OVERVIEW OF SYSTEMS DEFINITION ACTIVITIES FOR SATELLITE POWER SYSTEMS F. Carl Schwenk NASA Headquarters - Washington, D. C. The Satellite Power System, as shown .n the first viewgraph (Figure 1), is a means to gather energy in space and transmit that energy to earth in useful form. The concept shown is merely one of many possibilities for this concept. The Satellite Power System, invented in 1968 by Dr. Peter Glaser of the Arthur D. Little, Inc., has been the subject of design studies, symposia, program reviews by committees and, since 1977, the center of the activity known as the Concept Development and Evaluation Program. The purpose of this paper is to review what has been done during the last three years in the systems definition effort which has been the prime responsibility of NASA. In doing so, it is wise to refer to the total SPS effort conducted by NASA, which began in 1972, because the present lore of SPS is the summation of all past work. Prior to the Concept Development and Evaluation Program, the scope of SPS activities were as described on the next viewgraph (Figure 2). This chart, taken from a Table of Contents of a report on SPS prepared in early 1977, shows that the studies conducted by NASA included considerations of the role of SPS as an energy system, space technology, ground systems, environmental concerns, and comparisons. During these pioneering efforts many versions of SPS were studied and contributed to the present state of systems definition in SPS. Both photovoltaic and thermal energy conversion concepts were studied. One important aspect of SPS is getting it into space in the first place. Various transportation concepts for getting payloads into orbit at the lowest possible cost were studied, including modified shuttles, up-rated shuttles and a variety of advanced vehicle concepts generally known as Heavy Lift Launch Vehicles. These included ballistic single-stage and two-staged vehicles, winged two-stage vehicles for ease of recovery, and single-stage-to-orbit vehicles which could take off from conventional airports. In addition to studies of systems, the early analysis of SPS considered related activities in construction methods, construction locations and facilities, and different transportation methods in space, including chemical rockets for all orbit-to-orbit transportation, and electric propulsion systems using both nuclear energy and solar energy as the prime power source. From all these studies, the result is a focus on the photovoltaic approaches as being the prime early candidates for an SPS configuration. The choice resulted from a combination of the many factors I've described and represents an integration of many elements of the study efforts. It should also be noted that the microwave power transmission system has received the most attention in SPS studies. The power transmission system dominates the system design, environmental concerns and, ultimately, implementation of the program. It sets power levels and, therefore, system sizes and masses. For good penetration through the atmosphere, the carrier frequency in the range of one to ten gigahertz is desired. Because great emphasis was placed on obtaining baseload capability in satellite power systems, a frequency of 2.45 gigahertz in an available industrial, scientific and medical (ISM) band of the electromagnetic spectrum was selected for most of the study effort. Consideration was given also to frequencies of 5.8 and 8.0 gigahertz. For the 2.45 giga-
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