2
Responses to Parts 1 and 2 of the Statement of Task

The first section of this chapter describes the overall picture that emerged from briefings to the committee on the Air Force HyTech Program. Next, the committee presents its question-by-question responses to Part 2 of the Statement of Task. The committee’s responses are based on its expertise and the technical information gathered during the study.

OVERALL PICTURE

The opening paragraph of the Statement of Task indicated that the committee should concentrate on the “strategy and content” of the Air Force HyTech Program (see Box 2-1 for a summary of the program and Appendix B for a detailed description). Part 1 of the Statement of Task asked the committee to focus on the “technologies needed to demonstrate a hypersonic, air-breathing missile concept, using hydrocarbon-based propulsion technology for the Mach 8 regime, in time to achieve an initial operational capability of 2015 or sooner.”

When this study began, many committee members assumed that the HyTech Program was a component of a broader program to demonstrate the technologies for a hypersonic, air-breathing missile system capable of speeds up to approximately Mach 8. During the initial visit to Wright-Patterson Air Force Base in July 1997, the committee learned that, because of very limited funding, the Air Force had decided to concentrate almost solely on the propulsion subsystem of a representative hypersonic vehicle and to conduct only a limited ground-test demonstration of a single Mach 8 hydrocarbon-fueled engine flow path. Under the circumstances, the committee considers that this was a wise decision, although full integration with a flight vehicle and flight testing, which are especially important for a compact missile system, were not included in the program. The HyTech Program now has only one propulsion contractor, which limits the alternatives for the engine design.1 The program should consider using design reviews by independent experienced engineers, who could challenge the configuration selections and accompanying analyses and suggest alternatives for evaluation. Collateral development of the airframe, munitions, and the guidance and control and navigation system, as well as their integration, will have to be the subjects of follow-on programs. To the committee’s knowledge, the Air Force is not pursuing these collateral areas.

Because Part 1 of the Statement of Task asked the committee to emphasize the “assumptions that underlie technical performance objectives and the operational requirements for hypersonic technology,” the committee assumed that the Air Force had established a valid operational requirement (with specific performance objectives) for this kind of missile. To the committee’s surprise, no operational requirements for a system using this technology have been established, although the committee found several statements by the Air Force and the Navy describing missions for which an air-breathing hypersonic missile would be valuable. In the absence of operational requirements, there has been considerable speculation in the technical community about the operational parameters of such a weapon. The Air Force and Navy have established only general technical performance goals that clarify the parameters of the missile systems (e.g., top speeds of Mach 8 and Mach 6, respectively).

The potential improvements in warfighting capability offered by a hypersonic, scramjet-powered, air-to-surface missile (e.g., speed, standoff range, and kinetic energy) could be substantial. However, the technical challenges escalate as the maximum Mach number increases (e.g., from Mach 6 to Mach 8), and the development of a vehicle with a maximum speed of Mach 8 would require significant technological breakthroughs. Thus, the Air Force will need a carefully planned technology validation program that includes a prototype flight vehicle to control risks. The committee proposes a validation program in this chapter. The committee believes that, if the completion of the HyTech Program by 2003 is followed expeditiously by prototype flight testing,

1  

See discussion of the original plan in Appendix B. Also, see response to Question 2d for a discussion of the engine performance goals.



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Review and Evaluation of the Air Force Hypersonic Technology Program 2 Responses to Parts 1 and 2 of the Statement of Task The first section of this chapter describes the overall picture that emerged from briefings to the committee on the Air Force HyTech Program. Next, the committee presents its question-by-question responses to Part 2 of the Statement of Task. The committee’s responses are based on its expertise and the technical information gathered during the study. OVERALL PICTURE The opening paragraph of the Statement of Task indicated that the committee should concentrate on the “strategy and content” of the Air Force HyTech Program (see Box 2-1 for a summary of the program and Appendix B for a detailed description). Part 1 of the Statement of Task asked the committee to focus on the “technologies needed to demonstrate a hypersonic, air-breathing missile concept, using hydrocarbon-based propulsion technology for the Mach 8 regime, in time to achieve an initial operational capability of 2015 or sooner.” When this study began, many committee members assumed that the HyTech Program was a component of a broader program to demonstrate the technologies for a hypersonic, air-breathing missile system capable of speeds up to approximately Mach 8. During the initial visit to Wright-Patterson Air Force Base in July 1997, the committee learned that, because of very limited funding, the Air Force had decided to concentrate almost solely on the propulsion subsystem of a representative hypersonic vehicle and to conduct only a limited ground-test demonstration of a single Mach 8 hydrocarbon-fueled engine flow path. Under the circumstances, the committee considers that this was a wise decision, although full integration with a flight vehicle and flight testing, which are especially important for a compact missile system, were not included in the program. The HyTech Program now has only one propulsion contractor, which limits the alternatives for the engine design.1 The program should consider using design reviews by independent experienced engineers, who could challenge the configuration selections and accompanying analyses and suggest alternatives for evaluation. Collateral development of the airframe, munitions, and the guidance and control and navigation system, as well as their integration, will have to be the subjects of follow-on programs. To the committee’s knowledge, the Air Force is not pursuing these collateral areas. Because Part 1 of the Statement of Task asked the committee to emphasize the “assumptions that underlie technical performance objectives and the operational requirements for hypersonic technology,” the committee assumed that the Air Force had established a valid operational requirement (with specific performance objectives) for this kind of missile. To the committee’s surprise, no operational requirements for a system using this technology have been established, although the committee found several statements by the Air Force and the Navy describing missions for which an air-breathing hypersonic missile would be valuable. In the absence of operational requirements, there has been considerable speculation in the technical community about the operational parameters of such a weapon. The Air Force and Navy have established only general technical performance goals that clarify the parameters of the missile systems (e.g., top speeds of Mach 8 and Mach 6, respectively). The potential improvements in warfighting capability offered by a hypersonic, scramjet-powered, air-to-surface missile (e.g., speed, standoff range, and kinetic energy) could be substantial. However, the technical challenges escalate as the maximum Mach number increases (e.g., from Mach 6 to Mach 8), and the development of a vehicle with a maximum speed of Mach 8 would require significant technological breakthroughs. Thus, the Air Force will need a carefully planned technology validation program that includes a prototype flight vehicle to control risks. The committee proposes a validation program in this chapter. The committee believes that, if the completion of the HyTech Program by 2003 is followed expeditiously by prototype flight testing, 1   See discussion of the original plan in Appendix B. Also, see response to Question 2d for a discussion of the engine performance goals.

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Review and Evaluation of the Air Force Hypersonic Technology Program BOX 2-1 Summary of the HyTech Program The HyTech Program was established in 1995 at the direction of the Secretary of the Air Force as a follow-on program to the National Aero-Space Plane Program. The HyTech Program focuses on the development of generic, hypersonic technology. The program was designed to follow a stepping-stone approach focusing initially on hydrocarbon-fueled scramjet missiles with speeds up to Mach 8. The speed of Mach 8 was chosen in part by technical factors and the results of studies indicating to the Air Force that this speed could have a significant payoff for projected missions (e.g., the ability to attack time-critical targets). The choice of Mach 8 also appears to have been driven by a desire to explore the upper limits of hydrocarbon-fueled scramjet technology. Originally, the program was intended to address primarily engine technology, with a companion program in airframe technology. However, soon after it began, the HyTech Program was restructured to concentrate its very limited funds exclusively on engine technology. (HyTech was given a nominal $20 million per year funding, but the actual funding has consistently fallen short of that figure.) Currently, engine development is the critical path of the program, which is managed by the Air Force and carried out by industry under contract. Two prime contractors working on two engine concepts constituted the technical core of the program when this study began. Since then, the Air Force has selected one contractor to continue the program. The selected engine concept is slated to be tested in ground-test facilities in the 2001 to 2003 time frame. The managers of the program were directed to coordinate their activities with other programs in the U.S. Department of Defense, as well as with other government organizations (e.g., NASA), industry, and academia, if doing so would accelerate engine development or extend the U.S. hypersonics technology base. the Air Force could have, by 2015, an initially operational air-breathing hypersonic missile with a maximum speed in the range of Mach 6 to Mach 8. The current HyTech Program, which is a propulsion technology flow path program for a missile boosted to Mach 4 and then accelerated by its scramjet engine to Mach 8, is not sufficient for the development of a design concept with the engine integrated into a missile system. The planned ground testing of a non-integrated propulsion subsystem should indicate potential engine performance, but only flight testing over a representative range of operating conditions will determine the operability, reliability, and durability2 of the engine in an integrated system. These parameters are prerequisites to determining the utility of the engine in an operational system. The HyTech Program will not, by itself, provide the basis for an operational missile system because critical enabling technologies for hypersonic, air-breathing missiles are not part of the program. To the committee’s knowledge, these critical technologies are not being pursued by the Air Force. These technologies will have to be matured and validated before the Air Force can proceed with a low-to-moderate risk acquisition program. (The critical technologies are discussed in the responses to specific questions.) To develop an operational hypersonic missile system, the Air Force would have to take a two-part approach. First, the HyTech Program would have to be expanded to include a full-scale, airframe-integrated, engine flight test program; if the critical enabling technologies were mature, an operational air-breathing hypersonic missile system could be developed with low-to-moderate risk and without concurrency (almost certainly for a speed of Mach 6 and probably for a speed of Mach 8). This expanded HyTech Program could lead to an initial operational capability by 2015. Second, the Air Force would have to establish operational requirements for the system. The committee believes the Air Force has not yet undertaken design and requirements trade-off studies in support of an air-launched hypersonic missile system. The committee recognizes the difficulty of attempting to make trade-offs between systems that have not yet been developed, especially systems that require technologies that are not mature enough to indicate potential risks relating to performance and affordability. Trade-off studies would be especially difficult for a Mach 8 missile, which would require technologies at the leading edge of development. The committee considered this point carefully when attempting to differentiate between a Mach 6 system, for which the committee believes the technologies are within reach, and a Mach 8 system, which would stretch current technological boundaries. The necessity of having data about the cutting edge technologies in hand for an effective engineering and manufacturing development phase is the basis 2   The word “durability” in this report implies that the missile system and its components must be designed to operate successfully for a one-time 12-to-15 minute flight. The committee points out, however, that durability considerations must go beyond the rigors of the boost, cruise, and terminal phase flight environments and include storage, handling, and in-flight carriage.

