2
Hypersonic Flight

INTRODUCTION

Air-breathing hypersonic flight has long been recognized for its potential to enable both advanced military missions and (possibly) efficient access to space. It offers two principal advantages: high speeds, with associated reduction in time to target, and the potential for substantial performance gains resulting from the high specific impulse (Isp) achievable with air-breathing propulsion. However, these potential advantages must be weighed against formidable practical challenges, such as those posed by internal and external aerodynamics, supersonic mixing and combustion, and the costs of development as well as the requirements for high-temperature materials, lightweight airframes, and stability and control. Over the past 40 years, a long series of development programs has been funded in the United States and in other countries. Although these programs have generated a great deal of theoretical and practical understanding, sustained air-breathing propulsion and flight at high Mach numbers has remained elusive. Clearly, much more work needs to be done along many fronts.

On the other hand, scramjet technology has matured significantly over the past decade or so, and the flight-testing of scramjet engines is imminent. Computational methods such as computational fluid dynamics (CFD), finite element model (FEM) structural/thermal analysis, and vehicle design/optimization methods have also matured significantly, making it more likely that highly integrated hypersonics systems can be designed to close with respect to performance/economic requirements. In addition, command, control, communications, computing, intelligence, surveillance and reconnaissance (C4ISR) technologies have developed to the point where the timeline for finding, selecting, and engaging targets (the “kill chain”) is getting short enough to take advantage of the rapid response afforded by hypersonic missiles and reentry vehicles.

Recognizing the potentially large benefits of air-breathing hypersonics technology and the magnitude of the efforts to develop this technology, the NAI brought together essentially all existing U.S. programs and attempted to lay out a high speed/hypersonics technology roadmap. However, this roadmap is more a collection of existing programs than a logical and complete plan for achieving military strike, global reach, and space access objectives. Each of the collected



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Evaluation of the National Aerospace Initiative 2 Hypersonic Flight INTRODUCTION Air-breathing hypersonic flight has long been recognized for its potential to enable both advanced military missions and (possibly) efficient access to space. It offers two principal advantages: high speeds, with associated reduction in time to target, and the potential for substantial performance gains resulting from the high specific impulse (Isp) achievable with air-breathing propulsion. However, these potential advantages must be weighed against formidable practical challenges, such as those posed by internal and external aerodynamics, supersonic mixing and combustion, and the costs of development as well as the requirements for high-temperature materials, lightweight airframes, and stability and control. Over the past 40 years, a long series of development programs has been funded in the United States and in other countries. Although these programs have generated a great deal of theoretical and practical understanding, sustained air-breathing propulsion and flight at high Mach numbers has remained elusive. Clearly, much more work needs to be done along many fronts. On the other hand, scramjet technology has matured significantly over the past decade or so, and the flight-testing of scramjet engines is imminent. Computational methods such as computational fluid dynamics (CFD), finite element model (FEM) structural/thermal analysis, and vehicle design/optimization methods have also matured significantly, making it more likely that highly integrated hypersonics systems can be designed to close with respect to performance/economic requirements. In addition, command, control, communications, computing, intelligence, surveillance and reconnaissance (C4ISR) technologies have developed to the point where the timeline for finding, selecting, and engaging targets (the “kill chain”) is getting short enough to take advantage of the rapid response afforded by hypersonic missiles and reentry vehicles. Recognizing the potentially large benefits of air-breathing hypersonics technology and the magnitude of the efforts to develop this technology, the NAI brought together essentially all existing U.S. programs and attempted to lay out a high speed/hypersonics technology roadmap. However, this roadmap is more a collection of existing programs than a logical and complete plan for achieving military strike, global reach, and space access objectives. Each of the collected

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Evaluation of the National Aerospace Initiative programs has elements that address some portion of the critical technologies, and each has a funding line. The collective funds, if properly applied, may be sufficient in the near term for a significant critical technologies program; however, as configured, it is not clear that the collected programs cover all critical hypersonics technologies. What is more, NAI has not projected funding after existing program budgets run out. The committee observes that sharply higher budgets will be required to support technology scale-up (i.e., beyond presently planned small-scale demonstrations), especially flight demonstration programs aimed at maturing air-breathing hypersonics technologies to the point where decisions can be made about the development of large-scale Global Strike/ISR aircraft and air-breathing space access vehicles. A realistic budget must be projected consistent with answering the critical technology challenges, and existing programs should be more closely aligned, taking advantage of the synergistic potential championed by NAI, so they can provide critical hypersonics technologies to the nation. This chapter begins by presenting the committee’s findings and recommendations, with brief discussions as appropriate. Technical and financial issues associated with the technologies that are critical to hypersonic flight are then discussed, together with recommended directions for future research and development. FINDINGS AND RECOMMENDATIONS Finding 2-1. The U.S. Air Force, the U.S. Army, and the U.S. Navy all see the possible benefits of applying air-breathing hypersonic propulsion technology to a broad range of warfighting missions, but none has yet developed formal requirements for such technology. Similarly, NASA sees potential in applying air-breathing hypersonics propulsion technology to space launch systems. Together, the DoD services and NASA are investing in the development of near- and mid-term hypersonics technologies under the NAI and are looking to 20181 as a point at which to assess whether hypersonic propulsion has sufficient system and/or operational benefits to warrant applying the technology to space launch missions (Sega, 2003a). The DoD services see air-breathing hypersonics technology as applied to hypersonic missiles and/or aircraft as tangible products along the path to 2018 (see Chapter 1 for more discussion of warfighter requirements). Discussion 2-1. Although they recognize the potential benefits of applying hypersonic air-breathing technology to weapon systems, the DoD services are expressing a wait-and-see attitude, keeping an eye on hypersonics technology development, and continuing to explore concept of operations (CONOPS) for hypersonics systems employing air-breathing propulsion (Morrish, 2003; Graff, 2003; Hickman et al., 2003; Walker, 2003). For example, the Air Force recognizes that its space access requirements (as well as its time-critical strike and global reach requirements) may someday be met by hypersonic air-breathing propulsion, but only after the technology has been sufficiently matured. In the meantime, the greater maturity of rocket technology allows it to develop near- and mid-term (~2010 and 2015, respectively) rocket-based solutions to satisfy Operationally Responsive Spacelift (ORS) requirements. By 2018, hypersonic air-breathing technology may be sufficiently mature and understood, under a properly developed and executed NAI plan, to make a full-scale development (FSD) decision on whether to promote air-breathing hypersonics technology to the next block or spiral of space access system development, one with an initial operational capability (IOC) no sooner than about 2025. 1   This date and others that appear in the report were established by NAI participants prior to the President’s announcement of a new mandate for NASA. How these dates will be affected by the new NASA mandate is not yet clear; however, some NAI schedule objectives might be significantly delayed. See “NASA’s New Space Exploration Mandate” in the preface.

