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;
• 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 (C) of the flow (B) that enters the engine core bypasses the combustor and is used as turbine cooling flow to cool the turbine;
• the turbine entry temperature, defined as the temperature at the inlet to the most axially upstream turbine rotor at a maximum power condition of the gas turbine engine, is in the range of from 1800K to 2100K;
• and
• at cruise conditions, the cooling to bypass flow efficiency ratio is no greater than 0.02.