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Review and Evaluation of the Air Force Hypersonic Technology Program for the committee’s opinion that flight demonstration should be the next phase of the HyTech Program. The committee believes that the problems raised by the cutting edge technologies could be overcome in the Air Force’s time frame. The Air Force can be reasonably confident that early, fundamental trade-off studies will yield useful conclusions, especially in terms of cost and mission effectiveness. The most basic trade-off study has yet to be done to determine the Mach number of the first scramjet missile system. Many parameters of the basic weapon system could then be determined, even with the current state of development of the various technologies. During one briefing, the committee was informed that a hypersonic missile with range parameters similar to the Air Force concept (i.e., 750 nautical miles) would be subject to the limitations of existing arms control treaties. The committee did not verify this information or attempt to determine how the design, development, testing, or deployment of a hypersonic missile might be influenced by specific treaty provisions. Nevertheless, because there is a possibility that arms control treaty limitations could affect either missile design parameters or the designation of the launch aircraft, the Air Force should consider all missile design parameters, all aspects of the development and testing program, and all steps leading to initial operational capability in light of applicable treaties. In the following sections, the committee responds to each question in part 2 of the Statement of Task. These responses contain findings, conclusions, and suggestions, the most important of which are drawn together and sharpened in Chapter 4, Conclusions and Recommendations. The committee also created a technological road map for an acquisition program that would lead to the subject missile system in response to Question 2e(i) and made some first-order mission analyses in response to Questions 2a(ii) and 2c(iii) (Appendix C contains supporting details). Both the road map and the analyses are offered as starting points for more extensive work and future decisions by the Air Force. MEETING OPERATIONAL REQUIREMENTS, QUESTION 2a(i) Will the Hypersonic Technology Program, as planned by the Air Force Materiel Command (all references to the hypersonic program are directed at this specific program rather than broader contexts), lead to a capability which will meet operational requirements for hypersonic technology applications? Summary Answer The Air Force HyTech Program, as currently structured, will not lead to an operational capability. Furthermore, the Air Force has not defined operational requirements for the system. Detailed Answer The Air Force HyTech Program described to the committee appears to be well thought out and has made efficient use of available funding. However, the program, as currently structured, will not enable the development of an initial operational capability by 2015. The program lacks the breadth, depth, and funding necessary “to demonstrate a hypersonic, air-breathing missile concept” (Statement of Task, Part 1). The HyTech Program includes a test demonstration of a flight-like configuration of each engine component in a wind tunnel or test cell environment in fiscal years 2001 and 2002. The program concludes with a ground test demonstration of a complete engine flow path with flight-like components in a free-jet wind tunnel in fiscal year 2003. The program apparently does not include plans or funding (which would be considerable) for flight tests. The committee was not informed of collateral plans to address in depth the critical technologies beyond the propulsion system (e.g., thermal structures or guidance and control systems) or of a comprehensive plan to integrate the diverse technologies critical to air-breathing hypersonic flight. Although the committee was briefed on mission needs (e.g., the need to strike time-critical and exceptionally hardened targets), there was no indication that the Air Force had conducted trade-off studies or assessments to determine if an air-launched, air-breathing hypersonic missile would be the best way to satisfy them. Recommendation. The Air Force should initiate trade-off studies for the design and requirements of a hypersonic missile system. Analyses should include the following parameters: targets, speed, range, survivability, lethality, aircraft compatibility, risk, and cost. These analyses would provide a basis for articulating valid operational requirements for a hypersonic missile system. The HyTech Program, which is focused on technologies for a hydrocarbon-fueled scramjet, is currently structured as though its goal is to provide an evolutionary improvement of the propulsion subsystem of a mature, state-of-the-art vehicle development process (e.g., manned, tactical aircraft). In fact, the United States has no capability to engineer or manufacture operational, hypersonic, air-breathing missiles. The HyTech Program is the descendant of ramjet and scramjet propulsion programs. Unfortunately, these early programs provide little data on the performance, operability, reliability, and affordability of hydrocarbon-fueled engines, especially in the Mach 4 to Mach 8 range. The committee believes that developing these data will require investments of money and time that are well beyond the HyTech Program. The HyTech Program does not have the funds to inspire confidence in the design and development of an operational propulsion subsystem. In fact, the committee has learned that the HyTech Program has actually received much less than

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Review and Evaluation of the Air Force Hypersonic Technology Program the planned $20 million per year (see Appendix B). If this situation persists, the current HyTech Program may not be able to meet its completion date of 2003, which would make an initial operational capability by 2015 difficult. The HyTech Program is just one of many ongoing laboratory Air Force programs. The deficiencies in the HyTech Program reflect both funding shortfalls and the highly competitive nature of these science and technology programs. In light of the Air Force’s lack of commitment to flight testing a hypersonic air-breathing missile, and of the United States’ limited experience with scramjet propulsion, the committee makes the following recommendation. Recommendation. The Air Force should commit appropriate resources to completing integrated airframe-engine flight testing. Flight tests are vital to demonstrating a hydrocarbon-fueled scramjet in the Mach 4 to Mach 8 regime. If the Air Force decides not to make this commitment, it should reevaluate its goals for the development of air-breathing hypersonics technology. Because critical technologies will have to be developed, integrated, and tested, the program will have to be much more substantial than an ordinary laboratory program. Recommendation. If the Air Force determines that there is a requirement for a hypersonic missile system, then it should establish a system-oriented program office to manage the design and development, integration, and flight testing of critical enabling technologies for a hypersonic missile system. The program office should report directly to a senior official in a weapon system organization and should have multidisciplinary participation, including experienced design engineers of air-breathing propulsion systems. The committee believes the Air Force must take these steps in the near term for the successful development and application of hypersonic technology by 2015. In its response to Question 2e(i), the committee provides a notional road map for an air-launched hypersonic missile that would be operational by 2015. Based on the road map, the demonstration of a Mach 4 to Mach 8 air-breathing missile configuration with first-generation capability (i.e., delivery of a useful payload over a useful range at an affordable cost) will require a joint, shared-risk venture among technologists, acquisition offices, and operators of the system. The development of a system with first-generation operational capability will take a strong team effort. TECHNOLOGIES OTHER THAN PROPULSION, QUESTION 2a(ii) What technologies (beside propulsion) should next be pursued, and in what priority, for a hypersonic air-to-surface weapon? Summary Answer Several critical enabling technologies besides propulsion will have to be developed for a hypersonic air-to-surface weapon. In order of priority, the five most critical technologies are (1) airframe and engine thermostructural systems; (2) vehicle integration; (3) stability, guidance and control, navigation, and communications systems; (4) terminal guidance and sensors; and (5) tailored munitions. Detailed Answer The committee believes that the development of a hypersonic vehicle will require much more than the current HyTech Program is scheduled to accomplish. Five critical technologies will also have to be developed. In order of priority, these are (1) airframe and engine thermostructural systems; (2) vehicle integration; (3) stability, guidance and control, navigation, and communications systems; (4) terminal guidance and sensors; and (5) tailored munitions. Each of these technologies is discussed below. Airframe and Engine Thermostructural Systems Hypersonic vehicles with air-breathing engines will have to fly at high dynamic pressures to capture sufficient air for a combustion rate of fuel that will produce thrust levels significantly greater than the drag of the vehicle. The combinations of Mach number and dynamic pressure will mean vehicle and engine flight environments with high temperatures and heating rates and large aerodynamic forces. (Figure 1-2 in Chapter 1 illustrates the dynamic pressures and total temperatures.) Figure 2-1 shows that the thermal environment ranges from temperatures of about 1,100°F at Mach 4 (the temperature of the incoming airflow on the radiatively cooled surface) to about 4,200°F at Mach 8. Figure 2-1 also shows the likely range of temperatures of critical components of the surface of an airframe structure that is not actively cooled, as well as of an airframe structure that uses liquid cooling at roughly Mach 6 to Mach 8. The temperatures in Figure 2-1 for the airframe structure that is not actively cooled agree with recent calculations based on computational fluid dynamics presented to the committee by the Air Force. These temperatures indicate the maximum temperature of the vehicle inlet compression ramp. Slightly lower temperatures would be expected for the swept leading edges of the control surfaces. Much lower temperatures would be expected on the remaining external vehicle structures. Some flight maneuvers could cause shock waves that could intersect the leading edges of the airframe and engine surfaces, creating localized regions of much higher temperatures. The committee was informed by the Air Force that the thermal and mechanical loads, which include heating of edges and amplification of shock waves at the missile’s

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Review and Evaluation of the Air Force Hypersonic Technology Program FIGURE 2-1 Airframe structural temperature requirements. Source: Ram air total temperatures for air in chemical equilibrium are from Pruitt, 1987. Temperatures of the cooled and not actively cooled airframe structures were calculated by the committee. surface, were compared with analytical predictions for engine components at Mach 5.6. However, the environment of the engine and airframe at Mach 6 to Mach 8 is not fully known because no extensive testing has been done under relevant conditions or in flight. The engine of an air-breathing hypersonic missile will operate in the most demanding environment in the structure. The effects of Mach number on the temperatures for the engine combustors and nozzles are shown in Figure 2-2; temperatures for the inlets are shown in Figure 2-3. As Figure 2-2 shows, at Mach 8 the maximum gas total temperature at the combustor exit could exceed 5,000°F, which is in keeping with estimates by the HyTech Program. Air and radiation cooling will reduce the structural temperatures in the engine combustor at Mach 4 to Mach 6 to a range of 2,100°F to about 3,000°F; a fuel-cooled engine combustor operating at Mach 6 to Mach 8 will be subject to structural temperatures of about 1,300°F to 1,500°F. The engine inlet, shown in Figure 2-3, will be subject to significantly lower temperatures. One major challenge in designing hypersonic vehicles is developing airframe and engine thermostructural systems that can withstand the thermal environments and the aerodynamic forces for the required life of the vehicle, which must have acceptable weight and damage tolerance and adequate operating margins. The Air Force must take into account the weight and cost associated with complex active cooling systems and with even more complex endothermic fuel-cooled systems. The hypersonic environment is extremely demanding for aircraft and space access vehicles that must be reusable and reliable over many flights during their lifetime. However, an expendable hypersonic missile would be used for only one short flight and would not be subject to the same creep and low cycle fatigue challenges. Therefore, thermostructural designs for a hypersonic missile could use well characterized, reliable, high-temperature materials, coatings, and processes, in combination with passive or active cooling. A hypersonic missile system will require the integration of materials with widely different thermal and mechanical properties into one structure. These materials must maintain their useful properties at elevated working temperatures; they must have high specific strength and stiffness, be oxidation resistant, and have high damage tolerance (fracture toughness) and adequate creep resistance to withstand the thermal and aerodynamic stresses. Perhaps most important, they must have several other desirable properties, including ease of fabrication, ease of joining and assembly, reasonable cost,