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Evaluation of the National Aerospace Initiative NASA is also aiming for an FSD decision in 2018 on applying hypersonic air-breathing propulsion to space launch systems (Lyles, 2003; Rogacki, 2003), but it views the technology as a potential contributor to its space access future; on the other hand, the Air Force views the technology as just one of several options that may or may not get applied to far-term launch vehicles. To support near- and mid-term air-breathing hypersonic applications and an FSD decision in 2018 on whether to employ hypersonic propulsion on a subsequent generation of space access systems, the DoD and NASA would have to complete the planning, funding and execution of the NAI. All requisite technologies and an associated plan must be developed sufficiently to permit hypersonic product development decisions to be made based upon real data. Recommendation 2-1. The DoD should continue to evaluate the unique capabilities achievable with air-breathing hypersonic systems as a means of satisfying warfighting needs. Despite the current lack of formal requirements, the DoD and NASA should also continue to invest in the development of air-breathing hypersonic technologies to meet their future capability needs. Finding 2-2. The objectives of the high speed/hypersonics (HS/H) pillar of the NAI are to develop near- and mid-term hypersonic technologies to the point where they could support emerging DoD mission capability requirements and, far term, to evolve an air-breathing, hypersonic, reusable space launch system for both NASA and DoD applications. However, the top-level HS/H plan (roadmap) for accomplishing these objectives does not appear to be coherent, comprehensive, or well communicated to decision makers and stakeholders. Discussion 2-2. The NAI program claims to rely heavily on a formalized decision process employing goals, objectives, technical challenges, and approaches (GOTChA) to establish the content of HS/H technology development and flight demonstration roadmaps in support of an air-breathing launch vehicle FSD decision in 2018 (see Figure 2-1). However, many elements of the NAI program were in place prior to GOTChA, and the process did not appear to change these preexisting elements. It is not clear that the process, as used by NAI, was effective in defining a comprehensive and compelling technology development program, from fundamental research to flight demonstration, that would sufficiently mature all technologies critical to operational hypersonic flight. Particularly noteworthy is the low level of fundamental research (6.1 and 6.2) in the NAI HS/H plan. Although two NASA university research, engineering, and technology institutes (URETIs) have been established to sponsor fundamental research in air-breathing launch vehicle technologies, this level of support is deemed insufficient to support NAI goals. Similarly, it is not clear that comprehensive plans yet exist to develop enabling technologies and critical system components to the point where operational hypersonic flight can be achieved in the time frame specified. And finally, recognizing the need to conduct flight demonstrations to advance hypersonics technology, it is not apparent that NAI has identified the set of flight demonstrations necessary and sufficient to support an FSD decision in 2018. In summary, the HS/H program appears to be primarily a collection of preexisting flight demonstration programs or concepts slated to be used to mature hypersonics technologies. Regard-less, the committee believes the NAI hypersonics plan to be potentially technically feasible in the time frame laid out, but only if the necessary planning occurs, adequate funding is available, the beneficial effects of synergy are successfully captured by the NAI partners, and science enables breakthroughs in the associated critical technologies. Recommendation 2-2. Starting with a defined and articulated vision, DoD and NASA should use a top-down process based on sound system engineering principles to determine the objectives, technical challenges, and enabling technologies, and the fundamental research, technology devel-

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Evaluation of the National Aerospace Initiative FIGURE 2-1 NAI integrated high speed/hypersonics and access-to-space ground and flight demonstration roadmap. SOURCE: Sega, 2003b. opment, ground testing, and flight demonstration plans required to mature enabling technologies to the point where they can be applied to operational hypersonic flight. The result should be a comprehensive integrated roadmap that assures sufficient maturation of all critical technologies prior to making FSD decisions on space access in 2018 and prior to proceeding with earlier possible hypersonics applications as well (e.g., missiles and aircraft). The end product should be a roadmap similar to the notional one presented in Figure 2-2 but also including detailed roadmaps for fundamental research and each of the critical technologies. Once complete, the plan should be clearly communicated to decision makers and stakeholders, including the public. Unless this detailed planning is done, insufficient information will exist upon which to base sound implementation and funding decisions. Finding 2-3. Based on information provided to the committee, the current level and stability of NAI funding for air-breathing hypersonics S&T are insufficient to achieve the NAI goals by 2018. Discussion 2-3. Using available data, the committee identified four critical and enabling technologies for air-breathing hypersonic flight that must be sufficiently matured to a technology readiness level (TRL) of 6 or 7 to support the near-term development of missile systems, the medium-term development of aircraft systems, and an FSD decision on access to space in 2018 (see Box 2-1).

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Evaluation of the National Aerospace Initiative FIGURE 2-2 Notional comprehensive hypersonic flight roadmap showing all requisite activities, from fundamental research to critical technology development to flight demonstration, for making a decision in 2018 on full-scale development of hypersonic air-breathing access to space systems. SOURCE: Adapted from Bowcutt, 2003. However, in the committee’s judgment, there is insufficient funding for maturing these technologies. The four critical technologies are these: Air-breathing propulsion and flight test, Materials, thermal protection systems (TPSs), and structures, Integrated vehicle design and multidisciplinary optimization, and Integrated ground testing and numerical simulation/analysis. A detailed discussion of each of these technologies, including assessments of current TRLs, technical feasibility in the NAI time frame, available and required budgets, and recommendations for near- and far-term technical emphasis, can be found in the next section, “Hypersonic Flight Critical Technologies.” Recommendation 2-3. DoD and NASA should complete an end-to-end cost estimate for the top-down program through the period of interest and then work to establish funding commitments consistent with the plan. Finding 2-4. The NAI has been very effective in fostering collaboration, communication, and advocacy of detailed technology development in air-breathing hypersonics across the services and

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Evaluation of the National Aerospace Initiative Box 2-1 Technology Readiness Levels Technology readiness levels (TRLs) refer to a system of metrics that is used to assess the maturity of a technology, compare the maturity of different technologies, and communicate technology maturity to others. The NASA TRL scale, which was developed for flight hardware items, can be seen at http://spacescience.nasa.gov/admin/pubs/handbook/OSSHandbook.pdf (accessed January 2, 2004). The committee used an extension of the standard NASA TRL scale to develop consistent TRL ratings for all the technologies reviewed, nonhardware items as well as hardware items. SOURCE: Bowcutt, 2003. NASA. While this has great potential for leveraging existing funding to the benefit of all stakeholders, there may be duplication in some of the contributing NAI programs. Discussion 2-4. The services, the Office of the Secretary of Defense (OSD), and NASA are highly engaged in formulating the NAI program. This has helped to tighten the coupling among the S&T communities within the services and NASA and encouraged the communication and collaboration that is now leading to a more efficient and rationalized NAI. This collaboration includes the warfighting community, although the formal process for formulating service requirements is not necessarily suited to the cross-agency focus of NAI. (See the discussion of warfighter requirements in Chapter 1.) Examples of cross-agency collaboration include the Air Force’s Hypersonics Technology (HyTech) and Single-Engine Demonstrator (SED) programs and NASA’s X-43C program. With the HyTech program, the Air Force is developing and ground testing a hydrocarbon scramjet engine. The Air Force SED program plans to flight test a single fixed-geometry HyTech engine

