A fuel flow control system, comprising: a centrifugal pump having a fuel inlet, a gas inlet, and an outlet; the fuel control system being characterised in that it further comprises: a gas inlet valve disposed upstream of the gas inlet and responsive to valve position commands to move between a closed position, in which inert gas is prevented from flowing into the gas inlet, and a plurality of open positions, in which inert gas may flow into the gas inlet; and a control coupled to the gas inlet valve and configured to supply the valve position commands to the gas inlet valve to command the gas inlet valve to selectively move to the closed position or one of the plurality of open positions, the fuel control system being further characterised in that: in the closed position, the centrifugal pump is configured to operate as a fuel pump, in any one of the open positions, the centrifugal pump is configured to operate as a fuel-gas mixer, discharge pressure at the outlet of the centrifugal pump varies with inert gas flow into the gas inlet, and the control is further configured to supply the valve commands to the gas inlet valve to thereby control the discharge pressure at the outlet of the centrifugal pump. a centrifugal pump having a fuel inlet, a gas inlet, and an outlet; the fuel control system being characterised in that it further comprises: a gas inlet valve disposed upstream of the gas inlet and responsive to valve position commands to move between a closed position, in which inert gas is prevented from flowing into the gas inlet, and a plurality of open positions, in which inert gas may flow into the gas inlet; and a control coupled to the gas inlet valve and configured to supply the valve position commands to the gas inlet valve to command the gas inlet valve to selectively move to the closed position or one of the plurality of open positions, the fuel control system being further characterised in that: in the closed position, the centrifugal pump is configured to operate as a fuel pump, in any one of the open positions, the centrifugal pump is configured to operate as a fuel-gas mixer, discharge pressure at the outlet of the centrifugal pump varies with inert gas flow into the gas inlet, and the control is further configured to supply the valve commands to the gas inlet valve to thereby control the discharge pressure at the outlet of the centrifugal pump. a gas inlet valve disposed upstream of the gas inlet and responsive to valve position commands to move between a closed position, in which inert gas is prevented from flowing into the gas inlet, and a plurality of open positions, in which inert gas may flow into the gas inlet; and a control coupled to the gas inlet valve and configured to supply the valve position commands to the gas inlet valve to command the gas inlet valve to selectively move to the closed position or one of the plurality of open positions, the fuel control system being further characterised in that: in the closed position, the centrifugal pump is configured to operate as a fuel pump, in any one of the open positions, the centrifugal pump is configured to operate as a fuel-gas mixer, discharge pressure at the outlet of the centrifugal pump varies with inert gas flow into the gas inlet, and the control is further configured to supply the valve commands to the gas inlet valve to thereby control the discharge pressure at the outlet of the centrifugal pump. in the closed position, the centrifugal pump is configured to operate as a fuel pump, in any one of the open positions, the centrifugal pump is configured to operate as a fuel-gas mixer, discharge pressure at the outlet of the centrifugal pump varies with inert gas flow into the gas inlet, and the control is further configured to supply the valve commands to the gas inlet valve to thereby control the discharge pressure at the outlet of the centrifugal pump.