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Review and Evaluation of the Air Force Hypersonic Technology Program FIGURE 2-2 Structural temperature requirements of the engine hot section. Source: Ram air total temperatures for air in chemical equilibrium are from Pruitt, 1987. Engine cycle temperatures for the HyTech baseline hydrocarbon scramjet engine were provided in Mercier, 1998. Temperatures of the cooled and not actively cooled engine structures were calculated by the committee. reproducible properties, and an adequate supply base with sizes, quantities, and shapes suitable for fabrication. Recent HyTech evaluations of subscale coupon-sized specimens (e.g., 1 in. × 2 in. × 4 in.) of candidate materials encompass a range of conventional materials (e.g., metallic alloys and superalloys) and advanced structural materials (e.g., composites, ceramics, refractory-based systems, and coatings). The facilities simulated, to various degrees, the relevant hypersonic engine thermal and flow path environment. Coupon tests so far have revealed that subscale specimens of many existing conventional and advanced structural materials and coatings can survive the simulated hypersonic conditions. More testing is being done. Advances in manufacturing technology and fabrication methods will be required for the routine production of cost effective, reliable structures in appropriate sizes. With tests under relevant conditions of structural components made of very dissimilar materials, the Air Force can make preliminary evaluations of their performance. To ensure that the structural designs can withstand the projected environment for the required time, actual conditions that the airframe and engine will encounter will have to be determined. Advanced structural materials often are subject to problems associated with scale-up because the properties of bulk materials processed in production quantities may not be identical to materials manufactured, synthesized, and tested in the laboratory. Other problems include the small number of materials suppliers and batch-to-batch and company-to-company differences in material properties. These problems are heightened with advanced materials, which may be particularly sensitive to minor changes in processing, chemistry, and secondary processing. Effective manufacturing technology and fabrication methods based on a systems approach will be necessary to minimize these problems. Thermostructural designs and technology developments for the engine are being pursued in the HyTech Program, which plans to fabricate the inlet and nozzle structure of either a carbon-carbon composite or a passively cooled ceramic composite (silicon carbide matrix with carbon fibers), both of which have sufficient thermal capability (with antioxidation coating) and would not require active cooling. In contrast, the conditions in the combustor would require active fuel-cooling of the metallic structure (known as Haynes 188).

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Review and Evaluation of the Air Force Hypersonic Technology Program Actively cooled engine structures, including combustors, have been used for many years in reusable systems. The Space Shuttle main engine, for example, has an actively cooled combustor and nozzle, cooled with the hydrogen fuel. The main combustor chamber operates at a pressure of 3,000 pounds per square inch and a temperature of about 5,500°F. The combustor material is NARloy-Z, a high conductivity, high strength copper-based alloy. The engine design life is 55 missions, or 450 minutes. A challenge for the hydrocarbon-fuel actively cooled system in the HyTech Program will be designing a system in which coking (i.e., carbon deposition) does not occur in the cooling passages, which would reduce the cooling of the structure. The design of the thermostructural system will require careful attention to the active cooling system, attachment techniques, and joining techniques for materials with different thermal expansion characteristics. The entire system will have to be tested and validated at the required operating conditions. On the leading tips and edges, and on portions of the control surfaces, the airframe will require materials that retain their strength at very high temperatures in the presence of high-temperature gradients and significant aerodynamic loads. Aerodynamic design and the associated pressure loads must be considered in combination with the thermal loads. The metallic structure for the nose will probably have to be actively cooled with the fuel. Uncooled ceramic structures could probably be used for the control surfaces, depending on their configuration and degree of sweep. If not, passive heat pipes or active fuel cooling could be used. For radomes or windows, active surface film cooling would probably be required to protect them from the heat. All protuberances would require thermal protection. The remainder of the airframe could be made entirely of metal, such as a titanium-based or nickel-based superalloy. Many of the internal systems would have to be insulated. Low weight and high strength design structures could be, for example, monocoque structures for circular airframe configurations or honeycomb-sandwich structures for other configurations. One of the primary challenges for the thermostructural design of the entire vehicle is optimizing the selection of materials and structural architecture so it can not only FIGURE 2-3 Structural temperature requirements of the engine cold section. Source: Ram air total temperatures for air in chemical equilibrium are from Pruitt, 1987. Engine cycle temperatures for the HyTech baseline hydrocarbon scramjet engine were provided in Mercier, 1998. Temperatures of the cooled and not actively cooled engine structures were calculated by the committee.

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Review and Evaluation of the Air Force Hypersonic Technology Program withstand the conditions of the flight environment but can also meet the missile size and weight requirements. Although technologies are available that can meet these requirements, vehicle design should be combined with cost-effective manufacturing techniques and fabrication methods, as well as with adequate testing and validation, to produce an affordable integrated vehicle design that can survive the hypersonic environment. Vehicle Integration Because no one has extensive experience with hypersonic vehicles powered by air-breathing engines, there are many uncertainties in vehicle integration. Nevertheless, the committee believes that, with careful attention to integration issues and the development of coordinated analysis tools, integration of the vehicle will be possible. Scramjet-powered missiles, by their very nature, demand high levels of vehicle integration. The airframe is part of the engine. Volume and weight constraints also make the job of subsystem packaging difficult and require complex component interactions, which can be crucial in the areas of the integration of engine and airframe and sensors and munitions. Vehicle-level requirements, including the requirements for acceleration and maneuverability, will have a first-order effect on the integrated vehicle-engine design. Integration of the missile with a carrier aircraft will place weight and size constraints on the missile design that may affect the engine configuration. Deployment of the missile from the aircraft will require that attention be paid to vehicle aerodynamics. Because a scramjet engine does not produce thrust in its ramjet mode until it reaches a speed of Mach 3 or higher, the scramjet-powered vehicle will be the second stage of the system; the first stage will use a rocket booster. The rocket booster, its integration with the missile, and the associated separation will obviously affect the integrated vehicle-engine design. Notwithstanding their superior performance, air-breathing engines are inherently more difficult to integrate into missiles than rocket engines. Depending on the configuration, integration can be similar to the integration of air-breathing engines in vehicles that operate at lower speeds. Consider the two notional missile configurations shown in Figure 2-4, for example. The axisymmetric design in Figure 2-4 shows that the engine airflow is captured near the front of the vehicle, transported internally through ducts, and exhausted axially at the aft end of the vehicle. This type of integration requires FIGURE 2-4 Comparison of moderately integrated axisymmetric design and highly integrated asymmetric design.

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Review and Evaluation of the Air Force Hypersonic Technology Program careful separation of thrust and drag forces during the development process, but the engine performance and vehicle control are relatively independent. By contrast, the asymmetric configuration suggests a highly integrated engine and airframe design.3 The development of the vehicle control system requires a detailed understanding of engine operation because the internal and external flow fields are coupled (i.e., through the engine and around the exterior of the missile). Ground test facilities will have to simulate both flow fields accurately to demonstrate the integrated engine performance accurately. The asymmetric airframe design offers a considerably higher level of performance but requires considerably more attention to vehicle-engine integration issues than the axisymmetric design. In addition to the issues of engine and airframe integration, several other integration issues must be addressed. If system studies indicate the need for a terminal guidance system, the vehicle design will have to accommodate forward-looking radomes or windows. If a seeker is required, it may affect the design of the scramjet inlet and overall vehicle aerodynamics. Cooling requirements or temperature limitations would affect either the design or the allowable trajectory. The integration of munitions in the missile would affect the allowable size and volume of subsystems and could require that the missile size and shape be modified. If the munition is to be separated before detonation, vehicle trajectories may have to be modified. The dynamic pressure levels of 4,700 to 26,300 pounds per square foot, depending on pay load separation altitude (see Figure 1-2), will significantly affect the vehicle structural and control authority requirements during the unpowered descent. Stability, Guidance and Control, Navigation, and Communications Systems Guidance and control encompasses the following phases of a hypersonic missile’s flight: boost phase, cruise phase, and terminal phase. Only the first two phases will be addressed here; the terminal phase will be addressed in the next subsection. Stability, guidance, and control during the boost phase are performed by avionics on the vehicle. The vehicle will be equipped with an inertial navigation system that tells the avionics the location and orientation of the booster. The avionics will use fins or a thrust-vectoring system to maintain the stability of the booster and control its speed and location (longitude, latitude, and altitude), which depends on the location of the target, and the proper velocity for ignition of the missile’s air-breathing propulsion subsystem. When the hypersonic vehicle separates from the booster, the scramjet engine will be ignited, and the missile will accelerate to the cruise phase. During acceleration to cruising speed and during the cruise phase, the vehicle will use onboard sensors to stabilize and maneuver during free flight until it is close enough to the target to enter the terminal phase. The global positioning system (GPS—a constellation of satellites that determines precise location) will provide navigation commands and guide the vehicle to the point where it will dive toward the target. GPS units can be designed to operate on the vehicle. Onboard inertial sensors will provide the vehicle-based body measurements (e.g., angular rates) necessary for stabilizing the vehicle in its six degrees of freedom. The scramjet engine is expected to exert significant, and varying, forces orthogonal to the primary velocity vector of the vehicle. Based on detailed knowledge of the scramjet engine’s thrust vector, the control surfaces can be properly sized and the flight control system modeled to maintain vehicle direction and speed. A hypersonic missile will operate mostly autonomously, with various portions of the flight specified in preprogrammed onboard memory. Informing the missile of the target location can vary from simple to moderately complex, depending on the overall system philosophy. If the missile flies autonomously from the launch aircraft to the target, then only simple communications will be required, namely, a means to tell the missile where the target is prior to launch. A hard-wired command link through a pull-away connector could be used for simple communications. In this scenario, the target would have to be located very accurately, perhaps 15 minutes before intended missile impact. Another possibility would be low-rate radio frequency updates during flight. In this scenario, the antenna system would probably be designed to look in a specific direction (e.g., back at the launch aircraft, at the sky, or both) to avoid jamming. Messages could be relayed via satellite. An operational hypersonic missile might have no need to transmit information back to a base station. The hypersonic vehicle guidance and control system consists of several technical components (see simplified block diagram in Figure 2-5). Among these are several types of onboard sensors: (1) vehicle attitude and stabilization sensors (e.g., angular-rate sensors, linear accelerometers, and vertical gyros); (2) vehicle navigation sensors (e.g., GPS and an inertial navigation system); and (3) target-seeking sensors (e.g., radar, infrared, or visible sensors). Air data (e.g., angle of attack, dynamic pressure) are also necessary for the guidance and control system. Conventional air data sensors will probably not be suitable, but new sensors for this high-speed, streamlined vehicle could probably be developed. Most likely, air data will be derived from the inertial data and a vehicle aero-model in software. The guidance and control system will have to be able to effect changes in the vehicle’s angular attitude and lateral, longitudinal, or vertical position (i.e., control surfaces, such as ailerons, fins, and 3   The asymmetric HyTech engine configuration, which is fully integrated into the airframe, is the baseline configuration considered in this study.

