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Future Air Force Needs for Survivability 3 Technological Setting This chapter describes the various factors that determine the speed and signature characteristics of an aircraft and its weapons systems, the current status of the technologies involved, and the U.S. Air Force’s goals for the future strike system. This assessment draws upon the expertise of committee members, as well as on the databases and analyses of their sponsoring organizations, to assess the feasibility of achieving various levels of speed and stealth in an aircraft with a 2018 initial operational capability (IOC). Finally, the chapter concludes by highlighting near-term research and development (R&D) needed to achieve technology maturity (technology readiness level [TRL] of 6) by 2009 (for the 2018 IOC time frame). It also indicates R&D needed for longer-term opportunities and programs for air vehicles in the 2025 IOC time frame and beyond. AIRCRAFT SYSTEMS In this context, aircraft systems are taken to include the airframe, sensors and apertures, propulsion system, weapons and payload, countermeasure systems, and situation awareness systems. In the following subsections, the technology challenges in these areas are addressed, together with near- and far-term technology opportunities. Various trade-offs among these systems must be made using systems engineering approaches in the design of the overall weapon system.
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Future Air Force Needs for Survivability Airframe Subsonic and low-supersonic airframe design for stealth vehicles is well understood. The issues of platform shaping, sweep angles, and inlet and exhaust shielding are determined by the system requirements. Issues of tails, vectoring, and payload carriage are also determined by the need to meet operational requirements in the pursuit of operational utility. Motivated by the objective of multimission aircraft, research begun in the early 1960s enabled the successful design of variable-sweep aircraft, including the F-111, F-14, and B-1. This “variable aerodynamics” innovation provided improved performance at supersonic as well as subsonic speeds. Although these aircraft demonstrated dramatic mission capability improvements, the escalation of the Soviet air defense threat drove the need for improved survivability and the shift toward stealth in the late 1970s and early 1980s, as exemplified by the F-117, B-2, and F-22. The F-22 uniquely combines high degrees of stealth, supercruise, situation awareness, and maneuverability for survivability. Near-Term Technology Needs Higher flight speeds add complexity to the design process because of the need to increase the high-speed aerodynamic efficiency of the platform while still meeting signature requirements. If long portions of a mission must be conducted at supersonic speeds using existing propulsion technology, the size of the aircraft must increase owing to the need to carry larger fuel loads. Variable-cycle engines offer the opportunity to reduce this platform growth because of their better fuel specifics (specific fuel consumption) in both the subsonic and supersonic speed ranges. While some of the radar cross section (RCS) signature requirements may be relaxed somewhat because of the higher speed, they are nevertheless still challenging for the design team. Systems engineering must be used to balance the conflicting approaches and requirements to yield the most cost-effective, operationally relevant system. Far-Term Technology Opportunities Adaptability of the platform shape—possibly using variable wing sweep, the evolving technology of morphing structures,1 or active flow- 1 In this report, morphing structures refers to wing structures that can change their shape to achieve optimal, uncompromising performance during complex military missions. The
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Future Air Force Needs for Survivability control techniques—offers possible solutions to the need to have the platform optimized aerodynamically in multiple speed ranges. Wing variability, while mature, must be examined closely from a signature perspective (sealing, cracks, and gaps) for its use in a 2018 system. One of the key reasons for the decline of variable aerodynamics during the beginning of the stealth era was the difficulty of integrating the moving surfaces, apertures, and seals of a swing-wing design in a form compatible with RCS requirements. Stealth designs had to make significant cost and/or design compromises (e.g., forgoing variable sweep) in achieving the best survivability owing to planform2 limitations in various areas of the flight envelope. In the future, however, a combination of stealth and variable aerodynamics may be feasible and could result in an aircraft that has the ability to optimize its signature as well as its aerodynamics during a mission. With the benefit of recent research, variable aerodynamics has the potential to re-emerge as a key enabler for mission flexibility and cost reduction in future strike concepts. Recent developments in wing morphing technology funded by the Defense Advanced Research Projects Agency (DARPA) have led to material and design concepts that could enable variable-sweep-wing designs.3 DARPA research efforts cover several applicable technologies, including planform change design technologies and various actuation technologies. Application of the morphing materials and concepts to a swing-wing design may provide the strike aircraft designer with the best of both worlds—a wing that can assume the low-sweep aerodynamics desired for takeoff, landing, cruise, and maximum persistence, as well as sweep-back to create the long continuous lines desired for a stealth design combined with efficient supersonic cruise that further enhances survivability. Evolving operational requirements demand the careful consideration of proven variable-aerodynamic design techniques to address future threat scenarios effectively. The performance characteristics demonstrated by the previous variable-sweep operational aircraft and other research indicate that there are size reductions and the associated cost reductions to future multimission aircraft that have the potential to provide unprecedented capabilities in responsiveness, persistence, and survivability. The combina- ability to change wing shape and vehicle geometry substantially while in flight allows a single vehicle to perform multiple mission tasks. 2 In aviation, planform refers to the shape and layout of an airplane’s wing. 3 See, for example, http://www.darpa.mil/dso/thrust/matdev/mas.htm. Accessed August 22, 2006.
