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76 HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION In addressing this charge, the committee has received briefings from cog- nizant staff of the Air Force and NASA, and has formulated a preliminary view of technical issues, the resolution of which it deems most crucial to realization of practical hypersonic flight. The engineers and scientists of your organization have much to contribute to this review. We would welcome the opportunity to meet with them and examine the major technical problems and possible solutions to them. We have discussed these issues among ourselves and have compiled a list of questions that would serve to identify most of the crucial problems as we see them at the present time. We share this list with you below in the hope that you will orient your briefings toward and be prepared to discuss these issues. We will be able to spend one day with your firm and would like to spend our time in technical discussions with your best research and engineering talent. We are interested in the technical substance of your program and wish to assure you that the results of our discussions will not in any way affect the competitive character of your company's efforts. We have presented a rather lengthy list of questions and must leave to your judgment the specifics of the agenda and the prioritization of our concerns. Feel free to add to the list other matters that you judge to be of comparable importance. We recognize that some of the questions posed here may be unanswerable at present. In these cases, we solicit your view as to whether the NASP program is structured to enable their resolution in the future. BROAD ISSUES The NASP program plays a central role in the nationts hypersonic research program. Although there are other programs of basic and applied research that are contributing to the technology base and to understanding of the behavior of hypersonic vehicles in flight, the X-30 is intended to be a key research vehicle, enabling exploration or the technologies critical to hyper- sonic flight. Similarly, in addressing technical risks and risk reduction for hypersonic applications, the NASP technology maturation program, though a subset of the overall hypersonic flight technology program, is of key impor- tance. To achieve its research objectives, the NASP program must provide access, for experimentation, to the high Mach number regions of flight that are not accessible in ground test, and must enable validation of those systems concepts and verification of solutions to those technical problems, that are unique to hypersonic flight. Thus, the committee judges the requirement for design of NASP to be sufficient understanding of the critical technologies to enable the design of a reusable vehicle (or vehicles) with reasonable assurance that they will be able to explore the flight corridor of interest for hypersonic flight vehicles. Tentatively, this corridor includes the Mach number range from 0 to 25, at associated altitudes such that the dynamic pressure is in the range from 500 to 200 psf. It includes steady state flight, as well as acceleration and deceleration, in the hypersonic regime. Since the X-30 is to be a research

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HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION vehicle, it may reasonably incorporate technologies that are not fully mature; however, given the high visibility and cost of the program, these will have to be sufficiently reliable in an experimental context to give reasonable assur- ance that the program can be completed, and realize its experimental objec- tives. We seek your assistance in identifying the key technology demonstra- tions that must be in place before design of the X-30 research vehicle, as well as those that will stem from its operation. TECHNOLOGICAL ISSUES The committee has identified a set of technical issues that it believes to be critical, and for which it seeks clarification of the present level of under- standing. The set is not claimed to be complete, and the committee solicits identification and understanding of issues or problems additional to those listed below. For each technical area, a brief statement of the issue will be followed by a set of questions to which we request your response, specifically for those that fall in your area of responsibility. We will also appreciate your views on issues outside your direct areas of responsibility, should you care to offer them. HYPERSONIC PROPULSION The committee considers the viability of the supersonic combustion ramjet to be a key issue for hypersonic applications, since it is the SCRAMJET that promises the high values of specific impulse at Mach numbers above 7, which are essential to global hypersonic flight or single-stage transportation to orbit. Yet from the information available thus far to the committee, serious questions of feasibility exist in a number of areas. The principal areas of concern are: a) Non-uniform flow in the engine inlet and hence into the combustion chamber b) Hydrogen injection and mixing c) Hydrogen-air reaction in the combustor d) The effect of combustor-exit non-uniformities on nozzle performance e) Gas flow and chemical recombination in the nozzle area f) Transient and unsteady behavior of the propulsion system The questions to which we request your response are: What candidate injection systems have you identified, and to what extent have you evaluated their performance with regard to fluid mixing, molecular mixing, and pressure loss? What do you consider adequate mixing, and how have you assessed the penalties of incomplete mixing? 77

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78 HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION 2) How are you addressing the reactive flow in the nozzle; in particular, to what extent are reaction kinetics included in nozzle performance calculations? 3) What calculation techniques are you using to determine the effects on the nozzle performance of nozzle-inlet thermal and composition stratifications resulting from non-uniform combustion or mixing? 4) How are you assessing the stability of the inlet-combustor-nozzle flow system and its response to small perturbations introduced for example by atmospheric disturbances, vehicle attitude changes, or vehicle deflections? 5) How are you addressing the issue of three-dimensional flow non uniformities in the engine inlet due to such phenomena as shock wave boundary layer interaction? 6) How are you addressing the operational transition between RAMIET and SCRAMJET operation? 7) How do you propose to adjust the engine configuration to varying flight conditions and to maintain regenerative cooling of critical areas during these adjustments? S) How are you assessing the engine off-design performance and its effect on vehicle drag and cooling? 9) How are you approaching the engine-airframe integration process, including the plume-slipstream interaction? LOW SPEED PROPULSION The principal area of concern here is the design of a low-speed propulsion that will provide adequate take-off and transonic thrust, sufficiently high specific impulse, high enough thrust to weight ratio, and be integrable into the SCRAMJET envelope without detriment to the RAMJET and SCRAMJET performance. The questions to which we are seeking answers are: What candidate systems are you considering and to what extent have you identified their performance characteristics including possible losses due to integration into the RAMIET and SCRAMJET? What analyses or experiments have you made that will permit you to assess the transition process from low speed to RAMlET operation? What techniques are you using to assess the stability and control of this transitional operation?

