The Structural Test Articles (STA) that will make up the first core stage of the Space Launch System (SLS) rocket is in the final phase of construction at the Michoud Assembly Facility (MAF) in New Orleans. Focus is now switching to the welds on the core that will create the first flight core that will launch Exploration Mission -1 (EM-1).

SLS Cores:

The work at MAF is gearing up to preparing to get the first flight Core Stage ready by the end of September next year, in order for it to be sent on its journey that will culminate in launching from Pad 39B at the Kennedy Space Center.

Even though the major elements are welded, they still have to be coated with primer and thermal protection, outfitted with all the internal structures and plumbing, and assembled into a rocket stage.

NASA and prime contractor Boeing are nearly done with welding all the barrels, rings, and domes into the five elements that make up a SLS Core Stage: from top to bottom, the forward skirt, the liquid oxygen (LO2) tank, the intertank, the liquid hydrogen (LH2) tank, and the engine section.

“We’ve only got eleven welds to go before all of Core Stage 1 is welded up,” said Steve Doering, NASA Core Stage and the Stages Element Manager for the SLS Program, speaking to NASASpaceFlight.com.

Ahead of the first SLS flight both flight and Structural Test Articles (STA), also referred to as qualification or ‘qual’ articles, are being welded together in the Vertical Assembly Center (VAC) in Building 110 at MAF; the welds remaining will finish the flight LH2 tank and assemble the LO2 tank structural test article, the flight LO2 tank, and the forward skirt.

The 130-foot long LH2 tank STA was recently welded and after being removed from the VAC, it was rotated to horizontal and moved into Area 6 of Building 103, where it is being outfitted with strain gauges and the associated wire harnesses and technicians are inside closing out the friction stir welds.

The flight articles will get additional sensors.

“We’ve got a whole series of what we call DFI and OFI,” Mr. Doering noted.

“The DFI is developmental flight instrumentation and the OFI is operational flight instrumentation and it is effectively a series of strain gauges, different kinds of temperature sensors, pressure sensors and that whole suite has been developed through the Chief Engineer’s Office to help gather data in flight to validate that the predicted environments are correct or if we have to update or change our environments so that we understand where our margins are.

“The DFI is only going to fly on the first flight and we’ll probably have some on the second flight. After that, the developmental flight instrumentation doesn’t get applied anymore, but a lot of the operational flight instrumentation which is the data you normally see in flight will remain.”

Engine section structural bolting:

While welding for the first Core Stage wraps up in the VAC, structural bolting has started on other elements.

As shown to the media during a tour of MAF’s gigantic Building 103, the major engine section STA structures, the engine thrust structure and the barrel, are being bolted together in the structural assembly jig in Area 51.

“What we’re doing now is we’re bolting all of the structural elements to the inside, so the thrust structure is being mated to the barrel, and then we have all of the support rings that go around it, the SRB fittings, and then the longeron supports on the inside,” Mr. Doering explained.

“[It takes] about 2500 or so bolts to bolt it all together, and then once that’s done it will come off of that jig here in late September and it’ll get mated up with the simulator and that was the big silver thing that was right across the aisle way from it.

“Its planned shipment to Marshall [Space Flight Center] is going to be this Fall.”

In addition to the engine thrust structure, the SLS booster aft attach points are located in the engine section.

“There’s an internal ring that goes the full circumference of the inside of the barrel and then they’ve got some attach points on the outside,” added Mr. Doering.

“All of those are being fitted up as we speak. And then there are some vertical support structures that help with the loads and then if you look at the picture of the rocket the feedlines for the oxygen tank come down along the outside of the tank and then they [re-]enter the [vehicle] at the engine section. So they re-enter to get mixed with the hydrogen to feed the engines.

“We call those the feedline downcomers; there’s a series of brackets that go on the outside of the engine section that hold that feedline in place and we’ll also be putting those on because those carry some load as well.”