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Future Air Force Needs for Survivability tion of variable aerodynamic and propulsion technologies can further build on that potential. Sensors and Apertures All future strike aircraft, weapons, and intelligence, surveillance, and reconnaissance systems will require sensors and apertures to complete their missions. By definition, these sensors must interact with the external environment; therefore, control of the sensors’ associated signature with minimal sensor performance degradation is essential to meet both survivability and sensor performance requirements. Typical sensors that are candidates for incorporation into future aircraft platforms include combat radar systems, radar altimeters, and electro-optical/infrared (EO/IR) systems. Radar systems have been successfully incorporated into both the B-2 and F-22 aircraft. As speeds increase and higher temperatures are encountered, the materials limitations associated with apertures must be evaluated. EO/IR systems have been deployed on existing aircraft, but the high temperatures encountered with increasing speed impact the aperture design, structural size, transmissivity, and the spurious signal input into the sensor. Sensor performance requirements for range, resolution, and processing speed can also be significantly increased by increasing speed. In addition to onboard sensors, future aircraft systems will require the reception of Global Positioning System signals and continuous communications capabilities. Connectivity to the Global Information Grid (GIG) will require continuous communications connectivity into the aircraft. Near-Term Technology Needs In the review of future missions, concepts of operations (CONOPS), and targets, there will be a need to increase the fidelity and capability of active sensors without increasing the susceptibility of such sensors to signal intercept that can lead to the detection and tracking of the aircraft. Far-Term Technology Opportunities Signature-reduction techniques in the visible and IR spectral regions represent long-term technologies that may significantly improve surviv-
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Future Air Force Needs for Survivability ability in the future. These techniques rely on the ability to sense the background environment from different perspectives. Propulsion Future aircraft systems will have an enhanced ability to strike time-critical targets. Propulsion will play a key role in that capability either by enabling high-speed platforms and/or by providing outstanding fuel-consumption characteristics that will allow vehicle persistence over targets. Engine companies and aircraft system contractors will need to study the benefits of both evolutionary and revolutionary engine technologies that can meet a 2018 IOC to understand the capability and return on investment that each technology provides. This activity will be a crucial part of the technology development and concept development and demonstration prior to the system definition and design (SDD) milestone. The challenges for the propulsion system in a future long-range strike vehicle will be many. Sustained operation at elevated vehicle speeds, when coupled with a requirement for high performance, is a challenge to the engine designer. The engine cycle (fan pressure ratio, bypass ratio, operating pressure ratio) that produces the high specific thrust (thrust per pound of airflow) to enable high-speed flight does not produce the low-fuel-consumption characteristics that will allow long loiter times. From a component design standpoint, high-speed flight will result in the compression system of the engine experiencing much higher temperatures. This will also result in a significantly higher temperature for the cooling air, which poses a challenge for cooling the engine hot section. The hot section will operate for extended periods at near-maximum flow-path temperature, meaning not only that oxidation and/or erosion will be a limiting factor, but also that component creep will become a more prevalent design consideration. Air vehicle thermal management is a challenge for today’s advanced aircraft systems and will be even more so for the next generation of long-range strike aircraft. Heat loads at elevated flight speeds will challenge the temperature limits of the fuel that serves as the primary heat sink. The engine lubrication system will also be taxed by the higher operating temperatures, and reductions in oil system leakage will be critical to long-duration strike missions. Another challenge of future systems will be to continue previous trends in advancing propulsion capability and, as a result, the capability of the vehicle system. Previous technology-development initiatives have been
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Future Air Force Needs for Survivability successful in developing new materials, increasing module performance, and increasing component efficiencies. Further efforts on materials and components can bring about additional improvements in engine capability. A fundamental technology challenge for all future propulsion applications is to find additional ways to improve the total vehicle system. Promising avenues to meet this challenge include focusing on aircraft-system-level improvements such as adaptive cycle engines. Propulsion is a critical barrier to achieving a high-speed missile capable of cruising at hypersonic speeds. Ramjet propulsion systems have been highly evolved, and flight tests have been carried out in several demonstration programs at such speeds. Supersonic combustion ramjets (i.e., scramjets) have also been investigated for propelling high-speed missiles (see the section below on “Weapons”). Technology challenges include the following: generating stable combustion in the high-speed