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The RS-25's notoriously slow, expensive turnaround was blamed on its complexity. The Raptor appears about as complex.

Both use regenerative cooling.

Both use preburners to drive turbopumps.

While the Raptor has no need for the low pressure pumps, it does run at a much higher pressure.

While the Raptor does not need the seal between LOX pump and turbine, it does have a chemically active flow of hot oxygen coming from the LOX preburner.

While Raptor does not have embrittlement from hot hydrogen, hydrocarbon fuels tend to burn hotter.

Why does the SSME require several months for turnaround, whereas the Raptor needs no time at all?

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    $\begingroup$ The fastest that a Raptor engine was turned around between two flights was 33 days for Raptor SN6 between the 20m and 150m hops of Starhopper. That was also the only time a Raptor engine was turned around between flights, all the other flights were always done with new Raptor engines, they were never re-used. Flying to 20m (that's meters, not miles) is a lot different than flying to orbit. $\endgroup$
    – Jörg W Mittag
    Commented May 14, 2023 at 14:09
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    $\begingroup$ One of the factors is going to be incremental improvement; there were only ever about 50 SSMEs, but there have been hundreds of Raptors produced already. They've also had more Starship and Super Heavy prototypes to refine the engine bay with than Space Shuttle hulls. Ultimately, like Jörg's answer to another question, it seems like they're able to prioritize reuse and it was secondary before. $\endgroup$
    – Erin Anne
    Commented May 15, 2023 at 0:39

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The SSME was the first (and I believe, is still the only) first stage engine reused after returning from orbit.

By the end of the program, SSME's turnaround was much shorter than its "notorious" reputation would have one believe. (See https://space.stackexchange.com/a/15308/50958)

As the AR-22, the engine achieved 10 long duration firings in 10 days

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  • $\begingroup$ I'd forgotten all about Phantom Express. Neat project $\endgroup$
    – Erin Anne
    Commented May 21, 2023 at 3:12

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