In article mn.e03c7dc2f59dd83e.127094@snitoo,
says...

Brian Gaff brought next idea :
The whole issue of reuse of engines though, seems to be more of a problem
than one might think. The SMEs needed a lot of work done to validate them,
and I just wonder if they have factored the cost of this in sufficiently.
Brian

The SSMEs are more complicated than most rocket engines, aren't they?
And even with that, the 'tween-flight work was considerably reduced by
the time of the 2nd return-to-flight. (Actually, well before then, but
I don't have those flight numbers memorized.)

Yes. SSME's are high chamber pressure, high ISP, staged combustion,
LOX/LH2 engines. They're arguably the most exotic, routinely flown,
liquid fueled rocket engine in US spaceflight history.

The SpaceEx engines are supposed to be one of the simpler designs,
aren't they?

Yes. They're LOX/kerosene engines that do not push the envelope in
terms of performance. If you want LOX/kerosene engines that are higher
performance, you can buy them from the Russians like the US does for
Atlas V's first stage.

Jeff
--
" Ares 1 is a prime example of the fact that NASA just can't get it
up anymore... and when they can, it doesn't stay up long. "
- tinker