Hypersonics and Hypersonic Airbreathing Technologies

1. Hypersonics and Hypersonic Airbreathing Technologies Review, Status, and Outlook January 20, 2006 Marcus Lobbia Marcus Shaw

2. 2 Outline Introduction Challenges and Technologies Studies and Recent Programs Required Advances and Resources Outlook, Summary, and Conclusions

3. 3 Hypersonic Applications � General Most hypersonic vehicle concepts fall into a specific band when plotted on a velocity-altitude plot. This is due to the fact that hypersonic speeds are typically only possible at lower densities (i.e. higher altitudes). For a fixed Mach number, the lower the altitude, the higher the temperatures and forces that will be encountered. The figure above shows several examples of potential hypersonic airbreathing vehicles. At the lower Mach numbers, potential applications include cruise missiles, global reach weapons, and transport aircraft. At high Mach numbers, accelerator missions (e.g. Two-Stage-To-Orbit vehicles) are commonly proposed, as these vehicles can use airbreathing propulsion to attain a significant fraction of orbital velocity.Most hypersonic vehicle concepts fall into a specific band when plotted on a velocity-altitude plot. This is due to the fact that hypersonic speeds are typically only possible at lower densities (i.e. higher altitudes). For a fixed Mach number, the lower the altitude, the higher the temperatures and forces that will be encountered. The figure above shows several examples of potential hypersonic airbreathing vehicles. At the lower Mach numbers, potential applications include cruise missiles, global reach weapons, and transport aircraft. At high Mach numbers, accelerator missions (e.g. Two-Stage-To-Orbit vehicles) are commonly proposed, as these vehicles can use airbreathing propulsion to attain a significant fraction of orbital velocity.

4. 4 In Hindsight� What went right X-15: hypersonic flight testing Space Shuttle: reusable reentry technologies X-43: scramjet operation What went wrong X-30, X-33, etc. met program cancellation Goals were too ambitious Required technological advances not achieved Decline in public/government interest Many of the hypersonic vehicle programs in the past of made a number of promises on how they would revolutionize some aspect of civil or military utility. In many cases, substantial technological progress was made, as demonstrated by the X-15, Space Shuttle, and X-43. On the other hand, many programs ended in failure after promising too much. In these cases, required technological advances were not achieved (e.g. composite fuel tank for X-33), and/or public/government interest declined substantially over the life of the program (e.g. X-34). These all leads to the primary point that development of hypersonic vehicles and systems is not a trivial task � underestimating the challenges can often lead to program failure or cancellation.Many of the hypersonic vehicle programs in the past of made a number of promises on how they would revolutionize some aspect of civil or military utility. In many cases, substantial technological progress was made, as demonstrated by the X-15, Space Shuttle, and X-43. On the other hand, many programs ended in failure after promising too much. In these cases, required technological advances were not achieved (e.g. composite fuel tank for X-33), and/or public/government interest declined substantially over the life of the program (e.g. X-34). These all leads to the primary point that development of hypersonic vehicles and systems is not a trivial task � underestimating the challenges can often lead to program failure or cancellation.

