Areas Needing Continued Technical Development
Chapter 3 describes the need for focused research initiatives to pursue breakthrough opportunities. A broad range of technical activity must be supported to ensure that the technologies discussed in this chapter are mature enough to convince industry to develop them and meet designer’s needs at the time a supersonic transport is laid down. Technology requirements include reducing propulsion noise and controlling emissions of carbon dioxide, various oxides of nitrogen, and, possibly, water vapor (at very high altitudes); structures and materials that can withstand the more hostile environment resulting from supersonic flight; cockpit displays and controls compatible with the special design features and operational requirements of commercial supersonic aircraft; and systems and procedures to address safety issues associated with cabin depressurization and exposure to ionizing radiation during high-altitude flights. Although advances in the above technologies may not be necessary to develop an SBJ, they are critical to the ultimate goal of developing a large commercial supersonic transport. Each of these topics is discussed in more detail below.
PROPULSION EMISSIONS AND NOISE
Significant advances in propulsion technologies over the levels currently available are needed to meet environmental limits for NOx, CO2, water vapor, and airport noise. At the same time, the propulsion systems must be lightweight and demonstrate both high propulsive efficiency and low fuel consumption if the supersonic transport is to have enough range and payload to be an economic success.
Gas Turbine Engines
Key propulsion system parameters include TSFC (thrust-specific fuel consumption), thrust-to-weight ratio, engine life, noise, and emissions. As shown in Table 2-1, goals for these parameters are easier to achieve for aircraft with lower cruise speeds and smaller size. The committee believes the propulsion system technology available today is sufficiently advanced to allow industry to develop the propulsion system for an SBJ with a Mach number of about 1.6. On the other hand, achieving the goals for TSFC and thrust-to-weight ratio for a Mach 2.4 HSCT would require greater propulsive efficiency and component performance, including an engine nozzle that weighs 75 percent less than current, state-of-the-art designs. Some type of variable cycle engine, as described in Chapter 3, is likely to be needed if most of the propulsion system performance goals are to be met.
From an emissions point of view, supersonic aircraft differ from subsonic aircraft in three respects. First, a supersonic engine will generally operate at higher temperatures. Second, even if the propulsion goals for efficiency and emissions index are achieved (i.e., about 800 lb of fuel per passenger for supersonic flights of 5,000 to 6,000 NM compared with 500 lb of fuel per passenger for a subsonic flight of equal distance), a supersonic aircraft will create more combustion products than a subsonic aircraft. Third, the most fuel-efficient cruise altitude increases with speed. Supersonic aircraft will therefore cruise at higher altitudes than subsonic aircraft, and the products of combustion will be emitted into a different region of the atmosphere, with detrimental consequences, as described below.
While all of the differences become more significant as flight speed is increased, the most important issue is probably flight altitude. The altitude of the tropopause, which separates the troposphere from the stratosphere, changes with the season and latitude, varying between about 30,000 ft at the poles and about 60,000 ft at the equator. Most subsonic flight takes place in the troposphere (at altitudes below about 40,000 ft), whereas most supersonic flight takes place in the stratosphere. A commercial supersonic aircraft opti-
mized to fly at Mach 1.6 will operate at an altitude of about 50,000 ft, whereas a transport designed to fly at Mach 2.4 would ideally operate at about 60,000 ft.
Three issues associated with engine emissions are particularly important for a commercial supersonic aircraft: stratospheric ozone, climate change, and local air quality.
Atmospheric ozone concentration is shown in Figure 4-1, along with typical cruise altitudes for supersonic and subsonic aircraft. The concentration of ozone peaks at about 70,000 ft. This stratospheric ozone layer is important in that it absorbs ultraviolet (UV) radiation that can lead to skin cancer. Aircraft emissions that affect atmospheric ozone concentrations include NOx, water, and particles or particle precursors. Because of differences in atmospheric chemistry at different flight altitudes, emissions from subsonic aircraft tend to increase ozone concentrations, while those from supersonic aircraft reduce ozone.
Concerns about the effects of NOx emissions on ozone depletion have intersected with plans for the development of supersonic aircraft since the U.S. SST program in the 1960s and early 1970s. NOx takes part in catalytic chemical reactions that destroy ozone in the atmosphere. The IPCC Special Report on Aviation (IPCC, 1999) compared two scenarios for UV radiation in the year 2050. The first assumed an all-subsonic fleet of commercial airlines. The second assumed that a fleet of 1,000 Mach 2.4 HSCTs would replace 11 percent of the subsonic fleet. The net effect of the supersonic fleet is predicted to be a reduction in column ozone of 1.3 percent. The health impacts of this reduction were estimated by the Environmental Protection Agency: a total of 7,100 deaths in the United States among people born between 1980 and 2000. The uncertainty for this study was substantial, with a range of –5,000 to +36,000 (EPA, 2001).
Recent studies indicate that the effect of NOx is smaller than had been thought in the 1970s, and the use of combustors with an emissions index of about 5 (i.e., 5 g of NOx produced for every kilogram of fuel burned) may be sufficient to keep NOx within acceptable limits. Other studies, however, have raised another concern: Emitting water into the stratosphere destroys ozone because it affects the composition, growth, and reactivity of aerosol reactions and provides a source of HOx radicals that enhance ozone loss (Kawa et al., 1999). This is a serious complication, because water vapor is an essential combustion product of hydrocarbon fuels. Furthermore, the stratosphere naturally contains very
little water and the rate of atmospheric exchange between the troposphere and stratosphere is very low. Thus, water emitted by aircraft into the stratosphere can have significant, long-lasting effects on the environment.
The effect of emissions from supersonic aircraft flying at a given altitude and route structure will be roughly proportional to the amount of fuel burned. If the size or number of supersonic aircraft is smaller than assumed in the IPCC report, the effect on column ozone would be less. The depletion of stratospheric ozone due to emissions from a fleet of supersonic aircraft could also be mitigated by cruising at a lower altitude. Extrapolation of the data plotted in Figure 4-2 suggests that ozone depletion could be eliminated by reducing average cruise altitude from 60,000 ft to about 50,000 ft. However, the uncertainty of results produced by current atmospheric models is still substantial. Continued development of these models is crucial to ensure that the environmental impacts of a future fleet of commercial supersonic aircraft can be accurately predicted.
Achieving an emissions index of 5 represents a reduction of 70 to 90 percent relative to current engines operating at similar conditions. NASA’s HSR Program conducted small-scale steady-state and transient tests of combustor rigs to demonstrate that emissions indexes below 5 g/kg could be achieved. However, the program was terminated before ground and flight tests of an integrated engine could demonstrate the performance of the low emissions combustor technology in a flight engine that could also meet other essential performance parameters, such as thrust-to-weight ratio, TSFC, and reliability.
Another option for mitigating the effect of supersonic aircraft emissions on ozone would be to reduce the amount of sulfur in jet fuel. This would reduce the production of aerosol particles and increase fuel costs. The IPCC report assumed no change in the current jet fuel sulfur content, which is 0.05 to 0.07 percent by weight.
Climate changes can occur as a result of radiative forcing (i.e., changes in the global balance between incoming solar radiation and outgoing infrared radiation). Figure 4-3 shows the estimated impact on radiative forcing of a mixed commercial fleet of supersonic and subsonic aircraft in the year 2050, and Figure 4-4 compares the radiative forcing in 1992 with the radiative forcing in 2050 for both an all subsonic fleet and a mixed subsonic-supersonic fleet. With the mixed supersonic-subsonic fleet, radiative forcing from aircraft is
estimated to be 42 percent greater than with an all-subsonic fleet, because each supersonic aircraft at 62,000 ft increases radiative forcing by a factor of about five compared to equivalent subsonic aircraft flying at lower altitudes. However, as shown in Figures 4-3 and 4-4, the uncertainty associated with the estimates is very large.1 The estimated radiative forcing effect of the mixed fleet in 2050 ranges from about 0.15 to 0.6 watts per square meter. Reducing this large uncertainty is essential to developing a better understanding of the environmental challenges posed by commercial supersonic aircraft.
The disproportionate effect of the supersonic transport fleet on radiative forcing is primarily due to larger emissions of water in the stratosphere and the lack of vertical mixing between the stratosphere and troposphere. As indicated in Figure 4-3, there is a large uncertainty (by a factor of about 3) surrounding the climatological impact of radiative forcing from aircraft water emissions. Better atmospheric models are needed to assess potential climate changes resulting from supersonic civil transports.
If the effects of water emissions in the stratosphere prove to be unacceptable, a technological breakthrough would be required. Some options to eliminate water emissions include the following:
Emit water in a form that settles into the troposphere (e.g., as large droplets or ice particles).
Remove and collect water from emissions and store it until it can be released into the troposphere.
Use a fuel that does not contain hydrogen so that no water is formed.
Generate thrust without chemical reactions onboard the aircraft.
The committee did not identify any efforts to develop any of these approaches for supersonic aircraft applications, and even with a focused effort a practical system is unlikely to be demonstrated for decades. Alternative propulsion technologies are discussed in more detail below.
Local Air Quality
The primary concern with aviation’s effect on local air quality is the emission of NOx and unburned hydrocarbons
For the scenarios depicted in Figures 4-3 and 4-4, the bars indicate the best estimate of forcing, while the line associated with each bar depicts a two-thirds uncertainty range, meaning there is one chance in three that the true value falls outside the ranges shown. Available information on cirrus clouds was judged to be insufficient to determine either a best estimate or an uncertainty range; the vertical line indicates a range of possible best estimates. For the supersonic scenarios in 2050, supersonic aircraft are assumed to replace part of the subsonic fleet, reducing emissions from subsonic aircraft by 11 percent (IPCC, 1999).
near the ground (below 3,000 ft), because both can lead to ozone formation. (Near the ground, ozone is viewed as a pollutant because it is harmful to inhale and contributes to the formation of smog.) Assuming that future supersonic aircraft will not use afterburners (which are very fuel inefficient), a key difference between a supersonic aircraft and a subsonic aircraft with the same payload is that the supersonic aircraft will burn more fuel because aircraft designed for high-altitude supersonic flight are generally much less efficient during low-speed operations near the ground. Future supersonic aircraft are unlikely to receive exemptions from the environmental standards imposed on subsonic aircraft, so the need to minimize the impact on local air quality provides another reason to develop advanced supersonic propulsion technology.
Community noise is dominated by aircraft operations during takeoff, climb, and landing approach. It can be reduced by improving (1) propulsion systems, (2) the aerodynamic design of aircraft, and (3) the manner in which aircraft are operated.
Community noise from supersonic aircraft results primarily from mixing of the high-velocity jet exhaust with ambient air. Noise from the rotating blades of fans and compressors radiates from the inlet and contributes to the overall engine noise signature. Federal and local rules limit noise and, in some cases, restrict or prohibit operations of certain aircraft because of the noise they make. If future supersonic aircraft are required to meet the same standards as subsonic aircraft, they may need to be at least 3 to 4 dB quieter than current Stage 3 standards during takeoff, climb, and approach to landing.
At cruise Mach numbers of about 1.2 to 2.0, engine cycles with high bypass ratios (1.5 to 3.0), including variable cycle engines, can be used to reduce community noise while still meeting thrust requirements during takeoff and climb and propulsion efficiency requirements during cruise. This becomes more difficult for aircraft with cruise Mach numbers above 2.0; cruise efficiency requirements mandate the use of lower bypass ratios and, thus, higher jet exhaust velocities.
Meeting noise standards then becomes increasingly difficult without a nozzle that is too large and heavy to be economical.
Other alternatives for meeting noise standards include the use of inlet choking to reduce inlet-fan noise on landing approach, injection of fluids into the exhaust, and active controls. NASA should consider expanding subscale acoustic tests on these alternatives, along with experimental activities on acoustic liners for exhaust ejectors.
