Decadal Survey of Civil Aeronautics: Foundation for the Future (2006)

A total of 20 R&T Challenges were prioritized in the materials and structures Area. Table C-1 shows the results. The R&T Challenges are listed in order of NASA priority. National priority scores are also shown.¹ This appendix contains a description of each R&T Challenge, including milestones and an item-by-item justification for each score that appears in Table C-1.²

Integrated vehicle health management (IVHM) refers to the ability to monitor, assess, and predict the structural and material health³ of primary and secondary structures for individual missions and lifetime durability using networks of onboard sensors. A fully integrated approach to IVHM relies on a multidisciplinary set of analysis, testing, and inspection tools, including miniaturized sensors and distributed electronics; sophisticated signal processing; data acquisition, integration, and database maintenance; artificial intelligence; damage science; and the mechanics of structures and their failure.

In addition to the obvious benefit of increasing safety and reliability, IVHM holds the promise of reducing vehicle cost, weight, and maintenance downtime, as well as speeding the introduction of new material systems and structural concepts. Real-time onboard sensor systems that monitor the actual state of materials and structural components enable more efficient use of material, including novel concepts. Moreover, with a national fleet of aging aircraft and infrastructure in an industry with low profit margin, IVHM is increasingly important due to its ability to increase safety and reliability. IVHM promises low-cost, real-time sensing and inspection methods to detect damage before catastrophic failure occurs.

IVHM currently means putting a variety of sensor systems onboard an aircraft, along with the artificial intelligence that automatically interprets the various sensor output streams. These data are used to provide input to prognostic systems that then draw conclusions about structural integrity issues. Two main features distinguish the next generation of IVHM from traditional nondestructive evaluation (NDE): (1) Sensor packages will be very small and exceedingly lightweight and (2) the reliance on humans to interpret the sensor output and assess the impact on structural integrity will be reduced or eliminated. Sensors and software are available, e.g., fiber-optic (Wood et al., 2000; Stewart et al., 2003; Carman and Sendeckyi, 1995) and piezoelectric (Lin and Chang, 2002; Giurgiutiu and Zagrai, 2002). The next major hurdle is integrating IVHM systems in flight structures. Laboratory tests have demonstrated that several classes of IVHM systems are available. Downselects to designs appropriate for aircraft structures are needed.

Three classes of IVHM systems warrant attention over the next 10 years. The first class includes fiber-optic sensor systems that can use multiplexed fibers attached to or embedded in the structure, each with numerous multiphysics sensing sites interrogated in turn by a single electro-optic module. The second class includes locally self-powered, wireless microelectromechanical sensors of various types tiny enough that very large numbers of sensors become practical. Each sensor mote performs a point measurement, so many are used to effectively cover large areas. The third class includes discrete active or passive remotely powered sensor modules (e.g., by means of guided-wave ultrasonic or acoustic emission) that may be large compared to sensor motes but can interpret multimode vibrations or multiphysics parameters (temperature, stress, humidity, etc.) that propagate over relatively long distances within the key structural

elements. Over the near term, IVHM sensors are more likely to be discrete items integrated with the structure rather than just another function of the structural materials themselves. Over the next decade, however, there is much work to be done in making things small enough, smart enough, and with enough multifunctionality to be acceptable to the airframe designers and owners.

Successful application of IVHM also relies on continued research and refinement in fundamental structural mechanics and the mechanics of damage and failure for accurate interpretation of IVHM sensor data and to support autonomous decision making for damage recovery and mitigation.⁴

NASA’s research program should target key applications where IVHM is most likely to make a difference rather than develop monitoring systems for unforeseen problems in existing structures. Detecting and characterizing multidamage states, composite debonding, corrosion, long-term fatigue, and impact damage are all important areas that can be mitigated with IVHM. Assessing the integrity of structural repair patches and finding latent faults in aging wiring are also key issues that need to be addressed over the next decade. IVHM approaches for aging aircraft are typically in response to a problem that has been identified. Regardless of the particular problem, however, the goals are still the same: putting a variety of sensor systems onboard aircraft along with the artificial intelligence that automatically interprets the various sensor output streams to provide input to prognostic systems that then assess structural integrity and makes life predictions. Key milestones include

• Develop lightweight sensor networks that characterize the state of materials and structures over large areas.

• Develop very-low-power or self-powered wireless sensors capable of operation in harsh environments.

• Develop artificial intelligence to automatically assess structural integrity from sensor responses and implement damage mitigation protocols.

• Develop components and sensors that are cost competitive and available from multiple vendors.

• Flight test full-scale IVHM systems to detect multisite damage.

Capacity (9): IVHM increases operating flexibility by permitting a wider range of operating conditions and environments due to increased confidence in the actual state of the structural elements. Monitoring system performance in real time allows one to do new things with confidence, such as enabling larger aircraft sizes and new aircraft concepts (for example, V/STOL aircraft, variable-cycle engines, and new structural configurations). These new designs may operate in currently unexplored regimes of operation and greatly increase the flexibility of the air transportation system. In addition, health monitoring could reduce maintenance time and costs. Aircraft could report the predicted lifetimes of their own parts and report the need for replacement parts. IVHM could quickly diagnose root problems, minimizing flight delays (Powrie and Fisher, 1999; Simon, 2000).

Safety and Reliability (9): Early detection of impending failures in aircraft materials, structures, and wiring is critical for avoiding fatalities as a part of the aging aircraft program. IVHM also reduces time lost to scheduled maintenance and reduces the likelihood of unscheduled downtime.

Efficiency and Performance (3): Once confidence in IVHM systems has been established, they could allow aircraft to operate closer to performance margins and with greater structural efficiency, which would reduce operating costs. IVHM will change future aircraft designs by reducing the margin of safety required in design, thereby reducing weight and increasing performance. It will also allow better design of engine fan blades, which now must be overdesigned for fear of fatigue failure. Finally, IVHM could increase the efficiency of aircraft maintenance, reducing operating costs and downtime.

Energy and the Environment (1): This Challenge will increase structural efficiency, which could save energy and thus reduce environmental effects, but overall, the impact is indirect.

Synergies with National and Homeland Security (9): DoD and others are already supporting research relevant to this Challenge but NASA has an opportunity to contribute when it comes to civil aeronautics and with a focus on the more sophisticated sorts of sensor systems that require a high-level understanding of the measurement physics involved. Reducing downtime has a significant impact on mission availability. Because IVHM systems for civil aviation will have to comply with stringent national and international certification requirements, advances will likely appear first in military aircraft.

Support to Space (3): IVHM could allow spacecraft to operate safely closer to performance margins and to be confidently designed with greater structural efficiency. However, differences in operating conditions mean that aeronautical IVHM systems would only be partially applicable to space vehicles.

Supporting Infrastructure (9): NASA has many experts in technical fields related to IVHM. Facilities of particular relevance include the fiber-optic draw tower at Langley’s Nondestructive Evaluation Sciences Branch and the NASA Dryden flight research facility, which provides a platform to test various IVHM approaches in flight.

Mission Alignment (9): IVHM is a natural part of NASA’s aeronautics mission and also dovetails nicely with

NASA’s multifunctional materials and multiphysics analysis research, although over the near term IVHM sensors are more likely to be discrete items integrated with the structure rather than just another function of the structural materials themselves. Over the next decade, there is much work to be done in making things small enough, smart enough, and with enough multifunctionality to be acceptable to the airframe designers and owners.

Lack of Alternative Sponsors (1): DoD and others are supporting IVHM research, including applications for a wide variety of nonaerospace industries. Significant industrial commitment related to this Challenge is also evident.

Appropriate Level of Risk (9): By targeting key forward-looking applications where IVHM is most likely to make a difference over the medium term, rather than focusing on solving previously unforeseen problems in existing structures by coming up with monitoring systems for them, this Challenge faces high risk.

Use of adaptive materials and morphing structures to change the aircraft shape (outer mold lines) and functions on demand represents a revolutionary approach for enabling optimal performance over a range of flight missions. Efficient, multipoint adaptability allows optimal performance for a variety of diverse, often contradictory, mission objectives (Lin and Crawley, 1995). Historically, morphing devices have included retractable landing gear, flaps, slats, and spoilers, all of which allow aircraft to land at lower speeds and to cruise at higher speeds. Adaptive materials have been used to reduce vibrations, eliminate noise, or control local air flow features such as separation. More recently, wings with the ability to drastically change planform area and shape have been proposed, and a few advanced concepts have been built. Adaptive materials are important elements of a morphing aircraft structure due to their ability to change or alter material properties and structural shapes using energy inputs such as light, heat, and electric or magnetic fields. Adaptive materials include heat-activated shape memory alloys like NiTiNOL; ceramics (e.g., lead zirconate titanate); photonically activated, lightweight, flexible shape memory polymers; electrically activated piezoelectrics; and magnetorheological fluids. These materials, some of which are self-sensing and self-actuating, can be developed into motors, combined with mechanisms, or distributed as actuators to produce highly efficient, lightweight airplanes. The success of morphing designs requires new adaptive materials and mechanisms, as well as innovative aircraft designs.

Morphing aircraft predate the field of adaptive structures. Fighter aircraft with variable swept wings first appeared in the 1960s. These aircraft were required to take off and land on aircraft carriers and achieve efficient supersonic speeds, yet they weighed less than comparable aircraft with fixed-wing designs. The field of adaptive structures evolved in the 1970s with work on vibration suppression for optical devices using piezoelectric materials and continued into flutter suppression work in the 1980s, some of which was conducted at NASA Langley. More recent design and testing of adaptive aeronautical structural components, such as rotor blades, have also demonstrated promise for allied areas such as vibration and noise control.

Recent morphing wing designs include planform area changes of up to 50 percent. These concepts, along with adaptive materials, expand possibilities for advanced aircraft. This includes providing new opportunities to expand wing area on demand, providing local flow control devices for drag reduction, and reducing vibrations and noise (inside the cabin and externally). These combined and synergistic developments allow innovative aircraft designs that operate efficiently over a wide range of speeds, for example, to land at very low speeds, loiter for long periods of time, and cruise efficiently at both subsonic and supersonic speeds.

Morphing structures with adaptive materials are not limited to wing surfaces. They could also be used to enable engine inlets that dramatically reshape or alter flow characteristics over a wide range of speeds. However, this would require development of high-temperature adaptive materials. These and other applications provide opportunities for revolutionary changes in future aircraft structures, although the costs of incorporating such technology have not yet been determined.

DARPA’s Morphing Aircraft Structures Program has sponsored several wind tunnel experiments at NASA Langley Research Center’s Transonic Dynamics Tunnel. These tests, which cost more than $35 million, produced a wide range of experience and identified innovative aircraft and rotorcraft concepts and critical materials technologies for future work. DARPA has also sponsored flight research on morphing technologies applied at the system level. Although the results were promising, these tests also showed that the technologies relevant to this Challenge are still immature and require additional research.

Adaptive materials have received a great deal of attention during the past decade, mostly directed toward the development of piezoelectric devices (Chaplya and Carman, 2002; Ritter et al., 2000), metallic shape memory alloys (Shin et al., 2004; Lim and McDowell, 1999), and ferromagnetic actuations (e.g., both magnetostrictive (Moffett et al., 1991) and ferromagnetic shape memory alloys (Ullakko et al., 1997)). However, to date, few viable aeronautical concepts have been created. The principal missing technologies are self-actuating adaptive materials with sufficient power output per unit mass and new materials that can be easily molded and remolded, stretched, or reshaped drastically. Current materials are not suitable for large actuation or elastic strains required for commercial aircraft. The requirements for load-carrying efficiency (high stiffness) and actuatable structures (low stiffness) have resulted in quick fixes using stiffened polymers as wing panels.

