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I was thinking about it for a while and realized I really don't know. I'm guessing it's:

  • The RS-25s
  • The SRB segments
  • SRB seals/joints
  • SRB nosecone assemblies +(avionics/separation motors/housings)?
  • SRB decouplers?
  • SRB ground support/pad mounts?
  • Engine controllers?
  • Tank pressurization systems?

What else? I know the main tank's been almost completely rebuilt from the ground up. I'm sure the avionics are completely new. I'm guessing the SRBs are completely the same (for now). Orion & the second (third?) stage are all new (at least to STS hardware).

How much of this legacy system is actually legacy?

Note: I don't mean previously-flown hardware, just hardware which is "rolling" off the same production "lines". At some point the question breaks down if everything's hand-built and a decade's passed since components have been put together.

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Partial answer

  • The SSMEs / RS-25 are mostly reused but some parts have been redesigned. (Since SLS expends the RS-25s, in the unlikely case that it flies a lot, they plan to gradually improve new engines they build.) Two are leftovers from the shuttle program that never flew. enter image description here

https://www.nasa.gov/centers/stennis/news/releases/2021/NASA-Fires-Up-Artemis-RS-25-Rocket-Engines-with-New-Components

https://ntrs.nasa.gov/api/citations/20170008958/downloads/20170008958.pdf

enter image description here

Image source Where are the lost RS-25D Block 2 engines? Note: "30 years of flawless performance" is not true. 5 pad aborts, one in-flight shutdown, a myriad of smaller problems.

  • Some of the SRB segments are reused, some are or will be new. (Since SLS expends the SRBs, in the unlikely case that it flies a lot, eventually they will have to make new segments.) It does not say specifically, but I do not think they made any changes to the seals / joints.

enter image description here

Image source Northrop-Grumman, sadly I lost the link.

  • The engine controllers are new.

https://www.rocket.com/article/rs-25-main-engine-controllers-tested-sls-debut

  • The SRB forward skirt is legacy hardware with some additional insulation added.

enter image description here

https://ntrs.nasa.gov/api/citations/20180001125/downloads/20180001125.pdf

  • The booster separation motors are legacy designs.

https://www.northropgrumman.com/wp-content/uploads/SLS-Motors.pdf

This slide summarizes the situation with the boosters:

enter image description here

https://www.nasaspaceflight.com/2021/07/srbs-from-shuttle-to-sls/

The Thrust Vector Control System is basically identical to the STS version.

Are the APUs on the SLS SRBs planned to be hydrazine powered?

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    $\begingroup$ Is it weird that I want to tell someone to install 2062 and 2063 in opposite positions, rather than adjacent? $\endgroup$
    – Russell Borogove
    Jan 26 at 20:34
  • $\begingroup$ @RussellBorogove depends on whether SLS can make it to any orbit on just 2 engines, regardless of where they are in the cluster I guess - if not, its an automatic abort anyway, surely? $\endgroup$
    – Moo
    Jan 26 at 23:41
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    $\begingroup$ Forget failure, if for some reason these new engines perform better/worse you will have asymmetric thrust. Computers can probably correct for this, but if you can avoid problems why not? $\endgroup$
    – Bojan Kogoj
    Jan 27 at 9:53

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