A fuel tank inerting system for an aircraft, the system comprising: a fuel tank (202, 302, 402, 502, 602, 702, 802, 902, 1001, 1102, 1202, 1302, 1402); a first reactant source fluidly connected to the fuel tank, the first source arranged to receive fuel from the fuel tank; a second reactant source (220, 320, 420, 520, 620, 720, 820, 920, 1020, 1120); a catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322) arranged to receive a first reactant from the first source and a second reactant from the second source to generate an inert gas that is supplied to the fuel tank to fill a ullage space of the fuel tank; and a back pressure flow restrictor positioned within or along a fuel tank supply line and downstream of the catalytic reactor, the back pressure flow restrictor arranged to maintain high-pressure operation of the catalytic reactor; a heat exchanger (232, 332, 432, 532, 832, 932, 1132) 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 an inert gas and a byproduct; an injector pump arranged to receive the first reactant and the second reactant and to supply a mixture of the first reactant and the second reactant to the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322); and an inert gas recycling system located downstream of the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322) and upstream of the fuel tank (202, 302, 402, 502, 602, 702, 802, 902, 1001, 1102, 1202, 1302, 1402), wherein the inert gas recycling system is arranged to direct a portion of the inert gas to the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322). a fuel tank (202, 302, 402, 502, 602, 702, 802, 902, 1001, 1102, 1202, 1302, 1402); a first reactant source fluidly connected to the fuel tank, the first source arranged to receive fuel from the fuel tank; a second reactant source (220, 320, 420, 520, 620, 720, 820, 920, 1020, 1120); a catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322) arranged to receive a first reactant from the first source and a second reactant from the second source to generate an inert gas that is supplied to the fuel tank to fill a ullage space of the fuel tank; and a back pressure flow restrictor positioned within or along a fuel tank supply line and downstream of the catalytic reactor, the back pressure flow restrictor arranged to maintain high-pressure operation of the catalytic reactor; a heat exchanger (232, 332, 432, 532, 832, 932, 1132) 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 an inert gas and a byproduct; an injector pump arranged to receive the first reactant and the second reactant and to supply a mixture of the first reactant and the second reactant to the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322); and an inert gas recycling system located downstream of the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322) and upstream of the fuel tank (202, 302, 402, 502, 602, 702, 802, 902, 1001, 1102, 1202, 1302, 1402), wherein the inert gas recycling system is arranged to direct a portion of the inert gas to the catalytic reactor (222, 322, 422, 522, 622, 722, 822, 922, 1022, 1122, 1222, 1322).