Meeting noise standards will also require careful attention to operational procedures and low-speed aerodynamic characteristics. For example, noise could be reduced by changing Federal Aviation Regulations to allow the use of advanced/automated takeoff and climb procedures, including automatic variation of throttle setting. One- or two-segment, steep-decelerating landing approaches could also be used to reduce approach noise.
Aircraft designs with low aspect ratios and low wing loading are compatible with steep approaches to landing and have a large ground effect when the airplane is about a wingspan from the ground, increasing lift by about 20 percent. New, more complicated flight procedures will only be allowed, however, if flight control systems can handle the additional complexity without compromising flight safety.
The normal glide path for commercial transports preparing to land is between 2.5 and 3 degrees. A glide slope of 6 degrees or more is possible now at airports equipped with microwave landing systems. The steeper approach is quieter because it allows (indeed, it requires) reducing engine power, but it also requires the aircraft to stay within a much tighter approach envelope and creates a more difficult vertical navigation task. One solution would be to develop a steep approach guidance and control algorithm (with capabilities similar to the approach and landing mode energy management display on U.S. space shuttles). Such a system would (1) monitor the approach angle to make sure it is not too steep to safely land the aircraft on-speed and within the designated touchdown zone and (2) provide guidance to the pilot or autopilot based on aircraft energy state, configuration, and power settings.
Alternative Propulsion Technologies
In the long-term it may be possible to develop new propulsion systems that can be adapted to improve the performance of commercial supersonic aircraft or lessen their environmental impacts. Alternative propulsion technologies include pulse detonation engines, hydrogen-fueled engines, and fuel cell-based propulsion systems.
Pulse detonation engines (PDEs) use detonation waves to initiate rapid combustion of a pre-mixed fuel-oxidizer mixture contained in an array of tubes that are open at one end and closed at the other. Over the past 5 years industry and government laboratories, including NASA, have studied PDEs in configurations and test conditions consistent with aeronautical applications. For PDEs to become practical, however, internal losses must be reduced, the frequency of operation must be increased (to improve overall efficiency), and environmental performance must be improved. Detonation combustors tend to operate with a stoichiometric fuel-to-air ratio to increase thermal efficiency, but this may produce an unacceptable level of harmful emissions. Experimental PDEs are also intrinsically loud. Nevertheless, both the aircraft engine industry and the USAF are studying PDEs as augmenters for turbine-based engines (so-called hybrid PDEs).
Hydrogen-fueled turbine engines are attractive in that (1) modifying jet turbines to burn hydrogen is not technically difficult (the USAF did it in 1970 with a T-39) and (2) the exhaust product is mostly water vapor and NOx. (Even though hydrogen fuel would contain no nitrogen, NOx would be created by the high temperatures of the combustion process, which combines some of the oxygen and nitrogen in the air.) A key challenge in developing hydrogen-fueled engines is finding a compact way to store hydrogen. The volume required to store hydrogen in a gaseous form at reasonable pressures is so large that the slender shape of supersonic aircraft presents a serious structural challenge. Even in liquid form, the energy density of hydrogen by volume is one-fourth that of conventional aviation fuels. Also, although the energy density of hydrogen by weight is nearly three times as great as that of conventional aviation fuels, potential weight savings are offset by the additional weight of the high-pressure gas or liquid cryogenic systems needed to handle the hydrogen fuel and the additional aircraft structure needed to accommodate the large hydrogen fuel tanks and fuel handling systems. Two new means of storing high-density hydrogen may turn out to be useful as aviation fuel: slush hydrogen and gelled liquid hydrogen. The latter may have metallic additives as well, though such additives may create emission problems. The density improvement offered by these forms of hydrogen, as well as their advantages and disadvantages for aircraft application, is not yet clear.
In the early 1970s a NASA-funded study by Lockheed examined the potential of hydrogen-fueled supersonic aircraft with cruise speeds of Mach 2.2 and 2.7. The aerodynamic weight and propulsion characteristics of a previously established design for a Mach 2.7 supersonic cruise vehicle fueled by liquid hydrogen were critically reviewed and updated. NASA explored the effect of fuel price and noise restrictions on vehicle design and operating performance. The study concluded that, compared with equivalent aircraft powered by conventional jet fuel, aircraft fueled by liquid hydrogen offered potential advantages in performance, cost, noise, pollution, sonic boom intensity, and energy utilization (Brewer and Morris, 1975). However, the idea was apparently set aside. In 1997 Boeing indicated that it had preliminary designs for a hydrogen-fueled airplane, but it has no
plans for pursuing this technology. The European Union, however, is supporting a partnership of 35 organizations coordinated by Airbus that is carrying out a 2-year systems analysis of subsonic commercial aircraft that could be fueled by liquid hydrogen. This project will examine the technical feasibility, safety, environmental compatibility, and economic viability of using liquid hydrogen as an aviation fuel for a wide variety of subsonic commercial aircraft, from business jets to very large, long-range commercial transports. Environmental concerns that bear examination include the effect of water vapor emissions (including contrails) and the environmental impacts of a hydrogen fuel production industry. The European effort, however, may finesse the latter concern by assuming that the production of hydrogen will be powered by renewable energy sources (EADS, 2001). A key economic issue is the potentially prohibitive cost of building cryogenic systems at airports and elsewhere for transporting and storing liquid hydrogen, combined with the long-term costs of maintaining and operating two fuel systems—one for hydrocarbon and one for hydrogen.
Fuel cells could be used to produce power for an electrically driven propulsion system. The key challenge for a practical fuel cell propulsion system is increasing power density (in terms of weight and volume). Other issues include peak power demands, power management, and the type of propulsion devices. The power density of state-of-the-art fuel cells has increased by a factor of 7 during the past 5 or 6 years, and a fuel-cell based propulsion system having twice the power density of current fuel cells could become technically feasible for a small, general aviation aircraft. However, the energy density needed for a commercial supersonic aircraft is orders of magnitude higher and would probably require fuel cells with room-temperature superconductors. Also, a fuel-cell aircraft would probably be powered by hydrogen and so would need to address the challenges already discussed for that fuel. One advantage of fuel cells would be the potential to collect the water vapor in the emissions (if the elimination of water vapor becomes a requirement, which is conceivable for stratospheric aircraft). Although this capability is difficult to imagine in a system suitable for a commercial supersonic aircraft, it would be easier to capture water vapor from the emissions of a fuel cell than from the exhaust of a turbine engine.
