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4 Fuel and Energy Fuel and ammunition constitute the major share by weight and volume of materiel consumed by deployed forces. This chapter is concerned with the efficient provision and effective use of fuel as a fundamental energy resource for an AAN force. The significance of fuel and energy as logistics burdens cannot be overemphasized. For an Army heavy division, fuel accounts for 70 percent of the weight of re-supplies; even for light divisions, fuel accounts for around 30 percent (Mann, 1997; Petrick 1990~. Moreover, the availability of fuel (through resupply lines) dictates the tempo of battle. An AAN battle force to minimize the dependence on resupplied energy' must be as energy efficient as possible. There are three logical approaches for the battle force to meet its energy needs: to increase the total energy supply, to decrease the total energy demand, or to increase the efficiency of energy utilization and management. These three nonexclusive approaches are discussed separately below. INCREASING THE ENERGY SUPPLY A fuel is a substance that stores energy in a readily usable form. Because energy cannot be created, the supply of stored energy (fuel) available to an AAN battle force can only be increased in two ways.2 Either more stored energy (more fuel) can be transported to the battle force, or energy from a source material available at the staging area can be converted to a stored form of energy (fuel) usable by the battle force. The challenge is to perform one or both of these operations without increasing other logistics burdens in the process. One strategy for transporting more stored energy is to transport a fuel that has a higher energy density (more energy per unit mass of fuel). The same amount of energy (or more) can then be supplied to the battle force for a given weight and volume of fuel. The committee reviewed two candidates for increasing energy density: hydrogen and nuclear fuel. Although both have a higher recoverable energy per unit mass than 'The related but distinct issue of electric energy management (i.e., optimal design of electric power transmission lines, generators for power conversion, switching gear, and electrical energy storage) was covered extensively in the STAR 21 study (NRC, 1993a). 20f course the law of energy conservation (energy can be converted from one form to another but cannot be created or destroyed) holds true for nuclear reactions and high-energy physics only if matter is understood as a "condensed" form of energy, in accordance with the relation between energy E and mass m: E = mc2. This reminder is relevant to the energy supply for AAN missions if nuclear energy is considered as a source. The energy from a "nuclear fuel" comes from converting a tiny amount of the mass in the nucleus of an atom into a huge quantity of other forms of energy, some of which could eventually be converted to the electrical output of a reactor-generator system. 48
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FUEL AND ENERGY 49 petroleum fuels, there are also significant complicating factors, including associated logistics burdens. For reasons developed in later sections of this chapter, neither of these candidate fuels by itself offers a plausible alternative to the traditional fuel supply system, which is based on transporting and consuming refined petroleum fuels.3 She inherent difficulty in the second approach, producing fuel from available materials is that it requires a farce source of energy near the Point of use to convert the . · . · . ~ . _ · . · . . ~ ~ . . .. ~ . _ . ~ _ ~ source material Into fuel. In special situations te.g., during the Persian Cult war, when large reserves of crude of! and the refineries to convert them to usable fuels existed in the theater), a battle force may already have a primary energy source and the means of converting it to fuel at the staging area. in most cases, though, the energy source and the conversion facilities must be transported at least to the staging area, which creates another set of logistics burdens. In short, the second approach cannot be depended on for all AAN mission scenarios unless it also includes the first approach transporting an energy source with far more energy for the weight and volume than petroleum fuels have. The committee considered a strategy that not only combines both approaches but also might overcome the obstacles to the use of nuclear and hydrogen fuels, provided that a number of unsettled issues can all be resolved favorably. In this high-risk, but very high-payoff, strategy, stored energy in the form of nuclear fuel would be shipped to the staging area where lightweight, modular, transportable nuclear power plants would con- vert it to electrical energy. She electrical energy would then be converted by electrolysis of locally obtained water to stored energy in hydrogen, and hydrogen would replace pe- troleum fuels as the principal battlefield fuel for the AAN force. She obstacles to success of this strategy illustrate why increasing the energy supply is not the best approach to reducing logistics demand. A coupled nuclear-electric-hydrogen fuel supply system depends on technolo- gies associated with using nuclear fuels and hydrogen fuel. The component technologies are discussed in detail below to clarify the comparative advantages and disadvantages of the coupled fuel-system strategy. Of the conventional hydrocarbon fossil fuels (oil, coal, natural gas, and their distillates and derivatives), diesel fuel (LOPS) has the highest energy content per unit volume. JP-8 is at least competitive with other fossil fuels and is better than many in terms of energy content per unit weight. The strategic, operational, and tactical logistics of using diesel fuel have been proven in war and in operations other than war for more than half a century. Diesel fuel is, therefore, a reasonable baseline against which to compare alternative fuels for the AAN battlefield and the logistical systems alternative fuels may require. Hydrogen as a Battlefield Fuel Storage Problem Interest in hydrogen as an alternative to diesel fuel is based on its very high energy content per unit weight (33.2 kwh/kg versus 12.8 kwhlkg for diesel fuel), which means that, considering fuel weight only, hydrogen offers the same energy as diesel fuel 3JP-8 essentially diesel oil- is the standard battlefield fuel. The ideal is to have a single battlefield fuel to simplify logistics.
