This reads a bit like a marketing spiel.
The nk33 has a very similar t/w at a higher isp and similar chamber pressure.
There is nothing magical about the GG cycle that makes it lighter. Pumps
drive the weight. Merlin at this point is running the upper limit of Pc for
what is conventionally accepted for GG engines - thus "modest" is not how I
would describe them.
Also, if spacex were interested in selling merlins, virgin would not be
currently recreating the Merlin 1a.
On Dec 29, 2015, at 10:59 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.
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