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Subsections
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.
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]
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.