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A engineering problem during construction of one of the shuttle-derived solid rocket boosters for SLS is causing delays.
[The] original test target of mid-2013 slipped when an issue with the aft segment [of the booster] was found. Inspection of the segment showed it was contained an area where propellant had debonded from the inside of the segment wall. Following analysis – which notably found no voids in the propellant itself – NASA decided to ask ATK to scrap the segment and cast a replacement.
Preparations … continued, with the shipping and integration of forward and center segments at the test site, while ATK went to work to replace the aft segment, following approval – post investigation – from NASA’s Marshall Space Flight Center. After ATK successfully cast the replacement segment in July, technicians carried out routine ultrasound and x-ray tests. Unfortunately, the tests showed this segment had also had similar voids. [emphasis mine.]
The Space Launch System (SLS), was mandated by Congress to use as much shuttle-derived components as possible in order to supposedly save money as well as employ as many of the companies that built those components as possible. In reality, however, every one of those components has required significant redesign to make them work in SLS. In the case of the solid rocket boosters, the four segment shuttle boosters were not powerful enough. They had to be expanded to five segments.
Moreover, it appears from this article it was other technically unnecessary changes to the boosters that are now causing this problem.
The likely cause of the voids points to a change in processing when using a relatively new material in the insulation lining for the segments, a material that has replaced the previous use of crysotile – the most common mineral form of asbestos. The replacement of asbestos in the insulation package has been noted as one of ATK’s long-term goals. Crysotile-free rubber insulation had already been qualified for Shuttle, but never used in flight. It was then baselined for Ares I and subsequently for the SLS boosters. The previous insulation material used on the four segment shuttle boosters became obsolete as the shuttle fleet retired.
I suspect the replacement of crysotile was done to merely eliminate the use of asbestos in SLS, something that didn’t really improve the booster’s capability. Such unnecessary changes, which I suspect are taking place throughout SLS, might explain why SLS is costing so much and taking so long to build.