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Review and Evaluation of the Air Force Hypersonic Technology Program FIGURE 2-5 Block diagram of a guidance and control system of a nominal air-breathing hypersonic missile. rudders, or thrust-vectoring could be used). Control algorithms will have to be developed for software that can take in information from all of the sensors, compare it with what the vehicle should be doing, and send commands to the control effectors to correct the vehicle’s operation. Another important feedback control system in the vehicle is the engine control system, which will regulate the fuel flow to generate thrust, and hence the speed of the vehicle, and perhaps also cool the engine. The flight computer will contain the engine control system algorithms. The guidance and control system and the engine control system will have to be closely and intricately coordinated. Adding to the complexity will be asymmetric geometries and active cooling systems. Typical engine control effectors will be fuel pumps and valves; engine sensors will measure fuel flow, temperatures, and pressures. As far as the committee could ascertain, the Air Force has done no significant work so far on the guidance and control system, sensors, control effectors, or control algorithms for an air-launched, air-breathing hypersonic missile. The NASA HYPER-X program4 addresses a few of these issues but does not include work on relevant overall guidance and control issues important to a hypersonic missile system. The primary uncertainty in the guidance and control system prior to the terminal phase is in the control algorithms. The committee believes that substantial work on the design, analysis, and simulation remains to be done. Some of the work done on fighter aircraft and experience with other missiles may be applicable, but stabilization during the boost phase, during separation of the missile from the rocket booster, during scramjet ignition, and during re-ignition if the scramjet experiences a flame-out, are issues that have yet to be addressed. Another issue is the transition from the cruise phase to dive, at which point the dynamic pressure will be increasing, the speed will be decreasing, and the control authority of the control effectors will be increasing. The missile may have to have maneuvering capability to hit the target. The control algorithms will have to change the control loop characteristics dynamically (e.g., by scheduling the gain of the loop) during the dive. The design and validation of the control algorithms will require realistic simulation of the scramjet’s performance (including forces and moments) and of the vehicle’s aerothermodynamic environment throughout all engine operating conditions and all phases of the missile’s flight. The development of the simulation will be a significant project in its own right. The main uncertainty in the navigation system is the effectiveness of the GPS, which could be jammed by an enemy. A supplementary inertial navigation system will probably be used to provide continuity in case the GPS is not available and during intervals when the onboard GPS receiver is recontacting satellites. Studies will have to be conducted to determine how well the GPS and inertial system combination will perform in various scenarios. Because of the high speed of the hypersonic missile, the missile will travel through the zone of effective jamming rather quickly, thereby reducing the GPS-dropout interval, which could readily be filled in by the inertial navigation system. During 4   The HYPER-X program includes the development of technology and flight validation at Mach 7 and Mach 10 for a hypersonic aircraft configuration using a hydrogen-fueled, airframe-integrated scramjet. The program is discussed in more detail in the answer to Question 2d.

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Review and Evaluation of the Air Force Hypersonic Technology Program the cruise phase, the missile would be flying at high altitude with the GPS antenna5 on the top of the vehicle (shielded from an aircraft jammer), which should also reduce the chances of jamming. The GPS may not be as reliable during the dive because attempts at jamming will probably be intensified, and fewer GPS satellites will be in view of the antenna. Another concern with GPS is the potential destruction of GPS satellites by an adversary during a conflict. The loss of a single satellite would have no effect, but the loss of several would. A backup inertial navigation system, which would be less accurate, could also be incorporated. The main uncertainty in the radio frequency communications system involves the antennas, which must withstand the aerodynamic heating. The antennas may be on the underside of the vehicle, which is probably where the engine will be, and may protrude from the surface of the vehicle. Terminal Guidance and Sensors The terminal phase will begin at a designated point when the vehicle executes a high-g downward maneuver to head for the target. One concern during the terminal phase is the durability of the control surfaces, which will be subjected to aerodynamic heating and high dynamic pressures. Similar surfaces have been successfully developed for use under similar conditions on the Space Shuttle and on maneuvering re-entry vehicles. The terminal phase will require a combination of existing technologies for high-speed (e.g., reentry) vehicles and new technologies. Regardless of the target, the terminal guidance and control system poses several difficult challenges and stresses for the system. The maneuver from level flight at an altitude of approximately 100,000 feet into a steep-angled dive and striking the target on the ground will take less than a minute with speeds varying from Mach 8 to Mach 4 and will require very high accuracy. Control of the missile will be similar to the control of a re-entry vehicle, but with a more stringent accuracy requirement. Moreover, finding and hitting mobile, imperfectly located targets will require that the missile have target-seeking sensors and probably a large maneuvering footprint. Therefore, the committee believes that significant work, which is not yet funded, will be required for the development of an effective terminal sensing, guidance, and control capability. Proponents of an air-breathing hypersonic missile claim that one of its main advantages is that it can be used to destroy time-critical targets (i.e., it can be fired “from a fighter aircraft outside a heavily defended target area and yet reach time-critical targets, such as mobile launchers, before they could move any significant distance” [see Appendix B]). Whether the target is mobile or fixed, the committee believes (for reasons explained below) that sensing and guidance in the terminal phase of flight will be necessary. The requirements for hitting mobile, time-critical targets are considerably different than they are for fixed, hardened targets. Therefore, these two cases are treated separately below. Mobile, Time-Critical Targets. These targets include mobile tactical ballistic missile launchers and mobile air defense missile systems. Mobile systems are not hard to destroy if they can be hit; however, they are difficult to find and, once they have been found, they can be moved before they can be hit. Some mobile systems can be moved into or out of action in as little as five minutes (Yefremov and Svirin, 1998). To protect the aircraft launch platform and still provide coverage of the target, the range for a U.S. hypersonic missile may have to be 500 nautical miles or more. The range and average speed of the missile will determine the time line. It takes a missile with a top speed of Mach 8 about 12 minutes to fly 750 nautical miles, which is approximately one nautical mile per second. If the target is more than about 300 nautical miles away, the mobile target can be moved during the flight of the missile. Therefore, the missile will require a highly accurate terminal guidance and sensing capability that can direct the missile to the target area, which might be several nautical miles in diameter, and can subsequently search for and select the most important target. A hypersonic missile system should be able to operate in any weather. Cloud layers can interfere with the operation of optical or infrared sensors, and a cloud layer could mask the target until the final few seconds. If the target area has a diameter of several nautical miles, once the target is located, the missile (or its munition) must be capable of enough lateral acceleration to reach it. Developing a system with this level of agility will certainly be challenging. Radar sensors can be used to penetrate cloud cover and have the potential for longer range target detection. Synthetic aperture radars can be used to form images of stationary targets on the ground with resolutions on the order of one foot. Images of moving targets can be formed with Doppler tracking of strong scatters from a target. For a synthetic aperture radar on the missile6 to work effectively, the missile must either do a fly-over or must spiral toward the target. The deployment of munitions from the diving hypersonic missile would transfer this problem to the munitions, which would avoid many technical problems with the sensors, including heating of the infrared window and radome-induced thermal distortions, but would create new challenges for the munitions and their deployment. The committee is aware that the Air Force is developing “smart” submunitions that 5   Use of the GPS assumes the availability of antennas that can withstand the heat. 6   A synthetic aperture radar on another platform could be used to provide the missile with target information. However, the other platform would have to operate far enough from the target to be out of harm’s way.

