Phase II Final Report - NASA's Institute for Advanced Concepts

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Planetary Exploration Using Biomimetics An Entomopter for Flight on Mars heat from the source to the hydride. Hydrides constructed of heavy metals, such as vanadium, niobium, and iron-titanium, will release hydrogen at ambient temperatures. This avoids the heating issue, but these hydrides tend to be heavy and reduces the percentage weight of hydrogen carried. Other hydrides constructed of lighter materials will need to be heated from an auxiliary source until the temperature is sufficient to release hydrogen. Table 3-19 shows a list of potential metal hydride storage materials and the density of the hydrogen stored within the material. It should be noted that the density given in this table is for the hydrogen alone and does not represent the hydride material or any other ancillary components needed for the storage system to operate. Table 3-19: Metal Hydrides and Hydrogen-density Capability [4] Metal Hydride Hydrogen Density (kg/m 3 ) Magnesium (MgH 2 ) 109 Lithium (LiH) 98.5 Titanium (TiH 1.97 ) 150.5 Aluminum (AlH 3 ) 151.2 Zirconium (ZrH 2 ) 122.2 Lanthanum (LaNi 5 H 6 ) 89 Even though the storage density of hydrogen in a metal hydride is high, the total mass of the system is large. Table 3-20 lists the specifications for commercially available state-of-the-art metal hydride storage tanks. Table 3-20: Commercially Available Metal Hydride Hydrogen-storage Specifications [90] Volume H2 (m2) Mass H2 (kg) Metal Hydride Tank Mass (kg) Percent Mass of H2 of Total Tank Mass 0.042 0.0036 1 0.36% 0.068 0.0058 0.86 0.68% 0.327 0.0273 6.1 0.45% 0.906 0.0767 16.78 0.46% 1.274 0.1078 24 0.45% 2.547 0.214 36 0.59% Other systems under development can offer higher mass fractions of hydrogen. These systems promise mass fractions up to 5% (6 kg hydrogen storage for a 120-kg system mass). [79] 186 Phase II Final Report
Chapter 3.0 Vehicle Design 3.5 Fuel Storage and Production 3.5.3.1.1.3 Carbon Nanotubes Carbon nanotubes are tubular carbon structures on the order of 2 nm in size. These structures are theoretically capable of storing hydrogen within the tube structure. The storage mechanism is similar to that of metal hydrides, except the hydrogen-storage capability is much greater. It is theorized that carbon nanotubes can store anywhere from 4% to 65% of their weight in hydrogen. This technology is very new and still in the development stage. However, if it can live up to its projected potential, this would by far be the lightest, most efficient way to store hydrogen. 3.5.3.1.1.4 Glass Microspheres Glass microspheres store hydrogen in tiny hollow spheres of glass. If heated, the spheres’ permeability to hydrogen will increase. This provides the ability to fill the spheres by placing the warmed spheres in a high-pressure hydrogen environment. The hoop stresses achievable for glass microspheres can range from 345 Mpa (50,000 psi) to 1,034 Mpa (150,000 psi) [216]. The corresponding maximum pressure sustainable by the micro-sphere is calculated in the same manner as that for any other sphere. This is given by Equation 3-42, which would be solved for P for a given wall thickness (on the order of 0.68 µm). On the high end, this is comparable to the stress achievable with carbon fiber tanks. Once cooled, the spheres lock the hydrogen inside. The hydrogen is released by subsequently increasing the temperature of the spheres. This method of storage provides a safe, contamination-resistant method for storing hydrogen. The fill rates of microspheres are related to the properties of the glass used to construct the spheres, the temperature at which the gas is absorbed (usually between 150° C and 400° C) and the pressure of the gas during absorption. Fill and purge rates are directly proportional to the permeability of the glass spheres to hydrogen with increases with increasing temperature. At room temperature, the fill/purge rate is on the order of 5,000 hours, at 225° C, it is approximately 1 hour; and at 300° C, it is approximately 15 minutes [216]. This dramatic increase in hydrogen permeability with increasing temperature allows the microspheres to maintain low hydrogen losses at storage conditions while providing sufficient hydrogen flow when needed. Engineered microspheres provide the greatest advantage for high-density storage of hydrogen. If the engineered microspheres can achieve the high hoop-stress values suggested above, it is estimated that a bed of 50-mm-diameter engineered microspheres can store hydrogen at 62 Mpa (9000 psi) with a safety factor of 1.5 and a hydrogen mass fraction of 10% [216]. This produces a hydrogen density of 20 kg/m 3 . There is a trade off between storage pressure and mass fraction of hydrogen stored. At lower storage pressures, the mass fraction of hydrogen stored increases but the overall hydrogen volumetric density decreases. This is caused by the increase in the glass-sphere wall thickness needed to withstand the increase in storage pressure and maintain the same factor of safety. Figure 3-151 shows how the hydrogen mass fraction and volumetric density change for various storage pressures [216]. Although the storage capability of glass microspheres looks impressive there are a number of drawbacks to their use from a system standpoint. The main issue is that, to get the hydrogen into and out of the spheres, they must be heated. This heating takes considerable energy and time to accomplish. The higher the heating, the quicker the hydrogen will purge from the spheres. From a system standpoint, however, this heating, which can be significant, must be accounted for. 187
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Planetary Exploration Using Biomime
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Table of Contents Table of Contents
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List of Tables List of Tables Table
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List of Figures List of Figures Fig
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List of Figures Figure 3-54: Pressu
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List of Figures Figure 3-138: Stere
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List of Figures Figure 4-24: Averag
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List of Contributors List of Contri
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Executive Summary Executive Summary
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Chapter 1.0 Introduction Chapter 1.
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Chapter 1.0 Introduction Figure 1-2
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Chapter 1.0 Introduction 1.1 Histor
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Chapter 1.0 Introduction 1.2 Origin
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Chapter 1.0 Introduction 1.3 Missio
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1.3.3.1 Surface Imaging The Entomop
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Chapter 2.0 Entomopter Configuratio
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Chapter 3.0 Vehicle Design 3.1 Wing
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3. Density: 1.40E-2 kg/m 3 4. Press
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Chapter 4.0 Entomopter Flight Opera
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Chapter 5.0 Potential Payload Funct
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Chapter 6.0 Media Exposure 6.1 Intr
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Chapter 6.0 Media Exposure 6.3 Onli
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Chapter 6.0 Media Exposure 6.6 Spec
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Chapter 7.0 Conclusion Chapter 7.0
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Appendix A: Mars Atmosphere Data JP
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Appendix A: Mars Atmosphere Data Ge
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Appendix A: Mars Atmosphere Data Ge
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Appendix A: Mars Atmosphere Data Ma
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Appendix B: Sizing Results Appendix
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Appendix B: Sizing Results 30 Wing
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Appendix B: Sizing Results 16 Wing
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Appendix B: Sizing Results Length 0
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Appendix B: Sizing Results Lenght 0
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Appendix C: List of References Appe
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Appendix C: List of References 41.
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