Executive Summary

This study was undertaken in response to a request by the U.S. Air Force that the National Research Council (NRC) examine whether the technologies that underlie the concept of a hypersonic, air-launched, air-breathing, hydrocarbon-fueled missile with speeds up to Mach 81 can be demonstrated in time to be initially operational by 2015. To conduct the study, the NRC appointed the Committee on Review and Evaluation of the Air Force Hypersonic Technology Program, under the auspices of the Air Force Science and Technology Board.

BACKGROUND

Since 1935, engineers have been developing technologies for hypersonic aircraft and missile systems that can fly faster than the speed of sound in the atmosphere. More than 30 years ago, the rocket-powered X-15 reached hypersonic speeds (i.e., greater than Mach 5) in the atmosphere. However, other hypersonic projects (e.g., the ambitious national aerospace plane) have not led to the development of an operational aircraft.

Hypersonic systems would have many benefits. For example, a hypersonic missile capable of an average speed of Mach 6 (i.e., approximately one nautical mile per second at the planned operating altitudes) could strike a time-sensitive target 250 to 500 nautical miles away in four to eight minutes.

Hypersonic Technology Program

The Air Force Hypersonic Technology (HyTech) Program was established in 1995. Originally, the program was focused primarily on scramjet2 engine technology, with a companion program in airframe technology. Soon after it was begun, however, the program was restructured to concentrate its very limited funds exclusively on developing scramjet engine technology. Two prime contractors investigating two engine concepts constituted the technical core of the program until early 1998, at which time one concept was selected. The selected engine concept is slated to be tested in ground test facilities in the 2001 to 2003 time frame.

Technical Problems

The characteristics of an operational, air-breathing, hypersonic missile will be determined by a combination of the desired capabilities, the necessary technologies, and the resources allocated by the Air Force. The magnitude of the technical problems for hypersonic vehicles depends on the maximum speed of the vehicle. From a technical standpoint, technology for a missile with Mach 8 speed would have to overcome several difficult problems to become operational by 2015. These technical problems are more challenging than those for a missile with a maximum speed of Mach 4 or Mach 6 and will require several step-changes in the technology.

Many examples could be cited showing how the difficulty of the technical problems escalates as the Mach number increases. The stagnation temperature of the oncoming air flow, for instance, increases from about 1,100°F at Mach 4 to about 2,500°F at Mach 6 and to about 4,200°F at Mach 8. The temperatures after combustion inside the engine are even higher (e.g., more than 5,000°F at Mach 8). No materials are known or projected that would be practical for a scramjet engine and could survive without active cooling at the maximum internal engine temperatures reached during Mach 8 flight in the atmosphere. At Mach 8, for example, an endothermic fuel-cracking system is necessary to cool some parts of the missile so they can survive the demanding flight environment.

1  

“Mach 8” means eight times the speed of sound.

2  

A “scramjet” is a supersonic combustion ramjet (see Glossary).



