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Using a solar power system for most deep space missions is not as attractive as previous published studies have led readers to believe. While improvements in solar power system technology continue to expand the applicability of solar power to missions to regions of the solar system that farther away from the sun, there are still many scientifically desirable deep space and planetary surface missions that must have radioisotope power systems to be feasible. A process is presented in this paper that will allow a designer or technologist to perform an accurate comparison of solar and nuclear power system alternatives at the system and mission level. The process includes comparison of power system performance, plus comparison of the impact of the alternative power systems on the rest of the system/mission. Impacts on subsystems including attitude control and thermal management, plus impact to the launch vehicle selection are discussed. While there will always be some subjective elements of a comparison of alternate power system technologies, the process described here will produce a valid comparison in terms of quantitative parameters, such as system mass and mission cost.

The US National Research Council (NRC) recently published a report titled thermionics Quo Vadis? An assessment of the DTRA's advanced thermionics research and development program. The report presented the conclusions of an independent assessment of the DTRA program by a thermionics committee established by the NRC. As members of the NRC thermionics committee, the authors will present a summary of the findings and recommendations made in the report, along with a summary of the process used to arrive at them and selected supporting information. Although thermionic systems have the potential to satisfy many future power system needs, other power conversion technologies are also being developed. In relation to these other potential technologies, the committee believed that thermionic technology may offer equal or superior merit for specific missions. The future sponsor should continue to evaluate and develop the possibilities of thermionic systems despite the challenge of preserving, continuing, and advancing this technology in the near term.

In response to market dynamics, communication spacecraft power level requirements continue to grow, accelerating the evolution towards higher efficiency photovoltaic devices. Light concentrating arrays enable the cost-effective implementation of recently developed high-efficiency solar cells while providing high array efficiency, protection from space radiation effects, and plasma interaction minimization for high voltage arrays. The linefocus concentrator concept delivers two added advantages: (1) low-cost mass production of the lens material and, (2) relaxation of precise array tracking requirements to only a single axis. New array designs emphasize light weight, high stiffness, stow-ability, and ease of manufacture and assembly. The linear refractive concentrator can be designed to provide an essentially flat response over a wide range of longitudinal pointing errors for satellites having only singleaxis tracking capability. In this paper we address the current status of the SCARLET (Solar Concentrator Array with Refractive Linear Element Technology) concentrator program with special emphasis on cost and mass performance for geosynchronous missions. Copyright C1996 by the American Institute of Aeronautic! and Astronautics, Inc All right* reserved. INTRODUCTION Revenue producing payload capability for next generation communication satellite systems will continue to grow as market pressures drive customer demand for greater return on investment (ROI). This trend is driven by competition from emerging entrants into the commercial space segment, as well as from an expanding terrestrial infrastructure. Payload capability is expressed at the spacecraft level as number of transponders, power of transponders, and time on station in revenue producing mode. These factors drive solar array power requirements sharply upwards. Recent announcements for next generation communication spacecraft have suggested power levels up to 15 kW per satellite. Spacecraft system level trades often indicate the use of a high-efficiency solar array system to accomplish this design requirement without mitigating the spacecraft's revenue enhancing goals. If standard silicon cell rigid array technologies are employed, array mass, stowed volume, and deployed area may quickly erode the desired ROI enhancement. Standard bus designs were developed with silicon cell technologies for standard launch vehicles with standard faring shroud sizes. When power levels are roughly doubled, the twice-as-big solar array on such a standard bus seriously erodes launch vehicle mass and volume margins. This trend will serve to limit spacecraft power levels and hence

In response to published reports about the decline in funding for space nuclear power, the Board of Directors of the American Institute of Aeronautics and Astronautics (AIAA) approved a position paper in March 1995 that recommends (1) development and support of an integrated space nuclear power program by DOE, NASA and DoD; (2) Congressional support for the program; (3) advocacy of the program by government and industry leaders; and (4) continuation of cooperation between the U.S. and other countries to advance nuclear power source technology and to promote safety. This position paper has been distributed to various people having oversight of the U.S. space nuclear power program.

The Ballistic Missile Defense Organization sponsors an aggressive program to develop and demonstrate electric propulsion and space power technologies for future missions. This program supports a focused effort to design, fabricate, and space qualify a Russian Hall thruster system-on-a-pallet ready to take advantage of a near-term flight opportunity. The Russian Hall Effect Thruster Technology (RHETT) program will demonstrate an integrated pallet design in late FY95. The program also includes a parallel effort to develop advanced Solar Concentrator Arrays with Refractive Linear Element Technology (SCARLET). This synergistic technology will be demonstrated in a flight experiment this summer on the Comet satellite. This paper provides an overview of the RHETT and SCARLET programs with an emphasis on electric propulsion, recent progress, and near-term program plans.