An aircraft gas turbine engine (20, 120, 220) comprising: a combustor; a turbine section downstream of the combustor; a nozzle section downstream of the turbine section; a heat exchanger; and a fuel supply line for conveying fuel through the heat exchanger and to the combustor; wherein the heat exchanger includes a fuel-cracking catalyst, the fuel-cracking catalyst comprising: a protonated solid acid support; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support, the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites, and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites. a combustor; a turbine section downstream of the combustor; a nozzle section downstream of the turbine section; a heat exchanger; and a fuel supply line for conveying fuel through the heat exchanger and to the combustor; wherein the heat exchanger includes a fuel-cracking catalyst, the fuel-cracking catalyst comprising: a protonated solid acid support; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support, the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites, and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites. a protonated solid acid support; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support, the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites, and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites.