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Review and Evaluation of the Air Force Hypersonic Technology Program 3 Other Applications of Hypersonic Technologies In Part 3 of the Statement of Task, the committee was asked to consider applications of hypersonic technology for 2015 and beyond: To the extent possible, identify technology areas that merit further investigation by the Air Force in the application of hypersonics technology to manned or other unmanned weapon systems by 2015 or beyond. FUNDAMENTAL CONSIDERATIONS The application of air-breathing hypersonics technology to future manned or unmanned weapon systems was too big an issue to be systematically and comprehensively considered in the limited time for this study. The committee does not have enough information to make recommendations regarding the hypersonic weapon systems capabilities the Air Force might require in the twenty-first century. But the subject is clearly important and worthy of an in-depth study once operational premises and priorities have been better defined by the Air Force. In this chapter, the committee briefly discusses the possible applications of air-breathing hypersonic propulsion, as well as two approaches the Air Force could adopt, either (1) the expansion of the hypersonic technology base without developing real systems, or (2) the evolutionary development and deployment of systems to meet clearly stated Air Force requirements. The committee also provides a four-part process to guide the Air Force’s long-range development of future hypersonic systems. Recommendation. The Air Force should work on the evolutionary development and deployment of systems to meet clearly stated requirements. HYPERSONIC VEHICLES Hypersonic vehicles propelled by air-breathing propulsion systems, in addition to the scramjet missile currently under consideration, might include higher Mach number, long-range missiles; theater-reach and global-reach aircraft designed to deliver weapons or for reconnaissance missions; and space launch vehicles. Two Air Force goals that might benefit from air-breathing propulsion are global reach and access to space. These are not mutually exclusive. For global reach, a vehicle must be able to reach any military target on the globe quickly and carry out a critical military mission. Whether global reach may be best obtained by vehicles that operate primarily inside or outside the earth’s sensible atmosphere depends on the specific mission requirements. The pay loads of a global-reach hypersonic vehicle could range from missiles to reconnaissance equipment to orbiting systems. Because an enemy could attack U.S. space-launch facilities or space-based assets, access to space from military bases that survive an initial attack (especially if the United States has lost some of its space infrastructure) could be a critical capability. Access to space includes the capability to repair or replace satellites, to add mission-specific satellites, and to defend U.S. space assets. Access to space will rely, in part, on rocket propulsion systems, including the propulsion systems of orbiting vehicles. TECHNOLOGY AREAS FOR FURTHER INVESTIGATION In Chapter 2, the committee discussed the required technologies for an air-breathing hypersonic missile boosted to Mach 4, at which point the scramjet would take over and accelerate the missile to a top speed of Mach 8. The technologies developed in the HyTech Program will be important for all future hypersonic vehicles, especially if the technologies have been validated in a flight test program. The technologies might be directly applicable to some types of vehicles and might provide a valuable information base for building others. Potential vehicles that would directly benefit from the technologies developed by the HyTech
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Review and Evaluation of the Air Force Hypersonic Technology Program Program include hydrocarbon-fueled hypersonic aircraft and space-launch vehicles operating in the nominal Mach 4 to Mach 8 range. NASA’s HYPER-X Program and Advanced Reusable Transportation Technology Project (described in Chapter 2) are developing engine and airframe technologies that will also benefit the development of hypersonic systems. Future investigations should focus on the development of low-speed (zero to Mach 4) propulsion system technologies, such as turboramjets, ejector ramjets, and pulse detonation engines. For manned systems, the emphasis should be on the development of life support systems to protect and sustain pilots and crews in the demanding thermal environment of hypersonic flight. Vehicles with maximum speeds of about Mach 8, such as long-range missiles, aircraft, and space-launch vehicles, should benefit from the information gained in the HyTech Program and, depending on the specific concept, might directly use some of the technologies. Vehicles with top speeds above Mach 8 will require cryogenic hydrogen-fueled propulsion systems because of hydrogen’s additional cooling capacity and energy content. Higher speed vehicles, whether dual fueled or hydrogen fueled, will require the development of higher Mach number propulsion system technologies. Three critical technologies—airframe and engine thermostructural systems; vehicle integration; and stability, guidance and control, navigation, and communications systems—will also have to be investigated further (see detailed response to Question 2a(ii)). These vehicles will require durability, as well as reliability. Reusable space-launch vehicles would require the same technology development as lower-speed aircraft but would have much more demanding performance, reliability, and durability requirements. To date, space access has been based on rocket-based propulsion systems. Advances in air-breathing propulsion system technologies may be useful for future space-access applications. Space-launch vehicles propelled by a combination of air-breathing and rocket propulsion would allow the substitution of higher payloads for onboard oxidizer not needed by the air-breathing engine. In addition, these vehicles would have other interesting capabilities, including gradual engine start-up and shutdown, horizontal takeoff and abort, subsonic or supersonic self-ferry, mission flexibility (e.g., rerouting and retargeting after takeoff, large launch windows, and substantial cross-range ability), and reusable structures. PROGRAM OPTIONS FOR FUTURE HYPERSONIC SYSTEMS If the Air Force chooses to pursue a broad range of hypersonic air-breathing technologies with a variety of potential applications, it could provide a technology base to support the design, development, procurement, and operation of hypersonic weapon systems to meet several mission needs. This type of program could pursue technology in the following disciplines: propulsion, propulsion and airframe integration, airframe and engine thermostructural systems, guidance and control systems, aerothermodynamic environment, human factors, and operations. Another option for the Air Force is to pursue the evolutionary development and deployment of hypersonic weapon systems powered by air-breathing or combined-cycle engines that derive from established capabilities and clearly stated Air Force requirements. The committee believes this narrower approach is the only one that will result in operational systems, which would capitalize on a technology base that supports them directly. In other words, the Air Force would start with a requirement, develop the technology to support a system to meet that requirement, and build and test (in flight) a prototype of the desired system. This approach is described below in terms of the notional components of a long-range planning process. The committee strongly recommends this approach. Otherwise we may never know whether or not air-breathing propulsion will be useful and affordable at hypersonic speeds. Based on presentations to the committee by the best technical experts, the committee believes that Air Force plans for future hypersonic weapon systems employing air-breathing propulsion should include at least four components. These components are based on the premise that hypersonic technologies should be developed to meet future weapon systems requirements. Recommendation. The Air Force should develop a long-range plan incorporating four components as a primary document to guide the development of future hypersonic weapon systems. The four components are: operational concepts for future systems and preliminary system designs; scramjet-powered weapon systems using hydrocarbon fuels; hypersonic weapon systems using hydrogen fuel; and combined-cycle systems for space access. Component 1. Operational Concepts for Future Systems and Preliminary System Designs The tasks in this component are not usually considered to be technologies, but in this context they are extremely important. A focused, prioritized list of requirements for the development of hypersonic technology will require practical operational concepts and the evaluation of several alternate preliminary system designs. The committee found the Air Force’s current plans for technology development to be fragmented because concepts and preliminary designs were not well developed. In other words, the Air Force has apparently not given system engineering and system design integration for hypersonic technologies a high priority or commensurate resources. The committee believes that system engineering and design integration should be leading the technology development
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Review and Evaluation of the Air Force Hypersonic Technology Program and providing a basis for prioritizing specific technology developments. Component 2. Scramjet-Powered, Hydrocarbon-Fueled Weapon Systems The committee believes that emerging scramjet propulsion technology will support the development of Mach 6 to Mach 8 class hypersonic weapon systems. The committee suggests that tentative operational requirements be established, that operational concepts be formulated, and that competitive preliminary design studies be carried out for an unmanned reusable weapon system in this speed range. The purpose would be to generate focused requirements for the development of hypersonic technology. An advanced concept like an uninhabited combat air vehicle would provide a solid foundation for the development of a manned air-breathing hypersonic weapon system, which could conceivably be a requirement in the early part of the twenty-first century. Component 3. Hypersonic, Hydrogen-Fueled Weapon Systems The same approach should be applied to a hydrogen-fueled hypersonic weapon system. The process should begin with a definition of Air Force requirements, followed by user-driven operational concepts with a focus based on competitive system design studies. This class of weapon system would be able to take full advantage of NASA’s development of propulsion systems. Formulation of this weapon system would lead to preliminary system designs for both unmanned and manned weapons operating above Mach 8 and would illuminate and crystallize the priorities for hypersonic technologies for hydrogen-fueled weapon systems of this class. Based on the vision statements, strategic plans, and study results made available to the committee, the Air Force should consider hydrogen-fueled hypersonic weapon systems. The tasks outlined here are the logical next steps in that direction. Component 4. Combined-Cycle Systems for Space Access The capabilities derived from the previous components, as well as from NASA’s Advanced Reusable Transportation Technology Project, would provide a basis for a dual-mode scramjet with small, fully integrated rocket ejectors installed in the flow path for low-speed propulsion. Low-speed propulsion could potentially improve performance significantly over pure rocket engine systems for space launch because it would use atmospheric oxygen during the boost phase. The gross weight of the launch vehicle would be lower than for a vehicle with pure rocket propulsion because not as much oxidizer would have to be carried onboard. SUMMARY The committee considered possible roles that hypersonic vehicles might play in future Air Force capabilities, particularly global reach and access to space. The committee then identified two program options: (1) the broad pursuit of hypersonic technologies and (2) the evolutionary development of hypersonic technologies based on clearly stated requirements. The committee believes the latter option is the only one that will result in operational systems. On that basis, the committee provided a four-component long-range planning process to guide the Air Force’s development of future hypersonic systems.
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