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

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Planetary Exploration Using Biomimetics An Entomopter for Flight on Mars 3.5.3.1.2.1 Gelled Hydrogen To further increase the density of liquid hydrogen it is possible to produce a gelled liquid hydrogen that produces an increase in density over conventional liquid hydrogen. It is estimated that gelled hydrogen can produce a 10% increase in the density over standard liquid hydrogen. Gelled hydrogen is produced by introducing a gellant into the liquid hydrogen. This gellant can be either another cryogenic material, such as solid ethane or methane, or silica particles [91]. Gelling agents constructed from other types of hydrocarbons, such as ethyl alkoxides or hexyl alkoxides are also under development [205]. Figure 3-155 shows how the propellant density can increase with increasing gelling agent concentration. This figure is based on data obtained from Reference 289 for methane as the gelling agent. Gelled hydrogen reduces the boil-off rate by two to three times compared to standard liquid hydrogen. There is also a significant safety benefit to using gelled hydrogen: It has an inherently smaller spill radius due to its higher viscosity than liquid hydrogen, which minimizes its effect if spilled. The increased viscosity reduces the leak potential by making the hydrogen more resistant to seeping through small openings. The higher viscosity also reduces the amount of hydrogen slosh within the storage tank, reducing the chance for instabilities produced by the movement of the hydrogen. [205] 200 180 160 140 120 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 Percent Weight of Methane Added Figure 3-155: Theoretical Increase in Density with the Addition of Methane Gelling Agent [284] 194 Phase II Final Report
Chapter 3.0 Vehicle Design 3.5 Fuel Storage and Production 3.5.3.1.2.2 Cryocooler for Boil-off Compensation Based on the results shown in Figure 3-153, the boil-off rate for the liquid hydrogen is too high to be useful for a mission that can last for more then a year from launch to completion. A way to reduct boil-off therefore must be devised if liquid hydrogen is to be used. Previous work focused on eliminating boil-off in cryogenic systems by incorporating a cryocooler into the storage tank. [210]. The cryocooler is used to condense the hydrogen vapor that forms in the ullage (excess area in the tank not occupied by liquid hydrogen) of the tank. This vapor is generated by heat leakage into the tank through the insulation. By sizing the cryocooler to match the heat leakage, the system effectively can have no boiloff. A typical single-stage cryocooler for reaching liquid hydrogen temperatures is shown in Figure 3-156. The energy loss associated with the boil-off rates shown in Figure 3-153 are given below in Figure 3-157. This figure shows the amount of power needed to be supplied by the cryocooler Figure 3-156: Single-stage 20 o K Cryocooler [41] to compensate for the leakage of heat into the tank through the insulation. This power consumption is based on the latent heat of vaporization of hydrogen and the rate of heat flow into the tank, given in Equation 3-55. 195
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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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Phase II Final Report - NASA's Institute for Advanced Concepts

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