beleave me, this type of propusion has not been thaught up as far as I can tell
ok the gist is, you know how a gauss rifle works right? with rings of magnets accelatering a projectile
well think about, instead of rings of magnets, its a ring of trillions of subatomic blackholes, smaller than protons, because of Hawking radiation, they would disapate within microseconds, but before they do, they would enact a gravatational field much stronger than a single black hole and simmiler in dimention to the magnetic field for the gauss rifle, the net effect would be the equivalent of a gravity assist around a full size black hole, and with more rings, the gravity assist would become expanential
Like I said, unlike other propulsion systems, this would nagate the increase in mass because gravity would pull it at the same rate no matter what mass it is (thats the reasion why a golf ball and a bowling ball accelarate at the same rate)
how the ship would generate those rings would require a pretty detailed explanation of my theary of the degradation of subatomic radiation particles
of corse, like before, this is all assumeing that the known laws of physics hold, if they dont, than it is impossible to develop a relavant propulsion system untill the relavent information is givin