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50 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT at 38.5 percent of the weight. In practice, this reduction in logistics burden cannot be realized because even highly compressed hydrogen gas occupies a larger volume than the same weight of diesel fuel and requires a heavier and larger tank, resistant to hydrogen embrittlement, to hold it safely. Cryogenic storage of liquid hydrogen also creates a large burden in container weight, as well as the safety risks of venting hydrogen gas. In one study, the weight of a vehicle storage tank for 6.S kg of liquid hydrogen was estimated at 42 kg (Kuhn et al., 1996~. An advanced storage tank for gaseous hydrogen made of carbon-graphite composite wrap with plastic liners is estimated to weigh 6.25 kg per kilogram of stored hydrogen.4 Even assuming a 50 percent reduction in storage weight by 2025, liquid or gaseous hydrogen and storage tanks equivalent in energy content to 500 kg of diesel fuel would weigh about 792 kg (192 kg of hydrogen plus 600 kg for the tank). In addition to the weight disadvantage, the hydrogen system would occupy 6.1 times the volume (about 585 cm3/kWh) of its diesel fuel equivalent (96.3 cm3/kWh) respectively, assuming compressed gas stored at 1,000 atm. Storing Hydrogen at Moderate Pressure in an Absorbent Material The conventional methods of hydrogen storage assumed in the preceding discussion are based on either increasing pressure (compressing hydrogen gas) or decreasing temperature (liquefying hydrogen) to decrease the volume required to store a given weight of hydrogen "fuel." Significant progress has been made in the small-scare storage of hydrogen, at much Tower pressures than those used for compressed gas, to replace a heavy battery with an efficient proton exchange membrane (PEM) fuel cell. The hydrogen is absorbed into a material for which it has a high affinity but from which it can be released at a usable rate when the pressure is reduced (to atmospheric pressure). Metal hydrides have thus far been the most successful medium for storing hydrogen in this way. Metal hydride storage technology for hydrogen is becoming competitive in weight and cost efficiency with rechargeable batteries for some soldier-portable equipment (such as radios), but the energy density is far from competitive with diesel fuel for battlefield fuel requirements. It would take a major breakthrough in the amount of recoverable hydrogen stored in the absorbent material per unit weight and unit volume of the storage system for hydrogen fuel to be competitive with diesel fuel. Some preliminary, and still controversial, research suggests that a revolutionary breakthrough is possible that would enable the storage of nearly pure hydrogen at extremely high densities in a lightweight package. A new form of carbon "nanotubes" are reputed to hold up to 50 percent hydrogen by weight (at or above the density of liquid hydrogen) and at room temperature and 136 atm (Matthews and FeUkiw, 1997~. By comparison, state-of-the-art metal hydride storage systems typically hold only about 1 to 5 percent releasable hydrogen by weight! Several laboratories, including at least two in Europe, are working on safe, high- density storage. Hydrogen could compete with petroleum-based fuels as the standard fuel provided that all of the following conditions can be achieved through further research and development: 4 This estimate includes a safety factor of 1.5.
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FUEL AND ENERGY 51 . The storage technique or a similar concept can be scaled up to practical quantities (for battlefield vehicle range between fuelings). The performance parameters reported in this preliminary work~ensity, temperature, pressure, and percent hydrogen by weight are maintainable in a storage system that can be scaled to meet the fuel needs of AAN vehicles. The stored hydrogen can be easily extracted (reversal of absorption) and the storage medium reused for many cycles. Although this breakthrough in hydrogen storage might make hydrogen more competitive with diesel fuel on the battlefield, transporting the hydrogen to a suitable vehicle refueling site (an AAN staging area, for example) would remain an issue. Producing Hydrogen Fuel on Site from Water A potential advantage of hydrogen as a fuel is that it could be produced from locally available water by electrolysis if there is an energy source for the electrolysis. By the year 2010, an estimated 90 percent of the worId's population will be living in coastal regions, and the percentage is likely to increase by 2025 (Delmonico and Story, 1997~. If one assumes that the AAN staging area would be situated on a coast or near an estuary or other large body of water, the local water supply could provide a nearly inexhaustible source of hydrogen for an AAN force. If the energy source for electrolysis, plus the systems needed to convert it to energy stored in hydrogen fuel, could be transported to the staging area with less of a logistics burden than transporting the equivalent amount of diesel fuel, then the strategic logistics burden of supplying an AAN force with fuel could be reduced, as well as the operational and tactical burdens. Nuclear Fuel for Transportable Power Plants with High Power Density Energy density is the fundamental reason for considering hydrogen as an alternative to fossil fuels. Based on the enthalpy of combustion, a kilogram of hydrogen has the theoretical potential to supply 33.2 kW h of energy, compared with 12.8 kW h from a kilogram of diesel fuel. Yet this threefold advantage pates when compared with the theoretical energy density of a nuclear fuel, which is around 2,800 kW in/kg or more than 80 times the energy density of hydrogen (Paur, 1997~. In fact, the principal logistics burden in using nuclear fuel is not the weight of the fuel but the weight (and other burdens) of the system that would derive usable energy from it. This section discusses whether a "ready to run" nuclear power plant could be transported to an AAN staging area and, if so, what the logistics burdens would be. Nuclear reactors as energy sources are not viewed favorably in the United States for safety, economic, and environmental reasons. Yet the safety concerns are not insurmountable, and the economics of supplying energy for an AAN battle force are different from those of a commercial power plant supplying the national electrical grid. Two industrialized countries, lap en and France, rely heavily on nuclear power to supply their national electrical grids and have experienced no major problems. The Navy has used nuclear powered vessels (submarines, cruisers, and aircraft carriers) since 1954 without major incidents (Suid, 1990~. The Army even had a nuclear
