Boeing and NASA prepare for the assembly of the first SLS rocket

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Prime contractor Boeing recently discussed the current progress of assembly and production for the first Core Stage elements of NASA’s Space Launch System (SLS). With assembly of the major structures almost complete, all the different first-time pieces are working their way along parallel paths at the Michoud Assembly Facility (MAF) in New Orleans, Louisiana, as Boeing and NASA choreograph the development and testing work that will put together the first complete new rocket stage next year and certify it for its inaugural flight in late 2018.

Vertical Assembly Center (VAC) weld status:

Assembly of major elements of the first flight Core Stage and nearly identical qualification test articles at MAF is nearly complete.

The Core Stage consists of five major elements: the forward skirt that holds most of the stage avionics, the liquid oxygen and liquid hydrogen tanks that hold the cryogenic propellant, the intertank that serves as the structural connection between the propellant tanks and the SLS boosters, and the engine section at the aft end of the stage that holds most of the propulsion system, including the four RS-25 rocket engines.

Welding of the two liquid hydrogen (LH2) tanks, one for flight and one for structural qualification testing, was completed in the Summer in the large Vertical Assembly Center (VAC) at MAF. The 130 foot-long LH2 tanks are assembled using self-reacting friction stir welds.

The two propellant tanks are composed of domes on either end, barrels, and rings.

Those pieces were welded together in the VAC to create the LH2 and liquid oxygen (LOX) tanks; similarly, the two LOX tanks will be welded together.

Currently, welding of the LOX tanks is on hold as NASA and Boeing work through an issue with the quality of test welds that were done on a LOX weld confidence article early this year.

“We have a challenge with the liquid oxygen tank,” Exploration Systems Development (ESD) Deputy Associate Administrator Bill Hill noted in a NASA Advisory Council (NAC) Human Exploration and Operations Committee meeting last month in Houston, Texas.

“We’re doing the thickest friction-stir weld at five-eighths or 0.625-inches that anybody does. Most folks do a three-eighths or 0.375 (inch) thickness. We also on the liquid hydrogen tank do a half-inch thickness friction-stir weld.

“Our problem with the LOX tank is…we actually went through and did a weld-confidence article and passed everything but we’ve had some variation recently where we’re finding we’re introducing too much heat into the material as we’re stirring it and we’re actually melting some of the copper in the alloy, which increases the brittleness and decreases the tensile strength. So we’re working our way through that.”

NASA spokesperson Tracy McMahan said that the issue remains under review.

“NASA and Boeing want to ensure the welding parameters are correct before starting the welding process in the Vehicle Assembly Center. Engineers are still evaluating the data, but should know soon when welding on the LOX tanks will begin.”

In the meantime, welding of the forward skirt in the VAC was advanced in the weld sequence and recently completed. McMahan noted that it was removed from the VAC on December 1. “It is now in Area 15 to complete primer applications and begin integration work on the Factory Assembly Jig,” she added.

The forward skirt, the intertank, and the engine section are the “dry” structures of the Core Stage; the two propellant tanks that store the cryogens at pressure are “wet” structures.

The flight and qualification LOX tanks are the final articles that need to be welded together in the VAC for the first flight.

Fusion welds – plugging the holes:

When the propellant tanks come out of the VAC, they are not quite pressure-tight; the friction stir welds done in the VAC leave a small hole that has to be plugged.

“When we do the self-reacting friction stir welds in the Vertical Assembly Center, self-reacting is different than conventional,” Jackie Nesselroad said during an interview with NASASpaceflight.com. Nesselroad is Boeing’s Director of Production Operations at MAF.

“You actually drill a hole and you insert the pin and there’s the shoulder on the back and you perform the weld,” she explained. “So where the pin goes in, the pin comes out. So every self-reacting friction stir weld, when you’re finished you have a pin exit hole.

“And that’s what they were doing in the tank was they were plug welding for those pin exit holes on the qual LH2 tank.”

The exit/termination holes are about an inch in size, depending on the thickness of the material being welded.

Before the plug welding could be done, the LH2 qualification article had to be moved to a different area of MAF. When the tank was removed from the VAC in early July, it was first moved over to Cell A in Building 110, adjacent to the VAC.

Eventually, the tank was rotated to a horizontal position and moved from Building 110 next door to Area 6 in Building 103 for doing the plug welds and additional outfitting.

