Casimir Stardrive

You don't need any propellant for a spacecraft,
a solar sail and two electrically charged plates will
do (have to build it in orbit etc tho).

The first plate reflects the back plate (Casimir effect)
creating the first forward force; if the back
plate is reflective it will be accelerated back
(a moving reflective body will be accelerated depending
on velocity) where it will collide with the
first plate
=2nd forward force.

If a superconducting coil is used no further power
is required
=3rd and 4th forward forces ad infinitum.

OK, braking rockets, a second/third solar sail
or a second pair of plates to decelerate...and
increase the resistivity of the first coil/turn off
the current.