Morphing designs that change the aircraft shape require (1) development of new materials that can be turned on and off and (2) integration of these materials into novel mechanisms with embedded, distributed power sources that can activate the structures and move them efficiently from one point to another. These new materials will require lifetimes comparable to those of current materials.

Advanced aircraft concepts require designers to think differently about how aircraft and systems can be designed to demonstrate lower landing speeds, higher cruise speeds, and longer ranges than are possible today. A fundamental task is to characterize the mechanical response of these inherently nonlinear materials, including hysteresis, fatigue, long-term behavior, and damage behaviors. Analysis and design tools that accurately predict these responses will open the door to even more applications of these revolutionary adaptive structural concepts that could optimize performance and expand the flight envelope. Key milestones include

• Identify new morphing missions and designs for reconfigurable civil aircraft, including supersonic aircraft with low sonic boom.

• Develop the next generation of high-strain adaptive materials or devices that can be activated and deactivated for repositioning, with actuation deformation up to 100 percent.

• Develop novel integrated adaptive materials that allow wing surfaces and fuselages (including inlets) to rapidly change shape or alter load paths.

• Conduct scaled wind tunnel and flight tests on active, morphing aircraft to enable innovative, lightweight designs.

• Develop new, structurally integrated adaptive devices for flow control on a commercial aircraft to, for example, reduce drag and improve performance in off-design conditions.

• Develop analysis and design tools that account for and accurately predict nonlinear behaviors of adaptive materials and morphing structures.

Capacity (9): Adaptive materials and structures have already led to innovative solutions to aeronautical problems across a broad spectrum of requirements, including ice removal and vibration reduction. Morphing civil aircraft can adapt to a wider variety of landing and cruising speeds and may be able to fly at supersonic speeds over inhabited areas with minimal sonic boom.

Safety and Reliability (3): Adaptive materials are key elements of some aeronautical health monitoring concepts. Morphing airliners might also allow damaged aircraft to be reconfigured for safe flight.

Efficiency and Performance (9): Adaptive materials, as part of flow control devices and seamless control surfaces, can enhance wing lift. Morphing civil aircraft would be able to reconfigure for optimal performance at various altitudes, weights, and speeds.

Energy and the Environment (3): Adaptive materials reduce fuselage noise and drag, which indirectly reduces emissions. Active vibration suppression has been successful in reducing rotorcraft noise.

Synergies with National and Homeland Security (9): Adaptive materials and structures allow aircraft to perform tasks that are difficult or impossible for conventional aircraft, such as vibration control. Morphing aircraft are high on the DoD future concepts list because of their ability to adapt to unforeseen situations, to reduce the number of aircraft required for certain missions, and to operate over a wide range of conditions.

Support to Space (3): Adaptive materials have been used for vibration control for payloads delivered to space, for opening solar arrays from compact packages, and as adaptive optics.

Supporting Infrastructure (9): NASA has historically supported this type of technology and has a strong cadre of researchers with capabilities in flow control devices and adaptive materials. NASA has flight test organizations and facilities that uniquely support this type of activity. NASA Langley also has personnel developing new adaptive materials, including high-deformation piezoelectric actuators and piezo-fiber composites. More recently NASA Glenn has been developing high-temperature adaptive materials (e.g., piezoelectric and ternary nickel-titanium shape memory alloys).

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsors (1): Morphing structures and adaptive materials are currently being studied by academia and the DoD.

Appropriate Level of Risk (9): Some technologies related to this Challenge face very high risk, as evidenced by DARPA interest. However, recent government-sponsored research has reduced risk and clearly identified high-risk investments and issues in materials and structures that are appropriate for NASA to pursue.

Methods for simulation-based, multidisciplinary design and optimization (MDO) are at the very core of a philosophy that moves away from the build-test-build approach, which has proven to be expensive and ineffective in exploring the aeronautical design space. MDO processes develop synergistic benefits by integrating people, analytical tools, experimentation, and information to design complex structural components and systems that are characterized by strong

interactions. These approaches allow for development of optimal configurations, topologies, and dimensions for structural members and components to achieve design objectives, and they permit designers to examine the myriad what-ifs that characterize sophisticated designs with interdisciplinary trade-offs (Sobieszczanksi-Sobieski and Haftka, 1997).

Tools and platforms to enable the effective integration of analysis methods for the study of multidisciplinary interactions in structural design are the focus of this Challenge. A systematic analysis of uncertainty in all aspects of the design process is also important, as is increasing the level of detail in representing the structure. Uncertainties surrounding the predictive capabilities of physics-based models used in design—and their propagation in coupled systems—must be systematically included in the design process. Such design tools would greatly facilitate the process of sorting through design options and help identify and exploit interactions among multiple disciplines involved in the design process. The availability of MDO processes and analytical techniques enables experienced designers to collaborate with skilled analysts to identify and create innovative structural designs.

The rapid development of aerospace technologies has resulted in many new possibilities for aircraft design. Unfortunately, the benefit of these technologies, whether applied to conventional or radically new designs, is often slow to appear. MDO processes allow for the rapid identification of game-changing designs and design features, with significant potential impact on structural and material issues related to the design of a new generation of aerospace vehicles. The systematic inclusion of the effects of uncertainty, whether in loading, material behavior, or mission requirements, yields a rational approach for quantifying the risk associated with a certain design and allow for meaningful study of trade-offs among competing concepts. Inclusion of risk and reliability analysis in the design provides a time-dependent description of risk associated with structural and material systems in service, facilitating decisions that enhance vehicle availability and reliability.

After almost two decades of R&D, MDO processes for conventional designs have reached a high level of sophistication. In structural designs where the topology or outer mold lines are defined, a high level of success can be achieved by coupling analytical methods such as the structural finite-element technique with similar tools for load assessment. However, for designs with a multiplicity of topologies, some of which are not well-defined, and for problems where a large number of design parameters and constraints must be considered in the early stages of the design process, MDO methodologies are still underdeveloped (Giesing and Barthelemy, 1998). Major effort must also be directed at including the effects of uncertainty in the design process, as well as increasing the level of detail in representing the structure. Risk- and reliability-based design has emerged as a key research area, with industry, government, and academia focusing on developing robust methods that transition deterministic design tools to a nondeterministic environment. This effort has been primarily confined to disciplinary design with a dual focus: (1) developing approaches for modeling uncertainty in problem parameters and (2) seeking an efficient adaptation of design tools that account for the modeled uncertainties in a formal design process. New ways of formulating problems that incorporate quantitative reliability measures to facilitate effective design decisions have already been considered in this context. The extension of these approaches to large-scale structural and material design problems represents an entirely different level of problem complexity.

Significant new developments are required in both the platforms and the embedded tools that constitute the MDO process. Efficiency and effectiveness of the search process continue to be a problem, particularly in large-dimensionality problems and multimodal or disjointed search spaces. Existing problem formulations and search processes do not naturally allow for the emergence of radical design concepts and the associated design constraints. Current platforms are ill equipped to efficiently parse the vast amounts of data associated with the design process. Furthermore, not all methods are ideal for all problems. The goal of this Challenge is not to generate one perfect, all-encompassing algorithm but to use the most efficient and effective method or combination of methods for each problem. Proper algorithm selection in itself is an important research topic. There is a marked need for developing analysis modules for the search process to query. Some structural design issues, such as manufacturability, cost, repair, and environmental impact, are seldom represented in the design process. These analysis tools must be developed at multiple levels of granularity and precision, to coincide with the appropriate stage of the design process. The numerical efficiency of these tools is paramount, and alternative paradigms that take advantage of a new generation of parallel computational hardware must be sought (Giesing and Barthelemy, 1998; Sobieszczanksi-Sobieski and Haftka, 1997). Furthermore, existing analysis tools lack quantitative measures of prediction uncertainty. Uncertainty modeling in a data-lean environment, specifically for new concepts, continues to be an issue in this regard. There is a similar dearth of computationally efficient methods for reliability assessment, particularly in situations where uncertainty distributions do not conform to standard forms or where components or elements exhibit discrete behavior. The propagation of uncertainty in complex and highly coupled multidisciplinary systems needs to modeled, and tools for design and optimization in a nondeterministic environment continue to be computationally intractable, especially when applied to design problems involving a large number of nondeterministic variables, parameters, and design constraints.

New search methods and platforms that allow for an effective integration of analysis tools are required for the design of the next generation of aerospace structural and material systems. These methods would incorporate effective and

computationally efficient procedures and tools for quantifying risk and would be integrated with design optimization and decision-making tools and software at all levels of the design process. A systematic approach for modeling risk and uncertainty in complex coupled systems should be a key concern in this area of inquiry. Use of commercial tools in optimization is not enough to advance the state of the art in MDO. Optimization is only one piece of the analysis, design, and optimization triad. It is the tightly integrated development of analysis and optimization tools that furthers the potential of MDO methods. In the aerospace arena, such expertise is unique to NASA—additional gains can be realized with NASA working in close collaboration with researchers from academia and industry. A number of synergistic benefits could also be achieved by developing this approach in concert with health-monitoring technologies (see R&T Challenge C1). Key milestones include

• Develop multidisciplinary analysis tools that incorporate aerodynamics, structural dynamics, vibration, thermal response, and acoustic response with structural response to mechanical loads.

• Extend multidisciplinary tools to incorporate explicit mathematical modeling of design issues such as manufacturing processes, life-cycle cost, and repairability.

• Develop efficient approaches for multivariable optimization.

• Develop efficient and effective search processes for analysis of large complex systems.

• Develop approaches for modeling uncertainty in data-lean environments.

• Develop computationally efficient methods for reliability assessment.

• Develop systematic approach for modeling risk and uncertainty in complex coupled systems.

Capacity (9): This Challenge would support the development of new vehicle concepts and designs that are more efficient from the standpoint of speed, payload capacity, and fuel burn. The formal inclusion of multiple design objectives would permit the development of vehicles capable of operating in multiple operating environments and could alleviate traffic congestion in busy air corridors.

Safety and Reliability (3): Quantitative inclusion of risk and uncertainty in the design process could result in structural designs that promote vehicle safety and reliability. Temporal estimates of structural reliability might allow for improved practices in airframe maintenance and repair.

Efficiency and Performance (9): This Challenge will support development of structural components and systems that exploit synergistic benefits of multidisciplinary interactions to optimize explicit design criteria related to efficiency and performance.

Energy and the Environment (1): Lightweight structures would probably increase payload capacity but not necessarily reduce fuel burn of particular vehicles. MDO can increase understanding of structural acoustics and noise due to airflow, vibration, and structural dynamics but would not necessarily offer any solutions.

Synergies with National and Homeland Security (3): This Challenge would also improve the design and development of military aircraft.

Support to Space (3): The design of space structures has many of the same multidisciplinary elements as the design of advanced aircraft. The inclusion of quantitative measures of risk in the design decision process would be of particular relevance to the design of space structures.

Supporting Infrastructure (9): MDO tools and processes have a history of healthy development at the NASA centers. The multidisciplinary expertise required for the development of relevant platforms and tools—disciplinary experts, experimental facilities, and information technology support—are all available at NASA. Furthermore, NASA is uniquely qualified to conduct MDO research with aeronautics applications in mind.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission; it is a cross-cutting, enabling technology that supports many NASA missions.

