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Commercial Supersonic Technology: The Way Ahead 5 Findings, Conclusions, and Recommendations For decades, the speed of commercial aviation was constrained by the sound barrier. Even with the Concorde, supersonic flight was unavailable except on a few routes and only for those willing and able to pay the high airfares. Commercially successful supersonic flight will only occur when technology is developed and assembled into an aircraft that can be profitably manufactured in large quantities (i.e., hundreds of aircraft) and that is affordable for users and environmentally acceptable to society at large. Commercial supersonic aircraft could take many different forms. The path to success will depend upon the ultimate and intermediate goals selected and the time and resources set aside to achieve them. One approach would be to focus on entirely new forms of air vehicles, such as vehicles energized by laser beams from ground power stations. A second approach would be to pursue revolutionary new aircraft, such as a “morphing aircraft” with the ability to continuously modify the shape of its wings to optimize aerodynamic performance during all phases of flight. Either approach requires breakthrough technologies, and turning new technologies into an operational commercial supersonic aircraft is very expensive and takes decades of research and development to satisfy performance, economic, safety, and environmental requirements for aircraft and ground systems. Focusing NASA’s efforts to develop technology related to commercial supersonic aircraft on long-term, high-risk concepts would probably result in a research program that does little or nothing to enable the operational deployment of environmentally acceptable, economically viable commercial supersonic aircraft in the next 25 years or less. Because that is the time frame of interest for this study, the committee endorsed a third approach: investing in breakthrough technologies that could be applied to the design of a more conventional commercial supersonic aircraft. The committee assessed the ability of advanced technology to meet customer and design requirements for the three types of commercial supersonic aircraft described in Table 5-1. The committee concluded that foreseeable technological advances will be able to solve many key customer and design issues, particularly for commercial supersonic aircraft with cruise speeds of less than approximately Mach 2. But much work remains to be done. New, focused research is needed in several areas where existing efforts are unlikely to close the gap between the state of the art and aircraft requirements (Finding 1). However, new initiatives in these areas will be counterproductive if they divert resources from existing efforts that are also necessary to the development of a commercial supersonic aircraft (Finding 2). TABLE 5-1 Commercial Supersonic Aircraft: Three Notional Vehicles Supersonic Business Jet Supersonic Commercial Transport with Overland Capability High-Speed Civil Transport Speed (Mach no.) 1.6 to 1.8 1.8 to 2.2 2.0 to 2.4 Range (NM) 4,000 to 5,000 4,000 to 5,000 5,000 to 6,000 Payload (passengers) 8 to 15 100 to 200 300 Sonic boom low enough to permit supersonic cruise over land Yes Yes Yes, if possible
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Commercial Supersonic Technology: The Way Ahead Finding 1. An economically viable, environmentally acceptable supersonic commercial aircraft with a cruise speed of less than approximately Mach 2 requires continued development of technology on a broad front (see Finding 2). In addition, research in the following five areas of critical importance could lead to important breakthroughs, but only if current research is augmented by new, focused efforts (or significant expansions of existing efforts): airframe configurations to reduce sonic boom intensity, especially with regard to the formation of shaped waves and the human response to shaped waves (to allow developing an acceptable regulatory standard) improved aerodynamic performance, which can be achieved through laminar flow and advanced airframe configurations (both conventional and unconventional) techniques for predicting and controlling aero-propulsive servo-elastic and aircraft-pilot servo-elastic (APSE) characteristics, including high-authority flight- and structural-mode control systems for limiting both types of APSE effects in flight and tools for defining acceptable handling and ride qualities automated, high-fidelity, multidisciplinary optimization tools and methods for design, integration, analysis, and testing of a highly integrated, actively controlled airframe-propulsion system variable cycle engines for low thrust-specific fuel consumption, high thrust-to-weight ratio, and low noise Finding 2. An economically viable, environmentally acceptable commercial supersonic aircraft with a cruise speed of less than approximately Mach 2 requires continued advances in many areas, particularly the following: airframe materials and structures for lower empty weight fractions and long life, including accelerated methods for collecting long-term aging data and the effects of scaling on the validity of thermo-mechanical tests engine materials for long life at high temperatures, including combustor liner materials and coatings, turbine airfoil alloys and coatings, high-temperature alloys for compressor and turbine disks, and turbine and compressor seals aerodynamic and propulsion systems with low noise during takeoff and landing cockpit displays that incorporate enhanced vision systems flight control systems and operational procedures for noise abatement during takeoff and landing certification standards that encompass all new technologies and operational procedures to be used with commercial supersonic aircraft approaches for mitigating safety hazards associated with cabin depressurization at altitudes above about 40,000 ft approaches for mitigating safety hazards that may be associated with long-term exposure to radiation at altitudes above about 45,000 ft (updating the Federal Aviation Administration’s advisory circular on radiation exposure, AC 120-52, to address supersonic aircraft would be a worthwhile first step) Conclusion 1. Research and technology development in the areas listed in Findings 1 and 2 could probably enable operational deployment of environmentally acceptable, economically viable commercial supersonic aircraft in 25 years or less—perhaps a lot less, with an aggressive technology development program for aircraft with cruise speeds less than approximately Mach 2. The situation is somewhat different for commercial supersonic aircraft with cruise speeds above approximately Mach 2. The most efficient cruise altitudes are higher for higher cruise speeds. Aircraft with cruise speeds in excess of Mach 2 will normally cruise in the stratosphere, where engine emissions have a greater potential for climate change and depletion of atmospheric