Section 4.12: Phase 5A - Lunar Development
The Moon is obvious to anyone who has looked up at the sky. The same reasons for expanding civilization apply to it as to other locations. They include access to new energy and raw material resources for upgrading civilization, and to meet the other objectives listed at the start of Section 4.1.
However, we do not start developing the Moon right away for several reasons. First, we must travel through the Low and High Orbit regions to get there, then navigate the Moon's gravity field to reach closer orbits or the surface. This requires more energy and new transport equipment than the orbital regions. Second, the surface has different conditions than the orbital region around it, or the orbital regions around the Earth. So surface equipment also needs new designs. Third, developing the Moon is made easier with support from the previous orbital locations. So we delay the start of Lunar development until after Phases 4A and 4B have been started, and needed (R&D) for this phase is completed in Phase 0M. The Moon's surface is roughly the size of Africa and Australia combined, so it is much too large to be developed all at once. Once started, Phase 5A will continue in parallel with previous phases, with progressive upgrades and expansions over time, both in orbit and on the surface.
Our concept exploration for developing the Moon begins with describing the characteristics of the region and an industry survey to identify possible future activities. Motivations, economics, technology, and prior developments elsewhere will drive which of these activities can get started and when. Combining this information we can identify a development approach and specific Lunar projects, place them in approximate time order, and link them to other phases and projects. In the course of doing this, we identify needed R&D for Phase 0M, which is recorded in that section.
Lunar Region Features
The Lunar region includes the Moon itself, and orbits with average distances (semi-major axis) within 35,000 km of the Moon's center. At this distance the Moon's gravity exceeds the Earth's by 50% and is the dominant local force. Lunar orbits in general are unstable due to the influence of the Earth and Sun. The Moon also has mass concentrations from past impacts that create an uneven gravity field. Particular orbits, where these effects are minimized, can be stable for long periods. Otherwise propulsion must be used to correct them, or the orbit allowed to evolve over time. Since the Moon orbits the Earth, the Lunar region is embedded in the larger High Orbit region around our planet.
As noted above, horizontal surface area of the Moon is quite large, 37.93 million km2 or about one quarter of the Earth's land area. This does not include the sloped exposed area from large-scale topography (Figure 4.12-1) and numerous craters and other small-scale features. See USGS Topographic Map 3316 of the Moon, 2015 for a more detailed map.
The Moon orbits the center of the Earth-Moon system every 27.3 days with respect to the stars (inertial frame), and 29.5 days with respect to the Sun, which sets the length of the Lunar day. The Moon makes one rotation per orbit period, and therefore keeps approximately the same side facing Earth. It is not exact because the Moon's orbit is not circular, it has a slight residual pendulum motion, and the Earth's diameter is 1/30 of the Moon's distance. That size allows varying views of the Moon, depending on where you are. Orbit durations around the Moon vary from 108 minutes close to the surface, to 6.8 days at the upper edge of the region. The main environment and resource features of the region are as follows:
Temperature - The Moon has the same average distance as the Earth from the Sun, so the incoming Solar Flux of 1361 W/m2 is the same. That sunlight is partly blocked by the Moon's shadow in lower Lunar orbits, and blocked 50% or more on the surface on a monthly cycle. There is no significant atmosphere to moderate temperature changes on the surface. Objects exposed to space and direct sunlight can vary from 100-400K at the equator, and as low as 30K in shadowed polar craters. The surface is covered by particles separated by vacuum, and is an excellent insulator. Soil temperatures more than 30 cm deep are not expected to vary more than a few degrees from average, despite two week intervals of daylight and night. The average soil temperature is about 240K at the equator (Vasavada et al. 2012), and colder with increasing latitude and lower sun angles. Heat flow from the hotter interior has been measured at 0.008-0.03 W/m2, depending on location. The concentration of radioactive elements in the crust and good insulation properties leads to a somewhat uncertain thermal gradient of 10-50 K/km with depth.
Temperatures in orbit will mainly depend on the percentage of time in sunlight vs in the Moon's shadow, and how much light is reflected or emitted from the Moon at a given distance. The Earth appears about 2 angular degrees in size from the Lunar region, and thus fills less than 0.01% of the sky, and makes a small thermal contribution. The remainder of the sky is the cosmic background at near absolute zero (2.7K), which serves as an effective heat sink. Since orbits can be elliptical and change over time, the resulting thermal environment can vary significantly. The albedo, emissivity, and thermal properties of an orbiting object will determine how much heat is gained and lost from environment conditions, and what equilibrium temperatures they will reach.
Atmosphere and Water - As noted under temperature, the Moon's Atmosphere barely exists. It's total mass is less than 10 tons, which is the Earth's atmosphere over a single square meter. If pressurized environments are needed for people or equipment, the gases need to be produced locally, or imported. About 50 kg/s of solar wind particles flow through the region at high velocity. Since this is spread over an area of 3.85 billion km2 it would be difficult to collect in useful amounts. An estimated 600 million tons of water ice has been discovered in permanently shadowed craters near the poles. The remainder of the Moon's surface has 10-1000 ppm of chemically bound water in minerals. The dry and airless state of the Moon is due to the low escape velocity, coupled with extended periods of early heating from impacts, tides, and radioactive decay.
Ground Loads - Ground strength for surface structures and transport is adequate to excellent. The surface consists of broken rocks and dust of various sizes, which compacts a few cm but then can support heavy loads. Excavation, mining, or drilling would reach the limits of overlying rock strength at ~38 km. Depths below this require support structures. Much of the shallower depths consist of fractured material, and will also need support against movement and collapse.
Gravity Level - Surface gravity averages 1.625 m/s^2, or 1/6th of Earths, with a total variation of 0.0253 m/s^2 according to location (Figure 4.12-2). Free-fall conditions in orbit produce no effective acceleration. Structural support needed against gravity is therefore much lower than for Earth. The gravity levels required for long-term human health and plant and animal growth are not yet determined, but are likely to be more than natural surface or free-fall levels. Artificial gravity on both the surface and in orbit can be generated by rotation, otherwise stay times may be limited. Centrifugal forces from rotation will impose significant design loads on structures.
Radiation Level - Unprotected radiation levels on the surface and and in orbit are in the range of 0.1-1 mSv/day, which could reach lifetime astronaut exposure limits in a few years. A single powerful solar flare can deliver lethal amounts of radiation, although most are much weaker. Radiation also causes long term damage and electronic faults in equipment. The radiation mainly comes from the Sun and cosmic background. A meter or more of cover by Lunar soil or building underground can provide safe radiation levels on the Lunar surface. Bulk mass of various kinds can provide shielding in orbit.
Communication and Travel Times - Round-trip (ping) communication times from Earth to the Lunar region varies from 2.2 to 2.94 seconds, depending on where in the region, and the Moon's distance in it's orbit. This includes satellite relay time if you are communicating with areas behind the Moon that can't be seen directly from Earth. Travel time from Earth is nominally 3-4 days for people, by direct transfer orbit. Cargo delivery by electric tug is much more efficient, but also much slower. Slow transits without shielding or other mitigation would expose people to high levels of radiation while traversing the Van Allen belts.
Stay Time - Average stay times for people affect transport needs and the types of habitation required. Only nine trips with people have been made so far to Lunar locations, and those lasted only a few days each. These trips were made nearly 50 years ago. This data is too sparse and too old to establish an average for design purposes. Future projects will need to define stay times based on internal needs, until long-term habitation establishes an average.
Transport Energy - Reaching the Lunar region from the Earth's surface involves first getting to a high orbit that intercepts the Moon's location, then entering a stable orbit or landing on the surface. The first of these theoretically requires 62 MJ/kg, and entering orbit or landing requires up to 2.8 MJ/kg. These are ideal values for potential and kinetic energy. The actual expended energy depends on the details and efficiency of the transportation methods. The transport energies from Earth are high compared to the 10-20 MJ embodied energy of typical products or 15 MJ/kg of high energy chemical propellants. This favors local production where possible.
