Boeing finishes SLS LOX tank foam work, recovering from tube contamination issues

by Philip Sloss

Prime contractor Boeing recently completed Thermal Protection System (TPS) applications on the liquid oxygen (LOX) tank for Core Stage-1 (CS-1), the first NASA Space Launch System (SLS) Core Stage. The cryogenic propellant tank was moved out of Cell N at the Michoud Assembly Facility (MAF) in New Orleans on June 20, where spray-on foam insulation (SOFI) was applied to the outside of the tank.

Work on the critical engine section element was slowed earlier this year by issues with contamination of tubing, but NASA and Boeing are continuing to move forward with work on all the elements of the rocket for the first SLS launch. In April, foam applications on the Launch Vehicle Stage Adapter (LVSA) were completed at the Marshall Space Flight Center (MSFC) in Huntsville, Alabama.

The LOX tank is now in Area 6, where it is being prepared to be joined with the other two elements that form the upper half of the Core Stage.

CS-1 LOX tank TPS applications complete

Lying horizontally on factory ground support equipment (GSE) and transporters, the LOX tank was backed out of the building used by Boeing to apply SOFI to the stage’s two large propellant tanks. Cell N is located in Building 131 at MAF, adjacent to both the Building 110 high-bay and Building 103, the sprawling main building of the facility.

The factory transporters picked up the GSE attached to the tank, backed it out of the cell, and moved it into Area 6 of Building 103, where the next leg of work is being performed in preparation for “stacking” the tank with the other elements of the top half of the stage.

CS-1 LOX tank is backed out of Building 131, Cell N at MAF on June 20 after completion of SOFI applications. The forward end is closest to the vantage point, as the tank goes in the cell aft end first. Credit: NASA/Jude Guidry.

A system of roll rings and specialized Rotational Assembly and Transportation Tools (RATT) are attached to the propellant tanks to help apply both the SOFI for thermal protection and, before that, a coat of primer for corrosion protection. “These roll rings are three sections and it’s a neoprene [pad material] bolted up around the tank,” Steven Ernst, Boeing’s Core Stage Engineering Support Manager, said in an interview.

“They do attach, not structurally but for spacing reasons. They attach in a few places around the flange, but it’s a friction-type connection.”  The roll rings also attach to the RATTs, which can be used to move the roll rings and rotate the whole tank.

The RATTs can also be picked up by Boeing’s Manufacturing, Assembly, and Operations (MAO) Self-Propelled Modular Transporters (SPMT). Sets of two MAO SPMTs roll under and pick up each RATT. For large assemblies like an SLS Core Stage LOX tank (or larger), there are two sets that are linked together to both move and precisely position the entire assembly in tandem before setting it down again.

In Cell N, the SOFI was sprayed on top of the primer coat in two phases. First, the cylindrical barrels of the tank were covered in an automated, environmentally-controlled process where the GSE attached to the tank is rotated in front of a foam-spray gun until the barrel has the desired thickness of SOFI.

The tank RATTs are connected on both ends near each flange of the tank, which is a ring where the barrels and domes are welded together. One RATT is designated lead and the other a “follower.”

MAO SPMTs complete moving the CS-1 LOX tank into Area 6 in Building 103 on June 20. The tank is secured on both ends by white roll-rings attached to the blue RATTs. Credit: NASA/Jude Guidry.

“One is fixed,” Ernst explained. “It’s tied to the lead SPMT, that’s where the gear box is that drives the whole thing, so one is basically a slave. And how that goes is orientation specific. We do the aft end first into the [SOFI spray] cell, because we actually integrate these with the primer system and the SOFI system. So the control system for both of those cells is controlling the rotation of the RATT.”

After the barrels or “acreage” SOFI was sprayed, technicians manually sprayed the hemispheric domes on each end of the tank with a different SOFI formula suited for application in room temperature conditions. The tank will remain in the roll rings and RATTs while it is in Area 6, where the system is used to manually roll the tank to position different areas in front of a fixed work platform.

