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will be refined as the study progresses with a detailed mission analysis. Known omissions in the weight estimates of the electric system include the superconducting transmission lines (estimated at only 3 percent of the turboelectric system weight) and other power management and distribution components. If the motors and generators were cooled by liquid hydrogen (with only enough carried on the aircraft to provide refrigeration) rather than refrigerators, then the turboelectric system would weigh 2300 lb (1000 kg) more than the 16- engine system, and the required jet fuel is reduced by 4000 lb (1800 kg), or 9 percent (calculated from the efficiency advantage of the large engines, without accounting for the replacement of jet fuel energy with liquid hydrogen energy), and TOGW drops by 560 lb (255 kg). This estimate does not include corrections for the weight of the liquid hydrogen (which would provide about 5 percent of the aircraft’s fuel energy) and its tankage and accessories, compared to the corresponding weight reduction of the jet fuel, tankage, and components. (It may be noted that, for the same energy, liquid hydrogen has almost 4 times the volume but only one-third the weight of jet fuel.) A comparison between the turboelectric case and two large (presumably podded) turbine engines can be made based on the numbers in table 3. One can see that the entire refrigerated turboelectric system weighs 6300 lb (2900 kg) more than two large turbofan engine cores of 42 000 hp each (with no weight allowance for podding) and would be ~1 percent less efficient at takeoff because of the electrical losses. A liquid-hydrogen- cooled turboelectric system would weigh 3600 lb (1600 kg) more than the large turbofan engine cores. Thus, the propulsion system weight for an HWB using podded engines would be significantly less than either of the two turboelectric systems discussed, with consequent accompanying reductions in fuel burn. However, the use of two separate podded engines would provide no STOL capability and only limited noise reduction, two important corners of the trade space, and none of the other potential benefits and capabilities mentioned above. IV. Further Study and Research Directions As previously mentioned, the distributed electric propulsion concept is not limited only to HWB aircraft but also could easily be applied to other vehicle configurations such as traditional tube and wing aircraft and tilt rotor aircraft. However, in order to achieve maximum benefits, it will be necessary to design an aircraft with greater emphasis on propulsion airframe integration right from the conceptual design stage. Moreover, to achieve all the benefits described in the above sections, a diligent research and development effort is required on the superconducting system for aircraft application. Besides additional modeling and analytical refinement of the electromagnetic, structural, and thermal aspects of the superconducting motors and generators, development is required on subsystems and auxiliary systems. The largest potential technology development payoff is in reducing the AC losses in HTS motors and generators. Those losses must be well below 1 percent in each machine to keep the required refrigeration reasonable. (Note that large generators already exceed 99 percent efficiency, even at room temperature.) The several types of AC losses that occur in HTS materials can be reduced by reducing the size of HTS filaments in the composite conductor and twisting them. An order of magnitude or more decrease in size from present practice is required. Such dimensions (and smaller) have been achieved in the older low-temperature superconductors, indicating promising approaches for the newer HTS materials. In addition, the required refrigeration is proportional to the above losses, as is the required input power to drive the refrigerator and hence the refrigerator weight. Present cryogenic refrigerators of the required capacity have not been designed with low weight as an objective and must reach significantly lower weight per input power to be acceptable on aircraft. A factor of 3 to 6 reduction from the present best machines is desired. Improvements in refrigerator mechanical efficiency would also be effective but may be more difficult to achieve. As noted above, no refrigerators would be required on liquid-hydrogen-fueled aircraft or on ones carrying enough liquid hydrogen inventory to cool the electric components. A wide range of analyses and system studies would be beneficial. To determine the optimal fan pressure ratio and other propulsion system parameters, a detailed mission analysis is needed, which would include optimizing both the fan propulsor modules and the thermodynamic cycle of the engine. Other propulsor options, such as ducted-propeller systems, should be examined. In addition, the basic mission profile needs to be examined to determine the impact of cruise Mach number on mission fuel burn, block times, and direct operating costs for different fuel prices. The unique flexibility of the turboelectric propulsion system is well suited to the examination of a wide range of propulsion and mission options. V . Concluding Remarks Two novel transport vehicle concepts based on hybrid wing body configurations have been proposed under NASA’s Subsonic Fixed Wing project to achieve low-noise and cruise- efficient short take-off and landing (CESTOL). The first vehicle concept was a high subsonic short take-off and landing (STOL) capable hybrid wing body airframe with multiple, small, partially embedded conventional engines. The vehicle characteristics and performance data of that aerodynamically trimmed and low-noise concept vehicle were briefly reviewed in this paper. The present proposed vehicle is similar to the first but uses distributed superconducting electric fans, powered by two wing-tip-mounted turboelectric generators, to lower the fuel consumption, noise, and emissions even further, 9PDF Image | Distributed TurboElectric Propulsion for Hybrid Wing Aircraft
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