[AR] Re: Falcon Heavy use cases
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Thu, 8 Feb 2018 16:15:28 -0500 (EST)
On Thu, 8 Feb 2018, Henry Vanderbilt wrote:
A Raptor vacuum upper stage is not beyond credible...
Methane upper stages are far more an answer to potential in-situ refuelling
than they are any useful performance improvement on kerosene.
Agreed in general.
IIRC, potential methane vac specific impulse advantage over kerosene is
maybe 3%.
Depends on somewhat on assumptions, but yeah, it's not entirely negligible
but not large.
And arguably, you give that back on the extra mass of methane tankage
(liquid methane density is ~2/3rds that of kero, plus cryo insulation)
Methane is less dense than kerosene, but it also optimizes at a higher
mixture ratio, putting more high-density oxygen in the mix, so the density
difference at the stage level is less than you'd think.
And like LOX, on a flight stage, methane shouldn't require insulation
unless you've got some special constraint (like, say, fragile orbiter
tiles hanging downwind of the tank).
So, really not worth the effort to replace the F9H kero upper stage with
methane, in my view.
Agreed. Either replace it with hydrogen or leave it be. Opinions may
differ :-) on which of those two approaches is preferable, but replacing
with methane isn't really a competitor.
Henry
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