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Review and Evaluation of the Air Force Hypersonic Technology Program could significantly affect overall system performance. Unfortunately, funding limitations have precluded the development of multiple approaches as a risk reduction measure. The committee believes that the selection of only one propulsion system contractor has increased the program risk. In addition to the technology areas listed above, several other propulsion-related technologies will have to be developed before a Mach 8 missile is ready for final development. These technologies include a fuel control system (pumps, piping, valves, regulators, and bladders) capable of controlling both liquid and gaseous fuel at temperatures between −65°F and 1,000°F, a complete thermal management system, and an electronic control system coupled with an active missile control system. A significant amount of work will be required to model the dynamics of the propulsion system. The HyTech Program plans to culminate with only a ground-test, free-jet demonstration of an engine with flight-like components. Therefore, complete engine demonstrations will have to be conducted in subsequent programs. Because no flight demonstration is planned for the HyTech Program, follow-on programs for the development of prototypes will be necessary before the Air Force can commit to the development of an operational system (see Question 2e(i)). The ground test demonstration will not be able to validate technologies in a complete simulation of the flight environment (e.g., the effects of simulated air, Reynolds numbers, and angles of attack) or to develop the testing methodology for integrating the engine into an airframe (e.g., test techniques and force and moment accounting schemes) or to evaluate missile concepts throughout the operational envelope (e.g., maximum and minimum altitudes, g-loading, and off-design performance). All of these will require further development after completion of the HyTech Program. Because the HyTech Program has been structured to develop the technologies for a missile that can cruise at Mach 8, the committee concluded that certain technologies, such as endothermic fuel-cooled engine structures, a two-phase fuel control system, and a cold-start combustion system, will have to be developed. All of these technologies are challenging, and their development may be expensive. The risk and cost associated with the development of hypersonic air-breathing systems increase significantly with higher cruise speeds. Scramjet technology or existing ramjet technology using (nonendothermic) fuel-cooled metallic structures could be used for Mach 4 to Mach 6 systems. But systems with a maximum cruise speed of about Mach 6 to Mach 6.5 will require a scramjet; nonendothermic fuel cooling or uncooled ceramic composite materials could be used. Above Mach 6.5, active cooling using endothermic fuels will be required. A missile designed for Mach 6 or Mach 6.5 will not survive operations at a significantly higher Mach number. A Mach 8 missile, although heavier, more costly, and less efficient than a Mach 6 to Mach 6.5 missile, could be operated at a lower Mach number to increase its range (see Figure 1-1). System studies should be done to determine the maximum required Mach number for the missile because it will affect both the capability and affordability of the system. Roughly speaking, Mach 6 to Mach 7 (nominally referred to as Mach 6.5) represents a boundary above which the technological challenges increase significantly and at which the technologies being addressed by the HyTech Program will be required. At speeds of less than Mach 6.5, the engine could be operated with a hot structure, which means the engine operation could be separate from the cooling requirements of the vehicle. Instead of endothermic fuel-cooled engine structures, for example, uncooled ceramic composite materials and structures,8 liquid fuel control systems, and liquid fuel ignition and combustion could be used. These technologies would probably be easier and less expensive to develop. The Air Force should complete the analyses and establish an operational requirement for a hypersonic missile so that the technologies being investigated by the HyTech Program can be affirmed or the program can be modified. PROPULSION UNCERTAINTIES, QUESTION 2c(i) What are the salient uncertainties in the propulsion component of the hypersonic technology program, and are the uncertainties technical, schedule related, or bound by resource limitations as a result of the technical nature of the task (e.g., materials sources, qualifications of support personnel, or technology driven costs that affect affordability), to the extent it is possible to enunciate them? Summary Answer The significant technical uncertainties in the overall propulsion system derive from budgetary limitations, are manifested by a lack of focus on risk reduction and on flight demonstration, and cannot be resolved until the current program is completed in 2003. Additional uncertainties exist in the areas of weight, reliability, and affordability. The HyTech Program has not adequately addressed trade-offs at the system concept level between propulsion system capabilities, mission performance, and reliability and affordability. Detailed Answer The uncertainties in the component performance and engine operation of the propulsion system fall into four broad categories: low-speed engine operation; high-speed engine operation; high-speed performance; and the engine thermostructural system. Each of these categories is addressed below. 8   The use of ceramic composite materials would presume they are available in the sizes, shapes, and quantities required and can be manufactured reliably, reproducibly, and cost effectively.

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Review and Evaluation of the Air Force Hypersonic Technology Program At low speeds, the air-breathing engine must operate from the end-of-boost condition, which the HyTech Program has set at Mach 4. Critical technologies to be addressed include starting the inlet, maximizing the inlet contraction, realizing diffuser performance, developing an engine cold-start capability, piloting the combustor process, and providing an effective flame-holding mechanism. The air-breathing engine must operate up to the high-speed cruise condition, which the HyTech Program has set at Mach 8. Critical engine technologies to be addressed include boundary layer transition in the inlet, combustion piloting at high altitude, integration of engine operation and the flight control system (see response to Question 2a(ii)), and matching the heat load on the vehicle and engine with the cooling capacity of the endothermic fuel system. During this study, the committee was briefed by representatives of industry and the HyTech Program on an array of fuels and fuel blends. The committee was not able to delve deeply into the question of the fuel composition but notes that the choice of fuel is an important factor in the successful operation of a scramjet propulsion system. In addition to cooling capacity, other important fuel characteristics include energy density per unit weight and per unit volume, ignition limits, flame speed, and long-term physical and chemical stability. The Air Force should carefully consider these characteristics in system trade-off studies. The air-breathing engine must perform efficiently through a range of Mach numbers and at the high-speed cruise condition. Critical engine technologies include maximizing the inlet efficiency, minimizing losses associated with the combustor piloting system, minimizing the heat transfer and shear force losses in the combustor, and limiting recombination losses in the nozzle over the operating range of Mach numbers, altitudes, and angles of attack and sideslip. Engine concepts incorporate a combination of materials to satisfy thermostructural design requirements. Significant technology risks are associated with the design and fabrication of an actively cooled engine combustor. However, the risks are mitigated for single use, short duration vehicles like missiles. Although pursuing multiple solutions could alleviate potential problems, budget limitations have restricted investigations, and the current program does not include flight demonstrations. Therefore, many uncertainties will not be resolved until after the HyTech Program is completed in 2003. In spite of system uncertainties in the areas of reliability and affordability, technology decisions are being made. Management of the HyTech Program should analyze and evaluate the trade-offs between maximum Mach number, mission performance, and the reliability and affordability of the propulsion system. Earlier, the committee recommended that the Air Force initiate trade-off analyses. The following recommendation is based on the responses to this question and to the previous question (as well as other parts of this report). Recommendation. The Air Force should expedite trade-off studies in three separate areas: (1) mission parameters, to establish operational requirements; (2) system concepts, to define candidate configurations with optimum ranges of performance, operability, reliability, and affordability; and (3) technology, to redirect the HyTech projects toward the most promising alternatives, if necessary. OTHER UNCERTAINTIES, QUESTION 2c(ii) What are the salient uncertainties for the other main technology components of the hypersonic technology program (e.g., materials, thermodynamics, etc.)? Summary Answer See the detailed response to the technology uncertainties under Question 2a(ii). TECHNICAL FOUNDATION, QUESTION 2c(iii) Does the program provide a sound technical foundation for a weapon system program that could meet operational requirements as presently defined? Summary Answer The current HyTech Program does not have the mandate or the funds to provide a sound technical foundation for a weapon system. The Air Force will have to conduct extensive trade-off studies before it can establish an operational requirement for a hypersonic missile system and determine specific design goals. As a result of concerns that the survivability of this class of missile had not been adequately analyzed, the committee performed an additional study of the survivability trade-offs. Detailed Answer The committee’s response to this question is a resounding “no.” The HyTech Program will not provide a sound technical foundation for a weapon system for reasons that have been explained in the Overall Picture section of this chapter and in the responses to Questions 2a(i) and 2a(ii). The formulation of operational requirements for an air-breathing hypersonic missile system will require comprehensive mission analyses. The definitions (CJCS, 1997) of two elements in the formulation of operational requirements are described below. A mission need statement is not system specific but defines necessary operational capabilities in broad operational terms. The operational capabilities and constraints are then studied during the concept exploration and definition phase of the acquisition process. An operational requirements document is a statement of performance and related operational parameters for the

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Review and Evaluation of the Air Force Hypersonic Technology Program proposed concept or system. The document is prepared by the user or user’s representative at each milestone of the acquisition process, beginning with the approval of concept demonstration. The committee was informed of some mission needs that could be met by a hypersonic air-breathing missile system. However, the Air Force has not performed trade-off analyses or studies that could lead to the establishment of an operational requirement for a specific type of weapon. As a result of concerns about the vulnerability of a hypersonic missile, the committee conducted an analysis (see Appendix C) to examine one of the main reasons given in support of a Mach 8 missile—namely, that it would be “nearly invulnerable to countermeasures because of the high speed” (see Appendix B). The committee attempted to determine if there were significant differences in the vulnerabilities of hypersonic missiles with top speeds of Mach 8, Mach 6.5, and Mach 4 to a surface-to-air defensive missile system. The summary result was that modern air defensive systems could successfully engage hypersonic missiles at all three speeds. Therefore, even missiles operating at high speeds may require radar cross-section reduction to reduce their vulnerability. The decreased vulnerability of a Mach 8 missile can be achieved by a Mach 6.5 missile with a moderate reduction in radar cross section. The Air Force has yet to determine the effects of various parameters (e.g., top speed, radar cross section, maneuverability, and altitude) on missile vulnerability. The Air Force will have to assess the full range of parameters (e.g., average speed over various ranges, maximum and minimum ranges of flight, appropriate standoff distances from the target before launch of the missile) and technical trade-offs (e.g., between airframe and engine thermostructural systems and speed) to establish operational requirements for the hypersonic missile system. INTERACTIONS WITH OTHER PROGRAMS, QUESTION 2d How does the Air Force hypersonic program interrelate with other Department of Defense hypersonic initiatives, e.g., the Defense Advanced Research Projects Agency’s Advanced Concept Technology Demonstration on hypersonic vehicles? Summary Answer The HyTech Program is neither formally coordinated with nor intentionally dependent upon hypersonic initiatives by the U.S. Department of Defense (DOD) or NASA, although relevant technical information is being shared. The committee encourages the Air Force to continue this exchange of information. Detailed Answer In addition to evaluating the HyTech Program, the committee received briefings on the Navy Hypersonic Weapons Technology Program and the Defense Advanced Research Projects Agency Affordable Rapid Response Missile Demonstrator Program. These DOD programs, the parameters of which are summarized in Table 2-1, are for vehicles that rely on hydrocarbon fuel. The committee also received briefings TABLE 2-1 Summary of Parameters of Various DOD Hypersonic Programs   Air Force HyTech Navy Hypersonic Weapons Technology Defense Advanced Research Projects Agency Program Main thrust of program Engine ground test 1998–2003 Propulsion airframe guidance and control and ordnance Build and demonstrate an affordable missile Propulsion Dual-mode scramjet Dual-combustion ramjet Dual-mode scramjet Fuel Hydrocarbon Hydrocarbon Hydrocarbon Mach number 4 to 8 5 to 6 6 to 8 Range 750 nautical miles, nominal 400 to 700 nautical miles 100 to 600 nautical miles Initial operational capability date 2015 2010 N/A Year start/stop 1995/continuous Fiscal year 1998/2003 Fiscal year 1998 Phase I/Phase II optional Funding goal $20 million per year nominal $8 million per year nominal Phase I–$10 million Phase II–$50 million Weapon cost goal not applicable $400,000 $200,000   Source: Information furnished by representatives of the Air Force, Navy, and Defense Advanced Research Projects Agency

