For very high delta-V missions (eg outer solar system) the Falcon family really starts to suffer from that fact that its second stage is still using RP-1/LO2 rather than LH2/LO2 with a consequently lower ISP. I wondered just how complex it would be to use a Centaur (or an Ariane 4 ESC or eventually ACES) as a third stage (or in place of the second stage) on a Falcon 9 or Falcon Heavy launch. There are obvious political issues, but how hard are the technical issues and how much would you gain.
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asked Feb 25, 2018 at 11:11
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Much more difficult then it sounds. You would need the ability to fuel the upper stage, which would require major launch pad reorganization.
SpaceX's answer is to use the Raptor engine/ methalox, which won't be quite as good as LH2/LO2, but it should be pretty close. The extra thrust and less complex system should more than make up for any losses as well. This will come with the BFR, which is expected in a few years.
answered Feb 25, 2018 at 12:41
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1$\begingroup$ Having driven a car powered by natural gas (mostly CH4) I can say that the storage requirements are pretty straightforward. As opposed to LH2 which is anything but. $\endgroup$– geoffcCommented Feb 25, 2018 at 14:13
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2$\begingroup$ Methane would still require launch pad reorganization. A full sized raptor would be a very bad engine for a falcon second stage, and even worse for a third stage. You want a small engine for better mass fraction. The nozzle would also have a worse expansion ratio. $\endgroup$– LexCommented Feb 25, 2018 at 22:50
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$\begingroup$ @Lex would a halfsize-ish (or less) methalox raptor make any kind of sense for a falcon [heavy] methalox upper? the bump from 348s to 375-382s doesn't seem like it would offer that much for all the additional reengineering. $\endgroup$– lamontCommented Mar 7, 2018 at 23:32