Improvements in aircraft on-board inert gas generating systems wherein generally excess or surplus compressed airflow from a radial flow centrifugal compressor, produced in order to keep the compressor from surging in predetermined “reduced speed/reduced pressure ratio” operating modes, is utilized for at least one of driving a turbofan for conditioning the air separator module inlet air, or warming up/heating an air separator module in the inert gas generating system, thereby utilizing at least some of the energy of this excess/surplus compressed air that would otherwise be wasted. Several preferential operational methods and structural systems are set forth.