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2 Lightweighting Airborne Vehicles 2.1 CURRENT STATE OF LIGHTWEIGHTING IMPLEMENTATION AND METRICS 2.1.1 Drivers of Lightweighting At the highest level, all aircraft designs are driven by performance, cost, and risk. The factors that affect each driver are these: • Performance: thrust, weight, lift, and drag. • Cost: research, development, test and evaluation (RDT&E); manufacturing; certification and qualification; and operations and support (O&S). • Risk: safety, resource availability, technical maturity, and schedule. Weight has been an important consideration in military and commercial aircraft design since the beginning of manned flight. It affects directly the amount of lift required to fly, which in turn affects the drag on the aircraft and therefore the thrust required to achieve the desired performance. Weight also has indirect impacts on the cost of the aircraft. The importance of lightweighting for military aircraft depends on aircraft type. Table 2-1 relates the attributes shown previously in Table 1-1 to various military aircraft types. The three main attributes (functional capabilities, operational capabilities, and survivability) correspond roughly to the three parameters driving aircraft design as defined above (performance, cost, and risk), with some overlap. Trainers, fighters, and attack aircraft are included in the “fighter” category; primary military vehicles, tankers, and transports are included in “transports.” “Primary” and “secondary” assess the relative importance of these capabilities for aircraft design, and the numbers 1 through 3 rank the importance of the attribute for the aircraft type. Military aircraft are driven by performance features (functional capabilities), which are strongly affected by weight. The O&S attributes of fuel consumption, maintainability, and the like are still primary design drivers for most aircraft and are related to weight in a lesser sense by virtue of fuel consumption and efficiency of the design. Thus, weight considerations are central to aircraft design. Survivability is a design parameter for all military aircraft but a primary driver only for fighters, which some - times depend on low observability to get to the target. It is not so important for unmanned aerial vehicles (UAVs) and remotely piloted aircraft (RPA) because, at least to date, they are not being used for first-strike operations. 29
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30 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES TABLE 2-1 Lightweighting Attributes for Military Aircraft Systems Capability Aircraft Type Summary Transports Aircraft General and Commercial (Tactical and Attributes Specific Attributes Fighters Bombers Helicopters RPA / UAVs Transports Transport) Performance Speed Primary Primary Primary Primary Primary Primary Maneuverability 1 1 1 1 2 1 Payload Range Effectiveness Operational Fuel Consumption Secondary Primary Primary Secondary Primary Primary Supportability Maintainability 3 2 2 2 1 2 Durability Reliability Repairability Survivability Ballistic Impact Primary Secondary Secondary Secondary Secondary Secondary Explosion 2 3 3 3 3 3 Damage Tolerance Observability NOTE: RPA, remotely piloted aircraft; UAV, unmanned aerial vehicle. However, at least two trends are forcing UAVs to become more reliable. When these aircraft begin to fly over populated areas, the potential for damage and injury to personnel on the ground when such aircraft fail in flight will be a concern. Moreover, the suite of sensors they carry is becoming increasingly expensive. UAV designers will need to begin paying as much attention to risk as designers of manned aircraft do. This risk tolerance must be traded off against the performance requirements for vehicles such as high-altitude, long-endurance (HALE) UAVs, where risk increases if design margins are reduced to achieve the lowest possible density, but flight over populated areas drives a desire for reduced risk. Generally speaking, lightweighting of military aircraft will therefore need to be done with an eye to retaining or improving survivability. 2.1.2 Historical and Current Lightweighting Early work on composite and hybrid material systems in transport aircraft attempted to match their proper- ties and design methods to those of aluminum. The results were nicknamed “black aluminum” structures, which ended up sacrificing many of the favorable characteristics of composites that had led to their adoption in the first place. For example, incorporating composites into structures originally designed for aluminum where transverse and shear stiffnesses had to be maintained meant that the tailored stiffness in bending of the composites could not be put to use. Rotorcraft have also taken advantage of lightweight materials and structural concepts. Lightweight components for the engine and transmission housings have been studied but are not seeing widespread use today; however, lighter-weight materials and designs are finding their way into the airframe and the rotors of advanced rotorcraft. 1 Composites with integral stiffening were examined in the NASA/Army-sponsored Rotary Wing Structures Technol- 1 J.K.Sen and C.C. Dremann. 1985. “Design Development Tests for Composite Crashworthy Helicopter. Fuselage,” SAMPE Quarterly, Vol. 17, No. 1, October, pp. 29-39.
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31 LIGHTWEIGHTING AIRBORNE VEHICLES Figure 2-1.eps FIGURE 2-1 Composites in U.S. Fighter aircraft. SOURCE: C.E. Harris and M.A. Shuart. 2004. An Assessment of the State- of-the-Art in the Design and Manufacturing of Large Composite Structures for Aerospace Vehicles, NASA-Langley. Available bitmap at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040086015_2004090422.pdf. ogy Demonstration (RWSTD) program.2 More recently, under Boeing funding3 composite rotor blades have been developed that offer significant cost savings compared with those previously developed for the AH-64 Apache attack helicopter. Rotorcraft development organizations have been among the first to develop and employ health monitoring systems as a means to ensure the integrity of advanced composite systems. 4 From 1970 to 2000, designers began to take advantage of the properties of resin matrix composites, which were introduced sequentially into the skins of the empennage (rear part or tail assembly), wings, and fuselages of military aircraft, as shown in Figure 2-1. After their successful implementation in military aircraft, these materi - als began to be introduced into commercial aircraft, as shown in Figure 2-2. Until recently, however, the cost of composite materials limited their application by commercial aircraft. Furthermore, the fuel savings that resulted from lighter weight were not sufficient to overcome the cost of the materials and the manufacturing processes. In the 1990s, manufacturing technologies for composite materials became more capable, less costly, and more pervasive, allowing production of parts around the world. At the same time, higher actual and predicted fuel prices made composites increasingly desirable for commercial aircraft. By the time the Boeing 787 was being developed, the costs of design, production, and operation allowed much greater use of composites in the airframe. This was aided by the use of advanced physics-based modeling and simulation for design, development and manufacturing. 5 As materials analysis and fabrication methods continue to improve, composite materials are being employed extensively, not just for lightweighting but also to improve impact resistance and resistance to fire, damage, light - ning strikes, ultraviolet (UV) degradation, moisture, and thermal degradation. Over time, experience has led to the preference for certain resin systems and fiber-sizing materials. Repair methods have been developed for composite structures and have been successfully used in both commercial and military applications. 2 Shawn M. Walsh and Bruce K. Fink. 2001. “Achieving Low Cost Composite Processes through Intelligent Design and Control.” Presented at the RTO AVT Specialists’ Meeting, Low Cost Composite Structures, Loen, Norway. May 7-11. Published in RTO-MP-069(II). 3 Jian Li, P.H. Jouin, and A.S. Llanos. 2010. “Durability and Damage Tolerance Enhancement Feature and Life Prediction Methodology for the Apache Composite Main Rotor Blade (CMRB) Root-end Fitting,” American Helicopter Society 66th Annual Forum, Phoenix, Ariz., May 11-13. 4 Michael L. Basehore and William Dickson. 1998. “HUMS Loads Monitoring and Damage Tolerance: An Operational Evaluation.” Presented at the NATO RTO AVT Specialists Meeting, Exploitation of Structural Loads/Health Data for Reduced Life Cycle Costs, Brussels, Belgium, May 11-12. Published in RTO MP-7. 5 Göran Fernlund. 2008. “Reduction of Risk and Uncertainty in Composites Processing Using Process Modeling and Bayesian Statis - tics.” Presented at the 13th European Conference on Composite Materials, Stockholm Sweden, June. Available at http://www.escm.eu.org/ ECCM13_broschure.pdf.
