Welcome back!
Does this mean I have to go back to signing myself "The Other Henry?"
I'm thinking I may go with "Henry's Evil Twin" instead...
I think you've nailed it: The chief reason to go with methane over
kerosene for fuel is if you're somewhere methane is significantly easier
to obtain than kerosene.
Mind, at this point that's not just the off-planet locations where
in-situ methane looks feasible. Anyone planning to fly out of the
continental US these days has to look seriously at LNG (whether refined
to pure LCH4 or not) over kerosene on grounds of both price advantage
and also more assured future availability. Case in point, Blue Origin
(and probably ULA also) with the BE-4.
Henry's Evil Twin <cackles maniacally, twirls moustache>
On 12/24/2015 12:34 PM, Henry Spencer wrote:
On Thu, 24 Dec 2015, Ian Woollard wrote:
This subcooled methane idea, how is it supposed to work? ... ran
cpropep-web and looks like you'll get nearly the same Isp as Kerosene.
But the propellant density and impulse density is unimpressive.
Depending on conditions and details, Isp can be noticeably better than
kerosene -- not a huge gain, but a useful one.
Methane's density is not impressive compared to kerosene, but as Jeff
Greason once observed, you get a lot of that back because CH4 optimizes
at a higher mixture ratio, so there's more dense LOX in the mix.
(Again, this depends somewhat on conditions and details.)
I can see some small wins, like you it helps having the two
propellants at
about the same temperature...
This may or may not be a win at all; e.g., it means doing cryo
conditioning on both sets of plumbing rather than just one.
but no big win. What am I missing?
The one big win is if you eventually want to refuel somewhere off Earth,
in which case it's a lot easier to make methane than anything resembling
kerosene. (Making hydrogen is easier yet -- indeed, making methane may
involve that as an intermediate step -- but it's much harder to store,
which is important since fuel-making is likely to be slow and you'll
have to accumulate fuel for a while. It's especially hard to store if
you're in an environment, like say the surface of Mars, where there's
enough atmosphere to ruin the effectiveness of MLI.)
That aside, yes, it's small wins rather than big ones. Maybe enough to
be interesting, depending on your intentions.
To be honest, Musk really needs a hydrogen upper stage.
Remember, he had a hydrogen-engine project at one point. Then it went
on the back burner. Then it went on the very back burner. Don't think
it's been heard from at all lately. He appears to have reached a
different conclusion about what he needs, probably because he's got
different ideas about what the crucial figures of merit are.
Henry
P.S. Yes, I'm back! :-)