Lack of Alternative Sponsors (3): Some support for this activity exists within the DoD and the aerospace industry, but there is a lack of infrastructure or research leadership in this area at these other agencies and organizations. Arguably, commercial tool set providers may be able to better accomplish this challenge.

Appropriate Level of Risk (9): This Challenge faces moderate risk. The research tasks are all plausible but not routine and would require NASA researchers to collaborate with academia and industry.

Over the past 50 years, polymeric composites have revolutionized and improved the performance of aircraft structures. Future needs for enhanced structural performance, high-temperature capability, and durability can only be met by the next generation of high-temperature, polymer-based composites. Next-generation composites will take advantage of improved polymeric matrices, new reinforcement materials, hybrid reinforcement approaches, improved joining technology, and science-based manufacturing with controlled three-dimensional placement of reinforcements. They will potentially lead to significant improvements in structural efficiency, safety, and high-temperature performance, as well as a reduction in data scatter, increased damage tolerance (e.g., resistance to delamination), and improved manu-

facturability (elimination of hand lay-up). These composites will likely incorporate adaptive materials and multifunctional concepts, thus serving as enabling materials for visionary concepts in nacelle components, wing structures, and fuselage materials.

Relevant research is currently directed toward development of higher temperature matrix materials; nanoscale reinforcements such as nanofibers, nanoclays, and carbon nanotubes; composites with multiple, different reinforcement fibers, and integration of adaptive materials to increase functionality. These materials have potential application in propulsion systems and for supersonic aircraft. Multiscale modeling efforts are also being conducted to guide the design and development of these materials across the nano-micro-meso structural levels.

The development of next-generation composites requires three capabilities to gain a complete understanding of the potential advantages of these materials: multiscale modeling, science-based processing techniques, and structural and mechanical testing. Multiscale models that link nano- and microstructure to structural composite response as well as the introduction of hybrid and multifunctional models are a critical concern. Research should also target (1) development of science-based processing techniques that account for resin chemistry, cure kinetics, and flow physics in guiding placement and distribution of the different reinforcement phases and (2) optimization of the reinforcement–matrix interface. Finally, structural and mechanical testing capabilities are needed to evaluate both the design and processing parameters. These three capabilities will enable the creation of effective life-prediction models and thereby eliminate statistical variation and defects. Key milestones include

• Demonstrate fabrication of composites with multiple different reinforcement fibers.

• Integrate adaptive materials to increase functionality.

• Develop techniques for manufacturing, processing, and dispersion of nanoscale reinforcements.

• Develop a fundamental understanding of how different kinds of reinforcements (e.g., nano, functional, or hybrid additives) affect the performance of polymers and composites.

• Improve damage tolerance for high-temperature polymers.

• Develop effective life-prediction models for polymers and composites.

• Investigate environment-friendly end-of-life reuse or disposal strategies.

Capacity (9): This Challenge has the potential to reduce structural weight, increase strength, and enable higher temperature multifunctional structures that will enable new vehicle concepts and increase capacity.

Safety and Reliability (3): Advanced composites will have improved damage tolerance, reduced delamination, and improved crashworthiness, which will moderately improve overall aircraft safety and reliability.

Efficiency and Performance (9): This Challenge has the potential to increase structural efficiency, increase aircraft range, increase the use of polymers in engine applications, and facilitate the development of advanced aircraft, such as an economically viable commercial supersonic aircraft.

Energy and the Environment (1): This Challenge contributes to this objective only indirectly, via improved structural efficiency.

Synergies with National and Homeland Security (9): Polymers and composites continue to be important for many DoD and DHS applications.

Support to Space (3): High-temperature polymeric composites are important for some space applications.

Supporting Infrastructure (9): NASA (particularly at Langley) has excellent infrastructure for composites research, including development of methodologies to deal with damage tolerance and fracture mechanics. These efforts are particularly important because composites do not conform to linear damage theory. Nanocomposite efforts at Glenn have been initiated and are focusing on high-temperature polymers.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission to develop new materials with improved structural performance and reduced weight.

Lack of Alternative Sponsors (1): DoD funded the first generation of composite materials, although it is not currently funding basic composites research. Industry has sponsored relevant research in recent years.

Appropriate Level of Risk (9): The level of risk is high. Fundamental research has been conducted at universities and government laboratories, but only at the small coupon level.

At this time, many passive and active control concepts are being pursued to control the interior and exterior noise of aircraft, including rotorcraft, over a wide range of flight regimes. However, efficient solutions have not yet been achieved. Local communities in this country and abroad are becoming extremely aggressive in passing stringent noise regulations. As a result, landings and take offs at many airports have been restricted. Complying with additional noise restrictions could impose an enormous weight penalty on many aircraft. Additionally, the flight envelope is often restricted to keep noise within acceptable limits. Regulations pertaining to noise levels have limited civil applications of helicopters even at conventional airports because rotorcraft are noisier than most commercial aircraft. In addition, lack

of public acceptance has been a major barrier to the widespread commercial use of helicopters to serve off-airport locations. Efficient methods to reduce noise will help expand the use of rotorcraft and fuel-efficient prop-rotor aircraft in commercial operations and increase the capacity of the air transportation system.

Reliable noise predictions are required to design efficient passive and active noise suppression devices. The problem is multidisciplinary, but with an important structural component. Advanced tools are required to accurately predict and alleviate noise, especially the aeroacoustic noise of rotorcraft. Research is needed to better understand the basic mechanisms of exterior and interior aircraft noise generation for different flight conditions. Current prediction tools for structurally transmitted noise are either finite-element-based (and therefore applicable only in the lower frequency range) or are energy- or power-based (covering a wider frequency range but with limited accuracy). Effective noise control techniques must take into account multiple types of aerodynamic and acoustic excitations. Therefore, structural prediction tools must be integrated with computational aeroacoustic and fluid dynamic prediction tools for a fully coupled solution to the problem of structural noise.

Suppression of exterior aircraft noise using smart materials holds great potential, but much R&D is needed. Advanced materials for larger, stronger fan blades and higher-temperature turbine blades, together with the development of very-high-bypass-ratio engines, is the biggest single factor in reducing external noise produced by jet aircraft. Good payoff can be achieved by developing locally morphing structures that smartly deploy themselves as needed, to reduce the noise generated by the propulsion system as well as the airframe. Variable-geometry-chevron nozzles, which could be driven by the shape memory alloy NiTiNOL, have been demonstrated to provide reduced noise during takeoff and then reconfigure themselves to a more efficient shape for cruise (Calkins and Butler, 2004). Smart materials can also be used to make fan duct liners that can adapt themselves (by changing cavity depth, face sheet porosity, etc.) according to the fan operating conditions to maximize noise reduction. Similar concepts can be applied to airframe noise suppression, where smart morphing structures could be integrated with noise reduction devices installed on aircraft high-lift systems (flaps and slats). These devices would operate at normal landing and takeoff conditions for noise reduction but could rapidly retract (or change configuration) as needed to increase lift and power during an emergency to maximize aircraft performance.

Noise experienced by flight crew and passengers is due largely to the excitation of the fuselage by the exterior flow. The fuselage structural design plays a key role in determining the amount of add-on noise control treatment needed to meet interior noise goals. Major strides in controlling noise in the aircraft cabin can be achieved using advanced structures and materials techniques.

The use of lightweight composite structural designs in commercial aircraft has greatly increased over the last few years. Unlike metallic structures, composite structures are excellent radiators of noise. Structural tuning concepts such as those pioneered in NASA-funded research for the reduction of turboprop tones may provide new opportunities to reduce noise while maintaining the strength and weight benefits of composite material systems. Experiments on current composite fuselage designs show that they would benefit from composite material systems with higher intrinsic damping. Work is needed to balance the structural and noise reduction requirements of honeycomb structures. Experiments have shown that partially filling cells with small loose particles can increase the damping properties of honeycomb panels. Other promising approaches include tailored lay-up using high-damping composite materials; nanotechnology to enhance structural damping; new acoustic and thermal insulation; morphing or tailored structures for achieving laminar flow and noise control; multifunctional composite structures (which offer improved noise control, strength, health monitoring, thermal insulation, and so on); and smart materials employing nanobiotechnology that can sense and respond to acoustic, elastic, thermal, and chemical fields in a positive, human-like manner. Key milestones include

• Measure noise signatures in controlled environments such as anechoic wind tunnels, for a range of flight conditions.

• Predict noise signatures using advanced multidisciplinary methodologies, validating against test data for level and maneuvering flight modes.

• Develop efficient structural solutions for interior noise control, i.e., structural optimization.

• Design non-load-bearing passive noise control.

• Design active controls for interior and exterior noise through smart structures technology.

• Develop low-noise rotors.

• Selectively flight test full-scale systems with noise signature measurement.

Capacity (9): Efficient suppression of noise would help to expand the flight envelope, including forward speed, rate of climb, descent, and flight course. Additionally, it would help to develop advanced vehicle concepts and next-generation heavy-lift rotorcraft. This will be a key step toward runway-independent aircraft for civil operations.

Safety and Reliability (1): This Challenge will have little impact on this Objective.

Efficiency and Performance (3): Efficient passive and active noise suppression devices could help to increase the flight envelope, particularly in congested areas.

Energy and the Environment (9): Reducing exterior noise will reduce the environmental impact of aviation.

Synergies with National and Homeland Security (3): Noise suppression will help to develop stealth vehicles, which is a key mission of DoD. In addition, noise suppression will enable increased use of military aircraft on bases near population centers.⁵

Support to Space (1): This Challenge would provide limited benefits to space operations.

Supporting Infrastructure (9): NASA has some unique experimental facilities to measure noise signatures, including the 20 × 24 × 30 ft Anechoic Quiet Flow Facility, the 27.5 × 27 × 24 ft Anechoic Noise Facility, and the 21 × 31 × 15 ft Reverberation Chamber at Langley, and the 40 × 80 ft/ 80 × 120 ft wind tunnel with acoustic lining at Ames.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsors (3): Prediction and measurement of noise is of low priority for industry. Also, industry infrastructure is inadequate for long-term systematic studies. There are some limited research activities in DoD laboratories.

Appropriate Level of Risk (9): This Challenge faces moderate to high risks.

The goal of this Challenge is to provide the underlying technologies for materials modeling tools that can predict properties of new high-temperature metallic materials and associated protective coatings. The effort would include generation of the necessary fundamental data, complemented by testing that simulates realistic jet engine operating conditions to validate the models. These tools would then be applied, in concert with industry, to the development of innovative propulsion materials. Typically, substrate materials are developed separately from environmental coatings and then integrated toward the end of the development program, or coating development follows substrate development. This stretches development time considerably, often to a decade or more. These modeling tools are expected to reduce development time for high-temperature materials and coatings by 50 percent (NRC, 2004).

Advanced high-temperature materials are critical to advancing the next generation of jet engines that will power subsonic and supersonic fixed-wing aircraft, while enabling reduced operating costs and improved engine safety and reliability. Improved metallic alloys are needed for high-temperature structural applications such as turbine disks, blades, and pressure cases. Increases in operating temperature of 50°C or more for jet engine parts is possible, but the length of time and cost to develop these materials, and the risk that success will not be achieved, have been a huge disincentive to aggressive development of innovative materials.

