Fuel tank inerting systems for aircraft are provided. The systems include a fuel tank, a catalytic reactor arranged to receive a first reactant from a first reactant source and a second reactant from a second reactant source to generate an inert gas that is 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, and a controller configured to perform a light-off operation of the catalytic reactor by controlling at least one light-off parameter and, after light-off occurs, adjusting the at least one light-off parameter to an operating level, wherein the at least one light-off parameter comprises a space velocity through the catalytic reactor.