A method of operating a hybrid-electric propulsion system for an aircraft and a hybrid-electric propulsion system is provided. The method includes: a) providing at least one thermal engine sized to provide sufficient thrust to the aircraft for at least one flight mission independent of a second non-thermal engine source of thrust for the at least one flight mission; b) providing at least one electric motor powered by a non-hydrocarbon fuel burning electrical energy source, the at least one electric motor configured to provide thrust to the aircraft for the at least one flight mission; and c) selectively providing thrust during one or more segments of the at least one flight mission using the electric motor.