To identify the barriers to developing supersonic aircraft in the next 25 years for which advances in technology are needed, it is first necessary to identify the key customer and design requirements for useful aircraft and translate them into technology needs. The approach used here assesses the capabilities desired, defines notional vehicles providing such capabilities, and analyzes the technology needs of these vehicles.
Determining the level of technology required to meet customer requirements, ensure economic performance, and comply with environmental regulations requires some consideration of potential products. The process for creating product specifications consistent with satisfactory levels of performance in the above areas is complex, and the committee made no detailed attempts in this direction. However, committee members did interview domestic and international representatives, including business jet manufacturers and operators, large aircraft manufacturers, airlines, and the U.S. Air Force (USAF). The results of these interviews, combined with other information collected by the committee (summarized below), provided general guidance regarding the probable future of commercial and military supersonic aircraft.
Prospective manufacturers, with encouragement from business jet fleet operators, view the SBJ as a near-term prospect, with a payload of perhaps 8 to 15 passengers, a cruise speed of approximately Mach 1.6 to Mach 1.8, and a range of 4,000 to 5,000 nautical miles (NM). Supersonic flight over land is essential for this class of vehicles, and the potential market is estimated to be at least 200 aircraft over a 10-year period. Economic factors are not nearly as limiting for business jets as for a commercial transport; prospective manufacturers believe the market will support paying about twice as much for a supersonic aircraft that can cruise at twice the speed of current subsonic business jets.
The consensus view of prospective manufacturers is that the key technology barrier for this class of aircraft is the elimination, or reduction to acceptable levels, of sonic boom for flight over land. Another barrier is the need for an engine that can operate for 2,000 hours between major overhauls. Significant advances in aerodynamic performance may also be required to achieve the desired aircraft range if the solution to the sonic boom problem imposes aircraft weight or performance penalties.
Interviews with USAF personnel indicated that a long-range strike aircraft with sustained supersonic flight would provide significant potential for improving USAF war-fighting capabilities through substantially increased sortie rates and rapid response times. Desired characteristics are a payload of 20,000 to 40,000 pounds, a cruise speed of Mach 1.6 to 3.0, and a range of 4,000 to 6,000 NM. The precise Mach number is strongly influenced by optimization of total payload delivery rate (lb-NM/hour) for a given fleet cost. Long range is needed to reduce the need for forward basing and minimize the impact of overflight restrictions by countries not involved in a particular conflict. However, military aircraft would not necessarily need to meet all of the environmental constraints that apply to commercial aircraft.
The key technical challenges for a supersonic strike aircraft are as follows: development of a stealthy configuration with a high lift-to-drag ratio (L/D) and acceptable take-off and landing characteristics; efficient and durable engines; propulsion-airframe integration; advanced airframe materials and structures; weapons integration; and producibility.
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Commercial Supersonic Technology: The Way Ahead 2 Technology Challenges To identify the barriers to developing supersonic aircraft in the next 25 years for which advances in technology are needed, it is first necessary to identify the key customer and design requirements for useful aircraft and translate them into technology needs. The approach used here assesses the capabilities desired, defines notional vehicles providing such capabilities, and analyzes the technology needs of these vehicles. CUSTOMER REQUIREMENTS Determining the level of technology required to meet customer requirements, ensure economic performance, and comply with environmental regulations requires some consideration of potential products. The process for creating product specifications consistent with satisfactory levels of performance in the above areas is complex, and the committee made no detailed attempts in this direction. However, committee members did interview domestic and international representatives, including business jet manufacturers and operators, large aircraft manufacturers, airlines, and the U.S. Air Force (USAF). The results of these interviews, combined with other information collected by the committee (summarized below), provided general guidance regarding the probable future of commercial and military supersonic aircraft. Supersonic Business Jet Prospective manufacturers, with