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Appendix D Materials Options for Fuel Efficiency and Protection To protect against diverse threats, such as automatic small arms, machine can- nons, shaped charges and other chemical energy projectiles, as well as other weapon systems, vehicle protection systems have become very complex. Army After Next (AAN) vehicles are likely to use different types of armor to protect different areas of the vehicle, partly to reduce weight for a given amount of protection and partly to combine projectile protection with Toad-bearing capability. These protection system components will consist of combinations of metals, ceramics, and polymers. Enabling research to improve the building blocks of a complex armor system will provide armor designers with options for lighter combinations of materials with superior protection and structural properties to use at different locations on the vehicle to meet varying protection requirements. One of the most controversial issues in optimizing system weight is whether and how an AAN combat vehicle should be designed to prevail in direct-f~re duels analogous to "tank-on-tank" warfare. The current technology of high-velocity projectiles, such as Tong rod penetrators (LRPs) that can defeat even the heaviest passive armor, has raised the stakes on the protection side in the armor-antiarmor spiral. For the critical mission requirements of an AAN battle force, do the tactical advantages of equipping a vehicle both to shoot these projectiles and to withstand hits from them in direct-fire (line-of- sight) combat outweigh the costs in vehicle weighty The Army's answer to this ques- tion will have profound consequences for the fuel efficiency and protection characteris- tics of AAN combat vehicles. According to vehicle armor engineers at the Army Research Laboratory (ARL), a vehicle that could withstand direct hits by advanced antiarmor rounds would have to weigh 45 to 50 tons, even with the best current protection technology (Havel, 1997, 1998~. Based on the information available to the committee, incremental improvements in current armor technology over the next quarter century will probably not protect a 15-ton vehicle from advanced high-velocity rounds. Even if a breakthrough in active protection systems could defeat advanced projectiles, the armor to protect the crew and vehicle Tom projectile fragments and lesser threats, such as a machine cannon firing armor-piercing projectiles, would significantly increase vehicle weight. Nor an extensive technical (unclassified) discussion of the role of kinetic energy penetration in the armor-antiarmor spiral, see NRC 1 993c, especially pp. 10-15 and 40-43. 2The weight implications include effects on vehicle mobility as well as on fuel consumption. See the discussion in Chapter 6 on gun systems and alternatives. 190
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APPENDIX D 191 A strategy to circumvent this problem, at least partly, and still give the AAN battle force a capability for direct-fire engagement of opposing armored vehicles would be to remove the crew from some or most of the combat vehicles exposed to direct fire. Unsteady, a squad of vehicles with one or more human commanders would include un- manned vehicles operating autonomously or semi-autonomously. An unmanned vehicle could be engineered with a very low profile that would greatly reduce its vulnerability to frontal attack.3 For offense against enemy armor, these low-profile vehicles could rely on top-defeat of opposing armor by precision-guided munitions, delivered with rocket or gun propulsion systems (see Chapter 61. Technologies for automated control systems ~ , , , ~ , , ~ , technology and sensors may mature suti~c~ently In the next two decades to make this strategy practical. In one scenario, a few manned vehicles would travel inconspicuously with the unmanned vehicles. The former would have superior protection systems at the expense of lesser armament and lethal power than the unmanned vehicles, which would be more lethally armed but would have less protection. For both the manned and unmanned vehicles in this squad, stealth technology (signature reduction, low physical profile), decoys, and agility would be major elements of a total systems approach to combined survivability and other performance objectives, including fuel economy. The best "system" approach to protection, even for protecting combat vehicles from high-energy rounds, may be to avoid being hit in the first place. Stealth, agility, better situational awareness, and longer lethal reach may become more important in AAN combat vehicles than the capability of surviving a direct hit. In principle an advanced active nrc~tection svelter is nrohahlv the heat anr~ronch ---r------r--7 ~ - - rim ------ I------ ~~ rim ~ ~~~~ ~~~~ _rr- - - ~ · ~ ~ · ~ _ . ~ . ~ . ~ ~ ~ .. ~ ~ . ~ ~ .. . .. - .~ to snlelclmg a 13-ton combat vehicle trom multiple hlgh-veloclty projectiles or other advanced, armor-defeating rounds. A major question, though, is whether all the component technologies for such a system are feasible within the development time for an initial generation of AAN combat vehicles. The concept would need a rapid, intelligent tracking system to determine the trajectory of the incoming threat, identify the type of threat, and deploy mass or energy sufficient to deflect or reorient the projectile before it hits the vehicle. Russia has deployed a simple active protection system in which orates are propelled from ' ' ' ' ~ ~ . r , ~ · ~, . ~. ~ are prope. .. .e~ ~ Irom more sophisticated active AAN vehicle. the vehicle to deflect or fracture an ERP projectile. A much ~rotechon system would have to he develoner1 for a 1 S-ton MATERIALS OPTIONS FOR VEHICLE PROTECTION Most of the materials now used for the building blocks in combination armors are based on mature, well characterized, but conventional technologies. Examples of conventional building-block materials include rolled homogeneous armor (RHA), which is a chromium-molybdenum steel alloy; ceramics based on alumina or silicon carbide; glass fibers; and various aluminum alloys (7039, 5083, 2519, and others). The heavy iron-based building blocks, such as RHA, are mostly iron with a small amount of carbon and alloying elements (chromium, molybdenum, nickel, and/or silicon). They are prepared using conventional steel production methods, followed by 3Combat vehicles less than four feet in height are nearly immune to frontal attack from current projectile weapons on most terrain.
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192 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT thermomechanical treatments to tailor the microstructure (possibly down to nanometer scale) to obtain the best combination of properties for the particular armor application. Other building blocks include alloys based on titanium, aluminum, cobalt, or tungsten, as well as various metal-matrix or ceramic-matrix composites. Certain armor applications make use of polymer-matrix composites in which the dispersed phase in the composite may be glass, graphite (or, potentially, high-modulus carbonitrides), ceramic fibers, or whiskers of silicon carbide or silicon nitride. These building blocks are not interchangeable. They provide different combinations of properties, such as resistance to ballistic impact versus weight, structural properties, and fabrication difficulty and are suited to different applications or to different combinations in complex structural- protection architectures. Titanium alloys, by virtue of their high specific strength (strength per unit mass), are promising building block materials for lightweight armor on combat vehicles. However, their susceptibility to spelling requires the extensive use of spell liners or other techniques, which reduces the potential for weight reduction (Thompson, ~ 998~. Curiously, the leading titanium alloy in current armor applications is Ti-6Al-4V, which has been the mainstay titanium alloy for many uses since the early ~ 960s. The committee was unable to determine whether this alloy is objectively superior to newer titanium alloys for armor applications or whether it was selected for its greater production history. Advanced titanium alloys, which are being evaluated by the ARL, may warrant devel- opment specifically for their superior protection properties, particularly if a composition and temper with superior ballistics characteristics and other advantages can be developed. The history of advanced titanium alloys illustrates some of the common obsta- cles to the widespread acceptance of many advanced materials. Difficulties in processing have added to the cost of these materials and their low fabrication reliability (high rejection rate). Alternative processes, such as the electrosTag remelting process devel- oped in the former Soviet Union for advanced titanium alloys, will be necessary to improve manufacturability and lower the cost for these and other innovative building- block materials. The electrosTag process yields high-quality rectangular ingots that can be rolled to armor plate gauges. By contrast, the conventional round ingots from vacuum-arc-remelting require expensive forging operations to shape them into rectan- gles before they can be rolled into plate. For titanium bar products, which might be used in structural members supporting armor plate, the plasma coal hearth process would allow casting closer to final thickness, thereby reducing costs for hot-working the cast- ings. In addition to exploring Tower cost processes, the minimum purity of an alloy that would provide adequate ballistic protection, including resistance to spelling, should be identified. Materials engineering teams will have to work concurrently on issues of availability, manufacturability, and reliability, in order to realize the multifunctional benefits (e.g., same or better protection at reduced system weight) of advanced materials. For scout vehicles or aircraft, where weight reduction is critical and protection requirements are less demanding, aluminum lithium alloys have promising potential for combined structural-protection advantages. Some of these high-strength lightweight alloys, such as 2195 and 2094, which were developed for the space program, have extremely high specific strength and toughness. The Army could leverage NASA's substantial investment in engineering and manufacturing development. To date, research on ordered intermetallics has not yielded strength-toughness combinations approaching those of the chromium molybdenum steels or titanium alloys, such as Ti-6Al-4V. However, modest progress has been made with the iron aluminide
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APPENDIX D 193 and titanium aluminide intermetallics, which may provide lighter weight alternatives to the currently accepted options. Ceramics with increased resistance to ballistic penetration are also options for the building blocks in vehicle protection systems. One expert noted that the Army' s reliance on large, costly empirical test matrices reflects an incomplete understanding of the defeat mechanisms of ceramic armor materials (Thompson, 1998~. This is an area in which modeling could help meet AAN survivability goals for a 15-ton vehicle. An expert at the ARL, who agreed with the general need for modeling, emphasized that current materials models and computer codes cannot simulate the wide range of candidates for metallic and ceramic materials in protection systems (Havel, 1998~. Current ceramics options with good ballistic penetration resistance per unit mass include TiB2 and B4C. Unfortunately, they are both expensive because of their boron content and relatively limited production volumes. In addition, hot pressed or hot isostatically pressed ceramics of a given composition generally perform better than less expensive sintered ceramics. Research on new processing methods will be necessary to reduce the cost of high-performance ceramics of a given composition. According to an armor specialist at the ARL, new ceramic materials suitable for armor applications may either remain undeveloped or fait to make the transition to affordable production unless parallel commercial applications are found (Gooch, 1998~. At current funding levels, successful commercial application will be necessary to drive development and production scaring. The 15-ton weight desired for AAN ground combat vehicles, coupled with con- straints on transport volume, will require protection structures with good mass efficiency (protection per unit mass) and volume efficiency (protection per unit volume). These generally conflicting, and therefore challenging, requirements could possibly be ~7 J By) C7 ~ 1~ - r ------a - ~ . ~ ~ . . . ~ . ~ .. . . . . . . . achieved by ceramics or ceram~c-r~ch metal composites, such as tungsten carbide, which possesses exceptional volume efficiency, or functionally graded ceramic-metal composites (Gooch, 1998~. Ceramic-metal composites with simple architectures (three- dimensional interpenetrating structures and laminates) are being developed to combine the toughness of the armor metals with the hardness of ceramics. Composites containing borides, carbides, or nitrides, many of which have not been fully evaluated because of cost and scare-up issues, may also prove to be useful in components of vehicle protection systems (Thompson, 1998~. Functionally graded materials, including intermetallics and ceramic-metallic composites, have demonstrated excellent protection characteristics in bench-scale testing of tiles. However, engineering development of promising candidates will be necessary to explore their performance in a system. In addition, the processing methods used to produce the prototype materials are too costly, for the reasons mentioned above for titanium alloys. To bring down the cost and ensure the availability of structural components, processing methods, manufacturing and fabrication quality control, and finishing techniques will have to be improved. NEW ARCHITECTURES FOR PROTECTION SYSTEM COMPONENTS Among the new scenarios for protection systems are (~) reactive armors that include systems with a surface layer of an explosive material that can counteract the force of the incoming projectile; (2) active protection systems that can detect the projectile and eject a plate or shot that can stop it away from the surface of the vehicle;
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194 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT (3) smart armors that can detect incoming projectiles and assess the damage they cause; (4) electromagnetic armor (or shielding); (5) surface geometries that can deflect projectiles and reduce the effects of impact; and (6) systems that incorporate stealth technologies that can reduce the vehicle' s electronic or thermal signature or alter its visible profile (making it harder to identify and target). New candidates for protection system components also include architectural concepts borrowed from natural biological systems that have evolved to absorb substantial amounts of kinetic energy from impact without catastrophic failure. Some of these concepts involve stopping projectiles without the brute protection characteristic of conventional passive armor materials. "Dynamic" materials could be developed that could change properties when they are hit to absorb significant amounts of energy by altering their lattice structure, their architecture at the micrometer and nanometer scales, their defect structure, or their bulk volume or shape. PROCESSING AND FABRICATION TEClINOLOGIES Along with developing new materials options for building blocks and new architectural concepts to use them, new cost-effective processing and fabrication tech- nologies will be necessary to keep protection system components affordable and available. Research in new processing technologies, including technologies that can pro- duce structures with nanometer-thick, ordered layers (nanostructures) of intermetallics, ceramics, and metals, could provide major breakthroughs in the next decade. Research on processing technologies that can produce scare-specific structures could lead to novel architectures in which ordered intermetallics, ceramics, and metals would form distinct structural patterns at different scales. A critical issue for the production of functionally graded or nanostructured armor materials is that processing technologies must be cost effective and amenable to scaling. Electron beam deposition, laser processing, and venous thermal deposition processes might be further developed to produce finely layered or functionally layered components Tom these matenals. REFERENCES Gooch, W. 1998. Personal Communication from Walter Gooch, Army Research Laboratory, to J. Pickens. May 1, 1998. Havel, T. 1997. Discussion with members of the Engagement Panel of the Committee to Perform a Technology Assessment Focused on Logistics Support Requirements for Future Army Combat Systems, at Army Research Laboratory, Aberdeen, Maryland, November 24-25, 1997. Havel, T. 1998. Personal Communication from Thomas Havel, Army Research Laboratory, to J. Pickens. May 1, 1998. NRC. 1993c. STAR 21: Strategic Technologies for the Army of the Twenty-First Century. Lethal Systems. Board on Army Science and Technology. National Research Council. Washington, D.C.: National Academy Press. Thompson, J. 1998. Personal communication from Dr. James Thompson, U.S. Army Tank- Automotive and Armaments Command, to J. Pickens. April 28, 1998.
Representative terms from entire chapter: