A fuel tank inerting system for an aircraft, the system comprising: a fuel tank; a first reactant source fluidly connected to the fuel tank, the first reactant source arranged to receive fuel from the fuel tank, the first reactant source having a first reactant control element controlling flow thereof; a second reactant source having a second reactant control valve controlling flow thereof; a catalytic reactor arranged to receive a first reactant from the first reactant source and a second reactant from the second reactant source to generate an inert gas to be supplied to the fuel tank to fill an ullage space of the fuel tank; a heat exchanger arranged between the catalytic reactor and the fuel tank and configured to at least one of cool and condense an output from the catalytic reactor to separate out the inert gas; a ram air control valve configured to control flow of ram air into the heat exchanger; a recirculation loop having a driving mechanism therein, configured to drive a flow of gas through the fuel tank inerting system; a flow control valve arranged between the catalytic reactor and the ullage space, the flow control valve configured to control a flow of inert gas into the ullage space; and a controller configured to control operation of (i) the first reactant control element, (ii) the second reactant control valve, (iii) the ram air control valve, (iv) the driving mechanism, and (v) the flow control valve, the controller configured to control a state of the fuel tank inerting system, wherein states of the fuel tank inerting system comprise an OFF state, a CIRCULATE state, a PRIME state, a CATWARM state, an ON state, a DEPRESSURIZE state, and a COOLDOWN state. a fuel tank; a first reactant source fluidly connected to the fuel tank, the first reactant source arranged to receive fuel from the fuel tank, the first reactant source having a first reactant control element controlling flow thereof; a second reactant source having a second reactant control valve controlling flow thereof; a catalytic reactor arranged to receive a first reactant from the first reactant source and a second reactant from the second reactant source to generate an inert gas to be supplied to the fuel tank to fill an ullage space of the fuel tank; a heat exchanger arranged between the catalytic reactor and the fuel tank and configured to at least one of cool and condense an output from the catalytic reactor to separate out the inert gas; a ram air control valve configured to control flow of ram air into the heat exchanger; a recirculation loop having a driving mechanism therein, configured to drive a flow of gas through the fuel tank inerting system; a flow control valve arranged between the catalytic reactor and the ullage space, the flow control valve configured to control a flow of inert gas into the ullage space; and a controller configured to control operation of (i) the first reactant control element, (ii) the second reactant control valve, (iii) the ram air control valve, (iv) the driving mechanism, and (v) the flow control valve, the controller configured to control a state of the fuel tank inerting system, wherein states of the fuel tank inerting system comprise an OFF state, a CIRCULATE state, a PRIME state, a CATWARM state, an ON state, a DEPRESSURIZE state, and a COOLDOWN state.