3
HIGH-SPEED CIVIL TRANSPORT AIRCRAFT

INTRODUCTION

The United States had a government-funded supersonic transport (SST) program during the 1960s. That program was canceled in 1971 because of increasing concerns over economic viability, environmental acceptability, and the inadequate state of the needed technologies. Technology studies for a future SST continued in the National Aeronautics and Space Administration (NASA) at a low level of effort during the late 1970s and early 1980s.

The White House Office of Science and Technology Policy in 1985 and 1987 identified national aeronautical research and development goals and laid out an action plan for achievement of those goals.1,2 One of the goals was technology development to support a long-range supersonic transport, which was referred to as "a great market-driven opportunity." The reports further recommended that industry and NASA determine the most attractive technical concepts and the necessary technological developments for future long-range, high-speed civil transports (HSCT).

This chapter discusses the Committee's findings and recommendations regarding the future of HSCT technology. The potential market for this class of aircraft is considered, and NASA's contributions to developing the needed technology are outlined. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits of research and technology development efforts for HSCT.

1  

Executive Office of the President. 1985. National Aeronautical R&D Goals: Technology for America's Future. Report of the Aeronautical Policy Review Committee, Office of Science and Technology Policy. Washington, D.C.

2  

Executive Office of the President. 1987. National Aeronautical R&D Goals: Technology for America's Future. Report of the Aeronautical Policy Review Committee, Office of Science and Technology Policy. Washington, D.C.



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Aeronautical Technologies for the Twenty-First Century 3 HIGH-SPEED CIVIL TRANSPORT AIRCRAFT INTRODUCTION The United States had a government-funded supersonic transport (SST) program during the 1960s. That program was canceled in 1971 because of increasing concerns over economic viability, environmental acceptability, and the inadequate state of the needed technologies. Technology studies for a future SST continued in the National Aeronautics and Space Administration (NASA) at a low level of effort during the late 1970s and early 1980s. The White House Office of Science and Technology Policy in 1985 and 1987 identified national aeronautical research and development goals and laid out an action plan for achievement of those goals.1,2 One of the goals was technology development to support a long-range supersonic transport, which was referred to as "a great market-driven opportunity." The reports further recommended that industry and NASA determine the most attractive technical concepts and the necessary technological developments for future long-range, high-speed civil transports (HSCT). This chapter discusses the Committee's findings and recommendations regarding the future of HSCT technology. The potential market for this class of aircraft is considered, and NASA's contributions to developing the needed technology are outlined. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits of research and technology development efforts for HSCT. 1   Executive Office of the President. 1985. National Aeronautical R&D Goals: Technology for America's Future. Report of the Aeronautical Policy Review Committee, Office of Science and Technology Policy. Washington, D.C. 2   Executive Office of the President. 1987. National Aeronautical R&D Goals: Technology for America's Future. Report of the Aeronautical Policy Review Committee, Office of Science and Technology Policy. Washington, D.C.

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Aeronautical Technologies for the Twenty-First Century Recommendations General NASA should be the primary contributor to technologies that identify and reduce the environmental impact of HSCT, including ozone depletion, airport noise and emissions, and sonic boom. Specific NASA should enhance its atmospheric research program to determine whether acceptable levels of ozone-depleting emissions from HSCT exist. If so, NASA should perform the necessary propulsion research and validation to meet those levels. NASA should work toward advances in HSCT propulsion technology that reduce noxious emissions and engine noise. The HSCT program should be tailored to ensure that propulsion, aerodynamics, structures and materials, and overall vehicle management and control technologies are adequately represented. Although industry is not currently planning HSCT supersonic flights over populated areas, NASA should continue an aggressive program to define acceptable sonic boom levels and should continue its program to investigate approaches to meeting those levels. CURRENT INDUSTRY STATUS In response to the Office of Science and Technology Policy reports, NASA initiated the High-Speed Civil Transport Study to investigate the HSCT technical feasibility, economic viability, and environmental compatibility, and to identify potentially high-payoff technology development for HSCTs. Original vehicle characteristics were not specified—industry contractors were encouraged to examine flight between Mach numbers of 2 and 25, with the associated vehicle requirements and characteristics. Those early studies determined that a market will exist for an HSCT.3,4 3   McDonnell Douglas Aircraft Company. 1989. Summary of High-Speed Civil Transport Study (NASA CR-4236). Prepared for National Aeronautics and Space Administration. 4   Boeing Commercial Airplanes. 1989. High-Speed Civil Transport Study Summary (NASA CR-4234). Prepared for National Aeronautics and Space Administration.