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32 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-2 Composites in commercial transport aircraft. SOURCE: Charles E. Harris, James H. Starnes, Jr., and Mark J. Shuart. 2001. An Assessment of the State-of-the-Art Figure 2-2.eps in the Design and Manufacturing of Large Composite Structures for Aero - bitmap space Vehicles. NASA-Langley. Available at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040086015_2004090422.pdf. 2.1.3 Current State of Metrics Until recently, weight was used directly in calculating aircraft costs at the conceptual design stage. Military and commercial data dating back to 1935 had been used to develop and refine an understanding of the relationship between cost and weight for “built-up” aluminum structures. Translating weight into cost worked well until about 1970, when composite structures began to see greater application in military aircraft. 6 Composite materials offer great potential for weight savings at the same time that they lead to improved per- formance for military aircraft and lower operating costs for commercial aircraft. However, metrics are not available to compare those benefits with the significantly increased production costs. The aluminum-based cost models are not valid for composites, and the uncertainty surrounding costs became greater in the late 1990s, when stealth materials were introduced in military aircraft such as the B-2 (Figure 2-3) to reduce radar signatures. The higher costs of composites had less to do with raw material costs than with the need to fabricate parts in expensive autoclaves using tooling that was unique for each part and each design. Projecting the costs of aircraft that incorporated composite materials meant developing very detailed cost models that incorporated the cost of tooling and layup and reflected the complexity of the parts and of their fabrication. It has taken a long time for such tools to become standardized in the aerospace industry, and they still are not as well validated as the cost models for metallic structures had been.7,8 The assessment of risk is a crucial metric for advanced technology. Risk drives cost and schedule as program managers attempt to achieve the weight and cost advantages of advanced technologies while reducing the risk 6 K. Zhou, C. Radcliff, T. Lenzm, and J. Stricklen. 1999. “A Problem Solving Architecture for Virtual Prototyping in Metal to Polymer Composite Redesign,” Proceedings of the DETC’99: 1999 ASME Design Automation Conference. September 12-15, Las Vegas, Nev. 7 S.A. Reseter, J.C. Rogers, and R.W. Hess. 1991. “Advanced Airframe Structural Materials: A Primer and Cost Estimating Methodology.” R-4016-AF. RAND. 8 Han P. Bao, 2002. “Process Cost Modeling for Multidisciplinary Design Optimization.” NASA Grant NAG-1-2195. Norfolk, Va.: Old Dominion University. June.
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33 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-3 Composite applications for the B-2 highlighted the need for new validated cost models. SOURCE: Available at Figure 2-3.eps http://www.aviationexplorer.com/Stealth_Principles_What_Makes_Stealth_Aircraft_Work.html. bitmap that technology maturation will lag, thus delaying the schedule and increasing the costs of implementation. While the relationship has been understood for a long time, only recently have tools become available that allow risk to have the same visibility as cost and performance (weight) in the manager’s evaluations. 2.2 BARRIERS AND KEYS TO SUCCESS Because aerospace applications, especially vertical lift vehicles like rockets and helicopters, can justify higher costs for materials that reduce their weight, cost is not as great a barrier as it is for land and sea applications. Developers will not seek to achieve lower weight without appropriate cost and risk assessments, but the more serious barriers to lightweighting in aircraft relate to technology and management. These barriers include the need for (1) new materials; (2) new manufacturing processes and equipment; (3) systems engineering approaches to handling the multiple demands placed on materials and structures by aircraft applications; (4) more rapid insertion processes that include advanced physics-based modeling and reduce the test burden; and (5) less risky transition methods that account for all the requirements of the new air vehicles. 2.2.1 Timelines for Materials Development The development of composite material and of its associated manufacture take much longer than the design cycle for new systems.9 As a result, manufacturers often use existing materials and manufacturing systems for which data exist rather than emerging materials and manufacturing systems that are promising but unproven. 9 C.R. Saff, G.D. Hahn, J.M. Griffith, R.L. Ingle, and K.M. Nelson. 2005. “Accelerated Insertion of Materials—Composites.” 46th AIAA/ ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference. April.
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34 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-4 MRL relationships. SOURCE: Jim Morgan. 2006. “Manufacturing Readiness Levels (MRLs) for Multi-Dimen - sional Assessment of Technology Maturity,” presented at the Air Force Research Laboratory Seminar/Workshop on Multi- Dimensional Assessment of Technology Maturity. Figure 2-4.eps 9-11. Available at http://www.dtic.mil/cgi-bin/GetTR Fairborn, Ohio. May Doc?Location=U2&doc=GetTRDoc.pdf&AD=ADA507087. bitmap A material is considered application-ready at technology readiness level (TRL) 6, 10 when manufacturing scale-up and fabrication trials are complete and typical design values are in hand. The TRL scale measures the maturity of a technology’s performance. The corresponding manufacturing scale of maturity is the manufacturing readiness level (MRL).11 MRLs were developed to assess the manufacturing maturity of a technology or prod - uct and the plans for its future maturation; to provide a common language to convey risk; and to understand the manufacturing risk associated with producing a weapon system or transitioning a technology into a weapon system application. The relationship between TRLs and MRLs and system acquisition milestones is shown in Figure 2-4. System manufacturers can begin to consider a material during the conceptual and preliminary design phases, but once the product goes to detailed design, the materials must be locked in for that design unless special provisions are made to keep the door open. Even that must be curtailed at the engineering and manufacturing development stage, when testing for “allowables”12 is done to develop data for materials in their as-fabricated condition for final design. (See the section “Materials Properties and Testing” in Chapter 1.) Strong demand for lighter materials is driven by their performance payoff. As shown in Figure 2-5, each new composite material has been introduced gradually. As the materials and their manufacturing processes mature and their capabilities are demonstrated, they begin to account for a growing portion of each aircraft type. New materi - als are used first in hardware that is not flight-critical, then in empennage structures, then in wings (where their payoff is usually greatest) and in other primary structures (fuselage skins and substructures). As shown in Figure 2-5, new materials are often first introduced in fighters, where performance is the main consideration, then in business jets and rotorcraft, then in the larger bombers, and, finally, in commercial aircraft. It takes 5 to 10 years from the availability of a new material to its introduction into aircraft empennage surfaces, depending on how quickly manufacturing processes can be developed that enable low-cost fabrication of parts; 10 See Chapter 5 of this report for a list of the DoD’s nine TRL levels; level 9 is the successful use of a system in mission operations. 11 Manufacturing Readiness Level (MRL) Handbook, version 2.01, July 2011. Available at http://www.dodmrl.com/MRL_Deskbook_V2.01. pdf provides best practices for MRL. 12 “Design allowables are statistically determined material property values derived from test data. They are limits of stress, strain, or stiff - ness that are allowed for a specific material, configuration, application, and environmental condition.” See p. 361 in ASM Handbook, Vol. 21, Composites, 2001. ASM International.
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35 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-5 Timelines for the introduction of composite materials into aircraft. Of the two circles, the first links the business jets, and the second indicates the single rotorcraft to separate them from the other aircraft shown. SOURCE: Developed based Figure 2-5.eps on information in (1) R.B. Deo, J.H. Starnes, and R.C. Holtzwarth, “Low-Cost Composite Materials and Structures for Air- bitmap craft Applications,” paper presented at the RTO AVT Specialists’ Meeting on Low Cost Composite Structures, Loen, Norway, May 7-11, 2001, and published in RTO-MP-069(II); (2) M. Buckley, “An Introduction to Composites at Airbus,” presented at HYBRIDMAT 4, 2007, available at http://www.adcom.org.uk/downloads/3D Preform Technologies for Advanced Aerospace Structures.pdf; and (3) C. Harris and M. Shuart, “An Assessment of the State-of-the-Art in the Design and Manufacturing of Large Composite Structures for Aerospace Vehicles,” NASA Langley Research Center, April 2001. Available at http://ntrs.nasa. gov/archive/nasa/casi.ntrs.nasa.gov/20040086015_2004090422.pdf. about 10 years from fighters to bombers; and then another 10 years from bombers to commercial aircraft. The transition from empennage structures (tail surfaces) to primary structures is about 10 to 15 years. 2.2.2 System Engineering for Multifunctional Design Multifunctional Structures Components and structural elements that serve multiple functions offer the potential to lighten a structure. One way to achieve multifunctionality is through functionally graded materials that have properties and material constituents on one surface that differ from those on another surface, or are stratified through the interior of a part. There are a number of common examples, such as alclad aluminum, which provides corrosion protection for the base aluminum; or interlaminar toughened epoxies like those used in the 787, which provide impact damage resistance to composite systems. Hybrid materials like GLARE13 that marry conventional aluminums with high- 13 R.C. Alderliesten. 2007. “On the Available Relevant Approaches for Fatigue Crack Propagation Prediction in Glare.” International Journal of Fatigue. Vol. 29, Issue 2, pp. 289-304.
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36 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES strain glass-fiber-based systems can provide significant increases in damage tolerance for aluminum. The goal of graded materials has always been to build a material with graduated properties so that it meets different require - ments in different locations within the material of component part. Multifunctionality also encompasses “intelligent material” design, using either inherently smart materials or composites designed with multifunctional attributes. Examples of such materials are conductive polymers and high- temperature-resistant ceramic materials. Combinations of metallic and ceramic materials that protect the metal with a high temperature resistant coating while retaining the strength and toughness of the metal have been attempted for years. Similarly, inorganic and organic hybrid materials offer “designed” thermal and electrical conductivity and improved mechanical properties. While they have not yet reached maturity, nanoparticle technologies may offer the breakthrough required to make these kinds of materials a reality. While progress has been made in the use of lightweight composites that are resistant to fire and tolerant of impact damage, there are other barriers to their use. Protection from electromagnetic effects (EME) is still an add-on system that is assumed not to carry loads, but because it strains along with the wing skin materials and has stiffness, it does carry a portion of the loads.14 Burn-through criteria now set the minimum gage for composites more often than do the loads. That is, the time to burn through a composite fuselage skin panel can define the required thickness of the material. Thus, improving fiber, resin, or sizing materials is insufficient without also addressing burn-through. To overcome these challenges to the use of composites, methods to predict their properties accurately will have to be developed. Testing will then be used not to characterize the materials, but to verify that they are behaving as predicted. The predictive capability cannot be limited to the simple geometries of the coupon and element tests—it must also be capable of predicting the performance of highly complex shapes, structures, and components not only under expected service conditions, but also under the worst case loading expected for off-nominal flight conditions. Design for Durability Today’s aircraft structures must meet a host of durability requirements while still carrying loads and being capable of deflections that shed loads and enhance the aerodynamic performance of the aircraft. Durability issues include burn resistance, damage resistance, EME tolerance, and resistance to UV degradation, chemicals, moisture, and extreme temperatures. Aluminum is affordable and has an excellent set of properties under these conditions that, until recently, made it the material of choice for large transport aircraft. Some successes have been achieved by a new approach to the design of structures such as wings, which would have not been possible with aluminum. For in-plane loads, composite laminates having more than 25 percent of the fibers running in the load-carrying direction have much greater resistance to fatigue damage than do metallic structures. Thus, good composite designs offer better durability than good metallic designs. Because composite materials allow for tailoring the twist of a wing as it bends under airflow, it has been possible to achieve better aerodynamic performance.15 This tailoring has been used on X-29, F/A-18, and 787 aircraft to achieve better performance at lower weight. And only in recent years have the tools required to predict loads and perform these design and analyses been generally available to aircraft design teams. 2.2.3 Shorter Insertion Time for New Technologies If allowables for composites must be determined based on testing, it would be beneficial to focus it on the means of the properties and not on their distributions, which take hundreds of tests to determine. The ability to predict strength based on material parameters such as resin make-up, fiber properties, sizing capabilities, vari - ability of processing parameters, and so on might significantly reduce the amount of testing needed for composite materials. This is discussed further later in this chapter in the section on ICME for composite materials. One ele - 14 R. Jones. 1998. Mechanics of Composite Materials. 2nd Edition. Materials Science & Engineering Series. 15 M.J. Patil. 1997. “Aeroelastic Tailoring of Composite Box Beams.” Georgia Institute of Technology. Published through the American Institute of Aeronautics and Astronautics.
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37 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS ment of the ICME approach is the Advanced Mean Value Algorithm, 16 which requires only 10 or fewer specimens to accurately determine the mean of the property distribution.17 If the expected variation in properties is known from chemistry and mechanics, the allowables could be determined much more rapidly and from far fewer tests. Another way of hastening the determination of allowables will be the development of semi-empirical approaches to predicting the strength of a composite material for a wide range of geometries, layups, and loadings. Today’s methods for doing this are of two kinds. One is focused on using laminated plate theory 18 to convert the data from element tests to pseudo-lamina in situ properties, then applying them to structural strength predictions. This works well when the loads are applied in plane and when the damage state of concern is the same as that incorporated in the test. Whenever these conditions are violated, which happens all too often, additional tests are required to validate the more difficult details of the design. It can be very difficult for these tests to recreate the load or damage conditions of concern because the structure being modeled is so complex. The second method19 used is to intrinsically model the structure, including the layup, the geometry, the damage, and the loadings and perform a damage tolerance analysis on it. In general, this takes a very detailed finite element model, far more detailed than those normally used to predict internal load and strain distributions. Often because of the damage, loading, or geometry, these analyses must be performed using nonlinear methods, which increase run times and the complexity of interpretation. Such models cannot be run for every condition the aircraft might undergo: Because they are simply too complex and too time-consuming to validate and perform, they are run only for the most critical or complex cases. A third possible way of surmounting the test barrier is to use existing tests and data to cover new materials, as was done using the Advanced General Aviation Transport Experiments (AGATE) process for general aviation vehicle design20 and the Composite Materials Handbook 17 (CMH-17) methods for shared data. 21 2.2.4 Accelerating the Transition from Laboratory to Product The difficulty of bringing a new technology to the marketplace has been dubbed the Valley of Death, because so many promising technologies die before they are used in products. The Valley of Death occurs when technology is developed to some extent, and then a search begins for applications. Since the key requirements of candidate applications were not taken into account during the technology development process, oftentimes the development has to restart. Aircraft are no exception, and the difficulty of this transition inhibits the greater use of lightweight - ing technologies. If technology is developed in response to a defined need, there is no Valley of Death. Therefore, the key to truly accelerating technology transition is to create a mechanism for technology pull. Advanced technology demonstrations (ATDs) are one strategy used by DARPA and the military services to provide the pull required to transition technology from the laboratory to deployable vehicles. Their purpose is “a demonstration of the maturity and potential of advanced technologies for enhanced military operational capability or cost effectiveness. ATD are identified, sponsored, and funded by Services and agencies.” 22 Here, DARPA has had a number of successes. However, the process is flawed when the prototype aircraft are given limited operational assignments after meeting only a few design requirements—i.e., before they are battlefield-ready. As described in the second part of Section 2.5.2, the Predator and Global Hawk Unmanned Aerial Vehicles were ATDs that were deployed in Iraq and Afghanistan. Their ability to persist over targets and deliver 16 Y.T. Wu, H.R. Millwater, and T.A. Cruse. 1990. “Advanced Probabilistic Structural Analysis Method for Implicit Performance Functions.” AIAA Journal, Vol. 28, No. 9, pp. 1663-1669. September. 17 E.J. Gumbel. 2004. Statistics of Extremes. Mineola, New York: Dover Publications. 18 C. Kassapoglou. 2010. Review of Classical Laminated Plate Theory. Published online at http://onlinelibrary.wiley.com/doi/ 10.1002/9780470972700.ch3/summary. 19 E.J. Gumbel. 2004. Statistics of Extremes, Mineola, New York: Dover Publications. 20 For more information on the AGATE shared database process, see http://www.compositesworld.com/articles/agate-methodology-proves- its-worth. Last accessed October 19, 2011. 21 For more information on composite properties, see http://www.compositesworld.com/columns/shared-composite-material-property- databases. Last accessed October 19, 2011. 22 Information on ATDs is available at https://dap.dau.mil/glossary/pages/1414.aspx. Last accessed October 19, 2011.
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38 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES small, accurate weapons was valuable to the war effort, but because of the limited design requirements, these UAVs failed to meet normal reliability and supportability goals typical for fielded aircraft. 23 2.3 LIGHTWEIGHTING OPPORTUNITIES FOR AIRCRAFT Although the aircraft industry has been seeking lightweight structures for more than 100 years, there are still a surprisingly large number of opportunities to reduce weight through exploiting emerging technologies to refine materials, manufacturing, design, and configuration. 2.3.1 Opportunities in Materials It was recognized 20 years ago that high strain-to-failure graphite fibers would be relegated to research labo - ratories unless high strain-to-failure resins were developed simultaneously. Today, several promising materials could someday produce better, lighter structures. Carbon Fiber Development As shown in Figure 2-6, the use of carbon-fiber composites for lightweighting in commercial and military airplanes has grown since the early 1970s. The success of these composites illustrates the need for parallel devel - opment of a wide range of ancillary technologies (such as sizing [a chemical coating] for fibers, tooling for the system, and manufacturing processes for the application) and their eventual convergence. These developments took many years, and each needed sustained support. For instance, although high-strength carbon fibers made from polyacrylonitrile (PAN) were first created in the 1960s by Aksanti in Japan and then in the Royal Aircraft Establishment (RAE) at Farnborough in 1963, the applications at that time, such as in sports equipment, were structurally non critical and did not take full advantage of fiber compositing. Similarly, the resins available early on in composite development had lower strain to failure than the fibers themselves, again limiting the performance capabilities of fiber composites. In parallel, successful implementation has also required extensive investments in the educational, industrial, and research infrastructures, both here and abroad. The DARPA Advanced Structural Fiber program24 seeks to increase fiber strength and stiffness by reducing defects in the fibers through advanced processing and by applying atomic control on a massive scale. Before such fibers can achieve meaningful weight savings, resins will need to be developed that can carry the fibers well and without microcracking.25 Each development related to composite materials has lent a capability for the aircraft. Durable epoxies have led to lightweight structures with high stiffness. Thermoplastics, while initially hard to fabricate, became an enabler for toughened systems that have allowed more damage tolerance. Thermoplastics have also provided better compatibility with high stiffness and strain fibers to enable very low thickness to chord wings for fighters and bombers. Eventually, low-cost fiber made it affordable to introduce composites into commercial aircraft. Chemistry- and physics-based analytical approaches of integrated computational materials engineering (ICME) are replacing much of the testing done in the 1980s and 1990s. These approaches could reduce the time it takes to put new multifunctional materials to work in aerospace applications. Alternative Composite Polymers Work in the 1980s showed that thermoplastic resins offered much higher durability than conventional epoxy resins. At that time, however, thermoplastic resins were comparatively expensive, sensitive to degradation when 23 E. Bone and C. Bolkholm. 2002. “Unmanned Aerial Vehicles: Background and Issues for Congress.” Report for Congress, Order Code RL3187. April. Available at http://www.fas.org/irp/crs/RL31872.pdf. 24 For information on DARPA’s Advanced Structural Fiber program, see http://www.darpa.mil/Our_Work/DSO/Programs/Advanced_ Structural_Fiber_(ASF).aspx. Last accessed October 19, 2011. 25 H.G. Chae and S. Kumar. 2008. “Materials Science: Making Strong Fibers.” Science, Vol. 319, Issue 5865, pp. 908-909.
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39 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS Figure 2-6.eps FIGURE 2-6 Multidisciplinary approach to carbon fiber development. bitmap exposed to aircraft fluids, and difficult to work with because their processing takes place at very high temperatures. Instead of using thermoplastics outright, researchers used toughened epoxy resins to gain some of the strain-to- failure and toughness characteristics that the systems then lacked. Today, pultrusion26 and other ways of injecting thermoplastic resins into parts have progressed to the point where interest in such materials is growing. If these new processes overcome the barriers to the use of thermoplas - tics and are accompanied by new approaches to solvent sensitivity, the strain-to-failure, durability, and damage tolerance of thermoplastics could improve dramatically. One of the main attractions of pultrusion is its simplicity of tooling and low labor requirements. At first glance, pultrusion appears to be a straightforward process: reinforcing fibers are saturated with a thermosetting resin matrix and pulled through a heated die, as shown in Figure 2-7. It turns out that successful pultrusion requires excellent control of the staging temperatures and the tension in the system, as well as control of the state of the material as it is pulled through the process in order to achieve the desired results. It is, in short, more of an art than it appears. Nano- and Multifunctional Materials Damage in aluminum—for example, cracks or dents—becomes evident before it limits structural performance. Less accessible areas inspected frequently enough to prevent cracks of critical size from forming. 26 Pultrusion is a continuous molding process for composite materials that mechanically aligns long strands of reinforcements for a composite material and then passes them through a bath of thermosetting resin. The coated strands are then assembled by a mechanical guide before the curing process. More recently, pultrusion has been used with thermoplastic matrices such as polybutylene terephthalate (PBT) either by impregnating the glass fiber with powder or surrounding it a sheet of the thermoplastic matrix, which is then heated to fuse the polymer and fibers. From “Putting It Together—the Science and Technology of Composite Materials.” Australian Academy of Science. 2000. Available at http://www.science.org.au/nova/059/059glo.htm and http://www.acmanet.org/pic/products/description.htm.
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50 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-12 X-43A Scramjet after separation from Pegasus booster. SOURCE: NASA Dryden Research Center Photo Col - lection, available at http://www.dfrc.nasa.gov/Gallery/Photo/X-43A/Large/ED04-0082-4.jpg. Figure 2-12.eps bitmap which they would be exposed. Now that these UAVs ATDs: Adequate Testing with are fielded, limitations in their design capabilities with Rapid Transition to Production respect to battlefield needs are becoming apparent. Still, their ability to persist over targets and deliver small, accurate weapons to minimize collateral damage The problems with the Predator and Global Hawk on the battlefield suggest that more testing of have been valuable, and these capabilities more than ATDs could improve battlefield readiness. How- compensate for their shortcomings (e.g., performance ever, the performance of these UAVs also shows problems under varying weather conditions). that the ATD program is, on balance, an effective way to deploy systems with new capabilities. 2.5.2 Super Lightweight Tank for the Space Shuttle, using Aluminum-Lithium To place the International Space Station modules in orbit, the weight of the space shuttle had to be signifi - cantly reduced.49 The successful development of a new super lightweight tank (SLWT) to replace the lightweight tank (LWT) provided 50 percent of the performance increase required for the shuttles to reach the International Space Station. The weight reduction achieved was made possible by the development of the 2195 aluminum-lithium alloy. When work on the new alloy began, there was a strong bias against aluminum-lithium (Al-Li) alloys because the previous generation of aluminum-lithium alloys (2090 and 8090) had demonstrated poor fracture toughness. This toughness issue was overcome by a new formulation and new processing technologies. This produced an alloy— Al-Li 2195—with improved properties and a lower density than the 2219 alloy previously used for the external tank of the shuttles and reduced the weight of the external tank by 7,500 lb (3,402 kilograms). For comparison, the SLWT weighs 36,123 lb, the LWT weighs 43,623 lb, and a Standard Weight Tank weighs 52,589 lb. The lithium in Al-Li 2195 made the initial welds of the external tank far more complex. The repair welds 49 For more information, see “Analysis of International Space Station Vehicle Materials on MISSE 6,” available at http://ntrs.nasa.gov/ archive/nasa/casi.ntrs.nasa.gov/20100033233_2010034433.pdf. Last accessed October 19, 2011.
