NASA and Space Launch System (SLS) Core Stage prime contractor Boeing recently resumed welding elements for the launch vehicle\u2019s first flight after a technical issue suspended welding last year. A change to a welding tool in the Vertical Assembly Center (VAC) at the Michoud Assembly Facility (MAF) in New Orleans, Louisiana, had unintended consequences that in part disrupted the assembly and production schedule for the Core Stage and helped push the forecast target date for the first SLS launch on Exploration Mission-1 (EM-1) into 2019.<\/p>\n
Weld strength issue:<\/p>\n
The Core Stage consists of five major structural elements \u2013 the liquid oxygen (LOX) and liquid hydrogen (LH2) propellant tanks are the \u201cwet\u201d structures, while the engine section, intertank, and forward skirt are \u201cdry.\u201d Four of the five elements are welded, the bolted intertank structure being the only exception.<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-150051-350x254.jpg\")
Welding of the structures is performed using a process called \u201cself-reacting friction stir welding,\u201d where a spinning pin is pushed into and moved through the material (an aluminum alloy) at high forces. In contrast to conventional friction stir welds, self-reacting friction stir welding uses two shoulders on opposite sides of the weld rather than a large structure\/anvil on the opposite side of the pin.<\/p>\n
\u201cBasically what you have is\u2026a root shoulder, which bolts to the weld head, you have the pin that runs through the middle of it, and you\u2019ve got a crown shoulder and so the whole thing spins but the root gets pushed into the article and the crown that gets attached to the pin gets pulled into the article \u2013 so it\u2019s self-reacting, it squeezes it together,\u201d Steve Doering, manager of the SLS Stages Element Office, explained in an interview with NASASpaceflight.com. \u201cSo it\u2019s a very complicated thing.\u201d<\/p>\n
\u201cAnd then inside, the part that\u2019s in the metal piece, in between the root and the shoulder has a thread pattern on it like a bolt \u2013 similar to a bolt,\u201d he continued. \u201cAnd what that thread pattern does is stir the material up when it plasticizes. So it mixes it and gets it uniform and you don\u2019t get pockets of alloy, pieces of the alloy components leeching out and moving in places. It keeps it homogeneous.\u201d<\/p>\n
In the case of the Core Stage structures, the technology needed to weld the thickness of the material for the two propellant tanks is pushing the state of the art. \u201cThere\u2019s going to be a lot of PhD\u2019s coming out of this, in the weld arena,\u201d Doering noted. Both the Core Stage LH2 and LOX propellant tanks are thicker welds than anyone is currently doing. The engine section and forward skirt dry structures are of a more typical thickness seen elsewhere in the industry.<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-151519-350x253.jpg\")
Welding of the domes, barrels, and rings that make up the major structural elements is done in the large VAC in Building 110 at MAF, and by the beginning of 2016 the first weld confidence articles (WCA) were being completed.<\/p>\n
The full-scale welding is done in a multi-step process, as Doering explained: \u201cThe way we qualify our weld schedules is we do a whole series of panel welds, about three or four feet long. [We] pull them apart, break them, do tensile testing on them, do the metallography breakdown, then we do a weld confidence article \u2013 and that gets us the full-scale weld and then we cut out a bunch of pieces from that weld and we check all those. And all those came back good.\u201d<\/p>\n
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The VAC welds around the circumference of the structures, joining domes, rings, and barrels to other barrels. On inspection and evaluation of the early, completed full-scale welds a couple of issues were identified, one that affected the welding pin and one that affected the welded material.<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-145947-350x281.jpg\")
\u201cAfter we did the weld confidence article for the oxygen tank, which is our thickest material, we found two things that we learned as we\u2019re pushing the state of the art for these welds, self-reacting friction stir,\u201d Doering explained.<\/p>\n
\u201cYou\u2019ve got two barrel panels that are welded together each made up of eight panels. And so [at] the vertical intersection as the circumferential weld comes around, we were seeing some indications [of] some really small but significant voids in the weld land at the intersection. And so we were trying to figure out what\u2019s causing that.<\/p>\n
\u201cThe other thing that we found, which is related to the pin was we were getting microcracks at the base of the pin where it meets the root shoulder.\u201d<\/p>\n
Doering explained the rationale for making the change: \u201cWith these microcracks at the pin at the root shoulder we had a higher propensity for potentially breaking a pin. Think of this as a big drill bit which you\u2019re moving it sideways through the metal or through the wood \u2013 whatever material it is \u2013 and it\u2019s easy to break. You break drill bits all the time when you\u2019re at home and you\u2019re drilling through something and you\u2019ve got a little torque and it snaps because it\u2019s very brittle.\u201d<\/p>\n
\u201cBreaking a pin is not a huge deal in a circumferential weld, but it requires a repair, it\u2019s time,\u201d he continued. \u201cWhen you don\u2019t do a controlled shutdown of the weld head then it locks it in there, so you have to drill it out. It poses some problems that we would rather not have out in front of us to go work. That\u2019s what drove the need for the change.\u201d<\/p>\n
\u201cSo we made a very small change to the threading,\u201d Doering noted, \u201cwhere we knocked off the very points of two of the threads and we tapered them a little bit to give more material back there at the root. And then we did another series of test panels and they were testing good. And then as we were going through the process after that, we found a way to solve intersection voids, which in the end became unrelated to the pin.\u201d<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-145718-350x250.jpg\")
After the modifications to the pin were made to address the risk of breaking the pin, a new set of test welds were performed and passed testing, and full-scale welding in the VAC resumed, with most of the welded elements being completed by the end of the summer last year with the two LOX tanks, the thickest of the welds, up next. By that time, though, the Core Stage team had begun seeing random, infrequent strength failures in test panels welded with the modified pin tool.<\/p>\n
\u201cWe were finding random areas in which the strength of the weld is below the design requirement,\u201d Doering said. \u201cAnd it is not predictable and it\u2019s not something you can find through any kind of NDE, non-destructive evaluation technique. When you dissect the weld itself and do the metallography, the materials analysis on it, you can see what the issue is \u2013 it creates a little brittle layer at the very top which you rely a lot for your strength on elongation of the material.<\/p>\n
\u201cIt\u2019s kind of like a rubber band,\u201d he explained, \u201cyou stretch it and stretch it and it does great and then all of a sudden it gets to the point where it doesn\u2019t want to stretch any more and [snap] it fails immediately. Because one little spot starts to open up and once it opens up the rest opens up like a zipper. It\u2019s the way the strength issue works here with these.\u201d<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-151639-350x305.jpg\")
The plan to weld the LOX tanks and finish the VAC welding of all the flight and structural qualification articles necessary for the first flight last year was suspended in order to investigate the issue. Lots of test panels were welded to try to characterize the problem, which would appear unpredictably. \u201cAbout every fifteen panels or so we would get some bad data,\u201d Doering explained. \u201cAnd it\u2019s really not repeatable \u2013 it\u2019s not like it\u2019s every fifteenth one, it\u2019s on average about every fifteenth one.<\/p>\n
\u201cAnd we were spending a long time researching it and creating samples,\u201d he noted, \u201c[because] we have to make a lot of samples if we\u2019re only getting failures one out of every fifteen.\u201d<\/p>\n
After doing all the test panel welds and research, the current understanding is that the original pin design is the only one known to meet the design strength requirements.<\/p>\n
\u201cWe still don\u2019t know enough about self-reacting friction stir welding at these thicker welds, what the sensitivities are that affect the strength of the weld,\u201d Doering explained, \u201cwe have a point solution [where] we have data that says that it works \u2013 that\u2019s the RPMs (revolutions per minute), the forces, and the\u2026pin design.\u201d<\/p>\n
\u201cBut it\u2019s a point solution,\u201d he noted. \u201cSo [if there is] any deviation from that, there\u2019s not enough either industry data or research data at production levels or anything out there to tell us what the real sensitivities around that are. So because of that, the only way to guarantee that you have a good weld on your tank is to destroy the tank \u2013 there\u2019s not an NDE technique to verify that this particular random mechanism we saw on the [modified] pin exists anywhere in the weld. You can\u2019t tell unless you take it apart.\u201d<\/p>\n
![](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/01\/2017-01-09-194707-350x264.jpg\")
In addition to going back to the original pin design, they have added an extra step to the process before making the full-scale weld. \u201cOn the VAC prior to doing the weld we are doing what we call pre-qualifying the pins,\u201d Doering explained, \u201cbecause there\u2019s always some variability in your tooling. [If] I have two different pins, there will be some variability in them \u2013 [because] they\u2019re manufactured items.\u201d<\/p>\n
\u201cAnd because we don\u2019t yet know what all the sensitivities are, I\u2019m pre-qualifying my pins. So I\u2019ve got a handful of pins, I go do welds on the VAC just prior to doing my weld and then I pull [test] all the samples I get back. If they pull good\u2026then now I\u2019ve got a good pin, now I\u2019ve got a good weld schedule, my risk to having a bad weld when I actually weld the tank becomes very low.\u201d<\/p>\n
Although research on doing these high-thickness self-reacting welds will continue it\u2019s not known how long it will take to better understand the dynamics of the issue.<\/p>\n
