I mentioned a number of likely factors in SpaceX's decision to go with a
gas-generator engine for F1/F9. Engine T:W is only one of them (and
probably a minor one, as subsequent discussion seems to indicate.)
Biggest factor was I suspect greatly lower overall development
time-and-cost-to-market for gas-generator, absent previous experience
with staged-combustion.
As you point out in your previous post, previous experience with
staged-combustion is a huge help in developing a new one without much
expensive time spent destroying test engines. And as you were much too
polite to point out, but I will, the vast majority of practical
experience developing successful staged-combustion engines currently
lives in Russia.
In a perfect world, US launch vehicle developers could plan to buy the
best engines around with no worries about future political problems or
other supply or price surprises. It is alas, an imperfect world.
Mind, even in a perfect world SpaceX might have decided to develop their
own engines (and perhaps hire experienced Russian S-C engineers?) since
doing the maximum possible of their manufacturing in-house seems to be
part of their approach to keeping costs down. I would not be shocked if
SpaceX's internal production cost for Merlin 1D is not considerably
lower than even the relatively reasonable prices currently charged for
functional Russian equivalents (but as mentioned previously SpaceX is
unlikely to reveal those costs.)
Henry
On 12/29/2015 5:57 PM, Alexander Ponomarenko wrote:
I am curious, why T:W of booster LPRE is important? Or more important
than specific impulse? The only reason I see now could be the cost:
theoretically, the lightweight unit should be a bit chipper compared to
its more heavy equivalent. But does it matter on that scale (e.g. 450 kg
vs. 550 kg)?
Just FYI: vac T:W of latest RD-275 seems to be comparable (T:W~175). The
chamber pressure of RD-275 is comparable to that of NK-33.
Note that T:W of version of NK-33 which includes gimbaling unit and
thrust frame should be even lower. Does T:W of Merlin 1D includes
gimbaling unit and frame?
On 12/30/2015 12:17 AM, Henry Vanderbilt wrote:
NK-33 vac T:W, 138. Merlin 1D, vac T:W 180.
Mind, NK-33 in fact does have a higher chamber pressure, 2150 psi to
1410 psi for Merlin 1D.
Chamber pressure does seem to be a significant factor in engine
weight, yeah.
On 12/29/2015 1:48 PM, David Gregory wrote:
It was stated that GG engines are lighter than alternatives. At a
given chamber pressure, that's not the case.