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HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION BOUNDARY LAYER TRANSITION The problem of predicting boundary layer transition for the external flow, for the internal flow on the inlet, and in the nozzle at flight Mach numbers above 10 may lead to unacceptable uncertainties in the skin fraction drag, heat transfer, and boundary layer thickness. It would appear that the location and extent of transition must be reasonably well predicted under conditions of real, reacting gases, three-dimensional flows in pressure gradients, and with non-uniformities, unless large margins in drag and heat transfer are acceptable for the experimental vehicle, which seems unlikely. The questions we hope you will address are: 1) What techniques are you using to predict the locations of transition in each of the critical areas of the vehicle? 2) To what extent have these techniques been verified experimentally, including the effects enumerated above? 3) If we must accept large uncertainties in transition locations, what are the resulting penalties to the vehicle performance? 4) To what extent can laminar flow be realized over large portions of the vehicle in the presence of practical roughness, by shape changes for favorable pressure profiles, by control of surface smoothness and temperature, by selection of low Reynolds number flight paths, etc.? Are you exploring such possibilities? VEHICLE HEATING Our experience thus far with hypersonic vehicles suggests that locally high heating will result from complex three-dimensional flows in corners, from shock-on-shock interactions, and from other phenomena that result in very thin shear layers. At the very high Mach numbers these phenomena will be complicated by the effects of nonequilibrium chemistry and perhaps radiation. Since we do not have and do not expect to have the capability to fully explore these complex phenomena experimentally, it would seem necessary to rely heavily on extrapolation by computational fluid dynamics techniques from the available data base. Our questions are: 1) What data base have you for addressing these issues? 2) What areas on the vehicle have you identified as especially critical from the viewpoint of heat transfer, and how are you predicting the heat transfer at these points? Are you counting on non-catalytic coatings to reduce surface temperatures in certain areas? 3) What computational techniques are you using for dealing with the three-dimensional, reacting flows at critical points? How have these techniques been verified? 79

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80 HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION 4) Are there critical reaction rates that, you believe, are not yet known to sufficient accuracy? 5) Have you considered the effects of shock-on-shock interactions such as may arise from a bow shock crossing a wing or inlet leading edge shock, if this occurs within the pitch, yaw and Mach number envelope of your vehicle design? STRUCTURES AND MATERIALS The X-30 research vehicle and NASP technology maturation program are to explore structural concepts that will cope with the rigors of flight at hypersonic velocities and with the attendant structural integrity issues. Structural concepts must address, for examples, the choice of materials, thermal protection, active cooling, insulation, configuration, and fabrication. Structural integrity requires an assessment of the ability of the concept to meet the critical flight conditions with positive margins of safety. This requires knowledge of failure modes, which means the gathering of a data base from experiments. Much emphasis is being placed in the technology maturation program on developing new materials that will have better high temperature properties than are presently available. In comparison with the materials data required for subsonic airplanes, the hypersonic airplane data requirement is much more extensive because, e.g., thermal, chemical, oxidation, and other data at elevated temperatures are required. We wish to ascertain whether these materials will be ready for the X-30 vehicle. A relatively small fraction of an airplane is critically loaded in tension, yet the materials community appears to be using specific tensile strength as a principal figure of merit for new materials. The more prevalent modes of failure are those due to buckling, fatigue, and fracture caused by small flaws. It is the committee's perception that these modes of failure are not well understood for many of the materials projected for use in the NASP. Additional areas of concern are the fracture toughness at room temperature of the high temperature materials, and the effect of fastener holes on the strength and toughness of the advanced filamentary composite materials. Our questions are: 3 ~On w hat data base do you draw for design, considering the issues discussed above? How has it been validated? What major structural tests do you envision for validation of the structure of the NASP during design and prior to flight test? 3) Do the verification tests require thermal simulation? If so, what facilities are available or must be provided? A re the available computational techniques adequate for prediction of the structural deformations due to aerodynamic loads and heating'

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HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION 5) What are the critical flight conditions for the various structural components? 6) What are the critical modes of failure, i.e., what structural behavior determines the sizes of the various structural components? 7) What are the projected methods of fabrication? Will your concept meet the letter and/or spirit of specifications such as MIL-STD 1536A and MIL-A 83444? 9) What minimum or representative sizes of test materials are contem plated for materials evaluation? What minimum or representative sections are contemplated for systems? 10) Of the several concepts for reusable thermal protection systems, such as radiatively-cooled structures with internal insulation, external insulation, or connectively cooled structure, which do you consider most feasible for each of the critical heating areas of the NASP? CONTROL The control system for the X-30 will have to integrate control of the vehicle attitude, engine geometry and fuel supply, center of gravity, and the trajec- tory to high degrees of precision because of the sensitivity of the hypersonic vehicle to all of these. It may also have to actively control lower frequency structural and shock modes, maintain shock positions in the inlet so as to maximize performance and avoid upstart, counter any thrust malalignments, etc. Precision require- ments for sideslip and angle of attack control are at least unusual and perhaps unprecedented. Because the degree of interaction of controls and guidance with the aero- dynamics, propulsion, and structural aspects of the vehicle are to some extent configuration-specific, we are interested in insights you have gained from your studies. Some questions are: 1) How have you addressed the problem of simultaneous control of flight path, aerodynamics, propulsion, and damping of structural and slosh modes? 2) What vehicle configuration (aerodynamic, structural, propulsion, etc.) and control system features have been governing factors in your design? 3) How have the requirements for control been explicitly considered in your design studies? 81