In contrast to the Shuttle External Tank which had a single, 17-inch diameter LO2 feedline for three engines, the SLS Core Stage has two 16-inch diameter feedlines to feed oxidizer to the four RS-25, Shuttle-heritage engines.

Intertank structural assembly:

The intertank that sits between the two cryogenic tanks is the only one of the five major elements that is not welded; preparations to begin assembly of the flight intertank were just about complete.

“The panels you saw loaded in the jig last week were there doing fit up assessments, to make sure that the forces I need to use to get them lined up aren’t greater than what the jig itself can support,” Mr. Doering said.

“That was a successful test, so they should be offloading all of those panels from the jig, they’re going to set them aside, and then the thrust structure which was kind of off to the side in the back – that gets loaded into the jig first and lined up and centered and then you build the panels around it.

“We’ll be loading the flight thrust structure for the intertank into the jig and then start bringing the thrust panels, which go on first.

“They’ve got the holes that fit on that solid rocket booster fitting, those will get loaded in and then we’ll start populating it with the rest of the six panels until we get a full assembly and then we’ll start doing the match drilling in assembly for bolting it together. I think there are 300 bolts on each of the vertical joints between each panel.”

Getting STAs ready for foam:

It’s not just the flight Core Stage elements that will be sprayed with foam.

All the structural test articles will get the same thermal protection system (TPS) applications; for most of the elements, that means foam will be sprayed to cover them after a primer coat is put on first for corrosion protection.

The LH2 tank STA will be the first to go through the primer and foam spray process.

“There’s two real reasons for that,” Mr. Doering explained, referencing why the STAs are getting the same TPS sprays.

“One is that we want to test like we fly, so we’ll put it in the same configuration as flight – but we’re also going to cool it down to cryo temperatures.

“In this particular case, we can’t use hydrogen – because that’s a lot of hydrogen in a test environment that’s got flammability concerns for the test stand – so we will fill it with nitrogen, and that gets us to cryo temps of about a hundred degrees less than hydrogen, but that’s close enough that we can close the rest of it by analysis.

“So for the two tanks – the hydrogen tank you saw and the oxygen tank – those two tanks will both have primer and foam sprayed on them before they get mated up to their simulators and shipped to Marshall.”

The primer and foam sprays will be done in Cells P and N, respectively, located in Building 131 at the huge New Orleans facility.

“As a matter of fact, right now in Cell P, where we do the primer spray, we’re doing the development work for that,” Mr. Doering noted.

“And the tank that we’re using for that is our oxygen weld confidence article. We didn’t need to build a whole tank to get a weld confidence done, but we built a full tank so that we could exercise all of the TPS application for tanks on a full tank before we started doing it on either the qual or the flight units. So that’s in work now.”

The current schedule forecasts that the LH2 tank STA will start the primer and foam spray process at the end of September, once the development sprays are done and the tank is configured and ready.

The whole process will take a while, especially for the LH2 tank which is much longer than the other elements and by itself takes up almost two-thirds of the Core Stage length.

“The hydrogen tank is probably going to take on the order of two months for prime and foam and of course the oxygen tank will be a little bit less than that,” Mr. Doering added, using an analogy to explain the length of the process.

“You know the set up in getting ready to spray is your time-intensive, your labor-intensive process; actual spraying – it’s kind of like when you’re painting your house, right? All the time it takes you to tape and put ground clothes down and get everything ready takes two days, but actual painting may take you two hours.”

Engine section gets cork and paint:

The engine section is the one element that won’t get foam. Both the STA and the flight article have received a primer coat and instead of foam, they will have a layer of cork and a layer of white paint applied.

“It’s getting cork on it and the reason we’re not doing foam on the engine section is because the cork has a much higher tolerance for heating,” Mr. Doering explained.

“It sits right down there next to the engines so it’s got a significantly higher heating environment and so the cork is the material of choice there. We could have used foam but the thickness that we would have had to apply would have been both technically challenging and we would have had to work a whole bunch of design issues because of that thickness of foam.