The issue does not impact the strength of welds at smaller thicknesses, where there is more industry and research data, and so does not affect the engine section or the forward skirt. The welding of two rings to the forward skirt barrel was done in the VAC last fall to allow production work on that element to move forward.<\/p>\n
Given the desire to continue assembly and production and that the original pin design produces strong welds, VAC welding resumed early this year with a second, full-scale LOX weld confidence article demonstrating the dome to barrel and barrel to barrel welds at that thickness. That article was completed a week before the tornado that hit MAF on February 7 shutdown operations again.<\/p>\n
After cleanup and infrastructure repairs, welding of the LOX qualification article began in April.<\/p>\n
\u201cI\u2019ve got a dome and two barrels sitting in the VAC right now,\u201d Doering said at the time of the interview. \u201cWe\u2019ll be done with the LOX qual tank towards the end of May \u2013 I\u2019ll be done with the last weld May 4th, then it\u2019s going to be a week or so before I get it out of the VAC into Cell A, get the break-over brackets on it and get it out of there. And then we\u2019ll start loading the flight tank and so it\u2019s going to be a little over a month and a half to weld the full flight tank.\u201d<\/p>\n
Hydrogen tank impact:<\/p>\n
Although the weld strength issue stopped welding the qualification and flight articles of the LOX tank before it could start, the issue wasn\u2019t caught until after both LH2 tanks were welded with the modified pin tool last summer. The implications of the two tanks possibly having below design strength welds disrupted the original, post-weld plans.<\/p>\n
![](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/01\/2017-01-09-194338-350x219.jpg\")
The LH2 qualification tank, which will be used for structural testing at Marshall Space Flight Center in Huntsville, Alabama, was welded first and after setup and configuration was taken to Building 451 in December of last year both for proof testing of the welds and to qualify the test facility and procedures for subsequent flight tanks. Hydrogen tanks are proof tested by pressurizing them with nitrogen gas while a hydraulic test rig applies loads to the structure.<\/p>\n
\u201cWe wanted to wring out\u2026the control system \u2013 451 was another building that was made bigger to fit the hydrogen tank,\u201d Doering said. \u201cThe control system is all new, the reaction fittings are all new, along with all the actuators. We didn\u2019t want to put the flight asset in there to try to use it for the first time, so [using] the qual article [first] was also trying to wring out the pressurization and the actuation of the control system in 451.\u201d<\/p>\n
Originally, the plan included a test case to pressurize the qualification tank to slightly above flight pressure to help as a part of that \u201cpathfinding\u201d work; however, the discovery that the welds may be below design strength forced plans to be reconsidered.<\/p>\n
\u201cWe couldn\u2019t say with any real degree of certainty that these welds would make it to [flight pressure],\u201d Doering said. \u201cIn a pneumatic test, pressurizing it like that, it\u2019s like a balloon\u2026there\u2019s a good portion of the community that thinks it will survive, there\u2019s another portion of the community that says you don\u2019t know enough to be able to say that, [and] there\u2019s another portion of the community that says\u2026\u2019no way.\u2019<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-154152-350x266.jpg\")
\u201cWe do know from the testing that we can support lower strength limits, but not the full design strength limits for flight,\u201d he continued. \u201cFor structural qualification, we are not pressurizing the tank to full flight pressure. We don\u2019t need to because structural qualification is primarily going through the buckling and compression loading. The proof test that we do out here in 451 for the pneumatic proof for the hydrogen tank pressurizes to flight pressure, plus margin above that.\u201d<\/p>\n
As a result, the program decided instead to proof test the qualification tank to the pressure it will see during the structural testing at Marshall in the future, which is lower than flight pressure. \u201cWe just need to ensure that it will survive the pressure that we use for structural qualification, we do not need to do the full tension case, which is what you get in proof,\u201d Doering explained.<\/p>\n
\u201cFor structural qual, since it\u2019s just compression and buckling, we don\u2019t pressurize it all the way. And so we got comfortable, based on the data that we had that says this tank will survive without any question at [structural test pressure], there\u2019s no reason for us to not utilize this programmatic critical asset to go through qual.<\/p>\n
\u201cWe\u2019re going to get everything we\u2019re going to get independent of the weld strength, because the weld strength is the tension strength, not anything else. It doesn\u2019t affect buckling, it doesn\u2019t affect compression, it doesn\u2019t affect any of those other things. We going to go through a full qualification series for compression and buckling in the test stand with the qual tank. So for what it\u2019s needed for, it\u2019s got no issue.\u201d<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-154734-350x241.jpg\")