Boeing’s Vito Neal entered the tank in August to do the welds.

“First, we have a foam ladder that goes over the manhole and it drops down into the dome to the orthogrid, so we push all that down (into the tank),” Neal explained when speaking with NASASpaceflight.com.

“I have to take all my weld leads and I throw all that (inside), (then) I get into the tank and we also have a monitor on the outside. We have a sniffer that detects carbon dioxide or any other agents in the air (that would force us to) have to leave the tank.

“So we put all that down in there and then I drag all my leads to whichever termination hole that I’m welding. The guy that is with me is…a fire watch and also making sure that nothing happens and assisting me with all the (welding) materials.”

Nesselroad provided an overview of the process: “So there are two methods to perform what we call plug welding, there’s fusion, which is what we were doing on the liquid hydrogen (qualification) tank and there’s friction,” she explained.

“Friction plugging uses the same type of technology that friction stir weld does. We actually have a robot arm that’s qualified and programmed to perform those. When we do fusion plug welding, it is performed using a TIG welder and a welder – a person who is certified – and they actually apply the traditional type of weld TIG process, where they are using a filler material and there’s heat applied.”

“TIG means tungsten insert gas,” Neal noted. “You sharpen the tungsten to a point, a real sharp point and that’s where the arc accumulates from.” In addition to being the only person certified to do the SLS fusion welds, Neal, a U.S. Army veteran who was born and raised in nearby Slidell, Louisiana, has shuttled back and forth between MAF and NASA’s Marshall Space Flight Center in Huntsville, Alabama, in the past few years working to help develop the welding techniques for the Core Stage.

There are six exit holes that must be patched in the LH2 tanks; to plug the hole, Neal has to work both inside and outside the tank on the inner mold line (IML) and outer mold line (OML), respectively.

“I…make one pass on it, then I take another one, I make a hot pass it’s called, so I kind of crank up the heat and then I fuse it all together. Then I go on the opposite side and I kind of back grind it (the plug) a little bit to make sure there’s no joint left, then we get X-ray (inspection). X-ray comes out and (they) verify that there are zero defects. So after I pass X-ray, I go on the opposite side and I make two passes, (then) X-ray (inspect again).

“So I stagger two passes from the IML (inner mold line) to the OML (outer mold line), vice versa until I get it flush. Depending on how the “mood” of X-ray is, usually I can do one plug in say 12 hours, maybe a two-day period.”

Neal noted that the work on the outside of the tank requires an access stand.

“It’s called the diving board, and it’s this big, tall structure that goes to the level of the top of the tank and there’s a diving board that I crank down. It’s (manually operated) – I just crank on this wheel and it lowers down.

“When it lowers down, I’ve got a fall harness on and I’m laying on the diving board. I’m actually laying on my stomach and I’m hovering over the hole and welding the hole like that. So we go (back and forth) on the IML, OML, to distribute the heat on both sides and that minimizes the distortion of (the weld).”

Friction plugging the flight tank:

Elements of the LH2 tank flight article went into the VAC immediately after the LH2 qualification article and the full tank was lifted out of the VAC in September. After the flight tank was moved over to Cell A, an anti-vortex baffle assembly was installed inside the aft dome, so Neal has to enter the tank from the opposite end.

“I’m going to enter at the forward manhole on the flight article because the baffle is already installed,” he explained.

When NASASpaceflight.com spoke with Nesselroad in October, the LH2 tanks were being shuffled around to allow work to continue on both in parallel.

“It’s (the LH2 qual tank) in Area 6, but actually today it is going to be moved to Area 47-48, which is our final assembly area, because we have a few things we have to finish up, but we also want to get our Core Stage-1 (flight) LH2 tank moved out of Cell A and break it over and then take it into Area 6 so we can start working on it,” she noted at the time.

“So we’re putting it in 47-48 because we have a few things that we’re wrapping up as it relates to test readiness and we want to keep moving on Core Stage-1. Normally it would go straight from Area 6 out to (Building) 451 for (proof testing).”

Nesselroad also noted at the time that the work to certify the automated friction plug weld technique was nearly complete and it was recently approved for use. Plug welding using the friction stir technique began in the last few weeks on the LH2 flight tank. Neal noted that he is also the certified operator for the robotic tool that performs the friction welds.

