A rotary fluid drive system comprising: a first body and a second body, with a selected one of the bodies being coaxially disposed inside the other body to define a working fluid space therebetween, and with the second body being rotatable relative to the first body about a rotational axis; a plurality of gates supported by the first body, wherein each of the gates is configured to swing or pivot about a pivot axis oriented parallel to the rotational axis; a plurality of torsion rods, wherein each of the torsion rods extends through a longitudinal pin bore provided in one of the gates and has a longitudinal axis aligned with the pivot axis of the corresponding gate, wherein each of the torsion rods is configured to bias and swing the corresponding gate about the corresponding pivot axis towards the second body; at least one lobe provided on the second body; one or more fluid inlet ports directing fluid flow into the working fluid space; and one or more fluid outlet ports axially spaced from the fluid inlet ports and directing fluid flow out of the working fluid space; wherein: for each of the gates, the first body defines a gate pocket into which the associated gate can swing when contacted by the at least one lobe; and the rotary fluid drive system defines a fluid path through which a working fluid can enter and exit the drive system, wherein the fluid path includes the one or more fluid inlet ports, the working fluid space, and the one or more fluid outlet ports, such that a flow of a working fluid along the fluid path will cause rotation of the second body relative to the first body. a first body and a second body, with a selected one of the bodies being coaxially disposed inside the other body to define a working fluid space therebetween, and with the second body being rotatable relative to the first body about a rotational axis; a plurality of gates supported by the first body, wherein each of the gates is configured to swing or pivot about a pivot axis oriented parallel to the rotational axis; a plurality of torsion rods, wherein each of the torsion rods extends through a longitudinal pin bore provided in one of the gates and has a longitudinal axis aligned with the pivot axis of the corresponding gate, wherein each of the torsion rods is configured to bias and swing the corresponding gate about the corresponding pivot axis towards the second body; at least one lobe provided on the second body; one or more fluid inlet ports directing fluid flow into the working fluid space; and one or more fluid outlet ports axially spaced from the fluid inlet ports and directing fluid flow out of the working fluid space; wherein: for each of the gates, the first body defines a gate pocket into which the associated gate can swing when contacted by the at least one lobe; and the rotary fluid drive system defines a fluid path through which a working fluid can enter and exit the drive system, wherein the fluid path includes the one or more fluid inlet ports, the working fluid space, and the one or more fluid outlet ports, such that a flow of a working fluid along the fluid path will cause rotation of the second body relative to the first body. for each of the gates, the first body defines a gate pocket into which the associated gate can swing when contacted by the at least one lobe; and the rotary fluid drive system defines a fluid path through which a working fluid can enter and exit the drive system, wherein the fluid path includes the one or more fluid inlet ports, the working fluid space, and the one or more fluid outlet ports, such that a flow of a working fluid along the fluid path will cause rotation of the second body relative to the first body.