Some areas of the tank still need additional processing that requires access under the foam; besides areas under and around the roll rings, circular “sensor islands” around the barrel circumference were masked off prior to the foam sprays. The wiring runs were routed prior to foam application, leaving only access to the islands in the locations of different operational and development instrumentation sensors.

After the sensor installation is completed, some additional foam will be manually sprayed to cover or “closeout” those areas. Other areas were sprayed and then trimmed down.

Circular “sensor islands” are seen in areas around the circumference of the LOX tank acreage during the June 20 move. Those areas are being processed now in Area 6. Credit: NASA/Jude Guidry.

While the LOX tank was in Cell N the robotic system was also used to precisely machine down the foam in specific locations. “We’ll actually put a cutting head essentially on it to do some of the trimming, which becomes really critical down along the systems tunnel area,” Ernst noted.

The systems tunnel runs hundreds of feet up and down the length of the Core Stage; it will be attached in final assembly. “It’s just taking enough off to smooth it out and get it to a prescribed thickness,” Ernst explained about the trimming.

“It was a little tricky [for] the first tank, because the tanks as they are sitting here horizontally aren’t perfect, they do sag a little, deform a little bit, so there was a lot of development work going on to get the program for the robot refined to accommodate for that, but that was all part of the process,” he added.

The MAO SPMTs from Doerfer Companies Wheelift Systems Group has been used by Boeing for several years inside at MAF for positioning a variety of support equipment. “We use them a lot,” Ernst said.

“A lot of the work stands you see out and around the factory, those are moved into position using SPMTs. All the equipment, all the large pieces were designed with the intent of moving them around with SPMTs. It’s a lot safer, it’s very precise because you can position them just right.”

Ernst noted that other movers don’t have the positioning precision of the Wheelift transporters. “It can be a little bit nerve-wracking, for instance, you saw the clean room out in Area 6, that platform [to] go into the [LOX tank] aft dome,” he said. “We’ve got to bring that thing within inches of the flight hardware and that would be something you wouldn’t really want to do with a tug. So those SPMTs have that level of control to precisely locate that.”

CS-1 LOX tank “spotted” in Cell N in late March prior to the beginning of SOFI applications. Credit: NASA/Jude Guidry.

The MAO SPMTs will also be used during final assembly to horizontally position the four RS-25 engines one at a time for installation in the aft end of the stage.

More recently NASA took possession of a heavier-duty set of Wheelift SPMTs that are needed for both the more massive final assemblies and for longer, overland transportation.

LVSA foam applications completed in April, headed to Florida

At Marshall in Huntsville, the LVSA for the first SLS launch on Exploration Mission-1 (EM-1) was moved from Building 4707 to Building 4649 for final outfitting in late June. The LVSA connects the top of the Core Stage to the bottom of the Interim Cryogenic Propulsion Stage (ICPS). At around 30 feet in length, the LVSA also provides room for the long engine nozzle extension on the ICPS upper stage.

NASA LVSA manager Keith Higginbotham said in an email that manual foam spray work on the LVSA was completed in Building 4707 on April 17. Welding of the flight article was completed last summer, when it was moved to Building 4707 for TPS applications. The weld lands were painted to complete the primer application, and then foam sprays began last year.

The LVSA for EM-1 is moved from Building 4707 to Building 4649 at Marshall on June 26. Credit: NASA/Tyler Martin.

The adapter will be outfitted with a pneumatic actuation system and a frangible joint assembly will be installed on the top of the adapter. The frangible joint will separate the top of the LVSA and Core Stage below from the bottom of the ICPS and Orion above.

Once outfitting work is complete, the LVSA will wait for a ride on NASA’s Pegasus barge from Marshall to the Kennedy Space Center (KSC) in Florida later this year. Plans are for Pegasus to next arrive at Marshall with the liquid hydrogen (LH2) tank structural test article (STA) being prepped at MAF. After offloading the STA, the LVSA would then be loaded on Pegasus, along with perhaps some additional equipment that needs to be returned to MAF (such as the SPMTs).

Engine section tube contamination recovery

Integration of the CS-1 engine section remains the primary critical path for overall stage assembly and Boeing employs around-the-clock work shifts specifically for the element. The effort suffered a setback earlier in the year when it was discovered that tubing that will be installed in the element was contaminated.

Paraffin wax is used during manufacturing of the tubes to prevent crimping while they are being curved, but the supplier of the tubing failed to completely clean them prior to delivery to MAF. The problem wasn’t discovered until a quality control inspection in February.

“Some lines ended up being more susceptible to the contamination that we found and [it] was really a sizing-type thing,” Rick Navarro, Boeing’s Director of Space and Launch Operations, said during an interview. “Above a certain size of line, the paraffin wax was more in use as a part of the bending process.”

CS-1 engine section at MAF in late February, around the time the tubing contamination issue was discovered. A significant amount of work has been completed inside since then, but most of the tubing is still outside, being staged for later installation. Credit: Philip Sloss for NSF/L2.

Most of the tubing that will be installed inside the engine section has bends, and investigations found a widespread problem of tube sections that were not cleaned correctly. “It ended up being an across the board thing that we separated by system: gaseous oxygen, liquid oxygen, gaseous hydrogen, hydraulics, thrust vector control,” Navarro explained.

Navarro said that they finally decided it was prudent to re-check all the tubing. “There is a priority that was figured out: in which order and [what] specific sequence you need to get the tubes back into service,” he noted.

“And by ‘back into service’ I mean whether it was getting a new tube from Core Stage-2, or getting a tube processed off-site, re-cleaned, and sent back to use, or getting a tube inspected. So we had a complete set of priorities that said in which order we needed the tubes to come to us.”

“Some of them for instance, had to go through some higher-temperature bake out, which we’re doing in a separate Boeing facility to bake out residuals,” he added. “Typically the inconel tubes are going through that process to bake out.”

The hardware going into the engine section is densely packed and there are specific installation sequences to provide adequate access for installing them. “Amongst all of them we had a priority scheme and that’s the way we’re getting them back and installed. I don’t think we’ve had a tube shortage in weeks.”

Recovering from the problem put the engine section further behind schedule and a recent estimate put completion of CS-1 five months later than the official date of December. Given there is little margin, the implication is that the forecast date for the first SLS launch is closer to mid-2020 than the end of 2019.

Boeing is working to try to recover some of the schedule and the engine section team has recently made visible progress. Management recently challenged them to install several large components, such as the composite over-wrapped pressure vessel (COPV) helium tanks, in two weeks time.

They finished all the work with a few days to spare.  “In fact, ten percent of the total build was done in the last two weeks, just based on the amount of installations that we had,” Navarro said at the time of the interview in early July.

A composite of all three CS-1 “forward join” elements, which now have their primary TPS work complete as they near work to stack them together. From left to right, the forward skirt, LOX tank, and intertank. Credits: NASA/Jude Guidry, Philip Sloss for NSF/L2.

Elsewhere on the Building 103 floor at MAF, work continues on the other CS-1 elements. In addition to the recent LOX tank milestone, the functional checkout of the forward skirt was completed in early July and that element is ready for stacking when the rest of the elements catch up.

Installation of the avionics boxes into the intertank is complete, essentially finishing the outfitting of that element. Functional testing was set to begin after the boxes were plugged into their wiring runs.

The flight LH2 tank is waiting for its turn in the line to get SOFI applications. Currently, it is behind the LH2 STA tank, which is now in Cell N.

Additional work on the LOX tank sensor islands is currently underway in Area 6 and an internal sensor mast will also be installed while the tank remains horizontal.  After additional preparations are completed, the forward skirt, LOX tank, and intertank will be stacked vertically later this year in Building 110.

The completed “forward join” of the upper half of the rocket stage will then be moved into the final assembly area for additional work.

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