source or by a bimodal power system, depicted in Fig. 4b. In a bimodal system, burst power and steady state power are provided using a single, Bimodal NDR. The NDR has this unique capability because the tie tubes in the support elements can be used as a second, independent flow circuit, with helium or a He-Xe gas mixture as the coolant and closed Brayton cycle working fluid. The magnitude of the steady state power output that can be obtained is quite flexible. Steady State Power Megawatts of steady state electrical power have been projected to be needed for a number of space missions, including orbital transfer, lunar base, and electrical propulsion for outer planetary expeditions. The NDR can be readily designed for such applications, simply by integrating the reactor with closed cycle turbo-generator- compressor, using helium as the reactor coolant and power cycle working fluid. The system is similar to the steady state portion of the bimodal power source. The only difference is that the thermodynamic cycle working fluid flows through the main coolant channels of the reactor as well as the tie tubes. Direct Thermal Propulsion Direct thermal propulsion is an obvious application of the NDR, since the NERVA technology was originally developed for that function. Fig. 4d is a schematic that illustrates such a system. The reactor coolant is hydrogen. The reactor-heated hydrogen passes through a thrust nozzle before exhausting to space. The performance of such a system is basically where it was at the end of the NERVA-Rover program, with a specific impulse of about 850 seconds. Dual Mode A dual mode power system is illustrated in Fig. 4e. It is analogous to the Bimodal power source. The only difference is that the open cycle burst power system is replaced by direct thermal propulsion. The steady state electrical power output in a dual mode system can be used for baseload, housekeeping, or for electric propulsion. The last is an attractive capability that could enhance a number of space missions. Ground-based Testing The NDR is the only multimegawatt reactor tested and demonstrated successfully for space power applications (Fig. 5). The NERVA-Rover program, during which twenty reactors were designed and successfully tested by Westinghouse, provides more than $1 billion worth of technology to NDR. All space vehicles undergo extensive ground testing to ensure successful operation in space. The testing sequence is development, qualification, and acceptance tests. Development testing is intended to validate hardware and design concepts and to assist in the evolution of designs from the conceptual phase to the operational phase. Qualification testing is conducted on flight prototypic components, subsystems and overall systems, and is designed to demonstrate that adequate margins exist in the flight design to assure that design specifications are met. Acceptance testing is designed to demonstrate the flight-worthiness of each deliverable item. Acceptance testing serves as a quality control screen to detect deficiencies of workmanship and material.
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