The objective of th is program was to develop a flywheel energy storage system capable of achieving its maximum energy density, while being capable of repeated high peak -power demands. The key to achievi ng this objective is the development of a composite hub capable of supporting an optimized high -speed composite rim. The existing Beacon Power commercial flywheel system uses a composite rim and aluminum hub. The aluminum hub is the stress limiting part of the rotor, which prevents the carbon fiber composite from reaching its maximum capability at a significantly higher operating speed. During this project a composite hub has been designed that will allow the rim to run closer to its maximum stress capa bility resulting in a more compact and lighter weight system. Compared to battery energy storage systems, the FESS is more reliable, requires less maintenance, has a much longer life, high er cyclic capability, operates with minimal degradation in performa nce with time and in extreme environments, and eliminates environmental problems associated with disposal of batteries. The flywheel advantage begins where battery performance falls off. During ride -through and distributed generation use, electrical syst ems can experience multiple charge and discharge cycles even within one minute. Flywheels are not sensitive to high rates of charge and discharge, beyond checking that the torsional stresses are acceptable and the parts won’t slip; and such rates have no noticeable effects on the life of the flywheel. In fact, the greatest overall efficiency for a flywheel is achieved at high charge and discharge rates. If the battery rapidly discharges, it will only be able to extract a small percentage of its stored energy and more batteries may be needed to meet power and life requirements to account for this. In comparison, flywheels are relatively insensitive to deep discharge. The typical depth of a flywheel discharge is 75% to 90% of the stored energy and there are only minor effects on life due to these depths of discharge. Flywheels will also interface with any primary energy device on a space platform such as photovoltaic cells, fuel cells, or chemical batteries, improving their overall efficiency and energy density. The most important advantage is that flywheels can be designed to have more than 100,000 charge and discharge cycles. Batteries have a typical life cycle time below 1,000 charge/discharge cycles. In comparison, a flywheel is designed for many thousands of cycles with minimal degradation in performance of life. The discharge time of a flywheel is the time it takes for the flywheel to decelerate from its maximum speed at full rated power. In general and unlike batteries, flywheels are well suit ed for equal charge and discharge rates. Under cyclic conditions, the energy -to -weight ratio of a chemical battery will be significantly less, possibly less than half, of the claimed energy density, whereas the flywheel’s energy -to -weight capacity would r emain nearly constant under these same conditions.