Endothermic fuel compositions comprising 50% or more by volume decahydronaphthalene, including cis-decahydronaphthalene, trans-decahydronaphthalene or a mixture thereof, for use as endothermic fuels in hypersonic vehicles and particularly for use in dual-mode ramjet or supersonic combustion ramjet air breathing engines. Methods for operating a ramjet or scram jet engine wherein the endothermic fuel is used for cooling the combustor and for combustion in the combustor.