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62 HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION lecture, environmental issues, sensors, 12) etc. 10) We recommend several more speci- fic actions in this broad framework. In all cases, the above parallel approach applies. a) An effort should be launched to explore and identify areas where control activities give some promise of reducing the effects of uncertain behavior in the engine. Effective quan- tities to be sensed or otherwise estimated and possible control system architectures are central issues. Studies of alternative mechanizations of sensors or their surrogates will be needed as part of this effort. In carrying this out, the need for sensors in ground testing should be examined as well, including applications to high- speed wind tunnels. b) In some areas, such as in fault tolerant and high throughput information systems architec- tures, there already exist government-supported technol- ogy programs. The hypersonic program offices should establish liaisons with these activities to promote their applications to hypersonic vehicles. The tremendous heat loads that will be a natural part of hypersonic flight, together with the uncer- tainty of the heat transfer prop- erties of new materials when com- bined in a specific design config- uration, lead to the possibility of broad uncertainties in the thermal environment for key information/- control equipment, including cables and hydraulics. Detailed attention should be placed on highly robust thermal control to accommodate the wide uncertainties in thermal environment. Another issue that will have to be examined early enough to enable pursuit of alternatives, if needed, is that of communications. Above approximately Mach 10 there is likely to be a layer of ionized particles over some sections of the aircraft. This phenomenon will be configuration specific. An investi- gation is needed to ensure that reliable communication pathways will be available. If a definitive answer to this is not available early, a technology program may be needed to assure additional options for the first flight test vehicles. As part of this communications assessment, an examination should be made of the relevance of pre- vious accomplishments in this area. 13) The flight program and control design philosophy should be config- ured to recognize the levels of knowledge about uncertainties in the flight control system equipment associated with each phase. Grad- ual work up through the speed envelope, is essential to reduce the risks in control systems for all their primary functions. 3.6 High temperature Materials, Cryogenics, and Cooling 1 ) The structural weight fraction required for single-stage near- orbital hypersonic flight will require stiff, ultra-light, high temperature materials to insure vehicle performance and usable payload over the intended range for desired missions. 2) The vehicle configuration and the trajectory flown will determine structural concepts and material requirements. Various portions of the vehicle will require different materials because of varying structural loads and temperature

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FINDINGS AND RECOMMENDATIONS isotherms that will be experienced b) in hypersonic flight. 3) Material requirements will cover the range from -268 C (cryogenic fuel tanks) to 2200 C for nose and wing leading edges. 4) The major new requirements for ultra-lightweight, high temperature, high stiffness, oxidation resistant materials will dictate the use of materials and material concepts now in the realm of emerging technol- ogies. , Those high performance materials that have been identified as having significant structural properties at the high temperatures of interest, and their ranks thin as temper- atures rise, do not have an adequate data base, nor are their failure mechanisms adequately understood, nor do we know their ability to maintain adequate properties for the necessary times at elevated temperature to insure a design with the necessary structural integrity for the intended missions. 6) We do not have enough data to develop design criteria to use these emerging materials. We do not know if these materials can be produced in the proper quantity, quality, and forms with consistent properties to insure a manufacturable design and to insure structural integrity of the proposed hypersonic vehicle. 8) Specific requirements for materials for hypersonic vehicles include: a) Reliable test data on such diverse variables as strength, modulus, structural stability, ductility, oxidation resistance, interphase reactions, high temperature creep, and joining. f) Creep and stress rupture tests for time periods of at least 100 hours, preferably for 1000 hours, in appropriate atmo- spheres. Fracture properties such as fatigue crack growth rates and fracture toughness values at appropriate temperature. d) Oxidation resistance up to about 100 hours over a range of temperature. e) Structural and property changes under the above conditions. Reproducibility of structures and properties from repeated fabrication or processing studies. g) Development of higher temper- ature test facilities (tension, compression, fatigue, thermal stress, etc.) than are now available. h) Availability of at least two material producers for each of the classes of materials selected is desired. i) Encouragement of alternate manufacturing or processing sources. 3.7 Structural Concepts 1 ) The expected performance of the propulsion sub-system of a hyper- sonic air-breathing vehicle dictates a fuel fraction in the neighborhood of .75 for orbital or near orbital single stage performance. For a useful payload function and a reasonable take-off gross weight, the structural weight fraction must be about .18. 2) Design philosophy and specifications play a dominant role in the struc- tural design, so the structural weight fraction is sensitive to the specifications imposed on the vehicle.

