>>11648438The work that went into Raptor was to get the alloys right, the mass as low as possible, the complexity as low as possible, in-flight restart capability, rapid throttling response, etc etc.
If NASA wanted to pay them a billion dollars for a Raptor variant that burned hydrogen, it would not take a restart from scratch in order to get that engine available. They wouldn't even need to work out the alloys needed fro the hydrogen rich side on their own, because NASA already found something good enough to make a reusable engine back in the 70's (and lest we forget, this would be an RS-25 replacement for SLS, meaning no reusability required).
Here are the exact changes needed to develop a hydrolox RS-25 replacement based on Raptor;
Swap all fuel lines for ones that are bigger to incorporate the increased volume of fuel required, as per hydrogen's low density and the engine's overall more fuel-rich design.
Replace methalox turbopump with hydrolox turbopump, this is where 90% of the dev time goes, it will use zero-embrittlement alloys and be physically much larger than Raptor's methalox fuel rich pump.
Adjust nozzle regenerative cooling gap thickness, again because hydrogen meme density
Adjust oxygen rich pump's fuel injector to accommodate hydrogen, no alloy changes needed except maybe for injector face which shouldn't encounter high oxygen environment anyway
Misc changes; Don't need air-restart, don't need fast throttling, don't need any reusability systems or design considerations beyond ability to test fire before expendable launch, don't need oxygen autogenous pressurization system, don't need fucken uuuuuh some other things I can't think of atm
