Hmm ...
"... using chemical propellants as fuel is out of the question ..."
Place: An underground bunker on Titan. Time: The near future.
A bottle green creature leans back in its seat and steeples four of its front appendages. A similar creature crouches before it.
"And the invaders? How are they progressing?"
"Reports say their mother ship is tearing up rock from beneath the surface, melting it and passing it to their attack ships. These ships in turn are vaporising the rock and blasting it down over the surface, before leaping away to attack another district."
"I see. A curious strategy."
"We are receiving similar reports from Europa and Enceladus, Emperor."
The bottle green creature sighs and furrows two of its brows.
"You realise, of course, this means war."