Similarly, space nuclear propulsion to date has not been able to meet the conditions for success that facilitated development of submarine nuclear propulsion. In fact, what is surprising is not that aerospace nuclear propulsion remains merely a promise rather than a reality, but that the promise has remained so attractive for so long. With the exception of a brief hiatus in the 1970s, aerospace nuclear propulsion has been a field of considerable, and essentially continuous, activity for nearly five decades. In the years immediately following the Second World War. a number of studies were conducted to evaluate the potential of nuclear propulsion to aircraft, missiles, and space flight. Although these studies were not followed with development programs, this was as much a function of the ascendancy of the manned bomber in this period, which frustrated the development of rocketry generally, as it was the result of the shortcomings of nuclear propulsion. From the early 1950s through the early 1960s, several billion dollars were spent toward the development of nuclear-powered bomber aircraft. Though this produced significant technological developments, chemical-propellant bombers and ballistic missiles ultimately proved more attractive to the military. With remarkable continuity in funding, interest in and support for nuclear rocketry waxed as the nuclear airplane program waned. Nuclear rockets were seen as the key to piloted missions to Mars, and an important contributor to extended human exploration of the Moon. Though the nuclear rocket program continued through 1972, testing over 20 nuclear reactors and engines, by the early 1960s it was clear that there was little political support for the ambitious and costly space voyages these rockets would support. Nuclear airplanes and rockets were not the only aerospace propulsion projects of this period. An active testing effort was undertaken for the development of a nuclear- powered intercontinental cruise missile. Other projects included nuclear powered helicopters and dirigibles. These projects came to naught following the cancellation of the nuclear rocket program in 1972. For the ensuing decade exotic skills of aerospace propulsion designers were preserved in the Clinch River Liquid Metal Fast Breeder Reactor program. The SP-100 Space Reactor The demise of the Clinch River project in 1982 coincided with a resurgence of interest in aerospace reactor systems, which was greatly animated by the emergence of the Strategic Defense Initiative. Strictly speaking, the SP-100 program was conceived before President Reagan proposed the Strategic Defense Initiative on March 23, 1983. And for the first two years of the program, DARPA rather than SDIO was the Defense Department sponsor of the program. Nevertheless, Star Wars quickly became the focus and the primary motivation factor behind the SP-100. Thus, according to one Energy Department official:
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