Lower pressure isn\u2019t an option for the LH2 flight tank, which must perform at flight pressures both in testing and in flight. The SLS Program developed and is working on multiple, parallel options for consideration that include repairs and\/or replacement of the already-welded flight tank.<\/p>\n
\u201cWe\u2019re looking at use as-is \u2013 can I get to the point where I\u2019m comfortable using that flight tank?\u201d Doering said. \u201cThe answer to that is probably not, just because the analysis tools don\u2019t exist yet to do this.<\/p>\n
\u201cThe industry says your weld is either brittle or ductile; we\u2019ve got this ductile weld with a real small piece of brittleness in it and there is currently no analysis tool that allows you to [analyze] this hybrid between a brittle and ductile weld \u2013 it\u2019s either fully brittle, which means it doesn\u2019t support the loads, or it\u2019s fully ductile, which if it was, it would [support the loads]. So we don\u2019t have a way to get in between, without destroying the tank.\u201d<\/p>\n
Proof testing the flight tank as-is would also carry additional risk \u2013 as was noted in the aftermath of the tornado, Building 451 is designed to dissipate the energy of a blast in case of a tank failure while it is pressurized with nitrogen gas and that scenario would also effectively sideline the proof test facility until it could be rebuilt and re-qualified.<\/p>\n
![](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/01\/2017-01-09-194206-350x230.jpg\")
\u201cSo the use as-is is probably unlikely,\u201d Doering added. \u201cWe could take it out to the parking lot and pressurize it to [flight pressure] and if it didn\u2019t break, we\u2019d say \u2018OK, what do we do now?\u2019 Well, we know it didn\u2019t break, but I don\u2019t know if I\u2019ve done anything to the tank that [might cause it to break] the next time I did it. So it\u2019s a circular argument that you can never get behind because you just don\u2019t have the data.\u201d<\/p>\n
Another option is to repair the flight tank first. \u201cWe have a couple of options that look promising to be able to repair this tank,\u201d he said. \u201cIf you get rid of that\u2026brittle surface on the outside of the weld, then you\u2019re back to a full ductile system [and] your tank will work \u2013 that\u2019s the theory.<\/p>\n
\u201cInitial testing has validated that theory, but we\u2019ve got to put it into full-scale production. So we\u2019re in the process of going through the development of a repair technique for that tank, the flight tank, so that I\u2019m not wasting this program asset.\u201d<\/p>\n
However, the repair option carries significant schedule uncertainty associated with the research and testing of repair techniques.<\/p>\n
![\"\"](\"\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2017\/05\/2017-05-08-134959-350x179.jpg\")
The favored option at this point is to use the next hydrogen tank, serial number three (S\/N 3), for EM-1. (For reference, S\/N 1 is the qualification tank, S\/N 2 is the original flight tank.) The constituent panels and gores for the second Core Stage have been at MAF since last year, and the work to weld those into the two domes and five barrels that make up a hydrogen tank is about half complete.<\/p>\n
\u201cIt\u2019s being welded up now on the Vertical Weld Center (VWC),\u201d Doering noted. \u201cTwo of the five barrel panels are done, one dome is finished and [for] the other dome the gores are being welded up now.\u201d<\/p>\n
Welding the full S\/N 3 hydrogen tank will begin as soon as both LOX tanks have cleared out of the VAC. \u201cI will probably start welding the first dome to barrel [in the VAC] while I\u2019m still pulling the last pieces off the dome weld tools and the VWC,\u201d Doering said.<\/p>\n
These options and program recommendations were expected to be reviewed at an agency-level meeting last Friday, but the outcome of that meeting is unknown at this time.<\/p>\n
(Images: NASA and L2 which includes, presentations, videos, graphics and internal \u2013 interactive with actual SLS engineers \u2013 updates on the SLS and HLV, available on no other site. Additional renders by L2 Artist Nathan Koga. The full gallery of Nathan\u2019s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*))<\/p>\n
(L2 is \u2013 as it has been for the past several years \u2013 providing full exclusive SLS and Exploration Planning coverage. To join L2, click here: \/\/www.nasaspaceflight.com\/l2\/)<\/p>\n","protected":false},"excerpt":{"rendered":"
NASA and Space Launch System (SLS) Core Stage prime contractor Boeing recently resumed welding elements for the launch vehicle\u2019s first flight after a technical issue suspended welding last year. A change to a welding tool in the Vertical Assembly Center (VAC) at the Michoud Assembly Facility (MAF) in New Orleans, Louisiana, had unintended consequences that […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[3718,7866,624],"class_list":["post-38815","post","type-post","status-publish","format-standard","hentry","category-news","tag-em-1","tag-maf","tag-sls"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/38815"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=38815"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/38815\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=38815"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=38815"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=38815"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}