Abstract: A variable capacity hydraulic constant stroke piston apparatus has an annular rotor rotatably mounted for rotation around an axis of rotation and having a plurality of radial cylinder bores therein spaced at intervals therearound with working liquid inlets and outlets at the radially outer ends thereof, a rotor drive for rotating said rotor around the axis of rotation, an eccentric cam positioned within the rotor and movable along a circular locus of eccentric positions concentric with said axis of rotation; a device for fixing the eccentric cam at one of the plurality of eccentric positions and for moving the eccentric cam around the axis of rotation between said positions; a plurality of pistons radially reciprocally movably mounted in corresponding cylinder bores in the rotor, the pistons having heads normally contacting an outer periphery of the eccentric cam and moving through a constant length stroke at each position of the cam; and working liquid current rectifying means connected between the inlets an

Abstract: An attitude control system for controlling a plurality of attitude control devices, which cooperate to establish the attitude of a vehicle includes redundant control units for outputting attitude control signals to each of the attitude control devices. Additionally, a plurality of discreet power servo units are provided which are located in proximity to and operatively connected to the plurality of attitude control devices. The redundant control units and the plurality of discreet power servo units are connected electrically and/or optically. Each of the discreet power servo units includes an adder for adding an attitude control signal and a positional feed back signal, a rotational drive source for providing a rotational motion in response to the output from the adder, a fluid pressure circuit for hydraulically driving an attitude control device coupled thereto, and a sensor for sensing the position of the attitude control device coupled thereto and for providing to the adder the positive feed back signal.