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Conceptual Designs for Volatile Mining Operations in Lunar Cold Trap Environments

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Conceptual Designs for Volatile Mining Operations in Lunar Cold Trap Environments 44th International Conference on Environmental Systems ICES-2014-195 13-17 July 2014, Tucson, Arizona Conceptual Designs for Volatile Mining Operations in Lunar Cold Trap Environments Kyle Kotowick ,∗ David Barmore ,∗ Lynn Geiger ,∗ Thomas Coles ,∗ and Jeffrey Hoffman y Massachusetts Institute of Technology, Cambridge, MA, 02139, USA Long-term lunar habitation is an extraordinarily expensive endeavor, but can be made substantially more feasible through in-situ resource utilization of volatile compounds. One of the most likely locations of such volatiles is in lunar cold traps (LCTs): crater interiors near the north and south poles that lie in permanent shadow due to the sun angle being constantly below the crater rim. With no sunlight reaching the surface, LCT temperatures lie in the 30-50 K range, allowing volatile compounds to permanently exist in solid states. The lack of cyclic sublimation leads to a possible accumulation of these volatiles, such as water and methane, over several billion years. Orbital instruments on LCROSS and LRO suggest that the upper layer of regolith within LCTs could contain up to 2-5.6% water by mass. The opportunity to extract significant amounts of water from LCTs would have wide- ranging applications, including life support resources and rocket propellant for local use or export to Earth orbit. This paper describes a conceptual design for a robotic volatile mining operation within Shackleton Crater near the southern pole, one of the most promising locations for volatile accumulation, with a focus on designs for handling the harsh thermal and lightless environment of LCTs. Specifically, this paper provides a thermal and power requirements analysis, proposes a base/rover architecture, and explores the tradespaces for mining rovers, power production and transmission systems, navigation systems, resource export systems, hibernation (wintering) options, and initial set-up options. The proposed overall design is comprised of a feasible selection for each of these, with particular concern for integration, mobility, and scalability. I. Introduction One of the greatest barriers to long-term lunar habitation is that of resource requirements, which require a substantial and extravagant ongoing financial commitment. Continuous supplies of oxygen and water are required for human life support systems,1 but could also be used for greenhouses,2 electrical power production with fuel cells and a supply of hydrogen,3 or electrical power storage with a reversible fuel cell. The expense would therefore be reduced significantly if the transport of these resources could be performed at lower cost or even rendered unnecessary. It will now be shown that the key to reducing transport costs is reducing the requirement for launching propellant from Earth; this would not only be significant for a potential future lunar colony, but also for existing commercial and government interests in space. The fuel/oxidizer combination offering the highest specific impulse is hydrogen/oxygen, which is commonly used in rocket upper stages to transport communi- cations satellites from a LEO parking orbit to GTO.4 Similarly, hydrogen/oxygen upper stages are often the best chemical propulsion choice for other in-space maneuvers, including trans-lunar injection and trans-Mars injection, as long as they occur soon after launch and hence do not require long-term storage of cryogenic propellants. However, although the high specific impulse means that the propellant mass is lower than for other fuel/oxidizer combinations, it is nevertheless generally the dominant contributor to the overall mass that must be launched from Earth for any of the missions just described. For example, the hydrogen/oxygen propellant mass for the Delta IV launch vehicle's 4 m upper stage is 20,410 kg, which is much larger than the stage's 2,850 kg dry mass or even the largest payload mass that it can place into GTO: 6,390 kg.5 The mass fractions on other hydrogen/oxygen stages are similar, as shown in Ref 6. ∗Corresponding Author, Graduate Student, Department of Aeronautics and Astronautics yProfessor of the Practice, Department of Aeronautics and Astronautics 1 of 37 International Conference on Environmental Systems In light of this, the costs associated with both space exploration and geostationary satellite launches could be significantly reduced if most of the upper stage propellant did not need to be lifted out of the Earth's gravity well, i.e. if the upper stage could be refuelled with propellant already in LEO (or even already in lunar orbit to return astronauts to the Earth after a lunar mission). It would then be possible to either carry a larger payload on the same launch vehicle or carry the same payload with a smaller first stage. The latter is particularly significant for the