Escape velocity from the Lunar surface is 2380 m/s, or 21% of Earth's. Therefore escape energy is only 4.5% of Earth's. Low orbit velocity is 1680 m/s, so the difference to escape is an added 700 m/s. Orbit velocity at the upper edge of the region is 375 m/s, and escape an additional 155 m/s. These velocities and the associated energy are relatively low. Internal regional transport and reaching the High Orbit and Inner Interplanetary regions are then fairly easy from this region.
Energy Supply - The Sun provides a nearly constant flux of 1361 W/m2 at 1 AU. The Earth's orbit is slightly elliptical, and the Moon's orbit brings it closer to and farther from the Sun than the Earth. So the available energy in the Lunar region varies somewhat from this value. In full sunlight this energy is sufficient to process about 2 tons/m2/year of raw materials. Collecting the energy with solar panels or concentrating reflectors may only require 2 kg/m2 of equipment. So the primary energy return can be very high. Total solar energy flow through the region is 5.24 million TW, which is vastly greater than current world energy consumption of about 20 TW. Localized areas on the Moon contain up to 10 parts per million of Thorium and Uranium, which may be useful for energy production.
Most of the Moon's surface is exposed to the Sun 50% or less of the time, depending on local topography. Higher latitudes get less sunlight per unit of surface area due to lower Sun angles. Orbits are exposed to sunlight 50% or more of the time, generally increasing with altitude. Dark periods are relatively short in orbit. Dark periods on the surface last half a Lunar day, or two weeks. Since direct sunlight isn't available for this time, alternate energy solutions are needed. Possible approaches include:
- Ceasing Operations - High power operations can simply be stopped when sunlight isn't available, and minimal support provided by conventional sources like batteries. One of the following options can be combined with reduced nighttime operations, but not stopping them entirely.
- Thermal Storage - This uses the vast amount of rocks and dust as the storage medium, and the natural vacuum as insulation. The material is heated during the day by sunlight, and the heat used to generate power during the Lunar night.
- Nuclear Power - The surface is already bathed in high levels of natural radiation, so nuclear sources are not as much an issue as on Earth. A variety of nuclear designs are possible, with different power levels, and fueled from Earth or local sources.
- Beamed Power - Lunar orbits receive sunlight when the surface does not. Energy can be directed to the surface using simple reflectors, microwaves, or lasers, with varying beam target size vs distance.
- Transmission Lines - East-West transmission lines can deliver power from areas in sunlight to those that are dark. In the worst case at the Equator the lines would have to be 2700 km long to provide continuous power. This is 1/4 of the Lunar circumference, with one line in each direction. These are very long distances, and would only make sense when large-scale development is present. Shadowed craters get no sunlight, but the crater rim may get adequate power levels. A shorter transmission line or reflector can then deliver that power to where it is needed.
Materials Supply - Lunar orbits are essentially devoid of raw materials, so projects located there must import what they need. The Moon's surface has a somewhat variable, and reasonably well understood, Geology. Our understanding is from a number of lander and orbital missions, some of which returned samples for analysis, and from Lunar meteorites which were thrown to Earth by impacts. Broadly, the surface composition is oxide minerals of silicon, iron, calcium, aluminum, and magnesium, in order of elemental concentration, with 3-4% other elements. There are few volatile (low boiling point) compounds left on the Moon. It formed in a molten state, suffered many high energy impacts, and is too small to keep an atmosphere, so these compounds mostly escaped. While molten, the denser materials sank to the interior, and lighter minerals accumulated near the surface. As the Moon cooled, the surface minerals crystallized according to melting point, then further sank or rose according to density. Radioactive elements were preferentially concentrated near the surface, leading to additional melting or prolonged cooling.
Since the Moon retains no atmosphere, it does not slow incoming objects, and weathering does not occur as on Earth. The original crust formed by heat was then heavily cratered and broken up, but fairly unmodified otherwise. The surface is covered by a 2 to 8 meter thick Regolith (Lunar soil) made from the original crust and impacting asteroids, which have been thrown around and mixed during crater formation. The fine regolith is interspersed with larger rocks and boulders (Figure 4.12-3). Because of the broken nature, the regolith is easily collected and moved. Since the layer is global on the Moon, large quantities are available. See the Lunar and Planetary Institute's Lunar Sourcebook (1991) for more details about the Moon's geology.
We can develop a list of possible Lunar activities by looking at all Earth industry categories, and identifying ones that can potentially operate there in the future. To these we can add any activities that are unique to the Lunar region. Existing industries are classified for statistical purposes by the North American Industry Classification System (NAICS). We will use their numbering system and sequence for our survey, and insert unique Lunar items where most appropriate.
11 - Agriculture: Local agriculture will be useful to the extent that people and other living things in the region need food, and it avoids transport from Earth or other space locations. Plants can also recycle waste products from people while producing food and other useful products. Filtered sunlight is widely available in Lunar orbit, but the long Lunar night on the surface, and lack of sunlight underground, may require artificial lighting for higher plants. Microorganisms may be more tolerant of long periods of darkness. Carbon (100-160 ppm), hydrogen (30-60 ppm), nitrogen (60-120 ppm) and water (10-1000 ppm) are rare in Lunar soils, and may need to be imported from other locations. See Fegley and Swindle, 1993.
21 - Mining: The Moon's surface is covered by an average of 10 million tons/km2 of already broken up rocks and dust, and 1.36GW/km2 of daytime solar energy is available to power the mining and processing operations. Mining appears feasible for local use, and to supply locations which take less effort to reach than from the Earth. Delivery of mined products to the Earth itself would be limited. The mineral oxides making up most of the Moon's surface are also widely available on Earth at low cost. There may be some rare materials that will be worth transporting that far, especially if very low cost transport is developed.
22 - Utilities: All activities in the Lunar region will need local power, which at first can be supplied by equipment such as solar panels and batteries brought from Earth. When higher power levels are needed, simple systems like concentrating reflectors and thermal storage can be made locally. As local manufacturing develops, more complex power systems can be built. Our civilization consumes a large and growing amount of energy. 4-15 times as much solar energy is available in the High Orbit region as locations on the Earth's surface, and it is not interrupted by night and weather. If that energy can be delivered at a low enough cost, there would be a large demand for it. The Lunar region is likely too far to efficiently generate and transmit energy to Earth, but can be a source of materials and products to do so in the High Orbit region. The available quantity of solar energy within reasonable transmission range (80,000 km orbit radius) is 27.4 million TW, or over a million times Earth's current total energy use.
23 - Construction: The Earth is large enough to support its human population from a purely physical space standpoint for the foreseeable future. At the density of Manhattan, a population of 10 billion would fit on 0.25% of the world's land area. Even though more space is not necessary, it would be desirable for a number of reasons. The Lunar region can supply some of that physical space. Activities in the Lunar region that involve people and other living things will require habitats to support them, since the natural environment cannot. Large orbital habitats and industrial facilities would be difficult to deliver in a single piece. Either on-site construction is needed, or the orbital equivalent of shipyards and tow ships. Large surface habitats and other facilities would be too difficult to move, and need local construction.
31-33 - Manufacturing: The range of possible manufactured products in the Lunar region is vast. The first to be made are likely to be those that use local resources, displace significant mass otherwise brought from Earth, and are fairly simple. Examples include bulk shielding, thermal storage mass for nighttime power, basic chemicals for propulsion and life support, and mineral products and metals for construction. Once starter sets of production equipment are delivered to the region, they can begin to bootstrap their own expansion, and increase the range of products made locally. Transport energies to and from nearby regions are low, so a wider range of material inputs and product exports for local manufacturing may be feasible.
42 - Wholesale Trade: Individual operations in the region will trade with each other and with other regions, according to the economic principle of comparative advantage. The Moon has large amounts of easily extracted raw materials. So those materials, or products made from them, are likely to flow to Lunar and Earth orbits where they are lacking, and where transport energy is lower than from Earth. Ownership and control of trade may be from Earth rather than local. Communicating with the ground is fairly easy from this region.
