Okay, here's a different idea for "propellant-less" propulsion: http://adsabs.harvard.edu/abs/2004AIPC..699.1138M It's based on some idea by a University of California physicist named Woodward, who himself preferred to use the term "Mach Effect" Basically, his idea was that if you have mass moving at relativistic velocity inside another mass, then the internal moving mass can make itself heavier or lighter depending on how fast it's moving. So then if you make the overall system oscillate (surrounding mass), and you time the movements of the inner mass with the oscillations, then you can selectively impart more momentum in one part of the oscillation than in another, for the overall system. This would create a net change in momentum for the system. Did anybody catch what I said there? Hmm, this one has me scratching my head. So where did the extra mass/momentum on demand come from? Well, it came from the relativistic motion of the inner mass. You can switch that motion on or off, depending on which way your overall mass-system is oscillating. If momentum is to be conserved, then where did the difference in momentum go? It went to, umm, space? It went to whatever it is that retards the motion of a relativistically-moving mass. The universe? Hmm, tell me, if I lift up a cyclotron that's whirling particles around at extremely high speed, will it feel heavier to me than if I lifted it up while it was turned off? Does anybody see what I'm saying here? In that sense, is a relativistically-moving mass really a closed system? Or is it somehow interacting with space itself, where space itself can't really be described as closed?