a housing including longitudinally extending tubular members interconnected to allow controlled relative bending thereof , during a drilling operation , a rotary drive transmitting torsion bar extending generally longitudinally within the housing to controllably bend in response to relative bending of said members , a rotary drill bit operatively connected to said torsion bar to be rotated as the bar rotates , a rotary drive operatively connected to the torsion bar to rotate the bar , means for controlling relative bending of the said members during torsion bar rotation and as a function of such rotation .