In article ,
says...

Jeff Findley wrote:
In article , says...

Well, there is that, but I was actually referring to the "no required
refurbishment" assertion.

As long as the Merlin engines were designed from the start to be
truly reusable (which I suspect they were), then I don't see why a
Falcon 9 V1.1 first stage would be much harder to quickly re-fly
than DC-X. DC-X proved it was possible two decades ago and that was
with the additional (cryogenic) headaches caused by using LH2 as a
fuel.

Fair enough - but in the case of DC-X they were able to look at the
engines etc after each of the tests. And I'm not questioning the
principle of "no refurb" reuse as much as the "confidence" assertion.
Unless one includes Grasshopper (which may indeed be applicable here
at least to a point) I would think that having confidence in knowing
there will be no refurb required would call for examining a few
successfully recovered stages. Sure, they've brought two stages to
sea level (more or less) but just how much of those stages have they
been able to examine?

It all depends on how confident SpaceX is with their hardware. Not only
does Grasshopper (and the follow-on Falcon 9R first stage test vehicle)
use the same engine(s) and other hardware as a flight stage, but SpaceX
has done numerous test stand runs of its Merlin engines. As a
consequence of all this testing, they know quite well if their engines
are capable of rapid reuse.

Also note that there is nothing fundamental that limits a liquid fueled
rocket engine to a single use. Even "expendable" engines are typically
qualified for longer burn durations and multiple starts to allow for
testing before flight. But, some engines do have design compromises
which limit this somewhat. For example, SSMEs leave much of the
hardware needed to start them at the launch pad, so even "air start" for
the initial Ares-I design was problematic. Other engine designs may
incorporate a pyrotechnic igniter which is single use.

Surely SpaceX has designed Merlin engines with enough margin for rapid
reuse since that was their intent from the beginning. That's one of the
reasons they didn't opt for a more complex, higher pressure, higher ISP,
regen engine design (e.g. RD-180 or SSME). If you're optimizing for
reuse, traditional measures of "performance" is an area where you
deliberately compromise.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer