Yeah, well, “Pain In The Arse” has 2 acronyms in rocketry – 1 is “PITA”, the
other is “$$$$”
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Randall Clague
Sent: Friday, 9 February 2018 11:27 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Falcon Heavy use cases
Hydrogen is a monster PITA. If only its upper stage performance wasn't all
that. But its upper stage performance is all that. My conclusion was that if
you can do it, you have to do it.
-R
On Thu, Feb 8, 2018 at 3:31 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx
<mailto:hvanderbilt@xxxxxxxxxxxxxx> > wrote:
GMTA!
On 2/8/2018 4:13 PM, Jonathan Goff wrote:
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 <mailto:hvanderbilt@xxxxxxxxxxxxxx>
<mailto:hvanderbilt@xxxxxxxxxxxxxx ;<mailto: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:
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
metal.
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