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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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Orion & the second (third?) stage are all new (at least to STS hardware).

Orion's service module, the European Service Module supplied by the European Space Agency (ESA) and built by Airbus Defence and Space based on the design of the Automated Transfer Vehicle (ATV) uses a Space Shuttle Orbital Maneuvering System (OMS) engine (Aerojet Rocketdyne's AJ10-190) for its main propulsion system.

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