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Review and Evaluation of the Air Force Hypersonic Technology Program Executive Summary This study was undertaken in response to a request by the U.S. Air Force that the National Research Council (NRC) examine whether the technologies that underlie the concept of a hypersonic, air-launched, air-breathing, hydrocarbon-fueled missile with speeds up to Mach 81 can be demonstrated in time to be initially operational by 2015. To conduct the study, the NRC appointed the Committee on Review and Evaluation of the Air Force Hypersonic Technology Program, under the auspices of the Air Force Science and Technology Board. BACKGROUND Since 1935, engineers have been developing technologies for hypersonic aircraft and missile systems that can fly faster than the speed of sound in the atmosphere. More than 30 years ago, the rocket-powered X-15 reached hypersonic speeds (i.e., greater than Mach 5) in the atmosphere. However, other hypersonic projects (e.g., the ambitious national aerospace plane) have not led to the development of an operational aircraft. Hypersonic systems would have many benefits. For example, a hypersonic missile capable of an average speed of Mach 6 (i.e., approximately one nautical mile per second at the planned operating altitudes) could strike a time-sensitive target 250 to 500 nautical miles away in four to eight minutes. Hypersonic Technology Program The Air Force Hypersonic Technology (HyTech) Program was established in 1995. Originally, the program was focused primarily on scramjet2 engine technology, with a companion program in airframe technology. Soon after it was begun, however, the program was restructured to concentrate its very limited funds exclusively on developing scramjet engine technology. Two prime contractors investigating two engine concepts constituted the technical core of the program until early 1998, at which time one concept was selected. The selected engine concept is slated to be tested in ground test facilities in the 2001 to 2003 time frame. Technical Problems The characteristics of an operational, air-breathing, hypersonic missile will be determined by a combination of the desired capabilities, the necessary technologies, and the resources allocated by the Air Force. The magnitude of the technical problems for hypersonic vehicles depends on the maximum speed of the vehicle. From a technical standpoint, technology for a missile with Mach 8 speed would have to overcome several difficult problems to become operational by 2015. These technical problems are more challenging than those for a missile with a maximum speed of Mach 4 or Mach 6 and will require several step-changes in the technology. Many examples could be cited showing how the difficulty of the technical problems escalates as the Mach number increases. The stagnation temperature of the oncoming air flow, for instance, increases from about 1,100°F at Mach 4 to about 2,500°F at Mach 6 and to about 4,200°F at Mach 8. The temperatures after combustion inside the engine are even higher (e.g., more than 5,000°F at Mach 8). No materials are known or projected that would be practical for a scramjet engine and could survive without active cooling at the maximum internal engine temperatures reached during Mach 8 flight in the atmosphere. At Mach 8, for example, an endothermic fuel-cracking system is necessary to cool some parts of the missile so they can survive the demanding flight environment. 1   “Mach 8” means eight times the speed of sound. 2   A “scramjet” is a supersonic combustion ramjet (see Glossary).

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Review and Evaluation of the Air Force Hypersonic Technology Program Statement of Task In discussions with the Air Force about this study, it was evident to the committee that the time has come for the Air Force to decide whether or not hypersonic technology can lead to a militarily useful product within a reasonable time. The main element of the Statement of Task was that the committee evaluate the HyTech Program by (1) focusing on “the technologies needed to demonstrate a hypersonic, airbreathing missile concept” and (2) emphasizing “the underlying strategy and key components of the program, the critical technologies that have been identified by the Air Force and by other sources, as appropriate (e.g., advanced propulsion systems using ramjet and scramjet technologies); and the assumptions that underlie technical performance objectives and the operational requirements for hypersonic technology.” The committee was also asked to make recommendations based on its evaluation of the program. To perform its task, the committee had to address specific questions, such as whether or not the HyTech Program would lead to a capability that could satisfy the operational requirements for hypersonic technology applications; what technologies (besides propulsion) should be pursued next, and with what priorities, for a hypersonic, air-breathing, air-to-surface weapon; and whether or not the technical components of a hypersonic Mach 8 regime propulsion technology program have been identified and are in place. The committee was also asked to estimate reasonable milestone dates for the development and initial production of a hypersonic missile system. PRINCIPAL FINDINGS When the committee began this study, many members assumed that the HyTech Program was a component of a broader program to demonstrate the technologies for a hypersonic, air-breathing missile system capable of speeds up to approximately Mach 8. Instead the committee found that, because the program has very limited funding, the underlying strategy has been to concentrate on the propulsion subsystem of a representative hypersonic vehicle and to conduct a limited ground-test demonstration of a single Mach 8 hydrocarbon-fueled engine flow path. Under the circumstances, the committee considers this a wise decision. However, the committee recognizes that the program does not include either full integration of the propulsion subsystem with a flight vehicle (which is especially important for a compact missile system) or flight testing. Furthermore, the Air Force’s reliance on one propulsion contractor limits the alternatives for the engine design. To the committee’s knowledge, the collateral technologies required for the entire missile system are not currently being developed. Because the words “operational requirements” were emphasized in the Statement of Task, the committee was surprised to learn that the Air Force has not established operational requirements for a hypersonic missile system, although both the Air Force and Navy have established some general performance goals (e.g., top speeds of Mach 8 and Mach 6, respectively). The speed, standoff range, and kinetic energy of a hypersonic, scramjet-powered, air-to-surf ace missile could potentially enhance the warfighting capability of the U.S. military. Reaching a maximum speed of Mach 8 will require significant technological breakthroughs. Thus, controlling risks will be crucial for the technology validation program, including flight testing of a prototype vehicle. The committee proposes a carefully designed program below. The completion of the HyTech Program by 2003, followed expeditiously by flight testing of a prototype vehicle, could enable an operational, air-breathing hypersonic missile with a maximum Mach number in the range of 6 to 8 by 2015. SUMMARY OF RESPONSES TO QUESTIONS The Air Force HyTech Program, as currently structured, will not lead to an operational capability because the Air Force is not developing several critical, enabling technologies for the realization of an operational hypersonic air-to-surface weapon. The HyTech Program is investigating many of the propulsion flow path technologies that would support the development of a Mach 8 missile. Nevertheless, because of budgetary limitations, there are still several significant technical uncertainties about the overall propulsion system. These uncertainties are manifested by the program’s lack of focus on reducing technology risk and by the lack of flight tests. The current HyTech Program does not have either a mandate or funds to provide a sound technical foundation for a weapon system. The Air Force will have to conduct extensive trade-off studies before it can establish operational requirements and determine specific design goals for a hypersonic missile system of the kind envisaged in the HyTech Program. Because of the committee’s concern that the Air Force has not adequately analyzed the survivability of this class of missile, the committee undertook an additional study, which indicated that even at Mach 8 speed the missile will be vulnerable to surface-to-air missiles. The committee’s experience indicates that it will take until 2015 to develop, test, and produce the type of missile contemplated by the Air Force. A prototype missile phase would have to be initiated in 2003, and prototype flight testing would have to be completed by 2007 to reduce the risk for the engineering and manufacturing development phase. The HyTech Program is not formally coordinated with or intentionally dependent on hypersonic initiatives of the U.S. Department of Defense or the National Aeronautics and Space Administration, although they are sharing relevant technical information. Organizations throughout the world have developed expertise on scramjet-powered hypersonic vehicles, but no system-level hardware seems to be available.

