An autorotation assist system for a rotor helicopter comprising: a motor-generator (212, 412) or a hydraulic pump-motor configured to attach to an engine; a controller (216, 416, 516) connected to the motor-generator (212, 412) or the hydraulic pump-motor; one or more sensors coupled to the controller (216, 416, 516), wherein the one or more sensors detect one or more of a rotor rpm, an engine rpm, a speed, a descent rate and an altitude; a high capacity/high discharge battery system comprising a Lithium ion battery that is operable to recover energy in a short period of time to enable repeated autorotation assistance or a hydraulic accumulator coupled to the motor-generator (212, 412) or the hydraulic pump-motor, respectively; wherein, during a normal operation of the engine, the motor-generator (212, 412) or the hydraulic pump-motor is configured to store an energy in the high capacity/high discharge battery system or the hydraulic accumulator, respectively; wherein, upon a manual and/or an automatic control by the controller (216, 416, 516), the high capacity/high discharge battery system or the hydraulic accumulator is configured to discharge the energy to the motor-generator (212, 412) or the hydraulic pump-motor, respectively, and the motor-generator (212, 412) or the hydraulic pump-motor is configured to drive the rotor (108, 208, 408, 508); wherein the automatic control by the controller (216, 416, 516) is triggered to provide autorotation assistance based on an input from the one or more sensors; and wherein the manual and/or the automatic control by the controller (216, 416, 516) controls an output of the energy from the high capacity/high discharge battery system or the hydraulic accumulator. a motor-generator (212, 412) or a hydraulic pump-motor configured to attach to an engine; a controller (216, 416, 516) connected to the motor-generator (212, 412) or the hydraulic pump-motor; one or more sensors coupled to the controller (216, 416, 516), wherein the one or more sensors detect one or more of a rotor rpm, an engine rpm, a speed, a descent rate and an altitude; a high capacity/high discharge battery system comprising a Lithium ion battery that is operable to recover energy in a short period of time to enable repeated autorotation assistance or a hydraulic accumulator coupled to the motor-generator (212, 412) or the hydraulic pump-motor, respectively; wherein, during a normal operation of the engine, the motor-generator (212, 412) or the hydraulic pump-motor is configured to store an energy in the high capacity/high discharge battery system or the hydraulic accumulator, respectively; wherein, upon a manual and/or an automatic control by the controller (216, 416, 516), the high capacity/high discharge battery system or the hydraulic accumulator is configured to discharge the energy to the motor-generator (212, 412) or the hydraulic pump-motor, respectively, and the motor-generator (212, 412) or the hydraulic pump-motor is configured to drive the rotor (108, 208, 408, 508); wherein the automatic control by the controller (216, 416, 516) is triggered to provide autorotation assistance based on an input from the one or more sensors; and wherein the manual and/or the automatic control by the controller (216, 416, 516) controls an output of the energy from the high capacity/high discharge battery system or the hydraulic accumulator.