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Evaluation of the National Aerospace Initiative module in an expendable demonstration vehicle. NASA’s X-43C program plans to flight test three variable-geometry HyTech engine modules, also in an expendable demonstration vehicle. The goals and objectives of the X-43C and SED programs, as planned at the time this study was conducted, were similar. The most important technology demonstration for both the X-43C and SED programs is engine operability and performance across the operating range of the scramjet: from ignition, through ramjet-scramjet transition, to acceleration and cruise. Both programs share the same engine core—namely, the hydrocarbon-fueled scramjet of the Air Force’s HyTech program. A HyTech engine flight demonstration is particularly critical because of the use of hydrocarbon fuel and the need to use the endothermic capacity of the fuel to maintain heat balance during cruising flight (see discussion of hydrocarbon fuels in Discussion 2-7 and Appendix E). Other shared goals of the two programs are airframe, engine, and system integration; thermal protection system approaches; component durability; thrust, drag, and efficiency predictions; and flight control and management. The committee was impressed not only with the progress of the engine development program but also with the management structure and government participation in the program. In the committee’s opinion, the partnership between the government program managers and the contractors provides an excellent example of how such a management model can lead to rapid and continuous technology improvement. As far as this committee was able to determine, the primary difference between the planned X-43C and SED programs is the demonstration of multiple engine flow paths versus a single engine. The X-43C multiple-engine design addresses two propulsion technology issues not shared by the SED program: the simultaneous control of multiple engines and a variable-geometry inlet required to effect engine control. Under NAI, it might be possible for the Air Force and NASA to collaborate even further on an expendable vehicle flight demonstration. Instead of separate flight tests, the Air Force and NASA might be able to devise an alternative approach that would achieve the combined goals and objectives of the two similar programs. For example, development and incorporation of an engine control system that prevents engine flameout or inlet unstart and demonstration on a single-engine module in flight might mitigate the risk of multiengine operation sufficiently to permit the use of multiple engines in a later reusable flight demonstration. The committee notes that canceling or restructuring existing programs can be politically and contractually sensitive. The potential benefits of doing so must be weighed against the real-world costs. There can be strong, legitimate reasons for conducting development and test programs with similar objectives. Recommendation 2-4. NAI should continue in its role of advocating, communicating, and facilitating the elements of the nation’s HS/H and space access endeavors and should encourage a global view beyond the institutional constraints imposed on the individual partners. In this role, NAI should also identify and eliminate unnecessary duplication of planned activities to maximize the utilization of resources available for achieving NAI goals. Specifically, NAI programs should be critically reviewed to discern if closer collaboration and consolidation might eliminate potentially wasteful duplication. Finding 2-5. The high heat loads of air-breathing hypersonic flight place severe demands on the materials of which the vehicle, its propulsion system, and the mission sensors are made. Although significant advances have been made in materials for both active and passive thermal protection systems, much work remains to be done. The TRL assessments for materials presented to the committee by the Air Force and NASA were markedly different.

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Evaluation of the National Aerospace Initiative Discussion 2-5. Hypersonic flight poses well-recognized challenges in terms of materials that will survive the high temperatures and high heat loads imposed by flight environments. For example, at speeds above about Mach 8 (and at lower speeds inside the engine), some regions on hypersonic vehicles will exceed current material temperature limits of approximately 3000°F, making active cooling of even the most heat-resistant materials a requirement in these areas. Despite the challenges, much progress in materials development has been made. During the National Aerospace Plane (NASP) program, a dedicated NASP materials and structures augmentation program was funded as a part of the overall effort (SAIC, 1995). The extensive development and testing from this program have significantly extended the range of materials available for application to hypersonic vehicles, but much work remains to be done to fully qualify the wide range of materials required for all the anticipated needs of vehicles and their propulsion systems. Specific materials issues are discussed at greater length in the subsection on materials. NASA has developed a structured approach to classifying the TRL for various technologies including materials for hypersonics systems. This appears to be an excellent way to ensure that an objective cross-technology approach is used to judge the development of a given technology. In a presentation to the committee, a representative of NASA Langley (McClinton, 2003) provided data indicating that the TRLs for many of the materials required for a reusable Mach 7 hypersonic demonstrator vehicle are between 4 and 6. On the other hand, in a presentation made on the same day to the committee, a representative of AFRL (Evans, 2003) indicated that the technology readiness of the materials for the same vehicle and propulsion system was much lower, though the rating system used by the Air Force was not numerical and therefore difficult to correlate. Both NASA and the Air Force made the point that significant effort will be needed to develop materials for high heat flux applications such as leading edges and combustor panels. However, the committee tended to agree more with the Air Force assessment—that is, that the materials currently available are not sufficiently mature to ensure that a successful program can be executed with an acceptable level of risk.2 In spite of this situation, significant advances have been made in certain critical materials technology areas. Some of these advances were not presented in any detail to the committee, but they should be mentioned here as they could have great impact on the success of future hypersonics systems. Two examples of such technologies are toughened ceramic tiles and actively cooled carbon-silicon carbide ceramic composites. Both are recent advances that hold significant promise and will be discussed at greater length in the materials subsection of this chapter. Another important aspect of materials for hypersonic systems is their supply chain. Many of the specialized materials that have been studied for such applications have only been produced in limited quantities. Examples of such materials are high-conductivity composites, titanium matrix composites, and actively cooled ceramic composites. If these materials are to be used in large quantities, qualified sources of high-quality material will have to be developed. Recommendation 2-5. Aggressive materials technology development should continue at all participating NAI agencies, in line with NAI goals and objectives. All agencies and contractors working on technologies applicable to hypersonics, and in particular those working on the development and application of materials, should uniformly adopt the NASA TRL system of assessing technology readiness and reconcile differences in technology evaluations. 2   The committee discussed the reasons for the different perspectives of the Air Force and NASA and concluded that one reason was a difference in requirements and the other was a matter of communications. Suggestions for addressing the communication issue are proposed in Recommendation 2-8.

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Evaluation of the National Aerospace Initiative Finding 2-6. The NAI plan does not appear to contain sufficient activity in combined ground testing and numerical simulation for hypersonics technology development. Discussion 2-6. Only a few hypersonics high-enthalpy facilities exist in the United States. They all suffer from limitations. Facilities used to test scramjet engines for relatively long duration are vitiated (impure) air tunnels in which the free stream contains combustion products and in which the enthalpy is limited to below Mach 8. The shock tunnels and expansion tunnel at the Calspan-University of Buffalo Research Center, Inc. (CUBRC), the Caltech T5 shock tunnel, and the General Applied Sciences Laboratory (GASL) expansion tube range in enthalpy up to Mach 20 but are all short-duration facilities (1 to 10 ms) and also have other limitations, including free stream dissociation. Nevertheless, through a balanced combination of hypersonics high-enthalpy facility testing and numerical simulation, tunnel free stream conditions can be well characterized, and new and important effects can be discovered that will contribute to the development of vehicle design tools. New flow variables that come with high enthalpy, such as vibrational excitation and species concentration, can be measured by modern optical diagnostic techniques, which have, however, only been applied to high-enthalpy ground testing facilities in a few very limited cases. More detailed measurements in the free stream and in the flow fields of tested articles—particularly in the engine combustor of direct-connect tests—could provide essential data for validating simulation methods. A further technology shortfall is an inadequate knowledge of reaction rates, in particular the coupling of vibrational excitation, dissociation, and surface chemistry. The large number of dimensionless variables involved in high-enthalpy flows makes it impractical to develop a vehicle by testing in cold hypersonic and visciated air facilities alone. Ground testing in hypersonics high-enthalpy facilities using modern optical diagnostic methods, closely coupled with computational investigations of the same flow, is needed to develop numerical simulation tools that take proper account of high-enthalpy effects. Such tools can then be used with greater confidence in the design and preparation of flight tests. Recommendation 2-6. NAI’s plan should include as a substantial part of the HS/H pillar the development of combined ground testing and numerical simulation technologies which provide better and more reliable design tools for high-enthalpy flows. Such technology development requires much more 6.1 funding than is now available. Finding 2-7. In hydrocarbon-fueled hypersonic engines, the endothermicity of the fuel plays a critical role in cooling the system, limiting the maximum operational speed of systems using such engines. In contrast to this challenge are the operational benefits of storable fuels such as hydrocarbons, yet we find that no continuing fuels development program is being funded in the NAI that might increase the endothermic capacity of storable fuels and permit higher operational speeds. Discussion 2-7. Although not critical to the achievement of NAI’s present Mach number objectives for hydrocarbon-based, air-breathing hypersonic propulsion, a comprehensive research and development program in endothermic fuels could remove the Mach 7 upper speed limit imposed by relying on JP-7 fuel. Higher maximum speeds using hydrocarbon fuels might enable the application of such fuels for access-to-space missions while retaining their inherent logistical advantages. The maximum Mach number of a hypersonic cruise vehicle is determined by its heat-balance capability at sustained cruise speed. The heat has two sources: the recovery temperatures present in boundary layers over the vehicle and in its inlet, and the conversion of the fuel’s chemical energy to thermal energy via the combustion process inside the scramjet engine and in some cases extending into the exit nozzle. Clearly, flow path designs can play an important role in minimizing the heat

