A flywheel energy storage system configured to rotationally accelerate to convert electrical energy into kinetic energy during a charging mode and to rotationally decelerate to convert kinetic energy into electrical energy during a discharging mode, the flywheel energy storage system comprising: a rotating assembly affixed to a central shaft, having a plurality of magnets and a longitudinal axis about which the rotating assembly rotates; and a static assembly having a stator installed within the rotor and comprising stator windings, the static assembly configured to magnetically interact with the plurality of magnets of the rotating assembly; wherein the rotating assembly further includes: a rotor back iron supporting the plurality of magnets and disposed further from the longitudinal axis in a radial direction than the plurality of magnets, wherein the rotor back iron is formed of a material having a first stiffness, relative permeability of at least 10, and an electrical conductivity 10% or less than the electrical conductivity of magnetic steel and a composite structure supporting the rotor back iron and disposed further from the longitudinal axis in the radial direction than the rotor back iron; wherein the composite structure comprises a composite material having a second stiffness, which is greater than the first stiffness of the rotor back iron; characterized in that the rotor back iron material is flowable, which means that it has the shape-following characteristics of liquids, when exposed to forces higher than 500 g-force units. a rotating assembly affixed to a central shaft, having a plurality of magnets and a longitudinal axis about which the rotating assembly rotates; and a static assembly having a stator installed within the rotor and comprising stator windings, the static assembly configured to magnetically interact with the plurality of magnets of the rotating assembly; wherein the rotating assembly further includes: a rotor back iron supporting the plurality of magnets and disposed further from the longitudinal axis in a radial direction than the plurality of magnets, wherein the rotor back iron is formed of a material having a first stiffness, relative permeability of at least 10, and an electrical conductivity 10% or less than the electrical conductivity of magnetic steel and a composite structure supporting the rotor back iron and disposed further from the longitudinal axis in the radial direction than the rotor back iron; wherein the composite structure comprises a composite material having a second stiffness, which is greater than the first stiffness of the rotor back iron; a rotor back iron supporting the plurality of magnets and disposed further from the longitudinal axis in a radial direction than the plurality of magnets, wherein the rotor back iron is formed of a material having a first stiffness, relative permeability of at least 10, and an electrical conductivity 10% or less than the electrical conductivity of magnetic steel and a composite structure supporting the rotor back iron and disposed further from the longitudinal axis in the radial direction than the rotor back iron; wherein the composite structure comprises a composite material having a second stiffness, which is greater than the first stiffness of the rotor back iron;