44-45 - Retail Trade: We do not expect significant retail trade to develop in the region for a considerable time. Early populations in the Lunar region will be there for scientific and industry purposes. Their employers will likely provide for their basic needs, and even budget for some optional items like entertainment media. As permanent habitation develops and a non-working population accumulates, there will be more time and resources that can be applied to personal choices, and non-essential specialties. Retail trade may then develop.
48-49 - Transportation and Warehousing Transport is required from previously developed regions in order to start any activity in this region. Such transport is accounted for in earlier phases from their starting locations on Earth or in Earth Orbits. Transport within the Lunar region and to farther destinations is accounted for in this phase. At first the transport equipment is all made elsewhere, but over time propellants, structures, and more complex items can be made locally. Transport includes infrastructure such as spaceports and surface vehicle hangars. Warehousing includes all types of storage, which will start with bulk items like unprocessed rock and basic chemicals.
51 - Information Transmitting information through space requires no mass and little energy. So many satellite systems already exist in Earth orbit purely for communications, and nearly all have communications systems to support their primary purpose. Communications networks will be extended to the Lunar region once significant operations begin there. Networking, communications, and information technology for internal use within the Lunar region, and to Earth and other regions, will be extensive from the start. The equipment used is complex and fairly low mass. So it is likely to continue to come from Earth.
52 - Finance and Insurance This will most likely be supplied from Earth, since ownership rights, contracts, and money are all non-material relationships and can be transacted remotely.
53 - Real Estate, Rental, and Leases The Outer Space Treaty prohibits claiming celestial bodies but allows peaceful uses in space. International agreements for the Space Station, and orbital slots and frequencies for communications satellites have set precedents for ownership and use of equipment in space. What is not settled is territorial rights to less than entire bodies. For example, if someone builds a mining operation on the Moon, or a colony in Lunar orbit, will they own a territory around it in the sense of land rights on Earth? A reasonable answer is exclusion of others for technical and safety reasons. Thus, someone else cannot build so as to shadow your solar collectors or damage your equipment with rocket exhaust. Until the legal questions are settled, we cannot say what the scope of this industry category will be. Like Finance and Insurance, whatever the scope is, it will likely be handled from Earth for similar reasons.
54 - Professional, Scientific, and Technical: Some research and development, especially scientific, is best carried out locally in the Lunar region. Such activity began in the 1960's, to a large degree for political reasons, and continues to the present. Scientific and increasingly commercial activity is expected to continue in the future in the Lunar region. Most of the people involved in space activity so far have been on Earth, building the equipment and operating it remotely. This is likely to continue for some time, but increasing numbers of people will visit and work in the Lunar region. They will be concentrated in scientific and technical fields at first.
55-56 - Management and Organizational Support: Business management and administration will likely be mostly handled from Earth. Support activities like orbital debris collection and radiation remediation are tasks for Earth orbits. These in turn may be supplied from the Lunar region.
61 - Education: This will likely be done in advance or remotely from Earth at first. Local education will be mostly limited to training for industrial operations. General childhood and higher education will be deferred until permanent habitation develops with populations of young people to teach.
62 - Health and Social Services: Health monitoring and first aid capacity are needed for people in the Lunar region from the start. At first, this will be by training the crews themselves, with remote monitoring and advice. As the number of people grows in the region, more dedicated equipment and health specialists can be supplied. Telepresence, artificial reality, and haptic robots are not currently good enough to do health care remotely, and may not ever be so from Earth, due to speed of light delays. Automated health care carried out locally may be possible. These depend on future R&D, so for now health care concepts would involve existing methods. Nursing and residential care is assumed to be provided on Earth at first, by returning people there. As population grows in the region, local facilities may be established. We assume social assistance will be done remotely or not be needed. Basics like habitation and food need to be provided to everyone in the region for them to live at all. Return to Earth is an option for anyone who needs social support.
71 - Arts, Entertainment, and Recreation: Entertainment will start out as remote delivery or software for people in the region, because their energy and mass are low. Active recreation would start with necessary exercise for health maintenance. Early Lunar landing sites may be preserved as historical locations, and unique sports may develop in the Lunar region, such as rover racing or low-g gymnastics, but these are speculative at present.
72 - Accommodations and Food: Living space, food, and drink must always be provided for people in the region. At first these will be provided by sponsoring organizations and self-operated. As local capacity grows and people establish long-term residences, there will be room for temporary travel accommodations, tourism, and specialty food and drink locations. Space tourism to Low Orbit has already happened, and proposed to the Lunar region, but it is a very limited market due to extreme cost. It does demonstrate there is an interest. The existence of the Adventure Travel market makes it likely there will be more tourists once costs become more reasonable.
81 - Other Services: This category covers miscellaneous activities not covered elsewhere. Repair and maintenance is highly desired for the Lunar region from the start. This is because the equipment is either expensive to replace or life-critical. Personal services will start out self-provided until populations are larger. Private and civic organizations are not needed at first, can be extensions of Earth organizations, or self-organized locally.
92 - Public Administration: At first, most of this industry category will be handled from Earth. Fire and public safety will start out self-provided, and develop as specialties as populations grow. Environment quality and monitoring would be designed into Lunar systems as necessities. Publicly funded civil and national security space activities have been the only ones carried out so far in the region, and they are likely to continue to be important. Government budgets for these activities are finite, while business and private ones are not limited in this way, so we expect the latter to dominate eventually. For the sake of efficiency, public and private projects should be integrated so as to support each other.
Which of the above activities make sense for the Lunar region, and when, is a function of a number of factors. They include project motivations, their economics, available technology, and prior and parallel development. These drivers will change over time, affecting which of these activities get started and when.
Motivations to develop the region can be personal, organizational, or social. Curiosity drives scientific exploration of the Moon itself, and support of other science in the future. Safety from natural and human-created hazards is another motivation. It has driven orbital weather and defense systems in the past, and may drive asteroid deflection and solar blocking systems in the future. The natural Lunar environment is devoid of life and full of radiation. So moving hazardous activities off the Earth is another safety-driven motivation. The profit motive is ever-present in the business world. The unclaimed material and energy resources in the region are a potential source of profitable activity. [add list of human motivations and those that apply]
What projects are feasible, and when, is partly driven by available technology and support from previous phases. But it is also driven by the economics of the projects themselves. Economic principles like comparative advantage and returns on investment will still apply in the Lunar region. The first advantage of the Moon is relative closeness to the Earth, and to Earth Orbits which are already in use. Communications and transport times are shorter than for farther regions. Next is the low energy to reach nearby orbits, due to the Moon's smaller mass and position high in the Earth's gravity field. Transport "downhill" (towards the Earth) is eased by slow aerobraking with the Earth's atmosphere. Slow aerobraking uses multiple passes to lower heating and avoid damage. Transport "uphill" (away from the Earth-Moon system) is eased by using the Moon for gravity assists, when both departing and arriving. Lastly, large amounts of raw materials and solar energy are available in the Lunar region to enable projects.
While large amounts of materials are available in the Lunar region, they are not a complete source for all possible needs. The Lunar surface is deficient in volatile compounds, which escaped, and denser minerals and native iron, which sank to the interior. A fully developed economy would need to supplement local materials from elsewhere, driving trade. This includes Near Earth Asteroids, and from lower orbits or Earth. Near Earth Asteroids are a good choice for bulk materials because of the low energy to reach and return from them. Lower orbits and Earth are the likely source for higher value and finished goods, because of the more established industries there. In the long term, other sources may end up being economic. Costs tends to increase with the energy consumed in a task. So trade can make sense when the transport energy from elsewhere is less than the mining and processing energy from scarce local sources. Trade, in turn, drives a need for low cost transport.
Earth already maintains a complete civilization. So physical products from the Lunar region are not likely to compete with Earthly ones on cost. Exceptions include scientific and collectible samples, where their value specifically comes from their Lunar origin. Information is low mass and fairly easy to deliver, so may have significant economic value when coming from the Moon. Energy is a very large market on Earth, and solar energy is much more available in Earth orbit. If it can be delivered economically, that would be a spur to space industry in general, and for exports from the Lunar region to support it.