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Evaluation of the National Aerospace Initiative load by, for example, minimizing surface-to-flow area configurations. Low-heat-rejection materials and structures must be pursued for the combustor so that it can run hotter, significantly reducing the heat removal requirement over the present 2000°F obtainable using superalloys (cf. subsection on materials). In the end, only the fuel itself (and to a minor extent radiation) is available for cooling. More information on fuels and a possible fuels development program can be found in Appendix E. Recommendation 2-7. A long-term and sustained fuels program should be established to explore the Mach number limits of hydrocarbon and other storable fuels. Finding 2-8. No independent advisory board has been formed to help NAI establish and achieve program goals, objectives, and planning, as called for in the documentation provided to the committee (Sega, 2003a) and shown in Figure 2-3. Discussion 2-8. The committee was generally impressed with the progress that the NAI has made in facilitating the coordination of the participant’s activities. However, it believed that this coordination effort would benefit from periodic oversight of NAI by an independent group of experts. Recommendation 2-8. DoD and NASA should set up their planned advisory panels, steering groups, and revolutionary concepts panels to review the NAI program on a continuing basis. Related to this, NAI should form a critical technology coordination office for each of the four enabling hypersonic technologies, the purpose of which would be to enhance coordination of development efforts among the different agencies and contractors. Finding 2-9. Survival of a long-term initiative like the NAI depends on tangible technical products being produced along the way, yet many of the potentially viable weapons and other applications FIGURE 2-3 Structure of the NAI Executive Director’s Office. SOURCE: Sega, 2003a.

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Evaluation of the National Aerospace Initiative enabled by air-breathing hypersonics technologies, which are clearly available along the roadmap to space access, are not sufficiently (or equally) emphasized in the stated goals of the program. Discussion 2-9. One of the committee’s most difficult tasks was trying to concisely state the objective of the NAI. To sustain a national commitment to such a long-term goal, there must be a perception that the nation is continuing to benefit at each step of the way. Essentially all of the roadmaps reviewed by this committee indicated that there are many technologies in the early stages of development that could be matured and applied to DoD near- and mid-term products that address quick response and global reach. An example is a missile that could be developed based on the HyFly or SED demonstrator vehicles, both of which will fly in the next 2 or 3 years. Supporting this possibility, C4ISR, a critical enabler for the effective use of hypersonic missiles, has developed to the point where the timeline for finding, selecting, and engaging targets (the “kill chain”) is becoming short enough to take advantage of the rapid response afforded by hypersonic missiles. Despite the possibilities, these early spin-off products were not emphasized very strongly as being specifically part of the goals and objectives of NAI. Rather, they were discussed in a casual, offhand fashion. Recommendation 2-9. NAI should develop a concise description of its goals and objectives, giving equal weight to early applications achievable in the near and medium term, including hypersonic missiles and aircraft that are already part of the HS/H pillar. Recommendations 2-10, 2-11, and 2-12 are found in the first subsection of the section that follows, “Hypersonic Flight Critical Technologies.” HYPERSONIC FLIGHT CRITICAL TECHNOLOGIES This section covers the technologies that are critical for air-breathing hypersonic flight and discusses their current readiness levels. Because some of these technologies are analysis tools, processes, or ground tests—that is, not directly part of a flight system—the standard NASA TRL scale was extended to develop consistent TRL ratings for all technologies reviewed (see Box 2-1). The committee notes, however, that achieving high TRLs in the various technologies associated with air-breathing hypersonics does not guarantee that the system-level goals will be achieved. For example, one might be able to cool surfaces exposed to air with recovery temperatures of 6000 K, but will one be able to turn around a vehicle within 12 hours of such a flight? Can a reusable vehicle be refurbished for $1 million dollars or less? High TRLs help, but they do not guarantee successful system application or mission benefit. Before proceeding with detailed discussions of its technology assessments, the committee summarizes these assessments in Table 2-1. The committee divides the major HS/H technology areas identified by it into constituent technologies and provides its assessment of (1) the current maturity of the constituent technology, (2) the impact of the constituent technology on the major technology area, (3) the likelihood that the technology will achieve the desired results, and (4) the overall criticality of each of the five major technology areas. Both the impact and the criticality evaluations are based on the committee’s estimate of the anticipated contribution to the overall NAI goals—economics, safety, reliability, responsiveness, performance, and benefit to the industrial base. The impact was assigned descriptors according to the following (increasing) scale: negligible impact, little impact, moderate impact, significant impact, and extreme impact. Maturity levels are assigned numerical values corresponding roughly to the DoD S&T categories (not to be confused with TRLs, which are stated in parentheses): (1) basic research, (2) applied research, (3) advanced development, (4) demonstration/validation, and (5) engineering and manufacturing development.

