The structure weighs about 5.98X10E6 kg. This mass can be reduced by replacing some of the frames with lighter and less stiff members or by eliminating some frames if the lower load requirements of construction in GEO are considered. Based on the Grumman report, the natural frequency of the aluminum structure is estimated to be 1.3X10E-3 Hz, which is more than 100 times the orbital frequency. Such natural frequency, according to ref. 1, is adequate for control system stability. This estimate is obtained from the data provided in ref. 1 which is used to find the natural frequency of a structure with AR»4. This is probably a conservative estimate since the design described here has AR=2. One of the major concerns with the use of aluminum in construction of the SPS structure is the deflection of the structure due to the large thermal expansion of aluminum and the temperature differential between the front and back members of the structure. This problem was considered in a short study by Grumman(l), which considered a larger structure than the one investigated here. The Grumman study shows a deflection of less than 60 m at each end of a 10.7 km long satellite. The temperature differential used for this calculation was 55 C. The angular deflection of the structure is not significant according to the Grumman report. Doubts remain, however, as to the behavior of an aluminum structure in the geo-synchronous thermal environment. In response to this concern the following section provides a qualitative assessment of the problem. 3.5 PASSIVE THERMAL CONTROL Thermal control is a necessary concern in an Aluminum SPS structure because of the large temperature change experienced when the SPS is eclipsed and because of the high coefficient of thermal expansion for aluminum. The problem can be broken down into two general areas, total linear expansion and differential expansion. Total linear expansion is a measure of the overall change in size that the SPS experiences. This occurs when the SPS enters or leaves an eclipse. Leaving an eclipse, the SPS structure rises from the minimum to the maximum operating temperature in a relatively short period of time. This implies that the expansion also occurs on a short time scale. As a result, even though the total expansion may be small, some components may experience a high acceleration which increases the potential for damage to the system. For example, a typical 5 GW SPS design is 10,000 m long with 5E+07 kg mass distributed uniformly along its length. The CTE of aluminum is 2.23E-05-K at room temperature. The SPS passes from the Earth’s shadow into full sunlight in as little as 2 minutes. Assuming a temperature change of 100 K in the same time, the rate of temperature change of the aluminum structure must rise at 0.04 K/sec/sec or greater. The corresponding compression load at the center of the SPS is found to be 5.58E+04 newtons, or about 6.3 tons. Actual loads for the structure used here would be lower, since the structure’s temperature would not rise 100 X in two minutes. The force is proportional to the mass per unit length and to the square of the structure’s length, so the stress would be four times larger for a typical 10 GW SPS.
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