Fuel tank inerting systems are provided. The systems include a fuel tank, an air source arranged to supply air into a reactive flow path, a catalytic reactor having a plurality of sub-reactors along the flow path, and a heat exchanger. The sub-reactors are arranged relative to the heat exchanger such that the flow path passes through at least a portion of the heat exchanger between two sub-reactors along the flow path. At least one fuel injector is arranged relative to at least one sub-reactor. The fuel injector is configured to inject fuel into the flow path at at least one of upstream of and in the respective at least one sub-reactor to generate a fuel-air mixture. A fuel tank ullage supply line fluidly connects the flow path to the fuel tank to supply an inert gas to a ullage of the fuel tank.