[AR] Re: SpaceX F9 Launch/Update -- Live Link
- From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 29 Dec 2015 11:59:03 -0700
On 12/28/2015 9:53 AM, David Weinshenker wrote:
On 12/28/2015 07:22 AM, Henry Vanderbilt wrote:
This implicitly would be a gas-generator cycle engine, since all the
design sources you cite are, and as such could not serve as a drop-in
RD-180 replacement - the lower Isp would require major vehicle redesign
to increase propellant tankage. (Was an RD-180 replacement what we were
talking about in this thread? perhaps not...)
I was thinking more in terms of general-purpose engines for
large booster stages, (rather than necessarily an RD-180
"drop-in") - in which case, is there any real need for
staged-combustion cycles (or the unusually high chamber
pressures that they enable)?
For a first stage, wouldn't a simpler, lower-pressure cycle
be effective (if not as elegant), in the absence of unusual
requirements, such as the Shuttle locating the SSME cluster
on the aft of the orbiter fuselage (which drove the design
to high chamber pressures to make the engines more compact)?
Absolutely. SpaceX has made exactly that point with F9, which is
somewhat larger overall than it would be if Merlin was a
staged-combustion engine with ~10% better Isp - but that increased size
is almost entirely tankage (relatively cheap in aerospace terms) and
propellant (cost almost negligible in aerospace terms.)
Meanwhile, the overall size increase is somewhat reduced because
gas-generator Merlins are a lot lighter than staged-combustion
equivalents, development costs are significantly reduced because getting
staged-combustion engines ready for flight historically involves huge
amounts of (expensive) testing and tweaking, and overall production
costs are reduced because staged-combustion engines tend to be heavier,
more complex, and require higher-performance materials than
medium-pressure gas-gen engines like Merlin.
Arguably, this all is a significant factor in SpaceX's overall
significantly lower program costs for F9. I would be strongly inclined
to pay close attention to all this were I coming up with a
clean-sheet-of-paper heavy booster/engine combination.
The main practical rationale I could see for developing the
engine you describe would be if SpaceX for some reason wouldn't
sell you Merlins.
Is SpaceX planning to market the Merlins as a stand-alone product
(rather than just as 'captive' production for the "Falcon" vehicles)?
That's not really the correct question. Ask, would SpaceX talk to you
seriously if you showed up with a large checkbook and a requirement for
propulsion of the sort you're discussing. I suspect the answer would
depend in large part on how much of their development bandwidth you'd be
needing and on how likely you'd be to end up competing with them.
Henry
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