5. 5 Foreign Interest in Hypersonics Images: Japan � Illustration of the HOPE-X reusable spacecraft concept developed by JAXA (Japan). Currently the project is mothballed, but substantial hypersonic R&D (e.g. Hyflex experiment, etc.) was performed. Russia � Proposed hypersonic vehicle concept. France � A variety of hypersonic propulsion R&D projects have been performed in France (e.g. ONERA Scorpion hypersonic cruise missile development) Germany � Sanger II - proposed two stage to orbit vehicle. Air-breathing hypersonic first stage and delta wing second stage. The German Hypersonics Program and its Saenger II reference vehicle received most of the domestic funding for spaceplane development in the late 1980s and early 1990s. Advanced airbreathing space-access technology is being investigated in Russia, Japan, and France and by the European Space Agency. The Oryol hypersonic flight-test program, managed by the Russian Space Agency, focuses on the investigation of hypersonic airbreathing propulsion systems. Two conceptual designs are in work: the TSTO MiG design (MIGAKS) and the Tupolev Tu-2000, an SSTO concept. Both concepts employ horizontal takeoff and landing. To aid in the development of these concepts, Russia can rely on its unparalleled hypersonic groundtest infrastructure for supporting aerodynamic and propulsion development. Russia has also conducted four captive-carry flight tests of a hydrogen-fueled dual-mode scramjet in the Mach 3.5 to 6.5 range using the �Kholod� hypersonic flying laboratory. A second-generation flying testbed, termed IGLA, will expand the tested speed regime to Mach 12 to 14 for investigation of the hypersonic aerodynamic and propulsion environment. In addition to the hypersonic airbreathing engines, Russia has invested heavily in low-speed engines, such as the air-turbo ramjet, which are needed for space-access missions. 37 A major study of European reusable launch systems has been under way since 1993 under the Future European Space Transportation Investigation Program. The main consensus from the program is that the European RLV will not be an SSTO vehicle. France has teamed with Russia to investigate a Wide-Range Ramjet engine concept that operates between Mach 3 and 12 using a variable-geometry engine. This program aims at providing a ground-test engine to demonstrate the potential engine performance of an access-to-space vehicle. France and Germany have teamed on the Joint Airbreathing Propulsion for Hypersonic Application Research program, which aims at advancing dual-mode scramjet technology with the ultimate goal of flight-testing a vehicle between Mach 4 and 8. In Japan, a long-range program aimed at space-access technologies has been in place for the past two decades and continues today with the stated goal of developing a reusable SSTO vehicle with an airbreathing/rocket combined propulsion system. Technology development work includes activity associated with advanced turbo-engines (the Hyper program), combined-cycle engines (ATREX), and dual-mode scramjets. Japan has recently built several large ground-based facilities for investigation of scramjet engine operation at speeds of Mach 3 to 14. In weapon development, hypersonic research and technology is concentrated on hypersonic cruise missile (rather than aircraft) applications. Russia is the world leader in deploying operational ramjet-powered weapon systems, including the SA-4, SA-6, SN-22, and AS-17. Advanced technology development programs are under way to extend the operating range of ramjet-powered missiles and dual-mode scramjets to Mach 8. In France, the ramjet-powered missile ASMP is operational in a strategic air-to-ground role. Aerospatiale Matra is also competing for the BVR missile for the Eurofighter with a ramjetpowered air-to-air system. The Promethee missile, which is entering its second phase of development, is a hypersonic air-to-ground system with a cruise speed of Mach 8 and a launch weight of 3,750 lb. Both Aerospatiale and ONERA (Palaiseau) have extensive ramjet-scramjet test facilities staffed with experienced teams. India and China both possess operational ramjet-powered missiles. Although the two countries are recent entries to the hypersonics field, they are actively exploring scramjet-powered vehicles. India is believed to be developing a high-speed flight vehicle, which will be tested shortly. The last area to be considered concerns basic research and technology development, which offers the potential for radical improvement in hypersonic system design and performance. The majority of work in this area is being conducted in Russia. Technologies such as plasma aerodynamics and MHD control of flowfields, plasma-assisted combustion, onboard MHD power generation, and plasma-cloaking technologies are all under investigation. Work is under way at the Central Aerohydrodynamic Institute (TsAGI), Central Institute of Aviation Motors (CIAM), and several institutes of the Russian Academy of Science (Ioffe Physico-Technical Institute, High-Temperature Institute, and Moscow Radio Technical Institute) and Universities (Moscow State University and St. Petersburg State University). Although these technologies are relatively immature, they offer the potential to provide revolutionary improvements to vehicle performance in the hypersonic domain. Images: Japan � Illustration of the HOPE-X reusable spacecraft concept developed by JAXA (Japan). Currently the project is mothballed, but substantial hypersonic R&D (e.g. Hyflex experiment, etc.) was performed. Russia � Proposed hypersonic vehicle concept. France � A variety of hypersonic propulsion R&D projects have been performed in France (e.g. ONERA Scorpion hypersonic cruise missile development) Germany � Sanger II - proposed two stage to orbit vehicle. Air-breathing hypersonic first stage and delta wing second stage. The German Hypersonics Program and its Saenger II reference vehicle received most of the domestic funding for spaceplane development in the late 1980s and early 1990s. Advanced airbreathing space-access technology is being investigated in Russia, Japan, and France and by the European Space Agency. The Oryol hypersonic flight-test program, managed by the Russian Space Agency, focuses on the investigation of hypersonic airbreathing propulsion systems. Two conceptual designs are in work: the TSTO MiG design (MIGAKS) and the Tupolev Tu-2000, an SSTO concept. Both concepts employ horizontal takeoff and landing. To aid in the development of these concepts, Russia can rely on its unparalleled hypersonic groundtest infrastructure for supporting aerodynamic and propulsion development. Russia has also conducted four captive-carry flight tests of a hydrogen-fueled dual-mode scramjet in the Mach 3.5 to 6.5 range using the �Kholod� hypersonic flying laboratory. A second-generation flying testbed, termed IGLA, will expand the tested speed regime to Mach 12 to 14 for investigation of the hypersonic aerodynamic and propulsion environment. In addition to the hypersonic airbreathing engines, Russia has invested heavily in low-speed engines, such as the air-turbo ramjet, which are needed for space-access missions. 37 A major study of European reusable launch systems has been under way since 1993 under the Future European Space Transportation Investigation Program. The main consensus from the program is that the European RLV will not be an SSTO vehicle. France has teamed with Russia to investigate a Wide-Range Ramjet engine concept that operates between Mach 3 and 12 using a variable-geometry engine. This program aims at providing a ground-test engine to demonstrate the potential engine performance of an access-to-space vehicle. France and Germany have teamed on the Joint Airbreathing Propulsion for Hypersonic Application Research program, which aims at advancing dual-mode scramjet technology with the ultimate goal of flight-testing a vehicle between Mach 4 and 8. In Japan, a long-range program aimed at space-access technologies has been in place for the past two decades and continues today with the stated goal of developing a reusable SSTO vehicle with an airbreathing/rocket combined propulsion system. Technology development work includes activity associated with advanced turbo-engines (the Hyper program), combined-cycle engines (ATREX), and dual-mode scramjets. Japan has recently built several large ground-based facilities for investigation of scramjet engine operation at speeds of Mach 3 to 14. In weapon development, hypersonic research and technology is concentrated on hypersonic cruise missile (rather than aircraft) applications. Russia is the world leader in deploying operational ramjet-powered weapon systems, including the SA-4, SA-6, SN-22, and AS-17. Advanced technology development programs are under way to extend the operating range of ramjet-powered missiles and dual-mode scramjets to Mach 8. In France, the ramjet-powered missile ASMP is operational in a strategic air-to-ground role. Aerospatiale Matra is also competing for the BVR missile for the Eurofighter with a ramjetpowered air-to-air system. The Promethee missile, which is entering its second phase of development, is a hypersonic air-to-ground system with a cruise speed of Mach 8 and a launch weight of 3,750 lb. Both Aerospatiale and ONERA (Palaiseau) have extensive ramjet-scramjet test facilities staffed with experienced teams. India and China both possess operational ramjet-powered missiles. Although the two countries are recent entries to the hypersonics field, they are actively exploring scramjet-powered vehicles. India is believed to be developing a high-speed flight vehicle, which will be tested shortly. The last area to be considered concerns basic research and technology development, which offers the potential for radical improvement in hypersonic system design and performance. The majority of work in this area is being conducted in Russia. Technologies such as plasma aerodynamics and MHD control of flowfields, plasma-assisted combustion, onboard MHD power generation, and plasma-cloaking technologies are all under investigation. Work is under way at the Central Aerohydrodynamic Institute (TsAGI), Central Institute of Aviation Motors (CIAM), and several institutes of the Russian Academy of Science (Ioffe Physico-Technical Institute, High-Temperature Institute, and Moscow Radio Technical Institute) and Universities (Moscow State University and St. Petersburg State University). Although these technologies are relatively immature, they offer the potential to provide revolutionary improvements to vehicle performance in the hypersonic domain.

6. Challenges and Technologies

7. 7 HypersonicChallenges Many challenges High temperatures Propulsion Increased drag Vehicle design andtesting When designing hypersonic vehicle systems, many challenges are present that make the task unique compared to systems designed to operate in other regimes. Some of these challenges include: Very high velocities Shock waves, expansion waves Large acceleration/deceleration to and from cruise velocity High Reynolds numbers Strong viscous interactions Typically turbulent boundary layers Increased skin friction drag High stagnation temperature Leads to high surface temperatures Propulsion system requirements The 4 plots in the figure to the upper-right indicate how things get challenging with increasing Mach number for the areas of propulsion, materials, aerodynamics, and fuels. The lower-right figure shows how (in the ideal case) stagnation temperature increases to very large numbers as the Mach number increases. For vehicles designed to operate at hypersonic speeds, these high temperatures can lead to very challenging design issues, such as wing leading-edge thermal protection system design (an example of which is shown in the figure to the lower-left).When designing hypersonic vehicle systems, many challenges are present that make the task unique compared to systems designed to operate in other regimes. Some of these challenges include: Very high velocities Shock waves, expansion waves Large acceleration/deceleration to and from cruise velocity High Reynolds numbers Strong viscous interactions Typically turbulent boundary layers Increased skin friction drag High stagnation temperature Leads to high surface temperatures Propulsion system requirements The 4 plots in the figure to the upper-right indicate how things get challenging with increasing Mach number for the areas of propulsion, materials, aerodynamics, and fuels. The lower-right figure shows how (in the ideal case) stagnation temperature increases to very large numbers as the Mach number increases. For vehicles designed to operate at hypersonic speeds, these high temperatures can lead to very challenging design issues, such as wing leading-edge thermal protection system design (an example of which is shown in the figure to the lower-left).

8. 8 Turbojets, Ramjets, Scramjets Turbojet Three different types of high-speed propulsion systems are shown here: turbojet, ramjet, and scramjet. The turbojet is similar to a jumbo-jet turbofan, in that it uses compressors to slow the air (and increase its pressure) before mixing in fuel and burning the mixture. The high-temperature/high-pressure combustion products are then sent through a turbine, which powers the compressor (and can also provide electricity for other vehicle systems). Optional use of an afterburning can further increase the energy of the flow, which is then exhausted through the nozzle to produce thrust. Turbojets, although less efficient than turbofans, have the advantage that they can operate from static conditions on the ground to supersonic Mach numbers (and potentially higher if pre-cooling of the inlet air is performed). A ramjet is a simplified engine system, in that supersonic/hypersonic air enters the inlet/diffuser, and is slowed to subsonic speeds via several shock waves. The subsonic air is then mixed with fuel, and ignited by flame holders present in the combustor. The burned mixture then flows through the nozzle to produce thrust. Ramjets are less-complicated than turbojets, and can typically operate more efficiently at high supersonic Mach numbers. Unfortunately, they cannot operate at static conditions, and therefore typically require the vehicle to be accelerated to Mach 0.5 or higher via another propulsion system. Ramjets are often combined with a turbojet system (i.e. a turbo-ramjet), where the turbojet portion of the engine is closed off as the Mach number increases (e.g. the SR-71). When the vehicle velocity increases into the hypersonic regime, slowing down the flow to subsonic velocities can cause air temperatures >2500 K, which leads to dissociation of the chemical species and substantial reductions in combustion efficiency, not to mention creating extreme engine cooling challenges. Therefore, at Mach numbers >5, scramjets are commonly considered as an approach to inject and burn the fuel at supersonic speeds. A variety of research is currently ongoing in scramjet technologies (e.g. normal vs. transverse fuel injection, fuel types, etc.), and the propulsion system has been successfully validated to a limited extent (e.g. X-43A). A hypersonic air-breathing propulsion system usually implies a scramjet, although it should be noted that other types of propulsion (e.g. turbojet, rocket) will be needed to accelerate the vehicle to hypersonic velocities.Three different types of high-speed propulsion systems are shown here: turbojet, ramjet, and scramjet. The turbojet is similar to a jumbo-jet turbofan, in that it uses compressors to slow the air (and increase its pressure) before mixing in fuel and burning the mixture. The high-temperature/high-pressure combustion products are then sent through a turbine, which powers the compressor (and can also provide electricity for other vehicle systems). Optional use of an afterburning can further increase the energy of the flow, which is then exhausted through the nozzle to produce thrust. Turbojets, although less efficient than turbofans, have the advantage that they can operate from static conditions on the ground to supersonic Mach numbers (and potentially higher if pre-cooling of the inlet air is performed). A ramjet is a simplified engine system, in that supersonic/hypersonic air enters the inlet/diffuser, and is slowed to subsonic speeds via several shock waves. The subsonic air is then mixed with fuel, and ignited by flame holders present in the combustor. The burned mixture then flows through the nozzle to produce thrust. Ramjets are less-complicated than turbojets, and can typically operate more efficiently at high supersonic Mach numbers. Unfortunately, they cannot operate at static conditions, and therefore typically require the vehicle to be accelerated to Mach 0.5 or higher via another propulsion system. Ramjets are often combined with a turbojet system (i.e. a turbo-ramjet), where the turbojet portion of the engine is closed off as the Mach number increases (e.g. the SR-71). When the vehicle velocity increases into the hypersonic regime, slowing down the flow to subsonic velocities can cause air temperatures >2500 K, which leads to dissociation of the chemical species and substantial reductions in combustion efficiency, not to mention creating extreme engine cooling challenges. Therefore, at Mach numbers >5, scramjets are commonly considered as an approach to inject and burn the fuel at supersonic speeds. A variety of research is currently ongoing in scramjet technologies (e.g. normal vs. transverse fuel injection, fuel types, etc.), and the propulsion system has been successfully validated to a limited extent (e.g. X-43A). A hypersonic air-breathing propulsion system usually implies a scramjet, although it should be noted that other types of propulsion (e.g. turbojet, rocket) will be needed to accelerate the vehicle to hypersonic velocities.

9. 9 Hypersonic Propulsion Specific Impulse (Isp) is a measure of engine fuel efficiency The specific impulse (Isp) is a measure of propulsion efficiency, in that a higher Isp translates into less fuel required. As can be seen on this figure, rockets demonstrate a constant Isp independent of Mach number; unfortunately, this Isp is relatively low, due to the need to carry both fuel and oxidizer. The other propulsion systems shown on this figure are air-breathing systems � in other words, only fuel must be carried by the vehicle, as ambient air is used as the oxidizer. This leads to higher Isp potentials, as can be seen for turbojets in the low Mach number regime. As the Mach number increases, however, different types of air-breathing propulsion systems must be used, and the Isp tends to increase with Mach number. Finally, different fuel types can also affect Isp, where Hydrogen systems demonstrate a higher Isp (and may be required for high-Mach number systems). For the low hypersonic regime (Mach < 7), ramjets and hydrocarbon or cryogenic (H2) fuel can be used. For high hypersonic speeds (Mach > 7), the use of scramjets and rockets (along with cryogenic fuel) is most likely required. The specific impulse (Isp) is a measure of propulsion efficiency, in that a higher Isp translates into less fuel required. As can be seen on this figure, rockets demonstrate a constant Isp independent of Mach number; unfortunately, this Isp is relatively low, due to the need to carry both fuel and oxidizer. The other propulsion systems shown on this figure are air-breathing systems � in other words, only fuel must be carried by the vehicle, as ambient air is used as the oxidizer. This leads to higher Isp potentials, as can be seen for turbojets in the low Mach number regime. As the Mach number increases, however, different types of air-breathing propulsion systems must be used, and the Isp tends to increase with Mach number. Finally, different fuel types can also affect Isp, where Hydrogen systems demonstrate a higher Isp (and may be required for high-Mach number systems). For the low hypersonic regime (Mach < 7), ramjets and hydrocarbon or cryogenic (H2) fuel can be used. For high hypersonic speeds (Mach > 7), the use of scramjets and rockets (along with cryogenic fuel) is most likely required.

10. 10 Hypersonic Vehicle Fuel Choice High temperaturesrequire heat sink Advanced fuelpossibilities Endothermic Hydrocarbon fuels Examples: MCH, JP-7, JP-8 Cryogenic Hydrocarbon/hydrogen fuels Examples: Hydrogen, Methane For hypersonic vehicles, the fuel choice can have a large impact on the overall capabilities of the vehicle. Mach numbers >5 typically involve very high temperatures (as shown by the combustor adiabatic wall temperature in the above figure), which requires some type of active cooling system to be used for engine components and possibility other parts of the vehicle (e.g. wing leading edges). Similar to many rocket nozzle systems, the fuel itself can be used as a heat sink to absorb some of the high heat fluxes present on the vehicle. Two types of advanced fuels can be considered for this purpose: endothermic and cryogenic fuels. Endothermic fuels absorb heat through catalytic dehydrogenation � this is a chemical process in which the fuel is �cracked� into a different form by the heat input before combustion. These types of fuels are hydrocarbon-based, and are usually considered due to their storage characteristics (i.e. they can usually be stored at standard temperatures and pressures). The drawbacks to these types of fuels is that the fuel or its byproducts may be somewhat toxic, and combustion may require additional fuel addtivies. Cryogenic fuels absorb heat through normal heat transfer processes � they are stored at a very low temperature, and energy absorption translates into a higher temperature before combustion. Hydrogen and Methane are examples of cryogenic fuels, and they are desirable from the standpoint of their increased Isp (i.e. higher fuel efficiency). On the other hand, they cryogenic properties mean that the fuel storage system is more complicated (high-pressure insulated or actively-cooled tanks), and these fuels (e.g. Hydrogen) take up substantially more volume due to their low densities.For hypersonic vehicles, the fuel choice can have a large impact on the overall capabilities of the vehicle. Mach numbers >5 typically involve very high temperatures (as shown by the combustor adiabatic wall temperature in the above figure), which requires some type of active cooling system to be used for engine components and possibility other parts of the vehicle (e.g. wing leading edges). Similar to many rocket nozzle systems, the fuel itself can be used as a heat sink to absorb some of the high heat fluxes present on the vehicle. Two types of advanced fuels can be considered for this purpose: endothermic and cryogenic fuels. Endothermic fuels absorb heat through catalytic dehydrogenation � this is a chemical process in which the fuel is �cracked� into a different form by the heat input before combustion. These types of fuels are hydrocarbon-based, and are usually considered due to their storage characteristics (i.e. they can usually be stored at standard temperatures and pressures). The drawbacks to these types of fuels is that the fuel or its byproducts may be somewhat toxic, and combustion may require additional fuel addtivies. Cryogenic fuels absorb heat through normal heat transfer processes � they are stored at a very low temperature, and energy absorption translates into a higher temperature before combustion. Hydrogen and Methane are examples of cryogenic fuels, and they are desirable from the standpoint of their increased Isp (i.e. higher fuel efficiency). On the other hand, they cryogenic properties mean that the fuel storage system is more complicated (high-pressure insulated or actively-cooled tanks), and these fuels (e.g. Hydrogen) take up substantially more volume due to their low densities.

11. 11 Vehicle Design Integration Integrated design process necessary Flowfield/propulsion coupling Thermal considerations Stability and control Payload packaging Off-design performance Hypersonic vehicle design can be challenging due to the extensive coupling of the flowfield and various systems on the vehicle. Integration of the propulsion system requires not only extensive design and analysis of the flowfield entering the engines, but the engine surfaces themselves can substantially impact the lift/drag performance of the vehicle. Hypersonic Mach numbers imply high stagnation temperatures, which must be taken into consideration when designing the vehicle structure and thermal protection system. Large aerodynamic forces at these high velocities can cause substantial pitching vehicle pitching moments, which must be analyzed along with the vehicle center of gravity to come up with a controllable design. Integration of the payload (both volume and mass) can affect many of these same areas (e.g. stability, drag increase, etc.), and is an important area influencing the vehicle design. Finally, performance of the vehicle at non-design (or non-hypersonic) speeds may also be an issue depending on the application (e.g. landing gear systems for reusable hypersonic cruise vehicles, transonic performance, etc.).Hypersonic vehicle design can be challenging due to the extensive coupling of the flowfield and various systems on the vehicle. Integration of the propulsion system requires not only extensive design and analysis of the flowfield entering the engines, but the engine surfaces themselves can substantially impact the lift/drag performance of the vehicle. Hypersonic Mach numbers imply high stagnation temperatures, which must be taken into consideration when designing the vehicle structure and thermal protection system. Large aerodynamic forces at these high velocities can cause substantial pitching vehicle pitching moments, which must be analyzed along with the vehicle center of gravity to come up with a controllable design. Integration of the payload (both volume and mass) can affect many of these same areas (e.g. stability, drag increase, etc.), and is an important area influencing the vehicle design. Finally, performance of the vehicle at non-design (or non-hypersonic) speeds may also be an issue depending on the application (e.g. landing gear systems for reusable hypersonic cruise vehicles, transonic performance, etc.).

12. 12 Computational Fluid Dynamics (CFD) Continually advancing area Faster computers Better algorithms Much left to be achieved Images: Space Shuttle � full simulation of STS using NASA�s OVERFLOW code (chimera structured grid CFD). SR-71 � Unstructured CFD grid and simulation results using Cobalt software. Computational Fluid Dynamics (CFD) is an increasingly important aspect of hypersonic vehicle design an analysis. Due to the limitations of existing testing facilities, the ability to perform accurate computational simulations of these vehicles in their design environments can not only save time and money, but in some cases may be the only way to get usable results for specific environments (e.g. extremely high Mach numbers). Although various CFD methods exist, they typically involve discretizing a set of equations (e.g. Navier-Stokes) which describe the flow. The use of supercomputer systems is required, as these equations are solved on (typically) millions of grid points for full-vehicle simulations. Additions of turbulence and combustion models can enhance the accuracy of the flowfield being simulated. Modern grid generation methods (e.g. chimera-overset structured grids, unstructured/structured hybrid grids) can allow very complex geometries to be discretized. Flow simulation is an area that is still seeing constant research.Images: Space Shuttle � full simulation of STS using NASA�s OVERFLOW code (chimera structured grid CFD). SR-71 � Unstructured CFD grid and simulation results using Cobalt software. Computational Fluid Dynamics (CFD) is an increasingly important aspect of hypersonic vehicle design an analysis. Due to the limitations of existing testing facilities, the ability to perform accurate computational simulations of these vehicles in their design environments can not only save time and money, but in some cases may be the only way to get usable results for specific environments (e.g. extremely high Mach numbers). Although various CFD methods exist, they typically involve discretizing a set of equations (e.g. Navier-Stokes) which describe the flow. The use of supercomputer systems is required, as these equations are solved on (typically) millions of grid points for full-vehicle simulations. Additions of turbulence and combustion models can enhance the accuracy of the flowfield being simulated. Modern grid generation methods (e.g. chimera-overset structured grids, unstructured/structured hybrid grids) can allow very complex geometries to be discretized. Flow simulation is an area that is still seeing constant research.

13. 13 Aging Workforce Issues The workforce for hypersonic-related projects has tended to spike and dip in relation to major industry/government projects (e.g. Apollo, STS, NASP/X-30); in general, though, the overall number of personnel in hypersonic has decreased since the 1960�s. This is leading to a large number of experienced professionals are facing retirement (e.g. Apollo, STS), and can potentially cause a difficult-to-replace loss of knowledge and experience in many hypersonic technologies.The workforce for hypersonic-related projects has tended to spike and dip in relation to major industry/government projects (e.g. Apollo, STS, NASP/X-30); in general, though, the overall number of personnel in hypersonic has decreased since the 1960�s. This is leading to a large number of experienced professionals are facing retirement (e.g. Apollo, STS), and can potentially cause a difficult-to-replace loss of knowledge and experience in many hypersonic technologies.

14. Studies andRecent Programs

15. 15 National Aerospace Initiative (NAI) Objective: to renew American aerospace leadership Technology initiative, not a development program Started in 2001 by DoD and NASA Hypersonics is one of three technology �pillars�

16. 16 NAI Timeline �Flight demonstrate increasing Mach number each year, reaching Mach 12 by 2012�

17. 17 NAI Engine Capabilities/Off-Ramps

18. 18 National Research Council (NRC)Review of NAI Four areas identified that require maturation: Air-breathing propulsion and flight testing Materials, thermal protection systems, and structures Integrated vehicle design and multidisciplinary optimization Integrated ground test and numerical simulation and analysis

19. 19 Ongoing Demonstration Efforts Address Limited Areas of Hypersonics Research

20. Required Advances and Resources (For Operational Hypersonic Systems)

21. 21 ...Towards an Operational System NAC report on NAI identified advances required Technology development leads to decision in 2018 on full-scale development of operational system Areas identified by NAC report Fundamental research Critical technologies requiring maturation Air-breathing propulsion Materials, Thermal Protection, Structures Integrated vehicle design and optimization Integrated ground test and simulation Space access and mission application demos

22. 22 Technology Readiness

23. 23 Demonstrations Required To Operations

24. 24 Air BreathingPropulsion Critical Areas Hydrocarbon vs.Hydrogen fuels Engine materialscomponents Integration with low-speed propulsionsystem Flight testing From NAC report on NAI From NAC report on NAI

25. 25 Materials, TPS,and Structures Critical Areas Thermal ProtectionSystem (TPS) Actively-cooledcombustor panels Cryogenic tanks From NAC report on NAI From NAC report on NAI

26. 26 Integrated Vehicle Design and Optimization Critical Areas Multidisciplinary Design Optimization (MDO) Coupling of various disciplines (e.g. structures, fluids, propulsion, etc.) Variable-fidelity fluid dynamics analysis From NAC report on NAI From NAC report on NAI

27. 27 Integrated Ground Test and Numerical Simulation Critical Areas Detailed characterization of facilities (e.g. noise, flow purity) Validation of computational models vs. ground test facilities Experimental testing of hypersonic systems (e.g. integrated airframe/propulsion system) in existing facilities 10-year research program emphasizing high-enthalpy effects From NAC report on NAI From NAC report on NAI

28. 28 Resources Required to an Operational Hypersonic Air Breathing System $25B over 25 years (SAB-TR-00-03, Dec 2000) to achieve an operational space access system NAC review determined NAI under-funded US workforce and test infrastructure inadequate for developing operational system components

29. Outlook, Summary, and Conclusions

30. 30 U.S. Investment DoD posture Focus on operationally responsive capabilities Focusing on lower risk technologies (e.g. rocket propulsion) NASA posture Spacelift focused on human and heavy cargo lift to support exploration missions NASA re-working hypersonics program, but funding will be limited by national focus on space exploration missions

31. 31 Summary Past hypersonic program failures Overly-ambitious technology goals Divergence from critical-path technologies Frequent start/stop/redirection of programs Constrained budgets Lack or change of interest in project goals Dedicated large programs provide many benefits Stimulate and maintain public interest Create and sustain high-technology workforce (e.g. Apollo, NASP) Provide advanced technology development

32. 32 Conclusions of Recent Studies

33. 33 Conclusions of this Review Missiles are the most feasible application of hypersonic airbreathing technologies Likely multiple $B and 10 years away Hypersonic cruise vehicles will require many high risk advancements before becoming operational Short duration cruise at high per-flight costs possible by 2020 with around $10B in investment Truly operational hypersonic systems for reconnaissance, in-situ weapons release, and space access are likely tens of $B and 20 years away Many significant technological, operational, and workforce issues remain to achieve this goal NAI roadmap appears achievable path to success if sustained and funded properly

34. Backup Slides

35. 35