An on-board aircraft inert gas system, comprising: a source of hydrocarbon; a source of a gas comprising oxygen; a first fluid flow path between the source of gas comprising oxygen and an inert gas output; a reactor disposed along the first fluid flow path comprising an inlet that receives hydrocarbon and the gas comprising oxygen and reacts the hydrocarbon with the oxygen to produce an oxygen-depleted gas comprising water vapor, and an outlet that outputs the oxygen-depleted gas comprising water vapor; a first heat exchanger comprising a water-condensing heat rejection side disposed along the first fluid flow path comprising an inlet that receives the oxygen-depleted gas from the reactor and an outlet that outputs oxygen-depleted gas with a reduced water content, and a heat absorption side in thermal communication with a heat sink; a liquid separator that separates water condensate produced by the heat rejection side of the first heat exchanger from the oxygen-depleted gas with reduced water content; and a gas separator comprising a membrane permeable to water, comprising a first side of the membrane disposed along the first fluid flow path, the separator comprising an inlet on the first side of the membrane that receives the oxygen-depleted gas with reduced water content from the first heat exchanger and an outlet on the first side of the membrane that outputs dried oxygen-depleted gas, and a second side that receives water through the membrane from the oxygen-depleted gas with reduced water content, the separator comprising an outlet on the second side of the membrane that outputs a fluid comprising water; and wherein the heat absorption side of the first heat exchanger comprises an inlet in communication with a source of aircraft ram air. a source of hydrocarbon; a source of a gas comprising oxygen; a first fluid flow path between the source of gas comprising oxygen and an inert gas output; a reactor disposed along the first fluid flow path comprising an inlet that receives hydrocarbon and the gas comprising oxygen and reacts the hydrocarbon with the oxygen to produce an oxygen-depleted gas comprising water vapor, and an outlet that outputs the oxygen-depleted gas comprising water vapor; a first heat exchanger comprising a water-condensing heat rejection side disposed along the first fluid flow path comprising an inlet that receives the oxygen-depleted gas from the reactor and an outlet that outputs oxygen-depleted gas with a reduced water content, and a heat absorption side in thermal communication with a heat sink; a liquid separator that separates water condensate produced by the heat rejection side of the first heat exchanger from the oxygen-depleted gas with reduced water content; and a gas separator comprising a membrane permeable to water, comprising a first side of the membrane disposed along the first fluid flow path, the separator comprising an inlet on the first side of the membrane that receives the oxygen-depleted gas with reduced water content from the first heat exchanger and an outlet on the first side of the membrane that outputs dried oxygen-depleted gas, and a second side that receives water through the membrane from the oxygen-depleted gas with reduced water content, the separator comprising an outlet on the second side of the membrane that outputs a fluid comprising water; and wherein the heat absorption side of the first heat exchanger comprises an inlet in communication with a source of aircraft ram air.