Henrys,
And to be clear, I wasn't saying it would make sense for SpaceX to switch
to LH2 at this point--I was just saying that back when they decided to do
Falcon Heavy, or say the first time they almost canceled it, it might have
made more sense to do a LOX/LH2 upper stage instead. But at this point, now
that they have sunk the cost, and have Falcon Heavy flying, it wouldn't
make sense to go down that road.
~Jon
On Thu, Feb 8, 2018 at 3:57 PM, Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx
wrote:
To be clear, I'm not trying to say there's a commercial business case for
SpaceX to build its own LH2 upper stage for F9H. The near-term likely
markets, primarily LEO and GEO with occasional planetary probes, look to me
served fine by all-kero F9H as-is.
I can see no justification for SpaceX committing the funding and
development-bandwidth an in-house LH2 stage would take. (A lot of both,
especially as they'd be starting up on LH2 largely from scratch.)
I'm talking about the hypothetical case of a government initiative to
expand human operations to Luna and cislunar space, and to do so far more
quickly and affordably than a traditional NASA HSF program.
*They* would be well-advised to look at combining F9H (as by far the
highest-performance flight-proven lower stage available) with an
appropriately-sized long-endurance LH2 upper stage along the lines of ULA
ACES. (Or, to the degree it might be
nearer-term/lower-cost/mission-appropriate,
a stretch Centaur with some ACES enhancements.)
Henry V
On 2/8/2018 3:06 PM, Troy Prideaux wrote:
If what you're implying is true (and I'm of the perhaps naïve opinion
it's close to it), then it's also a good reason for not shifting to LH for
the upper stage.
Troy.
Just because a typical cost-plus vendor would cheerfully allow an upper
stage
engine's mechanical and procedural details to diverge radically and
expensively
from the sea-level version doesn't mean SpaceX would. A difference in
fundamental motivations: Cost-plus means higher cost equals *higher*
profits.
Commercial fixed-price, the opposite. SpaceX demonstrably understands
this.
So I'd be seriously surprised if SpaceX allowed the vast majority of vac
Merlin
parts and procedures to diverge at all from the sea level version, for
the precise
reason you cite, cost.
There's the vac nozzle extension, of course - but that's a passive
radiator,
simple formed sheet metal (relatively expensive metal, yeah) and I'd be
amazed
if it's involved during test-fires rather than bolted on after. Simple
mechanical
testing separate from the engine should reveal any nozzle extension
problems
far cheaper than doing upper-stage engine tests in (expensive!) separate
vacuum facilities.
The mechanical attachment provisions for the extension on the cooled
nozzle
are likely the biggest single physical part difference - assuming they
don't just
put those on all nozzles for commonality.
And ignition provisions may or may not be slightly different between sea
level
and altitude lightoff. I'd guess, not much mechanical difference at all
(maybe
some software procedural?) given that they relight core-stage sea-level
engines
in vac for the initial braking burns.
Bottom line, I doubt SpaceX allowed vac Merlin costs to grow very much.
Which all begs the real question, as far as I'm concerned: F9H cries
out for a high-energy upper stage.
In the best of all worlds, an ACES stage scaled for the F9H would
provide very, very interesting capabilities.
Yes, and now that you've suggested it, I'm going to have to model that
combination. I'll let you handle the politics of a ULA-SpaceX joint
venture.
I'd be fascinated to see what you might be able to share there.
Intuitively, it should make modest difference in F9H payload to LEO, with
increasing payload mass benefit the farther/faster the mission.
As for the politics, the obvious way to bypass that is if the government
is the
too-high-volume-to-ignore customer for both SpaceX and ULA. Or for
SpaceX
and Blue Origin - a BE-3U might actually be sized about right for an
F9H-lofted
upper stage. And Blue also has current hydrogen-stage experience. Not
to
mention an existing stage flying in roughly the right size range - maybe
check
out performance of a stripped-down BE-3U New Shepard on top of an F9H
also?..
(All this is in the context of a government program aimed at significant
results
in relatively few years, of course. In a context either purely
commercial, longer-
term, or both, there's far less likelihood of such multi-vendor booster
cooperation - all three mentioned have their own long-term
high-performance
integrated booster plans.)
Also, Schilling's three rules of space launch propulsion:
metal.
1. It is foolish to use anything but cheap, dense propellants in your
Earth launch stage. You need thrust against gravity, and you
shouldn't much care about weight when it's just cheap rocket fuel and
sheet
2. It is foolish to use anything but LOX/LH2 for your orbital
insertion stage. You need Isp to build delta-V, and every pound of
"cheap"
propellant has to be lofted halfway to orbit by an expensive booster.
3. It is foolish to use different propellants on different stages of
your rocket, because that makes every bit of hardware and every
operational procedure a complete duplication of effort.
Now go design a not-foolish space launch vehicle. Elon has made his
choice, and in my experience most rocket scientists are fairly
stubborn about which of the three rules is "obviously" wrong or at
least less important than the other two. Rocket plumbers may be more
pragmatic, of course, but I don't take Elon to be a plumber.
Well, Delta 4 certainly illustrates Rule 1. With considerable help from
Aerojet
doing a seriously piss-poor job of balancing sea-level and vac
performance in
the RS-68, in my view... (Their answer: Neither!)
I tend to agree with Rule 2. Working with LH2 isn't trivial, but XCOR
served as
proof it's far short of fluorinated devil-juice.
Rule 3 is I think overstated. I can see the advantages of embracing it
for SpaceX
in getting this far, in that it reduced demands on their finite
engineering/development bandwidth to a (barely) sustainable level while
still
allowing them to field a (very) commercially viable initial low-orbit
transport
system with useful (albeit not optimal) GEO capability.
But I think SpaceX's apparent continuing embrace of carved-in-stone Rule
3 may well provide their currently-behind rivals with competitive
windows they
can occupy and expand. We'll see.
Henry V