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64 3) Present vehicle specifications that insure structural integrity are the result of years of service exper- ience on subsonic and slightly supersonic aircraft. Their impact, if imposed on hypersonic aircraft, must be studied and evaluated with a fresh (i.e., zero-base) outlook because the weight increases that result from these historical speci- fications may keep the hypersonic aircraft on the ground. Structural failure modes depend on the local details of the design, in - addition to material properties and applied stress levels. These, in turn, are driven by the configura- tion of the aircraft and the internal layout. 5) Tensile strength is only one dis- criminant of material goodness for aircraft, and it is misleading to rank materials using tensile strength. 6) An extensive plumbing system for active cooling will form a large part of the structural weight fraction of the vehicle. 7) The large fuel fraction and the use of low density fuel (i.e., hydrogen) means a large portion of the air- frame will be tankage. 8) Thermal stresses are driven by temperature gradients. Heat flow in the structure will be affected by the conductance of structural joints, viewing angles of colder regions to hotter regions, and the thermal characteristics of the materials. 9) New structural design concepts are required to meet the unique en- vironments encountered by hyper- sonic aircraft. Active cooling, insulation, thermal protection, etc., are added to meet the traditional HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION requirements of strength, aeroelas- ticity, and longevity. Structural design concepts must be proven by the fabrication of hardware which are then tested to the simulated environment. 10) The structural integrity of aircraft are verified by full-scale testing statically and under cyclic loads to determine the longevity and durability. It does not seem practical to simulate the temper- ature environment of hypersonic aircraft. 11) Hypersonic aircraft will be hot on the outside and very cold in some parts of the inside, which presents an unusual and severe environment for sensors, fiber optics, cables, etc. 3.S The Role of CFD 1) CFD has become a principal tool for aerodynamic and propulsive-flow design of hypersonic vehicles because computers can simulate simultaneously the hypersonic flight parameters of velocity, free stream density, physical scale, and free stream thermochemical state. Present ground-based experimental facilities cannot simulate together all of these parameters, especially for high altitude hypersonic flight conditions involving non-equilibrium air chemistry. 2) Current supercomputers and numerical methods can simulate 3-D flows using the Reynolds-averaged Navier-Stokes equations, but these equations require that the turbu- lence viscous stresses and heat flux be modeled, and that the location and extent of transition from laminar to turbulent flow be known a prlorl.

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FINDINGS AND RECOMMENDATIONS 3) Present modeling of turbulence stresses and heat flux, and mod- eling of transition location, are the principal current limitation of CFD. Present validations of CFD code are relatively limited for hypersonic flows. Turbulence models for attached wall-bounded flows appear more acceptable than for compres- sible-flow mixing processes such as are involved in combustors. 4) Present CFD results appear accept- able for nozzle flows provided the initial entrance conditions and all relevant reaction rates are known, but CFD is incapable of determin- ing whether or not relaminarization takes place on a nozzle wall. 5) - Current CFD simulations of the intense aerodynamic heating for shock on cowl lip shock conditions agree well with experimental results, but both results correspond to the lower altitude portion of the hypersonic fight corridor where shock thickness is negligible. For the high altitude flight conditions wherein the thickness of a shock wave can become sizable compared to the lip shock detachment dis- tance, neither computations nor experiments have yet been made. The present limitations of CFD technology are not inherent to CFD itself, but are a consequence of the state of supercomputer develop- ment, which forces the use of a time -averaged ("Reynolds- averaged") form of the Navier-Stokes equa- tions. The long range future of CFD has the potential of using the time- dependent Navier-Stokes equations wherein turbulent eddies are directly computed rather than modeled. This could greatly reduce the limitations and inaccuracies of CFD, but will require more power 65 ful supercomputers than those now available. 8) The future realization of advanced CFD involving the direct numerical simulation of transition and turbu- lence would have important long- range consequences to the Air Force in the design of future hypersonic vehicles (and also aircraft and turbine engines.) 9) Greater emphasis should be placed on the modeling of compressible turbulent-mixing flows. Increased effort should also be given to modeling of hypersonic turbulent boundary layers in the presence of heat transfer and pressure gradi- ents. 10) High altitude shock-on-shock heating on a cowl lip, the most intense local heating expected on vehicles such as NASP, must be investigated by direct simulation Monte Carlo methods, appropriate new experiments, and continuum equations more advanced than Navier-Stokes. 11) The direct numerical simulation of turbulence in compressible flows should be an important part of the long-range technology development program of the Air Force. 3.9 Test requirements 1 ) Research on facilities is needed to identify their effect on the simu- lated gas flow and its interaction with real gas kinetics. 2) Direct connect testing of scramjet combustors cannot be done with conventional nozzle concepts and high enthalpy sources. 3) While it appears that facility capability is sufficient in some