Climbing the Mountain of Space

Elon Musk wants a hydrogen upper stage. Elon Musk will develop a
hydrogen upper stage engine. Elon Musk is smart enough to understand
that if the hydrogen upper stage is powerful enough, he can cluster up
ground started variations of the hydrogen engine on a large core stage,
and boost that large core stage off the pad with two Falcon 9s, and then
recover the Falcon 9s and continue on to low Earth orbit by sequentially
shutting down engines until only a single upper stage engine is running,
creating the world's first fully reusable interplanetary launch vehicle.

Upper stage engines and fuel tanks will be extremely useful in space.

http://webpages.charter.space/tsiolkovsky/

Ceres or Bust.