Research engineers at NASA Glenn Research Center and at U.S. Flywheel Systems, Inc., with their partners from TRW, Texas A&M University, the University of Texas Center for Electro-mechanics and the Boeing Company, combined their expertise for over 5 years to improve the flywheel technology that enabled this feat.

The team looked at everything from the materials used, to the types of bearings, to more efficient motor/generators, to the algorithms, or equations, used for system control.

Team members developed or identified high-strength carbon fiber/epoxy composites for the rotor, low-loss magnets for the bearings, high-speed electric motor/generators for energy conversion, and computer algorithms for motion control. It all came together late last December at the California facilities of U.S. Flywheels, the company that built the system.

"The flywheel energy storage system represents a revolutionary step in energy storage technology. We're thrilled to have met our sustained operating speed goal," said Raymond Beach, NASA Glenn principal investigator and team leader for flywheel development at Glenn.

The flywheel is a kind of mechanical battery that converts energy to mechanical motion and, when needed, converts that motion back to energy.

On the ISS, electricity from solar arrays will run the motor that spins the wheel. Then, during the shade period of the orbit, the spinning wheel will turn the motor, now acting as generator, to make the electricity that powers science equipment and life support systems.

"The process is very efficient. More than 85 percent of the energy put into the wheel comes out," Beach said. At full operating speed, the flywheel rotor's linear velocity is two and one half times the speed of sound (or 1875 mph), and if allowed to spin down without load would spin for more than 12 hours before coming to rest.

Flywheels have several advantages over the chemical batteries currently proposed for the ISS. They can be designed to have a lifetime that matches that of the ISS; chemical batteries planned for ISS will last only about 5 years and must be replaced during the mission.

They operate effectively over a wide temperature range; chemical batteries operate well only within a narrow 0 to 10 C (32 to 50 F) range. They are more efficient, returning more of the energy put into them than do chemical batteries.

They can also provide more power, for example, during Shuttle docking periods, because of their higher energy density; that is, they store more energy per pound than do chemical batteries.

These advantages mean lower costs of operation for the ISS, and, in particular, they mean more mass can be devoted to science experiments and facilities and even to astronaut quality-of-life payload.

The next step in the development of the energy storage system is to build and endurance test an engineering model. "We expect to be testing all Summer," said Timothy Tyburski, project manager for the ISS flywheel program at Glenn, "and to have a flight system built and tested by late 2004.

Ours will be one of the first technology demonstration missions for Station." In addition to its science missions on the ISS, NASA Human Exploration and Development of Space Enterprise will sponsor technology missions to prove revolutionary technical advances and space flight concepts.

A companion flywheel effort at Glenn involves a combination flywheel energy storage system and attitude control system. State of the art satellites use chemical batteries for energy storage and low-speed flywheels for attitude control. Advanced flywheel energy storage systems will combine these two functions at a fraction of the mass of current technology.