encouragement from business jet fleet operators, view the SBJ as a near-term prospect, with a payload of perhaps 8 to 15 passengers, a cruise speed of approximately Mach 1.6 to Mach 1.8, and a range of 4,000 to 5,000 nautical miles (NM). Supersonic flight over land is essential for this class of vehicles, and the potential market is estimated to be at least 200 aircraft over a 10-year period. Economic factors are not nearly as limiting for business jets as for a commercial transport; prospective manufacturers believe the market will support paying about twice as much for a supersonic aircraft that can cruise at twice the speed of current subsonic business jets. The consensus view of prospective manufacturers is that the key technology barrier for this class of aircraft is the elimination, or reduction to acceptable levels, of sonic boom for flight over land. Another barrier is the need for an engine that can operate for 2,000 hours between major overhauls. Significant advances in aerodynamic performance may also be required to achieve the desired aircraft range if the solution to the sonic boom problem imposes aircraft weight or performance penalties. Military Strike Aircraft Interviews with USAF personnel indicated that a long-range strike aircraft with sustained supersonic flight would provide significant potential for improving USAF war-fighting capabilities through substantially increased sortie rates and rapid response times. Desired characteristics are a payload of 20,000 to 40,000 pounds, a cruise speed of Mach 1.6 to 3.0, and a range of 4,000 to 6,000 NM. The precise Mach number is strongly influenced by optimization of total payload delivery rate (lb-NM/hour) for a given fleet cost. Long range is needed to reduce the need for forward basing and minimize the impact of overflight restrictions by countries not involved in a particular conflict. However, military aircraft would not necessarily need to meet all of the environmental constraints that apply to commercial aircraft. The key technical challenges for a supersonic strike aircraft are as follows: development of a stealthy configuration with a high lift-to-drag ratio (L/D) and acceptable take-off and landing characteristics; efficient and durable engines; propulsion-airframe integration; advanced airframe materials and structures; weapons integration; and producibility.
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Commercial Supersonic Technology: The Way Ahead Supersonic Commercial Transport The desired payload for a high-speed civil transport (HSCT) would be about 300 passengers, and the desired range would be at least 4,500 NM, with 5,000 to 6,000 NM preferred. Sonic boom reduction would not be required to provide trans-oceanic service. The minimum expected cruise speed is approximately Mach 2.0, based in part on earlier studies of aircraft utilization on long international routes. A cruise Mach number close to 2 might be acceptable to users (i.e., airlines) and would also mitigate some of the technological challenges and environmental concerns associated with Mach numbers of 2.4 and greater. A large transport, such as an HSCT, that can fly at Mach 2 or greater with little or no sonic boom, a capability necessary for overland operations, is not viewed as technologically feasible. One alternative for providing supersonic airline service over land would be to develop a large commercial supersonic aircraft with a cruise speed close enough to Mach 1 to avoid creating a sonic boom that would propagate to the ground. Boeing is currently conducting design studies for such an aircraft, which could probably be developed without government research into the breakthrough technologies that are the subject of this report. A second alternative for improving the economics of a large supersonic transport would be to design it for flight at a high cruise speed over water and a lower (but still supersonic) cruise speed over land. This scenario, however, raises questions about performance efficiency during long flight segments at off-design speeds. Early in the HSR Program, for example, Boeing explored the option of aerodynamically shaping a Mach 2.4 aircraft to produce a low-level boom for overland flight at Mach 1.6 to Mach 1.8. This effort was dropped when it became clear that the necessary design modifications would significantly degrade performance at Mach 2.4 and reduce the overall economic viability of the aircraft. It might be worthwhile, however, to reexamine this area using advanced technologies and lower cruise speeds (for example, Mach 2.0 over water/Mach 1.2 over land instead of Mach 2.4 over water/Mach 1.6 over land). A third alternative would be to build a commercial supersonic transport with a payload capability similar to that of a military strike aircraft (equivalent to 100 passengers), a range capability similar to that of an SBJ, and a cruise speed of approximately Mach 2. Compared to an HSCT, the reduced size and potentially lower speed would make it much more feasible to develop the technologies necessary to reduce