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Evaluation of the National Aerospace Initiative (Rogacki, 2003; Lyles, 2003), where declining budgets after FY 2006 are evident. Clearly, significantly increased budgets will be required to execute the entire NAI plan to 2018, and the funding will have to come from the Air Force and/or NASA. Materials, Thermal Protection Systems, and Structures NASA and Air Force representatives presented the state of materials development to the committee. They made clear that although significant progress has been made in recent years, additional progress must be made rapidly if the schedule laid out in the NAI program is to be kept. Because of the broad array of structural requirements, heat loads, and environments in a typical hypersonic vehicle and propulsion system, a full treatment of this topic would require a dedicated report. Instead, this report will focus on three important areas of materials technology, as follows: Thermal protection systems (TPSs), Actively cooled combustor panels, and Cryogenic tanks. The viability of hypersonic flight vehicles depends, to a large extent, on the availability of lightweight TPSs. Such systems may be either cooled (active) or uncooled (passive), and both ceramic and metallic TPS approaches have been successfully applied. Most of the windward surface of the space shuttle orbiter is subjected to moderately high heat loads during reentry, and it is protected by silica foam insulation tiles (Korb et al., 1981). Although these tiles have proven to be reliable in service from a safety perspective, they are extremely fragile and subject to damage from impact by rain, ice, and other objects. They also require frequent recoating and repair and are therefore not suitable for use where rapid response is a requirement. Several approaches have been taken toward designing a more rugged passive TPS system. One of the more promising uses a Nextel-fiber-toughened outer layer bonded to conventional silica foam tiles using a monazite (LaPO4) powder binder (Davis et al., 2000). The resulting aluminamonazite ceramic composite has been demonstrated to provide the necessary toughness and damage tolerance for an impact-resistant thermal protection system. Metallic TPSs have also been investigated and tested, but the operational experience is much less extensive than with the ceramic and ceramic composite systems. One advantage of the metallic TPS approach is that active cooling can be incorporated for the highest heat flux areas, such as leading edges and nose cones. In the propulsion system of a hypersonic vehicle, active cooling of certain components such as the cowl lip and combustor panels will be necessary. Although extensive testing of various candidate designs has been accomplished, no single solution that solves all the operational requirements has emerged (Sillence, 2002). In the combustor section of a hypersonic propulsion system, active cooling is necessary, even at modest Mach numbers. Figure 2-6 shows the wall temperature of uncooled combustors (the adiabatic wall temperature) as a function of Mach number, along with the maximum use temperature of a variety of candidate combustor materials (Faulkner et al., 2003). Metallic combustor panels using modifications of existing rocket combustion chamber designs have the appeal of considerable operational experience but tend to be too heavy. Recently, several ceramic matrix composite (CMC) designs have been fabricated and tested (McClinton, 2003). Although some of these concepts show significant promise, none completely satisfy the requirements and all need significant additional effort to reduce risk to the point where they could be incorporated into an operational hypersonic propulsion system.

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Evaluation of the National Aerospace Initiative FIGURE 2-6 Combustor wall temperature as a function of Mach number. Active cooling is required in the combustor. Asterisk denotes theoretical structural limit; oxidation limit is much lower. SOURCE: Faulkner et al., 2003. Cryogenic tanks have received considerable attention in recent years, and both metallic and composite designs that meet design criteria are available. The development of friction stir welding (FSW) technology has greatly improved the producibility and quality of advanced aluminum tanks, while advances in the understanding of large composite structures’ manufacturing has improved the efficiency and integrity of composite tanks. The remaining step is to produce and flight test a fully reusable, full-scale test article through multiple cycles (Chase et al., 2002). For materials, TPSs, and structures, air-breathing approaches to hypersonic speeds are far more demanding than rocket-based approaches owing to sustained, high-aerodynamic-heating environments, particularly above Mach 8. There seems to be a validation gap between numerous new high-temperature materials and their confident application. Further, NASA seems to be much more optimistic than the Air Force about the technological readiness of many of these materials. This suggests the NAI program needs Better coordination and agreement between the DoD and NASA on the definition of material readiness and A reinvigorated basic and applied research budget to move these materials further along in their TRLs. The feasibility of achieving adequate TRLs in the near term appears good for a Mach 8 missile. As the vehicle size increases, the risk increases appreciably, particularly at the higher Mach numbers. Table 2-3 quantifies this in terms of committee TRL estimates. Explicit estimates of the funding needed depend to a large extent on vehicle Mach number. To attain Mach 8 in the near term on a scale suitable for a missile application with an acceptable level

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Evaluation of the National Aerospace Initiative TABLE 2-3 Estimates of Current TRL Status for a Number of Critical Airframe and Engine Materials   Mach 3-7 Hydrocarbon Mach 3-7 Hydrogen Mach 7-14 Hydrogen Engine materials Combustor panels (metallic) 7 (6) 5 (4) 4 (3) Cooled CMC panels 4 (3) 3 (2) 3 (2) Cowl lip 5-6 4 3 Injectors 5-6 5-6 4 Seals 4 4 3 Sensors 5 4 3 Airframe materials Cryogenic tanks (Al) 5 (4) 5 (4) 4 (3) Cryogenic tanks (Gr-Ep) 4 (3) 3 3 Leading edges 5-6 5-6 4 TPS 8 (7) 8 (7) 7 (6) Structure 6 (5) 6 (5) 4 (3) NOTE: Numbers in parentheses are for large-scale, reusable applications. SOURCE: McClinton, 2003; Bowcutt, 2003. of risk, a modest increase in the present investment levels is probably adequate. However, to attain Mach 15 in a large-scale vehicle by 2018 would probably require increasing the materials technology investment by a factor of 5 to 10, resulting in a total investment level of $150 million to $200 million per year. Integrated Vehicle Design and Multidisciplinary Optimization Integrated vehicle design and multidisciplinary optimization have been identified by the committee as a critical technology for the HS/H pillar of the NAI. Air-breathing hypersonic vehicles will consist of highly integrated systems. At hypersonic Mach numbers, much of the airframe must act as the inlet and nozzle for the propulsion system. The airframe must also mitigate the effects of large propulsive lift and pitching moments and large Mach and dynamic pressure variations in flight. The shape of the vehicle will determine the vehicle structure, the type of integrated thermal protection system and its material, the control system, and the flight mechanics and trajectory. The flight trajectory will in turn determine the aerodynamic heating loads that will influence vehicle aeroelastic behavior, aeropropulsive performance, TPS, and hence empty weight. The empty weight of the vehicle is also directly related to the structural shape, and the vehicle airframe will affect the fuel and payload volume since, unlike conventional aircraft, the majority of the volume in a hypersonic vehicle must accommodate fuel. The application of multidisciplinary design optimization (MDO) is vital to obtaining robust vehicle designs that satisfy all constraints, including off-design performance. MDO can also allow the designer to investigate systematically the complex interaction of design trade-offs between the necessary disciplines, changes to the objective function, the sensitivity of the design to various parameters, and uncertainty. MDO in engineering design is essentially the solution of single or multiple optimization problems constructed from a coupled system of single-discipline, black-box legacy computer codes as opposed to the solution of a single tightly coupled code. MDO objective functions and constraints depend not only on optimization variables but also on ancillary variables such as solutions

