An overview of electrical power requirements for each mission of a baseline and alternate plan for space activities in the 1990-2035 timeframe is presented. The specific missions included low earth orbit (LEO), geosynchronous earth orbit (GEO), lunar, Mars, and asteroid related projects

Conway, Edmund J.

English

NASA Technical Reports Server

N93- 1 906
_a
Power System Requirements
Edmund J. Conway
We estimated the electrical power
required for each mission in the
baseline model (fig. 1) and in the
alternative model (fig. 2), according
to the specific energy-using
activities and operations shown.
We then identified appropriate
technologies to meet these power
requirements, using such criteria
as, Can the technology fully meet
the requirement? and, Can the
technology be ready at least
5 years before the mission? In
some cases, there were competing
technologies for the same mission.
Low Earth Orbit (LEO)
The initial space station, scheduled
for the mid-1990s, will have
75-300 kW (electric) of continuous
bus power. Mid- to late 1980s'
solar photovoltaic technology is the
only proven power-generating
option available. However, solar
photovoltaic systems require large
arrays and consequently produce
substantial drag. To provide power
above the 75-kW level, two
technologies could compete: solar
dynamic (solar thermal with
Stirling-, Brayton- or Rankine-cycle
conversion) and nuclear thermal
(with thermoelectric, thermionic,
or dynamic conversion). Both
technologies are now in
developmental phases.
A second-generation space station
appears in the baseline model at
2008. It would be needed for
large-scale space processing of
terrestrial materials. Space
Station 2 would require from one
to tens of megawatts. Such a
mission would provide a major
pull on the power-generating
technologies. The current choice
would appear to be some type of
nuclear power system.
For power requirements above
1 megawatt, serious technology
issues also arise in electrical power
management (high voltage and
current) and thermal management
(how to dispose of 1 MW of low-
temperature heat). Electrical power
management would require both a
new philosophy and some new
technology. Thermal management
would require such new technology
as a large liquid droplet radiator.
https://ntrs.nasa.gov/search.jsp?R=19930007717 2020-03-17T09:35:13+00:00Z
Geosynchronous Earth
Orbit (GEO)
By the late 1990s, a
geosynchronous experimental
science platform would require up
to 10 kW. This requirement could
be met by solar photovoltaic
power. Advanced lightweight
power generation and storage
systems might be required if the
present limitations on payload
mass to GEO have not been eased
significantly. Such systems,
including those with gallium
arsenide solar cells and high
specific-power chemical storage,
are in the research stage now.
By 2004, a GEO shack or
temporarily inhabited repair shop
on the platform will allow for
human-tended and interchangeable
experiments. To operate in the
repair shop, the human tenders
would need additional power, on
the order of 10 kW. This power
could be supplied by the visiting
spacecraft. Solar photovoltaic
technology, similar to that already
mentioned for the platform, could
be used.
A manned GEO station could be
required beyond 2010. The power
level anticipated and the enabling
technology are similar to those of
the LEO growth space station.
Thus, geosynchronous Earth orbit
provides no new power challenges.
5
Moon
An orbital lunar mapper in the mid-
to late 1990s has only small power
requirements, which can be met by
1980s' technology. An unmanned
surface explorer (compare fig. 3),
beginning in 2004, would require
only a few (2-5) kilowatts
continuously, for movement,
surface coring, analysis, and
telemetry. A radioisotope
generator (compare fig. 4) with
dynamic conversion is the
technology of choice.
Figure 3
Lunar Rover Used on the Apollo 17
Mission
An automated unmanned version of this
rover might be useful on future lunar
missions. While seemingly simple, this
Apollo Rover contained many of the
elements necessary for a completely
unmanned rover- a sophisticated
redundant power system, power steering,
automatic thermal control, a dust control
system, and a self-contained navigation
system which kept track of the location of
the Rover at all times.
The Apollo 17 Rover, using two 36-volt
silver-zinc batteries rated at 121 amp-
hours each, traveled a maximum distance
from the Lunar Module (LM) of 7.6 km. For
long unmanned traverses, battery power
would probably not be practical because
of the relatively low energy density of
batteries.
A completely automated rover with an
artificial intelligence (AI) system or a
teleoperated rover are two possible
versions for future appfications.
Figure4
a. Radioisotope Thermoelectric
Generator
This radioisotope thermoelectric
generator (RTG) was the power source
for the Apollo lunar surface experiments
package (ALSEP) on the Apo//o 16
mission. This power generator contains
fins for radiating away excess heat. On
this mission it powered an active seismic
experiment (see accompanying fig.), a
passive seismic experiment, a surface
magnetometer, a heat flow experiment,
and the central control and
communications station.
b. Mortar Firing Assembly for the Active
Seismic Experiment
['h_s assembly in the ALSEP was designed
to fire four grenades out to a maximum
d_stance of 1.5 km. The grenades were
designed to explode on impact, generating
a sezsm_c signal which would be picked up
by a string of three geophones. On the
actual mission, only three of the grenades
were used and the maximum distance
traveled was about 900 m. This experiment
determined the thickness and seismic
velocity of the near-surface structure at
the Apollo 16 site.
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By 2010, a lunar camp, to be
inhabited only during the 2 weeks
of lunar day, would initially require
25 kW, supplied by a solar
photovoltaic system. This initial
power level could be augmented
during future visits using similar or
improved photovoltaic technology.
Or the lunar camp's power system
could grow, in the same manner as
that of the space station, to include
solar dynamic or nuclear supplies.
The initial power level is suitable for
crew life support, lunar science,
and light work, but it does not
provide the storable energy for
heat and life support during the
lunar night. For full-time habitation,
the camp and later the base would
rely on nuclear power supplying a
few hundred kilowatts. (See the
analogy in figure 5.) High power
requirements away from the base
for transportation or mining could
be supplied by a separate source
or by transmission. Point-to-point
beamed transmission along the
surface or between surface and
space is possible.
Figure 5
a. Spartan Lunar Base
Theearly lunar base may consist of
several modules similar to habitation and
laboratory modules for the space station,
which can be transported to the lunar
surface and covered with lunar regolith
for radiation protection. In manyways this
early base would be like the American
Stationat the SouthPole, which is
probably the closest thing we have to a
base on another planet.
b. South Pole Station
The South Pole station is continuously
occupied, but crewmembers arrive or
depart only during the summer season,
While the occupants can venture outside
with protective clothing ("space suits")
during the winter, they are mostly
dependent on the shelter provided by the
geodesic dome and the buildings within
the dome, much as they would be at a
Moon or Mars base.
Analogous to the Antarctic winter is the
lunar night. More power would be
required for heating and fighting in both
cases. Even more important on the
Moon, solar power would not be available
at night unless massive storage was
provided. Continuous occupation of a
lunar base would probably rely on
nuclear power.
Photo. Michael E. Zolensky
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|Figure 6
Unmanned Mars Lander
In one concept (above), an unmanned
Mars lander is bringing in a scientific
package and ascent system while a
small rover is parachuted to the surface
in the distance. The rover could then
travel to the lander in the foreground,
collecting samples along the way. The
rover would deriver the samples to the
ascent system, which would take them
into martian orbit and start them on
their way back to Earth.
Much of the basic technology for such a
mission was developed and successfully
tested by the Viking lander (right). The
Soviet Luna missions successfufly
returned lunar samples to Earth in
the early t970s. Electrical power
requirements for such missions are
quite small compared to those for any
manned mission.
Mars
The baseline and alternative
scenarios identify only one mission
to Mars by 2010, the Mars sample
return. This mission would require
only very limited power, which
could be provided by current
technology--a radioisotope
thermoelectric generator. The later
Mars site survey rover would have
power requirements similar to the
lunar surface explorer (2-5 kW)
and, like it, would rely on a
radioisotope generator with a
dynamic converter. (See figure 6.)
10
Asteroids
The alternative model (fig. 2)
includes unmanned exploration of
an asteroid beginning in 2005.
This involves activities and power
requirements similar to those for
the earlier lunar surface explorer
and could be handled by a similar
system.
Mining (not included in the
scenario) would require power on
the order of 10 MW. A nuclear
reactor power system developed
for general application to industrial
processing in space would be
utilized. See figure 7 for a
medium-range application on one
of the asteroid-like moons of Mars.
Figure 7
Phobos Deimos Hot Drill
The Phobos/Deimos (PhD) "hot drill" is
designed to melt its way into the regofith
of one or the other of these sate/rites,
liberating volatiles (mainly water) as it
goes. Water could be trapped and
efectroly-zed into hydrogen and oxygen for
use as propeflants to refuel the martian
lander or the Earth-Mars vehicle.
Artist Pat Rawlings
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