A gas turbine engine comprising: a compressor section ; a combustor fluidly connected to the compressor section by a primary flow path (C), wherein the primary flow path is defined by an inner diameter flow path wall and an outer diameter flow path wall; a turbine section fluidly connected to the combustor by the primary flow path (C), wherein the turbine section includes a first portion and a second portion, the primary flow path (C) through the first portion has a high pressure relative to the primary flow path through the second portion; a thermally isolated cooling plenum radially inward of the primary flow path (C) of the first portion of the turbine section , wherein the thermally isolated cooling plenum is defined by a forward wall , a shaft structure , an aft wall , and an inner diameter wall of the primary flow path (C), and wherein air in the thermally isolated cooling plenum is thermally isolated from air in the primary flow path (C); first and second rotors disposed in said first portion of the turbine section and extending through said inner diameter flow path wall into said thermally isolated cooling plenum , each of said first and second rotors comprising an internal cooling system operable to receive cooling air from said thermally isolated cooling plenum and expel spent cooling air into said primary flow path (P), and each of said first and second rotors comprising a flow path opening in a portion of a root portion of the rotor to allow a cooling flow to pass therethrough; and at least one cooling air system operable to provide cooling air from a single source to the thermally isolated cooling plenum such that, as the cooling air enters the thermally isolated cooling plenum , a first portion of the cooling air enters the first rotor near an inner diameter of the primary flowpath (C) and a second portion of the cooling air branches radially inward, flows toward the shaft structure , and passes through the flow path opening of the first rotor . a compressor section ; a combustor fluidly connected to the compressor section by a primary flow path (C), wherein the primary flow path is defined by an inner diameter flow path wall and an outer diameter flow path wall; a turbine section fluidly connected to the combustor by the primary flow path (C), wherein the turbine section includes a first portion and a second portion, the primary flow path (C) through the first portion has a high pressure relative to the primary flow path through the second portion; a thermally isolated cooling plenum radially inward of the primary flow path (C) of the first portion of the turbine section , wherein the thermally isolated cooling plenum is defined by a forward wall , a shaft structure , an aft wall , and an inner diameter wall of the primary flow path (C), and wherein air in the thermally isolated cooling plenum is thermally isolated from air in the primary flow path (C); first and second rotors disposed in said first portion of the turbine section and extending through said inner diameter flow path wall into said thermally isolated cooling plenum , each of said first and second rotors comprising an internal cooling system operable to receive cooling air from said thermally isolated cooling plenum and expel spent cooling air into said primary flow path (P), and each of said first and second rotors comprising a flow path opening in a portion of a root portion of the rotor to allow a cooling flow to pass therethrough; and at least one cooling air system operable to provide cooling air from a single source to the thermally isolated cooling plenum such that, as the cooling air enters the thermally isolated cooling plenum , a first portion of the cooling air enters the first rotor near an inner diameter of the primary flowpath (C) and a second portion of the cooling air branches radially inward, flows toward the shaft structure , and passes through the flow path opening of the first rotor . <EOS>