High-speed flight is a major technological challenge for both commercial and business aviation. As a first step in revitalizing efforts by the National Aeronautics and Space Administration (NASA) to achieve the technology objective of high-speed air travel, NASA requested the National Research Council (NRC) to conduct a study that would identify approaches for achieving breakthroughs in research and technology for commercial supersonic aircraft. This report documents the results of that effort. It describes technical areas where ongoing work should be continued and new focused research initiated to enable operational deployment of an environmentally acceptable, economically viable commercial aircraft capable of sustained supersonic flight, including flight over land, at speeds up to approximately Mach 2 in the next 25 years or less. In particular, sonic boom is the major barrier to development of supersonic business jets (SBJs) and a major, but not the only, barrier to the development of supersonic transports with overland capability. While NASA should have its eye on the grand prize—supersonic commercial transports—it is still quite appropriate for NASA to conduct sonic boom research, even when related to SBJs. The report also identifies other critical areas where technology development is needed to support the development of commercial supersonic aircraft with cruise speeds beyond Mach 2.
STUDY OBJECTIVE AND APPROACH
The objective of this study was to leverage the results of NASA’s High Speed Research (HSR) Program, other research, and related studies to identify breakthrough technologies for overcoming key barriers to the development of an environmentally acceptable, economically viable commercial supersonic aircraft with minimal sonic boom (to enable supersonic flight over land). The scope of the study included both small aircraft (that is, SBJs) and large transports. The focus of the study was on high-risk, high-payoff technologies where NASA-supported research could make a difference over the next 25 years. The committee did not focus on any specific vehicle configuration, market segment, or technology readiness level (TRL), although it believes that, to have practical value, government-funded research should advance to a TRL of at least 6 before industry can be expected to incorporate new technologies into commercial aviation products.1
The committee considered which of two options it would focus on:
revolutionary new types of aircraft that are fundamentally different from existing aircraft
vehicles that more closely resemble existing aircraft
Both options require breakthrough technologies, but developing a new vehicle concept into an operational commercial supersonic aircraft would take decades of research and development to satisfy aircraft performance, economic, safety, certification, and environmental requirements. Focusing on revolutionary vehicle concepts to develop commercial supersonic aircraft would probably give NASA a research program that does little or nothing to enable operational deployment of commercial supersonic aircraft within the next 25 years. Because that is the time frame of interest for this study, the committee concentrated on identifying areas where breakthroughs and focused investments are most likely to achieve the ultimate objective of sustained commercial supersonic flight, including flight over land, in a more timely fashion.
As the study proceeded it became clear that the statement of task presented procedural challenges. Breakthrough tech-
nologies are likely to be guarded as proprietary and competition-sensitive and are not available to groups such as NRC committees, which work in a public forum. This was true for much of the results of the HSR Program, which is not yet public information, as well as details of ongoing work, such as the Quiet Supersonic Platform Program funded by the Defense Advanced Research Projects Agency (DARPA). Because of its limited access to detailed technical data, the committee was reluctant to accept the role of recommending which individual technical concepts and approaches, many of them in the early stages of development, should be funded. Fortunately, it was not necessary for the committee to do so. As demonstrated by the Quiet Supersonic Platform Program, once the government identifies specific areas of interest and allocates development funding, industry and other research organizations are able and willing to provide detailed, innovative research proposals within the framework of a competitive government acquisition program, where competition-sensitive information is more likely to be protected. Therefore, with the concurrence of the study sponsor, the committee carried out the intent of the statement of task using the following approach:
Identify the technical barriers to sustained commercial supersonic flight, including flight over land.
Characterize the gap between the state of the art and the technology required to overcome each barrier.
Establish the feasibility of closing each gap by considering if at least one promising approach is available.
Identify what would have to be demonstrated to show that the gap has been closed.