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Aeronautical Technologies for the Twenty-First Century Benefits of High-Speed Civil Transport Research and Technology Development Cost/Convenience Competitive operating costs Fuel consumption as low as possible Maintenance and reliability competitive with advanced subsonic aircraft Predictable airframe and component life Adequate comfort levels for crew and passengers Technology validation (to reduce development costs) Environment Compatibility Acceptable noise (airport noise and sonic boom) Acceptable emissions Aircraft Performance Improved fuel economy (over first-generation supersonic transport) Improved speed and range Because of the distances corresponding to the identified markets, practical limits on vehicle operations that limit aircraft productivity, and the increase in required technology development, risk, and cost with increasing Mach number, it became apparent that only speeds at or below Mach 3.2 were feasible. More detailed analysis has indicated that for the near-term, economic competitiveness is enhanced at even lower cruise Mach numbers.5 Economic viability requires that the cost and risk of developing advanced technology be balanced with the expected economic return when compared to the competing advanced subsonic systems of that time frame. The HSCT studies identified barriers to the potential operation of a supersonic transport. These barriers included adverse environmental impact, constraints on operations due to the environment, and insufficient technology to provide vehicle performance adequate to ensure economic viability. NASA instituted the High-Speed Research Program,6 which involves NASA and industry together doing the research required to quantify and remove these barriers, and to develop a technology base to ensure that U.S. industry is in a strong competitive position for the design and development of an HSCT. 5   Samuel M. Dollyhigh. 1989. Technology Issues for High-Speed Civil Transports (SAE Paper 892201). Society of Aerospace Engineers. 6   NASA Office of Aeronautics, Exploration and Technology. 1990. High-Speed Research Program. Program Plan. Washington, D.C.

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Aeronautical Technologies for the Twenty-First Century The Concorde, a first-generation SST built by a British and French consortium, is generally considered to be a technical success but an economical failure. A small number of these planes are in service, catering to a small segment of the first-class market, where very high fares and government subsidies support the substantial operating costs. The Soviets built an airplane very much like the Concorde (the Tupolev-144), which had very limited operation. Currently, many research and development programs relative to a second-generation supersonic transport are underway in foreign countries. The French are working on the second-generation Concorde.7 The Japanese, although currently limited in aeronautical technology capability, have the interest and capital to pursue an advanced supersonic transport. Representatives from the former Soviet Union are pursuing a cooperative venture with Gulfstream Aerospace in this country for the development of a supersonic business jet, as well as various cooperative ventures with the Europeans. Even if environmental problems are solved, bringing an HSCT from initial research and development to operation will still require a substantial investment. The demand for an HSCT fleet, although substantial, may support only one development program. MARKET FORECAST National and international air transportation systems are growing. Increases of about a trillion revenue passenger-miles are projected for each decade between now and the year 2020, resulting in two trillion revenue passenger-miles in 2000, three trillion in 2010, and four trillion in 2020. Much of this growth is projected to be in international markets (Figure 3-1). The North Atlantic market is expected to double by 2005, but more importantly the Pacific market (with its longer ranges) is forecast to quadruple during this time to about the same size as the North Atlantic market. The Europe-Asia market is also projected to experience significant growth. Based on these market projections, if a reasonable fare premium (10–20 percent) is assumed, an HSCT is optimistically thought to be able to attract about 300,000 passengers per day away from advanced subsonic transports by the year 2000, and about 600,000 passengers per day by 2015.8 This translates into a worldwide potential market for 600 to 1,500 HSCTs, depending on economic returns, aircraft specifications, operating constraints, and subsonic flight over land. Although these optimistic projections depend strongly on various economic factors, the number of aircraft is sufficient to be economically viable for an airplane manufacturer. 7   Ph. Poisson-Quinton. 1990. What Speed for the Next Generations of High-Speed Transport Airplanes—Supersonic or Hypersonic? France: ONERA. 8   Boeing Commercial Airplane Company. 1989. High-Speed Civil Transport Study. NASA Contractor Report 4233.

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Aeronautical Technologies for the Twenty-First Century FIGURE 3-1 International passenger traffic. BARRIERS To be successful, the HSCT must be economically viable, environmentally compatible, and technically feasible. Economic viability will be attained if the value of the HSCT to the airline is at least as great as the price charged by the manufacturer and the cost of ownership. Environmental acceptability includes no significant effect on the ozone layer, acceptable sonic boom levels, and community noise levels that meet future regulations. Economic Viability Currently envisioned HSCT configurations, which take advantage of either current or very near-term technology and could be introduced in the 2005 time frame, cruise at speeds of Mach 1.6-Mach 2.5, have ranges between 5,000 and 6,500 nautical miles, carry 250 to 300 passengers, and operate in the existing infrastructure (airports, runways, and air traffic management system) to maximize productivity. Since the HSCT will probably be 50 to 100 feet longer than today's Boeing 747, innovative ground operation and passenger loading/unloading

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Aeronautical Technologies for the Twenty-First Century techniques may be required. Additionally, the HSCT must permit rapid turnaround time to allow airlines to use the aircraft to its fullest potential. The value of an HSCT to the airline must be at least as great as the price charged by the manufacturer. Value to the airline is dependent on the cost of owning and operating an HSCT fleet, on being able to attract a sufficient number of revenue passengers, and on attaining high productivity. The costs of ownership and operation are dependent on the aircraft technology level, the number of aircraft produced, and outside economic factors (e.g., fuel cost and interest rates). Productivity increases as the airplane's speed, capacity (number of passengers), and rate of utilization (flights per day) increase. The number of passengers depends on several factors, including the market, the perceived benefit of this type of flight, and the ticket cost. The rate of utilization of an HSCT may be limited by scheduling and turnaround times. As noted in the introductory section of this chapter, much of the growth in the air travel market is projected for the long-range international market. The flight times on these routes are long (13 to 14 hours) for subsonic aircraft; the HSCT could cut this time in half. This reduced flight time is attractive to passengers for both the actual time saving and the decreased fatigue. However, reduced flight time will not be sufficient to lure the traveler away from the advanced subsonic transport unless the fare is competitive. Figure 3-2 is an estimate of how much extra the passenger is willing to pay for faster service. The figure shows how many available seat-miles can be captured as a function of fare premium. For example, a fare premium of 10 percent would capture a total of approximately 300 billion seat-miles annually, whereas a fare premium in excess of 20 percent would result in only about 200 billion available seat-miles. To attract the discount coach passenger market, the fare premium must be no more than 5 to 10 percent (the discount coach market accounts for about 40 percent of the overall market). Productivity for airline operators increases as a result of the HSCT speed advantage. For example, an HSCT traveling at twice the speed of a subsonic aircraft can carry a comparable load of passengers twice the distance in a given time (if the time to climb and descend, as well as time on the ground for servicing and refueling, is a small part of the total travel time) or can make twice as many trips over a given distance in a given time. For a successful HSCT, increased productivity would be due to a greater number of seat-miles deliverable each day compared to subsonic aircraft of the same capacity, provided the air traffic management (ATM) operations, airport operations, maintenance, and turnaround times can be held low enough so as not to offset the greater productivity enabled by higher-speed flight. Balanced against this are the direct costs per seat, fuel consumption per seat-mile, and maintenance costs per seat-mile, which are higher for an HSCT than for competing subsonic aircraft. Environmental Viability Economic viability is a necessity for the HSCT; so is environmental compatibility. Atmospheric emissions and community noise constraints must be satisfied or there will not be an HSCT program. The issue of sonic boom can be dealt with by allowing supersonic flight only over water (the current rule), but there is a resulting economic penalty in having to fly at subsonic speed over land.

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Aeronautical Technologies for the Twenty-First Century FIGURE 3-2 Market capture. The emissions from any aircraft engine include nitrogen oxides which, together with sunlight, generate ozone and other strong oxidants in the lower atmosphere. In the stratosphere, however, nitrogen oxides destroy naturally occurring ozone; unfortunately, stratospheric ozone destruction cannot be compensated by its production at lower altitudes. This is especially a problem with supersonic aircraft whose optimum cruise altitude coincides with the region of highest ozone concentration. One potential solution is to reduce the cruise Mach number (from Mach 3.2 to the Mach 1.6–2.4 range) and, hence, reduce the optimum cruise altitude, thereby reducing the effects on ozone. Continuing atmospheric studies are needed to understand and determine the emission levels, if any, that do not reduce ozone concentration. Technology development (primarily low-emission combustors for the engine) focused on reducing nitrogen oxides to that level is necessary to meet this challenge. To maintain high levels of productivity, the HSCT must be able to operate out of the same airports as competing subsonic transports. This implies that the HSCT must produce community noise levels no higher than its subsonic competition. Those levels vary widely by locality but are well-defined in most developed countries, and at this time the HSCT cannot meet them. Furthermore, by the time an HSCT is flying, ground noise limits may be even more restrictive as individual communities become increasingly involved in setting their own noise levels.