flow path over a wide speed and altitude range, developing high-temperature structural materials and insulators, and developing efficient cooling techniques that allow thermal balance in the overall weapon system. The fuel consumption and efficiency of such designs must be considered in light of the overall size and weight of the weapon. Before a ramjet or scramjet is ever started, the vehicle must be moving with a high velocity, so the envisioned high-speed weapon systems are two-stage systems with integral rocket boosters. An alternative approach to a ramjet or scramjet system is an all-rocket propulsion system, which can be either single-stage or multiple-stage. Solid rocket propulsion technology is highly mature and little development risk exists, but the limited specific impulse of solid rocket systems results in shorter ranges for volume-constrained systems compared with air-breathing systems. An additional issue associated with rocket-propelled weapons capable of operation at long range is that the propulsion system burns quickly, resulting in flight along a predictable ballistic trajectory, which makes the weapon susceptible to intercept by advanced SAM systems, unless it has a terminal maneuvering capability. Near-Term Technology Needs The very high performance levels of existing propulsion capability were achieved through investments made in the Integrated High Performance Turbine Engine Technologies (IHPTET) program, which funded innovative research from 1985 to 2005. By any measure, the IHPTET
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Future Air Force Needs for Survivability program was a great success. It transitioned many technologies and enabled the advanced capability that currently exists in engines such as the F414 and F119 as well as the engines currently under development for the Joint Strike Fighter, the F135 and F136. The follow-on program to IHPTET, called the Versatile Affordable Advanced Technology Engines (VAATE) program, expands the focus of technology development to include items that can drive the performance and affordability of the entire vehicle system. Unfortunately, since the VAATE program’s initiation, its funding has been curtailed (by 40 to 50 percent). It will be critical to restore that funding to the levels originally planned in order to enable the timely development and transition of propulsion technologies (increased hot-section capability, improved materials, and thermal-management system) needed for future strike aircraft. Under the VAATE program, the propulsion community is executing technology-development efforts necessary for aircraft to achieve long life at sustained high temperature. The solution is not a single technology, but rather advances in the state of the art along a broad front. A new generation of high-temperature, creep-resistant disk materials is required for the turbine disk to have sufficient life in this environment. Other materials will have to be employed in new locations—for example, the use of the advanced turbine disk materials in the aft stages of the compressor. Turbine airfoils will require advances in cooling technologies to allow the more efficient distribution of cooling air. In addition, advanced coatings will be required to better insulate hot structure from the gas path. There must also be changes in the combustor to improve efficiencies by reducing the variability in the spatial temperature profile entering the turbine. Less variability allows the turbine to run closer to its temperature limit; alternatively, that capability can be traded for increased service life. These solutions alone may not produce sufficient life in the turbine section of the engine. There may be an additional need for technologies that can reduce the temperature of the turbine cooling air. One system approach would be to reduce the temperature of the cooling air using a fuel-air heat exchanger. This approach requires the development of new, highly reliable high-temperature heat exchangers configured for the demanding propulsion thermal and acoustic environment. Fuel nozzles and seals will have to be redesigned for the higher temperatures, as will the control system. Meeting future aircraft range and loiter requirements will require engines with much improved fuel-consumption characteristics. Improved component aerodynamics and sealing to reduce parasitic leakage will be
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Future Air Force Needs for Survivability necessary, as will technologies that allow the engines to operate with tighter clearances between the airfoils and the engine casings. Advanced controls will include sensors that provide real-time information on the performance of the engine. With those data, the control will be able to use effectors to optimize the performance of the engine for reduced fuel consumption. Additionally, a new augmenter development program will be needed to study and define potential operability and performance characteristics of the supersonic, long-cruise application for IOC in 2018. Long-range strike platforms are being investigated that could fly several thousand miles, with a substantial portion of that mission at supersonic speeds, and then could loiter for hours. Today’s propulsion system design space will constrain the designer’s ability to support this type of mission. To enable this revolutionary performance, adaptability will be required of the next generation of engines and airframes in terms of engine cycles, innovative integrated architectures, and morphing configurations. The propulsion community is positioned, if adequately funded, to bring variable-cycle engines—with their significant air-vehicle system advantages—to support the propulsion needs of a 2018 IOC long-range strike system. For example, DARPA and the Air Force are considering the initiation