Boeing, NASA managing SLS Core Stage hardware traffic ahead of first vehicle stacking

by Philip Sloss

LH2 qualification tank and STA

The LH2 qualification tank is currently set up in the Final Assembly area, getting ready to be stacked with its simulators to form the LH2 STA. “We’re working through [installing] all the internal qual instrumentation and getting that all checked out,” Ernst said.

NASA and Boeing went back and forth on whether or not to put SOFI on the qualification/STA tanks, ultimately deciding to run through the full, automated acreage spray process on the barrel of LH2 qual tank. After the CS-1 LOX tank cleared Cell N in late June, the LH2 qual tank took its place.

LH2 qual tank is moved from Cell N to Final Assembly area at MAF following SOFI application to the barrel section. Credit: NASA/Michoud.

“The one thing that we wanted to do with that tank was to spray that barrel, because that was the first time that we had ever sprayed something that large in the history of spaceflight,” Michael Alldredge, NASA SLS Thermal Protection System (TPS) Team Lead, explained. “And by that large, I’m talking about something that length and that diameter and that much foam.”

“We wanted to make darn sure that the system we have out there would actually go the distance, because you don’t want to find out we’re two feet short or something like that on the flight tank. That’s a really bad day [if that happens].”

“We have processes to go fix that, but we really don’t want to find that out on the real deal, so we said ‘look, we’ve got to get that qual tank in there,’ so we can at least prove out that the system will do what it’s supposed to do,” he added. “In fact we did that, it worked beautifully, we got all the test data off of it. We just took [a look at] that with our test community last week, everything worked out exactly like we planned.”

The automated acreage spray applies the foam in one, long session, taking almost three hours. “You want to make sure that your delivery system has the capacity to hold enough foam to do it, that it will last,” Alldredge said.

“That’s a long time for the equipment to do what it does and you make sure that you get the product that you want out of it. So we met all of those objectives beautifully when we did that test spray.”

LH2 qual tank in Final Assembly area at MAF, August 13. The forward dome is in the foreground in this view. Credit: Philip Sloss for NSF/L2.

With the confidence-run of the acreage spray completed, the tank was moved to Area 47 in the Final Assembly area in July with the domes of the tank only covered in primer.

“The reason we didn’t insulate the domes is because the test team went back and looked at their test profile again and said ‘hey we don’t have to do a cryogenic case for that,'” Alldredge said. “So we really don’t need the domes to be insulated to meet all their test requirements. [If] we don’t have to do it for a test requirement, there’s no point in tying up the cell, exerting the effort to go do that.”

LH2 flight tank foam sprays in Cell N

Following shortly behind the LH2 qual tank, the CS-1 LH2 flight tank was spotted in the SOFI spray facility last month to begin setups and other preparations for its full spray work. At the time of the question and answer session last week, Alldredge said the acreage spray on the tank in Cell N was imminent.

The CS-1 LH2 tank is backed out of the primer spray facility in Cell P at MAF, April, 2018. Credit: NASA/Jude Guidry.

“We’re in the process of checking off the boxes to get ready,” he noted on August 13. “Our target is to spray by the end of this week. Another thing that we learned on the hydrogen qual tank is how long it takes to heat that up.”

“We have to pump hot air into the tank to get it up to about 130 degrees (Fahrenheit), that’s our target temperature, [to] get the skin up that high,” he explained. “And so you’ve got this really, really, really long aluminum tube laying on its side, and it’s rotating for you which is nice, but you’ve still got to get a uniform heat profile through that because of the two-to-one polyisocyanurate foam.”

“It needs the heat on the substrate, whereas the one-to-ones (foam formulas) are a little more friendly in that you can spray at room temperature.”

Following the acreage spray, the domes will be sprayed with that manually applied, “room temperature” foam. “The way we do that is we actually slice it up into pie-shaped sections and so we’ll spray one at a time,” Alldredge said. “It’s a few hours to spray each one of these because you’re building up so much thickness on the domes. It’ll be a multi-shift process.”

Pie-shaped, test-spray foam wedges (dotted with rectangular sample cutouts) on one of the domes of the LOX weld confidence article (WCA), as seen at MAF in February, 2018. Credit: Philip Sloss for NSF/L2.