To provide a framework for analyzing the technology needs for commercial supersonic flight over the next 25 years, the committee defined a set of three notional supersonic vehicles:
Small. An SBJ with about 8 to 15 passengers, a range of 4,000 to 5,000 nautical miles (NM), a cruise speed of about Mach 1.6 to 1.8, and sonic boom low enough to enable supersonic flights over both land and water.
Medium. An overland supersonic commercial transport with about 100 to 200 passengers, a range of 4,000 to 5,000 NM, a cruise speed of about Mach 1.8 to 2.2, and sonic boom low enough to enable operations over both land and water.
Large. A high-speed civil transport (HSCT) with about 300 passengers, a range of 5,000 to 6,000 NM, and a cruise speed of about Mach 2.0 to 2.4.
The committee also compared the technical challenges for commercial supersonic aircraft with the likely challenges for a military supersonic strike aircraft. A strike aircraft would have to overcome many of the same challenges as a commercial aircraft—for example, a high lift-to-drag ratio and acceptable takeoff and landing characteristics; efficient and durable engines; and advanced airframe materials and structures. A strike aircraft would need to meet these challenges while also meeting military requirements for stealth and weapons integration, but without necessarily meeting all the same environmental constraints.
For each class of aircraft, the committee used a combination of engineering judgment, historical trends, and simplified equations to identify key challenges and the research areas required to overcome those challenges. While noise and emissions are certainly major barriers to the development of an HSCT, significant advances in the traditional aeronautics engineering disciplines, such as structures, propulsion, and aerodynamics, are still required to close the business case and certificate new systems. Supersonic transports with overland capability (and military strike aircraft of comparable size) will require improvements in the four major factors related to economics (lift-to-drag ratio, air vehicle empty weight fraction, specific fuel consumption, and thrust-to-weight ratio) equivalent to about 10 percent over the present state of the art in each parameter, as well as additional advances related to the environment and certification. For SBJs, most parameters are already within the state of the art. HSCTs, on the other hand, will require significant advances, equivalent to about 15 percent for each of the four major economic parameters. Affordable supersonic flight is an exercise in integration: A viable commercial supersonic aircraft cannot be achieved until solutions to the individual technology challenges are brought together in one integrated airframe-engine design.
The committee also validated the importance of cruise speed as a key factor in determining the technological difficulty associated with development of a commercial supersonic aircraft. NASA’s HSR Program, which ran from 1985 to 1999, envisioned an HSCT with 300 passengers and a cruise speed of Mach 2.4. In 1997 the NRC concluded that the focus on Mach 2.4 was too aggressive and probably not justified by the business analysis. The study concluded that an aircraft with a cruise speed of Mach 2.0 might have a net productivity similar to that of a Mach 2.4 aircraft and would have an easier time overcoming some of the most difficult economic, technological, and environmental challenges.
As cruise speed increases, the most efficient cruise altitude increases also, and the technical challenges to developing an economically viable and environmentally acceptable commercial supersonic aircraft increase significantly above approximately Mach 2. For aircraft with cruise speeds less than Mach 2, an NOx emission index of 15 appears satisfactory, and water vapor emissions are unlikely to pose difficulties at the associated altitudes. Aircraft with cruise speeds in excess of Mach 2 will normally cruise in the stratosphere, where engine emissions have a greater potential to cause climate change and depletion of atmospheric ozone. At higher speeds, the NOx emissions index may need to be as low as 5.
In addition, water vapor, which is benign in the lower atmosphere, may have significant, long-lasting effects in the stratosphere. Also, at higher speeds air friction creates higher temperatures. For cruise speeds above approximately Mach 2.2, new structural materials are needed to meet requirements for strength, weight, and affordability. The noise suppression problem also becomes more challenging as speed increases, because cruise efficiency requirements mandate the use of engines with lower bypass ratios and, accordingly, higher jet exhaust velocities and more noise. As a result, larger nozzles are required to meet community noise standards. Without advanced technology, the nozzles of aircraft with cruise speeds above Mach 2.2 will probably be too large and heavy to be economical.
