Re: Fifteen years earlier
"The Grasshopper is not capable of getting to orbit either. "
True. It's a control systems and concept of operations demonstrator.
"The DC-X flew higher, for longer and further crossrange than the Grasshopper has."
So far.
" I suspect the goal of a orbit-capable rocket's first stage landing as the DC-X and Grasshopper do is physically impossible due to mass fraction constraints,"
Then by extension you would have considered the ultimate goal of the DC-X programme (sometimes called the DC-1), a single stage to orbit vehicle with no major maintenance between flights, to have been absurd
"The RS-68 Block 1 motors used at the start of the Shuttle program were supposed to be rated for ten flights before rebuilds were necessary; in the real world they were rebuilt after every flight. "
The RS-68 did not exist during most of the Shuttle programme.
It's the expendable engine on the Delta IV and as such has 1 use, although like all liquid rocket engines it's capable of multiple test firings.
The manufacturer ID # for the Space Shuttle Main Engine (which I think is what you're talking about) is RS-25.
You might like to review the rocket equation, the thrust to weight ratio of the Merlin 1d and approximate current payload fraction of the F9. It's all down to how much of the orbital velocity each stage has to supply.