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A fuel system for a gas turbine engine that utilizes a centrifugal pump is presented. The system includes a fuel metering valve that is adapted to set a metered flow of fuel, and a throttle valve that is adapted to accurately control pressure drop across the fuel metering valve. The throttle valve h

A fuel system for a gas turbine engine that utilizes a centrifugal pump is presented. The system includes a fuel metering valve that is adapted to set a metered flow of fuel, and a throttle valve that is adapted to accurately control pressure drop across the fuel metering valve. The throttle valve has at least two variable orifices and a compensation chamber between the variable orifices. The throttle valve includes a differential valve piston slidable in a valve body. The differential valve piston comprises working surfaces of at least two different diameters such that changes in chamber pressures effect different axial forces upon the piston.

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What is claimed is: 1. A fuel metering unit for a fuel system for regulating fuel flow in a gas turbine engine pumped by a high pressure centrifugal pump and delivered to a nozzle outlet passage, the fuel metering unit comprising: a fuel metering valve and a throttle valve arranged in fluid series,

What is claimed is: 1. A fuel metering unit for a fuel system for regulating fuel flow in a gas turbine engine pumped by a high pressure centrifugal pump and delivered to a nozzle outlet passage, the fuel metering unit comprising: a fuel metering valve and a throttle valve arranged in fluid series, the throttle valve having a valve body, a double diameter valve member and at least two variable orifices, the valve member being movable in the valve body to vary the size of the variable orifices, the variable orifices arranged in fluidic series with a compensation chamber defined therebetween such that a fluid control pressure develops in the compensation chamber when fuel flows through the throttle valve, fluid control pressure acting upon the valve member to control position of the valve member. 2. The fuel metering unit of claim 1, wherein the throttle valve includes a differential valve piston slidable in a valve body, the differential valve piston comprising two different diameters to provide differently sized working surfaces across the throttle valve. 3. The fuel metering unit of claim 2, wherein the differential valve piston further comprising first and second lands in spaced relation such that the throttle valve defines at least three chambers, including a first chamber subjected to fluid pressure upstream of the fuel metering valve, a second chamber subjected to fluid pressure downstream of the fuel metering valve, and the compensation chamber intermediate of the two variable orifices, the compensation chamber arranged in fluidic series with the fuel metering valve and an outlet passage, the outlet passage adapted to convey fuel toward the gas turbine engine. 4. The fuel system of claim 1, wherein the variable orifices change in flow area at different rates when the throttle valve moves between a plurality of positions. 5. The fuel system of claim 4, wherein the variable orifices are in series between the fuel metering valve to include an upstream variable orifice and a downstream variable orifice, the upstream variable orifice having a larger flow area than the downstream variable orifice during operation of the throttle valve. 6. The fuel system of claim 1, wherein the throttle valve comprises a spring urging the throttle valve to a closed position preventing fuel from flowing to the nozzle outlet passage, and wherein the fuel system is free of a separate shut off valve. 7. The fuel system of claim 6, further comprising a electrically operated valve fluidically adapted to receive a high pressure fuel output from the high pressure centrifugal pump during operation, the electrically operated valve having an on position adapted to connect the high pressure fuel output to an actuation chamber in the throttle valve and a off position adapted to disconnect the actuation chamber from the high pressure fuel output and connect the actuation chamber to a sump or boost pressure, the high pressure fuel output when initially communicated to the actuation chamber via the on position of the electrically operated valve acting upon the throttle valve against the spring to move the throttle valve from the closed position to an open position, the sump or boost pressure when communicated to the actuation chamber via the off position of the electrically operated valve allowing the spring to move the throttle valve from the open position to the closed position. 8. The fuel system of claim 1, wherein the throttle valve is arranged downstream of the fuel metering valve. 9. The fuel system of claim 1, wherein the throttle valve is biased by a spring, the spring providing a variable biasing force determined by the position of the throttle valve, wherein the two variable orifices change in flow area when the throttle valve moves when in operation whereby fluid pressure in the compensation chamber is changed in a predetermined manner determined by a selective sizing of ports for the variable orifices, the change in fluid pressure being correlated to changes in the variable biasing force wherein the variable biasing force and the fluid pressure in the compensation chamber generally increase and decrease in unison. 10. The fuel system of claim 9, wherein changes in fluid pressure in the compensation chamber further compensates for fluid forces such as Bernoulli forces generated when fluid is flowing through the throttle valve. 11. The fuel system of claim 1, further comprising a servo controller positioning the fuel metering valve.