future of space exploration, as the NASA Space Launch System vehicle currently under development will be extremely expensive; this is not only because of its size and performance, but also because its missions will be relatively infrequent, resulting in a slow learning curve for efficient operation and a significant impact from fixed costs. These issues would be addressed by the use of a smaller existing commercial launch vehicle instead; such a vehicle would be more than sufficient to launch the individual components of an exploration architecture, but it would not be able to carry the large propellant mass required and hence would require refueling.7 Refueling is a considerable technical challenge, not least because cryogenic liquids must be stored for a considerable time before being offloaded. However, there has been significant work on propellant depots in recent years with concepts involving sun shields attached to lengthened ULA rocket upper stages.7, 8, 9, 10 Sun shields are not perfect, but cooling to lower-than-normal temperatures before launch is one approach to allowing for a long storage time before any significant propellant boil-off occurs.11 Even though there is existing interest in refueling to improve launch vehicle capability, the propellant would nevertheless still need to be transported from the Earth at considerable expense. The problem could be solved by transporting it from the Moon instead; it would still not be free, but the significantly lower gravity of the Moon would simplify the problem. With this as motivation, this report is concerned with the extraction of water from the Moon's surface; it could then be electrolyzed to form hydrogen and oxygen propellant for transport to the depots described above. Water extraction is one aspect of in situ resource utilization (ISRU): the use of local (lunar) materials. In the context of a lunar colony, water extraction could serve to not only reduce transportation costs, but also to meet the colony's water and oxygen requirements with local resources without transport from Earth.12 One recent study estimated that the use of ISRU would result in a reduction of the requirement for mass launched from Earth by up to 90%, when applied to both propellants and colony resources.13 The presence of water on the Moon was first suggested in 1961,14 but compelling empirical evidence first arrived in the form of neutron readings made with a detector on the Lunar Prospector spacecraft in 1998-1999.15 These neutron readings identified the presence of hydrogen, thereby indicating a high probability for the presence of water. Further evidence came with the analysis of the top 1-2 mm of soil by a spectrometer on the Chandrayaan-1 spacecraft16 and an impactor that it released. Final confirmation arrived when LCROSS impacted the permanently shadowed Cabeus crater17 and the accompanying LRO spacecraft provided detailed radar, altimeter, thermal, and neutron data to more precisely describe the water distribution.18 Note that earlier work had indicated the presence of blocks ice on the Moon by inference from radar measurements;19 however, this inference was later demonstrated to be incorrect, as the signal that supposedly corresponded to ice was in fact due to rough terrain, such as that found on crater walls.20, 18 Lunar Cold Traps (LCTs) are regions that are in permanent shadow, found in craters sufficiently close to the poles for the sun angle to always be lower than that of the crater rim; some cold traps have not seen the Sun for billions of years and have accumulated water ice deposits in that time.21 It is from these deposits that ice could be mined for use as propellant or in support of a colony. This paper presents a conceptual design for a lunar volatile mining project. It begins by describing the environment of the LCTs, followed by a general overview of an operation architecture and base layout. The proposed technical design is then described, starting with the rover, which uses JPL's ATHLETE as a baseline. The ATHLETE rover's abilities are assessed in light of the requirements of an LCT mining operation, with an emphasis on the thermal and power systems. This is followed by a discussion of the requirements for winter hibernation, along with the energy collection system to be employed. Finally, ideas for potential future work on this project are presented. II. Lunar Cold Traps II.A. Environment Both the north and south polar regions of the Moon contain craters with floors in permanent shadow from a highly oblique angle of insolation. Because the lunar axis of rotation is inclined by only 1.5◦ from a normal 2 of 37 International Conference on Environmental Systems to the ecliptic,22 the insolation does not vary much over the course of a year and so temperatures reach as low as 29 K in places, see Figure 1.21 The temperature in these craters is approximately 40 K on average, giving them the name Lunar Cold Traps (LCT). Cold trap areas cover roughly 5100 km2 in the south polar region and 2600 km2 in the north.
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