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ABSTRACT situation demands the ability to integrate highly- There is currently a renewal of world-wide interest coupled and interacting elements in a fundamental in hypersonic flight. Vehicle concepts being and optimal fashion to achieve the desired considered range from cruise missiles to SSTO and performance. Some crucial technology needs are TSTO vehicles. The new characteristics of these found in propulsion-airframe integration and its role vehicles are that they will be powered by air- in configuration definition, hypersonic boundary- breathing engines and have long residence times in layer transition and its impact on vehicle gross- the air-breathing corridor. In the Mach 4-7 regime, weight and mission success, scramjet combustor waverider aircraft are being considered as mixing length and its impact on engine weight and, candidates for both long-range and short-range CFD (turbulence modeling, transition modeling, etc) cruise missions, as hypersonic missiles, and as high as a principal tool for the design of hypersonic LID highly maneuverable vehicles. This paper will vehicles. Key technology implications in thermal discuss the potential for near-term and far-term management, structures, materials, and flight control application of air-breathing engines to the above systems will also be briefly discussed. It is concluded waverider vehicle concepts and missions. In that most of the technology requirements in the Mach particular, the cruise mission is discussed in detail 4-7 regime are relatively conventional making cited and attempts are made to compare and contrast it applications near-term, yet offering very significant with the accelerator mission. Past criticisms levied advancements in aircraft technology. against waveriders alleged low volumetric efficiency, lack of engine/airframe integration studies, poor off- I. INTRODUCTION design performance, poor take-off and landing capability, have been shown by on-going research to Following nearly 15 years of relative inactivity, sev- be unfounded. A discussion is presented of some of eral countries have initiated research and technology the technical challenges and on-going research aimed programs to develop various types of at realizing such vehicles: from turboramjet and aero/spacecraft. Their ultimate goal is direct, scramjet technology development, propulsion- reliable and easy access to space (Ref 1). In Europe, airframe integration effects on vehicle performance, Russia, Japan, India, etc., hypersonic programs are aeroservothermoelastic systems analysis, hypersonic directed toward vehicles such as Hermes, Sanger, stability and control with aeroservothermoelastic and Hotol, Hope, Hyperplane, etc. Concepts for propulsion effects, etc. A unique and very strong hypersonic vehicles powered by air-breathing aspect of hypersonic vehicle design is the integration engines are also evolving as part of these programs. and interaction of the propulsion system, In the U.S., the National Aerospace Plane (NASP) aerodynamics, aerodynamic heating, stability and program is developing the key technologies required control, and materials and structures. This first - for the successful operation of air-breathing order multidisciplinary hypersonic aerospace vehicles. The general goal of
ese e a e I e a io a Gas u i e a Ae oe gi e Co g ess a E osi io Co og e, Ge ma y u e 4, 2 is a e as ee acce e o u ica io i e a sac io s o e ASME iscussio o i wi e acce e a ASME ea qua e s u i Se em e 0, 2 the NASP program is the construction and testing of stands at Mach 5.2, held by a ramjet-powered Martin an experimental, fully-reusable vehicle, designated Marietta ASALM missile that disobeyed a fuel shut- the X-30, that will be used as a manned demonstrator down signal. The cruise application, specifically for for hypersonic flight. The vehicle will use Mach 4-7, is the focused topic of this paper. The hydrogen-fueled, air-breathing ramjet/scramjet purpose in addressing this application separately is engines and will be capable of horizontal takeoff and that there are technologies which are specific to landing. It will be designed to expand the envelope Mach 4-7 cruise. There are also technologies which of high speed flight in and beyond the atmosphere to are common to cruise and other hypersonic missions. the point that access to low earth orbit can be These are discussed in this paper. The paper offers an in-depth investigation of the most pertinent achieved. The uniqueness of NASP lies in its highly- Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 integrated scramjet, special structures and materials technology issues associated with hypersonic cruise requirements, absence of suborbital stages, and and also addresses issues associated with one class of inherent payload return capability. NASP addresses vehicles which is fundamentally suited for hypersonic SSTO (single-stage-to-orbit) as well as aspects of cruise waveriders. A summary of the status of several accelerator/interceptor applications, but does waverider technology for air-breathing missions is not adequately address the technology aspects unique provided so that the extremely important to cruise applications. For either class of vehicle, technologies that must be developed for the benefit of however, a tradeoff between engine thrust-to-weight the entire hypersonic vehicle development ratio and specific fuel consumption is necessary. community are stressed with a perspective of applicability to real missions. Also, some of the In this paper, the prospects for air-breathing hyper- technologies for waveriders are actually specific to sonic vehicles, especially hydrocarbon-fueled Mach waveriders, and must be addressed in the context of a 4-7 v r d r aircraft, are examined. Prospects for waverider discussion. Because air-breathing generic air-breathing hypersonic vehicles (Mach 5- hypersonic vehicles are multidisciplinary systems, 25) were previously discussed in Refs 2 and 3. brief discussions about the technical areas which Figure 1 shows flight block time as a function of impact the design of such vehicles are presented. global range fraction for several cruise Mach numbers. The shaded regions indicate the global Mach 4-7 is an attractive segment of hypersonics as range fractions and Mach numbers believed to be of it offers a combination of very high speed capability primary interest for future high-speed civil transport and use of relatively conventional technologies. This aircraft and military.reconnaissance/strike aircraft. range of speeds offer dramatic reductions in travel Cruise Mach numbers of 2 to 3 for civil aircraft, time compared to subsonic or even supersonic speeds which are consistent with Concorde and current as shown in Fig. 1. This range of speeds may also be operational military aircraft, would reduce the block very competitive (depending on mission) with higher time for trans-pacific flights from about 14 hours to speeds of Mach 8 to 12 over distances up to 12,000 about 4-6 hours. Mach 5-6 military aircraft could nmi (or half the way around the earth). Such speed provide worldwide coverage in about 4 hours. Mach capability has a number of useful applications numbers greater than about 10-15 provide an including first stage of a two-stage-to-orbit vehicle, insignificant improvement in block time for both interceptor, recce, and transports. civil and military aircraft. Much greater distances Though the need for high speed travel has remained are required for these higher speeds to pay off in roughly constant constant over time (perhaps block time. The Earth simply isn't big enough to increasing with the advent of integrated global socio- warrant aircraft that cruise faster than about Mach economic ties), the availability or maturity of 10-15. technologies for the Mach 4 to 7 range has been As already noted, most national and international progressing. In the past few years several key hypersonic activities are focused on transatmospheric enabling technologies have matured to significant flight with the goal of constructing air-breathing levels. These technologies include high temperature single-stage-to-orbit second generation shuttles or hydrocarbon fuels, high temparature materials, two-stage transports that incorporate an air-breathing practical combined cycle engines (e.g. turboramjets first stage. An obvious intermediate regime, and and scramjets), and highly efficient perhaps spin-off technology from the above concepts, vehicle/aerodynamic shapes. is the development of extremely high-speed civilian One element of the latter, which is a key driver in and military vehicles designed for sustained flight in the overall concept of a Mach 4 to 7 vehicle is the the range of Mach 4-25. At the high end of this advent of modern efficient waverider concepts. anticipated air-breathing flight corridor, sustained These concepts have the potential of excellent levels orbital flight at Mach 25 is now routine. At the low of performance in at least three key areas of I , end, the speed record for air-breathing vehicles
2 volumetric efficiency, and stability and control. a sound interpretation of the Breguet range equation. Though the shape of candidate Mach 4 to 7 This classification of aircraft shapes is shown in Fig waverider vehicle concepts are relatively simple, 2 (Ref. 5). The progression of optimum "cruising" they are nonetheless the result of extensive analytical engines as a function of flight speed is also shown. optimizations involving highly non-linear For classical and swept-wing aircraft, the means for relationships. These concepts have only recently providing volume, lift, propulsion, and control are matured as a result of advances in 1) analytical distinct; they do not typically interact. As flight techniques which account for viscous effects on speed increases, the situation changes dramatically, rather complex aerodynamic shapes, 2) leading to a completely different aircraft design computational resources (e.g low cost philosophy. Many examples of classical, swept, and workstations ), 3) efficient linking of CAD and slender wing aircraft already exist. For the Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 analytical systems, 4) recent subsonic, supersonic waverider class, flat-top waverider principles were and hypersonic wind-tunnel tests of basic non- used in the design of the half-million pound XB-70 integrated cone-derived shapes, and 5) propulsion Valkyrie. This aircraft achieved an LID of about 7 integration technologies which apply to candidate at Mach 3 in test flights in the mid-1960's. The XB- waverider shapes. As a result of the aforementioned 70 program demonstrated the high LID potential and factors and technical progress to date, the viability of the practical applicability of waverider concepts to near-term development of a Mach 4 to 7 waverider full-scale cruise aircraft. vehicle appears to be quite high. Achievement of this, however, is contingent upon continued successful With the rekindled interest in hypersonics, a large research and development in several pertinent areas. spectrum of military and civilian applications of both hydrogen-fueled and hydrocarbon-fueled air- Hypersonic vehicles will operate at flight conditions breathing aircraft are being considered. Some of very different from those experienced by subsonic or these applications are accelerator/interceptor aircraft, low supersonic vehicles. Our experience with wind advanced reconnaissance aircraft, strategic cruise tunnel facilities, computational capacity and flight aircraft and missiles, highly-maneuverable research has provided the framework for the low interceptor aircraft and missiles, long-range speed area. This is not the case for air-breathing hypersonic hydrogen-fuelled commercial transports, hypersonic flight, where verification and single-stage to orbit vehicles (SSTO), and the first development through experiment will not be possible stage of two-stage-to- orbit vehicles (TSTO). in ground facilities, current or in the foreseeable Detailed studies of possible missions for Mach 4-7 future.The problems of hypersonic flight are aircraft have been presented in many references primarily associated with the high stagnation (e.g. Ref 6). temperatures/enthalpies associated with very high speeds. These higher temperatures result in high As previously indicated, the primary applications for heat transfer to the vehicles, changes in the gas hypersonic air-breathing propulsion include single- characteristics around the vehicles, and changes in stage-to-orbit (SSTO), air-breathing launch vehicles, the chemical constituents of air, which can directly long-range cruise vehicles, and hypersonic missiles. affect the classical combustion phenomena. For air- While many of the research topics associated with breathing hypersonic vehicles the severe operational these missions are similar, some very fundamental environment, the need to totally or partially integrate differences exist. The SSTO mission is an accelera- the propulsion system with the airframe, the ultimate tion-type mission which covers a very wide Mach disciplinary coupling of the vehicle subsystems, and number range, requiring a large movement of the long residence times in an environment at high variable geometry to accommodate low speed, levels of aerodynamic heating and dynamic pressure ramjet, and scramjet cycles in the same engine introduce technical and sociological problems that hardware. A turboaccelerator as a low-speed will be with us for a long time to come. It will be accelerator option affects the nature of the variable seen in this paper that in the Mach 4-7 range most of geometry requirement and perhaps suggests an the needed technologies are conventional or near over/under turbojet/scramjet propulsion system maturity making applications in this range short- similar to that which might be employed in a long- term. range cruise vehicle. The SSTO mission also introduces unique problems at high Mach numbers II. PROJECTED APPLICATIONS which include forebody/inlet and viscous interactions as well as nozzle frozen chemistry effects. The long- Kuchemann (Ref. 4 ) classifies the progression of range cruise missions share the variable geometry cruising aircraft configurations in terms of: classical requirements of the SSTO mission but without the straight wing aircraft, swept wings, slender wings, complexity of very high Mach number limitations. and waverider aircraft. His classification is based on However, high aerodynamic and propulsion
3 efficiencies are required at the cruise Mach numbers, flows and, above about Mach 10, the forebody leading to different solutions to the aerodynamic flowfield. A key feature of any hypersonic flight shape (such as waverider configurations) and the vehicle is that its technology and the methodologies propulsion/aircraft integration. In addition, smaller employed are specific to the application or mission. vehicles designed for interceptor missions may Familiar examples, even though they are supersonic require dense hydrocarbon fuels and thus share fuels examples, are the F-15, the Concorde, and the SR-71. and combustor research topics with hypersonic All are supersonic aircraft but their shapes are missiles. If it uses rocket boost to hypersonic speeds, different because of their different missions and the missile mission provides a potential propulsion operational requirements. This configuration- system simplification by allowing a near point design mission coupling will be even more pronounced for Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 scramjet engine. The resulting fixed-geometry hypersonic aircraft. This feature for the "confined design allows more geometric freedom, making flowfield configuration (waverider)" which is square-to-circular engine designs possible, which considered best-suited to a high-speed cruise mission would improve structural efficiency . A focus on is addressed further in the following sections. SSTO includes most research topics such as variable geometry requirements, large integrated forebody III. WHAT ARE WAVERIDERS? and aftbody surfaces, and high Mach number inlet viscous and nozzle chemistry effects. Low-speed A waverider is a supersonic or hypersonic vehicle geometry problems include turbojet/scramjet which has an attached shock all along its leading installations typical of cruise vehicles if low-speed edge. Because of this , the vehicle appears to be accelerator cycles are expanded to include turbojets. riding on top of its shock wave -- hence the term A topic not covered by SSTO research, except waverider. Since the shock wave is attached at the possibly by technology transfer, is integrated leading edge in the cruise condition the upper and propulsion systems for high efficiency cruise at lower surfaces have independent flowfields and thus speeds of Mach 4-8, which may use hydrocarbon can be designed separately. This is in contrast to a (possibly endothermic) fuels. This will be the focus more conventional hypersonic vehicle, where the of this paper. We will explore the status of thermal shock wave is usually detached from the leading management systems and detailed structural analysis edge. The aerodynamic advantage of the waverider capability, the readiness of endothermic fuels for is that the high pressure behind the shock wave under these applications, the degree or level to which air- the vehicle does not "leak" around the leading edge to frame-propulsion integration should be pushed, and the top surface; the flowfield over the bottom surface also outline some of the major differences between is contained, and the high pressure is preserved. cruise and SSTO aircraft. Accelerators are essentially driven by thrust minus drag, Isp, fuel fraction, and Waveriders are generated from known flowfields efficient flight for Mach 0-25. Cruise vehicles are established by basic shapes, such as wedges, cones driven by lift-to-drag ratio, Isp, volumetric and power-law bodies, different from that of the efficiency, and a design optimized for the cruise waverider itself. For example, the waverider concept Mach number. Some of these requirements and was first introduced by Nonweiler [7] in 1959, who drivers for cruise and accelerators are presented in generated caret-shaped waverider from the two- Table 1. It should be emphasized that major dimensional flowfield behind a planar oblique shock shortcomings in ground-test capability exist for both wave associated with a wedge. Cone-derived missions. waveriders including viscous effects were analyzed in Refs.8-9, and a generalized approach based on For turboramjet systems questions concerning arbitrary 3-D shocks is outlined in Ref.10. The whether tandem or over/under arrangements are essence of the idea is that an inverse procedure could optimum need to be resolved in addition to fuel be used in a straightforward way to deduce non- issues. Aerodynamic design of of inlets, nozzles, and linear large disturbance supersonic and hypersonic the thermal analysis of these components will be a flowfields, and the relatively complex three-dimen- major activity. For advanced hypersonic airplane sional finite-span configurations from which they configurations, the external geometry is highly 3-D arise. It offered prospects for rational design in this and the flowfields possess strong complex wave strongly non-linear regime by using a method giving systems (Fig. 3). The flows are dominated by configurations with exactly predictable properties viscous effects; shock/boundary layer interactions developed from use of basic simple known occur; the boundary layers will be laminar, flowfields. Excellent and authoritative surveys of transitional, and turbulent. For vehicles with air- waverider research have been given by Townend breathing propulsion, the external and propulsion [11], Roe [12], and Schindel [13 1. system flowfields are strongly coupled. Also, real- gas effects may affect the combustor and nozzle
4 and propulsion are highly integratd. Some of The recent "1st International Symposium on the advantages of the waverider class are (Ref.15): Hypersonic Waveriders" (Ref.14) provided many excellent papers. Results of the symposium are also • No lateral flow spillage summarized and assessed in Ref 15. Development of - Attached leading edge shock waverider configurations with anhedral/dihedral, • Volume generates all-postive lift advances in synergistic propulsion-airframe Clean blended shapes produce low drag integration methodology, waveriders with high • (i.e. efficient lift) volumetric efficiency, and ingenious extensions of the inverse design method were a few of the signifi- • Balance achieved between viscous & inviscid cant results presented. Procedures to alleviate the losses Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 leading edge heating problem without incurring lift - Product of waverider optimization code and drag penalties were outlined. The progress • Synergistic propulsion integration noted at the symposium indicates that an air- - Forebody precompression of inlet flow breathing, passively-cooled waverider vehicle using - Natural base area for nozzle integration hydrocarbon fuel could be built within a decade. This could be a small cruise vehicle operating in the Several significant technologies pertinent to the Mach Mach 4-7 range at an altitude of around 100 kft. It 4-7 range are currently being pursued outside of the would be used as a flight test vehicle. ongoing SSTO and TSTO programs - including Multidisciplinary optimization of such a vehicle will turbomachinery-based combined-cycle systems, still be a challenge, though. The required hydrocarbon fuels (possibly endothermic), advanced technologies, however, are relatively conventional. aerodynamic concepts, advanced analytical methods, thermal management systems, certain high Past criticisms levied against waveriders - alleged temperature materials and vehicle/pilot interface low volumetric efficiency, lack of engine/airframe systems. Applications under consideration include integration studies, poor off-design performance, long-range cruise vehicles, missiles, interceptors, and poor take-off and landing capability, were shown to the first stage of a two-stage-to-orbit mission. These be unfounded. With this in mind, the following are very different missions which will lead to section addresses the technologies associated with air- significantly different vehicles. breathing hypersonic waverider development, and also addresses several of the key technology issues For waveriders to become viable aircraft for these related to almost all air-breathing hypersonic applications, certain challenges must be met. Some vehicles. key challenges are summarized below. IV. MACH 4-7 WAVERIDER • Using volume efficiently TECHNOLOGIES - Volumetric efficiency - Conformal tankage As indicated earlier in Section II, Kuchemann (Ref. 4) describes how air-breathing aircraft can be • Structural efficiency classified into three categories according to their - Large wetted area and compound curves shape: wing-body (or swept), slender-wing, and - Can be simplified without reducing waverider aircraft. An understanding of one class performance does not necessarily allow one to design an effective • Propulsion integration vehicle in another. In this section, the potential for - Propulsion interactions waverider aircraft in the Mach 4-7 range is briefly - Combined aeropropulsion optimization examined. Development of operational aircraft • Stability & control capable of extended and efficient operation above Mach 4 will require a substantial leap in aerodynamic • Transonic drag design philosophy and concepts, not a mere - Synergistic low and high speed nozzle design evolutionary progression from the super-sonic cruise - External burning aircraft which have been evolving over the past four • Multidisciplinary optimization decades. The waverider is a true aircraft concept which satisfies this need. It offers Of these, propulsion is a major enabling technology the methodology for rational design in this for Mach 4-7 vehicles. Various combined-cycle sys- strongly non-linear regime (hypersonic) by tems have been studied and will continue to be using a method giving configurations with studied and developed for this speed range (Ref. 5). exactly predictable properties developed Subjects of study include engine cycles, fuels, from use of basic simple known flowfields. thermal management, and propulsion integration. Also, the means for producing lift, volume The combination of a turbojet and ramjet appears a
5 prime candidate for the Mach 4-6 range, with opera- Scramjet propulsion is one of the driving tech- tion above Mach 6 probably requiring a scramjet. nologies that must be developed to make the The development of hydrocarbon-fueled scramjet hypersonic air-breather a reality. Encouraging technologies with dual mode combustor results have been obtained to date through both ramjet/scramjet engine cycles have allowed experimental and computational efforts. However, consideration of high speed hydrocarbon fueled scramjet-related test capability has been constrained vehicles. Also, work on endothermic hydrocarbon by a combination of available power, facility size, fuels has shown potential for extending the heat sink structural and thermal limits of hardware, and the capacity of these fuels. few practical techniques for heating test gases to the
high temperature required for high-speed flight Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 Propulsion system integration is another key chal- simulation. To prove scramjet operability and per- lenge. This includes the design and functional formance, high Mach number data obtained in interaction of the inlet, engine, nozzle and related facilities with reasonable test durations in relatively airframe components such as the forebody. The clean, undissociated test flows, and large scales is most fundamental aspect of this area is whether the required. Tests with vitiated air must consider the propulsion system is mounted as a distinct module or effects of vitiation on inlet aerodynamics and it is highly integrated with the airframe. Figures 4 reaction kinetics in the combustor and thrust nozzle. and 5 capture the basic elements of a hypersonic air- Despite these limitations, current programs have breathing propulsion system and emphasize that the advanced the state of scramjet feasibility to the point propulsion system is an integral part of the vehicle, where a Mach 4-7 scramjet engine can be realized not just an add-on package. with acceptable risk. The Mach 4-7 range, the area of primary interest, Various combined-cycle engines have been studied as offers significant speed advantages over supersonic potential candidates for use in the Mach 4-7 regime speeds with only a modest increase in technology (Ref.5 ). These systems are based on hydrocarbon requirements. Relevant technologies are relatively fuels or a combination of hydrocarbon and hydrogen conventional and lower cost compared to technology fuel. The dual-mode turboramjet appears to be the requirements for higher speeds. Table 2 summarizes engine of choice for hypersonic vehicles in the flight some of the technologies applicable to the Mach 4-7 regime. Fuel heat sink capacity and ramburner range of interest. It is intended to develop the temperature issues indicate that a switch to the waverider using these technologies and tools. scramjet mode at around Mach 6 is desirable. The storability of hydrocarbon fuels, the advances being IV.1 AIR-BREATHING PROPULSION made with endothermics and the existing logistics The potential performance of conventional engine infrastructure make these fuels essentially the only cycles using hydrogen fuel is shown in Figure 6. choice up to about Mach 6. The volumetric The corresponding performance for hydrocarbon inefficiency of hydrogen makes it an unlikely choice fuels is also shown. As flight speed increases, the for hypersonic cruise aircraft in the Mach 4-6 range. turbo-accelerator engine class is supplanted firstly by However, hydrogen is still an excellent candidate for the subsonic combustion ramjet and secondly by the Mach 6-8 scramjets because of the heat sink capacity supersonic combustion ramjet. Consequently, for a and the very high values of heat of combustion. hypersonic flight vehicle operating at a maximum The mechanical packaging of the turbojet/fan and speed above Mach 7, a multi-mode propulsion system ramjet engines essentially takes on one of three is required. Given the limitations of materials, such forms. These configurations are referred to as "the an engine system might operate as a turboaccelerator over-and-under, the wrap-around, and the tandem" to speeds of the order of Mach 4, then transition to depending upon the method of integration of the core subsonic ramjet operation to about Mach 6, and then turbomachinery and the ramjet flowpaths (Refs. operate as a supersonic combustion engine for speeds 6,16a). Each has advantages and disadvantages. The to and above Mach 7. Figure 6 offers a simple com- actual selection of a particular arrangement depends parison of various engine cycles based on fuel on a number of important design issues, such as specific impulse (Isp) and is therefore appropriate to propulsion-airframe integration (in particular the assess cruise flight performance where the Breguet degree to which the integration should be taken). equation is used to compare ranges. However, as Trade studies are currently being pursued within noted in Ref. 5, for acceleration missions the programs by the Air Force and NASA Lewis. performance of an engine must be compared on the Candidate existing turbojet engines have been basis of both its specific impulse and its corre- identified for the Mach 4-7 application. The sponding thrust-to-weight ratio. turbojet's weight becomes a liability during operation
6 of the ramjet; hence, future investments should be all vehicle components, influences the choice and made to develop a more advanced light-weight selection of materials, affects combustor mixing turbomachine. efficiency, controls the heat loads a vehicle must sustain, and may determine overall mission success Cooling a Mach 4-7 hypersonic aircraft is a key or failure. The calculation and prediction of this technical challenge. Analyses and development of condition at hypersonic speeds is currently based on fuel/thermal management systems for such empirical correlations that use existing data and the hypersonic vehicles has been underway for some wide safety margins required force excessive vehicle time. Excellent research is progressing in this area, weight and performance compromises. These with significant systems simulation for representative correlations cannot be generalized in any meaningful trajectories. The recent detailed work by Bergholz way. A sustained effort is necessary to develop a Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 and Hitch (Ref. 16a) and Petley (Ref. 16b) for general design code formulation for boundary layer example, illustrate the integrated nature of thermal transition and high-speed mixing phenomena. This balance for these vehicles. For the waveriders under requires the "quiet tunnels" capabilities, major consideration, the airframe would most likely be efforts in theoretical studies, significant advances in built primarily with hot structures. Since these non- intrusive diagnostic instrumentation, and the vehicles have large wetted surface area thermal man- conduct of several carefully constructed flight agement problem may take on a different dimension research experiments. if extensive areas have to be actively