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Review and Evaluation of the Air Force Hypersonic Technology Program on two NASA programs that involve hydrogen fuel (HYPER-X and the Advanced Reusable Transportation Technology Project). Some technical challenges are common to all of these programs (e.g., air-breathing propulsion at hypersonic speeds), but each program also faces specific technical challenges (e.g., speed and type of fuel). Each program is discussed briefly below. HyTech Program The HyTech Program is summarized in Box 2-1 and described in more detail in Appendix B. The objectives of the program are to develop and demonstrate air-breathing, storable-fuel (hydrocarbon), scramjet propulsion technologies for missile (and aircraft) applications at speeds of Mach 4 to Mach 8. Performance goals have been established for specific impulse, specific thrust, and durability consistent with requirements for expendable hypersonic air vehicles, with a projected initial operational capability in 2015. The program goals are to demonstrate, by 1998, stable scramjet combustor operations between Mach 4 and Mach 8 with 90 percent of the final specific-impulse goal; to demonstrate by 2000 95 percent of the final specific-impulse goal; to demonstrate by 2001 scramjet structural durability for 12 minutes; and to demonstrate by 2003 integrated engine performance at 100 percent of the final specific-impulse goal. The program will demonstrate technologies through ground tests in appropriate facilities that can simulate Mach 8 flight conditions. The Air Force hopes these tests will be sufficient for a follow-on flight demonstration program that could eventually lead to a practical application. The HyTech Program was initiated in fiscal year 1995 at the direction of the secretary of the Air Force with a nominal $20 million per year funding profile, although the program has experienced a shortfall every year. Navy Hypersonic Weapons Technology Program In its draft mission needs statement, Tactical High-Speed Strike Capability, dated April 28,1997, the Navy stated that the capability to attack, destroy, and hold at risk short-dwell, time-critical targets at long standoff ranges is critical to joint strike operations, joint littoral operations, and joint suppressions of enemy air defenses. Navy studies have shown that covering 80 percent of the spectrum of time-critical targets requires engagements at ranges of up to 600 nautical miles and speed requirements of Mach 3.5 to Mach 7, depending on the launch point. The Navy has concluded that current state-of-the-art missile technology will not support the demonstration and validation or engineering and manufacturing development of a high-speed weapon that meets the mission needs. The thrust of the Navy Hypersonic Weapons Technology Program is to develop and demonstrate technologies for a hypersonic strike weapon in the concept exploration and definition phase. By fiscal year 2003, the Navy hopes to demonstrate enabling technologies for a hypersonic air-launched or surface-launched weapon that meets the Navy’s requirements. Specifically, the goals are to demonstrate9 critical technologies in the areas of propulsion, airframe, guidance and control, and ordnance for a hypersonic strike weapon that will have initial operational capability by about 2010. The weapon would have an average speed of Mach 5 to Mach 6, a range of 400 to 700 nautical miles, a cost of less than $400,000 per unit, a circular error probable of less than 10 feet, and the ability to deliver ordnance that can penetrate 18 feet or more of concrete. The principal configuration is an axisymmetric, dual-combustion ramjet10 with a hot structure. The ramjet will be demonstrated in a free-jet test configuration. Navy project funding starts in fiscal year 1998 and ends in fiscal year 2003. The program is funded at a nominal $8 million per year, which is even lower than funding for the HyTech Program. Defense Advanced Research Projects Agency Affordable Rapid Response Missile Demonstrator Program This concept definition program relies on the design tools and hardware being refined by Air Force, Navy, NASA, and industry programs to develop the basis for an affordable hypersonic missile. The program objective is to build and demonstrate in flight a test vehicle that will enable the development of an affordable, Mach 6 to Mach 8, scramjet-powered, hydrocarbon-fueled missile to support rapid-response, long-range (100 to 600 nautical miles) missions against time-critical (two to eight minutes) targets. In addition, this missile would enhance advanced penetrators with much higher impact velocities for the destruction of hardened and deeply buried targets. The emphasis of the program is on affordability. Program goals include the demonstration of affordable manufacturing processes to produce units with an average flyaway price of $200,000; the development of a concept of operations with the warfighting user; the demonstration of propulsion performance compatibility with tactical aircraft and the Navy’s vertical launching system; and the achievement of cruise speeds of Mach 6 to Mach 8 with a maximum range of 600 nautical miles. The program is divided into two phases of 18 months each, the second phase of which is optional. In the initial phase, critical risk-reduction measures will be taken, including detailed cost estimates. The second phase, if implemented, would include the assembly and flight of demonstration vehicles. The cost of the initial phase is projected to be $10 million; the cost of the second phase is projected to be $50 million. 9   This means a physical demonstration that provides a reasonable expectation (i.e., low-to-moderate risk) that the technologies are in hand. 10   A dual-combustion ramjet is a hybrid engine that combines the features of a ramjet and scramjet engine to operate over a wide Mach number range.

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Review and Evaluation of the Air Force Hypersonic Technology Program NASA HYPER-X Program The HYPER-X Program is intended to demonstrate and validate the technology, the experimental techniques, and the computational methods and tools for design and performance predictions of hypersonic aircraft using airframe-integrated, dual-mode, hydrogen-fueled, scramjet propulsion technologies. The program strategy is to evaluate the performance of scramjet-powered research vehicles at Mach 7 and Mach 10; demonstrate controlled, powered air-breathing and unpowered hypersonic aircraft flight; provide ground and flight data to validate computational methods, prediction analyses, test techniques, and operability for future hypersonic cruise and space-access vehicles; execute an affordable plan focused on key technologies using existing designs, design methods, databases, and off-the-shelf hardware and systems wherever possible; and conduct three flights of very short duration. The flight schedule is as follows: flight 1 (Mach 7) is planned for January 2000; flight 2 (Mach 7) for October 2000; and flight 3 (Mach 10) for September 2001. The total program budget is $170 million, starting in fiscal year 1997 and ending in fiscal year 2001. NASA Advanced Reusable Transportation Technology Project The Advanced Reusable Transportation Technology Project is one element of the NASA Advanced Space Transportation Technology Program. The primary goal of the project is to demonstrate technologies mature enough to reduce the development risk of hydrogen-fueled, rocket-based, combined-cycle propulsion systems for future launch vehicles. Most of these systems are ramjets or dual-mode scramjets with small, fully integrated rocket ejectors installed in the flow path to provide low-speed propulsion. This type of propulsion has the potential to improve performance significantly over pure rocket engine systems for space launch because it uses atmospheric oxygen during the boost phase. Compared to pure rocket propulsion, the gross weight of the launch vehicle should be lower because less oxidizer has to be carried by the launch system. The project has the following milestones: test critical propulsion component technologies by the end of 1998; develop a flight demonstrator engine by 2003; and conduct a flight demonstration by 2004. Comparison and Interrelationships of Programs Table 2-1 summarizes various parameters of the DOD programs discussed above. The vehicles being developed in DOD programs are limited in speed to Mach 6 or Mach 8, whereas the short duration NASA flight test program for hydrogen-fueled vehicles is intended to reach Mach 10. The NASA HYPER-X Program is currently the only U.S. hypersonics program that is funded for flight tests. The committee believes this flight experience will be valuable for all of the DOD programs. Although there are some interactions among these programs, they are not part of an overall DOD or national strategy. Each program has its own objectives, goals, and milestones; however, Appendix B suggests that the goals of the HyTech Program “are fully coordinated” with the other programs and “complement their activities.” The committee found no evidence that the HyTech Program is formally coordinated with (i.e., operating under a rigorously structured arrangement controlled jointly by the Air Force, other DOD entities, and NASA) or dependent on current DOD or NASA initiatives. However, much relevant technical information is being appropriately shared. The exchange of information should be encouraged, especially with the NASA HYPER-X Program, which will be the first U.S. flight test of scramjet propulsion. MILESTONE DATES, QUESTION 2e(i) From an engineering perspective, what are reasonable milestone dates for a hypersonic missile system development program leading up to production, i.e., concept development, engineering and manufacturing development, etc. For example, with a 2015 target date for operational capability, does the current program have a coherent plan and road map to build and test a Mach 8 regime hydrocarbon-fueled scramjet engine? Summary Answer The committee finds that initial operational capability for a hydrocarbon-fueled scramjet missile system in 2015 is technically feasible. The committee’s experience indicates that it will take until 2015 to develop the type of missile contemplated by the Air Force with moderate risk. A prototype missile phase will have to be initiated in 2003 and prototype flight testing completed by 2007, which would reduce the risk of entering the engineering and manufacturing development phase. Figure 2-6 is the committee’s suggested road map, which includes a complementary program to the current HyTech Program that will be necessary to reach initial operational capability by 2015. Detailed Answer During this study, the Air Force presented a road map for the current HyTech Program (see earlier description and Appendix B). The program ends in 2003 with limited ground demonstrations of a Mach 8 scramjet. Although the program makes wise use of available funding, it does not provide realistic criteria for the engineering and manufacturing development phase of an acquisition program. The committee did not find a road map for achieving operational capability. The committee, therefore, developed its own road map in keeping with the Statement of Task to show the required steps to an initial operational capability of an air-launched,

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Review and Evaluation of the Air Force Hypersonic Technology Program FIGURE 2-6 A six-phase road map to achieve initial operational capability of a Mach 6 to Mach 8 hypersonic missile system by 2015. scramjet, hypersonic missile system by 2015. The current Air Force program plan does not include flight testing of prototype vehicles to demonstrate the readiness of the scramjet engine technology for a Mach 8 missile system. The committee believes that this is a crucial step. The committee also believes that the competitive tests of prototype missiles would be extremely valuable for achieving the technical performance and cost objectives of the operational missile system. Prototype test flights would also be the most important factor for justifying the engineering and manufacturing development program. The time spans and scope for the committee’s road map are based on the experience of generally comparable past programs. In other words, the committee did not take the 2015 date as a given, but the committee’s experience indicates that it will take that long for the Air Force to establish an initial operational capability. Overall, the committee’s proposed schedule is of moderate risk (e.g., it has virtually no concurrency; many believe that the lack of concurrency is a priceless asset). However, a major feature of the program is the development of a prototype missile; to ensure that the engineering and manufacturing development phase incurs only moderate risk, the prototype missile program will necessarily incur significant risk. The road map is based on several general principles, which will make it easier to obtain funding from DOD and Congress. The first is minimal concurrency. The second is an orderly funding profile with no precipitous changes. The third is the ability to change the direction of the program, if necessary. In preparing the road map, the committee recognized that the current NASA HYPER-X Program, which is based on hydrogen-fueled propulsion, and other planned DOD programs will contribute substantially to the technology base for a hydrocarbon-fueled hypersonic missile system and will reduce the risk of an Air Force program. The Defense Advanced Research Projects Agency is formulating a very aggressive advanced concept technology demonstration program for a hypersonic missile, with an intense focus on the cost of a missile system. However, at the time of this report, the committee did not have enough information to ascertain how an advanced concept technology demonstration would feed into the road map. The six phases of the road map (depicted in Figure 2-6) are described below, followed by a rough cost estimate for the entire program. System Specification Development (An Iterative Process, 1998 to 2007) This is the classical iterative process for evolving a technically sound weapon system design that meets critical

