Also, what effect does the free expansion of gas in a vacuum have on rocket propulsion in space?
I'm not a rocket scientist, so I won't have a definite answer for this.
However, the net thrust generated by a rocket engine is governed by the following equation:
F
n = mv
e-opt + A
e(p
e - p
amb)
Where m is the mass flow of the exhaust, v
e-opt is our effective jet velocity when atmospheric pressure is equal to the pressure at the nozzle's exit, A
e is the flow area at the nozzle exit, p
e is the pressure at the nozzle's exit, and p
amb is atmospheric pressure.
As p
amb in a vaccum would be 0, while all other variables would stay constant at all times given a steady, constant stream of exhaust, a rocket in space would be more efficient than a rocket in the atmosphere.