requirements. In this sense, for instance, for the past decade the particle bed reactor has been a solution in search of a problem. The bewildering sequence of missions that have been attached to this reactor concept over the years is perhaps the clearest example of how (potential) capabilities drive requirements, in contrast to the more (naively) logical presumption that requirements should drive capabilities. While proponents of aerospace nuclear technology may have properly identified the locus of their discomfort, it is less clear that they have properly diagnosed the nature of their malady. Is this a problem which admits the possibility of a solution, or is this a dilemma from which there may be no escape? The solution usually suggested is to hatch the egg — creating reactors which will call into existence users. But four decades of effort render this solution questionable: • Aircraft nuclear propulsion systems were close to perfection when the program was terminated in 1962, and yet no users have come forward in the following three decades. • Nuclear rocket propulsion systems were ready for flight testing when the program was terminated in 1972, and yet no users have come forward in the ensuing two decades. • Over $400 million dollars has been expended by several agencies over the past decade on the SP-100 space nuclear power system, and yet the identity of potential users became increasingly elusive over the years. It is equally likely that the "chicken and egg" phenomenon is a dilemma for which there is no solution, particularly if the eggs produce turkeys when hatched. The continuing frustration of aerospace nuclear power and propulsion lies not in the absence of off-the-shelf reactors, but in the failure to meet the conditions that were the key to the success of atomic submarines. Aerospace nuclear reactors will remain artist's concepts as long as they continue to fail to respond to widely accepted requirements, fail to provide qualitative advantages compared to other technical approaches, and require major engineering innovations. The engineering challenges of this technology will be met only when clear user requirements are satisfied in a clearly superior manner. References [1] "Space Nuclear Power, Conversion and Energy Storage for the Nineties and Beyond," hearings before the Subcommittee on Energy Research and Production, Committee on Science and Technology, U.S. House of Representatives, October 8, 9, and 10, 1985, at page 68. [2] National Research Council, Energy Engineering Board, Committee on Advanced Space Based High Power Technologies, Advanced Power Sources for Space Missions, (Washington, National Academy Press, 1989), page 88.
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