space mission idea:
send up an HLS starship but with fins into low earth orbit.
fully refuel it.
orbit a starship without its fairing and vacuum engines replaced with nuclear thermal ones. this will be a booster.
refuel the booster.
attach the booster to the first starship.
use this setup to get to titan (saturn’s moon). both stages land near a methane lake so they can somehow autonomously refuel.
the people inside can step outside and breathe in a -179°C nitrogen atmosphere approximately 1.5x denser than earth’s. their lungs may freeze, however. other things would probably also happen but your lungs (and other bodily things) would freeze before anything else.
get back to earth.