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by danpalmer 1021 days ago
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.
2 comments
Rebelgecko 1021 days ago
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
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danpalmer 1021 days ago
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).
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mcny 1021 days ago
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.

Right now it looks and smells like corruption.

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danpalmer 1021 days ago
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.

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DennisP 1021 days ago
Methane is a lot more dense than hydrogen. You'd have to completely redesign the rocket.
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