Table 1. Solar array flight experiment (SAFE) objectives. • Demonstration of functional operation of the wing deployment and packaging system — Survival of launch loads — Multiple extensions and retractions — Complete retraction with automatic reapplication of preload — Operation of mast with maximum temperature gradient across canister — Survival of landing loads • Electrical performance — Measure on-orbit performance of solar cell panels • Thermal performance — Obtain electrical and mechanical performance data during stable operation, deployment and retraction under various sun angles • Dynamic performance — Obtain mode shapes and frequenceis of the array wing when excited with the orbiter VCS — Measure dynamics using three independent instrumentation techniques: accelerometers, photography, and optical displacement measurement 5. FUTURE DEVELOPMENT In anticipation of greatly increasing power needs for the 1990s, both NASA and the Air Force have funded Lockheed to investigate larger and lighter-weight solar arrays. On a study just completed for the NASA Marshall Space Flight Center, three array concepts were developed for space power sources in the 300 kW to 1 MW range. Important to the study was how much power could be taken up in a single shuttle load. Figure 13 illustrates our planar concept where arrays, using the SEPS technology but much lighter, would be fitted together to form a platform approximately 200 x 190 ft. All of this 428 kW system could be carried in a single shuttle trip. Concentrators are of interest to solar array designers because they reduce the number of expensive solar cells needed for a specific power output. Figure 14 shows our low Concentration Ratio (CR) concept whereby the solar cells “see” the equivalent of three sun intensities (after reflector losses) on their surface — and they consequently put out approximately three times as much power. The reflectors in this concept are made of aluminized plastic and fold up along with the solar cells. Each reflector is 10 ft2 and in one shuttle load 62,500 of them could be deployed, in five modules, to form an array 250 x 250 ft. This array would produce 300 kW if silicon cells were used, and 600 kW if GaAs solar cells were used. Gallium arsenide cells, as mentioned earlier, do not degrade in output as much at the higher temperatures resulting from sun concentration. The final “Cassegrainian” concept, shown in Fig. 15, is considered a high-CR approach since the cells, which have to be GaAs because of the high operating temperature (170°C), see the equivalent of 67.5 suns of their surface. In this case a parabolic reflector concentrates the sun onto a secondary reflector and then onto the cells. The large reflector is 45 ft in diameter, and the cell area is 11 ft2. Power output of one module is 12.8 kW and 15 of them can be packaged in the shuttle to assemble a 192 kW system or orbit. The concepts discussed above are within the state of the art of today’s solar cell and solar array technology. Advancements far beyond these are possible, however, which can significantly increase efficiency of energy conversion and also greatly
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