McMahan explained that performing friction stir welds is not only faster than the manual fusion welding, but inspection is also faster. Although both welds are tested using fluorescent dye penetrant inspections, friction welds are additionally inspected using phased-array ultrasonic testing (PAUT) versus the much longer X-ray inspection that is also needed for fusion welds.

The dry structures of the Core Stage that are welded have the same exit holes, but Nesselroad explained those don’t need to be welded.

“We plug the holes in the tanks, for obvious reasons, but we don’t have to friction or fusion plug the holes in the dry structures – the forward skirt and the engine section – we do that mechanically. We put like a fastener in them.”

In addition to the plug weld work he is doing, Neal is also getting ready to begin welding propulsion system elements in the engine section early next year.

“I’m in the process right now of training (new techs) on orbital tube welding. I was the only orbital tube welder here right now, but I’m in the process of training new techs on doing that. So we’re going to be doing the orbital tube welding of course in the engine section for all the tube-work that’s going to be in there.”

Developing the methods and the process of building the Core Stage.

NASA and Boeing are preparing the qualification articles of the stage elements as a part of the development of the Core Stage and certification for its first flight.

Nearly identical to the flight articles, those elements will go through much of the same production processes as the flight elements; then, while the flight articles are assembled into the first flight Core Stage, the qualification articles will be structurally tested in separate test stands at Marshall next year.

Completion of the development of the different stages of the production process is going on in parallel with the work to assemble the first stage elements. After welding of the propellant tanks is complete and they are pressure-tight, they then go through a proof test to verify their structural integrity.

After the tanks pass their proof test, they then get application of a coat of primer first and then a coat of the thermal-insulating foam that helps keep the propellant in the tanks in the optimal temperature range for powered flight.

While they are welding some tanks and finishing the plug welds on others, Boeing is also working with NASA to start proof testing of the LH2 tanks, and to finish readying the techniques for spraying primer and foam on the tanks when they get to that stage of production.

Proof test preps:

Preparations for the first proof test, which will be done on the LH2 tank qualification article, are close to completion.

The tank was rolled from the Final Assembly area of Building 103 out to Building 451 on December 10 for mating and integration with the test bed.

“We did install instrumentation on the tank that will provide real-time data to our test engineers during the proof test,” Nesselroad added.

“We also install proof-test covers, obviously (over) the manholes – the big holes at the ends of the domes. We have to have covers on them when we go to proof test and they are considered a tool – they’re not a flight cover.”

Core Stage LH2 tanks are tested pneumatically in Building 451 by pressurizing them with gaseous nitrogen, while loads are being simultaneously applied by a hydraulic test rig.

LOX tanks are tested hydrostatically; they are filled with water in those tests which will be one of the other cells in Building 110.

Primer, foam spray development:

After proof test and inspections, the LH2 qualification tank will be rolled back from Building 451 to Building 103 and prepared for primer and foam sprays.

In parallel with development work on welding and proof testing, work is continuing to develop the techniques for applying primer and foam to the large propellant tanks.

The liquid oxygen weld confidence article that came off the VAC earlier this year is being used as the pathfinder for development of the primer and foam sprays, which are being done in cells adjacent to each other in Building 131.

“Development sprays” of primer on the LOX weld confidence article were completed in October in Cell P and it is being readied to go into Cell N for foam sprays.

Engine section work continues:

Meanwhile, build up of the engine section structural/qualification and flight articles continues in another area of Building 103.

“We’re working them both at the same time – the engine section structural article is made in two major pieces,” Nesselroad said.

“The first thing you do is the thrust structure and then the thrust structure goes to the assembly jig (AJ) and is joined up with the barrel section, where you begin the second phase of the structural assembly. That is well underway for the qualification article, the thrust structure and barrel are in the AJ and (workers there are) drilling holes and installing fasteners.

“The thrust structure for Core Stage-1 (the flight article) is complete – it’s all built up and once the qualification article comes off the AJ, we’ll jump right into Core Stage-1.”

Recent schedules forecast completion of structural assembly for the qualification article at the end of the year, with installation of test instrumentation to be completed by the end of January, and finally the article will be stacked with a simulator in mid-March.

It will then be barged from MAF to Marshall for structural testing.

(Images: NASA, L2 and 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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