Attitude Control The problems of station keeping are similar to the ones experienced by telecommunications satellites, though on a different scale (assuming that the structure is sufficiently rigid). Due to the high level of accuracy required, a three axis electrical propulsion stabilization system is recommended. The use of electric propulsion not only minimizes the overall mass but reduces the number of different subsystems. Fly wheels could also be considered mainly because of their clean properties but the mass of these devices for a huge spacecraft and the necessity to desaturate them periodically are significant drawbacks. Furthermore, assembly considerations favor a set of modular independent propulsion packages requiring only electrical connections. For guidance considerations, the high accuracy required favors the use of a star tracker for the GEO case. For the lower orbits, pointing requirements are relaxed as the beam steering can compensate for the pointing variations of the platform. Mass Assumption The mass of the basic framed structure is comprised of the truss elements and a wire frame which further stiffens the structure. The mass of the trusses is obtained from Space Station Freedom dualkeel 5 m design. The basic mass is 18 kg/m leading to 1800 kg for the basic 100 m segment. For the planar array, the result for the exterior frame is 7200 kg. For the prismatic geometry, the total length is 900 m resulting in 16200 kg. To rigidify this frame and tallow attachment for the solar cells array, a 5 m x 5 m squared lattice of kevlar wires is used. The linear mass of the kevlar wire is 5 g/m resulting in a negligible mass contribution to the total structural mass. The antenna subsystem dominates the mass of the array. Both conventional tube derived and solid state transmission arrays are considered. The conventional array is comprised of heavy elements such as tubes, phase shifters and wave guides. The design assumes an array of 100 radiative elements. The mass of each set (amplifier and wave guide) is of the order of 10 kg, and the coupled radiative elements sum to 1000 kg. The slot antenna is constructed of 100 square panels (10 m x 10 m) made with 0.5 mm thick aluminum. The average weight for a panel is then 270 kg, with a total element mass of 271. The phased array antenna assumes 1000 radiative elements, the power per element is lower but it should be pointed out that, basically, the gyrotrons are identical to the previous case, and that the phase shifters are much more demanding than for the slot antenna. Phase shifter being usually made by using a set of different wave guides switched on and off their weight is very rapidly increasing with the number of phase step they provide. Then, an optimistic evaluation (within the frame of the present technology) leads to account each radiative element at 20 kg. The total amount is then 201. The constraints of rigidity and of thermal conduction on the antenna are roughly comparable to ones for the slot antenna. The total mass of the phased array, using conservative technology is 471 (based on 4.7 kg/m^2). Various types of solar arrays were considered. GaAs were assumed with both a near term and midterm (10+ year) values. In the near term, Si cells with an efficiency of 14% had mass figure of 2.15 kg/m2. This figure would include some rigid structure for deployment. For the mid-term, the use of 20% GaAs cells at 0.3 kg/m2 was regarded as attractive. A basic 100 m x 100 m area results in a mass of approximately 201 for the near term and 3 t for the mid-term (flexible array). For the prism structure, the mass is double this at 401 for the rigid (near-term) and 6 t for the flexible (mid-term) array. The mass of the electrical subsystems, including attitude control electronics, is given as 2000 kg,. This is regarded as a very conservative as this mass is loosely coupled to the size of the spacecraft. The electrical subsystem architecture would be similar to the one used for a telecommunication satellite, (the power subsystem excepted.) It would then be meaningful to use systems already developed. The power conversion subsystem mass is largely taken into account in the mass of the antenna, as it includes the full set of tubes and phase shifters (which represent the most significant, contribution). The fuel consumption is given as follows: Orbit transfer: from LEO 350 km to 36000 km, assumed to be hybrid: chemical and electrical propulsion using a LOX, LH2 engine with a 450 s ISP, and an 4500 s ISP for the electrical thruster. Station keeping ; Electric propulsion system only. 10 years of operation.
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