Look, ma! No thrusters! Nearly all spacecraft in existence today
work based on Newton's third law — for every action there is an
equal and opposite reaction. Expulsive explosion of pressurized
stuff out the spacecraft's back creates a reaction force that
accelerates the spacecraft forward.* This means that real-life
and conceivable future spaceships are mostly fuel tanks with a
controlled explosion chamber in the back, with comparatively
little volume for basic life support and human habitation. Most
Science Fiction stories, however are about humans, not reaction
mass, so Science Fiction writers would prefer spacecraft to have
much more of their volume dedicated to human activities. They
typically want their spaceships to be able to operate within
walking-distance of the cantina, rather than turning half the
town into slag as our heroes flee Mos Eisley. A reactionless
drive or reactionless engine is a piece of advanced technology
invented to make life easy for those Science Fiction For our
purposes it is suff