The use of flywheels for energy storage was probably the second thought after the wheel was invented. With the recent developments in composite materials, magnetic materials and the use of microprocessors, flywheel energy storage has wide applications in many facets of our lives. For space vehicles, two counter-rotating wheels are used to produce a flywheel energy storage system (FESS). The initial-flight opportunities for this flywheel technology can be used for energy storage in conjunction with existing battery and power subsystem designs. There are also possibilities that this pair of counter rotating FESSs can replace one of the attitude control wheels in the attitude control-system (ACS) wheel set. This paper describes the potential combined battery/flywheel energy storage system for aerospace application. Battery charging control schemes and solar array regulation can be augmented with a flywheel system to improve spacecraft performance, allow an alternate energy storage source for single battery systems, reduce the size of the solar array and be used as one of (3 or 4) attitude control wheels.