Computer-implemented methods of enabling optimisation of trajectory for a vehicle

A computer-implemented method of enabling optimisation of trajectory for a vehicle such as an aircraft. The method comprises determining a trajectory for the vehicle 112 using: an algorithm; a vehicle model defining path constraints for the vehicle through space 98; a propulsion system model defining parameters of a propulsion system of the vehicle; an objective function defining one or more objectives; and controlling output of the determined trajectory 114. The trajectory may be output to an automated vehicle control system such as an automatic flight control system. Vehicle operational parameters including vehicle orientation demand and propulsion system thrust demand may be determined 118 for control of the vehicle 120. A derate of the propulsion system may be determined using a navigational model 100. The trajectory may be for a take-off phase or a climb phase of an aircraft.