This is a good point, but to extend it a bit, changing the fuel means changing the chemistry of the whole system, changing tradeoffs and thresholds, changing capabilities, all sorts. Rockets very much need to be designed around a particular fuel, it defines so much of the rest of the rocket either directly or indirectly.
I imagine it also impacts all the other infra (eg, what reagents dou need to store at the launchpad, what do you need to store nearby, do you need cryogenic cooling, etc
It does! The take-off temperature is a very big concern, how much chilling of the rocket needs to happen etc. The ground support equipment I gather is also complex, although I think storage and transport of all those sorts of fuels around tank farms is much more of a solved problem, I suspect you could buy much of it off the shelf (not that NASA would, but SpaceX seem to be buying regular commercial equipment for theirs).
At a savings of USD 80 million or more per engine, I'd say it is well worth it to push deadlines or whatever we need to do to consider the cheaper options.
I think it's a false comparison to say 80m per engine today.
When SLS was being architected 10+ years ago, I believe the idea was that the RS-25 engines would be cheaper, and there were spares around. Also there were no $1m engines, not even close. Perhaps there might have been a competitor for 1/2 the price, but would that be worth it over an engine that you already have flight ad maintenance experience with? Probably not, it's certainly not a convincing argument.
Lastly there weren't any Methane engines – both of the hottest engines in the market now (pardon the pun) are Methane based, the Raptor and BE-4, but the decision to use Hydrogen as the fuel for SLS was set in stone years ago and unrealistic to change.
Basically there weren't convincingly better options at the time the decision was made, and changing that decision now would mean basically going back to the drawing board on the entire rocket.
And the whole reason for the hydrogen burning was to keep the space shuttle contractors jobs. Once again it's not a technical reason but a pork barrel one.
Possibility of corruption aside, these are clones of the Saturn 5 rocket engines and tech just scaled up - the whole thing is built for hydrogen which is clean burning (water is the byproduct) but you'd think SpaceX could take the design and make something for 20mil
The RS-25 is related to the J-2 used in S-II and S-IVB upper stages - a very successful engine. The STS program came up a new design based on a high-pressure combustion chamber running at 3,000 psi (21,000 kPa) for higher performance.
The first SLS flights will use available Block II RS-25D engines left over from the shuttle program, and when those run out (and if SLS is still flying) the rocket will switch over to the RS-25E, a cheaper, expendable version.
The F-1 engines used on the first stage of the Saturn V were built to burn Kerosene, and the RS-25 series has no common heritage.
How so? Other than sharing a fuel type, and thus surely things learned during J-2 development and operation influenced the RS-25 development and operation, so far as I know the two engines are completely different. Different cycles, different power packs (e.g. turbopump), wildly different packaging, different head pressure, different chamber pressure, throat, etc etc etc. I'm pretty sure that the J-2 did not cool the bell with the fuel, though I could be mistaken. Are the combustion chambers similar? What makes the two engines related, other than the fuel type and of course manufacturer?
SpaceX is on record with not wanting to use hydrogen for the "pain in the ass factor".
It boils at a lower point than oxygen, so you have to insulate the tanks from each other, it's very.. undense (sparse?) per volume so you need bigger tanks, and it's the smallest molecule that exists and makes leaks and shipping harder.
Methane is slightly less efficient but way easier logistically.
There are some alternatives to O2, various molecules that contain oxygen which react readily. But those same properties that make them good oxidizers (namely, reactivity) also make them toxic and difficult to handle.
Liquefied, not compressed. Pressure vessels are heavy, just liquefy it by getting it really cold. It only needs to perform for 10 minutes in booster stages.