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52 REDUCING THE LOGISTICS BURDEN FOR THE ARMY~FTER NEXT power program in the 1960s, which was also run without incident. The reasons given for te~inating the Army program were that (1) there were no major cost savings for nuclear power over petroleum-based fuels in the Cold War mission scenarios of that era, and (2) there was no operational requirement at that time for an unlimited supply of fuel within operational distance of the battlefield (Suid, 1990~. The very different mission scenarios that have led to the AAN battle force as a concept of operations for first-in forces suggest that it is time for the Army to reassess the nuclear option as a primary energy source. Several technological developments since the 1960s have brought fieldable nuclear reactors closer to realization. The compact nuclear power source (CNPS), devel- oped by Los Alamos in 1987, tackled many of the technological problems associated with fielding a lightweight nuclear reactor: portability, up to 18 months of unattended operation, a 20-year cycle time between refuelings, absence of weapons-grade material, operation under harsh environments (280 km/in winds, -50°C temperatures), and "walk- away safe" system design (i.e., minimal risk to the environment or local population, even if no action is taken during an accident) (Los Alamos National Laboratory, 19889. The 20-kW power output of the current generation of CNPS reactors is not on the scale of the projected power requirement for an AAN battle force of 88 MW.s Still, the field rugged- ness of CNPS technology is worth examining for applicability to designs for centralized power plants with high energy densities. (The option of dispersing Tower wattage nuclear power plants on the battlefield was not consistent with the AAN operational concept and was not evaluated by the committee.) For the purpose of analyzing the logistical implications of the nuclear power option at a rudimentary level, the committee assumed that nuclear reactor modules for producing electrical energy from the primary nuclear fuel source would be available in the AAN time frame with effective power densities of 0.4 kW/kg. Although they have never been built, there appear to be no technological barriers to building modular, scalable plants in the 10 to 100 MW range with this power density (NRC, 1989; Angrist, 1982~. The exploratory discussion in the next section is based on this assumption. Coupled Nuclear-Electric-Hydrogen System Generating hydrogen from locally available water would have logistical advan- tages tor a primary battlefield fuel, assuming that the revolutionary technology for safe, high-density storage of hydrogen can be realized. The primary energy source for producing hydrogen on site could conceivably be a transportable, modular, nuclear power plant. In effect, a fraction of the energy released from nuclear fission would be converted to electrical energy, a fraction of which would then be converted to chemical energy stored in the hydrogen. If AAN battlefield systems use fuel cells (which convert stored energy in a chemical fuel to electricity) as their primary energy conversion de- vice, hydrogen would be a convenient battlefield fuel because the proximal fuel for fuel cells is hydrogen (see Box 4-1~. AAN systems that use internal combustion engines could also burn hydrogen fuel, although their energy conversion efficiency would probably be less than systems that use fuel cell conversion. 5The derivation of the estimate of nominal power output is contained in the section on "Coupled Nuclear-Electric-Hydrogen System" that follows.
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FUEL AND ENERGY To illustrate the scale of this coupled nuclear-electric-hydrogen en- ergy system and the issues involved, the committee developed the following rough estimate of the reactor capacity required for an AAN battle force unit of 8,000 troops. In a ~ 987 report, the Army Science Board estimated that a light division of 10,277 troops would con- sume 165,000 kg (165 metric tons) of fuel per battlefield day (ASB, 1987~.6 Assuming a lower heating value of 12.8 kWh per kilogram of diesel fuel, this fuel consumption corresponds to an energy requirement of 2,!10 MWh. Prorating this estimate for an AAN battle force of 8,000 troops gives a ballpark estimate of 1,640 MW h for the daily energy required, or an average power demand 68.5 MW. Assuming peak power does not greatly exceed the average and a 78 per cent conversion efficiency for convert ing electrical power to hydrogen, an SS MW nuclear reactor would be required to produce enough hydrogen to meet the energy needs of the nominal battle force. Using the committee's estimate of 0.4 kW/kg for a nuclear power plant in the 10 to 100 MW range, this nuclear power plant would weigh about 220 metric tons and would have to be modularized for shipment to the staging area. The weight of the electrolysis unit, which would produce hydrogen from water, must be added to the weight of the power plant, which converts the nuclear energy to electrical energy. Based on today's commercial technology, an electrolysis unit weighing 100 metric tons produces about 75 kg/in of hydrogen. The 68.5 MW of power translates to a hydrogen production rate of about 2,100 kg/in, which means 28 units would be needed. With today's electrolysis technology, then, the weight of the total nuclear- electric-hydrogen energy supply system would be 3,020 metric tons. The AAN battle force's mission would have to last 19 days for this weight burden to break even with the strategic logistics burden of transporting diesel fuel. The break-even point improves considerably if one anticipates that a more efficient electrolyzer can be developed based, perhaps, on the same technology as fuel 53 BOX 4-1 Fuel Cells A primary advantage of using a fuel cell over a combustion engine to power an electrical generator is that the fuel cell can convert more of the stored energy in the fuel to electrical energy. Fuel cells have been used for more than three dec- ades in space applications, and they are being considered for use as residential and commercial electric power generators, as well as generators for electric vehicles. The focus of recent research has been on developing compact fuel cells that are weight-, volume-, and cost-competitive with traditional power sources, including batteries. Fuel cells are categorized generally by electrolyte and operating temperature, from solid-oxide fuel cells that operate at 1,000°C to proton exchange membrane and direct methanol fuel cells that operate at 25- 90°C. Current research is focused on reducing the cost and complexity of fuel cells powered by hydro-arbon fuels through more active electrocatalysts, lower cost materials of construction, and the development of fabrication processes suitable for mass production (NRC, 1 997a). 6The figure in the report was 57,000 gallons of POL (petrol, oil, and lubricants). For simplicity, the committee has converted this amount to kilograms and assumed a slightly rounded-down amount as the fuel component. Data for both light and heavy units show that the ratio of troops to vehicles (both ground and air) has remained relatively constant, at about 4 to 1. Proration on the basis of relative troop size is, therefore, reasonable.
