be changed whenever the drill string is tripped out of the wellbore which occurs several times during drilling of extended reach wellbores fig3 illustrates a cross sectional view of a portion of the lower section 70 of the drill string 30 which comprises an inner drive train 260 the inner drive train 260 comprising a drive sub 200 a flex shaft 220 and the power unit 57 is connected to the upper section 40 of the drill string 30 fig1 adjacent the inner drive train 260 is the outer housing 210 which rotates in response to the fluid flow through the power unit 57 when the power unit comprises either a mud motor or turbine fig4 illustrates the fluid path which originates from the surface into the drive sub 200 through the flow ports 200 and through the chamber of the power unit 57 which comprises a stator housing 230 and a rotor 240 utilization of this fluid path allows for rotation of the outer housing 210 of the lower section 70 of the drill string 30 the fluid path continues through the lower section 70 of the drill string 30 to the bottom hole assembly 80 fig5 illustrates an alternative fluid path this fluid.