nuclear airplane proponents in the 1950s and the space nuclear power proponents of the SDI era, advocates of nuclear propulsion for human space exploration were hard-pressed to demonstrate clear-cut advantages to their preferred solution. Meanwhile, nascent support for human exploration of Mars largely evaporated. While most recent work on space nuclear propulsion systems has been of a theoretical nature, one concept, the particle bed reactor, has proceeded to hardware fabrication. This concept, first identified in the late 1950s, and subsequently elaborated in the early 1980s, received nearly $200 million in funding from the Strategic Defense Initiative under the highly classified (in fact, unacknowledged) Timberwind program. However, in 1991 this program was largely declassified, and transferred to the Air Force as the Space Nuclear Thermal Propulsion program. Various strategic defense applications were considered for the particle bed reactor, including generation of electrical power, propulsion of anti-missile interceptors, and use as a high-performance launch vehicle upper stage. Although the complete nature of the SDI Organization's interest in this technology remains shrouded in classification, analysis based on open sources suggests that unique contributions of the particle bed reactor to strategic defense missions are rather difficult to identify. That this judgment was ultimately shared by the SDI Organization is indicated by its decision in 1991 to discontinue funding the project. Despite these setbacks, proponents of aerospace nuclear power and propulsion remain undaunted. Their persistence in the face of this history is not easy to explain. Equally if not more difficult to explain is the secrecy surrounding the Timberwind program, and the SDI Organization's interest in this project. The Chicken and Egg Syndrome Both of these phenomena are largely rooted in the observations contained in the 1989 report of a panel of the National Research Council: "History has shown that it takes longer to develop a nuclear reactor system... than to develop a space mission. Hence today's civil and military space project managers cannot include any nuclear reactor space power system — or any other system -- in their mission planning until that system has been developed and tested. This dilemma is sometimes referred to as the "chicken and egg syndrome. "[2] The persistence of the aerospace nuclear power community, and the curious history of the Timberwind project, derive from this "chicken and egg" dilemma. Long and hard experience has revealed the transient and ephemeral character of mission requirements, in contrast to the protracted realities of developing workable aerospace nuclear reactor systems. This has engendered a belief within the aerospace nuclear engineering community that if only reactors could be developed, users would emerge to claim them. Thus, mission analysis becomes an exercise in articulating not-implausible pretexts for continued reactor development, rather than a process for establishing firm
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