As of 2016, launch costs from Earth to the Lunar region were about $55 million/ton. This is somewhat higher than the price of gold/ton, which severely limits what kinds of projects are feasible. The raw wholesale energy to reach the Moon is only $1000/ton, which shows how much room there is for improvement. A number of new transport systems are in development. The SLS may eventually reach a cost of $1 billion/flight for 55 tons to the region using all chemical propulsion, or $18 million/ton. The SLS and an efficient electric tug from low orbit could deliver about 105 tons for $1.2 billion, or $11.5 million/ton. The Falcon Heavy can lift 64 tons to Earth orbit, and with an electric tug could deliver 50 tons to the Lunar region for about $200 million, or $4 million/ton. These are improvements, but still leave costs very high. For comparison, the mid-2017 price for silver was $0.5 million/ton. Any transportation measured in multiples of a precious metal price is still an impediment to significant development.
We therefore have a strong incentive for two kinds of technical improvements: systems with even lower transport costs, and use of local resources to avoid transportation. Both require extensive R&D to get the new technologies ready before attempting large-scale lunar development. Both improvements are useful for other locations. So we expect their development to be started in earlier program phases, and upgraded as needed for Lunar development.
Prior and Parallel Development
The technology improvements noted above will not all happen at once, so different lunar activities and projects will become feasible at different times. This will be in parallel with other program phases on Earth and in space, and other development in the Lunar region either before or in parallel. To the extent that transport costs to all parts of space are still high, it favors high value/low mass activities, and those where economic returns do not matter, such as publicly funded research and exploration. Low and high Earth orbits will likely continue to be more developed than the Moon, and therefore drive the improvements in transport and resource use. High thrust systems may be adapted from other regions for initial Lunar surface access. Improvements like Skyhooks, which are initially developed for Earth orbits, can later be adapted for Lunar surface access. Asteroid processing may start at L2, which is outside the Lunar region, but additional locations near or on the Moon may be added later. These locations can use the same or upgraded equipment.
We can now start to combine the above information into a general approach for Lunar development, and identify specific projects. We can provide initial concepts for these projects, which will gives a sense of their scale and main features. But this is merely a starting point, and does not exclude alternate ideas. It also barely begins optimization and integration of the projects to each other and other projects elsewhere.
The Moon's surface is the size of two continents on Earth, and the orbital region has a cross section of 3.85 billion km2, or 7.54 times the Earth's total surface. This region is far too large to develop all at once, or by a single project or organization. Our general approach is then to identify a number of smaller tasks and projects. They can be individually carried out by different organizations and put into a logical sequence. They would interact with each other when they exist at the same time, with other program phases, and with the rest of civilization.
The tasks can be grouped by start times into preparation, orbital, and surface development. There are already a number of current and near-term projects aimed at the early tasks. We note these next, and their existence should be integrated into plans for later projects. We don't have an integrated sequence yet for specific longer-term projects. So they are listed by primary function (production, habitation, transport, or services) and location (orbital or lunar surface) instead. Within each category we list them them in approximate start time order. We expect earlier projects to overlap later ones in the same category, rather than be completely replaced.
Preparation - Planning and designing future Lunar projects requires understanding the relevant features of the region in general, and of specific operating locations. Preparation for Lunar development therefore involves tasks like exploration, surveys, prospecting, and site investigation. Scientific exploration of the Moon began as soon as telescopes were available to observe it in detail. It accelerated once rocketry enabled placing instruments in orbit and on the surface, and bringing back samples for analysis. The orbital region is now well enough understood to begin projects there. For the surface, surveys and prospecting of the Moon as a whole are ongoing, but detailed work for future surface activities is incomplete. We assume devices like surface rovers will be sent before substantial development of a particular location. They would do detailed surveys, prospecting, and subsurface investigation.
Orbital Development - Lunar orbit is easier to reach than the surface, especially at first. So we assume orbital development begins earlier than the surface. Orbital locations generally don't need physical preparation, as they are devoid of natural materials. Use of a given orbit can thus begin with delivery of equipment. Smaller satellites, such as for communications relay, can be delivered as a complete unit from Earth. Larger installations like an inhabited station can use sections produced on Earth, and assembled in Earth orbit or in their final location. In either case, an electric tug can deliver it slowly but efficiently. People would first travel by faster chemical rockets to avoid radiation exposure. The same electric tugs can fetch asteroid rock from nearby orbits. It can first be used as-is for bulk radiation shielding. Processing equipment brought from Earth can then start making simpler products, like propellants, oxygen, and water, out of the asteroid materials.
We don't yet know the best path for further development. A lot more R&D is needed on this subject, but for example we can follow history and use iron and steam to bootstrap a growing economy. Metallic asteroids contain mostly iron, alloyed with varying amounts of nickel and cobalt. Chondrite asteroids have up to 20% water and carbon. The carbon can be added to the iron to make steel, and the water heated with focused sunlight for steam power. A starter set of production tools would convert the metal stock to parts for power units and more production machines, bootstrapping their own expansion. They can also make structural parts, pressure vessels, and other items to widen the range of production processes. This may be a simpler way to bootstrap than producing solar panels at first. Later mining and transport from the Lunar surface would increase the quantity and diversity of source materials. Rare materials and hard-to-make items will continue to come from Earth, but a decreasing percentage as production builds up. New people, remote control of Lunar systems, and support services are still be provided from Earth.
Surface Development - Lunar surface work starts later than in orbit, but continues in parallel afterwards. Sites on the surface are typically not ideal in their natural state. So construction sitework can begin after prospecting and site investigation. This would use some combination of robotic, automated, and remote controlled machines delivered from Earth. Propellants produced in orbit make their delivery easier. Some kinds of projects don't need large numbers of people. They can continue with remote operation, and perhaps short visits by crew for maintenance. Short visits and smaller long-term populations can feasibly be supported by deliveries from elsewhere. Delivering everything for larger populations and industrial operations is inefficient. Local mining, processing, and fabrication would be built up over time to support them.
Like for orbit, the best path for growth on the surface needs extensive R&D. It could begin with delivery of ready production equipment, then use starter sets to bootstrap further expansion. Depending on size and cost, local production of simple products may be started as early as the remote operation stage. These can be stockpiled for later development. Even after people are supported part or full time on the surface, you can continue to use machines to augment their work, using self- or remote-control. Depending on the level of automation, there may be high numbers of such machines per person. This approach will leverage the limited early human population.
Current and Near-Term Projects:
Transport from Earth - Direct transport from Earth is being worked on by NASA through their Space Launch System (SLS) and Orion Spacecraft projects. The SLS provides the capacity to deliver large payloads to the Lunar region (and other destinations), and Orion for carrying people. Other Chemical Rockets, which are in varying stages of operation and development, can also deliver payloads to the region. Only the larger ones can deliver significant payloads unaided. The Scaled Composites Stratolaunch carrier aircraft is part of a hybrid turbofan/chemical rocket transport system and can potentially support Lunar missions. The combined cycle SABRE airbreathing/rocket engine for the Reaction Engines Skylon spaceplane is in early stages of development. Skylon is intended to carry 15 tons to low orbit, enough support Lunar missions.
Transport from Low Orbit - Chemical rocket performance is limited by the available energy of the propellant. It is used for the Earth to Low Orbit transport segment because it provides high enough thrust to overcome the Earth's gravity and prevent the trajectory from intersecting the surface again before reaching orbit. Once Low Orbit projects are built in Phase 4A, slower but much higher performance alternatives can be used, and transport infrastructure built up from Low Orbit to the Lunar region. Even with chemical propulsion, system cost can be lowered compared to the current practice of single-use transport. This is possible if a cheaper propellant source is available or the hardware can be used multiple times. Multi-use orbital transport vehicles are generally referred to as Space Tugs by analogy to tugboats, which push ships and barges around.
Missions to the Lunar Region - There have been 64 successful public Missions to the Moon or involving the Lunar region since 1959. Notable among them were the Apollo missions which carried people and brought back 382 kg of samples. About 20 more public and private probes are under development, intended to launch between 2017 and the 2020s. Two short-term crewed missions are also in development. A dozen more probes and crewed missions are currently proposed but not yet in development. The NASA Deep Space Gateway is a crew-tended station planned for the 2020's in a halo orbit near the Moon. The European and Chinese Space Agencies have proposed Lunar surface bases by the 2030s. Both may involve international and private participation. We expect public projects in the region to continue to evolve, but be limited by available funding.
As noted in the general approach above, a lot of research and development is needed before we can make definite plans for long term Lunar development. What we can do now is identify potential projects and what work is needed to prepare for them. They can be put together into a plan, but this should be considered preliminary and very likely to be revised as time goes by. Projects are grouped by function and location for convenience, then in approximate time order.
Lunar Orbit Production
Lunar orbits require less energy to reach than the surface, and are easy to reach from the High Earth Orbit region because the Lunar region is embedded in it. So we expect orbital production to precede surface production. Early production can be an outgrowth of public projects such as the Deep Space Gateway. These projects will begin with assembly of pre-made components brought from Earth. Local production can then be added incrementally. Production can start with pre-made tools and equipment, then bootstrap its own expansion by using the seed factory approach. A starter set of equipment is used to make some parts for its own expansion, with the remainder imported. Over time, more can be made locally, to the limit of what makes economic sense. Production outputs can be used locally in orbit, delivered to the surface, exported to existing markets in the High and Low Orbit regions, and exported to farther destinations. We do not expect much export to Earth from the region because civilization is well developed there and costs are likely lower. Major production steps include:
Materials and Energy Sources - There are no significant materials sources in Lunar orbit, so they must be imported. Despite greater physical distance, Near Earth Asteroids are the easiest source of materials at first. This is because electric propulsion can be used, which is very efficient, and the Moon can be used for gravity assists, reducing propulsive delta-V. Neither is available for early transport to and from the surface. Asteroids also have more varied compositions than the Lunar surface, allowing a wider range of products. When transport infrastructure such as catapults and skyhooks become available (see Lunar Transport below), the lower cost, higher volume, and shorter delivery time from the Lunar surface becomes an advantage, shifting the balance to more local materials supply. Some materials are rare in both asteroid and Lunar ores, and would still need to be supplied from Earth.
Orbital locations have solar energy available 50-100% of the time, mostly as a function of altitude, while surface locations have it 50% or less, decreasing with latitude. Since the embodied energy to make products is typically larger than the kinetic energy to move materials from the Lunar surface to orbit, production can generally be faster in orbit. Most of the markets for orbital production will be local or other orbital regions, and require less energy for delivery than from the surface. Orbital production should continue to be favored for these reasons.
Materials Processing - Some products, like bulk radiation shielding, don't require further processing, just delivery. But most require conversion of raw materials to finished stock by mechanical, thermal, chemical, electrical, or other methods. Processing can start with the easiest and simplest methods. An example is water extraction from Chondrite type asteroid rock, which only requires heating a container using concentrated sunlight, and a shadowed condenser to collect the evolved vapor. Metallic asteroids are dominated by native iron-nickel-cobalt alloy, which makes up more than 95% of their mass. However individual samples vary in composition, and may have undesirable trace elements, lack other desirable alloying elements, or contain rocky inclusions. The Stony-iron group have higher percentages of silicates and less native metal. Processing is therefore generally needed to produce uniform alloys in desired sizes and shapes. The stony fraction of stony-irons, and the pure Stony or S-Type asteroids are typically silicate minerals which contain various metallic elements. These require extensive processing to extract desired products.
Orbital Fabrication - There are many known fabrication processes to convert stock materials into finished parts. These include all the known methods used on Earth. To these we can add some unique methods which take advantage of zero gravity, vacuum, and full-spectrum solar energy. Which ones are suitable for Lunar orbit and in what sequence to develop them will requires a lot of R&D. At a minimum, processes known to work on Earth can be used unchanged in space by providing a normal atmosphere and artificial gravity. However, these may be quite massive and inefficient in orbit.
Orbital Assembly and Construction - This can begin with prefabricated elements delivered from Earth and assembled in orbit, such as space stations and larger and more powerful satellites. As local production develops, the orbital products are less constrained by launch mass and cost from Earth, so they can use simpler and heavier designs. The Lunar orbit region is a useful location to combine raw materials supply, energy supply, and parts delivery from Earth or lower orbits. It may therefore develop as a major assembly and construction site. Large projects like space colonies may be too massive to move once constructed. In that case, smaller elements may be produced in Lunar orbit, then delivered to their final location for construction and installation.
Lunar Surface Production
Production systems on the surface can make some items without using imported materials. The relatively limited variation in Lunar ores means a wider range of products will need significant outside supplies. Landing on the Moon requires significant propellant at first, so imported supplies have a higher cost. So early production will favor those items that can be mostly or entirely made locally. Once efficient two-way transport is available (see Transport below), large amounts of raw materials and unfinished goods can be exported for orbital production. More finished goods and materials can also be imported for surface production. This allows a wider range of items to be made on the surface, and results in a trading network that benefits both locations. The preferred locations for surface production depends on availability of energy and material sources, local conditions, transport capacity, and the intended destinations and uses for the products. For example, a scientific outpost may not want industrial operations nearby if they cause unwanted disturbances. Possible products include:
Minimally Processed Regolith - Early Lunar mining and construction would be in support of near-term public scientific and exploration projects such as a surface station. This would include clearing and leveling building sites and access paths; then excavation, placement, and covering of station elements with unprocessed local regolith for radiation, thermal, and debris protection. Debris protection is both from natural meteoroids, and materials thrown by lander exhaust. Covering may be by simple loose piling of material over a support structure, and not require processing beyond sorting for rock size. Paving and blocks may be produced by melting the soil with concentrated sunlight.
Water Extraction - The Moon is devoid of known liquid water, but Lunar Water is known to exist in two main forms. The first is chemically bound as hydrates and hydroxide minerals. High temperatures are required to extract the water from this source. The concentrations are ~10-1000 parts per million. The second is water ice trapped in permanently shadowed craters near the poles, where temperatures are low enough to preserve it. Asteroids can contain up to 20% water, and Hydrogen can be combined with abundant mineral oxygen to produce 9 times its mass in water. So import is an alternative to the low concentrations and limited locations of local sources. In the long term, water is abundant beyond the mid-Asteroid Belt Frost Line, where temperatures have stayed low enough to retain solid ice. If low cost transport is available, they would be a preferred source over the limited Lunar sources. Of course, Earth has a great deal of water, but transporting it requires a lot of energy.
Native Iron Production - An early production path may be to mine and process native Lunar iron, which is present at about 0.5% of Lunar soil. If this process works, it may be feasible with fairly simple and low mass equipment, giving an early return on hardware delivered to the Moon. The soil is sieved for small grains, and iron-bearing particles are selected by magnetic separation. The remainder are used to create sand molds. A robot smooths a patch of ground, spreads the non-iron grains over them, then compacts and makes depressions for molds. The iron-bearing particles are placed in the mold cavities, and a large concentrating dish focuses sunlight on the cavities, scanning them sequentially if needed. The iron particles are often attached to other mineral grains, but being denser it will sink to the bottom of the mold, and glassy slag will rise to the top. The result would be basic shapes like plates and bars, with slag stuck to it on both sides from melting.
Depending on the melting point of iron vs other minerals, it may be necessary to select for refractory ones for the molds. One way to do this is to heat samples until the less refractory components sublimate. Another is to prospect for suitable refractory source rocks. A third approach is to bring a highly refractory crucible in which the grains are melted, and pouring off the slag and iron separately. In that case the mold is hot for a short period and will not melt much. R&D is needed to determine which, if any of these approaches will work. The cast metal stock will need to be sand-blasted to clean off adhered slag and mold grains. Fortunately there is no shortage of mineral grains on the Lunar Surface to do that with. You now have an inventory of iron stock to use for all types of construction and parts making.
