Generally its believed to require far to much mass-energy equivalence to be practical but some researchers appear to have discovered a way to get a tiny curvature of space with considerably less mass energy. (Nanometers at this point)
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Are you saying that since M = E/C2 then you are not going to get enough bang for the buck? Too much E to get a bit of M? If so, how do you get around that except by the expenditure of time?
Even small curvature “short cuts” over millions of miles would save much energy.