Abstract: An apparatus for steerable rotary drilling of a borehole having a wall in the earth comprising, a mandrel, having a central opening there through for the passage of drilling fluids, said mandrel having a lower connection for operatively connecting to a drill bit structure, said mandrel having an upper connection for operatively connecting to a drill string above said apparatus, and the mandrel having an intermediate portion, an outer housing surrounding longitudinal extent of said mandrel intermediate portion, a differential displacement drive within the outer housing, one or more pairs of radially-extensible, opposed side-force exerting elements controlled by said differential displacement drive to provide for side force exertion against the said borehole wall, said drive including a pair of pistons for activating each pair of radially-extensible opposed side-force elements, one or more displacement transducers for said pair of pistons, control valves within said outer housing for fluid pressure control of s