It would be advantageous if there were a power module which could serve in every application, at least in the vicinity of Earth. In the program under consideration, the main applications are power for equipment platforms and the space station, and ion propulsion. Units can be made in various sizes; a power output of 50 Kw for the largest unit is a value of interest. The lifetime without maintenance should be 300 days or so, for ion propulsion applications. For the largest unit, a weight of 1500 Kg would be desirable, and it should fit in the large object vehicle. For applications where more than 50 Kw is required, multiple units can be used.
The main contenders would seem to be a parabolic mirror and a heat engine, or a parbolic mirror and a shielded solar cell array. A possibility for a heat engine is liquid metal Rankine with hydraulic suspension of the turbine, in a sealed vessel with an exterior stator. Another is an MHD generator. For the solar cell alternative, the weight of the shielding must be kept reasonable (and there are other additional weights).
The requirement of overall system design applies especially to the power module. The mechanical and electrical connections to various components should be specified, for as wide a range of equipment as possible. A free flying power module with a "constraint box" is a seemingly interesting possibility; electrical coupling might use a wiper or microwave.
For LEO the plant size can be increased as necessary. For ion propulsion the specific power should be made as high as possible, and for a given value the mission time can be decreased by increasing the plant plus fuel mass. In Some Mars Trajectory Optimizations (PDF) the example of Hohmann transfer to Mars is considered, with a specific power of 25; the times are within reason, and times for high Earth orbit are a fraction smaller. Values greater than 25 lead to improved performance.
The time behind Earth is only a slightly greater drawback for ion propulsion than for LEO power. For low orbit missions the time is short, so doubling is not a hardship (and at 500 Km the time in sunlight is over 60%). For high orbit missions the flight time is less than twice the powered time (rough calculations for Mars Hohmann transfer suggest that the factor is under 1.5). Note that the operation time of the ion thrusters equals the powered time, so the disadvantages of increased mission duration are secondary.
The alternative of nuclear power is problematical. It has the greatest advantage in LEO, where its safety issues are greatest. It requires heat engine power generation to be competitive. The issue of power for LEO ion propulsion needs to be further considered, but with maximum ion mission times of 200 days or even higher a single standard module seems to suffice for LEO power.
Power in deep space, where some form of nuclear power seems necessary, is not a component of the backbone system. The standard module can be used in inner planet missions.