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Embodiments of a valve actuator are provided for adjusting the position of a main valve element in relation to the pressure of a fluid supplied by a supply duct. The valve actuator includes a housing, a control pressure valve disposed in the housing, and a piston slidably disposed in the housing and

Embodiments of a valve actuator are provided for adjusting the position of a main valve element in relation to the pressure of a fluid supplied by a supply duct. The valve actuator includes a housing, a control pressure valve disposed in the housing, and a piston slidably disposed in the housing and mechanically coupled to the main valve element. The housing has a control pressure chamber therein, which is configured to be fluidly coupled to the supply duct. The control pressure valve is configured to substantially impede fluid flow into the control pressure chamber until the fluid pressure within the supply duct surpasses a minimum actuation pressure. The piston normally resides in a first position and is configured to move toward a second position as the pressure within the control pressure chamber increases.

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1. A valve actuator for adjusting the position of a main valve element in relation to the pressure of a fluid supplied by a supply duct, the valve actuator comprising: a housing, comprising: an end wall;an inner structure fixedly coupled to the end wall and extending inwardly therefrom;an inlet conf

1. A valve actuator for adjusting the position of a main valve element in relation to the pressure of a fluid supplied by a supply duct, the valve actuator comprising: a housing, comprising: an end wall;an inner structure fixedly coupled to the end wall and extending inwardly therefrom;an inlet configured to be fluidly coupled to the supply duct; anda control pressure chamber fluidly coupled to the inlet through the inner structure;a control pressure valve disposed in the inner structure of the housing and configured to substantially impede fluid flow into control pressure chamber until the pressure of the fluid within the supply duct surpasses a minimum actuation pressure;a piston slidably disposed in the housing and configured to be mechanically coupled to the main valve element, the piston normally residing in a first position and configured to move toward a second position as the pressure within the control pressure chamber increases, the piston having a cavity in which the inner structure and control pressure valve are received when the piston is in the first position; anda piston spring disposed in the housing and biasing the piston toward the first Position, the pressurized fluid directed into the control pressure chamber through the control pressure valve exerting a cumulative force on the piston urging the piston toward the second position in opposition to the piston spring. 2. A valve actuator according to claim 1 further comprising: a return pressure chamber disposed in the housing; anda control orifice fluidly coupling the return pressure chamber to the control pressure chamber. 3. A valve actuator according to claim 2 wherein the control orifice is formed through the piston. 4. A valve actuator according to claim 2 wherein the return pressure chamber and the control pressure chamber are defined by the piston and by the housing. 5. A valve actuator according to claim 1 wherein the piston comprises: a first end portion extending through the valve housing and configured to be mechanically coupled to the main valve element;a second end portion; anda flange coupled to the second end portion. 6. A valve actuator according to claim 5 wherein the piston spring is compressed between an inner surface of the housing and the flange. 7. A valve actuator according to claim 1 wherein the control pressure valve comprises: a poppet valve element movable between an open position and a closed position; anda poppet valve spring biasing the poppet valve element toward the closed position. 8. A valve actuator according to claim 2 further comprising a cooling flow passage fluidly coupled to the return pressure chamber and configured to be fluidly coupled to the supply duct. 9. A valve actuator according to claim 8 wherein the cooling flow passage is formed through the housing and through the piston. 10. A valve actuator according to claim 1 wherein the inlet is formed through the end wall, and wherein the housing further comprises a seating surface provided on the end wall and contacted by the control pressure valve when in the closed position. 11. A valve actuator according to claim 1 wherein at least a portion of the control pressure chamber is located within the cavity of the piston. 12. A throttle valve assembly for use in conjunction with a supply duct fluidly coupled to a fuel pump, which draws fuel from a fuel tank when driven by an air turbine, the throttle valve assembly comprising: a ram jet flowbody having a main flow conduit extending therethrough configured to be fluidly coupled to the air turbine;a valve element disposed within the main flow conduit and movable between an open position and a closed position; anda valve actuator coupled to the ram jet flowbody, the valve actuator comprising: a housing having a control pressure chamber therein, the control pressure chamber configured to be fluidly coupled to the fuel pump by the supply duct;a control pressure valve disposed in the housing and configured to substantially impede fluid flow into the control pressure chamber until the pressure of the fluid at the outlet of the fuel pump surpasses a minimum actuation pressure;a return pressure chamber fluidly coupled to the control pressure valve;a return duct fluidly coupled to the return pressure chamber and configured to be fluidly coupled to the fuel tank; anda piston slidably disposed in the housing and mechanically coupled to the main valve element, the piston normally residing in a retracted position and configured to move toward an extended position as the pressure within the control pressure chamber increases. 13. A throttle valve assembly according to claim 12 wherein the open position and the closed position of the valve element correspond to the retracted position and the extended position of the piston, respectively. 14. A throttle valve assembly according to claim 12 wherein the supply duct fluidly couples the fuel pump to the control pressure chamber, and wherein the throttle valve assembly further comprises a cooling flow passage fluidly coupling the supply duct to the return pressure chamber and bypassing the control pressure chamber. 15. A flow regulation system, comprising: a flowbody having a main flow conduit extending therethrough;a valve element disposed within the main flow conduit and movable between an open position and a closed position;a valve actuator coupled to the flowbody, the valve actuator comprising: a housing having a control pressure chamber therein;a control pressure valve disposed in the housing and configured to substantially impede fluid flow into the control pressure chamber until the pressure of the fluid within the supply duct surpasses a minimum actuation pressure; anda piston slidably disposed in the housing and mechanically coupled to the main valve element, the piston normally residing in a first position corresponding to the open position of the valve element and configured to move toward a second position corresponding to the closed position of the valve element as the pressure within the control pressure chamber increases; anda supply duct fluidly coupled to the main flow conduit at a location downstream of the valve element and configured to direct pressurized fluid flow from the main flow conduit to the control pressure valve such that, after the minimum actuation pressure has been surpassed, pressurized fluid from the supply duct flows through the control pressure valve, into the control pressure chamber, and exerts a force on the piston urging movement of the piston toward the second position corresponding to the closed position of the valve element to maintain the pressure in the main flow conduit and downstream of the main valve element below a predetermined threshold value. 16. A flow regulation system according to claim 15 wherein the valve actuator further comprises a piston spring disposed in a return pressure chamber and biasing the piston toward the first position. 17. A flow regulation system according to claim 15 wherein the first and second positions of the piston are retracted and extended positions, respectively. 18. A flow regulation system according to claim 15 further comprising a return pressure chamber vented to a low pressure source. 19. A flow regulation system according to claim 15 wherein the valve actuator further comprises a control flow orifice fluidly coupling the control pressure chamber to a return pressure chamber. 20. A flow regulation system according to claim 19 wherein the control flow orifice is formed through the piston.