cooled. Research efforts for advanced fuels and heat Methods for the control of forebody boundary layer exchangers are continuing. flows are currently being studied. This includes especially viscous induced crossflow effects. It is IV.2 AERO/AEROTHERMODYNAMICS well-known that hypersonic boundary layers can be thick, and represent a substantial fraction of the Some of the major phenomena associated with shock layer that exists between a hypersonic shock waveriders include: and the generating surface. Because there is no apparent means to bleed off the boundary layer that Airframe/propulsion integration will form on the surface of a hypersonic vehicle, a Aerodynamic heating, especially leading edges hypersonic engine system must be designed to and nose operate with this thick incoming boundary layer. - Viscous interaction effects The control of the boundary layer thickness, and High altitude effects (Knudsen effects) whether the boundary layer is laminar or turbulent, Off-design performance will have a profound impact on the performance of a Stability & Control, etc hypersonic engine, especially under varying flight A database must be developed and methods must be conditions. Another boundary layer issue is limiting established for the full integration of air-breathing the boundary layer displacement thickness. This is propulsion systems into super/hypersonic vehicles. important not only for providing as uniform an inlet Current programs are conducting flowfield as possible to the engine, but also to avoid aerothermodynamics research in the definition of the undesirable coupling between vehicle attitude and forebody flowfield and boundary layer development, engine performance. Because the hypersonic three-dimensional inlet design and inlet location boundary layer thickness scales in inverse proportion and angle-of-attack effects, nozzle exhaust flowfield, to the surface angle of obliquity, changes in angle of afterbuming, thrust vector changes, and non- attack can translate into unity-order changes in axisymmetric nozzle effects on the vehicle afterbody. ingested mass flow, and therefore thrust, if the The relationship between airframe/propulsion inte- boundary layer approaching the engine is thick. gration effects and the stability and control and Another improvement to inlet flowfield boundary performance of such vehicles is a primary product of layer stratification can be sought by limiting this research. Wind tunnel simulation techniques, crossflow and three-dimensional boundary layer powered and unpowered cold gas test techniques, effects. One of the problems observed in com- and validated or proven CFD results will be used in putational solutions over proposed hypersonic vehicle developing ground-to-flight scaling procedures. configurations is a flow leakage from the upper Ongoing research in the areas of boundary layer surface down to the lower surface, leading to transition physics and modeling, turbulence physics dramatic thickening of the forebody boundary layer. and modeling, real gas effects and rarefied flows will Elimination of such leakage is a characteristic of the have applications to waverider aircraft. Boundary waverider design. In addition, the generally flat layer transition strongly dominates the design of a undersurfaces of the waverider should further limit scramjet engine inlet, affects the overall drag from undesirable three-dimensional thickening of the
7 forebody boundary layer. Improvements in lack of sensitivity existed. Even when the shock boundary layer displacement thickness and flowfield standoff distance was appreciable this independence stratification should therefore be realized with a persisted. Lead edge bluntness effects on caret waverider configuration, and this should be a wings were considered by Collingbourne and principal area of investigation. The boundary layer Peckham (Ref. 19). Estimates were made of the problem described above is not only significant for simultaneous decrease in lift and increase in drag for propulsion considerations, it is important in given nose bluntness. Their conclusions emphasize determining the pressure distribution over a the importance of designing for the smallest possible waverider and optimizing its configuration. Since a leading edge radii.
laminar boundary layer grows as the freestream Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 Mach number squared, the boundary layer From a practical point of view, the leading edge must thicknesses can be very large on a hypersonic vehicle be able to withstand large aerodynamic and flying at high speeds at high altitudes. This in turn aerothermal loads. Thus the structure must be leads to massive viscous interaction with the external designed to have the desired mechanical strength and inviscid flowfield, which markedly increases the also be able to cope with the aerodynamic heating in pressure over the surface near the leading edge, and such a way that the lead edge geometry is preserved, results in an increase in skin friction and heat the shock remains attached, and the wing structure is transfer. These thick boundary layers also change the protected from excessive heat. Note that the effective geometry of the configuration by adding to waverider is a vehicle for which the integrity of its its thickness. Viscous interaction effects can be geometry determines aerodynamic performance so particularly important on slender vehicles, of which that the use of conventional ablative coatings are waveriders are included. Viscous interaction effects ruled out. These would lead to significant changes in have recently been studied (Ref. 17). shape of the leading edge with subsequent adverse effects on performance. Various passive and active Previously, for missiles and entry vehicles, the struc- methods of cooling sharp leading edges have been ture could attain very high temperatures but, since proposed. Carbon -Carbon composite materials the rate of transfer to the interior is usually slow, appear to be a good choice. A passive cooling payloads and crew quarters never attain high method was proposed by Nonweiler (Ref.20a). In temperatures before the mission is over. However, this method, the wing structure is capped with a waveriders may operate in a severe heating sharp leading edge made of a highly conductive environment for relatively long durations, providing material and the heat is radiated away from both adequate time for interior temperatures to reach sides of the cap. Preliminary estimates and specific design limits. Aerodynamic heating to experiments have confirmed the feasibility of this leading edges, nose region or surface area over a method. A practical design based on this method critical component is also a very important design needs to be developed and evaluated although consideration. Typically, stagnation point heating is previous analysis and experiment have verified its the most critical of this type and is often easier to feasibility. A recent study (Ref.20b) study also calculate than heating to other portions of the concludes that radiative cooling would be sufficient vehicle. For waveriders, critical internal for the present applications. components will most likely be close to a hot vehicle skin at many locations on the vehicle and the critical IV.3 AIRFRAME/PROPULSION heating areas will be configuration dependent. INTEGRATION Leading edge heating protection of waveriders is an Propulsion /Airframe Integration (PA I) may be enabling technology that is currently being defined as that aspect of the design process that addressed. If, at the design condition, the shock addresses external features that directly influence or wave remains attached to the sharp swept leading are influenced by the propulsion system. For edge on the compression surface, then disturbances hypersonic air-breathing vehicles, designing the cannot propagate upstream of this shock. Thus the airframe separately from the engine is neither upper and lower surfaces of the waverider will be desirable nor simple, and the concept of aerodynamic independent of each other and each may be designed efficiency needs a new meaning. In fact, part of the separately. This is a basic property of external surface of the vehicle may contribute solely waveriders.Some blunting of leading edges will, to the creation of lift and bounding volume, while however, be required to avoid excessive local another part affects the airflow through the engine. temperatures. How sharp the leading edge has to be Also, the jet exhaust may be deflected from the to satisfy the above "independence " property has not direction of flight and consequently will participate been established. Previous work by Squire (Ref.18) in the creation of both thrust and lift. The dependent on this subject showed that for delta wings substantial variables in the aircraft design process influenced by
8 A are aerodynamic drag, lift, propulsive thrust, the design of highly integrated hypersonic systems volume, structural weight, and stability and control. are shown in Figure 4. The size and number of the The independent variables that influence the above engines must be sufficient to meet mission require- performance parameters include forebody design, ments. The scramjet inlet must be located within the inlet design, engine module location, nozzle design, forebody compression field to obtain maximum per- and the aircraft external configurational shape. formance. An effective precompression tends to Other issues that must be addressed include controls reduce the physical dimensions of the inlet. effectiveness, forebody-inlet interactions, inlet Reduction in inlet dimensions tends to reduce engine spillage effects, exhaust-plume interactions and weight and cowl drag. If the precompressed flow at boundary layer transition which was discussed the inlet face can also be made uniform, the increased earlier. complexity in inlet design required for efficient Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 operation in widely varying flows can also be The selection of the optimum airplane shape, in par- alleviated. However, the available shock-layer ticular the lifting surfaces and the deflection of the capture area decreases with Mach number so that the reaction jet, reduces to the problem of finding the inlet must capture most of the flow between the body shapes corresponding to minimum fuel-flow rate and and the bow shock across the entire fuselage span. satisfying other necessary requirements in the desired The engine size and the flowfield requirements are flight regime. Propulsion-airframe integration is the not the only considerations necessary for a good dominant consideration. It appears that the state of forebody design. Aerodynamic (including boundary the art in high Mach number vehicle integration is layer transition), structural, and internal volume represented by the 25-year-old supersonic SR-71 requirements are other constraints which must be design. Hence a void exists in the availability of incorporated early in the design process to achieve an hypersonic powered configuration data and also optimum configuration. Scramjet nozzle design, for experimental and computational methods for example, is primarily governed by thrust and evaluating installed vehicle performance. stability requirements. Thus, the location of the scramjet, orientation of the thrust vector, and the A common feature of hypersonic aircraft is the resulting trim penalties must be examined across the necessity to integrate the propulsion system with the entire flight envelope. The strong interaction vehicle airframe carefully to obtain optimum overall between the nozzle exhaust and the nonuniform flows performance. As shown in Figure 7 (Ref. 21), the surrounding the vehicle afterbody and external cowl size of the propulsion system relative to aircraft size must also be accounted for in the evaluation of nozzle increases rapidly with increasing flight Mach num- performance. bers, and the forces generated by the propulsion sys- tem become large when compared with aerodynamic This integration and interaction of the propulsion forces. Mutual interactions between these large system, aerodynamics, aerodynamic heating, stability forces are advantageous when the propulsion system & control, materials & structures, fuels, thermal is properly integrated with the vehicle airframe. management, energy availability, missions, Thus, the engine-airframe integration process trajectories, etc. are unique and very important represents a major design challenge to maximize the aspects of hypersonic vehicle design. This extremely performance of hypersonic air-breathing vehicles. interactive nature of air-breathing hypersonic For hypersonic air-breathers, it becomes increasingly vehicles requires a special in-depth multidisciplinary difficult to separate the airframe from the engine. A effort beyond typical "systems studies". It calls for part of the external surface of the airplane may be development of new analytical tools whose validity associated only with the creation of lift force and and/or accuracy are critical. It also requires the bounding volume, while part will be associated with development and application of optimization airflow through the engine. The jet exhaust, in techniques to analyze large interacting systems with general, may be deflected from the direction of extreme uncertainties and disturbances. Work along flight, and consequently will participate in the these lines is being conducted in on-going creation of both thrust and lift. programs.Trajectory optimization is an example of an integration issue. It requires a trade-off between The highly-integrated aircraft concepts depicted in dynamic pressure requirements for propulsion and Figure 4 are attractive because they provide both heating and pressure loads. Also, for vehicles maximum inlet capture area and maximum nozzle designed specifically for cruise, the acceleration and expansion area while maintaining minimum cowl deceleration phases of the cruise mission can be drag. However, the full benefits of these concepts can significant relative to the cruise phase. Optimization only be realized when the vehicle propulsion system of the vehicle design must hence be based on total is properly integrated early in the design process. mission considerations rather than on cruising flight Interactive constraints which must be considered in alone. Cross-discipline interactions, mostly
9 nonlinear, will generally have to be included in the values. Neither model nor facility sizes are serious optimization scheme. More will be said about this limitations in this speed regime. The degree of later, as they pertain to Mach 4-7 waveriders. exhaust simulation required to address this problem is unknown at this time. The ability to control or Forebody performance is one of the big unknowns in alleviate the adverse effects on a given configuration the aerodynamics of the vehicle classes under consid- is nonexistent. Numerical techniques have not been eration. Typical shapes produce complex and successfully applied to build an empirical approach. difficult to analyze flows whereas waveriders offer Therefore, a reliable numerical approach coupled more predictable flows. As indicated earlier, most with a substantial parametric experimental program effective integrated design concepts use the forebody could be of tremendous value in eliminating this par- as part of the inlet compression surface. Although Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 ticular performance limiter. The influence of large the geometric shape of the forebody may appear overexpanded nozzles again becomes a dominant relatively simple, it will have a significant effect on integration parameter at transonic speeds. The vehicle aerodynamics, aerothermodynamic, and nozzles tend to quickly separate downstream of the propulsive performance. The forebody must provide combustor exit, creating a huge increase in base drag the desired mass flow to the inlet as well as minimize and potentially drastic changes in pitching moment inlet flow distortion. Thus forebody shaping is a which compound the problem. It does not appear critical study area, as is the sensitivity of forebody feasible to eliminate this effect with external nozzle performance to geometric changes. The problem is contouring alone. further complicated by the fact that the flowfields of practical interest for hypersonic aircraft design are Installed performance is defined here as the predominantly three-dimensional, but relatively few integrated forces on a configuration with operating simple reference configurations and flowfields are propulsion system. It is a complete vehicle value available as either starting points or for purposes of rather than the traditional propulsion estimate. An comparison. However, the waverider class of vehi- accurate quantitative estimate of this parameter cles provides the link between supersonic aircraft and (including off-design sensitivities across the speed spacecraft and may be used to make systematic range) is critical to the success of any air-breathing studies of integrated designs. Propulsion-Airframe hypersonic vehicle. The maximum sensitivity integration studies on waveriders are being and/or uncertainty in performance for conducted by several research institutions and in hypersonic vehicles occurs at three sets of industry. At the University of Maryland, a novel conditions, namely: (1) takeoff (and landing forward- marching approach is used to develop as well, if powered), (2) transonic ascent, inlets and nozzle as an integral part of the waverider and (3) hypersonic powered flight beyond generation process. In other words the engine and Mach 5. nozzle are not just add-ons after the fact. An example of the type of results obtainable with the Maryland IV.4 S UC U ES MA E IA S code is shown in Fig. 8. At NASA Ames a research S UC U A Y AMICS program has been initiated to investigate firstly, the aerothermal field, and secondly, the optimization of Hypersonic vehicles demand unprecedented struc- integrated waveriders (Ref. 23 ). Some preliminary tural mass fractions under the most severe results of temperature distributions and optimization thermal and acoustic environments to meet all of the are shown in Figs. 9 and 10. currently envisioned goals. To design such structures, advanced materials with high specific Significant adverse ground effects occur during the strength and stiffness, high temperature takeoff maneuver for propulsion systems mounted on capability, resistance to thermal shock and the bottom of the fuselage (Ref. 24 ). Current thermal cycling, compatibility with hydrogen waverider designs may have this feature if the level and LOX, good thermal conductivity, of integration is pushed to the limit. With power off ductility, and resistance to oxidation must be the usual increase in lift is observed as the ground developed. They must be readily fabricated into plane is approached. With power on, however, the complex shapes, and have good fatigue/fracture highly overexpanded nozzles create a venturi effect characteristics and known and predictable failure resulting in large losses in lift and changes in mechanisms. The loads used to design such pitching moment. At high thrust levels, a strong structures must be known with exacting precision so suckdown effect could adversely affect take-off every bit of structural weight saving can be extracted performance. The magnitude of this effect can be for the airframe and propulsion systems. Advanced adequately assessed experimentally for a given instrumentation must be developed to measure tem- configuration; however, the experimental approach perature, high frequency pressures, heat fluxes and must include operating inlets to generate realistic strains at extremely high temperatures. Advanced
10 structural concepts that can be hot, insulated or design and for assessment and validation of CFD cooled depending on the structural application must codes. In addition, validated codes for the structural be designed, fabricated and tested to show viability response to such loads are required as well as test for actual applications and to demonstrate all other apparatus and techniques to validate proposed struc- disciplines required in the analysis, fabrication, and tural concepts. No data exists for fluctuating pressure testing of such structures. Materials issues and loads associated with combustion, plume dynamics or availability for the SSTO class of vehicles have been plume-boundary interaction at high Mach numbers. discussed in Ref. 25. Validated prediction methods for high frequency boundary layer loads, validated prediction methods Advanced high-temperature composites are for high frequency boundary layer loads, and vali- required for light-weight hot structures for dated integrated-flow-thermal-structural analysis Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 hypersonic vehicles. Thin-gage, light-alloy metal- capability are also needed. matrix composites offer significant potential for reducing the weight of airframe and engine Mach 4-7 cruise aircraft will be exposed to components which will operate at 600-1500 °F on maximum temperatures of 1000°F to 1400°F for hypersonic vehicles. Research is required to develop long durations at a time. Higher temperatures will these materials with high specific properties in thin occur in localized regions such as wing and cowl gages; to develop processing and joining technology leading edges due to shock interaction phenomena. to enable fabrication of complex hot structural Hot structures concepts are adequate and will be used components; and to develop life prediction because they are relatively simple compared to methodology to assure safety of flight. Similarly, actively and passively cooled structures. Although carbon-carbon and ceramic-matrix composites are experience with hot airframe structures is limited, required for structural components which will their use leads to lighter vehicles. The SR-71/YF-12 operate at 1500-3000 °F. Higher strengths in thin (to about 600°F) and the X-15 (to around 1200°F) gages and improved damage tolerance are key tech- are the only reusable vehicles that have flown with nology needs for these materials. Extensive testing hot structures (Ref.26 ). Carbon-carbon, ceramic and and analysis are required to establish the durability ceramic-matrix structures are slowly emerging, con- of materials and hot structures under the hostile strained by material properties and fabrication service environment conditions expected for • techniques. Structural concepts and materials hypersonic flight vehicles. selection are mission dependent. As indicated in Table 2, the Mach 4-7 range shows a heavy reliance Large regions of the airframe of advanced on advanced titanium alloys which are commercially hypersonic vehicles will reach very high tem- available to 1100°F, and and also the promising peratures as a result of aerodynamic heating. titanium-aluminide intermetallics which is a subject Depending on the particular vehicle, mission, and of much current development work. These are location on the vehicle, these temperatures can lower in density than the titanium alloys but possess exceed 3000°F. For temperatures below this level, a higher use temperature range from 1500°F to carbon-carbon composites are candidate materials 1800°F. because of their desirable combination of low weight and high-temperature strength and modulus. They For hypersonic airplanes, structural dynamic and are attractive even at lower temperatures because of aeroelastic areas of concern include vibration charac- their engineering properties. At temperatures above teristics (with interactions between structure, control about 1400°F even the most advanced engineering system, and fuel), propellant dynamics (fuel slosh- materials begin to decrease in specific strength and ing), lifting surface and panel flutter, control surface modulus relative to carbon-carbon composites. buzz, high intensity noise generated by engine and external aerodynamics, and so on. Details of some of Accurate loads are required if optimum weight struc- these issues are outlined in Ref 27. The high speeds, tures are to be obtained. For hypersonic vehicles elevated temperatures, and unconventional acoustic loads from the boundary layer as well as geometries and structural designs that are combustion and plume dynamics are design drivers, characteristic of hypersonic vehicles introduce and little is known about such loads for hypersonic- considerable complexity compared with conventional speeds and scramjet propulsion systems. Local aircraft. Figure 11 illustrates the effect of aerothermal loads are known to be design drivers aerodynamic heating on flutter characteristics of a from experience with such high speed vehicles as the generic hypersonic vehicle. These calculations were YF-12, X-15 and shuttle. A data base for generic based on the finite-element method and show that the detailed areas such as cowl lips, wing-cove junctions, frequencies were reduced by changes in material and deflected ramps/control surfaces is needed to properties (decrease in stiffness), but were increased provide the structural analyst a starting point for his by thermal stress effects (increase in stiffness). Thus
11 natural vibration characteristics are complicated by being conducted for the NASP program (Refs. 28 the fact that, for hypersonic configurations, the and 29 ). This work appears directly applicable to effects of aerodynamic heating on structural stiffness the Mach 4-7 waverider. A flight test simulator for has to be considered. Furthermore, a hypersonic cruise aircraft in this range would be extremely airplane may have a relatively high ratio of gross useful. Current research areas for flight systems are: weight to empty weight because of the high fuel (1) multidisciplinary modeling/controls fraction. Thus the vibration characteristics may integration, (2) guidance, navigation, and change considerably as fuel is consumed, leading to trajectory optimization and (3) pilot-vehicle complications in the design of the flight control interfaces. Multidisciplinary modeling of system. The possibility of very low structural weight hypersonic waveriders combined with advanced fraction also implies significant flexibility in the control theory will increase vehicle performance and Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1992/78941/V002T02A040/2401661/v002t02a040-92-gt-437.pdf by guest on 27 September 2021 vehicle structure. Elastic deformations of the reduce the likelihood of redesign after flight test. structure produced by aerodynamic loading will Disciplines that are closely linked and require control affect the loading and thus change the aerodynamic system solutions include: structural dynamics, stability derivatives of the airplane. These aeroelasticity, flight mechanics, flying qualities, considerations are complicated by high temperature propulsion, and thermal balance. Design and thermal gradient effects on the structural methodologies to simultaneously optimize per- stiffness. The design of the flight control system will formance of multiple, interacting control systems be impacted. The influence of fuselage flexibility is need development. Historically, piloted illustrated in the vehicle vibrational mode shapes aerospacecraft have had either rapidly-traversed shown in Figure 12 for a generic hypersonic shape. hypersonic regimes or have been automatically These calculations were performed for both carbon- controlled. Hence, appropriate design criteria for carbon and titanium-aluminide structures (Ref 27). flying qualities do not exist. The desired information Results for titanium-aluminide are presented in the and/or display formats that would enable a pilot to figure. Frequencies ranged from 3 to 9 Hertz.The effectively fly in hypersonic regimes have not been dominant flutter mechanism found was a coalescence determined. Since direct view windows are of modes 2 and 3, the fundamental wing-bending especially a liability for hypersonic vehicles in cost, mode and a highly-coupled wing-bending/second weight and effectiveness, alternative viewing schemes fuselage-bending mode, respectively. The visual must be evaluated. Manned cockpit aids such as appearance and overall character of these modes did fault-diagnosing tools and replanners for not change with material and temperature. However, accommodating failures and major disturbances are significant differences occur in both frequencies and desired. amplitudes between hot and cold. The first and third elastic modes exhibit significant fuselage motion. A unique and very strong aspect of hypersonic The interaction of these mode shapes (especially the vehicle design is the integration and interaction of the forebody) with the flow into the intakes, and hence propulsion system, aerodynamics, aerodynamic overall propulsion system is only very recently being heating, stability and control, and materials and addressed. structures. The technical problems faced by the designer are multidisciplinary to first - order, so that IV.5 FLIGHT SYSTEMS their proper resolution requires the ability to integrate highly-coupled and interacting elements in a Hypersonic vehicles will require unprecedented pre- fundamental and optimal fashion to achieve the cision to maintain nominal trajectories and local desired performance. The degree of coupling flight conditions so that maximum performance can intensifies with increasing Mach number. This is the be achieved from the aero/propulsive system. real problem with hypersonic vehicle design. Mission objectives will be threatened by atmospheric Obviously for some applications it will be necessary disturbances, modeling uncertainties, weak coupling to have satisfactory low speed, transonic, supersonic between disciplines and non-existent, off-nominal speed, and hypersonic cruise Mach number performance margins. Onboard flight systems will performance simultaneously built into the design. be required to solve these issues while providing the Fig. 13 gives a summary of some of the more flexibility and safety for manned operations. The important cross-discipline interactions as well as a development of manned aerospacecraft will provide a description of the multidisciplinary optimization whole new philosophy for flight qualifying and test- issues. The optimization process is generally assumed ing research vehicles. Flight system technologies to be numerical and methods must be available for will require significant research breakthroughs in analyzing forebody/inlet interactions, complete inter- understanding, approaches, algorithms, and nal flows (inlet, combustor, and nozzle), and equipment to enable the safe and accurate flight of nozzle/aftbody interactions, etc. The validity and/or aerospace vehicles. Current work in these areas is accuracy of analytical methods is the most important
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[20b] Vanmol, D. 0., Anderson, J.D. Jr.," Heat 8
Transfer Characteristics of Hypersonic Waveriders 6 with Emphasis on Leading Edge Effects," NASA CR- 189586. [21a] Edwards, C.L.W., "A Forebody Design Technique for Highly-Integrated Bottom-Mounted C v l Scramjets with Application to a Hypersonic Research tr n p rt M l t r r r f r n tr Airplane," NASA TND - 8369, December 1976. r r ft [21b] Small,W.J., Weidner, J.P., and Johnston, P.J., = 0 "Scramjet Nozzle Design and Analysis as Applied to M=20 a Highly Integrated Hypersonic Research Airplane" . NASA TND - 8334, 1976 . r ln . r Gl b l Atl nt f r n uaG igu e . ock ime o o g is a ce a e
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