Chemical Processing and Metallurgy - More complex processes may require more equipment, but yield more useful products from a given amount of Lunar ore. There is generally about 40% oxygen in any given soil, so any of several reduction processes can be used to extract it at a smaller scale. Oxygen is useful as a propellant, and for life support. More extensive mining and processing can supply finished materials such as refined metals and basalt fiber for local use or for export. Early markets will likely be in previously developed regions, and more solar energy is available in higher Lunar orbits. So we expect most materials by mass will be sent to Lunar orbit. The Moon's surface is not uniform in composition (Lawrence et. al. 1998), with higher concentrations of Iron, Potassium, Phosphorus, Rare Earth, and Thorium found mostly in the Oceanus Procellarum region. These are the result of fractional crystallization as the Moon cooled.
Helium-3 Mining - Mining of Helium-3 from the Moon has often been suggested for terrestrial power, because it has low radiation by-products in fusion reactions. There are several problems with this idea. The first is that Deuterium/Helium-3 fusion is about ten times harder than Deuterium/Tritium. We don't yet know how to sustain D-T fusion, much less do it economically, or the harder D-He3 fusion. The second is the abundance of this isotope is 15 parts per billion or less on the Moon. The abundance is 1000 times or more higher in Uranus and Neptune's atmospheres, requiring proportionally less mining and processing to extract a given amount. Although the outer planets are much farther than the Moon, if we need He3, then D-T fusion would already be solved, and it can power ships to reach to reach those planets. Since those atmospheres are mostly hydrogen, the Deuterium isotope is widely available, and both hydrogen and helium can be used as propellants. Thus mining the outer planets can be self-sustaining. The third problem is relative energy content. Pure He3 can supply 200 TJ/kg, but at Lunar concentrations the mined soil can only supply up to 3 MJ/kg in reaction energy. This is only about a tenth the energy in fossil fuels and a fifth that in wood. So it would hardly be worth mining on Earth, much less the Moon.
Silicon and aluminum make up 28% of typical Lunar soil. Assume only 10% of the soil mass is extracted to these elements and made into solar cells and structure for power satellites. The satellites will produce about 100 W/kg, so the net power per mined kg of soil is 10 W/kg. A nominal life for such cells is 15 years in orbit, during which they would produce 4.75 GJ/kg of energy. This energy can be beamed to Earth around the clock. So if you wish to mine the Moon terrestrial energy supply, you can generate 1500 times as much via solar power than He3 fusion. As noted above under Available Resources, some regions of the Lunar surface contain up to 10 parts per million Uranium and Thorium. These elements were concentrated in the crust by differential processes as the Moon cooled from a molten state. The mined ore has an energy content of about 800 MJ/kg, or hundreds of times the He-3 energy content. So even for Lunar or other space locations, He3 is not the most efficient source. Solar is even higher energy content, but there are situations where space nuclear power is useful. These include spanning the long night, shadowed craters, or for portable power on the Moon, and destinations far from the Sun where solar power is weak.
Surface Fabrication and Assembly - Like for other program phases, we use a bootstrapping process to build up our production capacity. We begin by importing ready-to-use tools and equipment, such as a mobile solar furnace for paving and block-making. We then add a starter set of machines (seed factory) capable of fabricating parts for additional production machines, as well as other end products. The starter set cannot make all types of needed parts, so the remainder continue to be imported. As the collection of machines grows, imported parts and materials decrease as a percentage, though they may increase in absolute amount. Parts are then assembled using some combination of robotics, remote-control, and direct human labor. Early assembly may be as simple as stacking blocks for a radiation shelter, but more complex products will need dedicated factory space. Robots and factory buildings are themselves products which can be fabricated and assembled, so the entire production system can grow itself over time.
Lunar Orbit Habitation
Lunar Surface Habitation
Habitats for near-term science and exploration missions to the Lunar surface will likely be based on pressurized modules, like the current International Space Station uses. They would be pre-fabricated and delivered from Earth. These may be either rigid-shell or inflatable. Due to high radiation levels, the modules would be shielded using arched supports covered by Lunar soil. Since low gravity is known to be harmful over extended periods, crew time would be limited to a year or less.
Long Term Habitats - Habitats for longer stays or permanent residence will require different designs. They may require centrifuges to create artificial gravity. An example would be a large habitat dome for spaciousness, and a centrifuge built around the rim for living quarters. Residents would spend enough time in the centrifuge to maintain health, but could work and enjoy the low gravity the rest of the time. We have essentially no data on how much gravity is enough between zero and 1.0. We know the body deteriorates over time in zero gravity. So as a worst case, people would need to spend most of their time in a one gee centrifuge of some type, but this subject needs more research.
Large habitats can be assembled from pre-made sections delivered from elsewhere, or use local production of structural elements. Until sufficient capacity is in place on the surface, it makes sense to do much of the work via remote-controlled robots. Since the natural environment is hostile to people, safety is a critical design factor. This would include multiple layers and compartments to contain atmosphere, and emergency breathing apparatus in multiple locations. It would also include failsafe or multiple life support systems. Failsafe design would still support residents in case power or equipment fails, though at reduced capacity or duration.
Lunar Orbit Transport
These projects include transport based in the Lunar orbit region.
Reusable Landers - Early landings on the Moon do not have the support of much infrastructure, so require high thrust chemical rockets to navigate the Lunar gravity field. For transport down to the surface, Carbonaceous type asteroids contain up to 20% carbon compounds and water. This can be reformed chemically to hydrocarbons and oxygen, which is a common rocket fuel combination. They would be produced at a high orbit location where there is full-time sunlight for power, then an electric tug delivers them to low Lunar orbit for efficiency. A low Lunar orbit fuel station then fills a reusable lander which delivers people and early cargo to the surface. Oxygen is the most common element on the Moon, and a number of ways of extracting it from oxide minerals have been studied. An oxygen plant to refuel for the return trip can reduce the round-trip mass ratio from 2.9 (LOX/CH4) to 1.96. This increases payload per trip from ~25% to ~40%.
Orbital Tugs - These are needed for efficient transport within Lunar orbits, and to and from these orbits to other regions. Transport to the High and Low orbit regions can be relatively low energy. In some cases the Moon can be used for gravity assists to lower orbits, and aerobraking used to lower apogee. Gravity assists can also be used to escape from the Earth-Moon system, but upwards transport generally requires more propulsion. Gravity and drag do not help in changing orbits around the Moon, but total velocity requirements are fairly low. We expect most of the propulsion will be electric.
Lunar Skyhooks - Lunar basalt and Carbon from asteroids carbon can be turned into high strength fibers to build a much larger type of orbiting centrifuge known as a "Skyhook". For a tip velocity equal to orbit velocity, the tip acceleration is much lower at a larger size. If it is set to 1 Earth gravity, a crew can live comfortably in orbit, and make trips to the Lunar surface with very low fuel use. The tip velocity cancels orbit velocity at the low point, so a lander can be dropped off at low altitude. It would not be dropped directly on the surface because of the variable gravity field and Lunar mountains and crater walls. The lander therefore uses a small amount of fuel to cover the last 10-20 km to and from the surface. A "frozen" orbit at 86 degrees inclination minimizes orbit perturbations and allows access to nearly all of the Lunar surface. However it limits the times when a particular location can be accessed. An equatorial orbit would have more frequent access but more limited surface coverage. Unlike Earth, the Moon rotates so slowly that the benefit of rotation velocity is negligible.
A large orbital Skyhook only makes economic sense if the traffic to the Lunar surface is large enough, so it would not be built right away. There is a one-time cost in mass to build a Skyhook, but afterwards it saves most of the required lander propellant. A Skyhook can be built incrementally, and provide partial savings before completion, which affects the economics. At the other end of the rotation, the centrifuge is moving faster than Lunar escape velocity, and can therefore send vehicles to a large range of orbits by choosing the radius and time of release. Catching and releasing vehicles affects the orbit of the centrifuge, but if traffic is balanced in direction it is a temporary change. If traffic is more in one direction than the other, the difference can be made up by electric propulsion. Since low gravity is known to be harmful, an orbital centrifuge can allow crew to mostly live in normal gravity, and operate equipment by remote control with short ping times. At the same time, the centrifuge can provide easy access to the surface when needed. So it is not purely a transport system.
Lunar Surface Transport
These projects include transportation systems which operate entirely on or near the Lunar Surface, and those which are based on the surface but transport to orbit or farther destinations.
- Surface Vehicles
These begin with rovers for science and prospecting purposes remote-controlled from Earth. They progress to mining and construction vehicles which are still remote-controlled or smart enough to self-operate. Once people start to be located on the surface regularly, we add unpressurized and pressurized crew transport, and eventually mobile habitation for extended stays.
- Lunar Catapults
These are systems intended to accelerate bulk materials to orbital velocity. Since they are not intended for people or delicate cargo, they can use high accelerations and therefore be physically compact. To lower the required power level they accelerate many smaller payloads to near-orbit, which are gathered into larger loads for a cargo tug. The two basic design approaches are centrifugal and linear, which are suited to different cargo volumes. Both are capable of delivering many times their own mass to orbit, and greatly reduce propellant consumption. The theoretical orbit velocity at the surface is 1680 m/s. However, the Moon is not a perfect sphere and has mass concentrations, so launch velocities of about 1750 m/s are needed to clear obstructions. Selecting a high point to launch from also helps. An unmodified trajectory will intersect the launch point again, so some method to circularize the orbit at the high point is needed.
Centrifuge Concept - The basic idea is an electric motor driving a rotor with long and short arms that are balanced. See for example Inertia II, but horizontal for the Moon to minimize support structure. Solar panels feed power during daytime to gradually accelerate the rotor until the tip of the long arm is moving at slightly above Lunar orbit velocity. The payload is released and coasts to a collection point in low Lunar orbit. At the same time, a counterweight is released from the short arm, which hits a hill behind the centrifuge. The reason for the counterweight is an unbalanced rotor would produce large forces on the centrifuge structure that could damage it. Whether the payload is sintered blocks of unprocessed surface material (regolith), or first separated to, for example, metals, is yet to be determined. Since there is no atmosphere to cause drag, spinning up the rotor can be done slowly, which keeps input power low. Rotor size can also be physically small, so this approach is suited to smaller delivery volumes that require less equipment to get started.
Centrifuge Design - Assuming a 50 meter radius on the long arm, and a tip velocity of 1750 m/s, the total stress along the arm is 156 g-km. High-strength carbon fiber has an ultimate strength of up to 386 g-km, so we are within available properties. Centrifugal forces are higher at the hub than the tip. That's because the tip only supports the payload, and the hub supports the payload + arm mass. Therefore the arm will be thicker at the hub to handle the increased forces. We don't want to work at ultimate strength, and allow a design factor of safety. We will 2.4 as a reasonable factor. This reduces our working strength to 386 g-km/2.4 = 161 g-km. The theoretical arm taper in area is then e156/161 = 2.635. In practice it will be somewhat higher due to non-structural overhead. An arm taper of 3-4:1 in area is a reasonable design solution. The circumference of rotation is 2 pi x 50m = 314 meters. A tip velocity of 1750 m/s then implies 5.57 rotations per second or 334 rpm. This is quite a reasonable number for a mechanical arm driven by an electric motor.
The short arm has a more slowly moving tip and therefore lower stress. The counterweight needs to be heavier than the payload to balance the forces on the rotor and anchoring structure. The result is the short arm will be the same mass as the long arm, but shorter and thicker. Since the counterweight is released much slower than orbit velocity, it will impact the ground some distance behind the centrifuge. It may self-destruct on impact, but there is no shortage of loose rock on the surface to replace it.
Assume we need 1 ton/day of bulk material in orbit. Since low Lunar orbits take 1.8 hours, we can launch 13.3 times a day to have the payloads arrive at a common collection point. Therefore each payload would be 75 kg. The kinetic energy of the payload is then 115 MJ, and the catapult requires 35 kW of average power at 50% efficiency. The other 50% goes into accelerating the counterweight and is wasted. 35 kW is half the output of one of the Space Station's four main solar array wings, so is a feasible power level. The Tesla Model S base model uses a 270 kW electric motor, so a 35 kW motor can be quite small and light. Since you are in Lunar night half the time, you need two catapults to produce the needed launch rate. It is hard to load payloads unless the centrifuge is stopped. So the flywheel energy in one catapult's arms just after launch can be transferred to the other catapult, rather than just dissipating it by braking. Regenerative braking, as this is called, is also common in electric cars. This makes the system more efficient. It is possible enough energy storage can be provided to last the Lunar night, or use a different power source that doesn't depend on sunlight. In that case only a single catapult is needed, but we will leave that choice to more detailed design.
Modern space solar arrays plus mounting and tracking on the Lunar surface would produce ~100 W/kg. 70 kW for two catapults would then require 700 kg of arrays. The catapult arms have a mass of 6-8 times the payload, so a complete device may be ~20 times the cargo mass, or 1500 kg/catapult. We will need a mining robot to gather raw materials, and equipment to weigh and package the payloads. A total mission mass allowance is then ~10 tons delivered to the Lunar surface. Since we deliver 1 ton per day, and nominal operating life for space hardware is 15 years, we deliver a total of 5,500 tons, or 550 times the system mass in payload.
Linear Accelerator Concept - These are known as Mass Drivers (Figure 4.12-N), and use a series of coils as sequential electromagnets to accelerate the payload. The high acceleration over a short distance requires high peak power. Since the acceleration is completed quickly, many smaller payloads can be launched in series. This approach is more suited to high volume delivery, where the large power supply is not a penalty.
Linear Accelerator Design - [TBD]
Orbit Circularization - The catapult places payloads in an orbit whose low point (perilune) is at the surface (or slightly below), and whose apolune is about 100 km higher. The low point needs to be raised so they don't hit the surface later, and this must be done at some other point in the orbit. Options include a small amount of propulsion on the payload, or a capture device in a circular orbit at the high point. The onboard approach uses a small solid or cold gas thruster. The design can draw from Rocket Assisted Projectiles developed for artillery, which experience similar high accelerations. Various Propellant Combinations can be derived from lunar materials, to avoid having to supply them from elsewhere. The payload itself has to survive high acceleration, and therefore can also supply the structure for the thruster. A capture device can be a bag or net, which is positioned with cables to meet an incoming payload, or is large enough to account for payload dispersion. The velocity difference between the capture device and payload is fairly low. Since the payloads are moving slower than circular velocity, the device faces forward and scoops them up, adding to their velocity. This slows the device, so it needs propulsion to maintain orbit. A single propulsion system in orbit may be simpler and cheaper than providing one per payload, allowing the payloads to be inert bulk material.
Cargo Tug Requirements - If the payloads have their own propulsion, they will end up in similar orbits, but somewhat spread out. A collection vehicle will chase them down and gather them into larger cargo loads. An electric tug then hauls large loads to a high orbit, such as Earth-Moon L2, a stable point on the far side of the Moon. This location is in sunlight all the time, so you have more energy to turn your raw materials into useful products. Electric propulsion is efficient, but slow. So the tug capacity needs to be matched to how much material is launched from the surface in the time it makes one round trip.
Lunar Orbit Services
Satellite Maintenance and Refueling Support - Most existing satellites are in the Low or High Orbit regions, because most of civilization is on Earth and that is what their services support. This situation should continue for some time. This project covers satellite support from the Lunar region to other regions, as well as internally within the region. With a few notable exceptions (the ISS and Hubble Space Telescope), most space projects have been single-use, because maintenance and refueling were too difficult or expensive. However replacing entire satellites when they break or run out of fuel is also very expensive, due to the high cost of new hardware and transport to space. So a maintenance and refueling capability is desirable. Raw materials from the Moon and Near Earth asteroids, combined with Lunar region energy, can support this capability, along with projects in the other regions and support from Earth. Support products include propellant supply, both for chemical and electric propulsion. Propellants are used to refuel satellites, remove orbital debris, and either transport maintenance equipment the satellites or bring the satellites to a maintenance location. They also include materials supply, including radiation shielding for people and equipment to support maintenance and bulk material for radiation belt depletion.
Lunar Surface Services
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Machine tools such as lathes and milling machines are designed to make metal parts from metal stock. Modern ones are computer controlled, and are themselves mostly made out of metal. Thus an initial set of machines supplied with raw stock can mostly make more machines. Given plans for a wider variety of machines, the initial set can increase in size in complexity. For example a forge press or rolling mill is useful for making some types of metal parts, but those are likely too heavy to bring to the Moon to start with. If you have a supply of raw materials, you can build them locally.
Standard assembly methods like bolts and screws can use robotic equipment. For some items that need to be pressure-tight processes like welding are preferred.
Welding - One process that may work on the Moon, since it is in vacuum, is to use articulating mirrors to direct a beam of concentrated sunlight at various angles. That could heat and weld parts to each other to make larger structures like habitats. If that does not work, then standard methods like vacuum plasma arc welding can be used.
Power and Heating
Electric Power - Solar power is abundant on the Lunar surface. While initial power devices will be brought from elsewhere, we can rapidly expand it using our metal supply. Since mass is not an issue for a stationary facility, you can deliver closed cycle generators based on a thermal cycle, and use concentrated sunlight to power it. The mirrors can be made locally of sheet metal with an evaporated coating of aluminum to make them more reflective. Later generators can be made entirely locally, but in the early stages making just reflectors is fairly simple. To supply power during the Lunar night, you can take a container filled with Lunar rock and a little gas for heat transfer, and surround that with more rock and insulation. Vacuum serves as a good insulator by itself. During the day you heat the container with concentrated sunlight, and at night run the thermal generator from the stored heat.
Process Heating - Many industrial processes need heating in some form. For this you can also use concentrated sunlight without an electrical generator.
Unprocessed Lunar rock can be used for things like shielding, and with melting can be cast into blocks for roads or landing pads, but for other elements and compounds some chemical processing will be needed, such as for reducing oxide minerals to metals and oxygen. Some elements are rare in Lunar rock and would have to be brought from elsewhere.
Some items need protection from daily temperature variations, radiation, meteorites, and human-caused hazards such as lander rocket exhaust. Most of that can be met with a structural metal shelter covered with sufficient Lunar regolith either in natural form or sintered into blocks.
A catapult system was mentioned earlier in the Space Elevator section as a method to deliver bulk Lunar basalt. This might be used to make high strength fibers for the Lunar Skyhook. With the Skyhook in place a bulk delivery method might not be needed any longer, but we will leave that choice to later analysis. Other bulk materials produced on the Moon could be delivered this way, but as a starting point for design, the calculations below assume basalt is the cargo.
Delivery Orbit - Any object thrown into an orbit from the surface will intersect the launch point one orbit later. Therefore your cargo needs a propulsion unit to raise the orbit, or you need something already in orbit to gather the cargo and change its orbit. One device in orbit is probably less expensive than one on every cargo, so for discussion we will assume a small Skyhook with a catcher device hanging down some number of kilometers from the main core. The offset allows the catapult to throw slightly slower than orbit velocity to match trajectory with the catcher. Missed catches will land somewhere else on the Moon instead of coming back to the launch site and doing damage. An equatorial location allows launching at about 108 minute intervals.
Tip Velocity = 1680 m/s - We don't yet know how much less than orbit velocity we will need, so for simplicity we assume the catapult reaches full orbit velocity.
Cargo Mass = 20 kg - Using basalt fiber as the main cable material, the Lunar Skyhook would have a nominal mass ratio of 20:1. If we want to deliver 10 ton payloads to the Lunar surface, that requires 200 tons of fiber. Assume we lost 1/3 of the basalt to processing, so the starter mass is 300 tons of raw basalt. We allow 10% extra time for maintenance and missed catches, so the average interval is 2 hours per cargo. If the catapult launches 20 kg at a time, that will be 43.5 tons per year if it only operates during the Lunar day, so about 6.9 years to launch enough cargo for the Skyhook construction. Larger versions would work proportionately faster.
Containers - Bulk Lunar rock will not hold together when tossed at high velocity. Two options are iron containers from Lunar rock, and Kevlar bags which are delivered and recycled.
- Iron Containers:
These would be made from local metallic iron, which could be used in orbit as another building material. Iron also gives the opportunity to fine tune the trajectory after the catapult throws the container using magnetic coils. Together with the catcher mechanically positioning itself during the 30-40 minute ballistic trajectory of the cargo, that should ensure a high percentage of catches. Lunar regolith has a density of about 3 g/cc, therefore a container holding 20 kg will have a volume of 7 liters. A cone 13 cm in radius and 42 cm tall has about this volume. The reason for a cone shape is so there can be a single attach point to the catapult. We restrict the container mass to 10% of the cargo, or 2 kg. The load will be constant across the cylinder height, so thickness will vary to maintain the stress, and the bottom will be domed for optimum strength, so we can model the structure as a 50 cm tall cylinder with an area of 4000 cm^2. With iron density of 7.8 g/cc, that gives a wall thickness of 0.64 mm. The cross section area is 5 cm^2, and with a working stress of 125 MPa, allowed load is 62 kN.
- Kevlar Containers:
Kevlar is a high strength material used on Earth. Bags made of Kevlar cloth with a capacity for 7 liters each would be delivered to the Lunar surface. A typical cloth ( 5 oz/yd^2, 650 lb/in strength ) of similar dimensions to the iron container would have a breaking strength of 21 kN, and a working strength of half that. We assume a 6 layer bag (or thicker cloth) to get the required strength, with steel wire mesh woven in for magnetic steering. Estimated mass is 0.45 kg/bag, or 2.25% of the cargo mass. Without an initial Skyhook the bags would need to be delivered with a lander, along with the rest of the catapult system. For 300 tons delivered, that would require 15,000 bags with a mass of 6.75 tons.
Radius = 1000 m - A force of 62 kN on a 22 kg loaded container implies a centrifugal acceleration of 2820 m/^2 ( 288 g's ). With a tip velocity of 1680 m/s, the radius is then 1.0 km, and it makes 0.267 rotations/s ( 16 rpm ). For accuracy and stability, the structure will need to be something like a truss frame, so it will support itself when not rotating. Added tension cables take up the acceleration load at full speed since cables can withstand higher stress.
Arm Mass = 92 kg - We used a working length of 126.4 g-km for carbon fiber in the Space Elevator section. We have an average of 144 g's over 1 km in the centrifuge, which produces a mass ratio of 3.1, and thus a cable mass of 2.1 times the loaded container. The centrifuge should be balanced so it does not move during operation, so a total of 4.2 times the container mass of 22 kg or 92 kg. The balance arm should be shorter, and release un-contained counterweight rock at the same time as the loaded container, but in the opposite direction, and aimed to hit a hill or travel far away ballistically.
Power = 11.5 kW - A 22 kg container moving at 1680 m/s has a kinetic energy of 31 MJ. If the catapult is operated once per 108 minutes when operating, it needs to supply 4.8 kW on average plus losses. This is doubled due to the counterweight rock. Additional energy is needed to spin up and down the rotating arm. If we use two catapults, the energy of one can be transferred to the other rather than wasting it as braking friction. The rotating arm has a mass of 4.2 times the thrown mass, and motor-generator conversions can be about 90% efficient. Thus we lose an additional 42% in energy in conversion losses. So the total average power supply needs to be 11.5 kW, and each motor-generator needs to transfer 30 kW when operating ( 40 hp ). An off the shelf 40 hp electric motor has a mass of 250 kg. Given the Lunar night, we need to double the solar array peak power to 23 KW. Modern arrays have a power level of about 100 W/kg, so the solar array mass would be 230 kg.
System Mass Estimate - Besides the bare rotating arm and power supply, there will need to be anchor structures, robots to collect the raw materials and put it in containers, the fine tuning magnets downrange, and communications and other support equipment. The total for this is unknown until more design work is done. For now, we will assume each of the two centrifuges will mass 1.25 tons total and total support equipment, including power, will be an additional 2.5 tons, for a total of 5 tons. Since the system can launch 168 times per lunar day, that means it can deliver 3.36 tons. Mass payback therefore takes 1.5 lunar days (1.5 Earth months).