New attitude to energy storage

By the summer of 2007, an AFRL team of scientists at Kirtland Air Force Base (AFB), New Mexico, intends to demonstrate a technology that combines satellite attitude control and energy storage.

For decades, engineers have employed flywheels as spacecraft positioning devices but, until recently, they have not given flywheels serious consideration for spacecraft power generation.

The successful demonstration of the Flywheel Attitude Control, Energy Transmission, and Storage (FACETS) system—which employs three flywheels, each spinning between 16,000 and 40,000rpm—could change that perspective.

Dr Jerry Fausz, FACETS program manager, said, ‘I’m definitely looking forward to demonstrating the combined energy storage and attitude control capability of FACETS and showing the feasibility of something that has never been done before.’

The FACETS program genesis dates back to the late 1980s’ Strategic Defense Initiative (SDI), also known as the Star Wars program. As part of the SDI’s space-based laser concept, AFRL scientists developed the Advanced Structures Experiment (ASTREX) spacecraft structural simulator to test the dynamics and control of large space structures. At the conclusion of the SDI test program, they moved the ASTREX structure to Kirtland AFB, where it has since become integral to the FACETS system demonstration.

‘The FACETS experiment represents the first full-scale, three-degrees-of freedom, mission-traceable ground demonstration, and introduces the Air Force to this new concept of combined energy storage and attitude control,’ said Dr Brian Wilson, a program team member. ‘Our successful demonstration will mature the technology through its adolescence. The excitement generated by this groundbreaking demonstration could lead to a flight experiment, further maturing the FACETS system.

‘These are the first steps necessary in the ultimate transition of the FACETS technology to military and commercial customers in the greater aerospace community.’

The recently completed mini-Agile Multipurpose Satellite Simulator (mini- AMPSS), a one ton structure mounted on the three-degrees-of-freedom ASTREX air bearing, serves as the test bed for the FACETS units (see Figure 1). Built under contract with Honeywell International, the FACETS units’ tri-flywheel arrangement will store energy as momentum and supply power through an electromagnetic drive system.

Figure 1

The FACETS units contain a device, similar to a car’s alternator, that will convert the flywheels’ rotational energy into the electrical power required for a spacecraft’s payloads (see Figure 2).

Figure 2

To provide a satellite’s—and in this case, the mini-AMPSS’—attitude control, the system can also change the flywheels’ speed and spin axes.

The research team will begin the trial by initially evaluating the power production of each individual flywheel unit on a rigid test stand. Testing will culminate with the team’s assessment of the power generation and the attitude control capability of the three FACETS flywheel units installed on the mini-AMPSS.

Because they will operate the flywheels at speeds up to 40,000rpm, project personnel incorporated several safety measures into the test facility to protect personnel and equipment from unexpected flywheel deterioration.

The team housed the entire test setup in a 16ft diameter, double-walled steel cylinder, filling the five-inch gap between the walls with sand. FACETS program objectives— providing energy transmission and storage, as well as the ability to maintain a satellite’s desired position— promise to significantly impact battlefield awareness.

‘The FACETS system can point the satellite like traditional attitude control systems, and in addition, its flywheels can provide the high peak power levels required by spacecraft payloads without the significant over sizing typical for traditional battery-based energy storage subsystems. The net result is a combined energy storage and attitude control system with dramatically reduced weight,’ explained Dr Wilson.

‘The frictionless magnetic bearings employed in the flywheel energy storage subsystem will also give a FACETS-equipped satellite the ability to operate on orbit for about twice as long as a satellite using chemical batteries’ added Dr Wilson

‘It is also important to note that flywheels provide clean energy storage in comparison to chemical batteries that contain caustic or toxic materials. Once flywheels are proven in their capacity to store and convert energy into electricity while simultaneously controlling satellite orientation, they will eliminate the need for heavy, chemical batteries on many satellite systems, thus significantly reducing spacecraft weight,’ said Dr Fausz.