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Evaluation of the National Aerospace Initiative to coupled systems of stand-alone solvers as surrogates—for example, discipline-specific partial differential equations, table look-ups, or other nonsmooth analysis codes. This has important implications for optimization since the user must be concerned with how data are transferred between surrogates and which data are transferred. In addition, the objective function and constraint values may be expensive to compute and may be nondifferentiable and discontinuous. Adding to the difficulties, many nonlinear programming techniques require that the user open the single-discipline solvers in order to evaluate internal parameter derivatives, if they exist. This can be costly and is generally resisted by the single-discipline specialist, though progress is being made in providing this necessary information via automatic differentiation. When applied to typical engineering problems, MDO will give a point solution—that is, the solution that satisfies an objective function and set of constraints. However, this solution is rarely the one that aircraft designers need. There is almost always more than one objective involving a mixture of continuous, discrete, and categorical parameters, and the objectives often conflict with one another. The solution is usually one that comes from studying the trade-offs between these competing objectives. It must also satisfy the problem constraints and give a robust estimate of the likely value of the objective function in the actual design. Moreover, the aircraft designer will often choose computational efficiency over finding a better solution, but obtaining efficiency is especially problematic when dealing with fluid dynamics. For example, varying the parameter values requires regridding the spatial domain around configurations and rerunning computational fluid dynamics (CFD) solvers. The effort expended to do this is dependent on whether the CFD solver uses a structured or an unstructured grid and on the degree of fidelity of the fluid flow equations being solved: Euler vs. Navier-Stokes, for example. Efforts are being made to automate the layout for both structured and unstructured grids. The model fidelity issue is sometimes addressed by using low-fidelity surrogates at the start of the optimization and increasing the fidelity as the optimal solution is approached, a procedure called management of variable fidelity surrogates. A method used to address the issue of fidelity is simply to increase the computational resources through inexpensive supercomputing or grid computing. In any of these approaches, determining the effect of a design parameter adjustment as it propagates through all of the surrogates is not straightforward. Arguably, the most critical hurdle in conventional aircraft design is the understanding, tracking, and management of uncertainty in and between surrogates within the MDO framework. In addition to the previously mentioned weaknesses in applying MDO to conventional aircraft design, the hypersonic vehicle designer must be able to construct high-fidelity fluid dynamics surrogates in the hypersonic regime. The construction of high-fidelity hypersonic fluid dynamic surrogates will require the following:3 Improved understanding and modeling of shock and turbulent boundary layer interactions, Measurements of nitrogen oxide and other species to support high-enthalpy wind tunnel studies, Accurate modeling of finite-rate chemistry effects on turbulent boundary layers, Validation and verification (V&V) of boundary layer transition models, including the effects of finite-rate chemical reactions, V&V of Reynolds-averaged Navier-Stokes (RANS) models for hypersonic flows, including flows with large favorable and adverse pressure gradients, V&V of finite-rate chemistry models, since the limits of kinetics models used for air, hydrocarbon-air, and hydrogen-air are not well known, especially under thermal nonequilibrium conditions, 3   Graham Candler, University of Minnesota, personal communication to committee member Kevin Bowcutt in October 2003.

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Evaluation of the National Aerospace Initiative TABLE 2-4 Committee Estimates of the Current Technology Readiness Levels of MDO Applied to High Speed/Hypersonics Vehicle Design MDO Components TRL Years to a TRL of 6 Nonlinear optimization 6-7 0 Automatic differentiation 6-7 0 Mixed-variable optimization 5 2 Grid computing 5 2 Automated data transfer between surrogates 4-5 5-7 Parametric geometry 4-5 5-7 Robust solutions through optimization 3-4 5-7 Automated grid generation 2-5 5-7 Management of variable-fidelity surrogates 2-3 5-7 Uncertainty management 2-3 5-7 High-fidelity HS/H surrogates 1-5 10-15 V&V of HS/H surrogates 1 10-15 Development of hybrid direct simulation Monte Carlo–Navier-Stokes (DSMC–NS) solvers for high-altitude aerodynamics, and Development of large-eddy simulation and discrete-eddy simulation (LES/DES) methods for highly compressible flows, including shock waves, fluid mixing, and chemical reactions. Current TRLs of the MDO components applied to aircraft design vary from high to low. Processes that are well understood and mature include nonlinear optimization techniques and automatic differentiation. Of moderate readiness are mixed variable optimization and grid computing. Automated data transfer between discipline-specific surrogates can be done regularly through the commercially available ModelCenter code4 and the publicly available Design Analysis Kit for Optimization and Terascale Applications (DAKOTA) code.5 Immature and low-TRL components include the determination of robust solutions through optimization, the efficient propagation of design parameter changes with faster automated regridding, management of variable-fidelity surrogates, and uncertainty management of the discipline-specific surrogates. The components of lowest TRL are high-fidelity HS/H surrogates and the V&V of these surrogates. The TRL for each of the MDO components is given in Table 2-4. The displayed TRL estimates are the product of this NRC committee only and are not based on government sources. Numerous researchers around the country are successfully addressing these challenges in MDO. Researchers include staff of NASA Langley,6 the AFRL,7 Sandia National Laboratories’ Advanced Simulation and Computing program,8 Los Alamos National Laboratory, and various universities around the country. The committee believes the number of years to a TRL of 6 shown in Table 2-4 will hold if these groups collaborate and if NASA’s uncertainty management and MDO efforts are funded over the next 10 years to a total of at least $200 million. It must be emphasized that the problems specific to hypersonic vehicle design are long term and will require several more years of adequate and sustained funding at a level of several tens of millions more dollars to fully solve. 4   Accessed at http://www.phoenix-int.com/products/ModelCenter.html. 5   Accessed at http://endo.sandia.gov/DAKOTA/software.html. 6   Accessed at http://mdob.larc.nasa.gov. 7   Accessed at http://www.va.afrl.af.mil/COE/MDT/mdt_index.html. 8   Accessed at http://www.nnsa.doe.gov/asc/program_overview.htm.

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Evaluation of the National Aerospace Initiative Integrated Ground Test and Numerical Simulation/Analysis Introduction Success in both external hypersonic flow and in air-breathing propulsion is critically dependent on the integration of ground testing (GT) and CFD for a number of reasons. At Mach numbers greater than, say, 8, the stagnation temperature is greater than 3000 K, and above Mach 12, greater than 7000 K. Materials from which ground test facilities are made cannot sustain associated heat loads for extended periods, so that short-duration methods such as shock tunnels and expansion tubes must be resorted to. Effects of prolonged heat loads on materials and engine control experiments are therefore inaccessible to such facilities. Also, only some of the relevant dimensionless parameters can be duplicated exactly in any one facility. The most successful experiments involving air-breathing combustion have, to date, been so-called direct-connect experiments, and accurate whole-model drag measurements in ground test facilities at high enthalpy are nonexistent. An urgent need in all high-enthalpy, short-duration facilities is adequate characterization of the free-stream conditions. In particular, the chemical composition and translational and vibrational temperatures are inadequately known. Diagnostic methods for measuring instantaneous planar distributions of particular chemical species concentrations and vibrational and translational temperatures do exist, but they are expensive and difficult to apply to large facilities. The need for such measurements applies equally to test article flow fields, of course, and is therefore not restricted to free-stream conditions. Computational techniques, while largely successful in many low-Mach-number flow applications, where high-enthalpy or finite-rate effects, and even high-compressibility effects, do not cause problems, suffer severely when the Mach number exceeds 8 or so. Chemical reaction rates have often only been measured at temperatures lower than those encountered in practice, so unjustified extrapolation is generally used. Furthermore, rates depend on the state of nonequilibrium that exists, and very little is known about that dependence. Coupling between vibrational excitation and dissociation can cause rates to be significantly changed, and models for these processes have not been adequately validated. Similarly, models for surface chemistry and the chemistry of hydrocarbon combustion are in need of validation. Large-eddy simulation of highly compressible, reacting turbulent flows involving shock waves is almost entirely undeveloped. The number of parameters for a vehicle development program is increased very substantially by increasing the flight Mach number from, say, 3 to 8 or more. This makes it impractical to employ flight testing for exploratory purposes. The philosophy for making significant progress must therefore be as follows: Characterize the free-stream conditions of short-duration facilities as accurately as possible, using modern diagnostic tools. Perform benchmark experiments in well-characterized flows, again using sophisticated modern diagnostics. Perform identical experiments in different facilities. Compute the benchmark experiments with existing numerical techniques to validate or establish doubts about the techniques. Use validated techniques for conditions that lie off or between benchmark conditions for development work (including different scales and different flow conditions). At the same time, it is most important that improved computational methods be developed, taking advantage of Moore’s law rates of increase in computational power, and that the most effective diagnostic methods be selected and new ones developed in appropriately funded 6.1 programs. The most effective funding for research of this type is presently provided by the Air

