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Next: 7.6.3 The DIPS Up: 7.6 Power Subsystem Previous: 7.6.1 Power Budget

Subsections

7.6.2 Power Elements

The next step in the preliminary design process for the power subsystem is selecting and sizing power sources. When considering the options (primary batteries, nuclear power sources, photovoltaic cells, etc.), several immediate decisions were made. As a group, the Asterius designers decided that the only realistic options for an interplanetary mission that has such high power requirements are nuclear power sources. The physical characteristics, including size and weight, in addition to electrical and programmatic features, allow the primary battery option to be discarded. Therefore, several space nuclear power sources were researched and debated.

7.6.2.1 Radioisotope Thermoelectric Generators (RTGs)

For power requirements less that about a kilowatt, RTGs provide an effective power source. RTGs have proven to be a reliable, long-lived power source for lunar and interplanetary missions. The RTG is built around the space qualified General Purpose Heat Source (GPHS). The present day GPHS RTG is basically an array of radiatively coupled thermoelectric cells enclosing a stack of GPHS blocks. MOD RTG represents the next step in evolutionary development from the present state-of-the-art GPHS RTG. RTGs have demonstrated outstanding reliability. Their thermoelectric conversion system is made up of multiple series-parallel strings of redundant elements which accommodate failure of any element in the string with only ``partial degradation.'' No open circuit failures have ever been recorded. Counting all the RTG powered missions flown to date, over 70 years of successful flight experience have been accumulated. [10],[11]

7.6.2.2 Dynamic Isotope Power Systems (DIPS)

At present, the most efficient converters of thermal energy are dynamic heat engines. When ``energized'' by an isotope heat source, the resulting ``power plant'' is known as a DIPS. A DIPS requires less isotope per delivered electrical watt because heat engines are three to five times more efficient then thermoelectric converters. DIPS development for space focused on turbomachinery-based heat engine converters, primarily the close Brayton cycle. For multi-kilowatt missions, a turbomachinery-based DIPS is significantly lower in mass that the equivalent amount of RTGs. However, the turbomachinery-based DIPS is not an effective competitor with RTGs for low-wattage missions (below 400-500 W). The fundamental reason for this is that turbomachinery does not scale well to power levels due to ``fixed losses such as bearing loss, windage, turbine tip clearance, etc.'' Generally speaking, DIPS unit sizes below 500 W are considered impractical because of scaling effects on overall converter efficiency. [10]

Because of the aforementioned facts, and due to the average and peak load requirements for electrical power, the power generation sources were selected and sized for Project Asterius. The power subsystem consists of power generation for all phases of the mission. Two separate power generation devices have been selected for the design of Asterius: the first, a 150 W RTG, located on the WISP, and the second, a 500 W DIPS, located on the SOM. Secondary batteries were also discussed throughout the group. However, because of the high efficiency rate of the DIPS and improbable degradation, as well as the extremely high mass needed, secondary batteries were dismissed as an energy storage option.