The noise emitted by commercial aircraft is a major inhibitor of the growth of commercial air transport and is a critical environmental issue in air transportation. A functionally-silent aircraft is envisioned to achieve a step change in airframe and propulsion system noise reduction (30 EPNdB). This thesis addresses the assessment of low-noise propulsion system concepts suggested to enable such a functionally-silent aircraft. The propulsion system concepts of ultra-high bypass ratio turbofan engines, a distributed propulsion system, and silent engine air-brakes are evaluated in light of their noise signatures and propulsion system performance. These propulsion system concepts are assessed using simple analytic models and existing semi-empirical noise prediction methods. Ultra-high bypass ratio turbofan engine cycles are predicted to achieve the 30 dB jet noise reduction goal. This noise reduction is possible with bypass ratios of order 70 for separate flow exhaust nozzles and of order 40 for a mixed flow exhaust nozzle. Such large bypass ratios result in a significant decrease in specific fuel consumption. However, the cost for a step change in jet noise reduction is a dramatic drop in specific thrust which requires larger engine diameters, and hence yields penalties in weight and installation drag. For a given noise goal, the study shows that an advanced technology engine cycle achieves a specific thrust similar to current technology engine cycles but enables a lower specific fuel consumption. In order to embed such ultra-high bypass ratio engines in a blended-wing-body airframe, a distributed propulsion system consisting of 10 core engines driving a total number of 30 fans via gearboxes and shafts is suggested.
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