Jon - true, since SpaceX got flying, they've been squeezing more
performance out of the Merlin. A lot more, to the point where the
choice between a pair of Merlins and, say, an RS-27a that would have
been a close thing when SpaceX was starting out is at this point an easy
choice (absent vehicle base-size issues, of course.)
But regardless, simpler and cheaper and lighter, but somewhat
lower-performance, than staged-combustion engines is still the point of
the (successful) design choice they originally made. "Simple" in the
absolute sense, maybe not so much, true. As for "cheap", I'd bet that
SpaceX's internal manufacturing cost on the 1D (not that they'd tell us)
would be at least an order of magnitude less than what AJR would ask for
a new-build RS-27a...
Interesting that they're looking at staged-combustion for their larger
follow-on booster engine. I hadn't seen that. I can see where
minimizing the cost of initial market entry is no longer such a large
consideration for them; they've definitely already made their initial
entry. I might speculate that Blue's expressed intent of BE-4 being
staged-combustion also could be a factor in this expressed interest,
mind. It'll be interesting to see how it all shakes out.
Henry
On 12/29/2015 12:18 PM, Jonathan Goff wrote:
Henry V.,
But SpaceX's gas generator engines aren't exactly simple low-performance
engines either. Maybe they started there, but they've been aggressively
pushing thrust and T/W ratio on those engines to the point where they're
running chamber pressures that are almost half what the SSME runs--much
higher than what I normally hear for gas generator designs. And they're
already looking at LOX/Methane staged combustion for follow-ons.
SpaceX's focus on simplicity and lower-performance seems to be rather
legendary -- half history, half myth.
I'm not saying that their engines are somehow bad (they're not!), just
that a lot of the mythology around them being simple, low-cost,
Minimum-Cost-Design exemplars seems to be dated.
~Jon
On Tue, Dec 29, 2015 at 11:59 AM, Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
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.
Henry