In providing a radiator system for the thermal engine concept, several design choices must be made early in the design program including: (1) Will the cycle working fluid be used as the radiator fluid also (direct radiator) or will a secondary coolant loop with an interface heat exchanger be provided? With the Rankine cycle the direct method would involve a condensing radiator, whereas with the Brayton cycle a gas radiator would be required for direct radiation; (2) Will the radiator reject heat from one or both sides?; (3) Long-term meteoroid protection of the fluid tubes must be provided for reliability purposes; (4) An extremely reliable fluid pump is required for extended operation. These questions will be discussed in more detail in the text of the report, but it should be noted here that whatever concepts are selected, the design of the heat rejection system in principle involves no major technological breakthroughs. The design of space radiators is a well- known technology, and techniques abound for this task (see, for example, refs. 5 and 6). Extreme care in designing the radiator subsystem is required, however, as this is the heaviest single component of the system. From a technological standpoint, the weakest component of the thermal engine system is the reflector-absorber subsystem. Both the reflector and absorber systems will require technological innovations to effect proper collection, concentration and absorption into the working fluid of the vast amount of solar energy required to power the system. Since weight is at a premium and the collector is the largest-area component, an extremely lightweight reflector system is needed with a reasonable reflectivity and concentration ratio. Since the absorber must operate at temperatures higher than the turbine inlet temperature, precautions must be taken to obtain a high-thermal-efficiency absorber with a minimum of re-radiation, reflection, and other heat losses. The one most significant technology breakthrough required for feasibility of the entire concept (be it the photovoltaic or the thermal engine) is the problem of how to design, erect and maintain large structures in space. This problem is addressed in another section of the report and will not be discussed here. From the thermal engine standpoint, however, the problem affecting all subsystems and components is one of a "scale-up" nature from the size of existing devices to the very large sizes (even with modularization) required for the SPS. In summary, from a purely technological standpoint, there appears to be no inherently insurmountable problem with the thermal engine concept, although many engineering problems will need to be solved to make the concept a workable system. This report will discuss these engineering problems and will evaluate where possible the solution proposed by the Boeing Company, as well as compare the thermal engine and photovoltaic concepts. It should be emphasized that in this brief study only an evaluation and discussion of concept is possible, since sufficient design details are not available at this point in time.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==