Some do not mention the obvious...
jd
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Jonathan Goff
Sent: donderdag 24 december 2015 20:39
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Re spacex falcon 9 landing
Welcome back Henry! You were missed!
~Jon
On Thu, Dec 24, 2015 at 12:34 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx> 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! :-)