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Aeronautical Technologies for the Twenty-First Century The propulsion system is the primary source of noise during HSCT operation: however, the choice of flight path can have an impact on the overall noise level at and around airports. Innovative propulsion concepts are required to reduce engine noise, and good high-lift aerodynamic characteristics can provide flight paths that minimize noise outside the airport boundaries. Interpretation of the noise regulations for HSCTs that would allow higher noise levels inside airport boundaries could result in decreased noise in the community around the airport. Sonic boom is probably the most controversial of the environmental issues. Yet it has a simple solution: never generate a sonic boom over land. However, significant costs are associated with scenarios that force supersonic aircraft to slow to subsonic speeds over land. If supersonic flights were allowed over land, the economic viability of the HSCT would be greatly enhanced because of the additional markets that could be served and would probably result in the construction of a greater number of aircraft. Additionally, designing an aircraft that shifts easily from supersonic to subsonic flight as it passes over land, while remaining competitive with advanced subsonic aircraft, is a significant engineering challenge. The alternative, of course, would be to provide an aircraft that produces a sonic boom level acceptable to the community. This is difficult, especially because the level the community can accept is unknown and surely varies from place to place. Current rules in most countries categorically prohibit aircraft from generating sonic booms over land. During the SST development program, there was a belief that a low sonic boom level (perhaps 1 pound per square foot) would potentially be acceptable for overland supersonic flight. However, there still is no standard defined for an acceptable sonic boom level, and it is doubtful that 1 pound per square foot would be acceptable today. Technologies to reduce the sonic boom level and shape the sonic boom signature so as to minimize the annoyance are required, along with a definition of the acceptable level. NASA'S CONTRIBUTIONS TO HIGH-SPEED CIVIL TRANSPORT Studies to date have shown that, if the cost is low enough, a substantial potential market exists for an HSCT. Foreign competitors are also working to capitalize on the market opportunity. The demand for an HSCT fleet, although substantial, may support only one development program. Potentially even worse for the United States than the loss of the HSCT market would be the associated reduction in the advanced subsonic transport market. The success of HSCT depends on economic viability and its environmental compatibility. However, current technology is insufficient to produce a feasible HSCT. An aggressive technology development program is necessary to optimize the likelihood of achieving an environmentally compatible and economically viable HSCT to meet market demand. The United States must be in a good programmatic and technical position to be a key player (either alone or in an international consortium) in order to participate in the HSCT program. Development of an HSCT represents a great deal more new development than the generally incremental improvements needed for short-haul and advanced subsonic aircraft. The primary issue facing the HSCT is its environmental compatibility—engine emissions, sonic

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Aeronautical Technologies for the Twenty-First Century boom, and engine noise. In light of this, the Committee did not go into detail in defining the cost, safety, and ATM system implications of an HSCT, so the following discussion concentrates on technologies that NASA can help develop to push the state of the art in performance and to meet environmental requirements. Still, it is the belief of the Committee that, since the ultimate goal of the HSCT is to capture a significant portion of the market currently held by subsonic transports and to open up new markets, the technologies that enhance safety and convenience, reduce cost, and allow the aircraft to mesh with the air traffic system cannot be ignored. So, the fact that these needs are not specifically called out in this section should not be construed to mean that they are unimportant—they were simply not dealt with at this early stage of HSCT development. Table 3-1 shows the current (1992) funding for HSCT by discipline. As shown in Table 3-1, the current emphasis in the NASA program is on the environmental compatibility of HSCT, and to a lesser extent on propulsion and materials technologies. Given the discussion in this chapter outlining the importance of solving the emissions and noise problems associated with supersonic flight, this emphasis is appropriate. TABLE 3-1 High-Speed Civil Transport Funding by Discipline Discipline Current NASA Program (millions of 1992 dollars, percent of total)a,b Environmental 59.9 64.5% Systems and operationsc 0.6 0.6% Aerodynamics 2.1 2.1% Propulsiond 19.4 19.4% Structures and materials 10.1 10.9% Controls, guidance, and human factorse 0.7 0.7% Total 92.8 100.0% Source: NASA Office of Aeronautics and Space Technology. a 1992 funding in real-year dollars, excluding fundamental research not tied to a specific application. b Percentages may not add to 100% due to roundoff error. c Includes both flight systems research and systems analysis studies. d Includes $16.5 million for propulsion enabling materials. e Includes the avionics and controls and cognitive engineering categories discussed in this report.