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Review and Evaluation of the Air Force Hypersonic Technology Program operational requirements (e.g., speed, range, and survivability) but also satisfies weight, volume, and, most important, production cost objectives. System specification development, which is continuously executed, is critical to the long-term viability of the program but has not been given enough emphasis or resources thus far. Producing a missile with a range of 600 to 750 nautical miles and a nominal gross weight of 3,000 pounds at launch will be difficult. Numerous trade-off studies and evaluations of many alternate system configurations will be essential to a balanced system design that meets all critical operational requirements (which might evolve) within acceptable weight and production cost constraints. The highest level of activity during this phase is estimated to occur from 1999 to 2003. System Concept Development (Two Competitors, 1999 to 2003) This phase is closely related to system specification development. Concept development studies should be competitive to evolve the most cost effective system design. Although plausible, preliminary system designs were presented to the committee, additional detailed engineering will be required before a detailed design of the prototype missiles can be initiated in 2003. An initial measure of effectiveness could be range with a fixed gross weight at launch, which could be changed later by specific measures of system effectiveness developed by the Air Force (e.g., in connection with the establishment of an operational requirement). A hypersonic missile poses a very difficult challenge in design integration. The best design will emerge only after several highly creative alternative concepts have been developed. Technology Risk Reduction (1999 to 2007) In addition to the scramjet propulsion system that is being developed under the HyTech Program, appropriate technology development programs should be initiated in the following areas in order to be ready for engineering and manufacturing development in 2007 (see response to Question 2a(ii)): affordable airframe and engine thermostructural systems (including a full definition of the environment to which these systems will be exposed); optimization of vehicle system design, integration, and performance; low-cost and integrated stability, guidance and control, navigation, and communications equipment; appropriate terminal guidance and sensing equipment to ensure accuracy in all weather conditions; and two types of tailored munitions (e.g., a lightweight, high energy, explosive warhead and a high-speed penetrating warhead for hardened targets). The Air Force will have to develop a high fidelity, full-mission simulation model for a hypersonic missile system. The highest level of activity for this phase will be from 2000 to 2003. Prototype Design and Flight Testing (2003 to 2007; First Flight in 2005) Prototype design and flight testing is a crucial and risky phase of the program. The committee is not proposing a specific program in terms of the number of prototype missiles or detailed flight test objectives. Three to five fully instrumented flight test vehicles will probably be adequate, especially if they can be recovered for inspection (water or ground recovery of some of the propulsion systems should be a program requirement). This phase must demonstrate that the integrated scramjet and missile structure and vehicle control system perform as predicted, repeatably, under flight conditions. Prototype flight tests will validate analytical predictions, confirm the results of simulations, and provide essential flight test data for low-risk engineering and manufacturing development. These prototypes must have prioritized, carefully selected, limited objectives. Munitions and target engagement capabilities are not required, nor is a production configuration of the solid rocket booster. This phase will also have important nontechnical value because it will provide convincing evidence to the DOD and the Congress that the Air Force can field a long-range missile with speeds up to Mach 8 to defeat specific threats. No matter how successful ground testing is, it will not demonstrate the level of technology readiness for entry into engineering and manufacturing development. Engineering and Manufacturing Development (2007 to 2012; First Flight in 2010) This phase would include the conventional steps (i.e., detailed production design, rigorous full-mission system simulation, initial tooling and test equipment, production of two small lots of missiles, complete ground testing, development flight testing, and initial operational testing and evaluation). The proposed schedule assumes that the baseline missile configuration will not include active sensors and that only two warhead options will be implemented, one for fixed above-ground targets and the other for hardened underground targets. The number of missiles assembled during this phase and the number of missiles that are flight tested with live munitions can only be determined after further analysis. A first approximation is 10 to 15 missiles without munitions and 10 to 15 with munitions. These numbers may seem low, but with rigorous full-mission simulation and very high fidelity modeling, the number of flight tested missiles can be kept to a minimum. Low-Rate Initial Production Leading to Initial Operational Capability (2012 to 2015) Initial operational capability requires that a specified number of operational missiles be in the hands of an operational command and ready for immediate use in combat. The

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Review and Evaluation of the Air Force Hypersonic Technology Program committee is not in a position to estimate an appropriate initial operational inventory. However, from the standpoint of industrial production, a reasonable number, based on the proposed road map, would be 30 to 50 missiles. Costs The committee’s very rough, preliminary estimate of the cost of the entire program is $750 million to $1.5 billion (in 1998 dollars). In production quantities (e.g., about 1,000 missiles), the committee believes the resulting missile will be considerably more expensive in 1998 dollars—by at least a factor of two—than the $200,000 goal for the vehicle contemplated in the Defense Advanced Research Projects Agency program (see Question 2d). FOREIGN HYPERSONIC APPLICATIONS, QUESTION 2e(ii) Are there foreign hypersonic technology applications that are significantly more developed than those of the United States, that, if acquired by the U.S. government or industry through cooperative venture, license, or sale, could positively affect the development process or schedule for Air Force hypersonic vehicles? Summary Answer Several organizations throughout the world have significant expertise related to scramjet-powered hypersonic vehicles. Although no system-level hardware seems to be available internationally, many technologies of potential use in hypersonic vehicles are being investigated. The committee believes that the Air Force should continue to evaluate potentially significant foreign technologies. Detailed Answer During this study, the committee was not informed of any hypersonic, scramjet-powered vehicle that has reached the operational stage or engineering and manufacturing development stage elsewhere. From the committee’s review of the development of hypersonic technologies abroad (based on information furnished by the Air Force), it is evident that technologies associated with vehicles capable of hypersonic flight are being actively investigated by several other countries (including the flight testing of large-scale scramjet propulsion systems), and several international collaborations are in the formative stages. Russia has the most significant technical capabilities related to hypersonic systems. The Soviet Union invested heavily in advanced air-breathing missiles and fielded several operational ramjet-powered systems (e.g., SA-6, SS-N-22, M-31) for use on land and at sea. The Soviet Union, and now Russia, also invested heavily in hypersonic technologies. Russian centers with strong programs in hypersonics are the Central Institute of Aviation Motors, the Central Aero-hydrodynamic Institute, the Central Institute of Machine Building, and the Institute for Theoretical and Applied Mechanics. The technologies being explored at one or more of these institutes include hypersonic aerodynamics, scramjet propulsion systems, endothermic fuel systems, ground testing facilities, measurement systems, and flight demonstration techniques. In addition to the classical hypersonic technologies, Russia has also invested in several novel technologies whose advocates claim will improve hypersonic systems significantly. At the present time, researchers in the United States are evaluating these technologies to ascertain their potential. Ramjet-powered vehicles originated in France, where the first flight of a piloted, ramjet-powered aircraft (the Leduc 001) took place in 1949. France has continued to investigate ramjet-powered vehicles, including the operational, ramjet-powered air-to-ground Missile Air-Sol-Moyenne-Portée. French experience with ramjet-powered vehicles has provided them with significant experience in the areas of high-temperature materials and flight testing. The Office National d’Etudes et de Recherches Aerospatiales and Aerospatiale have particular areas of expertise. Many other countries, including Great Britain, Canada, Australia, Germany, Italy, Japan, and China, also have some experience with hypersonics. The committee was not informed of any system-level hardware available on the international market. However, considering the diverse technologies that are associated with the development of hypersonic vehicles, the acquisition of foreign technologies has the potential to enhance the development of Air Force hypersonic vehicles. Acquisitions could be made through cooperative ventures at the basic technology level or through licenses or sales. The committee believes the Air Force should continue to evaluate foreign technologies, but the committee does not have the expertise to make specific recommendations involving cooperative ventures, licenses, or sales. Technologies that should be evaluated include scramjet technologies (e.g., fuel preparation, injection, mixing, ignition, and flame-holding); endothermic fuel and fuel-control systems; and advanced materials and structures. CONTENT AND PACE OF THE PROGRAM, QUESTION 2e(iii) Based on these assessments, the committee will make recommendations on the technical content and pace of the program. Answer If the Air Force determines that there is a requirement for a hypersonic missile system, the committee recommends that the Air Force adopt the road map in Figure 2-6. To achieve

