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> 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