Henry,
I think Falcon Heavy's evidence could cut against John's rule #3 as easily
as it cuts against #2. Falcon Heavy cost (according to Elon) at least an
extra $500M to develop, and almost got canceled 3x, and ended up being a
lot more operationally complex than they had originally assumed. They made
it work spectacularly well out of the gate, but couldn't they have achieved
basically the same result by developing a LOX/LH2 upper stage to replace
the current F9 upper stage? You'd probably have Falcon Heavy performance
that way with a single-stick vehicle. And if they were smart about how they
did it, I have a hard time believing it would've cost them more than it did
to develop Falcon Heavy. XCOR's ability to work with LH2 suggests it might
not be the cryogenic demon juice that it sometimes is considered in this
crowd.
SpaceX made Falcon Heavy work spectacularly, but I think a SpaceX like
startup that decided to suck it up and do LOX/LH2 for their upper stage
could have also done just fine, and likely be competitive with SpaceX. It's
not a dumb approach, but I don't think the LOX/Kero for both stages rule is
at the heart of SpaceX's success vs its competitors.
~Jon
On Thu, Feb 8, 2018 at 9:04 AM, Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Wed, 7 Feb 2018, John Schilling wrote:
1. It is foolish to use anything but cheap, dense propellants in your
Earth launch stage... 2. It is foolish to use anything but LOX/LH2 for
your orbital insertion stage... 3. It is foolish to use different
propellants on different stages... ...Elon has made his choice...
And given that Falcon 9 gets a substantial payload to GTO or a Mars
trajectory with two stages of LOX/kerosene, and that we've just had a
demonstration of how to get bigger payload and/or higher delta-V by just
piling up more LOX/kerosene, I'd say the case for rule #2 is really a
trifle weak...
*Provided*, that is, that you're designing the whole vehicle and there are
no externally-imposed size constraints. LOX/LH2 looks much more appealing
if you're trying to add an upper stage to an existing first stage, because
that limits upper-stage mass, and LOX/LH2 definitely delivers more
performance per kilogram (maybe not per dollar, but per kilogram, yes).
Some of the LOX/LH2-upper-stage fixation from the 60s came about because
both Centaur and the Saturn upper stages (as originally conceived) were
being added to existing boosters (Atlas and Saturn I respectively). Another
example is that LOX/LH2 is really appealing for air launch, where (unless
you feel like spending a *lot* of money and time on aircraft development)
your rocket is tightly constrained by what an existing carrier aircraft can
lift.
Henry