“Foam is effectively easier to apply than cork, but it comes with its own limitations. When I spray foam, I have to clear the area and I can’t do any other work. Cork has higher heating environment capability, but I’m also able to apply the cork to the outside while I’m working on all of the integration of the plumbing and sensors and avionics boxes on the inside. So I can do parallel work with cork that I can’t do with foam.”

As for the white paint, “the paint on the outside of it is there because cork has a tendency to absorb water, whereas the foam doesn’t,” Mr. Doering explained. “We don’t want to paint the foam because it serves no purpose, other than make the rocket heavier than it needs to be, but we do paint the cork because we don’t want it to absorb the water as it sits outside.”

STA priorities:

The order of which elements went into the VAC to be welded was juggled to optimize schedules for time and the same could happen with the upcoming primer and foam sprays in Cells P and N, respectively.

“We’re doing a dance right now between who gets to go into Cell P and Cell N when, based on when we need the data,” Mr. Doering said, referring to the data from structural qualifications that will be done at Marshall. “We will reserve the right, if you will, to go re-look – we continuously look – at our schedule.

“Right now…I’m going to do the hydrogen qual tank first, then I’m going to do the oxygen flight tank second, then the hydrogen flight tank third and then the oxygen qual tank last because I don’t need that structural data to support the Green Run.

“I want to get the hydrogen qual [tank] in work quickly, then I’ll do the flight units so I can do all the integration, then I’ll come back and do the oxygen qual.

“And in the meantime, I’m doing the cork installation on the engine section and I’ve got a whole bunch of spraying I’ve got to do on the intertank and the forward skirt and those happen in a different area.”

The Green Run test is a major milestone for Core Stage flight/launch readiness and the EM-1 flight stage will be structurally bolted to the B-2 test stand at Stennis Space Center in Mississippi at the same points it will be attached to the rest of the SLS vehicle.

The structural qualification tests that the STAs will be involved in at Marshall will not only clear the elements structurally for launch but also clear a subset of structures for the hot-fire test.

“We’re doing the hydrogen tank first because the data coming from the structural qualification test will support Green Run.

“Since the LOX tank is above the tie-in to the upper SRB fittings where we’re tying into the structural test stand (at Stennis), we don’t need to have the LOX [structural] qual complete before Green Run, but we do have to have the intertank, hydrogen tank, and engine section quals done and [certification] loads done before Green Run.”

Delivery, assembly schedules:

The engine section STA will go to Marshall first, but when the rest of the STAs are fully outfitted and stacked with their respective simulators, they will get a (Pegasus) barge ride from MAF to Marshall, currently targeted for Spring next year.

“It’s going to be springtime next year; they’ll come on two separate barge trips,” Mr. Doering explained, “because I can’t fit the hydrogen tank with simulators and the LOX tank with simulators on the same barge.

“So right now, we’re targeting the hydrogen tank to come first. Obviously, it’s coming through the factory first. And then when the barge comes back, we’ll load up the oxygen tank and bring it up here.”

Meanwhile, the flight elements will remain at MAF; after TPS application they will continue with sub-assembly outfitting separately until final assembly.

At over two-hundred feet in length, the Core Stage is too tall to be vertically stacked all at once, in even the monster buildings at MAF.

It will be done in two major sub-assemblies vertically in Building 110 cells next to the VAC, with a forward join of the forward skirt, LO2 tank, and intertank, and an aft join of the LH2 tank with the engine section.

Mr. Doering said the current schedule forecasts that stacking in the May-June time-frame next year. Once those are done, the two sub-assemblies will be rotated to horizontal and make the short trip on a transporter over to the Final Assembly area of Building 103.

The final join to make a complete Core Stage element will be done horizontally in Final Assembly, after which final outfitting of elements such as the four RS-25 engines will occur and checkouts of the integrated rocket will begin.

Completion of the fully-integrated Core Stage is still targeted for the end of September next year.

(Images: NASA and L2 – including photos from Philip Sloss (full collection and videos) and SLS renders from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)

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