of an Adaptive Versatile Engine Technology program that aims to select technologies and concepts that are promising for future adaptive cycle engines. As this program is currently envisioned, it will advance adaptive engine technology, but it will not support a 2018 IOC. This program, if reprioritized and started now, could, in the assessment of committee members from the propulsion industry, advance the technology to TRL 6 in time to start SDD in 2009 and IOC by 2018. It is the recognition of these challenges and the contemplated solutions that strongly argue for the development of variable-cycle engine technology. Since aspects of variable-cycle engines have been successfully demonstrated since the 1980s, the technology is available now to permit the packaging of multiple-cycle engines capable of sustained high speed and efficient loiter, if both are required for the long-range strike platform. Far-Term Technology Opportunities With declining stocks of oil, the continued availability of existing hydrocarbon fuels must be examined, and propulsion systems capable of operating on alternative fuels must be developed. While this issue is not specific to the question of speed and stealth, future aircraft systems will be
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Future Air Force Needs for Survivability significantly impacted by technology advances made in the area of aviation fuels. Desirable characteristics of new fuels include both additional cooling capacity and increased energy density. Flight at hypersonic speeds will require significant development of new propulsion systems. Hypersonic air-breathing propulsion solutions and aircraft development—other than a missile—are highly unlikely in the time frame necessary to support 2018 IOC systems, but they represent an area of promise for application to future systems. Turbine-based combined-cycle (TBCC) engines have been explored for application to hypersonic aircraft and space-access vehicles, but this class of engine technologies is immature relative to near-term needs. TBCC engines could be enabling to a future hypersonic strike aircraft, but there are a number of integration challenges with the TBCC, including flow-path optimization, transition Mach number between turbine and scramjet, and thermal management. Integrated vehicle-and-propulsion designs need to be made on the basis of system thermal-management considerations. The thermal margins for high-speed propulsion systems are small and unforgiving of poor integration decisions at the start of these highly integrated designs. In addition to propulsion solutions for the aircraft platform, there are several advanced engine technologies that could be brought to bear on the weapons that would be carried by a long-range strike aircraft. For instance, the pulse detonation engine (PDE) uses a detonation-based combustion cycle and has the potential to offer improved fuel-consumption characteristics and lower fabrication costs. If an engine can be built to fully utilize the efficiency gains of the constant volume cycle, relatively large gains in propulsion system efficiency and performance, which are application-dependent, can be realized. Expendable PDEs for supersonic missiles have the potential to operate at speeds approaching hypersonic, and they integrate well into small, space-constrained high-speed weapons currently under evaluation by the Air Force and the Navy. Vehicle-level analysis of these missiles has shown the potential to offer approximately 50 percent reductions in propulsion system cost compared with similarly performing supersonic turbojet or afterburning turbojet engines. Weapons Weapon technologies strongly impact air platform survivability and, ultimately, the mission effectiveness of the launch aircraft. As an example of this interdependency, the increasing of weapons payloads to achieve a
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Future Air Force Needs for Survivability FIGURE 3-1 Examples of existing weapon systems: (a) Guided Bomb Unit-28 (GBU-28); (b) joint air-to-surface standoff missile (JASSM). SOURCES: See http://www.fas.org/man/dod-101/sys/smart/gbu-28.htm, accessed August 22, 2006; and http://www.fas.org/man/dod-101/sys/smart/jassm.htm, accessed August 22, 2006. specified mission leads to larger air vehicles, larger propulsion systems, and increased signatures. Alternatively, a high-speed missile hosted on a standoff air platform may handle the same mission as a gravity weapon dropped from a stealthy air vehicle that is capable of persisting in a high-threat environment. In the subsections below, technology challenges associated with weapons are discussed, together with near-term and far-term opportunities for advancing the state of the art for weapons. The critical technologies for weapons include aerodynamics, propulsion, materials and structures, system integration, terminal sensors and apertures, weapons data links, type of warhead, and signature. Many issues associated with weapons technologies overlap those associated with aircraft, although the weapon’s single use and limited flight time significantly simplify the system design. The principal focus of this technology assessment concerns kinetic weapons, which can be subdivided into gravity or glide weapons (e.g., Guided Bomb Unit-28 [GBU-28]) and powered weapons (e.g., joint air-to-surface standoff missile [JASSM]) as seen in Figure 3-1. Gravity weapons are necessarily short-range systems that require the aircraft to closely approach the target, which may stress the aircraft signature requirements. With their short flight times and close spacing, gravity weapons are inherently difficult to intercept, so little attention is paid to signature issues for these weapons.4 4 Future bombs could have greater standoff distance, which affects survivability considerations.