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54 REDUCING THE LOGISTICS BURDEN FOR THE ARMY~FTER NEXT cells for vehicles. The U.S. Navy is already working to reduce the weight of H2O eleckolyzers used on submarines. The weight of an eleckolyzer unit could probably be reduced by at least a factor of 20, to about 5 metric tons apiece. Then the ensemble weight (nuclear power plant plus advanced eleckolyzers) becomes a more manageable 360 metric tons, or about the weight of 2.2 days' worth of diesel fuel. Any AAN operation that lasted more than three days would then have an increasing advantage in reduced strategic logistics burden with the coupled energy system (Figure 4-1~. This simplified analysis does not take into account the risks of going into a military operation with only one primary source of power, which might be vulnerable to attack, sabotage, or accident. If two 88-MW units were used, the crossover point for payoff would increase (although by less than a factor of two, because there would not have to be twice the number of electrolysis units). Another possibility is that a number of smaller reactor modules might be used at the storage area, with a smaller safety margin to cover the Toss of one or two. (The same approach could be taken with spare electrolysis units.) Even if the assumptions in the estimate are changed, this exercise illustrates several important points. First, a realistic approach to increasing energy supply while decreasing logistics burdens must take into account the entire system, from the materiel that must be transported to the staging area (the strategic logistics burden) to how AAN vehicles are powered and refueled (the operational and tactical burdens). Second, an energy system based on a single petroleum fuel (e.g., diesel fuel) is tough to beat without going to a more complex system (such as the coupled system discussed here) and 2500 _` U' 2000 C' .~ ID 01 500 lo ~5 ID o Q U) Ct~ me o ._ 500 o I .. ..... Nuclear-Electric-Hydrogen Fuel / (Current Electrolyzers) / / , . _ ~o~ 19 Days 2.2 Days l Nuclear-Electric-Hydrogen Fuel (Advanced Electrolyzers) I / . : _ . ~.. - - 0 10 20 30 Duration of Mission (days) FIGURE 4-1 Altemative energy systems for the AAN.
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FUEL 'AND ENERGY 55 making radical changes in many areas. Third, although the coupled nuclear-electric- hydrogen system has the potential to reduce fuel burdens substantially and appears worth evaluating, a significant number of research breakthroughs and technology develon meets, especially in safe, compact and lightweight means of hydrogen storage, would all have to fall into place. Finally, the committee believes that the most important lesson to be learned from this analysis is that reducing energy demand is a far more dependable strategy for meeting AAN energy needs than gambling that unrestrained energy demand can be met by increasing the energy supply. If the gamble did pay off, reducing energy demand without sacrificing other performance characteristics would make a radical al- ternative like the coupled nuclear-electric-hydrogen system easier to implement. REDUCING ENERGY DEMAND The principal consumers of fuel in an Anny division are vehicles. Reducing energy demand thus amounts to reducing the vehicular demand for fuel. In 1993, the Office of Transportation Materials reached the conclusion that "the current, most successful method for improving fuel efficiency is to reduce the weight of automobiles" (DOE, 1993~. This principle can be extended to all vehicles. The most direct way to reduce energy demand is to reduce the weight of vehicles. Four strategies for achieving this goal are explored in this report. The first is to develop a suite of minimally crewed or autonomous (unmanned) vehicles for the AAN battle force. This would reduce the large volume (and hence large weight) associated with larger crews. This strategy is detailed in Chapter 5. A second strategy is to replace currently used materials with materials that provide equivalent performance (toughness, strength, etc.) but weigh less. The third strategy is to select materials and design vehicle components and subsystems for optimized system performance. ("System performance" in this context includes fuel efficiency and other reductions in logistics burdens, as well as the traditional objectives of survivability and crew protection.) Fourth, incremental improvements can be attained with "low tech" solutions that affect how a vehicle is used rather than how it is designed or what armament and protection it carries. Idling discipline, towing vehicles, route selection, and fuel economy mandates during procurement were identified in two Army Science Board studies (ASB, 1984; ASB, 1987) as areas for reducing fuel consumption. The committee found little evidence that the Army has acted on these recommendations. Lighter Vehicles through Materials Substitution Vehicle fuel consumption is directly correlated with vehicle weight. The most likely candidate materials for reducing vehicle weight in the short term (primarily light but expensive substitutes for steel) are alloys of titanium, aluminum, or magnesium. The relative weight savings are approximately 43 percent, 66 percent, and 78 percent, respectively, when these alloys can be used instead of steel (see Table 4-1~. However, density (mass per unit volume) alone is not the deciding factor in selecting a material for a given application. A more precise design parameter is usually a material property critical to the demands of the application, such as stiffness (modulus) per unit mass or
