Page 190 - Intro to Space Sciences Spacecraft Applications

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Spacecraft Systems
used to control the distribution of the power is an important element of
this system. Solar, nuclear, and other power-generation sources have been
used onboard satellites. Examples of power systems used in spacecraft are
described in the following paragraphs. Since solar cells are the most com-
mon source of spacecraft power, they will be used in the detailed descrip-
tion of the design process.

Solar. There are two types of systems that rely on the sun’s radiations to
generate power: solar cells and heat exchangers. Solar cells directly con-
vert the sun’s photons (light) to electricity by their reaction to the incom-
ing photon flux. Current solar cell technology provides panels capable of
generating 140 watts or more of power per square meter of cells. The
angle of incidence to the sun is very important and there is a general dete-
rioration of the cells of up to 10% per year due to exposure to the space
environment. Heat exchangers use the sun’s total radiated energy (the
solar constant) to heat a working fluid which then turns a generator to pro-
duce electricity. Generated power quantities may be generally higher for
heat exchanger systems, but their disadvantage over solar cells is their
increased weight and complexity.

Nuclear. There are also two types of nuclear-related power generators which
can be used in space: fully critical nuclear reactors and radioisotope thermal
generators (RTGs). Nuclear reactors, like solar heat exchangers, use the heat
of the nuclear reaction to heat a working fluid which turns a generator to pro-
duce electricity. These systems can provide very large quantities of power for
missions with high power requirements, such as space-based radars. RTGs
also produce heat, but they are noncritical (and inherently safer) and produce
lower quantities of electrical power. Both sources are very stable, exhibit
long life, can operate far away from solar sources, and can be more compact
than solar cells (for maneuvering purposes). Disadvantages are that they are
generally very heavy, are radioactive, and are currently in disfavor as a result
of adverse public and political reaction to their use.

Chemical. This type of power source includes batteries, with which we
are all familiar, and fuel cells in which two elements are recombined with
a subsequent release of electrical power such as the oxygen-hydrogen fue’
cells currently used aboard the Space Shuttle. All spacecraft employ bat,
teries, at least as storage devices, using the major power-producing sys
tem to charge the batteries and subsequently withdrawing power from thc