string actuator 100 may be enclosed in a housing 110 that may have connections not shown separately at each longitudinal end for engaging the housing 110 to a segment of the dual drill string e g 1 in fig1 on one or both longitudinal ends thereof u201c engagement u201d may include metal to metal or other form of sealing between the housing 110 and each connected segment of the outer pipe as explained with reference to fig1 u201c engagement u201d may further include having an upper internal conduit 112 mounted in fixed longitudinal position within the housing 110 such mounting may include without limitation friction fit standoffs welding adhesive bonding etc the upper inner conduit 112 may be configured to sealingly engage the inner pipe 3 in fig1 to enable completion of the fluid return flow passage 5 in fig1 through the actuator 100 a fluid return flow passage formed by the components of the actuator 100 is shown generally at 113 and 113 a as will be further explained below the actuator 100 also may provide a fluid flow passage between the interior of the housing 110 and the exterior of the upper internal conduit 112 lower internal conduit 115 and additional components explained below thus the.