Alldredge said the last major work item in Cell N is precision-trimming of an area of the acreage foam: “The big thing that will be a challenge, but in a good way is the systems tunnel machining. Where the systems tunnel is, there’s a trench that we cut into the foam and it runs the entire length [of the LH2 tank].”

“Because this will be the first one we’ve done of this length, any kind of inflections in the way the barrel lays out, you have to take that into account,” he added. “Before we had joke about how we were having to cut a smiley face into it, but it doesn’t sag in the middle, it actually is a little bit high in the center.”

“The crew that does it is of course very methodical because once we get [the tank] welded up, once we get the foam on it, we sure don’t want to mess anything up. They are very, very, very meticulous about when we go in to cut, we’ll do skim passes to make sure the robot pathway is where we want it before we start cutting. It’s a very deliberate process.”

CS-2 hardware to join the Building 110 traffic

While the flight and remaining test hardware at MAF for the first SLS launch is progressing towards stacking, other parts of the facility were finishing modifications and upgrades. Some work was getting ready for final assembly of the first flight stage, other work was for manufacturing the next one.

CS-2 engine section barrel structure (foreground, middle) sits next to the CS-1 engine section (background, left) at MAF on August 13. Credit: Philip Sloss for NSF/L2.

Modifications to the final assembly area facilities were recently completed, looking ahead to that phase of work on CS-1. “Right now we’re doing some facility modifications that will enable us to be really effective when the entire vehicle is there,” Navarro said in early July.

“So when we do get the vehicle in there, we don’t have to go so serially.  We can work at multiple heights and on parallel activities. We also want to get a Pathfinder activity done and that’s also going to pay off later when we go to both Stennis and KSC.”

At the same time, the structural parts for Core Stage-2 (CS-2) are due to begin major assembly at MAF, which will also contribute to the traffic flowing in and out of Building 110, where the large Vertical Assembly Center (VAC) welding tool is located.

The VAC was used last October to weld an L-ring to the top of the CS-2 engine section barrel. Since then, it was taken offline to make changes to eliminate some of the major workarounds the team lived with while welding two LOX tanks and one LH2 tank in the VAC last Summer in 2017.

Black duct-work along the back wall feeds air-conditioning into the ground level of the VAC. “Registers” are set up on the different work levels of the large welding tool where air-conditioning can be hooked up. The new infrastructure was added to allow for longer periods of work to take place in the VAC during outside temperature extremes. Credit: Philip Sloss for NSF/L2.

“We learned last Summer…as we started working inside the tank, [that] to make our people safe we had to develop a rotation schedule [for] exposure inside the tank, [because] some of it is very manual activities,” Navarro explained. “We actually had to meter the amount of people available and for how long.”

Ernst noted that they reached combinations of temperature and humidity inside the tank structures during VAC welding last Summer where the technicians could only stay inside for fifteen minutes at a time and then would have to be outside for at least forty-five minutes before they could return.

Boeing and NASA are now looking to resume CS-2 work there next month with the start of welding of two L-rings to the top and bottom of the forward skirt for the second flight vehicle. The LOX tank elements would follow with the two domes and two barrels lining up to go into Building 110 to be put together.

Structural assembly work on the second Core Stage intertank and engine section will also be picking up in the last quarter of the year. Modifications to the intertank structural assembly jig following the two builds to support the first launch are complete, and the intertank thrust beam is loaded in the jig.

The intertank panels, which are bolted together versus being welded, will start being fit up and attached in the Fall.

CS-2 structural elements at MAF, August 13. Clockwise from top left: the forward skirt barrel, LOX domes and various barrels, intertank hardware in the structural assembly area, and LH2 gore panels in and staged around the Gore Weld Tool. Credit: Philip Sloss for NSF/L2.

The engine section barrel for CS-2 was moved from storage into the work area earlier in the year, where it now sits close to the first flight unit. In between the move, the barrel was fully painted with primer, with the weld lands around the circumference and the ring on top getting covered.

Based on lessons learned from assembling the engine section STA and the CS-1 flight structures, Ernst said that initial structural work for this unit is being split between the supplier and MAF. Initial drilling and bracket installs on the thrust structure for the CS-2 engine section are being done at the supplier before that is delivered to MAF. In parallel, some secondary structural drilling is being done on the barrel as get ahead work.

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