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56 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT TABLE 4-1 Densities of Elements That Form the Basis of Major Structural Alloys Theoretical Density Ratio Element Density~g/cc) Relative to Steel Be 1.85 0.22 Mg 1.74 0.24 Al 2.70 0.34 Ti 4.54 0.58 Fe (steel) 7.87 1.00 Ni 8.90 1.13 Cu 8.96 1.14 yield strength per unit mass7. Nevertheless, density (mass per unit volume) is often a useful initial indicator for assuring materials with broadly similar properties (such as metals) (Ashby, 1992~. Across-the-board incorporation of lightweight materials into Arrny vehicles will require three key advances: a decrease in the cost of lightweight materials, an increase in designer awareness, and accelerated development and empirical verification of new lightweight materials through M&S at the atomic and microstructural levels. Decreasing the Cost of Lightweight Substitutes The near-term incorporation of lightweight materials into Army vehicles will re- quire that their cost be reduced. At present, known substitutes that could substantially reduce vehicle weight are 3 to 30 times more expensive than the steel equivalent. The high cost for lightweight metal alloys of titanium, aluminum, and magnesium reflects one or two factors: the cost of extracting and refining the metal from the raw ore and the cost of processing the metal into its final shape. The auto industry, which is well aware of these cost obstacles, is attempting to promote strategies among its suppliers to lower the cost of extracting, refining, and processing these metals, particularly magnesium (DoC, 1995; Sherman, 19974. The Army can monitor developments and promote its re- quirements through continuing participation in programs involving industry, such as the Partnership for a New Generation of Vehicles (PNGV) and other venues. Non-metallic alternatives should be considered. The ceramic-organic hybrid composite armor developed for the composite armored vehicle prototype, exemplifies the potential of novel ceramic materials. The cost of composites is relatively high in limited quantities, and it would be useful if manufacturing and production trade-off studies were made comparing the cost of composite armors and metallic armors in large volumes. in evaluating alternatives, the cost-performance trade-off analyses should assess life-cycle costs, not just the up-front acquisition costs. (See discussion of life-cycle cost 7Yield strength, ~ / p, is an excellent design parameter for a stationary metal component. If the specific application for the component involves moving parts, where inertial forces become important, a more precise design parameter derived from the yield strength would be (~3 / p.
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FUEL AND ENERGY 57 models in Appendix C.) When life-cycle costs, including relative fuel costs, are used to compare materials options, increases in fuel efficiency alone from using lightweight materials would justify their use in some applications, even with high up-front costs. Information Resources for Improving Materials Selection A second obstacle to the Army's use of lightweight materials is the lack of awareness by system designers of the existence and properties of new alloys and other advanced composite materials. Materials that are not known include materials developed secretly by the former Soviet Union, such as aluminum foam and explosive armor coatings. Software that mines the internet for data can be useful in carrying out searches for information on novel materials. Often, the material selected for a particular application is the material the designer is most familiar with, either from prior experience or from engineering handbooks rather than the best material for that application (Ashby, 1992~. Although the Army already participates in many networks for research on new materials and applications development, it should not overlook the research activities of the other services. The commercial industrial sector, as well as the other services and defense agencies, would also have much to gain from developing better information resources. Therefore, the Army should leverage its resources in this area by encouraging and participating in partnerships and networks that would give systems designers and engineers at all stages in the development, testing, and acquisition process access to the best available information on less well known materials and compare them with commonly used materials. The Army will also need tools to help designers select from and understand this wealth of information; the development and widespread adoption of these tools could be encouraged through the same partnerships and networks. Modeling and Simulation Aids for Designing Materials Appendix C explains the long-term potential of M&S technologies for designing and evaluating new materials and for developing new ways of structuring known materials. M&S technologies will also contribute to the Army's search for substitute materials that could reduce the weight of vehicles while maintaining or improving other critical properties. The functional properties of a material can be altered, controlled, and even designed by manipulating its microstructure on multiple spatial scales, ranging from the interatomic scale (Angstroms) to the nanometer and micron scales. The timely design and assessment of new microstructures will require computer-based tools (1) to mode! new structures (for example, biomimetic materials that mimic properties of structures found in nature), and (2) to use structure-property relations derived from physical theory and materials engineering to predict the bulk properties of a modeled microstructure. Processing issues should be considered simultaneously to ensure that advances in materials design could be readily translated to fabrication. Integrating appropriate processing models into overall simulations will be key to using M&S tools for materials design. The importance of M&S tools for advanced materials design has also been recognized in peer-reviewed studies by the NRC National Materials Advisory Board, one of which concluded:
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58 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT . . . a more comprehensive materials system approach must be developed Mat incorporates materials modeling and processing considerations, and Mat allows designers to interact even more closely win materials developers to ensure the proper application of new materials. (NRC, 1993b, p. 50) The design of materials (as opposed to the selection of materials for design) by modeling and simulation has great potential for future commercial and defense applications. Through research grants, federated laboratories, and various cooperative efforts with the academic and industrial research communities, the Army is already a participant in this rapidly developing field. Whether the technology will mature in time to contribute to the design of the first generation of AAN-era vehicles is an open question. For the Tong term, however, the Arrny should continue to participate in information