ozone. Water vapor, which is benign in the lower atmosphere, may have significant, long-lasting effects in the stratosphere and pose a serious limitation on high-speed, high-altitude flight. Also, at higher speeds, air friction creates higher temperatures. For cruise speeds of about Mach 2.4, new classes of structural materials are needed to meet strength and weight requirements. The noise suppression problem also becomes more challenging. The optimal engine bypass ratio is lower at higher Mach numbers, which leads to increased jet exhaust velocities and greater take-off noise. This makes it more difficult to design a nozzle that provides adequate noise suppression without becoming so large and heavy that the vehicle becomes uneconomical. Finding 3. An economically viable supersonic commercial aircraft with a cruise speed in excess of approximately Mach 2 would require research and technology development in all of the areas cited in Findings 1 and 2. In addition, significant technology development would be needed to overcome the following barriers: climate effects and depletion of atmospheric ozone caused by emissions of water vapor and other combustion by-products in the stratosphere high temperatures experienced for extended periods of time by airframe materials, including resins, adhesives, coatings, and fuel tank sealants noise suppression at acceptable propulsion system weight
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Commercial Supersonic Technology: The Way Ahead Conclusion 2. Candidate technologies for overcoming environmental barriers to commercial supersonic aircraft with a cruise speed in excess of approximately Mach 2 are unlikely to mature enough to enable operational deployment of an environmentally acceptable, economically viable Mach 2+ commercial supersonic aircraft during the next 25 years. The environmental barriers associated with development of a supersonic transport with a cruise speed in excess of approximately Mach 2 are especially challenging. The most likely path to operational deployment of an environmentally acceptable, economically viable commercial supersonic aircraft within the next 25 years lies in research areas listed in Findings 1 and 2 as they would apply to an aircraft with a cruise speed of less than approximately Mach 2. Far-out technologies may be interesting and worth investigating, but they have little to do with development of a commercial supersonic aircraft in the foreseeable future. Breakthrough aviation technologies are generally proprietary, competition-sensitive, and/or classified. Few individuals or organizations are willing to divulge publicly detailed information on such technologies. However, once the government has identified areas of interest and funds an appropriate research program, industry will respond with detailed proposals. For example, DARPA’s Quiet Supersonic Platform Program is developing technology that would contribute to advanced supersonic aircraft with substantially reduced sonic boom, reduced takeoff and landing noise, and increased efficiency. Research areas include airframe shaping to reduce sonic boom, integration of low-specific-thrust propulsion systems, advanced inlet concepts, particulate injection into the engine exhaust, supersonic laminar flow, ceramics, and localized flow heating to increase virtual body length (and reduce sonic boom). Although detailed information about this research is still not publicly available, NASA has full access to it. In fact, the Quiet Supersonic Platform is managed by a NASA employee assigned to DARPA. Most of the technology challenges identified by the committee are unique to supersonic aircraft. However, in some cases the technological advances necessary for development of commercial supersonic aircraft would also improve the performance of subsonic aircraft. Where applicable, efforts to develop supersonic technology should take advantage of related efforts to improve the performance of subsonic technologies and the lessons learned that come from operational use of new technologies in subsonic aircraft. Recommendation 1. NASA should focus new initiatives in supersonic technology development in the areas identified in Finding 1 as they apply to aircraft with cruise speeds of less than approximately Mach 2. Such initiatives should be coordinated with similar efforts supported by other federal agencies (e.g., the DARPA Quiet Supersonic Platform Program). Recommendation 2. For the technologies listed in Finding 2, NASA should allocate most of the available resources on goals and objectives relevant to aircraft with cruise speeds of less than approximately Mach 2. NASA should focus remaining resources on the areas listed in Finding 3 (i.e., the highest risk areas for cruise speeds greater than approximately Mach 2). Again, NASA activities should be coordinated with similar efforts supported by other federal agencies. For each of the HSR Program’s critical technology elements, the general goal was to demonstrate a TRL of 6 (i.e., system/subsystem model or prototype demonstrated in a relevant environment). This goal was appropriate, in part because of the large investment required and the high risk that individual technologies might not lead to a viable commercial product. In fact, even if the HSR Program had been completed, additional fundamental technology development and validation would have been required to prepare and demonstrate that critical technologies were ready for use in a commercial transport (NRC, 1997). Maintaining a goal of TRL 6 for supersonic research is essential if the results are going be adopted by commercial product development programs. NASA should continue to recognize TRL 6 as the appropriate goal for supersonic technology programs intending to transfer research results to a commercial product. Recommendation 3. NASA and other federal agencies should advance the technologies listed in Findings 1 and 2 and Recommendations 1 and 2 to technology readiness level 6 to make it reasonably likely that they will lead to the development of a commercial product. CLOSING REMARKS Affordable, reliable, and safe air transportation is important to quality of life and economic growth. The global transportation infrastructure would be enhanced by the addition of a truly high-speed transportation element. If the United States intends to maintain its supremacy in the commercial aerospace sector, it has to take a long-term perspective and channel adequate resources into research and technology development. The technological challenges to commercial supersonic flight can be overcome, as long as the development of key technologies is continued. Without continued effort, however, an economically viable, environmentally acceptable, commercial supersonic aircraft is likely to languish. REFERENCE NRC (National Research Council), Aeronautics and Space Engineering Board. 1997. U.S. Supersonic Commercial Aircraft. Washington, D.C.: National Academy Press.
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