Although the International Space Station (ISS) flies in “space,” it does not orbit completely outside the atmosphere. Even at 350-km altitude, the ISS passes through the tenuous upper reaches of the atmosphere, and this atmosphere in fact causes a minute amount of drag on the space station. In order to prevent the orbit from decaying, rocket engines on the ISS are used to “reboost” the station, maintaining its orbit. The solar arrays, which provide power for the ISS, are the largest elements causing this atmospheric drag on the ISS.
ISS in orbit in its configuration after the 2003 Columbia disaster and before the space shuttle return-to-flight missions of 2005 and 2006. The solar arrays can be seen as the largest drag area on the ISS.
In 1991, Geoffrey A. Landis and Cheng-Yi Lu at the NASA Glenn Research Center (known then as the NASA Lewis Research Center) proposed a new method to orient the solar arrays on the ISS to reduce the drag they produce and, consequently, decrease the amount of propellant required to maintain the orbit. The concept was simple: orient the solar array to track the Sun while the ISS is in the illuminated part of the orbit (about 55 min per orbit), and then adjust the orientation so that the array is edge-on to the direction of flight (“feathered”) during the eclipse period (about 36 min per orbit). The researchers calculated that this would reduce the propellant required to compensate for the drag of the solar arrays by 18.5 percent. They also suggested an operating mode, “beta-controlled,” to decrease the drag area yet further: tracking the Sun in one axis, while flying with the solar arrays edge-on to the direction of flight, in effect “slicing” through the atmosphere like a knife. This mode of operation reduced the drag further, at the price of some decrease in power. Although the results were presented to the ISS power system group and published in the AIAA Journal of Propulsion and Power (ref. 1), the proposed fuel-saving orientation of the solar arrays was not implemented at the time because frequent shuttle flights to the ISS were anticipated to routinely supply drag makeup propellant, and the propellant savings were not expected to be worth the added complexity of operations.
The proposed orientation would face the solar arrays toward the Sun during the illuminated portion of the orbit and would turn them edge-on during the night (or “eclipse”) portion of orbit.
This situation changed after the Space Shuttle Columbia disaster. Routine access to the ISS was interrupted until the space shuttle return-to-flight missions in late 2005 and in 2006, and the propellant to maintain circular orbit suddenly became a critical factor. Consequently, the new orientation concept of the arrays was reconsidered. Renamed the “eclipse drag reduction configuration,” and tagged the “night-glider mode,” the new mode of operation was implemented following the interruption of shuttle service to the ISS (ref. 2). This mode of operation has now been demonstrated, along with the “Sun-slicer” mode (similar to the beta-control mode proposed earlier). The new orientation method has been successfully demonstrated in over 3 years of operation and is now being used routinely. The results have been exceptional, and the lower fuel requirement has been one of the key factors in the continued operation of the ISS without frequent space shuttle support. According to Fortenberry et al., “Use of these techniques can reduce the atmospheric drag on the ISS as much as 25 percent, resulting in up to 1000 kilograms per year savings of propellant and allowing this unused Progress vehicle up-mass to be reprioritized to carry research payloads” (ref. 3).
Last updated: December 14, 2007
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