The intensity of sonic booms increases with vehicle size, weight, and speed; developing a low-boom design suitable for supersonic cruise over land will be much harder for an HSCT than for smaller transports or an SBJ. Even so, the development of any economically viable commercial supersonic aircraft is far from trivial. High-risk investments are still required to develop and validate the design of a small supersonic aircraft with low sonic booms. Success in this endeavor, however, could support the eventual development of an HSCT with a low sonic boom by performing critical noise suppression experiments, testing public acceptance of sonic boom noise levels, and gathering critical data.
RECOMMENDATIONS FOR RESEARCH
The technology advances necessary to attain the economic and environmental goals will be easiest to achieve for an SBJ and most difficult for an HSCT. Surmounting the special technology challenges associated with cruise speeds greater than approximately Mach 2 will require long-term research. To achieve commercial supersonic flight by 2025, NASA’s primary focus should be to support (1) new initiatives and (2) ongoing research that would enable supersonic flight at cruise speeds less than approximately Mach 2, where the technology challenges are more tractable in the near- to midterm. Specific areas of research identified by the committee appear in the findings and recommendations below and are also shown in Table ES-1.
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 impor
tance 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
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).
In addition to focused research initiatives on the technologies listed in Finding 1, the development of economically viable, environmentally acceptable commercial supersonic aircraft also requires continued advances in other areas. A broad range of technical activity must be supported to ensure that other key technologies are mature enough to convince industry to take on development and to meet designer’s needs at the time a supersonic transport is laid down. Although advances in these additional technologies (see Finding 2) may not be necessary to develop an SBJ, they are critical to the ultimate goal of developing a large commercial supersonic transport.
Finding 2. An economically viable, environmentally acceptable commercial supersonic transport 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 thermomechanical 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)
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.
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.
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
Conclusion 2. Candidate technologies for overcoming environmental barriers to a 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 ultimate importance of the commercial supersonic aircraft to the U.S. air transportation system is expounded in the long-range technology plans and visions of NASA, the Department of Transportation, and the National Science and Technology Council. Fulfilling these visions of the future will require a long-term investment strategy that looks beyond the short-term economic factors that drive much industry-funded research. The importance of a long-term view is especially important with breakthrough technologies. Unfortunately, both government and industry are reluctant to make the long-term investments necessary to mature expensive, high-risk technologies. In particular, at a time when manufacturers require TRLs of 6 or higher to embrace complex new technologies in safety-critical aeronautics applications, NASA appears to be changing its technology investment strategy so that it reaches TRLs of only 3 or 4. The likely result is a technology maturation gap that could jeopardize U.S. leadership in aerospace technology. To avoid this result—that is, to allow promising technologies to make the transition from the laboratory to the marketplace—NASA must invest enough to achieve TRL 6. With past programs, such as the HSR Program, NASA adopted TRL 6 as the appropriate goal for commercial supersonic research, and the committee is concerned that NASA’s less ambitious goals for much of its ongoing aeronautics research is driven more by the need to curtail aeronautics research because of reduced funding than by an objective assessment of what it will take to achieve the government’s programmatic goals.
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.
In summary, the committee identified no insurmountable obstacles to the development of commercial supersonic aircraft and believes that a properly focused research effort by NASA could develop technological solutions to the key problems identified in Finding 1, thereby enabling a successful commercial development program by industry in the relatively near term, especially for aircraft with a cruise speed of less than Mach 2.0. Without continued effort, however, an economically viable, environmentally acceptable commercial supersonic aircraft is likely to languish. National indifference to supersonic technology development would jeopardize longstanding U.S. supremacy in the aviation business segment and significantly harm the nation’s economy. The United States is not the only sponsor of supersonic technology development, and once a commercial supersonic aircraft is developed, users in the United States and other countries will purchase it, regardless of where it is manufactured.