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51 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-13 NASA’s HELIOS, a type of high-altitude long endurance (HALE) vehicle. SOURCE: NASA Dryden Research Figure 2-13.eps Center Photo Collection, available at http://www.nasa.gov/centers/dryden/images/content/105841main_helios.jpg. bitmap Figure 2-14.eps FIGURE 2-14 TU delft–Delfly. SOURCE: Used with permission of www.DelFly.nl; available at http://www.delfly. nl/?site=Publications &menu=&lang=en. bitmap
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52 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES were difficult to make and the joints in the external tank had much lower strengths. In an effort to mitigate the increased production cost and regain the mechanical properties of the earlier Al 2219 external tank, the project began researching alternative welding techniques. The friction stir welding process produces a joint stronger than the fusion arc welded joint and has only three process variables to control: rotation speed, travel speed, and pres - sure, all of which are easily controlled. The fusion weld, in contrast, has to control many process factors, such as purge gas, voltage and amperage, wire feed, travel speed, shield gas, and arc gap. The increase in joint strength, combined with the reduction in process variability, provides an increased safety margin and a high degree SLWT: New Materials and of reliability for the external tank. Shuttle external tank Processes Go Hand in Hand weights are compared in Table 2-2. The successful development of aluminum lithium Advanced processing methods must often be 2195 illustrates what can be accomplished when fun- used for the advanced materials used for light- damental alloy development is integrated with pro- weighting. When fundamental alloy development, cessing and design. The addition of Ag to the alloy advanced processing, and design are addressed together, it allows consideration of tradeoffs and promoted a distribution of the T1 phase in the micro- thereby improves the final design. structure that gave good transverse strength, which had been the weak strength direction in earlier generation Al-Li alloys 2090 and 8090. Solving the transverse strength problem allowed this alloy to be used to fabricate section thicknesses required for the shuttle external tank. Friction stir welding also contributed to the success of this allocation as noted above. 2.5.3 Using Aeroelastic Tailoring of Composite Wings to Enable Higher Maneuverability X-29 Wings The thin forward swept wing of NASA’s X-29 (Figure 2-15) can be achieved only with tailored composite wing skins. Such a wing improves maneuvering per- formance because the ailerons are not “washed out” X-29, F/A-18, 787: Advantages by wing bending at high angles of attack as they are to Composite Wings on rear swept wings. The wings still provide the sweep required to achieve supersonic flight. Composites offer bending torsion relationships The composite wings of the X-29 were designed that allow tailoring wing bending to achieve to achieve flutter-free flight in the forward swept con- lightweight designs with better performance. The figuration. “State-of-the-art composites permit aero- significant laminate tailoring to achieve the neces- elastic tailoring, which allows the wings some bending sary stiffness in torsion offers performance char- but limits twisting and eliminates structural divergence acteristics that cannot be achieved any other way. within the flight envelope (i.e., deformation of the wing or breaking off in flight).”50 FA-18 E/F Wings The FA-18 E/F is a much larger aircraft than its predecessor, the FA-18 C/D. Its larger size was enabled by IM-7 fibers and the toughened resin systems required to exploit the additional strain and strength capability of those fibers (without being limited by resin strains to failure). The toughened resin allowed the strengths after impact damage to remain near those of open holes and thus not drive the design to an overly conservative strain level. The additional stiffness of the fibers made an expanded wing planform possible without a comparable increase 50 Federation of American Scientists. 2010. “The X-29.” Website of the Federation of American Scientists. Available at http://www.fas.org/ programs/ssp/man/uswpns/air/xplanes/x29.html. Last accessed December 7, 2011.
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53 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS TABLE 2-2 Space Shuttle External Tank Weights Weight (lb) Standard Weight Lightweight Super Lightweight Tanka Tankb Component Tank 1 Structure (LO2 and LH2 tanks, intertank) 52,589 43,623 36,123 2 Thermal protection (external foam, misc.) 5,959 4,823 4,823 3 Propulsion (feed, vent, pressure systems) 2,951 2,951 2,951 4 Power (cables, supports) 372 372 372 5 Controls 0 0 0 6 Avionics (instrumentation and supports) 68 68 68 7 Environment 0 0 0 8 Other (orb., SRB attachment, misc.) 6,676 5,954 5,954 9 Growth 0 0 0 10 Non-cargo (unusable and reserve LO2 and LH2) 8,209 8,209 8,209 TOTAL 76,824 66,000 58,500 aWeight reduction made in structure and SRB attachments. bReplace major portion of Al2219 components in lightweight tank with Li Al 2195 alloy. SOURCE: Prepared with information in the NASA paper “Super Lightweight Tank—A Risk Management Case Study in Mass Reduction.” Available at http://www.nasa.gov/externalflash/irkm-slwt/Text%20Case/SLWT%20RM%20Case%20Study%20Accessible%20Version.pdf. in wing thickness. The fibers also provided enough outer wing torsional stiffness to maintain the aerodynamic shaping of the very thin wing. 787 Wings The Boeing 787 uses composites to tailor its wings, allowing much more efficient cruise flight than had been possible with the earlier metallic wings. These tailored wings, which have raked wing tips, have been designed to optimize cruise performance, and they bend more than conventional wings. To maintain torsional stiffness for the outer wings and still provide the required bending stiffness, such a wing has a highly tailored layup along its span. 2.5.4 Commercial Aircraft Applications Extensive Use of Composites in the Boeing 787 The Boeing 787 (Figure 2-16) is the first commercial plane to have a composite fuselage. Composites were selected primarily to reduce weight, and the design offered significant weight savings. However, Boeing added weight back to the design to address lightning, noise, and manufacturing issues. The weight breakdown for the 787 aircraft is shown in Figure 2-17. For large commercial aircraft, small differences 787: Challenges of Large in weight add up quickly—for example, a reduction Composite Platforms of only 5 lb per floor beam makes it possible to carry an additional 250-lb passenger. The revenue added by Large-scale application of composites remains one additional passenger over the life of the vehicle is a new and difficult undertaking. Boeing’s break- equivalent to between 0.5 and 0.3 percent of the value through use of composites for the fuselage of the vehicle. reduced weight but suggests that multifunctional- While composites account for 50 percent by ity could have retained more of the lightweighting weight in the 787, the low density of these materi- benefits of a large composite platform. als means that the amount of composite material by
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54 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-15 X-29 aircraft, featuring forward-swept wings. SOURCE: NASA Dryden Flight Center Photo Collection, avail - Figure 2-15.eps able at http://www.nasa.gov/centers/dryden/images/content. bitmap volume is about 72 percent.51 Not only does the move to composites reduce weight, but going to one-piece barrel sections with integral stiffeners (see Figure 2-18) also reduces the number of fasteners required to produce the aircraft by roughly 50,000. These advances, combined with fuel-efficient and lighter engines, electronic systems in place of hydraulic systems, and a myriad of lesser weight-reducing technologies, make the 787 capable of flying more people longer distances with lower fuel consumption than any other airliner in the world. Weight determines the range of commercial aircraft and therefore the city pairs that can be served. A paper by Bernstein Research52 looked at Airbus estimates of the reduction in 787 range based on its estimates of 787 overweight condition. The assessment estimates that a 15,000-20,000 lb overweight condition will degrade the performance of the 787 “into a range near 6,900 nm, well below the promised 7,700-8,200 nm range.” Boeing states in the same article that early 787s will be overweight but that the company is making every effort to restore the promised performance. Composites helped with lightweighting, but the large scale of the composite structure encouraged a conservative design, with the result that so far the 787 has achieved only about half of the anticipated weight reduction. Advanced Materials for the Airbus A350 Weight has so much value for the commercial aircraft customer that Airbus was forced by its customer base to move from a hybrid metallic/composite baseline fuselage to an all-composite baseline fuselage. Design changes like these become tremendously expensive when the time and effort that goes into engineering, design and certifi - cation testing are taken into account. Because of the uncertainty surrounding today’s design and analysis methods for these material systems, much of the certification readiness testing must be carried out using large components 51 Justin Hale. 2006. “787 from the Ground Up.” Aeromagazine, Quarter 4, pp. 16-23. The Boeing Company. 52 Jon Ostrower. 2009. “Analysis: 787-8 Weight Examined (Update 1 with Boeing Comment).” Bernstein Research. May.
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55 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-16 Boeing 787, a primarily composite commercial airliner. SOURCE: Boeing. Figure 2-16.eps bitmap FIGURE 2-17 Boeing 787, which uses approximately 50 percent structural composites by weight. SOURCE: Boeing. Figure 2-17.eps bitmap
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56 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-18 787 composite fuselage structures. SOURCE: Justin Hale. 2006. “787 from the Ground Up.” Aeromagazine. Figure 2-18.eps The Boeing Company, Quarter 4. Pp. 16-23. bitmap like the barrel shown for the XWB A350 fuselage in Figure 2-19. A350: Uncertainty in Design and Analysis Tools 2.5.5 Composite Crew Module for the ARES I Weight- and performance-conscious commercial Launch Vehicle aircraft will continue the shift to composite fuse- Under the Constellation program, NASA devoted lages, but design and analysis tools have not considerable resources to reduce costs and lighten pay- yet caught up to the need. The design process loads through increased use of composites in future requires development and validation of more accurate and comprehensive design and analysis space structures. In 2006, the NASA Engineering and tools to realize the full potential of lightweighting Safety Center studied the feasibility of a composite of structures with composite materials. crew module (CCM) for the Constellation program crew exploration vehicle. Constructing a CCM was found to be feasible, but a detailed design would be necessary to quantify technical characteristics, particularly in the areas of mass and manufacturability. The CCM project was chartered in January 2007 as a partnership between NASA and industry that shared design, manufacturing, and tooling expertise.53 The project’s goals were not only to deliver a full-scale test article 18 months after project initiation but also to develop a network of NASA engineers with hands-on experience using structural composites in complex spacecraft design. The CCM was based on the architecture of Orion’s aluminum crew module. The design for the CCM con - 53 C. Collier, P. Yarrington, M. Pickenheim, B. Bednarcyk, and J. Jeans. 2008. “Analysis Methods Used on the NASA Composite Crew Module.” American Institute of Aeronautics and Astronautics. Available at http://hypersizer.com/pdf/AIAApaperCollierCCM107727.pdf.
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57 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-19 Test fuselage section for the Airbus A350 XWB aircraft. SOURCE: Airbus. Figure 2-19.eps bitmap sists of “a stiffened honeycomb sandwich, with carbon fiber/epoxy skins and aluminum honeycomb core.”54 Ares I: Composites Improve Launch Vehicle Performance Fiber SIM software calculated the shape and size of each ply segment to conform to the tooling and then exported that information to a numerically controlled Composite construction can be used to light- weight very large platforms and can prepare a cutting machine. cadre of engineers to apply that experience in One unique feature of the CCM design is the future projects. structural integration of the packaging backbone with the floor and pressure shell walls (Figure 2-20). This design provides a load path that accommodates load sharing with the heat shield, especially for water landing load cases. Another unique feature of the composite design is the use of lobes between the webs of the backbone. This feature puts the floor into a membrane-type loading, resulting in a lower mass solution. Connecting the floor to the backbone and placing lobes into the floor resulted in mass savings of approximately 150 lb for the overall primary structure. 55 As the design progressed and analysis became more mature, analyses were divided into three classes: • Analysis for optimization of sizing (a chemical coating), which included architectural trade studies, optimum honeycomb sandwich design, and optimum composite layups; • Analysis for failure margins of safety for large sheets of material, which included panel buckling, com- posite strength failure, and damage tolerance, and sandwich-specific facesheet wrinkling and core shear, and 54 Sara Black. 2009. “Simulation Simplifies Fabrication of All-Composite Crew Module.” High-Performance Composites. November. Avail- able at http://www.compositesworld.com/articles/simulation-simplifies-fabrication-of-all-composite-crew-module. 55 NASA Engineering and Safety Center. 2008. “Technical Update.” Available at http://www.nasa.gov/pdf/ 346545mainNESC08TechUpweb. pdf.
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58 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES FIGURE 2-20 Structural features of composite crew module. SOURCE: Team Gains Experience as It Builds Innovative Com- Figure 2-20.eps posite Spacecraft. Available at http://www.nasa.gov/ offices/nesc/home/Feature_6_090908.html. bitmap • Analysis for fabrication/manufacturing features, which included cutouts, sandwich ramp-downs, laminate ply drops, fabric ply overlap regions, and fiber angle alignment. The CCM is constructed in two main parts: an upper pressure cell and a lower pressure shell. The two halves are joined outside the autoclave to enable the packaging of large or complex subsystems. Building block tests of critical design and technology areas were conducted to validate critical assumptions and design allowables. Full- scale fabrication of the upper and lower pressure shells began in 2008. Successful testing of CCM was carried out in July 2009. Project director Mike Kirsch estimates that the CCM is 10-15 percent lighter than its aluminum counterpart on the Orion crew vehicle.56 2.5.6 Modernization of the Kiowa Warrior OH-58D The mission of the Kiowa Warrior, a single-engine, two-man helicopter, is “to support combat and contingency operations with a light, rapidly deployable helicopter capable of armed reconnaissance, security, target acquisition and designation, command and control, light attack, and defensive air combat mission.” 57 Originally deployed in 1969 to Vietnam as the OH-58A, the Kiowa was based on a successful commercial helicopter, the Bell 206. The OH-58D (Figure 2-21), prototyped in 1983 and first flown in 1985, is currently the dominant version of the helicopter, having flown more than 400,000 hours in Iraq and almost 39,000 hours in 56 For more information, see M. Kirsch, 2009, “Broad Based Teams, Case Study #1—Composite Crew Module.” Presented in Project Management Challenge 2009, Daytona Beach, Fla., available at http://pmchallenge.gsfc.nasa.gov/docs/2009/presentations/Kirsch.Mike.pdf; or B.A. Bednarcyk, S.M. Arnold, C.S. Collier, and P.W. Yarrington. 2007. “Preliminary Structural Sizing and Alternative Material Trade Study of CEV Crew Module.” NASA TM-2007-214947; AIAA-2007-2175. 57 For more information on the Kiowa Warrior, see http://www.fas.org/man/dod-101/sys/land/wsh2010/198.pdf. Last accessed October 19, 2011.