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Evaluation of the National Aerospace Initiative Force Office of Scientific Research (AFOSR); however, AFOSR’s annual budget has decreased (in inflation-adjusted dollars), from $456 million in 1965 to $218 million in 2000, while the number of topics supported has increased significantly. NASA resources are similarly small and, though apparently focused, are in fact quite heterogeneous because of the large number of principal investigators in each effort. Many of the scientific aspects of the challenges in computational methods are the same ones that also face scientists in the national laboratories of the DOE. It is therefore worth exploring collaborations with these persons and labs to leverage funding sources. Existing and Projected Ground Testing Facilities The facilities existing in the United States for testing at enthalpies corresponding to Mach numbers greater than 10 include the shock tunnels and the expansion tunnel at the Calspan-University of Buffalo Research Center, Inc. (CUBRC); the T5 hypervelocity shock tunnel at the California Institute of Technology; the expansion tube at the General Applied Science Laboratory (GASL); and the G-Range reflected shock tunnel at the Arnold Engineering Development Center (AEDC). All of these are short-duration facilities operating in the test-time range from 1 to 10 milliseconds, depending on enthalpy, with the shorter times corresponding to higher enthalpy. All of these facilities (except the G-Range) have been extensively used for both fundamental research and industrial testing. Characterization of the flows in these facilities is not sufficiently complete. The most detailed characterization has been done in one of the shock tunnels at CUBRC by comparing computational predictions of the tunnel nozzle flow and the flow over a sensitive shock-wave–boundary-layer interaction model with experimental measurements (Candler et al., 2002). Information about the composition and vibrational excitation of the free stream in all the facilities is indirect, via comparisons of experimental and computed flows over models using surface heat flux, interferometry, or pressure measurements. Existing optical diagnostic techniques should be used to measure composition and vibrational excitation directly (Bessler et al., 2002; Ben-Yakar and Hanson, 2002; Rossmann et al., 2002). The lack of such data is directly caused by the high cost of running the facilities and the high cost (or development cost) and sophistication of the diagnostics. Detailed information about flow noise is also lacking. The larger size of the facilities at CUBRC makes them more suitable for industrial testing. Also, the expansion tunnel at CUBRC is a facility in which effective reservoir pressures up to 600 MPa can be reached. The T5 at Caltech has mainly been used for high-enthalpy fundamental research, which has discovered important, previously unknown effects. An ongoing effort by Princeton University is to work toward a facility in which high-pressure air is first expanded to a low supersonic Mach number in a nozzle, then heated by magnetically controlled electron beams, and subsequently expanded to a high Mach number in the same nozzle. This device, if successful, would produce a hypersonic flow with virtually pure air and very low nitric oxide concentration for a projected run time of 10 seconds. So far, elements of this process have been demonstrated at below projected power, but the facility is very far from completion, and many details of the principle of operation are still uncertain. The facility may not be operational for at least 10 years. Facilities suitable for testing material properties under high heat load need to run for extended periods to give the heat time to diffuse into the solid. For this purpose arc tunnels provide a compromise. In such tunnels the gas is heated by an electric arc in the reservoir region of a continuous expansion through a water-cooled nozzle. Though the flow is continuous, the pressure in such facilities is much lower than in shock tunnels, so the free stream is much more highly dissociated and the flow field not as faithfully simulated. However, surface heat flux can be

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Evaluation of the National Aerospace Initiative approximately duplicated for sufficiently long times to test materials. The largest such facility in the United States is operated at AEDC. Longer test times (seconds rather than milliseconds) are also required for engine control development and investigation of aeroelastic phenomena. No ground test facilities exist at this point that can provide the necessary capability at Mach numbers greater than 8. Uncertainties in Computational Methods Some of the uncertainties in the application of computational methods to high-Mach-number flows have nothing to do with the algorithms employed. Rather they stem from the fact that the chemical reaction rates needed both for the dissociation and recombination in air and for combustion are not sufficiently precisely known. In particular, the reaction rates for gases in vibrational nonequilibrium (vibration-dissociation coupling) are subject to considerable uncertainty. The same applies to surface chemistry. Above Mach 14, oxygen dissociates, and surface recombination must be considered for all TPS material and integrated into computations. Neglect of this effect could result in insufficient thermal margin in the design of hypersonic vehicle thermal protection systems (Jumper, 1995; Jumper and Seward, 1994). The prediction of transition from laminar to turbulent boundary layer flow is critical to the design of hypersonic vehicles. While computational techniques have not succeeded in making predictions of transition at high Mach numbers, linear stability calculations combined with experiment have yielded insights showing that vibrational excitation can strongly influence the path toward, and therefore the location of, transition (Johnson et al., 1998; Adam and Hornung, 1997; Stuckert and Reed, 1994). However, much more work is needed in this area to gain sufficient confidence for design. Computational techniques need particular efforts in the simulation of turbulence. In particular, LES techniques for high-enthalpy reacting flows must be developed in order to improve predictions of important quantities in turbulence. Roadmap In accordance with the philosophy outlined earlier, the roadmap for combined research in ground testing and computation should proceed along the following lines, which can be related to the notional timeline shown in Figure 2-4. One purpose of combining ground testing with computation is to produce validated computational tools that can subsequently be used in design. To get there, identical benchmark experiments must be performed in the different facilities. The free stream of the facilities must be characterized accurately by combining modern diagnostic measurements with computation. Computations of the benchmark flows have to be made and compared with the data from experiments. Time required, 7 years; cost (facility upgrades and characterizations, benchmark experiments, computation), $80 million. For each of the vehicles in NAI, external flow and integrated airframe-propulsion testing at the correct enthalpy and covering the parameter space is necessary in combination with application of validated tools to gain more confidence. The facilities most suited for this work are the AEDC tunnels in the Mach 5 to 8 regime and the CUBRC shock tunnels above Mach 8. Time frame, 2006-2015; cost, ~$1 million per vehicle per test. Full-scale engine testing is not possible in currently existing ground test facilities, especially above Mach 8, but missile-scale engines can and should be directly ground tested at all designed-for operational speeds. Engines for hypersonic vehicles larger than missiles, on the other hand, can only be tested at subscale. Properly validated analysis tools must therefore be used to

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Evaluation of the National Aerospace Initiative FIGURE 2-7 Perceived facility and instrumentation upgrade needs and costs ($7 million over 2 years). SOURCE: Holden, 2003. supplement engine ground testing. The time and cost profiles are contained in the subsection on propulsion. A 10-year research program on hypersonic flows, emphasizing high-enthalpy effects on each of the following: Transition from laminar to turbulent boundary layer flow, Turbulent boundary layers and shear flows, Shock wave-boundary layer interaction, Fuel injection, mixing, and reaction with air, and Integrated airframe-propulsion performance and operability. Full advantage should be taken of modern optical diagnostics and computation for all of these. Time, 10 years; cost, $50 million. Technology Readiness Levels TRLs in the integrated ground test and numerical simulation/analysis area are as follows: High-enthalpy ground test facilities (subject to earlier described limitations): 5-6 (see Figure 2-7 for perceived upgrade needs and costs), Optical diagnostic methods: 5-6, Computational methods: 4-6 (high-enthalpy LES, 4; others higher), Chemical reaction rates at nonequilibrium conditions: 4-5, and Surface chemistry: 4-5.

