supported by an interagency agreement with the Department of Energy (DOE) and Sandia National Laboratory (SNL), and (c) a NASA Lewis Stirling Power Generating System (SPGS) project that is supported by an interagency agreement with DOE and Oak Ridge National Laboratory (ORNL). The ASCS project is based upon the use of current technology to demonstrate a system on-sun that is capable of generating 25 kW of electricity within DOE's long-term cost constraints [1]. The SPGS project is based upon the synergistic characteristics between space power and residential/com- mercial heat pumps. These characteristics include high efficiency, low vibration, potential for long life and high reliability, and independence of heat source or fuel. Due to a length constraint, the discussion contained in this paper will be limited to NASA's High Capacity Power element whose overall goal is to develop the technology base needed to meet the long-duration, high-capacity power requirements for future NASA space missions. Need for Space Power NASA's space power technology history has concentrated on systems delivering less than 10 kW, as shown in Fig. 1. The exception was Skylab, which was designed to deliver nearly 20 kW to the user. Power requirements of NASA's missions, in the past, have been met almost exclusively by photovoltaic (PV) and electrochemical storage systems. Over the next several decades, the amount of electric power in space is expected to grow immensely. Tomorrow's space platforms will continuously require hundreds of kilowatts; and some will periodically consume many megawatt-hours. These space platforms will include manned space stations, communication stations, surveillance platforms, and defensive weapons. These large power systems will be quite different from today's solar arrays. These projections of space power growth tend to show broad trends as shown in Fig. 1. These broad trends are a direct result of uncertainties in future mission capabilities and needs. It is, however, clear that future space power needs may be several orders of magnitude greater than anything that has been accomplished to date. The challenge for the space planner is formidable—to select power technologies that can meet the projected trends and adapt to multiple users. NASA Lewis, as the primary NASA center for space power research and technology, has contributed significantly to these technologies. Only recently, we have expanded our research into technologies which offer promise of hundreds to thousands of kilowatts of electrical power in space. These expanded research areas cover a broad base of advanced technology. The first step toward this research and technology advancement is through CSTI. This 5- year technology program is the precursor to NASA's bold new missions. The resulting technology will enable and greatly enhance NASA missions while restoring the Agency's technical capability. The CSTI program not only focuses on space power but also on transportation systems, operations and science. CSTI started in 1988 and will end in 1992 and at that time should have generated critical data from which the Agency can make decisions on new initiatives. One space power candidate for these bold missions is the free-piston Stirling engine. The free-piston Stirling is a rapidly emerging technology which has only recently attracted considerable attention because of the successful Space Power Demonstrator Engine (SPDE). A recent scaling study indicates that it may be possible to build a free-piston Stirling engine/linear alternator system with up to 500 kWe per cylinder
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