The abilities to estimate materials properties and model complex phase fields, microstructures, and materials processing are advancing through the use of new computational tools and powerful desktop computers. However, application of this work to guide materials research is still in the beginning stages. NASA has the opportunity to be a leader in this technology.

The key to improving engine efficiency lies in developing turbine materials systems (i.e., alloy substrates for the turbine blade, disk, and shroud, plus necessary environmental coatings) that possess structural performance at higher temperatures while maintaining stability for tens of thousands of operating hours within an environment that is highly oxidative, corrosive, and erosive. Long development times would be reduced by the ability to conduct many experiments computationally, analogous to what is currently done by the tools for computational fluid dynamics. Key milestones include

• Define required models and a model integration strategy to provide necessary functionality for simulations.

• Select models for further development, based in part on how well they are aligned with materials systems that provide greatest benefit for propulsion systems.

• Develop models for selected substrates and associated environmental coatings; determine all the physical parameters required by the models.

• Validate the models by applying them to the development of new materials that are selected in concert with industry.

Capacity (3): Advanced high-temperature alloys with environmental protective coatings are one enabler for supersonic flight. These materials will increase maintenance intervals and could contribute to more flexible flight operations. Materials modeling will enable these materials to be developed faster, at lower risk.

Safety and Reliability (9): This Challenge will resolve safety and reliability issues with engines that operate at higher temperatures by improving the inherent capability of materials to withstand degradation modes.

Efficiency and Performance (3): Higher-temperature materials will directly improve the aerothermodynamic efficiency of the engine, thereby reducing fuel burn. These materials also reduce maintenance costs. Higher temperature materials will enable supersonic propulsion systems. The

next generation of a turbine materials system resulting from this Challenge is anticipated to increase operating temperatures by about 50°F within 5-10 years, resulting in only a moderate impact on this Objective.

Energy and the Environment (1): This Challenge has little impact on this Objective.

Synergies with National and Homeland Security (9): This Challenge would benefit efforts by DoD to develop improved, long-life propulsion systems for supersonic and hypersonic flight.

Support to Space (3): This Challenge would benefit efforts to develop air-breathing access-to-space turbine engines. Advanced modeling tools might be used to guide development of specialized alloys expressly tailored for space launch systems.

Supporting Infrastructure (9): NASA Glenn Research Center has a cadre of experts in the modeling and development of advanced turbine alloys and coatings. NASA Glenn also posseses unique specialized testing capabilities that simulate severe engine conditions, such as foreign object damage, creep, fatigue, and various environmental conditions. NASA also has outstanding electron microscopy equipment and facilities for high-temperature materials characterization.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission. In the past, NASA contributed significantly to this field.

Lack of Alternative Sponsors (3): The Air Force, Navy, and DARPA have all conducted work related to this Challenge. However, the DoD effort is not aimed at commercial engine or rotorcraft applications and is only likely to fund point solutions to specific problems.

Appropriate Level of Risk (9): This Challenges faces high risk.

Because the aerodynamic environment surrounding an aircraft is unsteady, the aircraft experiences significant vibratory loads that need to be either isolated or absorbed to minimize their impact on passengers and key structural components and instruments. Also, unsteady aerodynamic forces couple with structural and inertial forces, resulting in potentially catastrophic aeromechanical instabilities. This Challenge will minimize the impact of vibratory loads using innovative passive and active techniques. It will also examine innovative techniques to increase the aeromechanical stability margins of aircraft in all flight modes. Minimizing vibratory loads enhances ride quality, increases the structural life of components, and improves handling. Aeromechanical stability (aeroservothermoelasticity) is the key to expanding the flight envelope.

Current aircraft use numerous passive devices to isolate or absorb loads. Prediction of vibratory loads, especially in rotorcraft, is far from satisfactory. Mechanisms of vibration in maneuvering flight are not completely understood. Vibration is a nonlinear-coupled phenomena that involves unsteady aerodynamics and wakes; nonlinear structural deformations and inertial couplings; and interactions between the flow and the structure. Aeromechanical stability can be a major issue with new configurations and expansions to the flight envelope.

Advanced CFD methodology needs to be coordinated with (1) comprehensive structural analyses to predict aeromechanical stability, vibratory loads, and vibration signatures at different stations in the airframe and (2) systematic validation against test data. The development of comprehensive MDO studies focused on inherently stable, low-vibration aircraft is another area worthy of research. Experimental issues involve the performance of systematic wind tunnel and flight tests using dynamically scaled and full-scale models. Problems also stem from a fundamental mismatch in basic structural models and reduced-order control models. Key milestones include

• Predict vibration using advanced CFD methodologies and validate experimentally.

• Predict aeromechanical stability for advanced configurations and expanded flight envelope (including hypersonic flight) and validate experimentally.

• Measure vibratory loads and vibration signatures under controlled wind tunnel environments for a range of flight conditions.

• Develop novel techniques for control-oriented modeling.

• Selectively flight-test full-scale systems, measuring vibration signatures and damping levels at level and maneuvering flight conditions.

• Innovate and employ active or passive techniques to minimize vibration and increase stability margin.

• Develop MDO techniques to develop low-vibration, stable systems.

Capacity (3): This Challenge would help increase payload and expand the flight envelope.

Safety and Reliability (9): Minimizing vibratory loads will increase structural life of components and improve reliability of controls and instrumentations. Aeromechanical stability margins are important for flight safety.

Efficiency and Performance (3): Increased stability margins would result in moderate increases in airspeed, payload, and structural efficiency.

Energy and the Environment (1): Reducing vibratory loads might have some impact on noise, but the effect would be minimal.

Synergies with National and Homeland Security (9): Vibratory loads have direct impact on DoD missions, espe

cially the accuracy of some aircraft weapons. This Challenge would also increase the life of structural components and would help increase the maneuverability of military aircraft.

Support to Space (3): Active and passive vibration suppression techniques and stability augmentations could also apply to vibration suppression for access-to-space vehicles.

Supporting Infrastructure (9): NASA has unique experimental facilities that are important for full-scale and smallscale model testing. These include the 40 × 80 ft/80 × 120 ft Wind Tunnels at the National Full-Scale Aerodynamics Complex at Ames and the Transonic Dynamics Tunnel at Langley for Froude- and Mach-scale testing.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission to improve aircraft performance.

Lack of Alternative Sponsors (3): Prediction and understanding of vibration and aeromechanical stability is already of moderate priority to industry and DoD.

Appropriate Level of Risk (9): This Challenge faces moderate to high risk.

Recently, NASA conducted a systematic preliminary design study (Johnson et al., 2006) on a high-speed, heavy-lift civil rotorcraft with a cruise speed of 250 knots (Mach 0.6). It was “neighborly” quiet, economically competitive with a Boeing 737 aircraft, flexible in cruise altitude, and runway independent. Potential rotorcraft configurations include tiltrotor, tandem-rotor compound, and an advancing blade concept (compound coaxial rotor). The development of a next-generation, cost-effective, high-speed rotorcraft that can meet these technology goals, faces numerous difficulties and barriers, which include scaling effects, aeromechanical efficiency, power-train limitations, structural efficiency, and life-cycle cost. Overcoming this Challenge will be a giant step toward the development of a runway independent aircraft that can efficiently expand capacity.

For the development of a high-speed, heavy-lift, efficient rotorcraft, a multidisciplinary aeromechanics research program needs to be established. It should encompass innovative rotor designs, active vibration and load control, variable-speed rotor technologies, active noise control, rotor morphing, lightweight and crash-absorbing airframe technologies, advanced variable-speed transmission systems, diagnostics and prognostics of drive trains and rotor head systems, increased autonomy and maneuverability, and enhanced handling quality. Because of an enormous increase of dynamic loads at high-speed flight (due to increased aerodynamic asymmetry), advanced composite designs with high damage tolerance are key structural issues. This provides opportunities to incorporate many disruptive and nondisruptive technologies in rotorcraft design, with an enormous payoff in performance and life-cycle cost compared with existing helicopters. Key milestones include

• Develop comprehensive aeromechanic analyses for high-speed rotorcraft that include tilt-rotor, tandem-rotor compound, and compound coaxial rotors for level and maneuvering flight conditions.

• Develop aeromechanics and technology tools for the drive-train system and other key components necessary for variable-speed rotors.

• Design and develop lightweight, crash-absorbing composite airframes.

• Develop technology for all-weather rotorcraft operation.

• Develop advanced composites with high damage tolerance for use in large dynamic structural components.

• Reduce required shaft power by 15 percent from current levels using elastically tailored composite blades, active structural and flow control, and advanced airfoils.

• Reduce life-cycle cost using health and usage management systems, low-cost tailored airframes, and lightweight low-vibration rotors.

Capacity (9): The performance of a next-generation, revolutionary civil rotorcraft greatly increases the capacity of the air transportation system.

Safety and Reliability (1): This Challenge involves the use of many disruptive technologies and is unlikely to significantly increase safety or reliability.

Efficiency and Performance (3): This Challenge would increase the speed, range, payload, and structural efficiency of rotorcraft.

Energy and the Environment (1): A next-generation rotorcraft should be designed to fly at an altitude of 15,000 ft or higher, to reduce noise on the ground. A variable-speed rotor could also be used to reduce external noise. However, although these efforts would reduce the noise produced by rotorcraft, it would only reduce them to levels comparable to fixed-wing aircraft. Therefore this Challenge will not reduce the overall noise produced by the air transportation system.

Synergies with National and Homeland Security (9): High-lift, high-speed rotorcraft will have an enormous impact on emergency evacuation, heavy-lift operations, and other military applications.

Support to Space (1): This Challenge is not relevant to this Objective.

Supporting Infrastructure (9): NASA has unique experimental facilities that are important to rotorcraft full-scale and small-scale testing. These include the 40 × 80 ft/80 × 120 ft

Wind Tunnels at the National Full-Scale Aerodynamics Complex at Ames, the Drop Test Facility and Transonic Dynamics Tunnel for Froude- and Mach-scale rotor testing at Langley, and the Power-Train Full-Scale Test Facility at Glenn.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission to improve aircraft performance.

Lack of Alternative Sponsors (3): The U.S. Army is the primary supporter of research on high-speed rotorcraft, but the Army will not investigate issues related to civil applications.

Appropriate Level of Risk (9): This Challenge faces high risk.

Advanced structural ceramics, including oxide-, carbide-, nitride-, and boride-based systems, are characterized by high strength, stiffness, hardness, corrosion resistance, and durability. Furthermore, they retain these properties at high temperatures, making them ideal for a wide range of demanding applications, including engine components for subsonic aircraft (combustor liners, exhaust-washed structures, high-temperature ducts, heat exchangers, and nacelle insulation) and airframe and propulsion systems for high-speed vehicles. Due to their inherent brittleness and low fracture toughness at ambient conditions, they are often combined with other materials to produce composite systems that take advantage of the high strength of small-diameter ceramic fiber reinforcements or used as nonstructural coatings for thermal management. In general, each class of material exhibits inherent benefits and limitations for particular use environments (NRC, 1998).

The primary benefit of structural ceramic materials is increased thermal capability, which improves propulsion system efficiency, increases lifetime, enables higher operating speeds, and expands the margin of safety in airframe applications. The selection of the ceramic system and its use temperature is dictated by the chemistry of the operating environment and the minimum acceptable lifetime of the component. Oxide composites with operating temperatures as high as 1200°C and lifetimes of thousands of hours in highly oxidizing combustion or reentry environments are very suitable for some engine components, warm structures, and thermal management components.

