A Case Study in NASA-DoD - The Black Vault

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around 300 n mi, an optimized PRU may provide up to 30 percent more
power than the direct energy transfer systems for arrays with long
thermal time constants (T). This is because the array is more efficient
at lower temperature when it first comes out of eclipse and the
PRU takes full advantage of this. For an array like Skylab, the thermal
constant is about 20 min. Thus, it takes 60 min (3T) to get to 90 percent
of the final AT, and this is the whole illumination period. For
lightweight arrays such as the Flexible Roll Up Solar Array (FRUSA),
the thermal time constant is only a few minutes and the improvement
over a direct energy transfer system in low earth orbit may be no more
than 5 to 10 percent.
OVERVIEW
Because many maximum or average power levels can be defined for
each space vehicle, Table B-2 summarizes some of the more useful values.
Shorter-term peak power levels available for experiment packages may be
limited by a variety of considerations unrelated to the factors that
dominate in Table B-2. The regulated 28 V ±2 percent power supply for
experiments on the AEM, for example, is limited to 50 W maximum; however,
the regulator can supply 120 W at 28 V ±5 percent for up to 4 sec.
Short-term peak power levels may be limited by the excess output of the
solar array, by the battery energy storage capacity, by the surge current
limit of the battery, or by the peak power handling capability of
some component in the power conditioning subsystem. Short-term power
levels--that is, those lasting seconds to minutes--are generally only
a factor of 2 to 10 times the average power level, but only penalties
such as cost, weight, or reliability inhibit the use of larger factors.
Because the complete power subsystems of the STPSS and MS are not as
well defined as for the AEM, and no power-time profiles are available
for each experiment, no short-term peak power summary is shown.
There is no doubt that the peak power tracker design of the MKS
can squeeze more power out of a given array in a low altitude orbit
than a direct energy transfer system, but the primary justification
for its use on the IMS is that the array characteristics and array integration
into the space vehicle need not be optimized--any handy oversized
array is acceptable and can easily be integrated. In this case,