Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/STATION/STFMEDEA.CN4

Text File | 1992-09-09 | 17KB | 365 lines

SPACE STATION FREEDOM: Lewis Research Center

The Lewis Research Center was established in 1941 at
Cleveland, Ohio adjacent to the airport.  It was one of
three centers operated by the National Advisory Committee
for Aeronautics (NACA) nationwide.  The center was named for
George W. Lewis, NASA's Director of Research from 1924 to
1947.  The Center developed an international reputation for
its research on jet propulsion systems in the new jet age.

Lewis' original objective was aeronautics propulsion
research.  The Engine Research Laboratory, as it was first
called, was responsible for creating technology to improve
aircraft engines and components, studying fuels and
combustion, and performing fundamental research in those
areas of physics, chemistry, and metallurgy relevant to
propulsion.

In October 1958, the NACA Centers became the nucleus of the
National Aeronautics and Space Administration (NASA).
Today, Lewis scientists, engineers, technicians and support
personnel number about 2,700 people and occupy 100 buildings
and 500 specialized R&D facilities spread out over 360
acres.  In addition to office and laboratories for almost
every kind of physical research such as fluid mechanics,
physics, materials, fuels, combustion, thermodynamics,
lubrication, heat transfer, and electronics, Lewis has a
variety of engineering test cells for experiments with
components such as compressors, pumps, conductors, turbines,
nozzles, and controls.

A number of large facilities can simulate the operating
environment for a complete system: altitude chambers for
aircraft engines, large supersonic wind tunnels, space
simulation chambers for electric rockets or spacecraft, and
a 420-foot-deep zero-gravity facility.  Some problems are
amenable to detection and solution only in the complete
system and at essentially full scale.

The combination of basic research in pertinent disciplines
and generic technologies with applied research on components
and complete systems has helped Lewis become one of the most
productive centers in its field in the world.

Whereas Lewis engineers have continued their traditional
work in aircraft propulsion, they have utilized their
expertise in space propulsion, space power and satellite
communications.  They have also applied this fundamental
knowledge to terrestrial applications such as solar and wind
energy, automotive propulsion, advanced technology
batteries, fuel cells, and biomedical engineering.  Some of
the unique facilities supporting programs and basic research
include the following:

Propulsion Systems Laboratories
8-by 6-foot Transonic/Supersonic Wind Tunnel and 9-by
15-foot V/STOL Subsonic Wind Tunnel
10- by 10-foot Supersonic Wind Tunnel
Icing Research Tunnel
Engine Research Building
High Pressure Facility
Vertical Lift Facility
Electric Propulsion Laboratory
Rocket Engine Test Facility
Zero-Gravity Facility
Energy Conversion Laboratory
Power Systems Facility
Materials and Structures Laboratory
Materials Processing Laboratory
Basic Materials Laboratory
Central Process Air System
Research Analysis Center
Flight Research Building (Hangar)
Technical Services Building
Plum Brook Space Power Facility


The new Power Systems Facility will test the Space Station
Freedom Power System.  Lewis is responsible for the
end-to-end electric power system architecture for the
station including solar arrays, batteries, and common power
distribution components to the platforms.

Contemporary and future programs at Lewis will continue to
develop technologies important to the Nation.



Space Station Freedom Unique Activities  (Summary)

Solar Arrays
A series of eight solar array wings will be utilized to
provide electric power aboard the space station during its
early years.  Each 34-by 108-foot wing consists of two
blanket assemblies, each covered with 14,592 solar cells.
The modules are located on the transverse boom, outboard of
the truss element alpha gimbals.  Each one consists of  an
integrated equipment assembly (radiator panels, energy
storage, DC electronics thermal control assemblies and AC
power) and truss members.

Batteries
The energy obtained from the sunlight will be stored in
Nickel-Hydrogen batteries for later use when the station is
in the Earth's shadow.  A battery pack is made up of 30
Ni-H2 cells, wiring harness, and mechanical/thermal support
components.  On discharge, this operates near 28 volts which
allows the flexibility to connect several packs in series to
obtain a high voltage system for the space station and
platforms, and lower voltage for the platforms or other
station applications.

 Power Management And Distribution (PMAD)
The 20 kHz Primary PMAD system is designed specifically to
meet aerospace system requirements.  The system is based
upon rapid semiconductor switching, low stored reactive
energy, and cycle by cycle control of energy flow which
allows the tailoring of voltage levels.  The PMAD system
will deliver controlled power to many scattered and
different user loads.  The high frequency AC power system
was selected to provide higher efficiency, lower cost, and
improved safety.

