Efficient aircraft engine

A gas turbine engine for an aircraft comprising:

• an engine core comprising:
• a turbine, a combustor, and a compressor, the turbine comprising a first turbine and a second turbine and the compressor comprising a first compressor and a second compressor;
• a first core shaft connecting the first turbine to the first compressor;
• a second core shaft connecting the second turbine to the second compressor, the second turbine, second compressor, and second core shaft being arranged to rotate at a higher rotational speed than the first core shaft, the gas turbine engine further comprising:
• a bypass duct radially outside the engine core and defining a bypass flow;
• a fan comprising a plurality of fan blades;
• and
• a gearbox that receives an input from the first core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the first core shaft, wherein:
• part of a flow that enters the engine core bypasses the combustor and is used as turbine cooling flow to cool the turbine;
• wherein the second turbine comprises (i) at least one row of stator vanes in which a most axially upstream row of stator vanes in the second turbine is metallic, and (ii) at least one row of rotor blades in which a most axially upstream row of rotor blades in the second turbine is metallic, and
• wherein at a turbine entry temperature in a range of from 1800K to 2100K a cooling to bypass flow efficiency ratio is in a range of from between 0.005 and 0.02, wherein the cooling to bypass flow efficiency ratio is defined as a ratio of a mass flow rate of the turbine cooling flow to a mass flow rate of the bypass flow.