Defense Intelligence Reference Document Laser Lightcraft Nanosatellites

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inclination relative to the ecliptic, will reduce the maximum eclipse period to about one
hour. Inclusion of spacecraft batteries is then justified. Passive thermal control will be
used to keep the spacecraft electronics within 10°c of ambient temperature, and hence
will not require electric power for heating. Using such a scenario, a battery
requirement of about 2 amp-hours at 3.3 volts will allow full spacecraft functionality
during an eclipse. Twelve AA size Lithium-ion batteries meet the requirement and only
weigh 480 grams.
Circuits that have high current demands, such as thruster solenoids and fuses, need to
be augmented with components that have a lower power density than batteries, but
also have lower internal resistance. Ultra-capacitors are being explored for this
application.
Miniaturization of the power system electronics (PSE) to meet the weight and size
requirements of the nanosats is a considerable challenge. The ideal approach is to
eliminate the PSE completely, by having a fixed electrical load and batteries provide the
needed bus regulation. This yields a simplified system consisting of solar cells,
batteries, and minimal circuitry. A more immediate approach to miniaturization is to
produce hybrid modules that measure approximately 5.08 cm x 3.17 cm x 1.27 cm
and weigh 100 grams for each PSE component, namely the solar array regulator,
battery regulator, and low voltage power converter. The combination of these three
components into one module will reduce the size and weight another order of
magnitude.
THERMAL
Although an inclination change by 10° renders maximum shadows below two hours, we
evaluate the case of a maximum eight hour shadow for the purpose of generality.
Three thermal configurations are considered: (1) top and bottom of the nanosat are
insulated, the inside of the cylindrical solar array is not insulated, allowing internal heat
transfer between the internal equipment and the array; (2) the entire nanosat is
insulated, top and bottom as well as inside the solar arrays, except for a radiator on
top, sized to radiate the internal electrical dissipation; and (3) the internal equipment is
thermally isolated as well as possible from an "outside shell" with a controllable two-
phase heat transport device which can be "shut off" during Earth shadows, serving as
the only thermal coupling between the equipment and a radiator on the outside surface.
The key advantage of configuration (1) is its reliability, or robustness. Since the
temperature of the nanosat is set by a high energy balance (heat in - heat out)
dominated by the absorbed solar energy, the operational temperature of the nanosat is
relatively insensitive to top and bottom multilayer insulation (MLI) properties, or,
largely, to internal heat dissipation. However, the feature that yields the operational
reliability, i.e., the high energy balance, also results in a rapid drop in temperature
when the solar load disappears during the Earth shadow. During the maximum eight
hour eclipse used for this evaluation, it was found that internal temperatures dropped
by about 60 8
C, which would result in internal temperatures in the range of -30 8
C to
-40°C. At the same time, the solar arrays dropped to a temperature of about 60°C.
Based on past experience, these end-of-eclipse temperatures are reasonable.
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