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Future Air Force Needs for Survivability With respect to powered weapons (i.e., missiles), a large array of potential technologies and system solutions exist. Since missiles may fly for extended ranges within a heavily defended area, the survivability aspects of strike missile concepts are extremely important. Existing air-launched missiles are either turbojet-powered (e.g., air-launched cruise missiles, JASSMs) or rocket-powered (e.g., Maverick). The turbojet-powered systems are subsonic missiles that rely on low signature for survivability or fly within ground clutter at low altitude to evade radar tracking. Rocket-powered systems fly at higher speeds but are necessarily of shorter range. Near-term advances in weapons system technologies currently being investigated include terminal guidance systems for subsonic weapons, algorithms for the autonomous search and recognition of targets, and data links for communicating continuously with the weapons following release from the air platform. High-speed weapons potentially add to overall system survivability by enabling the attack of time-sensitive targets or targets outside the range of conventional glide munitions. The issues associated with development of high-speed missiles with significant range were addressed in a 1998 report of the National Research Council (NRC)5 and in the USAF Scientific Advisory Board (SAB) report published in 2000.6 The 1998 NRC report found that “completion of the HyTECH program by 2003 followed expeditiously by flight testing of a prototype vehicle, could enable an operational, airbreathing hypersonic missile in the Mach number range of 6 to 8 by 2015.” However, the report also found that many other technical challenges beyond propulsion, such as sensors, guidance and control, thermal management, and others, require emphasis and investment as well. The 2000 SAB report states that “long-range, high-speed air-to-surface missiles can have significant military utility, provided that targeting information is available to exploit their inherent advantages.” Given the potential utility of high-speed, long-range weapons, the committee conducted a brief evaluation of the status of weapons technology. The science and technology community within the Department of Defense (DOD) has been investigating high-speed missile technologies to enable a 5 National Research Council, Review and Evaluation of the Air Force Hypersonic Technology Program, National Academy Press, Washington, D.C., 1998. 6 United States Air Force Scientific Advisory Board, Why and Whither Hypersonics Research in the US Air Force, SAB-TR-00-03, HQ USAF/SB, Washington, D.C., December 2000.
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Future Air Force Needs for Survivability Far-Term Technology Opportunities The reader should be aware of the fact that imaging IR seekers are effectively used in modern short-range air-to-air missiles (SRAAMs) by the United States and by several foreign countries. As medium- and long-range air-to-air missiles are degraded in performance by lower aircraft RCSs, it is plausible that some U.S. adversaries might begin to look at multimode seekers that use multiple guidance modes. There are several multimode wavelength combinations that are possible. Trade-off studies should be done to postulate system concepts, simulation and modeling should be used to evaluate the potential threat to future Air Force systems, and countermeasures should be explored. If these threats begin to emerge, the Air Force should design and demonstrate countermeasures well ahead of their needs in the force. Situation Awareness A maxim of aerial warfare is that the side with the best situation awareness (SA), coupled with the best air vehicle, dominates the fight. This was true in the early 20th century and continues to be true today. In today’s context, good SA is required to provide timely and precise knowledge of the enemy’s defenses, targets, and intent. More precisely, SA is the temporal and spatial knowledge of what is in a particular environment, where it is located, and what the key characteristics associated with it are. This includes both friendly and threatening features of the environment. SA requires the gathering and processing of onboard and offboard sensor data to distill relevant information pertinent to the environment and to filter out what is not relevant to the particular role assigned to the participant. Today and in the future, the precision with which these measurements are made in terms of position and time must be increased to support the timelines of modern engagements and the targeting requirements of weapons. Survival in complex threat environments will require that SA sensors and operations not significantly increase the observables of the penetrating aircraft through emissions, communications, or changes to the aircraft design. With the advent of greatly improved sensors, broad bandwidth connectivity, and improved display capabilities, there is a need for “intelligent data management” systems on aircraft that process large amounts of SA data in the background and present the results to the pilot instantaneously so that rapid decisions can be made to enhance survivability.
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Future Air Force Needs for Survivability Some of the requirements associated with improving SA to meet the needs of the next-generation strike aircraft are the following: Multisensor data and information fusion; Expanded databases on all IADS elements; Cross-sensor cueing; Mobile networks that are trusted and/or secure, wideband, and self-forming; Apertures and processors compatible with the stealth and speed design; and Improved sensor aperture integration with advanced sensing modalities. Near-Term Technology Needs Commercial off-the-shelf technologies and government off-the-shelf technologies can provide some of the near-term technologies needed for the next-generation survivable aircraft. These include the following: Highly accurate low-probability-of-intercept (LPI) or passive sensing capability; Higher-resolution radar imagery; Electronic support measures (ESM) improvements in sensitivity, accuracy, and geolocation; and EO/IR sensors, apertures, and vehicle integration. Network-enabled platforms must meet all the requirements of the common DOD architecture for information exchange. This network will define with whom and why communications links are formed. Current technology provides line-of-sight (LOS) and beyond-line-of-sight (BLOS) low-bandwidth links with few information assurance provisions. The messages are not content-labeled, so classification and distribution become difficult. The definition of the role of each participant is unclear and is not adaptable to changing situations. The use of networks is currently restricted to preplanned static networks, which has a significant impact on the entire SA process.
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Future Air Force Needs for Survivability Far-Term Technology Opportunities There are several key offboard issues for both near-term and far-term systems: for example, whether offboard systems will be available when required or tasked, whether an onboard airborne sensor platform will be over the battlefield when needed, whether there will be enough satellite coverage to provide SA over the battlefield, and whether the information will be timely, accurate, and relevant to the battle. With continued R&D investment, onboard sensors will continue to improve and will be able to provide the essential SA capabilities required for IOC and to upgrade survivability as the threats evolve. These include the following: LPI radars with enhanced bandwidth and resolution features, Change-detection imaging, EO systems with enhanced sensing performance and modalities, Automatic Target Recognition/Automatic Target Correlation (ATR/ ATC) and other identification modes, and ESM systems’ improved geolocation, both single-ship and cooperative, LO antennas with enhanced performance features. In the future, there will be a need for an improved distribution system including BLOS and LOS high-bandwidth links, versatile networks, and secure data handling based on message content. Most important, there will be the need to develop fusion algorithms capable of producing higher levels of information content. In summary, tomorrow’s platforms will be able to exploit enhanced SA in future scenarios—for example, enhanced onboard sensors with LPI and passive techniques. Greater use of offboard sensor systems will provide both the precision and required timeliness. The committee believes that future air vehicles, when coupled with excellent SA, will continue to be survivable and effective against evolving threats. Systems Engineering Approach Required for Program Success The success that the United States has enjoyed in fielding low-signature vehicles depended on the systems engineering approach developed and executed by U.S. industry. The understanding of the complexities of stealth integration (e.g., weapons, sensors, antennas, SA, and enabling apertures)
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Future Air Force Needs for Survivability into the fundamental airframe/propulsion design paradigm is most challenging and represents a significant accomplishment of U.S. industry-government collaboration. The systems design approaches developed, though often somewhat unique to each company, are critical to the success of any future systems design and represent a national asset that must be protected. The challenge of increased speed with stealth will not change the basic approach to systems design, but will require even stricter adherence to the discipline of systems engineering. Clear technical and operational requirements must be established to permit the myriad of trade-offs required in any successful systems engineering approach. The committee believes that there is sufficient time available to field a next-generation long-range strike platform with a 2018 IOC if the decision is made soon. ELEMENTS OF SIGNATURE Radio Frequency Current and future threat radar systems operate over a broad frequency range. Early warning systems are typically at the lower end of the bandwidth, with tracking, fire-control, guidance, and fuzing functions operating in progressively higher frequency bands. The lower-frequency systems typically achieve the longest detection ranges and the higher frequencies typically achieve the best resolutions and accuracies. In all bandwidths, the performance levels of the threat systems vary significantly, with the later versions and newer generations showing significant performance improvement over the earlier systems. Foreign IADSs have significant integrated capabilities and will be more integrated in the future. The number of fielded advanced IADSs is proliferating around the world. The committee focused on the speed and stealth of the aircraft, though similar considerations of Mach number and signature also apply to the weapons. As discussed in the next chapter, higher speed allows a design with less stringent signature requirements for equivalent susceptibility. This is clear from first-order analyses; however, higher-speed designs require more detailed and careful trade-off studies considering all frequencies projected for future IADSs as well as the influence of air vehicle configuration changes on the angular dependence of the signature. Omnidirectional or “fuzz-ball” models are not sufficient. It is imperative that the more detailed signature models be used in all levels of analysis, and higher-fidelity engagement
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Future Air Force Needs for Survivability models and simulations are required as well, to ensure valid conclusions and attrition rate assessments in campaign analyses. The far-term application of advanced techniques for aircraft signature reduction may substantially change the design trade-off between speed and stealth. With these alternative means to absorb and disperse radar RF energy, the signature of the aircraft potentially becomes less sensitive to material and shape and more compatible with high-altitude supersonic to hypersonic flight. Infrared The roles, capabilities, and limitations of IR sensors in airborne interception are well understood in the United States and in other countries. Short-range (within visual range) air-to-air missiles (SRAAMs) since the first Sidewinder have used IR seekers. Technology for IR seekers has advanced through about five generations to the imaging IR seekers in use today. Aircraft fighters and interceptors that carry SRAAMs have relied primarily on radars to detect and track the target. The missiles are launched after first acquiring the target with the missile seeker. Some of those aircraft have been fitted with infrared surveillance and tracking sets to aid in the acquisition, identification, and tracking of opposing aircraft. The roles, capabilities, and limitations of IR sensors in ground-based air defense systems are not so well characterized or understood. While there are some systems equipped with IR sensors, their ability to detect lower-signature vehicles could be degraded by fog, clouds, precipitation, and background conditions, and there may be other reasons for their use. Examples of IR long-range surveillance and tracking systems have been deployed in some countries. An IR-assisted IADS is still considered well within the technological capabilities of several adversaries and remains a potential future asymmetric threat to the survivability of U.S. aircraft when the atmospheric conditions permit its use. IR sensors in all three waveband windows of the atmosphere (long wave [LWIR], medium wave [MWIR], and short wave [SWIR]) have made significant advances and are incorporated into a wide range of threat missile systems to provide detection, tracking, and guidance at short range or at very high altitude above the weather and most cloud formations. The more sophisticated and robust applications are multimode, multispectral, and/or have improved counter countermeasures (CCM) capabilities, which improve their lethality.
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Future Air Force Needs for Survivability IR signature continues to be an area that should be considered for increased resources to further the technology development. Additionally, a methodology is needed to evaluate sensor designs and capabilities and how they might be exploited by the enemy. This evaluation should model and evaluate IR sensors and systems in the context of engagements and should consider tapping the expertise of the missile defense community in this area. Visual Signature Visual signature sources can provide foreign elements an additional source to enable alert, detection, and cueing. Susceptibility to visual signatures can be significantly impacted by factors such as weather, daytime or nighttime, and viewing aspect. However, in operationally common situations, in favorable environmental conditions, detection ranges can be useful. The visual signature provides both airborne and ground-based threat system operators with valuable knowledge that can then be applied in the effective operation of their systems. This is particularly true in engagement scenarios with airborne interceptors at altitudes above inhibiting weather conditions. The committee believes that the Air Force should hedge against the possibility that future asymmetric threat responses will involve exploitation of visual signatures, by thorough assessment of the potential utility and development of CCM technology and concepts. Visual signature control continues to be an area that should be considered for future development. Other Signature Elements: Electronic Emissions and Acoustic Signatures Electronic emissions will continue to become a more stressing element of signature as the “network-enabled” requirements for more robust data communications become more demanding. Past solutions involving shutting down communications and going silent in the high-threat region will not meet the network-centric GIG connectivity demands of the future operational environment. More in-depth assessment of this type of detectable signature source and the technologies and electronic techniques to control these emissions is needed. For the operational altitudes currently envisioned for the next-generation strike systems, subsonic acoustic signatures are not currently
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Future Air Force Needs for Survivability considered a significant contributor to aircraft susceptibility. However, there is the potential that threat acoustic detection systems could be much more effective against supersonic aircraft that are generating a supersonic boom. Further assessment of this potential threat is merited at all flight speeds. TECHNOLOGY FEASIBILITY ANALYSIS Signature Technology Matrix Readiness Versus Speed The committee assessed the readiness and potential of the technologies that impact signature across a speed regime from subsonic to speeds approaching hypersonic. The technology readiness was assessed in five groupings: airframes, propulsion systems, sensors and apertures, countermeasures and electronic warfare, and weapons. This assessment was conducted in a three-step process. In the first step, the committee generated a list of the significant technologies that impact the aircraft and weapon system and conducted a baseline assessment of their readiness to support a 2018 IOC (TRL 6 by 2009 SDD milestone) for the next-generation long-range strike system. In the second step, the committee’s baseline assessment was distributed to seven organizations to collect their readiness assessment. The seven organizations included three airframe companies (Boeing, Lockheed Martin, and Northrop Grumman); two engine companies (General Electric and Pratt & Whitney); and two government organizations. The final step in the assessment process consisted of compiling the data and generating a committee consensus view of the technology readiness. For the majority of the assessed technologies, agreement existed in the technology readiness among the assessing groups. However, divergence in the assessment of the technology readiness was observed in several areas. There was very good agreement between the committee’s baseline assessment and the subsequent assessment of experts in the seven organizations listed above on the high-level readiness assessment of many of the sensors and apertures-related signature technology elements. There was reasonable agreement on most of the remaining technology elements. Technology readiness was judged to be significantly impacted by the speed range. The significant conclusion is that, generally, solutions are considered to exist, but there is uncertainty in the assessment of risk associated with the implementation in the required time frame.
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Future Air Force Needs for Survivability The assessment of the onboard electronic countermeasures and electronic warfare-related signature technologies found very good consensus on all of the technology elements. There was very good consensus on most of the high-level readiness assessments of the weapons-related signature technologies. There was reasonable consensus on the remaining few technology elements. There were no areas of large variability in the assessments. SUMMARY OF SIGNATURE TECHNOLOGY READINESS From a high-level perspective, several general conclusions can be drawn from the assessment, as follows: As vehicle Mach number approaches the hypersonic region and temperatures rise, fewer signature control technologies are sufficiently mature to support an air vehicle development program that would meet a 2018 IOC. In the lower supersonic region, there are no major outstanding airframe issues associated with achieving the RCS signature levels required, and the airframe technology is mature enough for a 2018 IOC. At high Mach numbers where continuous engine augmenter operation will be required, the higher-temperature materials (both in the propulsion system and some parts of the airframe) needed to control signature require additional developmental investments to support a 2018 IOC. The readiness of signature-control technologies required for weapons at higher speeds is generally better than is the readiness of the corresponding technologies for aircraft, due primarily to weapons’ single-use, short-life operations. The readiness assessment concluded that the following technologies incurred the highest risk in the higher-speed ranges, in support of a 2018 IOC: High-temperature radar-absorbing structure, EO7 apertures, and Communication system apertures. 7 EO includes ultraviolet, visible, and IR.
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Future Air Force Needs for Survivability Achievable Signature Versus Speed In addition to the technology assessment discussed above, the committee assessed the overall projected signature levels achievable in the future as a function of flight speed compared with the Air Force signature goals and levels achievable today. The committee initially assessed the achievable signature levels as a function of speed, vehicle aspect, and frequency/wavelength using its own experience and knowledge base together with information provided in briefings to the committee. The committee’s assessment focused on demonstration of TRL 6 by 2009 in time for a 2018 IOC. The committee believes that the subsonic RF signature goals could be achieved. The committee believes that the goals for IR signature should be restated in terms related to specific postulated sensors, and in the presence of countermeasures, after further engagement analyses have been carried out to assess the utility of IR sensors with and without countermeasures. The committee made an assessment of expected signature growth as the Mach approaches the hypersonic region. With increasing speed, this expected signature growth is attributed to higher material temperatures coupled with the need for configuration changes, including those of inlets and nozzles. ONGOING RESEARCH AND DEVELOPMENT PROGRAMS Current R&D programs associated with high-speed vehicle development, initiated under the National Aerospace Initiative of the Director, Defense Research and Engineering, were reviewed. These efforts are aimed at maturing hypersonic technologies to support both high-speed missiles and aircraft. Propulsion and airframe technologies to support high-speed missiles operating in the Mach 3 to 7 speed regime will be flight-demonstrated within the next 1 to 4 years. RESEARCH AND DEVELOPMENT NEEDS AND OPPORTUNITIES In reviewing the Air Force’s current capabilities as well as requirements for platforms that are survivable, responsive, and persistent, the committee identified areas of likely technology needs. These needs, discussed below, are described relative to a time horizon that is both near term (defined as
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Future Air Force Needs for Survivability TRL = 6 in 2009 to support SDD with IOC in 2018) and far term (defined as technologies to support pre-planned product improvement [P3I] programs for the 2018 IOC platform or development of new platforms in 2025 and beyond). Near-Term Technology Needs Additional technology efforts are needed in the area of IR/EO and visible signatures as well as countermeasures to sensors operating in these wavelength regions. Fundamental investigations should be pursued into potential techniques for controlling signatures. Technical advances to allow greater utility of active RF sensors without increasing susceptibility should be explored. Shortfalls exist in propulsion technology in the areas of variable-cycle engines for missions requiring efficient operation over a wide range of operating conditions. Within the VAATE program, funding shortfalls exist that jeopardize planned gains in high-temperature, creep-resistant disk materials; advanced turbine-cooling techniques; thermal management; and power-generation systems. For sustained flight at supersonic speeds, temperature limits for signature control technologies should be extended. Additional research is needed on RF signature control technologies capable of sustained operation at elevated temperatures for high-temperature leading-edge materials, exhaust coatings, and engine seals. Additional countermeasure technology efforts are appropriate. High-speed weapons technologies can be developed in a time frame consistent with a 2018 IOC capability following successful completion of the ongoing flight experiments and demonstration programs. Technologies that should be considered to support the weapons system needs include submunition dispense techniques and the integration of terminal sensors. Finally, research into robust and accurate techniques for capturing the cognitive limitations inherent in an adversary IADS should be pursued. Mid- and Long-Term Technology Opportunities The postulated requirement for an IOC in 2018 with the start of an SDD program in 2009 constrains technologies to ones that are relatively mature and nearly available for transition. A number of technologies have been identified that may significantly impact survivability in the future,
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Future Air Force Needs for Survivability but which are not, in the opinion of the committee, likely to be sufficiently mature to support the start of an SDD program in 2009. These technologies may represent opportunities for P3Is or for application in the development of future systems. A number of promising propulsion technologies warrant investigation for application to future systems. Specifically, combined-cycle propulsion systems, high-temperature materials and structures, and advanced thermal management systems have the potential to significantly expand the operating capabilities of hypersonic platforms. The development of fuels with increased cooling capacity and/or higher energy density will lead to more capable aircraft and more flexible designs. In the area of airframe technologies, the development of morphing airframe shapes is one potential path to the development of an aircraft system that can operate with high performance in multiple flight conditions. Technologies associated with lightweight actuators and flexible skins may enable entirely new classes of aircraft. Active flow-control techniques for application to both aircraft and propulsion systems show promise for improving the robustness and performance of existing and advanced systems. These techniques may produce challenges for the LO systems designer, and appropriate design methods must be developed by the LO community. If future aircraft systems carry advanced directed-energy systems, light-weight power-generation systems and advanced thermal balance techniques beyond the present state of the art will be required. For weapons, needed areas of investigation include propulsion and airframe miniaturization technologies, novel seekers for enhanced target recognition, and advanced fuels, as well as issues arising from the specific type of warhead carried. These investigations will support future weapons that are smaller, more capable, and more autonomous in their operation.
Representative terms from entire chapter: