A gas turbine engine, comprising: a first rotor disk supporting a first array of rotor blades; a second rotor disk supporting a second array of rotor blades; and a thermally isolated area bounded at a fore location, an aft location, a radially inner location, and a radially outer location, the thermally isolated area having a single, common inlet for receiving a flow of cooling fluid, wherein the first and second rotor disks (70, 72) are provided within the thermally isolated area, wherein the thermally isolated area is arranged such that the flow of cooling fluid (F 1, F 2) exits the thermally isolated area via the first and second arrays of rotor blades (64, 68), characterised by the inlet is a tangential onboard injector (TOBI), and, downstream of the TOBI, the thermally isolated area is arranged such that a first stream (F 1) of the cooling fluid is directed into an internal passageway of the first rotor disk, where it is split into a first portion (F 1A) and a second portion (F 1B), and a second stream (F 2) of cooling fluid is directed radially inward along a fore surface of the first rotor disk before entering an orifice in a fore-extending flange of the first rotor disk, and then downstream of the orifice it flows through a passageway between a radially inner surface of the first rotor disk and the fore-extending flange of the second rotor disk to an orifice in the fore-extending flange of the second rotor disk where it splits into a first portion (F 2A), which flows into a space axially between the first rotor disk and the second rotor disk, and a second portion (F 2B) which flows through the orifice, and further comprising: an outer seal spanning between the first rotor disk and the second rotor disk, the outer seal directly contacting the first rotor disk and the second rotor disk, wherein interior orifices and radially inner orifices are formed in the outer seal to allow the second portion (F 1B) of the first stream (F 1) and the first portion (F 2A) of the second stream (F 2) to be combined and flow toward an internal passageway within the second rotor disk. a first rotor disk supporting a first array of rotor blades; a second rotor disk supporting a second array of rotor blades; and a thermally isolated area bounded at a fore location, an aft location, a radially inner location, and a radially outer location, the thermally isolated area having a single, common inlet for receiving a flow of cooling fluid, wherein the first and second rotor disks (70, 72) are provided within the thermally isolated area, wherein the thermally isolated area is arranged such that the flow of cooling fluid (F 1, F 2) exits the thermally isolated area via the first and second arrays of rotor blades (64, 68), characterised by the inlet is a tangential onboard injector (TOBI), and, downstream of the TOBI, the thermally isolated area is arranged such that a first stream (F 1) of the cooling fluid is directed into an internal passageway of the first rotor disk, where it is split into a first portion (F 1A) and a second portion (F 1B), and a second stream (F 2) of cooling fluid is directed radially inward along a fore surface of the first rotor disk before entering an orifice in a fore-extending flange of the first rotor disk, and then downstream of the orifice it flows through a passageway between a radially inner surface of the first rotor disk and the fore-extending flange of the second rotor disk to an orifice in the fore-extending flange of the second rotor disk where it splits into a first portion (F 2A), which flows into a space axially between the first rotor disk and the second rotor disk, and a second portion (F 2B) which flows through the orifice, and further comprising: an outer seal spanning between the first rotor disk and the second rotor disk, the outer seal directly contacting the first rotor disk and the second rotor disk, wherein interior orifices and radially inner orifices are formed in the outer seal to allow the second portion (F 1B) of the first stream (F 1) and the first portion (F 2A) of the second stream (F 2) to be combined and flow toward an internal passageway within the second rotor disk.