propulsion have generally waned before the required technologies could be brought to maturity. • While both atomic submarines and communications satellites promised revolutionary capabilities, nuclear aerospace propulsion has more often than not found itself in sharp competition with a host of non-nuclear solutions that offered comparable, or at least not drastically inferior, performance. The performance gains of nuclear propulsion too often seemed an inadequate reward for the effort expended. • In contrast to atomic submarines or communications satellites, which resulted from evolutionary extensions of the existing state of the art, aerospace nuclear propulsion has required major technological innovations. Most aerospace reactors would operate, for example, at temperatures far exceeding those of reactors intended for other applications. Modest evaluation in capabilities is purchased at the price of technological revolutions that have challenged the imaginations of a generation of reactor designers. Other Exceptions: RTGs, RORSAT Reactors The contrast between the successful development of atomic submarines and nuclear rocketry also illuminates the fruitful development of radioisotope thermoelectric generators (RTGs) and of the small nuclear reactors used aboard the Soviet satellites known in the West as Radar Ocean Reconnaissance Satellites (RORSATs). Since 1961, the United States has launched twenty-four spacecraft powered by RTGs in support of both military and civilian missions. These have included some of the space program's greatest achievements such as the Voyager probes in the 1970s. With respect to the outer planetary missions in particular, the choice of a nuclear power source was necessitated by the spacecraft's distance from the Sun. The Soviet Union launched an estimated 33 RORSATs between 1970 and 1988, when the program was terminated. The low orbit (around 260 kilometers), driven by the desire to optimize the satellite's radar capability, also dictated the selection of a nuclear reactor to minimize aerodynamic drag. In each case, a space nuclear power supply was clearly the best technical choice— or the only technical choice— to satisfy an established, highly valued mission requirement. For the same reason, the one space reactor flown by the U.S. did not inaugurate a series of reactor-powered missions. The flight-test of the SNAP-10A, launched in 1965, sufficiently proved the feasibility of developing, launching, and operating a space nuclear reactor. Yet the 500 Watts of electrical power generated by this reactor provided little incentive to displace the use of conventional, non-nuclear power sources. No follow-on systems were ever flown.
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