In low earth orbit satellite applications, spacecraft power is provided by photovoltaic cells and batteries. Unfortunately, use of batteries present difficulties due to their poor energy density, limited cycle lifetimes, reliability problems, and the difficulty in measuring the state of charge. Flywheel energy storage offers a viable alternative to overcome some of the limitations presented by batteries. This paper discusses the University of Maryland's operational 300 Wh open core composite

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... wheel, called the Engineering Development Unit, and the research being conducted to enhance its performance. Current research in this area focuses on the control system, the touchdown surface material, and the magnetic bearing actuator. FARE, Inc. has designed a state of the art 50 Wh flywheel energy storage system. This system, called the Open Core Rotator, is based on the Engineering Development Unit and will incorporate some of the innovative features of the University of Maryland system as well as those under development by its subcontractors. The OCR is intended to be a prototype energy storage device for low earth orbit satellite applications.