networks that can tap into this technology as it develops. The Army should also look for opportunities to participate in application-development efforts, particularly joint-service initiatives and other venues where Army contributions could be leveraged. Lighter Vehicles through Optimized System Performance The feasibility of designing vehicles for improved fuel economy by making them lighter has been well demonstrated by the research program of the PNGV (NRC, ~ 994a, ~ 996, ~ 997b, ~ 998; DoC, ~ 995~. in some instances, a material with less mass per unit volume can be substituted more or less directly. Often, however, the functional role of the component or structural material has to be rethought to take advantage of an inno- vation that achieves the same or better functional performance by several measures, including weight reduction. Some of these weight-reducing innovations in the commer- cial automotive industry (including heavy trucks, passenger vehicles, and light trucks) could be adopted either directly or with modifications by the Army. For combat vehicles the Army will have to develop its own system-optimizing approach, however, because some of the performance objectives, such as protection of the vehicle and crew from hostile action, are not major concerns for commercial vehicle designers. Traditional approaches to improving protection have tended to increase rather than decrease system weight. Innovative materials solutions that combine reduced system weight with increases in system performance will reduce AAN logistics burdens. System Optimization of Protection and Other Vehicle Weight Reduction Factors Protection of an AAN combat vehicle and crew must be assessed at the system level, along with other system performance goals, such as cross-country mobility, fuel efficiency, and lethal efficiency (i.e., one round, one hit, one kill). in addition, protection is itself a complex performance goal that includes survivability characteristics (such as shielding or active protection against ballistic impact, blast, heat, directed energy, and nuclear, biological, and chemical agents) and stealth characteristics (such as physical configuration and thermal and electronic signature). If structural and protective components of a vehicle are designed with distinct structures for each function, the system will be too heavy, too large, and too unwieldy to meet AAN performance goals. Optimizing an AAN vehicle for the full range of
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FUEL AND ENERGY 59 protective objectives and other perfo~ance criteria wit! require designing the structural components and the entire protective subsystem for multiple functions. Light, small, agile systems that require minimal field maintenance and logistical support will have to be designed at the subsystem, component, and materials design levels for structural integrity, shielding characteristics, and signature management. Instead of providing protective elements with distinct material components, such as armor plating or applique, the components of the protective system will have to be multifunctional. Similar principles apply to the design of AAN air vehicles (piloted or un- manned), soldier portable equipment, and other combat systems, as well as ground combat vehicles. For each application, the Arrny should consider the full range of op- tions to select the best "system solution" to protecting the AAN battle force while meeting other objectives. Vehicles, shelters, and soldiers will need lightweight, effective protection against diverse threats. Individual soldiers will need protection against small arms fire, but their body armor must not impede their fighting effectiveness. (System- leve! considerations for protective systems for AAN combat vehicles are discussed more fully in Appendix D.) EFFICIENT ENERGY MANAGEMENT The preceding discussions on increasing energy supplies and decreasing energy demands have illustrated the advantages of using a systems engineering approach to determine the best alternatives for AAN mission scenarios. In this section, a systems approach is described for improving the efficiency with which energy, however supplied, is used on the battlefield (e.g., transporting vehicles, soldiers, ammunition, or materiel; powering information systems; and energizing weapons). The focus of this section is on vehicle energy management, on the assumption that a highly mobile, mechanized battle force will rely on vehicle energy systems to supply energy for anything that does not have a self-contained energy supply. (Categories of materiel with self-contained energy supplies include energetics for projectiles "discussed in Chapter 6] and compact power for soldiers "discussed in Chapter 84~. Fuel Economy as a Functional Specification The committee believes that the best overall approach for achieving fuel . . . . . . . . . . ,% . ~ , economy In Army vetches may simply be to specify a maximum fuel consumption target along with other performance specifications to the vehicle manufacturer and allow the manufacturer to make the system trade-offs. In other words, the Army is likely to elicit a better outcome in response to a system-level functional specification than to a design specification. This approach would ensure that the appropriate engine (e.g., diesel, gasoline, turbine, or fuel cell) is chosen for the job and that lightweight materials are used where they are feasible and most beneficial. In procurements of ground vehicles, the Army already uses functional specifica- tions for some performance requirements, such as vehicle range (distance between refuelings), but no limits have been stated for the rate of fuel consumption. The com- mittee recommends that all vehicle procurements include an unequivocal, readily meas- urable limitation on vehicle fuel consumption. Although this functional specification