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59 MODEL DESIGN OPTIONS FOR FORECASTING SYSTEMS FIGURE 2-21 Bell OH-58D Kiowa military helicopter with full ammunition. SOURCE: Available at http://www4.army.mil/ Figure 2-21.eps armyimages/armyimage.php?photo=3873. bitmap Afghanistan (about 80 flight-hours per helicopter per month), accounting for more than half the total reconnais - sance/attack hours.58 The Army’s current fleet is 340 OH-58Ds, and many others have been sold internationally. The Kiowa Warrior was viewed as a temporary program, to be replaced by the RAH-66 Comanche—the Army’s next-generation, lightweight, low-cost armed reconnaissance helicopter. The Comanche program faced numerous difficulties—it underwent six restructurings and a drawn-out schedule, with inconsistent funding from year to year. When it was canceled in 2004, one of its performance shortcomings was not meeting its lightweight - ing goals. A second proposed replacement for the Kiowa, the Bell ARH-70, was canceled in 2008. The Kiowas were originally intended to be unarmed scouts, but with delays and then cancellation of the RAH-66 Comanche and the ARH-70 Arapaho, the OH-58D has become the Army’s armed reconnaissance helicop- ter. As these helicopters have aged, maintenance has become more difficult, and the technology outdated. Funding intended for the next-generation helicopters will be used instead for modernization. In October 2010, the OH-58D Upgrade: Improved Army announced specifications for the Fox model— Performance via Lightweighting upgrades to the OH-58D Kiowa Warrior, which will be designated OH-58F. Lightweighting is a viable strategy within a vehicle Many of the planned upgrades are aimed at reduc- modernization program. ing weight. One of the top priorities is an engine that will increase the power-to-weight ratio, which is particularly important in mountainous areas such as Afghanistan. Rolls-Royce is working to increase the power of the Kiowa’s current engine by 12 percent. At the same time, Bell is working with the Honeywell engine that was to be used on the Arapaho; it would add about 100 lb but provide 50 percent more power.59 58 M. Rusling, 2010, “With No Replacement in Sight, Army’s Oldest Helos Keep Going,” National Defense, April; and “Army New Kiowa Warrior FOX Model Increases Capability,” Defense Daily, October 27. 59 S. Trimble. 2010. “U.S. Army Announces New Fox Model for Kiowa Warrior.” Flightglobal, October 26.
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60 APPLICATION OF LIGHTWEIGHTING TECHNOLOGY TO MILITARY AIRCRAFT, VESSELS, AND VEHICLES The Kiowa Warrior’s machine guns will be replaced by Avenger’s M3P machine guns, which use a simple mount that avoids the need for a gun cage assembly. The new system is 65 lb lighter than its predecessor, which Ron Bridges of Redstone Arsenal estimates will save $20,000 per pound over the life of the helicopter.” 60 Lifeport Interiors Inc. has developed and tested lightweight floor armor; the installed kit weighs 8.6 lb and is capable of defeating 7.62 rounds.61 2.6 CONCLUSIONS Aircraft—both military and commercial—have long used lightweighting because of its strong connection to performance. Lightweighting not only reduces fuel costs but also determines range. Composites are increasingly replacing aluminum in aircraft. The great barrier to further lightweighting in aircraft is that the development time for new composites exceeds the development time for aircraft. The committee reached the following conclusions about lightweighting aircraft and the need to accelerate the materials development cycle: • Because the development of composite materials takes 10 years or more, materials must be under develop- ment long before aircraft definition takes place. New materials must always be in the pipeline and being brought to maturity to provide the capabilities foreseen in the strategic plans of the armed forces. • The long timeline for materials development also points to the need for computational materials engi- neering to bring the synthesis of new resins and fabrication processes into the digital age. Advancements in computational chemistry may make it possible to design the molecular structure of the resin to meet structural and other requirements for a given application. Processing science will be an integral part of this methodology to ensure that section thicknesses and shapes can be fabricated from the designed resins. • Currently available empirical analysis tools cannot replace the testing required to support composite applications for commercial aircraft. Better analytical methods are required and better definition of test variability is also required to allow testing for means and not for distributions. • Part of the materials pipeline should be devoted to the development of multifunctional materials and structures, and the processes by which such structures can be manufactured. The more requirements can be met with multifunctional materials, the less design complexity is needed for structural integration of multiple systems. Some of the key multifunctional capabilities that need to be met with new materials include strength and electromechanical energy dissipation and shielding; dynamic or sonic vibration damping; greater tolerance of extreme temperatures; and resistance to damage from hail or foreign objects. • Integrated computational materials engineering (ICME) is a promising strategy for improving the analyti- cal models of the strength of composite materials and the models for predicting the variation in composite laminates given the mean strength capability. First-level models for predicting the behaviors of today’s materials have been developed, and some elements of ICME, such as chemical modeling of resins, are already being used to develop the materials needed for tomorrow. However, much more is still needed to provide rapid, accurate, and reliable methods for predicting the performance of composite materials that would reduce the enormous amount of testing required to design aircraft today. • Another promising strategy for accelerating the introduction of new materials for lightweighting is the DoD’s advanced technology demonstration program, which needs to use a systems engineering approach that includes all known threats and all the requirements set for operational systems. The ATD demonstra - tion data can then be used to reduce the amount of testing required to certify the operational system. • Despite having pushed the lightweight boundaries for many years, the aerospace industry still has a long way to go before affordable lightweight structures can be easily designed, certified, and maintained. • Taking advantage of Title III funding could provide some relief to develop and mature advanced materi- als (e.g., titanium metal matrix composites) when they are too expensive to buy in large quantities. Such relief would no longer be needed once material costs are reduced to affordable levels. 60 K. Hawkins. 2009. “Kiowa Warrior Gains Firepower.” Redstone [Arsenal] Rocket, May 6. Available at http://www.army.mil/article/20656/. 61 “Army New Kiowa Warrior FOX Model Increases Capability.” Defense Daily, October 27, 2010.