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Evaluation of the National Aerospace Initiative REFERENCES Published Adam, P.H., and H.G. Hornung. 1997. Enthalpy effects on hypervelocity boundary-layer transition: Ground tests and flight data. J. Spacecraft and Rockets 34(5): 614-619. Bartolotta, P., and N. McNelis. 2002. RTA-Demonstrating TBCC Technologies. 1st AIAA/IAF Symposium on Future Reusable Launch Vehicles. Huntsville, Ala. April. Ben-Yakar, A., and R.K. Hanson. 2002. Characterization of expansion tube flows for hypervelocity combustion studies. J. Propulsion and Power 18(4): 943-952. Bessler, W.G., C. Schulz, T.Lee, J.B. Jeffries, and R.K. Hanson. 2002. Quantitative NO-LIF imaging in high-pressure flames. Appl. Phys. B-Lasers O 75(1): 97-102. Bradley, M.K., K.G. Bowcutt, J.G. McComb, P.A. Bartolotta, and N.B. McNelis. 2002. Revolutionary Turbine Accelerator (RTA) Two-Stage-to-Orbit (TSTO) Vehicle Study. AIAA Paper 2002-3902. 38th Joint Propulsion Conference. July 7-10. Candler, G.V., I. Nompelis, M.C. Druguet, M.S. Holden, T.P. Waldhams, I.D. Boys, and W.L. Wang. 2002. CFD validation for hypersonic flight: Hypersonic double-cone flow experiment. AIAA Paper 2002-0581. Chase, R., and Ming Tang. 2002. The quest of single stage earth-to-orbit: TAV, NASP, DC-X and X-33 accomplishments, deficiencies and why they did not fly. AIAA Paper 2002-5143. Curran, E.T. 2001. Scramjet engines: The first forty years. Journal of Propulsion and Power 17(6): 1137-1192. Davis, J.B., D.B. Marshall, P.E.D. Morgan, K.S. Oka, A.O. Barney, and P.A. Hogenson. 2000. Fuzzy logic control of structural vibration of beams. AIAA Paper 2000-0172. Hueter, U., and J. Turner. 1999. Rocket-based combined cycle activities in the Advanced Space Transportation Program Office, AIAA 99-2352, 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Los Angeles, June 20-24. Johnson, H.B., T. Seipp, and G.V. Candler. 1998. Numerical study of hypersonic reacting boundary layer transition on cones. Phys. Fluids 10(10): 2676-2685. Jumper, E.J. 1995. Recombination of oxygen and nitrogen on silica-based thermal protection surfaces: Mechanism and implications. NATO ASI Molecular Physics and Hypersonic Flows. Maratea, Italy, May-June 1995. Jumper, E.J., and W.A. Seward. 1994. Model for oxygen recombination on reaction-cured glass. Journal of Thermophysics and Heat Transfer 8 (July-September): 460-465. Korb, L.J., C.A. Morant, R.M. Calland, and C.S. Thatcher. 1981. The shuttle orbiter thermal protection system. Amer. Cer. Soc. Bulletin 60(11): 1188. McClinton, C.R., R.R. Reubush, J. Sitz, and P. Reukauf. 2001. Hyper-X program status. AIAA Paper 2001-1910. AIAA/ NAL/NASDA-ISAS 10th International Space Planes and Hypersonic Systems and Technologies Conference, Kyoto, Japan, April. Powell, O.A., J.T. Edwards, R.B. Norris, and K.E. Numbers. 2001. Development of hydrocarbon-fueled scramjet engines: The Hypersonic Technology (HyTech) program. Journal of Propulsion and Power 17(6): 1170. Rossmann T., M.G. Mungal, and R.K. Hanson. 2002. Evolution and growth of large-scale structures in high compressibility mixing layers. J. Turbulence Vol. 3, article no. 009. SAIC (Science Applications International Corporation). 1995. National aero-space plane advanced technology impacts, Final Report. Sillence, M.A. 2002. Hydrocarbon scramjet engine technology flowpath component development. AIAA Paper 2002-17-5158. Stuckert, G., and H. Reed. 1994. Linear disturbances in hypersonic chemically reacting shock layers. AIAA J. 32(7): 1384-1393. Waldman, B.J., and P.T. Harsha. 1990. The National Aero-Space Plane program. AIAA Paper 90-5252. Unpublished Bowcutt, K. 2003. Hypersonic Technology Status and Development Roadmap. Briefing by Kevin Bowcutt, Chairman, Boeing Technical Fellowship Advisory Board, to the Committee on the National Aerospace Initiative. September 3. Evans, D. 2003. National Aerospace Initiative: Materials and Processes. Briefing by Daniel Evans, AFRL, to the Committee on the National Aerospace Initiative. September 4. Faulkner, R.F., and R. Kazmar. 2003. Pratt and Whitney Hypersonics Overview. Briefing to the Committee on the National Aerospace Initiative on the occasion of its site visit. November 7. Graff, G. 2003. HyFly. Briefing by Gil Graff, Office of Naval Research, to the Committee on the National Aerospace Initiative. October 7.

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Evaluation of the National Aerospace Initiative Hickman, R., G. Keether, D. Boudreaux, P. Stewart, and G. Hernandez. 2003. Operationally Responsive Spacelift. Briefing by Robert Hickman, The Aerospace Corporation, to the Committee on the National Aerospace Initiative. October 7. Holden, M. 2003. Facility and Implementation Improvements. Prepared by Mike Holden, CUBRC, in response to questions by committee member Hans Hornung. Lewis, T. 2003. The Role of Turbine Engines in NAI: AFSTB NAI Review. Briefing by Timothy Lewis, AFRL, to the Committee on the National Aerospace Initiative. October 7. Lyles, G. 2003. NGLT Briefing to the Air Force Science and Technology Board. Briefing by Garry Lyles, NASA Marshall, to the Committee on the National Aerospace Initiative. October 7. McClinton, C. 2003. NASA Hypersonics. Briefing by Charles McClinton, NASA Langley, to the Committee on the National Aerospace Initiative. September 4. Morrish, A. 2003. DARPA Programs for the National Aerospace Initiative Objective. Briefing by Art Morrish, DARPA, to the Committee on the National Aerospace Initiative. August 5. Rogacki, J. 2003. The National Aerospace Initiative—A Synergistic Partnership between NASA and DoD. Briefing by John “Row” Rogacki, NASA, to the Committee on the National Aerospace Initiative. August 5. Sega, R. 2003a. National Aerospace Initiative—NRC Briefing. Briefing by Ronald M. Sega, DDR&E, to the Committee on the National Aerospace Initiative. August 6. Sega, R. 2003b. National Aerospace Initiative—NRC Briefing. Briefing by Ronald M. Sega, DDR&E, to the Committee on the National Aerospace Initiative. November 10. Walker, B. 2003. National Aerospace Initiative and the U.S. Army. Briefing by Billy Walker, U.S. Army Aviation and Missile Command, to the Committee on the National Aerospace Initiative. August 6.