It appears that in its effort to finally assembly SLS’s core stage so that it can be tested prior to its first launch, probably in 2021, NASA and Boeing recently experienced an issue that required “corrective action.”

Neither NASA nor Boeing have provided any detailed explanation for what the issue was, but according to the story at the link,

…one source suggested to Ars that Boeing technicians are having difficulty attaching the large rocket engines in a horizontal configuration rather than a vertical position. NASA and Boeing made a late change to the final assembly process, deciding to mate pieces of the core stage horizontally rather than vertically to save time. However, this source said horizontal mating of the engines has created problems.

Despite this, NASA officials said progress is being made. “NASA and Boeing are expected to have the first engine soft mated to the core stage next week,” Tracy McMahan, a spokesperson for Marshall Space Flight Center, said on Saturday. “However, there are many steps in engine installation that have to occur before the installation is complete.”

I applaud them for finally trying to “save time,” a concept that up until now the entire SLS project has never had much interest in. That this effort ended up causing a delay however is symptomatic of the design of the project, which remains cumbersome and inefficient, a fault not of the engineering but of some fundamental decisions at the top by Congress and NASA’s management.