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8 The Committee on Hypersonic Tech- nology for Military Applications was formed in early 1987 by the Air Force Studies Board of the National Research Council in response to a request from the Air Force Systems Command. We were asked to evaluate potential appli- cations of hypersonic air-breathing vehicles to military missions, and to assess the status of those technologies critical to the feasibility of such vehicles. Further, we were asked to examine the full range of issues relevant to potential military hypersonic applic- ations, including the classes of missions and the types of vehicles that would be appropriate to them, at least to the extent that understanding of such issues is necessary to an evaluation of hyper- sonic technologies. We determined very early in our proceedings that a firm conceptual basis for military air- breathing hypersonic operations, that is, a set of mission scenarios, does not yet exist. Therefore, our discussion of potential missions will be relatively brief; the principal emphasis of our discussion will be the status of the critical technologies. In the time frame of the commit- tee's activities, the National Aerospace Plane program has dominated hypersonic research in the USA. With its focus on technology development for, and prelim- inary design of, a research vehicle capable of air-breathing operation to orbital speeds without staging, the NASP HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION INTRODUCTION 1 program has highlighted hypersonic technology requirements and provided the bulk of the support for hypersonic years. The program has led to a review and to some extent rediscovery of the rather exten- sive work done on hypersonic propulsion and configurations in the 1 960s and early l970s. It has, therefore, also been the source of much of the committee's information on the status of hypersonic technologies. Our assessment of hyper- sonic capabilities is thus to some extent in the context of the tentative require- ments set for the NASP research vehicle. However, other requirement statements, perhaps somewhat less demanding of the critical hypersonic technologies, may ultimately be of considerable interest to the Air Force. We have attempted to maintain this perspective in our deliber- ations. research for the last three The committee was briefed by all major government and industrial par- ticipants in hypersonic research and technology development, including the relevant units of the Air Force Wright Aeronautical Laboratories, the NASP Joint Program Office, and NASA Langley Research Center. Individual members of the committee had additional contacts during this period. Two members of the committee were members of the NASP Review Panel of Board, providing the Defense Science the committee as a whole with an insight into that group's findings. After the selection of con ~To assure the widest dissemination, a special effort has been made to publish this report without security restrictions. The committee believes that it has adequately examined and reported the state of the technology to provide the reader with an accurate assessment of hypersonic technology development without divulging classified information.

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INTRODUCTION tractors for Phase Il. the preliminary design phase of the NASP Program, we were briefed by the two propulsion contractors selected for phase 2B, and met informally with the other prospec- tive propulsion contractor who par- ticipated in Phase 2A. The committee was then briefed by the three airframe contractors selected for phase 2, part 2. To ensure that we and the several contractors engaged the issues that the committee believed were important, we prepared a questionnaire and sent it to the contractors through the director of the NASP Joint Program Office (see Appendix B). In general, the contractors responded quite adequately to our quer- ies, and added their own special points. Inevitably, many judgmental issues arise in any assessment such as this. We have tried to ensure that we under- stand the views of the hypersonic research and technology community, and to the extent that we agree, we have incorporated these judgments in this report. But the final responsibility for the findings and recommendations of this report rests with us, and where our views differ from others, our experience and judgment have prevailed. In addressing our charge, we have placed the major emphasis on evaluation of the technologies most critical for hypersonic flight. We judge these to be, in order of importance: . 9 throughout the flight corridor from takeoff to orbital or near-orbital velocity. We have evaluated these technologies against our perception of the needs for military hypersonic flight in the broadest sense. the supersonic combustion ramjet (scramjet) engine the technology of integrating such engines with the airframe, including the aerodynamics and control of the resulting "flying engines" the structural concepts and high- temperature low-density materials required to achieve the desired weight fractions guidance and control of such large, flexible, complex vehicles as result from the requirement to operate Also in keeping with its charge, the committee has examined the technology needs for the experimental hypersonic vehicle, the X-30, which is the focal point for the NASP program. We believe the design of the X-30 requires suffi- cient understanding of the critical tech- nologies to enable the design of a reusable vehicle (or vehicles) with reasonable assurance that they will be able to fully explore the flight cond- itions of interest for hypersonic flight vehicles. Tentatively, this envelope includes the Mach number range from 0 to 25, at associated altitudes such that the dynamic pressure is in the range from 500 to 2000 psf. It includes steady state flight as well as acceleration and deceleration in the hypersonic regime. Since the X-30 will be a research vehicle, we believe that it may reason- ably incorporate technologies that are not fully mature; however, given the high visibility and cost of the program, they will have to be sufficiently reliable in an experimental context to assure that the program can be completed and realize its experimental objectives. These requirements are considerably less demanding than those for an operational vehicle with the same envelope, in terms of the required reliability, durability, and safety of the vehicle. On the other hand, they are more demanding of oper- ational flexibility if the vehicle is to be capable of accessing all parts of the altitude-Mach number space that may be of interest for various possible oper- ational vehicles, any one of which may have to operate in a more limited envel- ope. As will be elaborated in the ensu- ing discussion, the research vehicle may require special features, particularly in

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10 HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION propulsion and control, that will not be required in an operational vehicle. Facilities for testing can be con- sidered a critical issue for hypersonics. Indeed, if the NASP program were to proceed in the classical way, with full verification of all major components and systems before flight, a major national commitment would be required, far exceeding that presently planned. We have not addressed this possibility. We have taken the view that the NASP pro- gram must depart from the classical approach by making much more extensive use of numerical modeling (made possible by recent advances in computational fluid dynamics), and by depending more on flight research. There is no other option, if the NASP program is to pro- ceed on the planned schedule. It is not clear that full validation testing is possible at the high Mach number limits of the envelope, but even if feasibility could be established, 10 or more years would be required to design, build, and activate such major facilities. In connection with the NASP program, we have therefore asked how existing facilities can best be used, and what component testing is feasible on the time scale of the NASP program. In the context of long term oper- ational uses of hypersonic vehicles, the issue of test facilities must be reexam- ined. If we are to build such vehicles, extensive testing capabilities probably will be needed to ensure their econom- ical development. This committee has not addressed this issue in depth.