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Review and Evaluation of the Air Force Hypersonic Technology Program The committee believes that the Air Force should continue to evaluate potentially significant foreign technologies. The implications for the Air Force support infrastructure of acquiring a hydrocarbon-fueled hypersonic missile are strongly dependent on the maximum speed of the missile. The implications include the amount of investment in ground testing and flight testing, as well as the analytical resources required to determine the performance and operability of the propulsion system. If the maximum Mach number is below 7, an existing test facility could be used with relatively minor modifications. The Air Force is also considering making modifications to at least one facility to support a Mach 8 capability. Regardless of the maximum Mach number, the Air Force must have the capability for periodic destructive testing of selected missiles from storage. The Air Force could benefit from using hypersonic vehicles after 2015, particularly for enhancing its global reach and access to space. Hypersonic technologies could be pursued either along a broad front or through a more focused program of evolutionary development to meet clearly stated requirements. The committee believes that only the focused program will result in operational systems. Therefore, the committee developed a long-range planning process with four components to guide the Air Force’s development of hypersonic systems for 2015 and beyond. CONCLUSIONS AND RECOMMENDATIONS Conclusion 1. The Air Force’s HyTech Program, which is a Mach 4 to Mach 8 propulsion technology flow path program, is necessary but not sufficient for the development of a scramjet engine as an integral part of a missile system. Although the limited testing (ground testing only) planned for the propulsion subsystem should indicate its potential engine performance, flight testing over a representative range of operating conditions will be necessary to determine the engine’s operability, reliability, and durability in an integrated system. These parameters are prerequisites to understanding the engine’s utility in an operational system. Recommendation 1. The Air Force should commit appropriate resources to integrated airframe-engine flight testing, which is vital to demonstrating a hydrocarbon-fueled scramjet in the Mach 4 to Mach 8 range. This recommendation (and the related recommendations that follow) assumes that the Air Force will decide that a hypersonic air-breathing propulsion capability is a potential candidate for fulfilling future system needs (e.g., as part of a hypersonic missile or space access application). If the Air Force is not willing to commit to flight testing, it should reevaluate its goals for the development of air-breathing hypersonic technology. Conclusion 2a. The HyTech Program itself will not provide the basis for an operational missile system because the development of critical enabling technologies for hypersonic air-breathing missiles are not included in the program and, to the committee’s knowledge, the Air Force is not pursuing them. These critical technologies will have to be mature and validated before the Air Force can proceed with a low-to-moderate risk acquisition program. Conclusion 2b. Besides propulsion, the five most critical enabling technologies for air-breathing hypersonic missile systems, in order of priority, are (1) airframe and engine thermostructural systems; (2) vehicle integration; (3) stability, guidance and control, navigation, and communications systems; (4) terminal guidance and sensors; and (5) tailored munitions. Conclusion 2c. If the HyTech Program were expanded to include a full-scale, integrated airframe-engine flight test program, and if the critical enabling technologies were mature, an operational air-breathing hypersonic missile system could be developed with low-to-moderate risk and without concurrency in support of an initial operational capability by 2015. Recommendation 2a. If the Air Force determines that there is a requirement for a hypersonic missile system, then it should establish a system-oriented program office to manage the system design and the development, integration, and flight testing of critical enabling technologies for a hypersonic missile system. Recommendation 2b. The program office should establish a road map to reach initial operational capability by 2015. The road map should include six phases: (1) system specification development; (2) system concept development; (3) technology risk reduction; (4) prototype design and flight test; (5) engineering and manufacturing development; and (6) low rate initial production. Conclusion 3. The Air Force has not established operational requirements or conducted design and requirements trade-off studies in support of an air-launched hypersonic missile system. Recommendation 3. If the Air Force intends to pursue the development of an air-launched hypersonic missile system as a viable candidate to meet its future warfighting needs, then it must initiate design and requirements trade-off analyses in the following areas: targets, speed, range, survivability, lethality, aircraft compatibility, risk, and cost. Conclusion 4. The risk and cost associated with the development of hypersonic air-breathing systems increase significantly with higher cruise speeds. Scramjet technology or existing ramjet technology with nonendothermic fuel-cooled metallic structures could be used for Mach 4 to Mach 6 systems. Systems with a maximum cruise speed of Mach 6 to Mach 6.5 will require a scramjet, which uses nonendothermic fuel cooling or uncooled ceramic composite materials. Mach 8 systems powered by a hydrocarbon-fueled scramjet will require endothermic fuel-cooled engine structures.

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Review and Evaluation of the Air Force Hypersonic Technology Program Recommendation 4. The Air Force should expedite trade-off studies in three areas: (1) mission parameters, to establish operational requirements; (2) system concepts, to define candidate configurations with optimum ranges of performance, operability, reliability, and affordability; and (3) technology, to redirect HyTech projects toward the most promising alternatives, if necessary. Conclusion 5. A hypersonic air-breathing missile will affect primarily one aspect of the Air Force support infrastructure, namely, ground testing facilities. Existing test facilities can support full-scale propulsion performance testing only at flight speeds up to approximately Mach 7. Existing test facilities for testing propulsion system operability and reliability are limited. The HyTech Program has plans to upgrade an existing facility for propulsion reliability testing at the Mach 8 cruise condition. Hypersonic missile systems will have no obvious implications for two other areas of the Air Force support infrastructure, high-speed computational facilities and test ranges. Periodic destructive testing of scramjet engines will be necessary in the future. Recommendation 5. The Air Force should begin planning for the ground test infrastructure to support the development and qualification of the operability, durability, reliability, and performance of integrated hypersonic propulsion systems over the Mach number range from the speed at the end of the rocket-boost phase to the maximum cruise speed. This infrastructure should be completed expeditiously. Conclusion 6. The Air Force has two broad options for the development of hypersonic technologies for 2015 and beyond. The first is to pursue a broad range of technologies covering a variety of potential applications. The second is to pursue the evolutionary development and deployment of hypersonic weapon systems that derive from established capabilities and clearly stated Air Force requirements. Recommendation 6. For 2015 and beyond, the Air Force should pursue the evolutionary development of hypersonic weapon systems and develop a long-range plan that incorporates the following four components: operational concepts for future systems and preliminary system designs; scramjet-powered weapon systems using hydrocarbon fuels; hypersonic weapon systems using hydrogen fuel; and combined-cycle systems for space access. Conclusion 7. The committee is not aware of any other nation that has operational hypersonic scramjet-powered missiles; however, several nations have been working on development, evaluation, and testing, including flight testing, for several years. The Air Force is monitoring foreign developments in hypersonics technology adequately.