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Katherine K. Martin
Media Relations Office

June 7, 2007
RELEASE : 07-019
Glenn Guides Electrical Requirements for Space Vehicles
Cleveland -- As the only research laboratory orbiting the Earth, the International Space Station is a work in progress, with constantly changing amounts and kinds of electrical power supply and demands.

These challenging tasks rely on expert advice from a team at NASA's Glenn Research Center called the System Power Analysis for Capability Evaluation (SPACE) Team.

SPACE also is the name of the software code developed by the team that predicts the maximum power level that the space station power system can sustain throughout a variety of conditions. The code was developed in the late 1980s to support the design of the precursor to the International Space Station (ISS), Space Station Freedom. Since then, dozens of engineers have contributed to its enhancement, which includes mathematical models of the solar arrays, batteries and power management and distribution equipment. Utilizing this code, the SPACE Team provides analyses that are required to certify space shuttle flight readiness.

The most pressing challenge addressed by the team involves the next shuttle mission, STS-117, during which a fifth and sixth set of solar arrays and batteries, or power channels, will be installed on the station. These new power channels, also called truss segments S3/S4, will become a part of station's electric power system, which was designed under Glenn's guidance in the 1990s. S3/S4 will be added to station's 11-segment integrated truss structure, the space station's backbone, and will provide one-fourth of the total power generation for the completed International Space Station.

Of the four channels already on the station, three are active and one is temporarily not in use, or dormant. The two new power channels added during STS-117 will require retracting a second solar array for the channels to track the sun. Additionally, two of the active channels must constantly send power to the dormant channels to prevent them from freezing or getting too hot.

To enable STS-117's mission to be successful, Glenn's SPACE Team advised NASA's Johnson Space Center, Houston, that the dormant channels temporarily receive less power and that loads normally shared among two power channels be fed by the more capable power channel.

The team also assessed the station's electric power system performance for joint operations when Atlantis is docked with the station during the STS-117 mission. This includes launch, power channel startup and deployment of solar arrays. With this complicated sequence of events, the station's power system will be transformed from three power channels to four with one channel going dormant and two new channels being used.

Later this year, the SPACE Team will continue its involvement with space electric power needs with the first docking of the European Space Agency's (ESA) Automated Transfer Vehicle (ATV) to station. The team's analyses will confirm whether the docking can be performed without depleting energy provided by station's batteries. The ATV will bring up supplies, re-boost the station, undock and burn up in the atmosphere six months later with trash from station.

A complex procedure, the ATV analysis takes into account how much solar power the station's arrays capture, including shadowing of the arrays by other parts of the station, vehicle attitude and the behavior of batteries, considering their age and capability, that store energy from the solar arrays.

An analysis is particularly needed because the ATV docking lasts five orbits, or about seven and a half hours. In this period of time, the solar arrays must be positioned to avoid damage from the ATV's thrusters and interference with communication antennas that guide the ATV. Because of these restrictions, the arrays cannot continuously track the sun, resulting in reduced power generation.

Detailed estimates of power availability during critical portions of the ATV flight are provided to Johnson Space Center, the European Space Agency, the Russian Federal Space Agency and NASA's other partners. This information is used to determine if expected electrical load distribution within station can be supported during ATV docking operations. If the analysis determines there is a shortfall in power needed, changes are made to the choreography of the docking sequence. Another energy balance analysis is then performed, with the result of a docking timeline that balances station's power needs with operational constraints needed for safe and successful ATV docking.

For more information about STS-117, Atlantis' next mission to the International Space Station, visit:

For more information about ATV, visit:


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