The idea of using fuel cells as the prime mover assumes that substantial advances in electric propulsion will have been made by the time fuel cell technology becomes a practical energy exchange device. Currently, conversion of electric power to propulsive power generally involves electric motors that are too heavy for aircraft applications. Electrically driven space propulsion engines have also been developed, but they have power densities that are many orders of magnitude too low for high-speed aircraft applications. Other, more radical approaches can also be imagined, such as zero-emissions aircraft with nuclear propulsion systems or beamed power systems using ground- or space-based power-beaming stations. Nuclear propulsion systems are unattractive, however, because of the need for very heavy radiation shields and the threat of radioactive contamination, especially in case of an aircraft accident. A power-beaming concept also raises fundamental issues, such as how to ensure continuity of power to a target moving at hundreds of miles per hour over large distances with a reliability of 0.999999999 (“nine 9’s”—the standard for safety-critical aviation systems).
The committee believes that the alternative propulsion concepts examined are generally worthy of basic research support because they could be useful in various power or propulsion applications. However, none is applicable only to supersonic flight, and successful application to commercial supersonic aircraft would almost certainly have to be preceded by success in other applications, such as ground-based power systems (fixed or mobile) and subsonic aircraft. Furthermore, the economic and environmental justification for developing these technologies for existing, large-scale applications (such as electric power plants, automobiles, or subsonic aircraft) is probably much stronger than it is for applications that have yet to demonstrate long-term economic success (such as supersonic aircraft).
In light of the above, the committee has concluded it would be inappropriate to use the limited resources available for development of commercial supersonic aircraft technology to support basic research in alternative power and propulsion systems that show no particular promise for or relevance to supersonic applications. Supersonic R&T should focus on basic and applied research projects with the potential for resolving the difficult problem of improving the environmental acceptability of gas turbine engines powered by conventional fuels.
For the next generation of supersonic commercial aircraft to meet environmental and economic requirements, propulsion materials and coatings must be superior to those used in the current generation of subsonic aircraft and in the Concorde. The degree of improvement is dependent on aircraft performance specifications, such as range, cruising speed, and size—an SBJ cruising at Mach 1.6 will need considerably less improvement than an HSCT cruising at Mach 2 or faster. More rigorous environmental and occupational safety regulations are expected to eliminate hazardous materials such as lead, chromium, and cadmium from high-temperature bearings and rotating surfaces. This could also prove to be a critical issue. Improvements are also needed in four other critical areas:
combustor liner materials and coatings to meet emission and durability requirements
turbine airfoil alloys and thermal barrier coatings to meet performance and durability requirements
high-temperature alloys for compressor and turbine disks to meet performance and durability requirements
strong, lightweight, high-temperature materials that will enable the propulsion system to meet noise and weight requirements
Other materials and coatings could also be critical to the next generation of supersonic propulsion systems. These include compressor and turbine seals with better resistance to erosion. Stiffer, lower density fan-containment materials could also prove to be critical, especially for cruise speeds in excess of Mach 2.
Propulsion materials are a high-risk technology for HSCTs. To reduce the weight and improve the performance of supersonic propulsion systems, materials must have low density, high strength, and long life at high temperatures. Extending the combustor temperature to 3000 °F or nozzle temperature to between 2300 °F and 2400 °F will be a very challenging materials problem.
Advanced materials such as polymers, intermetallics, metal matrix composites, and ceramic matrix composites (CMCs), coupled with innovative structural designs, could significantly reduce engine weight while improving engine aerothermodynamics. Furthermore, advanced materials have the potential to reduce aircraft TOGW, fuel consumption, emissions, and noise. New fibers and fiber coatings will also be required to withstand the extreme temperatures and long operational times of many engine components.
Titanium aluminide may be used to fabricate several critical components for engine nozzles. The potential weight savings of titanium aluminide over conventional superalloys make it a good candidate for high-temperature applications where high stiffness is required. To capitalize on the potential for this class of material, more research is required. Areas for improvement include low-cost material production, robust joining methods, and more comprehensive databases for materials properties . In addition, further development may allow PETI-5 (phenylethynyl-terminated imide), a high-temperature resin created by NASA during the HSR Program, to help satisfy the requirement for a high-temperature composite matrix resin and adhesive.
Lower manufacturing costs would help achieve the economics that commercial supersonic aircraft need to be successful. Isothermal rolling and forging, cold and hot spray forming, and laser powder deposition could reduce manufacturing costs substantially by reducing the buy-to-fly ratio for materials from 10-to-1 to 2-to-1, especially when combined with advanced modeling and computer simulations. Large, complex structural castings have long lead times and are expensive. A third of the cost is associated with inspection, repair, and rework. More modeling and computer simulation could reduce these costs. Casting with laser powder deposition to produce complex castings could lead to additional cost savings.
Combustor Liner Materials and Coatings
Current subsonic engines use state-of-the-art nickel-based superalloys with a thin ceramic coating for additional thermal protection. To meet the targeted NOx emissions index of 5, the volume of film cooling air in the engine combustor must be greatly reduced, though the actual requirements are strongly dependent on the combustor design concept. A final combustor design is required to determine how much the cooling air must be reduced—and how much the operational temperature of the liner must be increased. In any case, materials that can withstand considerably higher temperatures than nickel-based superalloys will probably be needed for combustor materials exposed to very high temperatures for very long times.
NASA’s HSR Program concluded that CMC was particularly promising as a combustor liner material. However, CMC is expensive and there is not yet an established manufacturing process to produce CMC liners. CMC is also brittle, which could create a durability problem. The Department of Energy has been supporting CMC development for industrial gas turbine applications, and it has achieved over 20,000 hours of operation at low temperatures (2200 °F to 2400 °F). This experience is generating an operational database that will reduce both the technical and financial risk of using CMCs in aircraft. Another promising but less well-developed liner material is a molybdenum-niobium-based alloy. Successful development of a liner using CMC or a molybde-num-niobium-based alloy would greatly advance efforts to satisfy emission requirements for future commercial supersonic aircraft.
Turbine Airfoil Alloys and Thermal Barrier Coatings
One of the key differences between supersonic engines and subsonic engines is the profile of temperature versus time during a typical flight. In a subsonic engine, critical engine components, such as the disks and the turbine airfoils, are at maximum temperature only during takeoff and climb, which typically last about 15 minutes. In a supersonic engine, critical engine components are at maximum temperature during cruise. Thus, a supersonic flight that lasts 4 hours subjects critical engine components to high temperatures 16 times longer than a subsonic flight of the same distance. Consequently, the creep and thermal mechanical fatigue properties of the airfoil alloys must be much more robust than the alloys used in subsonic engines, even if the maximum operating temperatures are the same. In fact, new supersonic engines are likely to have turbine inlet temperatures a couple
of hundred degrees higher than current subsonic engines. The combustor exit temperature will probably be between 3000 °F and 3500 °F. A new thermal barrier coating is needed to provide 200 °F to 300 °F of thermal protection. This will require a ceramic coating with thermal conductivity 50 to 75 percent lower than that of current thermal barrier coatings. The HSR Program made some progress in this area, developing an airfoil alloy and thermal barrier coating to a TRL of 3 to 4. However, this is still an important risk item, and the risk increases as the cruise Mach number increases.
High-Temperature Alloys for Compressor and Turbine Disks
To satisfy the performance, durability, and affordability requirements of an HSCT, the required pressure ratio will probably push compressor outlet temperature 100 °F to 300 °F higher than current subsonic engines. In addition, as discussed above, the disks will need to withstand elevated temperatures much longer than the disks in subsonic engines. The longer operational times at the higher temperatures will require the new disk alloys to have improved creep and cyclic fatigue properties. The HSR Program developed a new disk alloy with a 1350 °F limit and advanced it to a TRL of 3 to 4. However, no full-size disks were forged and no component tests were conducted to determine the ability of the new alloy to meet the required disk service life of 18,000 hours. The temperature limit needs to be increased by another 100 °F to 150 °F, forging and heat treatment procedures must be validated to ensure that the disks can be manufactured, and component tests must be conducted to establish disk life. The lower limit is probably adequate for SBJs, and the higher limit is essential for higher-Mach-number airplanes.
Materials to Meet Engine Noise and Weight Requirements
The extent to which a commercial supersonic aircraft will require new materials to meet engine noise suppression requirements is highly dependent on the type of aircraft and the engine design concept. The need for engine nozzles to suppress noise would be much less for a smaller aircraft or an engine design cycle with a high bypass ratio during takeoff and climb. The primary nozzle issue is achieving weight, cost, and size goals compatible with aircraft weight and performance requirements. Concerns about the durability of the components when exposed to high levels of acoustic energy and high temperatures are also well founded. The HSR Program developed large, thin-wall castings of nickel superalloys and titanium aluminide intermetallic materials along with CMC acoustic tiles to a TRL of about 4. Continued development is needed to fabricate a subscale nozzle with these materials and conduct component tests in an appropriately simulated environment.
AIRFRAME MATERIALS AND STRUCTURES
Material systems selected for commercial supersonic aircraft could deteriorate in the thermomechanical environment expected in service. Some existing materials, such as graphite epoxy, may be suitable for cruise speeds below Mach 2, while others, such as graphite/bismaleimides, may be suitable up to Mach 2.2, but testing is needed to verify that they will perform adequately after lengthy exposure to the high temperatures of supersonic flight. The airframe must perform its intended function throughout its time in service to ensure flight safety and to meet customers’ economic expectations. Much work has been done over the last 20 years or more in understanding mechanisms of deterioration due to temperature, oxidation, ultraviolet, and time for several composite material systems. Also, some aluminum-lithium alloys have been exposed to high temperatures for prolonged periods of time. Little is known, however, about the applicability of that research to other material systems that may have different deterioration mechanisms. In addition, some preliminary accelerated aging protocols have been developed for high-temperature material systems, but more data are needed to prove that the protocols are valid. Time is of the essence for providing these data. About 7 years are needed to conduct simulated lifetime testing for a vehicle designed for 60,000 hours of operation at design temperature (i.e., at cruise conditions). This makes it impractical to include complete lifetime testing in preliminary screening of candidate materials or the generation of design data. A long-term commitment is required to validate accelerated testing protocols.
Proof of Structure
Traditional aircraft design methods include numerous structural tests of varying complexity—including tests of complete airframes—to prove that structures are satisfactory from both a static and a durability perspective. Appropriate thermomechanical tests can be conducted on small test articles, but similar tests on complete airframes can be prohibitively expensive. In addition, airframe testing may have little value, because it is difficult to validate the accuracy of the simulated environment.
Nevertheless, the certification process—and good engineering practice—requires manufacturers to prove that the airframe will perform safely throughout its time in service. Airframe manufacturers have accumulated a wealth of knowledge from past test programs. Several thermomechanical tests were conducted on the Concorde airframe; the full-scale fatigue test proved to be particularly challenging because of problems with simulating the in-flight heat transfer rates. Testing will become even more complicated if composite material systems are widely used, because the relationship between deterioration under mechanical loads and
deterioration under thermal loads is not well characterized. This makes it impossible to simulate thermal effects using mechanical loads.
The fuselages of commercial supersonic aircraft will be long and slender. Designing an airframe stiff enough to avoid APSE problems and light enough to allow an economic payload will be difficult. Conventional aircraft materials and structures will not be able to provide adequate stiffness at an acceptable weight, especially for HSCTs. At a minimum, increased stiffness will be required at several locations on the airframe, such as outboard wing and forward fuselage. Some work has been done on aeroelastic tailoring but little on the scale that would be required on an HSCT. Also, work will need to be done to improve the producibility of large structures with unbalanced lay-ups and to characterize their structural performance.1 Hybrid laminates and materials with high-stiffness fibers, such as boron or high-modulus graphite fibers, are available, but more work is required to fully understand issues related to their performance in service. New fibers may need to be developed to provide increased stiffness without loss of strength or toughness. Large structures using the proposed materials and lay-ups will need to be manufactured and tested to prove viability.
Lightweight Structural Design Concepts
To meet weight and operational performance targets, lightweight structural design is essential. Sandwich construction has some potential, but it also has some limitations; moreover, existing data regarding its ability to reduce structural weight are inconclusive. This type of construction has been widely used in secondary applications on large commercial transports but is not often used for primary structure. Research supported by the HSR Program revealed that conventional skin-stringer construction would weigh less in areas subject to high loads and in wing and empennage structures having shallow box depth. The design information available for sandwich structure subjected to high loads is incomplete, especially where through-thickness effects are significant.2 Previous research revealed that these through-thickness effects can limit the design by, for example, requiring higher density cores to provide (1) adequate core shear strength and (2) sufficient tension strength between the face sheet and the core. This would increase weight, making sandwich construction less attractive.
Another significant challenge will be developing a means of damage prevention following the absorption of water. Past service experience on subsonic commercial transports shows that it will be difficult to prevent sandwich structure from absorbing water throughout the life of a fleet. The freeze-thaw-boil cycle associated with an airplane flying at subsonic speeds at altitude followed by acceleration to supersonic speeds will prove to be an exceptionally difficult problem. Indeed, the rudder of the Concorde was susceptible to water absorption, which caused in-flight separation of portions of the rudder. Water absorption, however, is not a new problem and has been experienced in a variety of airplanes. The current solution for new airplanes is to fill the voids with closed cell foam. While this slightly increases weight, it reduces the water absorption rate by several orders of magnitude.
In addition, design loads are limited by the location of structural joints. Choice of load levels must consider factors such as durability and bearing strength. More efficient designs of conventional structures as well as new structural concepts will increase the likelihood of meeting design weight targets.
There are many potential structural materials for commercial supersonic aircraft with cruise speeds of less than Mach 2, but few have been tested for long periods at the temperatures that would be experienced while cruising at supersonic speeds. Advanced materials may be advantageous in some structures for commercial supersonic aircraft with cruise speeds below Mach 2.0, but they are essential for faster aircraft. Graphite epoxies and aluminum cannot sustain the temperatures expected at cruise speeds above Mach 2.0 for long periods of time, and titanium is too heavy to use as the primary structural material. PETI-5 may be able to serve as the basis for composite materials for aircraft with cruise speeds between Mach 2.0 and Mach 2.4. Improved titanium alloys have also been developed for key structural applications (e.g., highly loaded wing spars). The HSR Program investigated a promising hybrid laminate consisting of titanium foil and high-temperature composite materials. While progress was made on different product forms of the PETI-5 material, most had not reached an advanced state of maturity when the HSR Program was cancelled. Life testing on some materials was in progress but was halted when it was still many thousands of hours short of reaching the equivalent of one design service lifetime. Accelerated aging protocols were postulated but not proven. No significant work was done on alternative fibers such as higher modulus graphite fibers or boron fibers for improved stiffness. Additional work was needed on the hybrid laminate, especially
on development of a robust surface preparation for the titanium foil. Much more work was also required to improve the cost and producibility of these materials for fabrication of airplane parts. Materials using boron fibers have proven to be especially difficult to fabricate, and loose boron fibers can present a health hazard during handling.
Adhesives that will perform at high temperatures for long periods of time are required, especially for operation above Mach 2.0. However, improved materials for lower temperature applications may also be advantageous, because many of the structural concepts that have been investigated rely on structural bonding.
Adhesives that perform consistently at high temperatures for long periods of time are needed to attain aircraft weight goals and ensure vehicle safety. Adhesives developed under the HSR Program were based on resin systems similar to those used in high-temperature polyimides, such as PETI-5. Early indications were promising, and both film and paste adhesives were developed. Other types of adhesives, such as a reticulate film adhesive, also need to be developed. While some work has been done on aging of adhesives, more data are needed. Only preliminary information on suitable accelerated aging protocols is available for these materials.
For sealants, the application of maximum concern is fuel tanks on aircraft with cruise speeds above Mach 2.0; currently available sealants are probably sufficient for lower speed applications. The key characteristic of a fuel tank sealant is its ability to cure effectively in a few hours in a confined space at temperatures that can be easily attained in a production environment. Fuel tank leaks result in excessive downtime for repair and dissatisfied customers, so long life is also important. Additional lifetime testing is required to characterize long-term performance of sealants in the operational environment of a commercial supersonic aircraft. Additional research is required to explore the deterioration mechanisms and rates of candidate materials. Accelerated aging techniques are also needed.
Some candidate materials have been developed for fillet seals (i.e., seals along the edges of parts and around fastener heads) in fuel tanks. Suitable materials have not yet been identified for fay surface seals (between overlapping or fitted parts), and the only ongoing research is a small, proprietary effort by Japanese industry and government.
An alternative would be to develop a different approach to sealing fuel tanks. For example, fuel could be stored in bladders inside each fuel tank, although this is probably not a viable approach for a new commercial supersonic aircraft because of increased weight and the difficulty of removing bladders for structural inspections and repairs.
Coatings are used for both appearance and thermal management. Without advanced coatings, it may not be possible to maintain fuel temperatures below acceptable levels for aircraft with cruise speeds above Mach 2.0. At Mach 2.4, fuel temperatures are predicted to reach the limit of acceptability: A maximum fuel tank temperature around 150 °F was assumed for design studies in the HSR Program.
Coatings with the desired emissivity and absorptivity are essential. The coatings must maintain these characteristics and remain aesthetically pleasing until the aircraft is repainted. The HSR Program viewed the development of advanced coatings as a low priority and did not fund research in this area. A small, proprietary, industry-funded study conducted concurrently with the HSR Program identified a few candidate materials (e.g., siloxane, fluoroethersilicone, and solgel with zinc ortho-titanate and titanium oxide pigments) that appeared to have many of the required characteristics, at least initially. Testing, however, indicated that the materials tend to turn yellow after only a few hundred hours at temperature. When the HSR Program was cancelled, the research on coatings was terminated before this problem could be solved. Additional resources are needed to more thoroughly investigate optical properties and to make sure new coatings can be applied easily and have adequate wear and erosion attributes.
COCKPIT SENSORS AND DISPLAYS
Supersonic flight imposes few unique requirements on avionics and flight deck systems. Government and industry are developing new avionics technology that will benefit commercial aircraft regardless of their range or speed. Indeed, advancements in avionics are often applicable to a wide range of commercial and military aircraft. The avionics and display technologies needed for a commercial supersonic aircraft are currently operational or in development at a TRL of at least 3.
New cockpit display technologies are of particular importance to the development of supersonic aircraft, because most supersonic aircraft designs feature a long, pointed nose for drag reduction. Additionally, most contemporary supersonic designs feature a modified delta wing optimized for high-speed flight that results in high angles of attack at lower speeds. The long nose and high angle of attack at low air-speeds impairs the forward visibility of the flight crew and requires the use of either a droop-nose design (like that of the Concorde) or advanced sensors and displays to restore visibility without the weight and mechanical complexity of the droop nose. Technology currently available or under development makes the second option feasible. For example, seamless merging of visual and radar imagery with information from a digital database has demonstrated a real-time, all-weather situational awareness display that provides the
crew with terrain data, aircraft flight path information, weather, and traffic avoidance advisories. Three-dimensional imagery has intriguing possibilities and is actually being used for medical applications, but the current state of technology is not suitable for near-term airborne applications.
The development of these so-called enhanced vision systems is an interdisciplinary task that requires a wide spectrum of different information technologies: (1) reliable camera systems to provide visual imagery, (2) data link technology for transmission of guidance information, (3) complex databases to provide terrain data for synthetic images and object signatures to support the imaging sensors, (4) high-performance computer graphics systems to render synthetic images in real time, (5) onboard imaging sensors, such as solid state infrared or imaging radar, to provide a real view through darkness and adverse weather, (6) knowledge-based image interpreters to convert sensor images into a symbolic description, (7) projection technology for panoramic or holographic displays, and (8) precision navigation tools tied into the Global Positioning System or equivalent systems that may become available.
Advanced cockpit sensors and displays could enhance safety by detecting and displaying images of objects that pilots would not normally be able to see when looking through the cockpit window of a conventional aircraft. Safety could be further enhanced by integrating features such as navigation enhancements and proactive systems to avoid controlled flight into terrain and runway incursions. Flight crews, however, may be uneasy about flying aircraft with only a computer-generated view of the outside world. One way to deal with this uneasiness during approach and landing would be to locate the cockpit windows at the lower front fuselage of the aircraft, instead of the traditional location on the upper front fuselage. This would give the pilots direct visual contact during critical phases of the approach, landing, and ground operation. During other phases of flight (takeoff, climb, and cruise), pilots would use an enhanced vision system with computer-generated displays for safety, guidance, and control functions.
Other sensor and display technologies could further enhance the operability of commercial supersonic aircraft. Some systems, such as a boom shadow tracking system, would be unique to supersonic aircraft.3 Supersonic aircraft would also benefit from upgrades to flight planning systems. Today, airline dispatchers and pilots consider weather and atmospheric conditions, especially winds and hazardous weather, to select safe flight routes that minimize flight time and fuel burn. Flight planning for supersonic flight operations would benefit from additional information, such as temperature anomalies and solar radiation. Routing could be further improved by increased use of real-time information to adjust routing in flight. Crews flying complex modern aircraft would also benefit from advanced cockpit concepts, such as a task saturation detection system. Conceptually, this system would detect stress and/or tunneled focus that might occur, for example, during abnormal or emergency flight conditions. If necessary, the system would assume temporary, but overrideable, control of certain flight functions to assure safety while allowing the crew to adequately handle the situation that captured their attention. This technology is currently at TRL 1 or 2.
Cosmic radiation is caused by high-energy charged particles from space (galactic cosmic radiation) and, to a lesser extent, from the Sun (solar cosmic radiation). Many of the particles are deflected by Earth’s magnetic field. This effect is strongest at the equator and weakest at the poles; at altitudes commonly used by subsonic aircraft, cosmic radiation is twice as high at the poles as it is at the equator (FAA, 1990).
Cosmic-radiation particles that enter the atmosphere collide with atoms and molecules of air. This reduces the energy of the primary (incoming) radiation while producing secondary radiation in the form of lower energy particles and gamma rays. At sea level, Earth’s atmosphere provides an effective shield against primary and secondary cosmic radiation, with a mass thickness of over 1,000 g/cm2. The shielding mass drops off to about 200 to 300 g/cm2 at normal cruise altitudes for subsonic aircraft (30,000 to 40,000 ft). At 65,000 ft, the shielding mass is 60 g/cm2, which reduces the primary proton flux to about half of the incident flux, the alpha particle flux to about a quarter, and the heavy ion flux to about 3 percent or less, depending on the mass of the cosmic radiation particle (Goldhagen, 2000). When the Concorde entered service in 1976, cosmic radiation was identified as a potential crew safety issue. British Airways installed radiation-monitoring devices in the flight deck area and determined that radiation dosage received during flights at 50,000 to 60,000 ft were about twice the dosage received at 35,000 to 40,000 ft, the cruise altitude of a typical airliner.
On average, people living in the United States are exposed to about 3.5 millisievert (mSv) per year. Over 80 percent of this exposure is from natural sources, primarily radon and radon by-products in the atmosphere. At sea level, less than 10 percent of natural background radiation is from cosmic radiation (ISU, 1996). In an aircraft, however, primary and secondary cosmic radiation are the main source of radiation exposure. The average exposure of a commercial pilot on a single polar flight has been measured at about 0.1 mSv (Blakely, 2000). The FAA’s advisory circular on radiation exposure (AC 120-52) estimates the annual exposure that crew members would experience if they worked full time
on one of 32 domestic and international routes. The largest annual doses, which were as high as 9.1 mSv, were for transoceanic flights at high altitude (up to 43,000 ft). By comparison, federal regulations require the nuclear power industry to ensure members of the general public receive no more than 1 mSv of additional radiation per year from nuclear power operations. The limit for radiation workers is 50 mSv per year (10 CFR 20).
AC 120-52 also addresses the health risk associated with the estimated exposures to cosmic radiation. If 1,000 crew members worked full-time (950 flight hours per year) for 20 years on a route with an annual exposure of 5 mSv, AC 120-52 estimates that about 6 might be expected to develop fatal cancers as a result. Based on public health statistics, about 220 of 1,000 Americans normally die from cancer, so exposure to 5 mSv annually would increase the chance of dying from cancer by 3 percent (FAA, 1990). The risk to passengers, who spend much less time in the air than full-time crew members, is much less.
Another early concern with high-altitude flight was the possibility of being exposed to excessive radiation as a result of solar flares. Conceivably, a strong solar flare could deliver a radiation dose greater than 10 mSv per hour to crew and passengers flying on polar routes. Concorde pilots have monitors so they can reduce altitude and latitude, if necessary. The Concorde routes are subpolar. Still, over the 25-year experience with the Concorde, no flight had to activate an emergency descent because of high radiation from a solar flare, even during peak years of the cycle.
The deployment of commercial supersonic aircraft should be accompanied by an update to AC 120-52 that extends the analyses of radiation exposure to the altitudes, routes, and flight times typical of supersonic aircraft, evaluates the need for tracking individual exposure by frequent flyers, and incorporates up-to-date knowledge regarding biological damage from highly energetic particles (which seems to be more severe than originally estimated in the 1960s) (Wilson, 2000). This update is necessary to ensure that aircrew and the flying public continue to enjoy a high level of confidence that high-altitude radiation does not threaten their health. Future research and analysis should assess the adequacy of current knowledge about high-altitude radiation with regard to the safe operation of commercial aircraft at various altitudes. Future research should also explore technological and operational approaches for reducing exposure to cosmic radiation, perhaps through lightweight shielding and/or the tracking of radiation doses accumulated by individuals and the exposure associated with particular flights (to make sure crew assignments do not result in excessive exposure to any individuals).
High-Altitude Cabin Depressurization
Cabin depressurization at altitudes higher than 50,000 ft would pose a safety risk to passengers and crew, especially in the case of a rapid or explosive decompression. Although the likelihood of a structural failure leading to cabin depressurization at cruise altitude is very small (failure of a compressor or turbine disk is the most likely cause), the effects could be fatal unless immediate and drastic action is taken. Above 50,000 ft, a person in good physical condition has a time of useful consciousness of only 5 to 12 seconds without supplemental oxygen. FAR 25.841 requires commercial aircraft to be designed so that occupants will not be exposed to a cabin pressure altitude that exceeds 25,000 ft for more than 2 minutes or a pressure altitude of 40,000 ft “for any duration” for any failure condition that cannot be shown to be “extremely improbable.”
This could be a difficult standard to meet for commercial supersonic aircraft cruising at altitudes well in excess of 40,000 ft. Efforts to satisfy this requirement should investigate the innovative technologies such as self-sealing materials to contain fuselage pressure leaks. Even if self-healing technologies cannot seal leaks completely, they could reduce the leak rate and provide extra time for an emergency descent. An automatic emergency descent mode for aircraft flight control systems, which could be triggered by unexpected loss of cabin pressure changes, might also be necessary to meet safety standards.
The FAA does not have certification requirements suitable for a future commercial supersonic transport. The Concorde was certificated using a set of special conditions. A preliminary set of rules, “Tentative Airworthiness Standards for Supersonic Transports,” was developed during the U.S. SST program in the early 1970s. The FAA also established certification teams to support the future certification of new technologies under development by the HSR Program. Ongoing efforts to develop new supersonic technologies should proceed in parallel with the development of new regulatory standards to ensure that the regulatory approval process does not impede the development of a commercial supersonic aircraft. For example, certification issues could be a major obstacle to the use of a windowless cockpit that uses advanced sensor and display systems to produce a computer-generated view of the outside world. National resource specialists (i.e., FAA specialists in specified disciplines) could be a valuable resource for planning and implementing changes to certification standards.
Blakely, E. 2000. “Biological Effects of Cosmic Radiation: Deterministic and Stochastic.” Health Physics: The Radiation Safety Journal 79(5):495-506.
Brewer, G., and R. Morris. 1975. Minimum Energy Liquid Hydrogen Supersonic Cruise Vehicle Study. NASA CR137776. NASA Contract NAS2-8781 with Lockheed Martin, California. (N76-17101).
EADS (European Aeronautic Defence and Space Company). 2001. Special: Cryoplane. Available online at <http://www.eads-nv.com/eads/en/index.htm?/xml/en/businet/airbus/cryoplane/cryoplane.htm&airbus>. Accessed on September 13, 2001.
EPA (Environmental Protection Agency). 2001. Human Health Effects of Ozone Depletion from Stratospheric Aircraft. Washington, D.C.: EPA.
FAA (Federal Aviation Administration). 1990. Radiation Exposure of Air Carrier Crewmembers. FAA Advisory Circular 120-52, March 5. Available online at <http://ntl.bts.gov/DOCS/Ac12052.html>. Accessed on September 13, 2001.
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