A Case Study in NASA-DoD - The Black Vault

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NOTES TO TABLE B-I (Cont.)
mounted to the six faces of the space vehicle; it should be noted that
not all solar cells are exposed to the sun simultaneously on this spacecraft,
therefore, about 1200/w of 382 W are available on this design.
JElectrical power consumption of the standard STPSS modules, excluding
experiments, is determined by the stabilization system used: spinning
spacecraft, 92 W; three-axis earth reference, 136 W; three-axis
stellar (and wheels), 185 W; three-axis stellar with hydrazine, 197 W.
kTRW does not recommend using batteries smaller than 20 Ah for
missions requiring less than 500 W because the nonrecurring costs
associated with designing a smaller capacity battery and with interface
redefinition would increase program cost by about $200K to $300K.
Recurring battery cost savings due to using the smaller battery are not
substantial, since, typically, cell hardware contributes only 20 percent
to battery total cost, with the other 80 percent due to test and quality
control requirements.
ZExcess power generated by the STPSS solar array is shunted into
resistive modules on the surface of the spacecraft and radiated into
space.
From Ref. 5:
mPage 22 says, "28 ±7 V dc negative ground."
nThe power subsystem can support an orbital average load of 1200 W
in any orbit from 500 to 1665 km and at geosynchronous altitude. This
includes being able to accommodate a peak load of 3 kW for 10 min, day
or night. These determine the peak and average power requirements of
the power regulating unit and batteries.
0 The choice of various numbers of batteries and two sizes allows a
large variation in battery capacities to be chosen to suit the particular
experiment: 20, 40, 50, 60, 100, or 150 W.
PThe most recent specification calls for a 60 percent depth of discharge
in synchronous orbit instead of 50 percent.
qThe baseline power module weighs about 254 lb, including the case,
louvers, and all module attachment hardware. The heat sink louvers,
which prevent thermal runaway of the switching semiconductors, weigh
12 to 13 lb. The weight of the power subsystem frame or box, i.e.,
without electronics, just structure, is about 54 ib; and the attachment
hardware is about 25 lb. Thus, the 254 lb power system module, excluding
thermal and structural elements, weighs about 262 lb. Each 20 Ah
battery weighs about 50 to 53 lb; each 50 Ah battery weighs about 100
to 110 lb. Thus, for the baseline case, the weight of the battery and
power conditioning is about 354 lb; and, for 3 x 50 Ah batteries, the
total weight can be as much as 585 lb. Note that these figures include
some structure but do not include the vehicle harness, i.e., power
distribution.