Nonoxide composites made of silicon carbide reinforced either with carbon fibers or a combination of carbon fibers and silicon carbide fibers are capable of operating temperatures of 1300°C-2500°C for short times in highly oxidizing environments or for much longer times near the lower end of the thermal range when protected with coatings. Furthermore, because nonoxide fibers exhibit higher strength and better strength retention than oxide fibers, they are being widely researched for applications in combustion environments as well as for hot structures of reentry or hypersonic vehicles.

Refractory metal (e.g., hafnium or zirconium) carbides and borides are capable of surviving thermal excursions up to 2000°C-2500°C for short times with little material recession, making them strong candidates (in either monolithic form or as a composite matrix) for the sharp leading edges of hypersonic vehicles.

During the last 10 years, significant progress has been made in the processing, development, and demonstration of many ceramic systems for specific applications. Oxide composites deriving damage tolerance from highly porous matrices have been commercialized, and other systems with novel fiber coatings have been demonstrated in subscale testing for reentry vehicle thermal protection systems. Silicon carbide matrix processing approaches have advanced significantly, with systems produced by chemical vapor infiltration, melt infiltration, and preceramic polymer infiltration all having been demonstrated in subscale testing for jet or rocket engine components. NASA Glenn has led efforts to fabricate and test jet engine components such as exhaust nozzle liners, combustor liners, and turbine airfoils with silicon carbide matrix composites. Rocket nozzles fabricated from silicon carbide materials have been rig tested, and NASA Ames has demonstrated the ability to reproducibly fabricate refractory metal carbide and boride systems.

Despite the above successes, component fabrication is not often taken much beyond the prototyping stage. Fabrication methods can significantly affect material properties, but it has not been possible to devote enough effort to understanding the effect of variations. In addition, new design rules and attachment methods must be developed to fully exploit the unique properties of these materials. There has not been sufficient component testing under actual operating conditions to validate design rules, nor are there sufficient mechanical and thermal performance data to fully characterize these ceramic systems. Therefore, there is currently low confidence in designing structural components with these material systems.

Advancing the state of the art for high-temperature ceramics suitable for aeronautical applications requires research in several key areas: fabrication and testing, modeling, and attachment methods. As mentioned above, insufficient fabrication and testing experience deprives designers of confidence in the long-term behavior of these materials and in the design rules for translating material characteristics into component designs. As a consequence, structures are typically overdesigned or constrained by existing designs for other classes of materials. At best, structures are rarely optimized; at worst, they fail when the material is inappropriately applied.

Additionally, modeling tools to predict the life of components made of these materials are inadequate. This leads to inaccurate performance and cost assessments and further limits the use of ceramic materials. Since these materials are only considered for niche applications, no economics of scale

can be anticipated. This problem could be alleviated to a large extent through the development of better design tools, a more thorough understanding of the effects of process variations, and more efficient commercial fabrication approaches.

Work is also needed to develop robust methods for attaching hot components to warm and cool structures and to develop textile approaches for integrating complex component architectures with key features such as stiffeners, sensors, and cooling features. Near-term milestones include

• Generate material property databases appropriate for design of a high-temperature ceramic component.

• Complete full-scale testing of at least one ceramic composite component with improved performance for subsonic aircraft applications (e.g., fairing heat shields, combustor liner, or turbine airfoil).

• Develop models to optimize a structure for a new, rather than an existing, platform.

• Model crack growth under actual operating conditions.

• Develop advanced ceramic composites for large surfaces and leading-edge components for supersonic and hypersonic vehicles and complete relevant environmental testing of subcomponents.

Far-term milestones include

• Flight test at least one ceramic composite component for improved subsonic flight vehicles and transfer the technology to industry.

• Verify model predictions of performance using flight test data.

• Extend model predictions to new flight speed regimes to optimize supersonic and hypersonic vehicle designs for hot structures and engine components.

• Demonstrate, through full-scale testing, at least one ceramic composite component for a supersonic or hypersonic platform.

Capacity (3): Ceramics, ceramic composites, and ceramic coatings could improve the operating temperatures of aircraft engines, leading to increased aircraft speed.

Safety and Reliability (1): This Challenge would increase safety margins for aircraft components such as nacelle insulation and burn-through shields, but the overall effect would be minor.

Efficiency and Performance (9): This Challenge would increase the operating temperatures of aircraft engines, resulting in higher efficiencies. They are an enabling technology for airframes and engines for hypersonic aircraft and would facilitate the development of commercial supersonic aircraft.

Energy and the Environment (3): High-temperature materials would enable engines to run hotter and to potentially decrease NO_x emissions.

Synergies with National and Homeland Security (3): This Challenge is relevant to military supersonic and hypersonic aircraft.

Support to Space (9): This Challenge is very relevant to access-to-space vehicles, as evidenced by the thermal protection systems developed for the space shuttle and more recent X-vehicles.

Supporting Infrastructure (9): NASA Glenn has materials expertise and unique facilities in which to test the performance of ceramic engine components under appropriate flow and pressure conditions for jet aircraft engines. It also has a full suite of materials characterization facilities, and facilities to evaluate component and material behavior in rocket engine environments. NASA Ames has unique environmental exposure facilities and arc jet tunnels capable of simulating hypersonic and reentry environments.

Mission Alignment (9): The development of high-temperature, ceramics-based systems is very relevant to NASA’s space exploration mission.

Lack of Alternative Sponsors (3): DoD continues to support relevant research. However, NASA is still positioned to lead in the development of these materials for the most extreme conditions.

Appropriate Level of Risk (9): This Challenge faces high risk.

Materials that possess multifunctional behavior combine electronic, magnetic, chemical, thermal, and mechanical properties at the macro, micro, or atomic level. These materials present unique opportunities for integrating communication, actuation, sensing, self-healing, and energy-harvesting functionalities into lightweight, load-bearing structures. Multifunctional materials enable a wide range of benefits, including improved aircraft telecommunications (wired, wireless, and optical); enhanced potential capabilities and flexibility for electronic and optoelectronic platforms, such as agile phased-array and multifunctional radar systems; structural prognosis and nondestructive evaluation; self-sensing and self-repair; camouflage and avoidance; and local power generation though energy harvesting. The use of structural elements to provide new functions to aircraft platforms increases structural efficiency and enables new aircraft capabilities.

A number of multifunctional concepts have been demonstrated, such as the conformal load-bearing antennas implemented in military vehicles. Additional work on materials capable of self-healing and local power generation has been conducted and is being integrated into

research aircraft. The first use of multifunctional materials in operational aircraft will likely occur in UAVs, where consequences of failure are low and issues such as energy harvesting are important. While the majority of research to date has been confined to coupled electromechanical domains, a much broader vision is possible. Recent discoveries of electrochromic, magnetoelectric, and thermomechanical materials show substantial promise for future multifunctional materials.

From a programmatic viewpoint, the research problems inherently involve multiple domains and require a multi-physics (electrical, magnetic, chemical, structural, thermal, and electromagnetic) approach. Additionally, facilities to fabricate and test either monolithic or composite multifunctional materials are rare. The main tasks are to develop new materials based on fundamentals or to design composite materials that couple functionality without disrupting structural performance. To date, little research has been conducted at the atomic or composite levels, with even less on fabrication and testing. Relevant research should be conducted in collaborative between universities and NASA. Key milestones include

• Develop a comprehensive analysis to predict the performance of selected monolithic and composite multifunctional materials.

• Use this analysis to guide parametric studies to explore and optimize material response with the goal of understanding the combined response of the multifunctional material.

• Fabricate materials according to model predictions.

• Evaluate material performance, both coupled and structural, and compare with analytical predictions.

• Integrate multifunctional materials into a structural component for benchtop verification.

• Conduct flight tests on a structural component.

Capacity (3): Multifunctional materials can increase the payload fraction of individual aircraft via improved structural efficiency, allowing a given airplane to carry more passengers or cargo.

Safety and Reliability (3): Multifunctional materials potentially increase aircraft safety and reliability. For example, self-healing structures provide the opportunity for in-flight repair.

Efficiency and Performance (9): Multifunctional materials can improve the efficiency of engines by evaluating the flow characteristics at or near the engine inlets and outlets and provide locally generated power by converting thermal energy into electrical energy.

Energy and the Environment (3): Multifunctional materials can harvest energy from the surrounding environment, by converting solar, mechanical, or thermal power into electrical power.

Synergies with National and Homeland Security (9): This Challenge is very relevant to remote observation, hypersonic flight, and aircraft protection (through radar absorption or cloaking).

Support to Space (9): Multifunctional materials support access to space by enabling structures with inherent sensing capabilities that can transmit data to a central location.

Supporting Infrastructure (3): NASA facilities at both the Langley and Glenn research centers are very relevant to this Challenge. Langley has developed relevant expertise through a recent morphing program. However, academia and the DoD possess similar facilities.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission to conduct fundamental aeronautics research.

Lack of Alternative Sponsors (3): DoD is interested in relevant research but not in applications to civil aviation. University-funded research in this area has demonstrated the viability of these materials but has not yet explored their relationship to aircraft systems.

Appropriate Level of Risk (9): Relevant technologies are still very immature, and this Challenge faces high risk.

Exterior and interior coatings can be designed to provide novel, yet beneficial functionality through the use of nanoscale fillers or self-assembled monolayers. Advanced coatings are attractive since they can be applied to existing structural components as an add-on technology.

This is a broad Challenge that may enable visionary concepts in aircraft design and improve structural efficiency and safety. Potential benefits include self-assembled monolayers for corrosion protection, soft polymeric coatings for noise reduction, ultrahydrophobic surfaces for drag reduction, coatings that enable aircraft to shed or melt ice without the use of deicing fluids, nanoparticle-filled coatings for wear resistance, electrically conductive coatings, and self-sensing and self-repairing surfaces. Development of novel coatings is an active research area in both academia and industry, but there is not a lot of research targeted at aeronautics or aerospace applications. Many potential systems have been demonstrated in the laboratory environment, for example, self-assembled monolayers for corrosion protection—while many others, such as deicing coatings, are under more advanced development. Key barriers for these coatings include development of nanoscale fillers with the appropriate functionality, processing and dispersion of nanoscale fillers, and

the high cost of many nanoscale fillers (e.g., carbon nanotubes). In addition, most of the coatings (e.g., self-assembled monolayers) need to scale up for use with larger structures such as an aircraft wing. Finally, most of the coatings are not yet durable enough to be used in aeronautical applications.

Novel coatings are an emerging field with significant opportunity for new materials to achieve the various functions described above. Key milestones include

• Develop more durable, environmentally stable formulations for novel coatings that can survive in an aircraft environment.

• Develop cost-effective methods of processing and applying novel coatings onto large aircraft structures.

Capacity (3): Novel coatings might reduce drag, which could increase speed and deicing or self-cleaning capabilities, which would, in turn, increase operating flexibility. In addition, these coatings might enable new aircraft concepts.

Safety and Reliability (9): Novel coatings may detect damage (e.g., color change), facilitate deicing, and protect against corrosion. In this way, they can lessen the risk of catastrophic malfunction, aircraft loss, and human injury.

Efficiency and Performance (3): These coatings can reduce drag, and they offer biomimetic functionalities that may enable new aircraft capabilities.

Energy and the Environment (3): These coatings could provide environmental protection, lead to noise reduction, and be used for harvesting energy. New corrosion-resistant coatings could replace the environmentally hazardous chromium and cadmium that is currently used.