LEWIS  RESEARCH  CENTER

Elements and Systems

Electrical Power System (EPS)
NASA's Lewis Research Center is responsible for the
end-to-end electric power system architecture for the space
station and for providing the solar arrays, batteries, and
common power distribution components to the U.S. Polar
Platform.  The EPS consists of power generation, energy
storage, and power distribution subsystems.

The EPS provides all user and housekeeping electrical power
and is capable of expansion as the station grows.
Initially, the EPS will generate 37.5 kw, which will
increase to a baseline value of 75kw.  Nickel Hydrogen
(Ni-H2) batteries store the direct current (DC) power
generated by the solar panels for use when the station is in
the shadow of the Earth.  The DC will be converted to AC for
primary distribution.  The EPS provides 20kHz, 208 volts,
single phase sine wave, utility grade power to station
elements.  The power is then converted to 129 volt DC and
distributed to users.

The most important design choice for the space station EPS
was the selection of the power generation and storage
system.  The possible options are all photovoltaic (PV), all
solar dynamic (SD) and hybrid (a combination of PV and SD).


Photovoltaic (PV)
A PV system has solar arrays for power generation, and
chemical energy storage (batteries) to store excess solar
array energy during periods of sunlight, and provide power
during periods of shade.  A PV system is generally
characterized by low development cost and high recurring
cost (due to maturity of solar array development and high
cost of solar cells and panels); low
efficiency-approximately 10 percent; and high drag from the
large solar array panel area required to capture sufficient
sunlight to meet required user power levels.

Initially, power for the space station will be provided by
eight flexible, deployable solar array wings.  This
configuration minimizes the complexity of the assembly
process by taking advantage of the technology demonstrated
on Space Shuttle Flight STS 41 B.  Each 32- by 108-foot wing
consists of two  blanket assemblies covered with solar
cells.  These are stowed in blanket boxes which are attached
to a deployment canister.  Each pair of blankets is to be
deployed and supported by a coilable, continuous longeron
mast.  A tension mechanism will supply tension to the
blanket as it reaches complete extension.  The entire wing
will be tied structurally to the transverse boom by means of
the beta gimbal assembly.  In order to provide the power
needed during the period of space station assembly, two
solar wings and other elements of the power system are
scheduled to be carried up on each of the first two space
station assembly flights.  These four wings will provide
37.5 kW of power.   The remaining four panels will be
delivered on orbit after the permanently-manned
configuration is reached.

 Batteries
Ni-H2 batteries will store the energy produced by the solar
arrays.  A battery pack is made up of 23 Ni-H2 cells, wiring
harness, and mechanical/thermal support components.  On
discharge, this operates near 28volts which allows the
flexibility to connect several packs in series to obtain a
high voltage system for the space station, or use of single
packs as a candidate for other low voltage applications.
Ni-H2 batteries offer minimum weight and high reliability.
During the eclipse periods, power is supplied by these
batteries.

Solar Dynamic (SD)
Solar dynamic systems use solar radiation to heat a working
fluid in a closed loop.  The fluid transfers work to a
turbine which drives an alternator, converting thermal
energy to mechanical energy to electrical energy.  Heat is
added to the fluid in a heat receiver which absorbs focused
solar radiation from a sun-tracking concentrator with a
reflective surface.  The receiver and concentrator are
oversized to allow excess thermal energy to be stored in a
melting salt as the heat of fusion when the system is in the
sun.  During solar eclipse, some of the salt solidifies,
releasing heat to the working fluid which continuously
powers the  turbo alternator.  Radiators are required
bysolar dynamic systems to reject the waste cycle heat to
space.  Solar dynamic systems are characterized by higher
development costs (because they have never flown in space
before) but lower recurring costs; slower performance
degradation due to aging; much higher efficiency than PV
systems, and much lower drag.  Extensive trade studies were
conducted comparing PV, SD, and hybrid EPS options during
the Phase B effort.  Although the hybrid option was judged
to be superior to either all PV or all-SD options, the
all-PV system was selected for development initially because
of low initial cost.

As the space station grows and the demand for electric power
increases, a solar dynamic system may be installed as a
complementary system to the photovoltaic power module.
This technology, far different from the photovoltaic system,
converts the Sun's rays into heat for the production of
power.  Heat is collected in a receiver which is located
near the focal point of a large parabolic mirror.  Power is
then generated exactly the same way as on an earthbound
power station: by heating a fluid, which in turn rotates a
turbine.  Since a heat/gas driven turbine is a much more
efficient power converter than a sunlight driven solar cell,
the mirror (the assembly with the largest area in the solar
dynamic system) would have to be only one third the area of
a solar array to generate the same amount of power to from
the Sun's light.

There are several different engines that can be used for the
generation of power within the solar dynamic system.  They
are similar in that they are "closed cycle," i.e., they
recycle the working fluid.  These engines are usually known
by the names of their inventor.  For use on Space Station,
the Brayton Cycle engine has been selected.

The energy storage device used for a solar dynamic power
system is superior to a photovoltaic system because heat is
stored rather than electricity.  Heat is cheaper and far
more simple to store for subsequent use.  Storage can be
accomplished by taking advantage of the heat, of fusion of
inorganic salts.  On the sunny side of the Earth, heat is
absorbed by the salt and it melts.  On the dark (cold) side
the salt freezes and gives up its heat to  the working fluid
of the engine, ensuring continuous operation.


Primary Power Distribution
The 20 kHz  Power Management and Distribution (PMAD) system
is designed specifically to meet aerospace system
requirements.  The system is based upon rapid semiconductor
switching, low stored reactive energy, and cycle by cycle
control of energy flow which allows the tailoring of voltage
levels.  The high frequency AC power system was selected to
provide higher efficiency, lower cost, and improved safety.

The overall distribution equipment will include cables, load
converters, transformers, regulators, switches and other
standard electrical equipment.  The overall distribution
subsystem will be composed of equipment necessary to
process, control, and distribute power to other station
subsystems, elements, and attached payloads.

The most significant PMAD design decision was the selection
of the primary distribution system frequency.  Both DC and
AC options were considered, and both high frequency
(typically 20 kHz) and low frequency (typically 400 Hz) AC
options were considered.  DC Primary distribution was not
selected because it had much higher weight and cost than
either of the AC options.

The performance of the candidate AC systems was relatively
similar and the choice was difficult.  All reactive
components (i.e. inductors, capacitors, transformers) are
much lighter for the 20 kHz system than for the 400 Hz
system.

The major discriminator between 20 kHz and 400 Hz was
electromagnetic interference (EMI).  Space station
experiments are sensitive to conducted and radiated EMI from
a 400 Hz system, including all of the harmonics up to about
10 kHz.  The weight of shielding and filtering required to
reduce the EMI from all of these frequencies to acceptable
levels in a 400 HZ system is prohibitive.  The EMI in a 20
kHz system is expected to be a more tractable problem.  In
addition to EMI considerations, audible noise from a 400 Hz
system may be objectionable to the crew.  As a result of
these considerations, 20 kHz was selected as the primary
distribution frequency.


Power Systems Facility (PSF)

The PSF will provide the capability for development,
testing, and evaluation of prototype power systems hardware
for the space station program.  The facility will be used to
test systems in support of both the baseline program and
evolutionary growth phase, to simulate anomalies during
flight, and support testing needs for future refinements.
The PSF will have a total of approximately 31,000 square
feet and will include a high bay test area with Class
100,000 Clean Room capability, a loading-unloading-workshop
area, laboratory rooms and support areas.  Solar dynamic
systems will be tested together with the power management
and distribution system.  Assembly and deployment tests,
optical tests, and vibration tests of concentrating mirrors
as large as 60 feet in diameter will be conducted in the
clean high-bay area.  The building site has been selected
for its close proximity to the existing solar array field in
recognition of the importance of using line lengths
representative of the space station electrical power
distribution system.  Electrical transient interactions are
very sensitive to line lengths and component separation as
well as the detailed characteristics of the power source.
While some studies will be done using the solar simulator,
others will require use of the outside solar array powered
by the sun.



Space Station Freedom Systems Directorate

NASA's Lewis Research Center in Cleveland, Ohio is
responsible for the Work Package 4 portion of the Space
Station Freedom Program.  The Space Station Systems
Directorate is responsible for the design and development of
the Electric Power System.  In effect, this Directorate is
the Space Station Freedom Electrical Power System Projects
Office.

The Project Control Office's responsibilities include
resources control, contracts, administrative services,
configuration management and technical documentation.  The
Systems Engineering and Integration Division performs system
engineering and analysis for the overall Electrical Power
System.  The Photovoltaic Power Module Division is
responsible for all activities associated with the design,
development, test and implementation of the photovoltaic
systems.   The Solar Dynamic Power and Propulsion Division
is responsible for hooks and scars activities in solar
dynamics and in supporting Work Package 2 in resistojet
propulsion technology.   The Electrical Systems Division has
responsibility for the power management and distribution
system development.  The Operations Division manages all
Directorate activities associated with Lewis space station
power system facilities and in planning electric power
system mission operations.

This organization currently includes approximately 200 civil
servants.  There are an additional 150 people in other Lewis
organizations working on such areas as test and evaluation,
construction and outfitting of the Power Systems Facility
and power related research.