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Review and Evaluation of the Air Force Hypersonic Technology Program initial operational capability by 2015, the program office recommended in response to Question 2a(i) should establish a road map similar to the one developed by the committee. The program should proceed step by step through the various phases, including flight testing, and should address all critical technologies. INFRASTRUCTURE, QUESTION 2f Are there any evident implications for the Air Force support infrastructure for a hypersonic missile system? For example, will other technologies need to be developed in parallel to support a hypersonic vehicle and are those likely to pose significant barriers to eventual success in demonstrating the missile concept or in fielding a viable weapon system by 2015? Summary Answer The implications for the Air Force support infrastructure of a hydrocarbon-fueled hypersonic missile will depend on the maximum speed of the missile. Some investment will be necessary in ground testing facilities, flight testing, and analyses to determine the performance and operability of the propulsion system. Ground testing facilities will have to support both technology development and demonstration and system development and qualification of a complete missile. Full-scale ground testing facilities are currently limited to about Mach 7, although modifications to at least one facility are under consideration to support a Mach 8 capability. If a maximum nominal Mach number of 7 or lower is selected, the only modification to a test facility might be to provide for hydrocarbon fuel testing at the NASA 8-Foot High Temperature Tunnel. Regardless of the maximum Mach number, a capability for the periodic destructive testing of selected missiles from storage must also be provided. Detailed Answer This question addresses several different aspects of the infrastructure support required for hypersonic air-breathing missiles, including ground testing facilities; ancillary test equipment and instrumentation and computational facilities; test ranges; and missile storage capabilities. The committee was advised that about 12 percent of the HyTech Program funding (approximately $12 to $13 million) has been allocated to test facilities and instrumentation for Mach 8 scramjet development. This allocation was based on an average projected funding of $16 million per year. Ground Testing Facilities Ground testing facilities will be required to test technologies in at least four areas: propulsion, fuels, thermal structures, and airframe-engine integration. The committee focused its attention primarily on existing facilities that have the capacity for testing full-scale, integrated propulsion systems for an air-launched tactical missile at true inlet temperature in the Mach 4 to Mach 8 range, in the altitude range of 50,000 to 100,000 feet, and for test durations of up to 12 minutes. The committee considered several facilities, including the Air Force Arnold Engineering Development Center Aerodynamic and Propulsion Test Unit and the NASA-Langley 8-Foot High Temperature Tunnel. Several special-purpose facilities that are only capable of supporting component technology development were also considered. The committee assumed that these facilities would not be affected by the realignment or closing of bases. The existing thermostructural and propulsion capability of the NASA-Langley 8-Foot High Temperature Tunnel can support the development and demonstration of several of the required hypersonic integrated propulsion technologies at Mach numbers of 4, 5, and, nominally, 7. However, the test duration time is limited to about two minutes, which is not long enough to demonstrate the reliability of the propulsion system in a single continuous test. None of the test facilities can support the critical integrated-engine demonstration phase of the HyTech Program at a higher Mach number than nominal 7. However, several facilities can support component or subscale testing above nominal Mach 7. Validation of integrated-engine performance at Mach 8 will require flight testing. The committee was informed that modifications to the Arnold Engineering Development Center’s facility have been planned and partially funded to increase the maximum Mach number to about 8 and the test duration to approximately 10 minutes. These modifications directly support the integrated-engine demonstration phase of the HyTech Program. A major uncertainty in the performance, operability, and reliability test data from all of the test facilities considered by the committee is whether or not the ground testing results (e.g., specific net thrust, combustion efficiency and stability limits, and combustor starting limits) simulate flight operations within usable limits of error. To date, the ground test results of scramjet propulsion systems have not been validated with flight data. The HYPER-X Program will provide flight data for hydrogen-fueled scramjets, but it will not provide data on reliability and range of operability. The committee was advised that tests by Russia and France have shown a substantial correlation between ground test data and flight test data for hydrogen-fueled scramjets. The high-temperature gas supplied to the engine inlet from each of the existing facilities is modified or reconstituted air rather than atmospheric air. These “pseudo-air” working fluids contain significant quantities of gaseous contaminants, and possibly particulate matter. The quantitative effects of these gaseous contaminants and particles on some parameters, such as specific net thrust, ignition limits, and heat transfers, are not currently known. These effects must

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Review and Evaluation of the Air Force Hypersonic Technology Program be defined to ensure that valid test data will be available for a system acquisition program. The NASA HYPER-X Program will provide some data for a hydrogen-fueled scramjet, but the validity of these data for a hydrocarbon-fueled scramjet has not been determined. The committee believes the HyTech hydrocarbon-fueled propulsion technologies should be flight tested to validate the ground facility data and test performance at flight conditions. The committee also evaluated test facilities that could support both technology development and follow-on system acquisition of the rocket booster, hypersonic airframe, and the related guidance, sensor, and munitions subsystems. Existing DOD, NASA, and industrial aerodynamic and aero-thermal facilities are adequate to support the development and qualification of the airframe, including lift and drag, stability and control, thermal protection, and structural integrity. But they are not adequate to test the full flight envelope of the integrated vehicle-engine, which will require flight tests. Existing facilities are adequate to support the development and qualification of the guidance, sensor, and munitions subsystems. Recommendation. The Air Force should begin planning for the ground test infrastructure to support the development and qualification of the operability, reliability, durability, and performance of integrated hypersonic propulsion systems over the Mach number range from the speed at the end of the rocket-boost phase to the maximum cruise speed. This infrastructure should be completed expeditiously. Ancillary Test Equipment and Instrumentation and Computational Facilities Current facilities have free-jet test sections with the size and strength to support full-scale propulsion testing in the nominal Mach number range of 4 to 7. However, the technology development and demonstration of a fixed-geometry, dual-mode propulsion system fueled with endothermic hydrocarbons will involve iterative design and test challenges to optimize the engine components, such as inlet, isolator, combustor, and nozzle, for flight in the Mach 4 to Mach 8 range. Developing a propulsion system that can maintain stable operation through at least two transitions will require iterative designs and tests. One of these transitions is from ramjet mode (subsonic combustion) to scramjet mode (supersonic combustion); the other is from liquid hydrocarbon fuel injection to two-phase (vapor and liquid) pyrolyzed hydrocarbon fuel injection. Ancillary test equipment is available to test the design optimization of the integrated engine and control system at discrete Mach numbers from approximately Mach 4 to approximately Mach 7. Equipment is also available to test the mode transitions at the component level. But testing mode transition of the full-scale integrated engine and engine fuel control system will require flight testing. Ancillary test equipment to support both technology development and follow-on system acquisition for the rocket booster, hypersonic airframe, and the related guidance, sensor, and munitions subsystems is adequate. Existing capabilities extend from typical subsonic air launch Mach number and altitude windows to well above Mach 8 and 100,000 feet in altitude. Because of the complexity of hypersonic test facilities and hypersonic test articles, automated control networks will be necessary to ensure safe and efficient operation. Most of the candidate test facilities have been operating successfully for decades. However, it would be prudent for the Air Force to assess the capability and reliability of the test control networks to limit the risk of disruptions or damage to test articles. The committee also evaluated the requirements for instrumentation and computational facilities to support development and demonstration testing. Existing instrumentation technologies in several areas (e.g., force, flow, temperature, and pressure) are adequate. The existing high-speed computational facilities are also adequate. The committee believes that existing facilities can support the computational fluid dynamics, computational structural mechanics, and system modeling, as well as the follow-on prototype and engineering and manufacturing development programs proposed by this committee. Test Ranges Several test ranges can accommodate a flight test program for a missile with a nominal maximum Mach number of 8 and a range of about 750 nautical miles. The options vary depending on the flight test requirements, such as whether the test vehicle is expendable or recoverable and whether it is air launched or ground launched. The flight test requirements are similar to the requirements for the NASA HYPER-X Program, which initially included flights planned for Mach 5 to Mach 10 with minimum to maximum trajectory ground ranges of 600 to 1,200 nautical miles. The requirements that might be similar for a missile test program included the following: conventional flight termination for the rocket-boost phase; flight termination capability throughout the flight; telemetry coverage throughout the flight (assuming data rates are consistent with the test range capabilities); ground-launched options using a rail launcher for better inclination toward air-breathing flight corridors; air-launched options from appropriate platforms; environmental impact statements for some ranges; subsonic operation prior to recovery sequence for recoverable test vehicles; and flight tests on Air Force, Navy, Army, or NASA test ranges. The ground-launched range options that were deemed feasible for a HYPER-X flight test vehicle boosted by a Castor IVB class booster included: the Wallops Flight Facility, from Virginia down the Atlantic Test Range; Vandenberg Air Force Base, from California down the Pacific Western Test

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Review and Evaluation of the Air Force Hypersonic Technology Program Range; Poker Flat Range in Alaska; and Wake Island to Kwajalein Atoll in the Pacific. None of these flight paths is toward a populated area. Both the Wallops facility and Wake Island have rail launchers and the required facilities, but both would require a water recovery. The Vandenberg and Poker Flat ranges would require environmental impact statements and, depending on launch trajectory, may require the installation of rail-launchers. Poker Flat would also require facility upgrades. All four test ranges can meet the requirements for telemetry and safety. The air-launched range options that meet the flight program requirements include: the Air Force Development Test Center (at Eglin Air Force Base) Missile Range from the Gulf of Mexico; Vandenberg Air Force Base/Pacific Test Range; Edwards Air Force Base/Utah Test Range; and Poker Flat Range in Alaska. All of these test ranges have ground recovery locations (pack ice location at Poker Flat). But ground recovery at the Air Force Development Test Center and Vandenberg would require flight toward a major population center. Water recovery is available at all ranges except Utah. All of the facilities are capable of air-launched tests and can meet telemetry coverage and range safety requirements. Missile Storage For an operational hypersonic missile to be affordable, it should not entail significant changes to the Air Force support structure in the field. An affordable hypersonic missile will have to approach the “wooden round” concept as much as possible. The Navy’s experience with the liquid-hydrocarbon fueled Tomahawk missile is directly applicable to the present situation. Tomahawk missiles have been stored for up to 10 years, with only periodic electronic system checks and computer reprogramming, and then fired successfully. The two aspects of missile support that must be considered are storage and systems checking. Experience suggests that the design problems with the subsystems (e.g., batteries, electrical and mechanical actuators, lubricants, seals, computers, and codes) can be solved, especially if the special circumstances are recognized from the outset. All aspects of the fuel system (i.e., the fuel, storage tank, pump, catalysts, and cooling system) will require special attention for proper storage. A system that uses endothermic fuels would be unique. Experience has shown that the subsystems can be periodically interrogated and computers reprogrammed. However, in contrast to the Tomahawk, which has a multiple-use turbojet engine, a scramjet propulsion subsystem will be designed for a life of one cycle. During testing, pyrotechnics will be fired, coatings will ablate, materials will heat up and yield, and catalyst beds will be polluted. This situation is similar to the situation of a solid-fueled rocket where the motor must be fired to be thoroughly tested. Special procedures, such as the random sampling program used for intercontinental ballistic missiles, must be developed for testing scramjet missiles. A good deal can be learned from tested vehicles even if they are not reusable. Selected scramjet propulsion subsystems and rocket boosters must be periodically removed from long-term storage and test fired to confirm system storage life. These test firings, combined with the required inspections of the post-test hardware, will provide an objective basis for extending or terminating storage. The propulsion subsystem components would be consumed in the test and inspection processes and could not be returned to storage.