Machian space drive ?

 In his lectures The Meaning of Relativity* Einstein stated that "the inert mass...increases when ponderable masses approach ..." So one would expect that when two masses are close to one another their masses increase slightly and when they are further apart their masses decrease slightly. If this is true one could oscillate a pair of masses transversely (say in the y direction) and using a third mass pull on them axially (say in the x direction) when they are close together and push on them when they are far apart. This cycle might produce a net motion of the whole system of masses in the x direction. It's assumed that momentum is conserved through the coupling with more distant matter. Of course the effect, if real, may be too tiny to be detectable. Also, Mach's principle might simply be wrong; at least this particular version of Mach's principle. My "x-files" contain papers from several authors proposing other sorts of Machian space drives.**

* The Meaning of Relativity, Princeton University, 1922, page 97.

** See, for example, Making Starships and Stargates, J. F. Woodward, Springer, 2013.