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60 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT would be critical for AAN systems, it would also be helpful for procurements before 2025. A specification might read as follows: The vehicle will consume no more than one gallon of fuel for each Free [for example] miles of travel on He Aberdeen Proving Ground level ground [for example] course and one gallon of Mel for each two [for example] miles on the cross-country For example] course, using a defined Almy duty cycle. During idling periods the vehicle will consume no more than six [for example] gallons per hour. A functional specification like this would ensure that the manufacturers paid attention to the rate offuel consumption and would force designers to consider the fuel burden as important as other military requirements. In addition, establishing a standardized test course and standardized test procedures specifically for fuel consumption would enable the Army to accumulate valid, comparable data on the fuel consumption rates of ground vehicles. These data could then be used for logistics analyses in force-on-force models, trade-off analyses, mission planning, and mission rehearsals (see Chapter 3~. A similar strategy is used by the U.S. auto industry to meet its fuel economy objectives.S The results can be seen in the figures for "highway miles per gallon" and "city miles per gallon" posted on the window of every new automobile. HYBRID VEHICLES Much has been said, in the Army and elsewhere, about fuel economy in vehicles with electric-hybrid power plants. These power plants consist of a prime mover, such as a diesel or turbine engine, that converts fuel energy to mechanical energy, and an electrical subsystem of one or more electric generator/motors and storage devices (typically, storage batteries or flywheels) to provide electrical energy. The electrical subsystem enables the prime mover to operate at peak efficiency more of the time. It can also be designed to recover kinetic energy from the moving vehicle during braking (regenerative braking). In this report, vehicles with this kind of hybrid energy supply and management system are called "hybrid vehicles." The committee found no reason to assume that a hybrid vehicle, which gains maximum fuel efficiency under stop-and-go driving conditions (such as those typical of a city bus), would have better fuel economy than a prime-mover-only power plant on an AAN battlefield. As far as the committee could determine, the Army has not yet determined the classes of vehicles, much less characterized the duty cycle (or cycles) for representative AAN mission scenarios. There may be other reasons for using a hybrid engine (e.g., silent watch capability or electrically energized armament), but a hybrid vehicle cannot be justified on the basis of fuel economy until the Army has a model to help determine fuel consumption during an AAN duty cycle. (See Appendix E for a discussion of the interrelationship of duty cycle and fuel economy). The committee questions whether AAN operations favor a hybrid vehicle design. The committee found no evidence that the Army has given the relevant factors the systematic, quantitative engineering analysis this important design choice deserves. To improve the fuel economy of the vehicle as a system, a hybrid vehicle would have to The committee wishes to acknowledge the technical and conceptual contributions on this topic of Dr. Wolf Elber, director, Vehicle Technology Center, Army Research Laboratory. Dr. Elber discussed issues of fuel economy with the committee at meetings in August and December 1997.
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FUEL AND ENERGY 61 save more in energy management efficiencies than the energy costs of the added weight of the power plant's electrical subsystem and from energy conversion losses. The efficiencies could come from (~) the operation of the prime mover, (2) the transfer of prime-mover energy to the drive sprockets or wheels and other energy demands, and (3) energy recovery from regenerative braking. An analysis of civilian-vehicle duty cycles has shown that hybrid vehicles do not break even in this balance unless the average power demand of the system in normal operation is (approximately) one-fifth or less of the peak power demand (see Appendix E). For AAN combat vehicles to move at high speeds over rough terrain, the average power demand of a hybrid vehicle would be substantially more than one-fifth of its peak demand. For a combat vehicle to sustain a speed of 130 km/in would require nearly full power for sustained vehicle operation (see Chapter 5~. Furthermore, most of the energy would go not into recoverable kinetic energy but into soil deformations, suspension losses, and aerodynamic Tosses. Although these preliminary considerations are based on rough approximations and general engineering experience, they imply that the fuel economy advantage often assumed for hybrid vehicles cannot be taken for granted in the AAN context. The point of this argument is not that hybrid vehicles should be ruled out but that systematic, detailed analyses that incorporate the relevant factors on duty cycle and nonrecoverable energy losses should be used to assess the potential benefits and make a rational decision. Other requirements of the vehicle duty cycle may make electric drive or an alternative vehicle configuration a better choice. For example, operational requirements for transporting troops or hardware may argue for placing the power plant and drive train components in a configuration for which an electric drive works best. If an electromagnetic gun, active protection system, or directed-energy armaments are part of the system design, which therefore would demand a great deal of electric power, a hybrid vehicle might be the best choice. In short, a rational choice of vehicle configuration is likely to depend more on the overall duty-cycle requirements of the vehicle than on fuel economy. SCIENCE AND TECHNOLOGY INITIATIVES TO REDUCE ENERGY-RELATED LOGISTICS BURDENS Based on the preceding analyses of the logistics burdens associated with the fuel and energy demands of AAN operations and the technological opportunities for reducing these burdens, the committee concluded that the Army should pursue the following areas of scientific research and technology development. The order of the numbered items reflects a rough order of priority. Increasing the Energy Supply I. Systems Analysis of Alternative Fuel Supply Systems. An efficient and reliable fuel supply system is critical for a modern land combat force. An investigation of significant changes in the existing fuel supply system to enable AAN operations and to decrease logistics burdens should be based on a mode! of the entire fuel supply system, including a realistic simulation of all significant logistics burdens and their effects on the supply system and the war-fighting organization. AAN wargames have begun to incorporate
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62 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT realistic modeling of fuel supply logistics at the strategic, operational, and tactical levels. An immediate objective should be to couple a fuel supply mode! to the fuel demands of the AAN system being used in a wargame. Systems to achieve this modeling capability could also be used for fuel system trade-off analyses, mission planning, and training exercises. 2. High-Density Hydrogen Storage. The Army should monitor the progress of research on high-density hydrogen storage. If new technologies can be scaled and have the potential for future applications, the Almy should foster industry partnerships, a joint- service program, or another mechanism for investigating the implementation of this new . . · .. . . .. . . - . . . . . · .. technology. A reasonable next step would be applied research to determine the feasibility of using technology based on high-density hydrogen storage to make hydrogen a battlefield fuel competitive with diesel fuel. 3. Modular Nuclear Power. The Army should investigate the feasibility of scaling modular, transportable, field-rugged, "walk-away safe" nuclear power plants to sizes suitable for use at an AAN staging area. A key consideration is whether an energy density in the range of 0.4 kW/kg can be maintained for modules of practical size (10 to 100 MW) that meet other critical performance, safety, and reliability objectives. 4. Fuel Cell Technology. The Army should continue its support of research and applications development in fuel cell technology. If the investigations into the feasibility of weldable nuclear power plants and high-density hydrogen storage lead to favorable results, the Army should expand its participation in and sponsorship of applied research and development for large fuel cells, perhaps through partnerships with the automotive industry. Assuming that the prerequisite investigations into nuclear energy as a primary source and hydrogen as a storable battlefield fuel are successful, the Army should investigate an electrochemical process for converting electrical energy to fuel energy in hydrogen (reverse of a hydrogen fuel cell). Reducing Energy Demand I. Information on :Lightweight Substitute Materials. The Army should find ways, through the information and research and development networks in which it already participates, to promote and support the development of information resources that would help system designers acquire data on lightweight materials. Information resources could include industry-wide databases of materials properties and user-friendly tools, such as graphical interfaces, for selecting and comparing data on substitutes for conventional materials. 2. Lowering the Cost of Lightweight Materials. The ongoing development of low-cost synthesis and processing technologies to extract, refine, and process lightweight materials (e.g. magnesium) warrants the Army's close observation, involvement, and encouragement. Similarly, the Army should monitor commercial developments for Tow- ering the cost of ceramics and organic composites. Work being undertaken with the encouragement of the auto industry should be monitored and supported, as appropriate. The Annoy can leverage resources by keeping abreast of progress by potential
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FUEL AND ENERGY 63 commercial users and by promoting joint-service and DoD-wide participation with industrial and academic partners. 3. Vehicle System Weight as a Design Criterion for Trade-off Studies. In an ideal scenario, vehicle system weight could be reduced without sacrificing other performance objectives. However, it appears that the stringent requirements proposed for AAN com- bat vehicles will probably require some trade-offs between system weight and other desirable but not mission critical-capabilities. The use of armor to protect against advanced high-velocity projectiles probably will be one of these capabilities. The general requirement of protecting combat vehicles against projectiles can be addressed in many ways and to varying degrees. The integrated, hierarchical modeling environment (dis- cussed in general terms in Chapter 3 and more specifically for vehicle design in Chapter 5) will be essential for evaluating the many technology options for AAN vehicles and for making sound decisions on the optimum strategy for balancing vehicle weight against effective protection. The lower levels of this hierarchical modeling environment could also assist designers in evaluating candidate materials for armor building blocks and new armor architectures. Feeding these vehicle protection designs up to force-on-force simu- lations in the hierarchy would allow the testing of assumptions about the most effective combination of characteristics in AAN wargames. ~. ~_ ~ . . . ~ ~ . ~ . ~ . Efficient Energy Management . I. Fuel Economy as a Functional Specification. Faced with expensive developmental trade-offs, contractors will not design fuel economy into Army combat vehicles unless the Army specifies clear and unequivocal limits for maximum fuel consumption and holds the contractors to those limits. All vehicle procurement contracts should specify readily measurable limits on vehicle fuel consumption. 2. System Analysis of Hybrid Vehicles. Whether an electric hybrid power plant im- proves the fuel economy of a vehicle depends on the vehicle duty cycle. Based on the qualitative descriptions to the committee of how an AAN battle force would operate and a general automotive engineering rule of thumb on the kind of duty cycle in which a hybrid power plant improves overall fuel economy, it seems unlikely that a hybrid vehicle would reduce fuel consumption in an AAN operation. However, a quantitative analysis based on realistic modeling of representative duty cycles and the distribution of energy sinks would resolve this issue. Furthermore, other factors in system design might favor a hybrid vehicle. Once again, this decision calls for a vehicle system trade-off analysis that realistically represents the duty-cycle demands on the power plant, includ- ing nonrecoverable energy losses during prolonged, high-speed, off-road operation.
Representative terms from entire chapter: