Efficient gas turbine engine

A gas turbine engine for an aircraft comprising:

• an engine core comprising:
• a first turbine, a first compressor, and a first core shaft connecting the first turbine to the first compressor;
• a second turbine, a second compressor, and 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 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:
• a turbine entry temperature (T0turb_in) is defined as the temperature (K) at the inlet to the most axially upstream turbine rotor in the gas turbine engine at a maximum power condition of the gas turbine engine;
• a core size is defined as CS=Wcompin·√{square root over (T0comp_out)}/P0comp_out
• where:
• Wcomp_in is the mass flow rate (kg/s) at entry to the engine core;
• T0comp_out is the stagnation temperature at exit to the compressor;
• P0comp_out is the stagnation pressure at exit to the compressor;
• and
• a fan to core efficiency ratio FC is in the range of from 1.9×105 mkg−1 sPa to 3.5×105 mkg−1 sPa, where the fan to core efficiency ratio is defined as
• FC
• =
• (
• Fan
• 
• 
• Diameter
• )
• ·
• T
• 
• 
• 0
• 
• 
• turb_in
• CS
• .