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.