug boats up the Mississippi River to the Ohio River and then to the Tennessee River. For this trip, the barge route went up the Tombigbee River to the Tennessee via the Tombigbee-Tennessee Waterway.
“We’ve prepared the Pegasus to be able to handle the Tombigbee,” Flores said before the trip. “We had some antennas and some things that were about four feet higher than they needed to be. It’s all about clearing the bridges. That helps us and then we understand how we can ballast a little bit further and get a little bit deeper and so we think there shouldn’t be an issue on the Tombigbee. That was always the problem was there’s more bridges and they’re all shorter.”
Intertank test stand
Once Pegasus arrived at the dock at Marshall, the SPMTs again picked up the STA assembly and made the drive to the intertank test stand in Building 4619. This is a much longer drive than at MAF. “They’re estimating that whatever it is — the six or seven miles that it is — is going to take us thirteen hours, so it’s going to be a long day,” Flores said prior to the trip. The roll was completed on March 6.
“We’ve been working on our test stand many months prior to this, so we had to do a lot of work to get the stand ready to accept that test article,” Haney said, picking up the story. “So once this gets to Marshall, it will be outside and we will break it over and then we have to do a lot of work.”
“We have a lower load ring that will attach to it and then we will have an upper load ring. When those are attached we’ll back it into the 4619 test stand and then we will continue with the build up.”
The STA is placed in the stand upside down, with the crosshead that moves to help apply loads pushing down on it during tests. “We’re saying roughly four weeks to finish up to get it ready to put into the test stand and then we won’t start testing probably until around August,” she noted. “So there’s a lot of build up that has to continue after that gets in.”
The test article is also covered with instrumentation and wire harnesses which have to be connected to a data acquisition system to record measurements during all the test runs. The structure itself will be surrounded by the test stand elements and hooked up to hydraulic actuators that will apply the different forces to the STA. The cryogenic piping in the STA will also be connected to liquid nitrogen supplies for chilling down areas in some test cases.
Haney explained how the test stand will structurally attach to the STA: “There’s six panels on each side and so then what we have is what we call a belt buckle — it’s an orange piece with a hole in it. There’s over a hundred hydraulic actuators for this whole test. About 24 of those, 12 on each side, they attach to that belt buckle.”
Referring to the covers over the SRB attach fittings, she continued: “That was what we call a ‘top hat.’ Those will sit in the belt buckle where we have all these hydraulic actuators that will apply the forces that we’ll see during launch and ascent.”
“So all the forces that it is going to see are going to get pushed into the 27-foot thick concrete floor,” she explained. “There’s lower load rings that are attached to the concrete and there’s an upper load ring that is attached to the crosshead — that’s what will push those forces on it.”
Dozens of tests
Once the STA is fully installed and the test stand is configured, a series of dozens of test cases will be run. “Right now it’s about forty-eight test cases,” Haney said. “As we get closer to test and we refine things, then we may do less. But whatever we do as a team we make sure that we have enveloped all the loads that we need to see, [so] that we can say that it’s qualified for flight.”
Preparing to run the cold test cases will generally take a long time. “If you’re going to run a cryo case, you have to get it cold enough,” she explained.
“So it could up to six hours just to get it cold enough and then you go run the actual test case. Depending on the case, it depends on how much load you’re adding and we add load in increments of 20 percent and up until we get to 100 or if we’re doing ultimate it would be up to 140 and then we back down the same way. And every time we increment 20 percent we flag a data scan, we look at it, we say we’re good to go, and we continue up.”
Limit load cases test the structure to 100 percent of the maximum loads (or combination of loads) expected to be seen in flight, while ultimate load cases test the structure to 140 percent of those maximum expected loads.
Similar types of forces will be put on the intertank article as were applied to the engine section. “It’ll be the same, but it may be different ways we do it,” she said. “It’s going to see a little higher forces than we saw on the engine section, remember the solid rocket boosters attach here and so it’s got to be beefy where it attaches to our intertank.”
As the Dry Structures Manager, Haney will be watching each test closely from the test control room. “I was there for every test we did on the engine section and I’ll be there for every test we do on the intertank,” she said.
When asked how long the test series will run, she said: “Probably until around the first of January. We don’t test every day, you know you run a certain number of test cases and then Boeing, our partners, they go off and they look at that data and make sure we got what we need.”
“Because it’s NASA and Boeing, hand in hand,” she emphasized. “When we add load, Boeing is going ‘yeah, that’s good, continue.’ So we’re all looking at it and making sure that it’s ready [to proceed]. We don’t run [tests] every day, but it’ll take about three to four months to do those.”
Engine section structural tests finish with ‘margin test’
The engine section STA tests were recently completed at Marshall in a load test area of Building 4619 near where the intertank STA will be tested.
One of the decisions that was pending for a long time was whether or not to do a “test to failure” after each STA completed its required series of tests. For the intertank STA, Haney said, “We haven’t decided that for sure yet, but more than likely we will.”
The engine section STA was the first to complete its full test series and after those were done, the decision was made to do a “margin test.” The structure was taken up to a factor of safety of 2.0 or 200 percent of the maximum expected loads. “We didn’t go to failure, but we did [take it] to 200 percent,” Haney said.
The 200 percent loads were held for five minutes. “I’ll say we were all holding our breath — ‘OK, we’re here, alright counting down,'” she added. “It went really well. It’s almost like, you got all worked up prior to the 200 percent mark and you’re like ‘that’s it?’ The engine section tests went very, very well.”
When asked if the structure ‘failed’ at 200 percent, she explained: “It was yielded in areas, so it did. But it wasn’t like you heard a loud crack. It was very good.”
Haney said the data from the margin test was very close to what was predicted by structural models. “Our Boeing guys did a really good job with their predictions. And our NASA guys, you know we’re all hand in hand doing this. But Boeing did a really good job with their predictions. And so that’s going to tell us a lot about future needs.”