the sonic boom enough to permit overland operations. This transport would be capable of transcontinental service as well as transoceanic service directly to and from noncoastal cities. A 1997 study by the NRC reviewed demand studies that predicted a market size on the order of 1,000 HSCTs, assuming that targeted levels of cost, performance, and environmental impacts can be achieved (NRC, 1997). The demand studies also assumed that sonic booms would prevent these aircraft from flying at supersonic speeds over land and that they could be operated profitably with a ticket surcharge of about 10 percent (relative to the price of travel on subsonic aircraft) for coach class travel and a surcharge of 30 percent for business and first class travel. However, generalizations in the assumptions on which some of the demand studies were based may have caused the studies to overstate the projected market size. The 1997 NRC study concluded that turn-around times seemed to be unrealistically low and that an aircraft with a cruise speed of Mach 2.0 might have a productivity similar to that of a Mach 2.4 aircraft (NRC, 1997). A cruise speed of Mach 2.2 or less would also be a more tractable goal than Mach 2.4 for the 25-year period of interest to this study. VEHICLE CHARACTERISTICS AND ECONOMIC GOALS The above-mentioned views of customer requirements form the basis for defining three notional commercial supersonic aircraft: an SBJ, an overland supersonic commercial transport, and an HSCT. These generic aircraft were selected as being representative of the complete spectrum of supersonic aircraft likely to be developed in the foreseeable future. For example, the committee determined that the speed, range, and payload characteristics of an overland supersonic transport would be similar to those of a nominal supersonic strike aircraft. The purpose of these notional vehicles is to help assess the need for advances in the technological state of the art; they are not intended to endorse any particular product or replace the need for detailed design and market studies to validate the vehicle performance specifications prior to advanced product development. Implicit in aircraft design specifications are costs—development, production, operations, and maintenance costs— that are considered affordable. Each of these costs depends on many factors. The 1997 NRC report identified 22 factors that impact vehicle affordability. For the purpose of this study, takeoff gross weight (TOGW) was used as one readily measurable indicator of aircraft cost for rapid assessment of configuration trade studies. Given this simplification, affordability is tied to the ratio of payload weight to TOGW that is considered to be economically viable (or militarily cost-effective) in the applications of interest identified above. For each of the three notional commercial supersonic aircraft, the committee used a combination of engineering judgment, historical trends, and simplified equations to identify vehicle characteristics and the technology goals that must be achieved to satisfy requirements for an environmentally acceptable and economically viable aircraft (see Table 2-1 and Figure 2-1). Overland transport aircraft (and comparably sized military strike aircraft) will require improvements equivalent to about 10 percent over the present state of the art in the four most important factors related to economics (L/D, air vehicle empty weight fraction, specific fuel consumption, and
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Commercial Supersonic Technology: The Way Ahead TABLE 2-1 Customer Requirements, Vehicle Characteristics, and Technology Goals for Economic and Environmental Performance of Notional Aircraft Supersonic Business Jet Overland Supersonic Commercial Transport High-Speed Civil Transport State of the Arta Customer requirements Speed (Mach number) 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 possibleb No Vehicle characteristics Payload weight fractionc ~0.07 0.15 to 0.20 ~0.20 Aircraft empty weight fractiond ~0.44 ~0.40 ~0.37 Vehicle empty weight fractione ~0.38 ~0.34 ~0.32 ~0.36 (larger aircraft) to 0.38 (smaller aircraft) Fuel weight fraction ~0.49 0.40 to 0.45 ~0.43 Takeoff gross weight (1,000 lb) 140 200 to 250 600 Technology goals Economic performance Lift-to-drag ratio 7.5 to 8.0 9 to 10 10 to 11 ~7.5 to 8.5 TSFC/M (lb/hr/lb/Mach number)f ~0.60 ~0.52 ~0.49 ~0.60 (Mach 1.6) to 0.55 (Mach 2.4) Engine thrust-to-weight ratio at sea level 5 5 6 ~4 (for large engines) to 5 (for small engines) Environmental performanceg Community noise less than Stage 3h less than Stage 3 less than Stage 3 Stage 3 Sonic boom overpressure (psf) <1 (with a shaped signature)i <1 (with a shaped signature) <1 (with a shaped signature)b ~2 for large aircraft, ~1 for small aircraft NOx emissions index at cruise (g NOx/kg fuel)j <15 <15 <15 (lower speeds), ≤5 (higher speeds) ~25k Water vapor emissions index (g water/kg fuel)l ~1,400 ~1,400 ~1,400 for lower speeds, possibly 0 at higher speeds ~1,400 aState of the art is estimated