the weapon system. For this reason, there are concepts being addressed which can deliver multi-megawatt levels of power and not produce effluents. Concepts of this type are also discussed in this section. SDI has the generic requirements, as all potential users do, of safety, lightest possible weight because of high launch costs, and extremely high reliability, both in the dormant stage and the high power ‘burst’ mode. NASA NASA is continuing an extensive planning effort which is directed toward establishing and maintaining permanent human presence beyond the confines of Earth’s gravitational field. The missions within this planning effort include expeditions to the Mars system, science outposts on the lunar surface, evolutionary bases on the lunar and Mars surfaces, and expeditions to near Earth asteroids. The potential leverage inherent in reduced mass associated with nuclear power and propulsion systems is being studied to provide a basis for directing appropriate nuclear technology research and development. The potential applications for nuclear technology can in general be classified in two categories; open cycle, short duration bursts for propulsion and closed cycle, steady state long term production of thermal or electrical power for surface support or electric propulsion. Specific applications could include: (1) direct thermal propulsion for manned and cargo vehicles to and from the Mars system; (2) electric propulsion for lunar cargo vehicles and Mars system cargo vehicles; (3) surface base electrical power for environmental and logistics support; (4) surface base electrical or thermal power for in situ resource processing; (5) orbital electrical power generation for subsequent beam transmission to space based or surface based loads. Generic requirements for power and propulsion systems pertinent to NASA are consistent with those for SDI aplications, in particular with respect to safety, reliability and mass. The presence of humans in proximity to nuclear power and propulsion systems gives additional emphasis to safety considerations in the design, deployment, operation and disposal of such systems for NASA applications. Technology Issues Most of the technology issues being addressed are a result of the need to develop high power systems with the minimum possible weight. With launch capacity limited to less than 30 000 kg at present in the shuttle, or a future capacity of 50-60 000 kg with the advanced launch system (ALS), and costs which are measured in thousands of dollars per kg, there is a tremendous motivation to make these systems as light as possible. This drive toward light weight systems is reflected in the use of high operating temperatures to increase cycle efficiency, the use of high strength, high temperature metals and composites, and the development of new and innovative concepts in the areas of heat rejection and power conversion/power conditioning. Safety is the paramount concern of all concepts being designed for power in space and this concern is reflected in all aspects of the design, development, testing, launch, usage and ultimate disposal of any proposed system. This concern has particular effects in the thermal hydraulic design of a system. Many of the proposed closed cycle
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