k magnetron r-f converter, proposed by Raytheon, has the potential to eliminate power processing requirements. The magnetron can be operated from a single 20 kV buss and has internal voltage tolerance (5 to 10%) which permits direct connection to a 20 kV solar array. The magnetron approach is expected to benefit the reference Boeing design due to their optimistic conductor designs and conservative projections for power processing performance. The opposite could be said for the Rockwell reference design. The magnetron may also favor solar thermal or nuclear concepts because alternating current (a-c) power received from the Brayton or Rankine machines can be easily rectified to 20 kV d-c. The solid state r-f converter concept, when operated from large area solar arrays or Rankine or Brayton machines, is penalized by low voltage requirements (~200 volts). Power processing, necessary for 100% of the power, incurs a 5 x 106 kg mass and 4% loss penalty. A similar penalty results from the use of low voltage solar arrays. Low voltage solar arrays (400 volts) have been considered as a contingency pending verification of the feasibility of multi-kV arrays. The power processing penalty would again be 5 x 106 kg mass and 4% loss. High voltage a-c transmission has been a continuing trade study option and although a-c transmission designs can reduce power conductor and switchgear requirements, they require 100% power processing at the load and have therefore been less attractive than d-c designs. The remaining element of the PDS, shown in Figure 1, is the ground rectenna system. The ground rectenna receives power from series connected diodes and provides switchgear, and power processors for connection to the utility grid. Equipment requirements have been considered to be relatively in-hand when compared with requirements of the orbiting satellite. The primary concern has been with overall satellite/ ground rectenna power management when (1) the SPS is off-line for scheduled maintenance, (2) the SPS or utility grid experience partial failures, (3) peak on slack load is demanded from the utility grid. The PDS required for the Electric Orbiting Transfer Vehicle (not shown in Figure 1) requires switchgear and power conductors to connect the solar array to the electric thrusters. Power processors may not be necessary, depending on system performance trade studies involving solar cell (Si or GaAs) characteristics as a function of altitude. Energy storage, to operate a minimal number of attitude control thrusters during shadow periods, is dependent on trade studies comparing electric versus chemical thrusters. The interaction of all satellite subsystems with the space plasma environment or other plasma created by electric thrusters or the outgassing of materials is included in the PDS investigations. This includes technology development to permit solar arrays to be operated in the multi-kV range; selection of insulation material to control spacecraft charging; and trade studies to optimize the location of electric thrusters and to select material with low outgassing characteristics. Hardware that might be applied to PDS designs exists as high power, high voltage ground utility equipment or as low power, low voltage satellite equipment. The
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