for technologies that have matured to a TRL of 6 or higher. bOnly if intended for supersonic flight operations over land. Otherwise, sonic boom levels are not limiting. cWeight of the payload divided by TOGW; payload is defined here as everything not necessary for controlled flight, including avionics (except the flight control system), mission equipment, and outfitting. dWeight of the aircraft with no fuel or payload divided by TOGW. eWeight of the aircraft with no fuel or payload or engines divided by TOGW. fThrust-specific fuel consumption divided by Mach number (TSFC/M) is inversely proportional to overall propulsion system efficiency. In principle, for a given propulsion system state of the art, TSFC/M varies approximately as the one-quarter power of the Mach number over the range of speed (Mach 1.6 to 2.4) of interest here. However, this has not been demonstrated in operational engines. gCO2 is an environmental constraint because it influences climate change, but CO2 is not listed here because it is likely to be controlled by limiting total fuel consumption, not by imposing limits on emissions by individual aircraft. hCurrent U.S. and international limits on noise for subsonic aircraft during takeoff, climb-out, and approach to landing are referred to as Stage 3 limits. Quieter Stage 4 limits are already under review. iSonic booms have a pressure wave with a very rapid rise time. Shaping of sonic booms would increase the rise time of the sonic boom pressure wave, reducing the effect of booms on people for a given overpressure limit. However, even with a shaped signature the maximum acceptable overpressure is unknown, although it seems certain to be less than 1 pound per square foot (psf). jNOx emissions are important to ozone depletion, local air quality, and climate change. kState of the art for supersonic engines is about 25. Most commercial jet aircraft have an NOx emissions index of about 7 to 15 (MCT, 2001). lWater vapor emissions in the stratosphere are important to ozone depletion and climate change.
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Commercial Supersonic Technology: The Way Ahead FIGURE 2-1 Weight distributions of selected aircraft. thrust-to-weight ratio), 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 even more significant advances, equivalent to about a 15 percent improvement for each of the four most important economic parameters. ENVIRONMENTAL GOALS Real or perceived environmental requirements result in technical barriers to the development of new products. The potential uncertainty associated with environmental effects and regulations pose nontechnical barriers as well, because industry is unlikely to commit to product development without certain knowledge of the environmental standards that must be satisfied. Some of the relevant environmental regulations are summarized in Table 2-2. In general, existing regulations were not developed with supersonic aircraft in mind and would have to be updated before a new commercial supersonic aircraft could be certificated. All of these issues will have to be coordinated domestically with the appropriate federal agencies and internationally with the International Civil Aviation Organization (ICAO). Technology goals associated with environmental performance of commercial supersonic aircraft are shown in Table 2-1. As discussed below, the major environmental concerns TABLE 2-2 Environmental Regulations Relevant to Commercial Supersonic Aircraft Environmental Issue Current or Expected Method of Control International Regulations and Authorities U.S. Regulation and Authorities Community noise operating restrictions Aircraft certification standards, ICAO (Annex 16, Vol. I) 14 CFR Part 36 and 14 CFR Part 91 Sonic boom Operating restrictions ICAO (Resolution A33-7) 14 CFR Part 91 Climate change Aircraft certification standards, market-based measures (emissions trading or charges) United Nations Framework Convention on Climate Change and ICAO (under Kyoto Protocol, if ratified) 14 CFR Part 34 Ozone depletion Operating restrictions Montreal Protocol Section 615 of the Clean Air Act Local air quality Aircraft certification standards ICAO (Annex 16, Vol. II) 14 CFR Part 34 and 40 CFR Part 87
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Commercial Supersonic Technology: The Way Ahead are community noise around airports, sonic boom (which prevents supersonic flight over land), climate change, depletion of atmospheric ozone, and local air quality. Reducing sonic boom enough to meet the needs of SBJs or medium-size overland transports could be very difficult. Continuation of ongoing research is likely to reduce emissions enough to meet goals both for local air quality and for emissions during cruise for aircraft with cruise speeds less than approximately Mach 2. Cruise emissions goals for aircraft with higher cruise speeds will be considerably more difficult to meet, and noise suppression will need attention at all Mach numbers. Community Noise Commercial aircraft must meet community noise standards for takeoff, climb-out (after takeoff), and approach to landing. For subsonic aircraft, requirements are defined by so-called Stage 3 standards. ICAO standards for supersonic aircraft are not in place, but ICAO Annex 16 Volume I indicates that “noise levels . . . applicable to subsonic jet aeroplanes may be used as guidelines.” Except for existing Concorde aircraft, federal regulations prohibit the operation of commercial supersonic aircraft unless they comply with Stage 2 noise limits (14 CFR Part 91). Federal regulations allow existing Concorde aircraft to operate as long as (1) the noise levels are shown to have been reduced “to the lowest levels that are economically reasonable, technologically practicable, and appropriate for the Concorde type design,” and (2) the aircraft does not take off or land between the hours of 10 p.m. and 7 a.m. (14 CFR Part 36). Environmental standards have historically been governed by what can be accomplished within the economic constraints of the industries involved, and more stringent standards are already under development by ICAO’s Civil Aviation Environmental Protection (CAEP) Committee: Stage 4 limits are likely to reduce noise limits during takeoff, climb, and approach to landing by about 3 dB at each point. Presumably, these new ICAO limits would be used as guidelines for revising community noise limits in the United States and other countries, but details, including how these standards would be applied to supersonic aircraft—would have to be coordinated with regulatory agencies, such as the FAA, in each country. Sonic Boom ICAO policy requires aircraft operators to ensure that “no unacceptable situation for the public is created by sonic boom from supersonic aircraft in commercial service” (ICAO Resolution A33-7, 1998). This seems to leave room for determining an “acceptable” level for a sonic boom. U.S. Federal Aviation Regulations (FARs), however, prohibit commercial flight at greater than Mach 1 over the United States and require that commercial supersonic aircraft, such as the Concorde, that fly to and from the United States impose flight restrictions to ensure that they do not “cause a sonic boom to reach the surface within the United States” (14 CFR Part 91).1 These regulations will have to be changed to allow supersonic flight over land. Except for supersonic speeds close to Mach 1 (i.e., Mach numbers between 1 and about 1.15), sonic boom reduction technology is unlikely to eliminate sonic booms in the foreseeable future. As discussed in Chapter 3, a more reasonable aim for sonic boom reduction is to produce “shaped” sonic signatures with an over-pressure limit somewhere between 0 and 1.0 lb/ft2 throughout the total sonic boom ground footprint (not just below the aircraft). Low-boom, shaped signatures are intended to reduce annoyance and minimize the possibility of damaging structures by reducing startle, rattle, and building vibrations. Additional research is needed, however, to ascertain the limits on sonic boom parameters, such as overpressure and rise time, that would be acceptable to the general public. Research must also validate the ability to design aircraft that (1) create shaped booms within acceptable limits and (2) still meet other economic and environmental goals. These advances are necessary for both the SBJ and the medium-size overland transport, since both require flight over land and would be beneficial for the HSCT. Climate Change Under the Kyoto Protocol, ICAO was given the responsibility for control of aircraft emissions that affect climate change. Accordingly, CAEP is examining methods to control aircraft emissions that can affect climate. The primary focus now is carbon dioxide (CO2). The Kyoto Protocol did not classify water or oxides of nitrogen (NOx) as greenhouse gases, but their treatment is being discussed in ICAO. Because of water vapor emissions, atmospheric models generally predict that an aircraft cruising in the stratosphere can have climate effects several times as severe as a similar aircraft flying at lower altitudes (in the troposphere), where water vapor emissions are benign. Emissions of CO2 are directly related to fuel consumption, and the methods of control being considered by CAEP are all market-based options, such as a system of CO2 trading (similar to current SO2 trading programs in the United States) or fees based on fuel consumption. If such programs are implemented in a way that increases fuel costs, they would increase economic barriers for supersonic aircraft, which consume more fuel per passenger seat mile than sub- 1 The FAA is authorized to allow supersonic flight over land for testing— if over-water flights are not practical (14 CFR Part 91). Also, military aircraft may fly at supersonic speeds in authorized training areas, but environmental impact statements may be required to establish or expand training areas if supersonic flight will be allowed at altitudes less than 30,000 ft over land (or within 15 miles of land) or at altitudes less than 10,000 ft over water (32 CFR Part 989).
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Commercial Supersonic Technology: The Way Ahead sonic aircraft.2 In addition, uncertainties about how CO2, water vapor, and NOx may be regulated in the future add to uncertainties about the economic viability of commercial supersonic aircraft. ICAO is currently developing a methodology to characterize aircraft emissions during the climb and cruise phases of flight, and the initial focus is on NOx emissions. The methodology will consider emissions and aircraft productivity, probably in a format such as grams of pollutant per passenger-mile or ton-mile of payload. Details of the methodology and a typical range of values for subsonic aircraft are expected to be discussed at the sixth CAEP meeting, which will probably occur in 2004, and a standard could be established in the 2010 to 2015 time frame. Supersonic aircraft developed during and after that time may be required to meet the same standards as subsonic aircraft. That could be difficult, because the increased fuel consumption and higher engine operating temperatures that occur during supersonic cruise tend to increase NOx emissions. Ozone Depletion The stratospheric ozone layer absorbs ultraviolet radiation that can cause skin cancer. Many compounds that deplete the ozone layer, such as chlorofluorocarbons, are controlled under the Montreal Protocol. NOx emissions by aircraft can impact atmospheric ozone in either positive or negative ways, depending on the altitude at which they are emitted. There is considerable uncertainty in the estimates of the actual impact, but it is generally agreed that emissions at altitudes above 50,000 to 55,000 ft will degrade the ozone layer to some degree. Concern about ozone destruction by NOx emitted from supersonic aircraft flying in the stratosphere dates back to the U.S. SST program in the early 1970s and was acknowledged by the HSR Program, which established an NOx emissions index goal of 5 (i.e., 5 g of NOx produced for each kilogram of fuel burned during cruise). This still appears to be an acceptable upper limit for commercial supersonic aircraft operating in the stratosphere. For aircraft at lower supersonic speeds (i.e., less than approximately Mach 2), NOx emissions would be less of a problem because of their lower cruising altitudes, and a higher emissions index might be acceptable. Recent analyses (e.g., Kawa et al., 1999) indicate that water emissions from a fleet of large commercial supersonic aircraft operating in the stratosphere would significantly deplete ozone, even if NOx could be reduced to very low levels. Although the analytical results have considerable uncertainty, effects would be substantially smaller for a smaller fleet of smaller aircraft and could be eliminated by flying at a lower altitude consistent with a lower cruise speed. Local Air Quality ICAO standards for both subsonic and supersonic aircraft cover emissions of NOx, unburned hydrocarbons (HC), carbon monoxide (CO), and visible smoke. Standards for supersonic aircraft were based on the Concorde emissions levels and therefore apply to an engine that uses an afterburner during takeoff. Engines with afterburners generally produce more CO and less NOx than typical subsonic jet engines. Presumably, the next generation of supersonic transports will not use afterburners, so emissions should be more like those of subsonic aircraft. Since the current supersonic standard was set, there have been substantial reductions in emissions of all of these species, and NOx standards for subsonic aircraft has been reduced twice. If a new supersonic aircraft is introduced into service, ICAO standards would very likely be reexamined in light of these improvements in subsonic engine technology. To minimize opposition on environmental grounds, new supersonic aircraft may be required to meet the same standards as subsonic aircraft. TECHNOLOGY CHALLENGES The key technology challenges that derive from the customer requirements, vehicle characteristics, and technology goals quantified in Table 2-1 are related either to economics, the environment, or certification: Environment benign effect on climate and atmospheric ozone low landing and takeoff noise low sonic boom Economics—range, payload, fuel burn, etc. low weight and low empty weight fraction improved aerodynamic performance highly integrated airframe/propulsion systems low thrust-specific fuel consumption (TSFC) long life Certification for commercial operations acceptable handling and ride qualities passenger and crew safety at high altitudes reliability of advanced technologies, including synthetic vision technical justification for revising regulations to allow supersonic operations over land These challenges are discussed in more detail in Chapters 3 and 4. All of the challenges must be overcome to enable viable commercial supersonic aircraft. Significant advances in the traditional aeronautics engineering disciplines, such as structures, propulsion, and aerodynamics, are still required 2 Passenger seat mile is a unit of transportation capacity, whereas passenger mile is a measure of service provided. An aircraft with 100 seats scheduled to fly a 200-mile route would have a capacity of 2,000 passenger seat miles. If 50 passengers make the trip, only half of the capacity is used (1,000 passenger miles).
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Commercial Supersonic Technology: The Way Ahead to create a situation in which (1) the economic factors will support a profitable business case and (2) certification issues for new designs and systems can be resolved. Of the various vehicle types examined by the committee, the technology advances necessary to attain the required economic and environmental goals will be easiest to achieve for an SBJ and most difficult for an HSCT. The economic goals for the SBJ are easiest to achieve because it can tolerate a very low payload fraction, a shorter range capability, and a modest cruise speed. Economically viable sonic boom reduction is also likely to be easier to accomplish for an SBJ than for a medium-size overland transport. The economic goals for the HSCT will be the most difficult to achieve because it needs the highest payload fraction and the longest range. Furthermore, if a cruise speed greater than approximately Mach 2 is selected, the environmental goals would be much harder to achieve. The above discussion has been limited to aircraft powered by gas-turbine engines using conventional hydrocarbon fuels. The potential use of other power plants and/or other fuels would change the tradeoffs among the various technology parameters, but it would be unlikely to reduce the gap between what is needed and what is currently available. For example, the use of liquid hydrogen as a fuel has been examined in the past and is a prospect for the future. Because hydrogen offers high energy per unit mass of fuel, it reduces TSFC to about 40 percent of the consumption for a conventional hydrocarbon fuel. However, hydrogen’s low density requires large fuel storage volumes, which would increase the air vehicle empty weight fraction, decrease the L/D, and increase sonic boom. Thus, in the terminology used here, the technology requirements would change, but the challenges would not diminish. Similar observations can be made about the potential use of fuel cells as aircraft power plants. In principle, fuel cells offer perhaps a 20 percent reduction in TSFC compared with turbofan engines, but at the expense of a considerably heavier power plant. Accordingly, the needed values of TSFC and thrust-to-weight ratio will change (the former will be lower and the latter higher than those shown in Table 2-1), but the challenges will not diminish. Certification and regulatory issues are especially important in two respects: (1) to enable the use of new systems, such as synthetic vision in the cockpit, that relate to passenger and public safety and (2) to allow overland supersonic flight if an aircraft with a low, but nonzero, sonic boom can be developed. REFERENCES Kawa, S., J. Anderson, S. Baughcum, C. Brock, W. Brune, R. Cohen, D. Kinnison, P. Newman, J. Rodriguez, R. Stolarski, D. Waugh, and S. Wolfsy. 1999. Assessment of the Effects of High-Speed Aircraft in the Stratosphere. Greenbelt, Md.: Goddard Space Flight Center. MCT (Ministério da Ciência e Tecnologia)(Brazil) 2001. Inventory of Emissions per Type of Aircraft in 1995. Available online at <http://www.mct.gov.br/clima/ingles/comunic_old/iac01.htm>. Accessed on July 10, 2001. NRC (National Research Council), Aeronautics and Space Engineering Board. 1997. U.S. Supersonic Commercial Aircraft: Assessing NASA’s High Speed Research Program. Available online at <http://www.nap.edu/catalog/5848.html>. Accessed on September 13, 2001.