From: jfindley@cinci.nospam.rr.com
In article , snidely.too@gmail.com
says...
>
> On Saturday, JF Mezei exclaimed wildly:
> > Today, NASA was asked how long it would take to relight the engines
> > should it be found that therte was no problem. The answer was about one
> > month, because of need to dry the engines, clean them etc.
> >
> > meanwhile in Boca Chica, SpaceX did 3 engine tess in the same day
> > (granted very short bursts).
> >
> >
> > I wonder how re-usable the SSMEs/RS25s would be if SpaceX would inherit
> > them.
>
> Very different design (by about 50 years), so extrapolation is
> unreliable.
SpaceX made a conscious decision to avoid liquid hydrogen. It has such
low density that it makes tanks and turbo-machinery oversized compared
to other fuels (increasing the dry mass of the stage). It's deeply
cryogenic requiring more insulation, among other things. And finally,
since it's the smallest molecule in nature, hydrogen has a tendency to
leak out of pretty much everything.
Kerosene was chosen as SpaceX's first fuel for a variety of reasons
(high density, room temperature handling, decent ISP) and it's worked
out quite well for Merlin. Liquid methane was chosen for Raptor for a
variety of reasons as well, but one of the most important is that you
can make it on Mars fairly easily (CO2 from the atmosphere and H2O from
the ground/permafrost at the poles).
I think it's fair to say that SpaceX wouldn't be interested in a LOX/LH2
engine that has to be ground started.
Jeff
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