Synergies with National and Homeland Security (1): Although the DoD sponsors similar research, civil aeronautic applications would have a different emphasis, and NASA’s research would not be applicable.

Support to Space (1): This Challenge has little impact on this Objective.

Supporting Infrastructure (3): NASA has infrastructure relevant to this Challenge but little ongoing research.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission of improving aircraft capabilities.

Lack of Alternative Sponsors (3): Novel coatings are currently being explored by industry, and there is some interest in this area from DoD, but neither places sufficient emphasis on civil aeronautics applications.

Appropriate Level of Risk (9): Basic research is being done at universities. High-risk research is necessary to mature and transition this research to aeronautic applications.

Load transfer in airframe structures is accomplished by joining discrete structural members. These joints add significant weight to the airframe, thereby reducing its efficiency. The broad classes of joining in airframe structure are mechanical fastening, adhesive bonding, and welding (for metallic structures). Significant advances have been made over the past decades in semiempirical analysis and design methods for structural joints. However, to make substantial progress, such as a 50 percent reduction in structural weight fraction, and to enable the widespread use of new joining methods, an initiative to develop a rigorous, all-encompassing simulation of performance is needed.

Joints in airframe structures are weight and cost drivers that often call for specialized treatments at the edges being joined. Improved joining technology promises significant payoffs in structural efficiency. Leakproof joints will eliminate parasitic seals in structures that must hold fluids. Efficient joints will improve airframe repairability and arrest damage propagation.

Several NASA and DoD programs have yielded test data and semiempirical models to analyze and design mechanically fastened and adhesively bonded joints. Welding of metallic structures is a well-defined and -characterized process. With the advent of ultralightweight structures using foam or honeycomb core sandwich panels, however, new mechanical fastening concepts with accompanying analysis methods are required. Friction stir welding of aluminum-lithium (Al-Li) structures needs to be modeled and a methodology for simulating complex structural arrangements needs to be developed. For adhesively bonded joints, surface preparation techniques and damage propagation arrest features need to be developed to ensure flight safety.

Currently, a rational methodology is required for adhesive surface preparation to ensure consistent, high-strength, certifiably bonded joints. A design and analysis methodology is needed for mechanically fastened joints in extreme environments along with substantiating test data. A fatigue performance and design methodology to resist cracking is required for friction stir welded joints in airframe-grade materials. Reliable, nondestructive inspection and evaluation methods are required for adhesively bonded joints. Some key milestones in advancing this technology include

• Develop certification methodology and tools for bonded joints.

• Fully characterize friction stir welding processes for Al-Li structural materials.

• Develop nondestructive strength assessment techniques for bonded joints.

• Develop modeling and simulation capabilities for mechanically fastened joints in extreme environments.

Capacity (3): Although this Challenge would have few direct effects on capacity, advanced joining technology is an enabler for new aircraft concepts, increased aircraft size, and increased operating flexibility.

Safety and Reliability (3): The ability to rigorously model, simulate, test, and verify joint concept performance translates directly into increased reliability and, hence, safety. This is especially significant for pressurized airframe structures.

Efficiency and Performance (9): Improvement in joint efficiency will increase structural efficiency and, thereby, aircraft efficiency and performance.

Energy and the Environment (1): Joining would only have a small, indirect impact on this Objective via improved structural efficiency.

Synergies with National and Homeland Security (3): This Challenge might have some impact on military aircraft.

Support to Space (3): This Challenge might have some impact on access-to-space vehicles.

Supporting Infrastructure (3): NASA Langley has some skills left over in this area, which it developed in the early 1980s in its Aircraft Energy Efficiency Program.

Mission Alignment (9): This technology is well aligned with NASA’s goals for improving airframe structural efficiency.

Lack of Alternative Sponsors (3): DoD organizations support the goals of this Challenge, although in recent years DoD has not supported relevant research.

Appropriate Level of Risk (9): Previous research addressing the very difficult task of modeling real airframe joint behavior has reduced risk to the point where this Challenge faces moderate to high risk.

Significant portions of aircraft structures will continue to be designed using metallic materials. New alloys that possess higher strength, inherently high fracture toughness, very high resistance to fatigue crack growth, and significantly improved corrosion resistance will enable much lighter, more efficient airframe structures with increased durability and reliability. It is important that these materials be developed along with the manufacturing science required to turn them into viable structural elements. Improved aluminum alloys have a history of very rapid insertion into aircraft applications if they provide significant performance and cost benefits, and can be recycled at the end of the aircraft’s life. New chemistries, an enhanced understanding of processing– microstructure–property relationships, and improvements in processing science are enabling the development of advanced aluminum alloys with lower density, higher strength, and greater stiffness. The entire metallurgy of titanium will be redone if meltless titanium processing is shown to be practical; progress to date is quite promising.

Materials modeling is just now becoming possible at the desktop, driven by new computational tools and ever-increasing desktop computer processing capability. The conventional approach for alloy development is highly sequential and typically occurs over a long period of time—a decade or more. Use of modern materials modeling tools would facilitate the development of advanced alloys, reducing time and effort by 50 percent or more. Applying modeling to guide the advancements of these materials is in the very early stages, and NASA has the opportunity to be the leader in this technology.

The ability to model complex phase fields, microstructures, and materials processing and to estimate the full gamut of materials properties will allow many alloy trials to be conducted by computer analysis. This model-based approach for designing materials will focus limited resources on the most promising new alloy candidates. Improved materials performance directly translates into higher structural efficiency and reduced product weight; reductions of up to 25 percent appear possible. Improved modeling could increase material reliability as well.

A key focus of this Challenge is developing an integrated set of physics-based models that accurately estimate material properties of new alloys, significantly accelerating the R&D of new aerostructural metals. This effort would include the generation of fundamental data, complemented by testing that simulates realistic jet engine operating conditions to validate the model.

Industry has recently made significant improvements in conventional aluminum alloys by incrementally improving chemistry and microstructural control. For example, alloy 2025-T3 has 15-20 percent better fracture toughness and twice the fatigue crack growth resistance of 2024-T3. New Al-Li alloys have lower density and higher fatigue resistance. For example, the Al-Li alloy 2097 that replaced 2124 in an F-16 bulkhead has three times better spectrum fatigue behavior, which allows approximately 5 percent higher spectrum fatigue stress. Materials modeling could leverage the results of this latest work.

In addition, the aluminum–magnesium–scandium family offers potential for high strength and outstanding corrosion resistance. Serious consideration of these alloys has been impeded by the high cost of scandium. However, a significant reduction in the price of scandium is anticipated because a very large new ore deposit and a new refining method are coming on line in Australia.

Other alloys under development are moderate-temperature, age-hardenable aluminum alloys that could retain their strength at operating temperatures up to 150°C and would therefore be useful for a Mach 2.4 aircraft. Laminated hybrids of aluminum sheet with fiber reinforcements have high fatigue resistance and

the potential for significant weight saving. The prospect of innovative chemistries that cannot be produced by conventional melting routes offers exciting new titanium materials. Meltless titanium processing has recently been demonstrated using several different processes, spurred by a current DARPA program. Advanced titanium alloys can exploit the meltless processing approach to enable very fine-grained structure with superb fatigue strength. Key milestones include

• Develop databases of the physical and mechanical properties needed for design of materials.

• Develop physics-based models that accurately estimate the properties of new alloys.

• Validate material models through alloy trials and material testing.

• Optimize new metal-processing techniques and scale them up to production size.

• Characterize new alloy families based on new alloying elements.

• Optimize and scale up processing techniques for the most promising new alloys.

Capacity (9): Lighter weight structures made from improved metallic structural materials will allow higher pay-load fractions, increasing the amount of cargo or passengers an aircraft can carry. Improved corrosion resistance and damage tolerance will reduce the time required for maintenance, improving 24/7 operational flexibility.

Safety and Reliability (1): Although these new structural alloys could provide improved reliability by enhancing corrosion resistance, fatigue strength, and inherent toughness, the focus of this Challenge will be on improved performance and efficiency rather than safety and reliability.

Efficiency and Performance (9): Better corrosion resistance directly translates into reduced maintenance and inspection costs. Better fatigue resistance will give the airframe longer life, lowering operating cost. Supersonic airplanes will greatly benefit from having affordable aluminum and titanium alloys as design alternatives to composites.

Energy and the Environment (1): This Challenge is not relevant to this Objective.

Synergies with National and Homeland Security (3): The DoD would consider using advanced airframe alloys for a variety of aeronautical systems applications, including supersonic aircraft. The range of DoD applications for advanced titanium alloys based on meltless processing would be considerable and would go beyond aeronautics. Supersonic airframes would also directly benefit from this technology.

Support to Space (1): Although these new structural materials might be considered for structural applications in access-to-space vehicles and satellite structures, these applications would require considerable additional effort for evaluation, first, and then for the development of specialized manufacturing methods.

Supporting Infrastructure (1): Within NASA, the core metallurgical expertise has not been refreshed as people left the organization and the emphasis shifted to composite airframe structural materials. Airframe alloy research can leverage the thermal structures research facility at Langley and the hypersonic materials environmental test facilities at Langley, Johnson, and Ames for evaluation of the higher temperature alloys.

Mission Alignment (3): Although NASA has contributed significantly to this field, it has since been adopted by industry, and the research is less aligned with NASA’s mission.

Lack of Alternative Sponsors (1): If this work were not undertaken by NASA, it would be done by industry and by DoD. The former is already leading the development of new chemistries and processing for advanced airframe alloys.

Appropriate Level of Risk (9): This Challenge faces high risk.

NDE is an interdisciplinary field that is concerned with the development of analysis techniques and measurement technologies for the quantitative characterization of materials and structures by noninvasive means. Ultrasonic, radiographic, thermographic, electromagnetic, and optical methods are employed to probe interior microstructure and characterize subsurface features. Currently available NDE instruments are compact, rugged, and can acquire large amounts of wide-area multiphysics data via sensor arrays. Recent better-than-Moore’s-law increases in computational hardware capabilities allow these data sets to be processed with compact, rugged, and inexpensive computers. To advance NDE capabilities beyond the paradigm of rendering high-quality imagery for humans to interpret, the missing piece, more and more often, is the understanding necessary to create multiphysics algorithms that would allow the enormously rich data sets to be automatically interpreted.

The primary goal of next-generation NDE is, therefore, to develop this enabling understanding and algorithms. In the short term, the goal is to create artificial intelligence that can provide a backup assessment, as is now done in x-ray mammography. In the medium term, the primary task of NDE technicians will be to transport and set up instruments; NDE measurements and interpretation will be fully automatic. In the long term, the instrumentation could be robotic, so that NDE inspections as well as interpretation would be automated.

Next-generation NDE improves safety and reliability by minimizing manufacturing defects and identifying in-service

flaws before they cause malfunctions. It enables 24/7 operation by minimizing downtime due to faults and has the potential to make just-in-time maintenance feasible. Most next-generation NDE technologies will find application in DoD and access-to-space vehicles, as well as in a variety of important nonaerospace industries. Next-generation NDE is synergistic and closely allied with IVHM research; NDE would provide inputs to prognosis and life-prediction systems.

The hardware that acquires the NDE data is becoming less and less interesting from a scientific perspective, and it is not the appropriate focus for NASA. At the same time, brute force computer image processing, rendering, and visualization is not the focus either. The human visual system is set up to deal with surfaces, not volumes of data, so it can be argued that existing NDE systems already generate data and imagery in quantities that strain or exceed human limits. Accordingly, current work is directed at bringing an understanding of the instrumentation and measurement together with sophisticated, multiphysics models of the probing energy–material interaction that is taking place.

NDE for complex materials and structures includes developing and fusing multiple sensor techniques that provide orthogonal information on the state of a material or structure, as well as improving data reduction techniques for quantitatively mapping measurements to accurately characterize material and structural integrity. Computational NDE involves developing and validating accurate multiphysics simulations to reduce the cost of optimization and automation of NDE techniques. Autonomous NDE involves the development of techniques that accurately characterize materials and structures (including damage) with minimal or no human interaction, including techniques that are self-calibrating and methodologies for assuring proper instrument setup. Key milestones include

• Demonstrate successful, real-time, fully automatic interpretation for various individual NDE techniques in targeted families of applications in the laboratory.

• Fuse multiple orthogonal NDE techniques.

• Adapt laboratory-tested NDE techniques to field-portable configurations that can be demonstrated outside of the laboratory.

• Implement large-area, multiphysics NDE techniques and instrumentation robotically.

• Transfer techniques and algorithms to IVHM efforts, including optimization of the output of next-generation NDE technologies for input to research on prognosis and life prediction.

Capacity (3): Current NDE relates directly to enabling 24/7 operation by reducing downtime due to faults. Next-generation NDE will be faster and more automated, and it will help to enable just-in-time maintenance.

Safety and Reliability (9): NDE directly supports safety and reliability by minimizing manufacturing defects and identifying in-service flaws before they cause a malfunction. Next-generation NDE is synergistic and closely allied with IVHM efforts and provides input to prognosis and life-prediction systems.

Efficiency and Performance (1): Although next-generation NDE imparts enough confidence to permit adopting new material systems and structural concepts as well as to operate safely closer to performance margins, the improvement over current NDE will have less of an impact on this Objective than other technologies.

Energy and the Environment (1): This Challenge is not relevant to this Objective.

Synergies with National and Homeland Security (3): Some NDE technologies are applicable to security screening areas, in particular the automated real-time interpretation of multiphysics measurements. Most next-generation NDE technologies will also find application in DoD aircraft and other vehicles, with moderate impact expected.

Support to Space (1): Most next-generation NDE technologies will find some application in access-to-space vehicles, but the impact will be minimal because the small number of reusable vehicles means that most NDE applications for space will be for manufacturing quality control and related issues prior to flight.

Supporting Infrastructure (3): NDE equipment and research facilities tend to be compact, relatively inexpensive, and broadly available, but NASA has some staff with unique expertise.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission in terms of (1) aviation safety (especially aging aircraft) and (2) facilitating development and insertion of new materials, structures, and processes by ensuring that they are manufactured according to specifications and are behaving as designed as they are put into service.

Lack of Alternative Sponsors (1): DoD and nonaerospace industries are supporting NDE research. Focusing on automated interpretation of multiphysics NDE measurement data would provide a niche where NASA could collaborate with academia without duplicating work by others.

Appropriate Level of Risk (3): Given that this Challenge encompasses many tasks that are standard industrial practice now, it faces low risk.

Improved aviation security requires that commercial aircraft be hardened against safety threats such as explosions and biological or chemical agents. Effective solutions will encompass detection, avoidance, and impact minimization of threats. The large number of potential threats and path-

ways for delivery dictates the development of multiple and varied technologies, ranging from sensors for threat identification and structural health monitoring to the development of highly durable blast containment systems and methods to accurately model structural damage due to blast events. Successfully hardening commercial aircraft increases their safety and reliability and contributes to on-ground safety by minimizing the use of aircraft as weapons.

Biological and chemical threat detection sensors and self-decontaminating coatings are in the early development stages. Hardening technologies developed for military aircraft such as fuel inerting and nonreflective, infrared-absorbing paint for signature reduction remain largely unimplemented for commercial flight because of their weight and/or cost. These considerations have also limited the use of blast containment technologies in commercial aircraft even though there has been a significant effort by the FAA and commercial companies to develop blast-resistant luggage containers, also known as hardened unit-loading devices.

Various schemes for improving the energy absorption capacity of the luggage containers have been investigated, including incorporation of honeycomb or foam elements. In addition, instead of using aluminum or fiberglass as the primary structural material, new containers could use (1) composites reinforced with fibers developed specifically for ballistic armor applications (e.g., Kevlar, Spectra, and Zylon) or (2) hybrid systems (e.g., a laminate composed of fiberglass and aluminum foil). FAA-certified designs have been developed and verified through blast testing. Inspection, maintenance, and repair methodologies, however, have not been adequately addressed. Similar concepts are currently under development for hardening overhead bin compartments.

Many of the impediments to incorporating threat-hardening technology in commercial aircraft stem from the constraints imposed by retrofitting existing aircraft and the difficulty of analyzing rapidly loaded, complex structures. The cost and weight of hardening existing aircraft is prohibitive. New aircraft designs will more efficiently incorporate features such as fuel protection filters; integrated threat detection (including biometric identification); health monitoring sensors; and highly durable, fire-resistant composite structures. Key milestones include

• Analyze, design, and test an optimized blast-resistant luggage container.

• Develop and validate accurate damage prediction models for blast events including shock overpressure and crack propagation due to hull pressurization.

• Integrate onboard biological and chemical sensors.

• Model and develop self-decontaminating coatings.

Capacity (1): Hardening of aircraft can reduce vehicle losses, but the weight increase associated with blast hardening in particular would decrease capacity.

Safety and Reliability (9): Hardening is a key strategy for improving aircraft safety by providing protection from onboard explosions and improving threat assessment.

Efficiency and Performance (1): Hardening would likely have a nominal or negative impact on this Objective due to increased weight.

Energy and the Environment (1): Hardening would likely increase structural weight and possibly volume, which could indirectly increase noise and emissions.

Synergies with National and Homeland Security (9): This Challenge is very relevant to DHS and DoD missions.

Support to Space (1): This Challenge has no impact on this Objective.

Supporting Infrastructure (3): NASA supported relevant research under its Aviation Safety and Security Program, where the topics included control systems to detect and compensate for vehicle damage; fuel protection; fire-resistant, damage-tolerant composites; and sensing of onboard chemical and biological contamination.

Mission Alignment (3): Although NASA’s 2003 Strategic Plan specifically discusses the need for NASA to “aggressively apply our expertise and technologies to improve homeland security,” this Challenge is more closely related to the mission of DHS.

Lack of Alternative Sponsors (1): The FAA, DHS, DoD, and industry are all sponsoring research for hardening technology.

Appropriate Level of Risk (9): This Challenge faces moderate to high risk.

Multiscale and multiphysics modeling and simulation encompass computational modeling of interdisciplinary systems at multiple spatial and temporal scales (e.g., by finite-element methods, molecular dynamics, and ab initio methods). With advances in the understanding of material and system behavior at multiple spatial scales (from atomistic to continuum) and time scales (from the period of atomic vibration to structural lifetime) when subjected to multiple physical stimuli (mechanical, thermal, electromagnetic, chemical), the promise of designing new materials based on atomic characteristics is emerging. Given a specific requirement (e.g., strength, stiffness, or piezoelectric coefficient), it may become possible to design a new material from the atomic level up that will meet the requirement, replacing the

make-it-and-break-it approach currently used to investigate new material systems.

Multiscale and multiphysics modeling has been examined for several years, but is still in its infancy. With recent advances in computational capabilities, there has been renewed interest in multiscale and multiphysics modeling. DoD has invested in this field through various programs, including the Multidisciplinary University Research Initiatives (MURIs), Materials Engineering for Affordable New Systems (MEANS) grants, and DARPA programs. Multiscale, multiphysics modeling has been identified as an integral component of the National Nanotechnology Initiative, which promises to invest close to $1 trillion per year in product development over the next 10 years. A number of leading universities are also beginning work in this area.

From a programmatic viewpoint, the research problems are inherently twofold. Overcoming this Challenge requires researchers familiar with multiple physical phenomona (mechanical, thermal, electromagnetic, chemical). In addition, it requires facilities with high-end computer facilities for storing, processing, and managing large data sets. Substantial computer power is required to construct even small-scale simulations of materials. Complex systems require tens to hundreds of simulations, requiring high-performance computing support (in general, hundreds to thousands of processors) to complete the simulations in a reasonable time frame. Similarly, each simulation can generate terabytes to peta-bytes of data. Thus, state-of-the-art visualization, data mining, and data analysis techniques are also critical to the success of this Challenge. Key milestones include

• Select an aeronautics-related material test problem.

• Develop multiscale, multiphysics modeling software representative of the selected test problem.

• Procure necessary computer facilities for the modeling effort.

• Measure critical parameters necessary for formulating the models, including measurements made using electron-based optics with the ability to perform energy dispersive spectroscopy or electron energy loss spectroscopy.

• Complete a multiscale, multiphysics analysis of the selected test problem.

• Validate modeling by comparing the results to those of an experimental development program.

Capacity (3): Multiscale and multiphysics modeling will enable the development of revolutionary new material systems. These new materials will be lighter, stiffer, and stronger than current material systems, leading to increased payload fractions, so that an aircraft of given size will be able to carry more passengers or cargo.

Safety and Reliability (3): Multiscale and multiphysics modeling will allow a complete understanding of the manner in which materials fail. This will improve safety and reliability by improving the ability to predict and account for structural failures.

Efficiency and Performance (3): Multiscale and multiphysics modeling will lead to new, possibly more efficient materials for aircraft propulsion systems.

Energy and the Environment (3): Multiscale and multiphysics modeling will improve the understanding of acoustic dampening properties and interactions between airflow and structures. These improvements will aide in the development of quieter aircraft.

Synergies with National and Homeland Security (3): Multiscale and multiphysics modeling is going on throughout DoD—for example, to create new materials that are immune to radiation effects for use in nuclear reactors. Nationally, much of the research relevant to this Challenge (including DoD-funded research) is conducted by universities, and it is still at a low level of technology readiness. Therefore, the impact on national security, at least in the near term, will be limited.

Support to Space (1): Multiscale and multiphysics models applicable to civil aeronautics focus primarily on interactions between mechanical, material, and flow phenomena. Since high temperatures typical of space reentry vehicles are a minor consideration, most of the research relevant to this Challenge would not be applicable to space applications.

Supporting Infrastructure (3): Facilities and personnel at NASA’s Langley and Glenn Research Centers are supporting research relevant to this Challenge. Superior facilities and personnel may exist at many universities, however.

Mission Alignment (3): This Challenge is relevant to NASA’s mission. However, the technology is in the early stages of development, and current research is rather generic. As these materials are advanced to the point where specialized research with a focus in aeronautical applications becomes necessary, the Challenge will become more relevant to NASA’s mission.

Lack of Alternative Sponsors (3): DoD, DOE, and some universities are conducting research relevant to this Challenge. However, without NASA’s attention, this research may never be used to develop materials and structure necessary for civilian aeronautical applications. Additionally, NASA’s expertise in multidisciplinary design and optimization means that NASA is well qualified to implement much of the work done elsewhere.

Appropriate Level of Risk (3): Relevant technology is currently at a very early stage of development; this Challenge faces very high risk.

The current state of the art in lightweight airframe structures is demonstrated by the Boeing 787, which makes extensive use of structural composites. Lightweight structures enable aircraft with longer range, more fuel efficiency, greater payload, and/or lower operating costs. Ultralightweight airframe designs would increase these payoffs.

Ultralight structures programs have been characterized by highly innovative concepts that test the boundaries of the possible. The Gossamer Condor demonstrated human-powered flight. Several NASA and DoD ultralight airframe programs have produced prototype high-altitude, long-endurance UAVs, such as Helios, which demonstrated solar-powered flight. Typically these ultralightweight aircraft have been point-designed for specific flight conditions. To achieve their objectives, they disregarded usual aircraft design practices such as minimum skin thicknesses, redundant load paths, and damage-tolerant design criteria. The designs demonstrated in these programs are not robust enough to meet the strict certification requirements for commercial aircraft. However, they do motivate pragmatic adaptations of ultralightweight structural concepts suitable for commercial application. Promising concepts for ultralight airframes include the use of foam or honeycomb core-stiffened structures with integral, durable damage arrest features; high-performance fibers for increased strength; directional tailoring and unitized construction; and structural optimization methods. Including adaptive materials for variable camber morphing wings may reduce control surface weight. Embedding multifunctional technologies, such as integral antennas or new flexible polymer solar cells, could reduce subsystem weights. The details of many of these concepts have been discussed in other, more highly ranked R&T Challenges. A new initiative is needed to integrate these concepts and the associated design and analysis technologies, along with the substantiating test data, to enable robust, ultralightweight airframe structures for commercial transport applications. Key milestones include

• Develop specific ultralightweight airframe concepts, leveraging lessons learned from experimental ultralight aircraft research to develop damage-tolerant, adaptive, and multifunctional materials.

• Develop a design and analysis methodology.

• Develop a structural optimization methodology.

• Perform verification testing.

• Demonstrate the potential of one or more concepts to reduce airframe weight by 40 percent.

Capacity (3): Reducing airframe weight can increase payload fraction, meaning that an aircraft of a given size will be able to carry more passengers or cargo. Ultralight airframes may enable new aircraft concepts and increased operating flexibility.

Safety and Reliability (1): This Challenge is not relevant to this objective.

Efficiency and Performance (3): Reducing airframe weight will help reduce aircraft operating costs and increase structural efficiency and performance.

Energy and the Environment (1): This Challenge would only have a small, indirect impact on this Objective via improved structural efficiency.

Synergies with National and Homeland Security (3): Some aspects of this technology, such as structures for micro-UAVs and long-endurance surveillance aircraft, are of great interest to the DoD and DHS.

Support to Space (3): This Challenge is relevant to the design of space structures.

Supporting Infrastructure (3): NASA Langley has some relevant skills left over from the Advanced Composites Technology Program of the 1980s.

Mission Alignment (9): This Challenge is well-aligned with NASA’s improved airframe efficiency goals.

Lack of Alternative Sponsors (1): DoD organizations support this goal, although in recent years there has been no investment in this area. Private entrepreneurs’ investment in innovative ultralightweight concepts is frequently motivated by setting new aviation performance records (speed, altitude, endurance, etc.).

Appropriate Level of Risk (3): Incremental research to reduce the weight of airframe structures has been under way for a long time. Therefore, this Challenge faces low risk.

Polymers that adapt their properties or alter their form in response to a change in their environment are known as functional or stimuli-responsive materials. Polymeric materials have demonstrated many responses coupled to a wide range of stimuli (temperature, pH, ionic strength, electrical potentials, and light). These polymers can provide unique functionality of great benefit to aeronautical and aerospace applications. They hold particular potential for achieving biomimetic functionality and sensing.

Potential benefits from this broad-based technology include new sensing capabilities, self-healing polymers for passive repair of damage, reversible liquid crystal adhesives, phase changing polymeric materials for managing interior temperatures, superabsorbent polymers for fire retardation, nanocomposite dispersions providing longer life and resistance to dirt (e.g., self-cleaning), ionic polymers for actuation, color change or other reaction to stress or an environmental threat, conductive polymers, and materials with energy-harvesting capabilities. Development of advanced

functional polymers is an active research area in both academia and industry. However, many current applications are not necessarily targeted for aeronautics or aerospace. Many potential functionalities (e.g., self-healing and self-cleaning) have been demonstrated in the laboratory environment, but only in small samples have been generated. Field testing remains to be done. There is also a need to improve the durability and environmental stability of these materials so they can survive at very high or very low temperatures and in corrosive conditions. It would also be helpful to reduce the need for expensive catalysts or other additives, which many advanced functional polymers require.

Functional polymers are an emerging field with significant opportunity for synthesis and characterization of new polymers to achieve the varied applications described above. Key milestones include

• Demonstrate cost-effective methods for processing larger quantities of functional polymers.

• Transition new functional polymers from research laboratories to the sizes needed for aircraft component testing.

• Develop more robust, environmentally stable formulations that can survive in aircraft environments.

Capacity (1): Advanced functional polymers may increase operating flexibility, but the impact on aircraft size, new vehicle concepts, and speed will not be significant.

Safety and Reliability (3): Advanced functional polymers can enable self-repair, improve damage detection and fire retardation, and simplify maintenance.

Efficiency and Performance (1): This Challenge has no impact on this Objective.

Energy and the Environment (1): Advanced functional polymers may provide some energy-harvesting capabilities, but the impact on noise, emissions, environmental hazards, and development of alternative fuels is minimal.

Synergies with National and Homeland Security (3): Chameleon-like functionality, self-assessment, self-repair, and autonomous functions are of interest to DoD and DHS, which also fund research relevant to this Challenge.

Support to Space (1): This Challenge has no impact on this Objective.

Supporting Infrastructure (9): NASA Langley Research Center has unique, relevant infrastructure for polymer research.

Mission Alignment (3): Development of functional polymers for aeronautics and aerospace applications is aligned with the NASA mission of increasing aircraft performance. However, the technology is in the early stages of development and current research is rather generic. As these materials are advanced to the point where specialized research with a focus in aeronautical applications becomes necessary, the Challenge will move into closer alignment with NASA’s mission.

Lack of Alternative Sponsors (3): DoD sponsors work in this broad area, but the focus is on military applications.

Appropriate Level of Risk (3): At this stage, with basic research mainly done at universities, this Challenge faces low risk. However, when the technology advances to the point where research results are ready to be transitioned to aeronautical applications, the risk will increase.

Engine nacelles and pylons are critical portions of aircraft structure. Nacelles enclose the jet engine and pylons provide means for mounting the engine on the airframe. The front portion of a nacelle also directs air into the engine inlet and thus affects performance of the engine. For large commercial airplanes, nacelle designs have not appreciably advanced for a generation. Consequently, nacelle structures have not taken advantage of recent developments in materials and structures technology. The result is nacelles that weigh more than they should, which decreases range, payload, and airframe fatigue life.

New structural concepts take advantage of modern analytic design tools and advanced structural materials to significantly reduce the weight while maintaining structural integrity and improving engine efficiency.

Nacelles and pylons are critical structures that affect airworthiness, so that any change from current practice must be well understood, analyzed, and validated to assure that it gives at least the same level of safety as existing structures, which have served well in many airplane applications. Key milestones include

• Define the attributes of new design concepts for nacelles that reduce weight and engine noise based on input from large and small aircraft manufacturers as well as NASA experts.

• Perform multidisciplinary design analysis to identify new structural concepts for both large and small airplanes.

• Validate the analysis via testing of subscale models of these new concepts.

• Test design at full scale.

Capacity (3): Lighter weight nacelles would allow a higher payload fraction, meaning that an aircraft of a given size would be able to carry more passengers or cargo. However, this particular Challenge may have more overall benefit for small airplanes than for large ones.

Safety and Reliability (1): This Challenge would ensure that pylons and nacelles are as safe and reliable as current

systems, but there would be no net improvement for aircraft or the airspace system as a whole.

Efficiency and Performance (3): This technology will leverage advanced analytic tools and advanced structural materials to significantly reduce the weight of these structures. As with capacity, effects may be more pronounced for smaller aircraft.

Energy and the Environment (1): This Challenge would offer little reduction in community noise.

Synergies with National and Homeland Security (1): DoD may gain the advantage of this technology once it becomes standard practice in the aeronautical industry. However, it is not likely to be active in advancing this Challenge. Furthermore, as noted before, larger aircraft, such as the tankers and transports the DoD would be interested in, would benefit less.

Support to Space (1): This Challenge has no impact on this Objective.

Supporting Infrastructure (1): NASA seems to have little or no infrastructure relevant to this Challenge.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission to improve aircraft performance.

Lack of Alternative Sponsors (1): Although there is currently no known effort within government or industry, this is the sort of research that should be pursued by original equipment manufacturers. If they feel the payoff is great enough, they will pursue the technology themselves.

Appropriate Level of Risk (3): Although there is currently no known effort within government or industry, this is the sort of research that manufacturers should support. There, too, if they conclude the payoff is high enough, they will pursue the technology themselves.

Modern airframes, whether composite or metallic, require repairs either to restore functionality or to extend their lives. To assess the repairability of structures and make correct repair-or-replace decisions, structural assessment methods and tools, tools for trade-off analyses, and repair technologies and processes are required. These methods, technologies, tools, processes, and analyses must be applicable to metallic, polymer composite, and ceramic composite structural elements.

The primary benefit of being able to assess repairability and repair airframe parts instead of replacing them is lower direct operating costs. Life extension via repair of aging aircraft also has a significant economic impact in the form of lower acquisition costs.

Several NASA and DoD programs and manufacturers’ repair manuals for aircraft owners and operators provide up-to-date tools and processes for making airframe repairs. The DoD has published a repair design guide for combat and transport aircraft. However, new algorithms are needed that incorporate validated analysis of crack growth criticality in metals, defect or damage propagation in composites, and mathematical models for stress corrosion cracking. Further, these algorithms need to be integrated with repair integrity evaluation analyses to provide a complete modeling and simulation capability that assesses the economic effectiveness of the repair method.

Fatigue and fracture analysis and databases for metals and composites used in airframe construction need to be developed, along with software enabling complete modeling and simulation of repairs, including cost considerations, to identify realistic trade-off alternatives. Key milestones include

• Conduct damage and damage growth analyses for metallic and composite structures.

• Collect a compendium of repair processes for metallic and composite structures.

• Demonstrate computer codes to model and simulate repairs for decision making.

Capacity (3): Effective airframe repairs will increase operating aircraft availability.

Safety and Reliability (3): Improved quality of airframe structure repairs will reduce the likelihood of loss or human injury.

Efficiency and Performance (3): This Challenge reduces aircraft operating costs and postpones the need for new aircraft purchases.

Energy and the Environment (1): This Challenge would only have a small, indirect impact on this Objective by extending structural life.

Synergies with National and Homeland Security (1): The focus of this Challenge would be on the needs of civil aircraft.

Support to Space (1): This Challenge focuses on repairability issues for air-breathing aircraft and would likely have little relevance for access-to-space vehicles, which operate in extreme environments.

Supporting Infrastructure (3): NASA Langley has some capability relevant to this Challenge as a legacy of several fatigue and damage propagation research programs related to metallic and composite airframe structures during the 1970s and 1980s. These skills would need to be augmented and updated.

Mission Alignment (3): This Challenge is aligned with NASA’s mission to improve aeronautical technology, but it

addresses issues with operational aircraft that are also industry’s responsibility.

Lack of Alternative Sponsors (1): This technology should be undertaken by industry; in the past, DoD has supported this goal as well.

Appropriate Level of Risk (3): This Challenge faces low to moderate risk.

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