A hydraulic power unit from which a hydraulic fluid may be discharged under pressure to the wing control actuators of a guided missile, the actual pressure at which such fluid is discharged during operation being changed in accordance with the requirements of the wing control actuators. In one of two embodiments disclosed the power unit includes two plenum chambers for the hydraulic fluid, means for independently varying the pressure on the hydraulic fluid in each one of such chambers using a source of substantially constant pressure for such purpose, and means for connecting the wing control actuators to either one of the plenum chambers as required during the flight of the guided missile. It is therefore an object of the invention to provide a hydraulic power unit wherein potential energy of the gas stored within such unit is utilized in a most efficient manner. it is an object of the invention to provide a hydraulic power unit wherein the duty cycle of such unit is matched to the duty cycle of a hydraulic device coupled to such unit. it is an object of the invention to provide a hydraulic power unit wherein the fluid contained within such unit is supplied to a hydraulic device, such fluid being supplied at a pressure level commensurate with the pressure level requirement of such device, thereby to utilize the potential energy of the stored gas in the most efficient manner. These and other objects of the invention are accomplished by providing a hydraulic power unit for supplying a hydraulic fluid stored in a plenum chamber to a hydraulic device coupled thereto, the pressure on the hydraulic fluid being matched to the requirements of the hydraulic device, thereby conserving the potential energy of the stored gas. The desired matching is accomplished by sequentially varying the effective area of a piston operating on the hydraulic fluid in the plenum chamber without changing the area on which the stored gas operates.