Computer Science – Performance
Scientific paper
Aug 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt........23z&link_type=abstract
Ph.D. Thesis Colorado Univ., Boulder.
Computer Science
Performance
Attitude Control, Bending, Satellite Attitude Control, Spacecraft Control, Spacecraft Structures, Temperature Effects, Temperature Gradients, Thermal Shock, Torque, Umbras, Active Control, Landsat 4, Solar Arrays, Topex
Scientific paper
Thermal elastic shock (TES) is a twice per orbit impulsive disturbance torque experienced by low-Earth orbiting spacecraft. The disturbance is attributed to the vehicle passage in and out of the Earth's umbra. During penumbral transitions, large flexible appendages undergo thermally induced bending caused by rapidly changing thermal conditions. Flexible structures, in particular solar arrays, experience rapid cooling during entrance and rapid heating at exit. TES has been observed during normal on-orbit operations of the LANDSAT-4 spacecraft, the Communications Technology Satellite and the Hubble Space Telescope. The fundamental equations used to model the TES disturbance torque for typical spacecraft appendages (e.g. solar arrays and antenna booms) are derived in detail. The time rates of change of the thermal gradient are shown to be the driving force behind estimating the TES disturbance torque. In order to predict the thermal response of a typical satellite solar array, a detailed thermal model is developed. The TOPEX spacecraft, designed using LANDSAT heritage, is selected to demonstrate the extensive thermo-mechanical modeling of the TES disturbance. In particular, the attitude pointing performance of the TOPEX spacecraft, when subjected to the TES disturbance, is analyzed using a three-axis, non-linear, time-domain simulation. Results indicate that the TOPEX spacecraft will exceed its roll axis attitude control requirement during prenumbral transitions, and remain in violation for approximately 150 sec each orbit until the umbra collapses. Fortunately, the attitude deviations are not large enough to cause the primary science instrument, a radar altimeter, to break lock, which would result in temporary loss of data. A localized active control system is proposed as a solution to minimize and/or eliminate the degrading effects of the TES disturbance. The feasibility of the control system is demonstrated by including it as part of a TOPEX vehicle design and re-evaluating the attitude pointing performance using the numerical simulation. Results show that the roll axis control requirement violation period could be appreciably reduced, for a TOPEX vehicle, if the torque authority of the control system reaction wheel is about 0.6 N - m.
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