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Aeronautical Technologies for the Twenty-First Century Environment As mentioned in the section on barriers to HSCT development, environmental impact may be the single most important discriminator in the feasibility of an HSCT. NASA should continue to pursue technologies that reduce the environmental impact of HSCT and, thereby, eliminate a major barrier to its development. In particular, efforts to define acceptable sonic boom levels and research in mitigating ozone depletion may well provide the key factors that make an HSCT possible. NASA can, of course, make a major contribution to propulsion systems technologies that reduce both noise and emissions. This will require extensive development in the areas of high-temperature engine materials, cooling techniques, thrust and fuel management, and cycle performance improvements. In addition, inlet, engine, and nozzle integration depends on both analytical and experimental development of propulsion performance and control systems optimization. In aerodynamics, development of a wing with good takeoff lift performance will help reduce community noise by allowing a steeper climb-out and concentrating the noise on airport property. Also, reduction of sonic boom levels without affecting overall performance is a matter that NASA is well suited to address. Aircraft Performance Meeting the performance objectives of HSCT will require advances in four general areas: propulsion, aerodynamics, materials and structures , and overall vehicle management systems. The latter area would include the areas that are described in later sections of this report on advanced avionics and control systems, and cognitive engineering. NASA should continue to tailor its HSCT program to ensure that these four areas are adequately represented. Advances in the propulsion system will require an improved mixed flow turbofan or variable-cycle engine using advanced materials that can provide greatly reduced emissions and propulsion system weight, and greater fuel efficiency over the entire range of flight conditions. However, the annual fuel consumption of an HSCT is still likely to be twice that of an advanced subsonic transport aircraft.9 Technology to improve aerodynamic performance (lift-to-drag ratio) is a constant goal in the development of all aircraft. For supersonic cruise vehicles, high lift-to-drag ratios are critical. Laminar flow control is an especially promising technology because it has the potential to minimize overall drag by directly reducing the friction drag component, which is a third or more of the total aircraft drag and indirectly reducing shock wave drag by lowering aircraft 9   For example, according to projections by the McDonnell Douglas Aircraft Company in 1989 (Study of High-Speed Civil Transports, NASA Contractor Report 4235) a Mach 3 HSCT would require approximately 0.2 pound of fuel per available seat-mile per nautical mile of range (lb/ASM-nmi). An advanced subsonic transport aircraft would require only about 0.05 lb/ASM-nmi.

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Aeronautical Technologies for the Twenty-First Century weight and volume, because less fuel and smaller engines are required to overcome drag forces.10 Improved low-speed, high-lift aerodynamic performance for highly swept wings typical of a configuration designed for supersonic flight can positively impact noise. Although the propulsion system is the major contributor to noise during takeoff, as noted earlier the vehicle flight path and operations also affect noise impact on the community. The key materials requirement for HSCT aircraft is for long-term stability of thermal and mechanical properties at sustained elevated temperatures and after years of routing operations over many thousands of thermal cycles. The challenge is to achieve this stability with the minimum material density while being able to fabricate and assemble the parts at reasonable cost. Up to Mach 1.8, existing conventional metals and organic composites provide adequate performance. At Mach 2.4, titanium, reinforced aluminum, and high-temperature organic matrix composites provide adequate performance. The long-term stability and other durability and damage properties of the materials required for use at Mach 2.4 are not well understood, nor are the producibility characteristics well established. From a structural standpoint, there are three primary issues. The first is to employ structural concepts that are economically producible with the chosen material systems. The second is to employ structural concepts that minimize the effects of thermal gradients in the structure. This issue becomes more critical as speeds increase beyond Mach 1.8. The third issue, which applies to HSCT aircraft that cruise above 40,000 feet, is to provide pressurized fuselage concepts that allow survivable cabin decompression scenarios. Aircraft systems development is needed to provide the capabilities to operate the HSCT near optimum efficiency and to maintain safe operation. The technology for the HSCT includes flight and propulsion control systems, displays and navigational systems, and flight management of the aircraft. To obtain good performance characteristics, an integrated flight/propulsion control system that addresses propulsion system control, propulsion effects on stability, control during low-speed flight, and acceptable flying qualities at all speeds must be developed. Advancements in integrated flight-critical control systems, fiber optics, synthetic vision, and integrated flight deck displays and management systems offer significant opportunities for implementing a vehicle management system that provides improved performance and safety for an HSCT. 10   NASA Office of Aeronautics, Exploration and Technology. 1989. High-Speed Research Program, Briefing Book. Washington, D.C.

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