The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources (2016)
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How? Things We Should Have Been Doing
Although several attempts to revitalize lunar exploration met with partial success, currently, the Moon is not a strategic destination for the United States. A general misunderstanding of the value of the Moon keeps stalling our plans to return. The Apollo program was a successful architecture for getting people to the Moon and that seminal experience still colors many viewpoints on how to approach a lunar return. The Apollo program was the product of historical circumstances born during a specific time and place. While that experience holds many lessons for us, we must resist using it as a guidebook to get back to the Moon.
Questions on how to extend human reach beyond LEO have preoccupied the space community for years, with widely varying opinions on the appropriate steps to take, the order in which to take them, and how to implement the specific technical needs of each phase of human travel in deep space. Although many of these choices are a matter of personal preference, there is a common set of requirements that any trans-LEO architecture must satisfy. In what follows, I will outline some of the basic challenges of human spaceflight, the specific issues confronting travelers beyond LEO, and how these issues can be addressed.1
Some Spaceflight Basics
Rocket engines work through combustion. The chemical energy stored in propellant is released and expelled through a nozzle at high velocity. We have many choices—the type of fuel and oxidizer, the engine configuration, fuel flow rates and the geometry of the combustion chamber, as well as the mixing ratios and the nozzle diameter to vary the amounts of power a given rocket engine can generate. Regardless of how we might vary these parameters, we remain fundamentally limited in what we can put into space from the surface of the Earth.
The principal limiting factors in spaceflight are the force of gravity and the amount of energy available for release in the chemical bonds of the propellant. We can do nothing about either of these two factors; they are dictated by nature. At best, we can be clever in our engineering by employing strategies like staging, and by varying the types of materials used to make structures. But varying these parameters work only at the margins, not at the fundamentals. Those fundamentals are described by something called the rocket equation, first formulated in 1903 by the Russian “Father of Astronautics,” Konstantin Eduardovich Tsiolkovsky. The rocket equation essentially says that for chemical fuels, a rocket must consist of about 80-99 percent propellant by mass. This depressing arithmetic informs us that the payload—the useful mass that we want to get into space—can be only a small fraction of the mass of the vehicle.2
This simple fact of life, one that astronaut Don Pettit aptly terms “the tyranny of the rocket equation,” means that going into space is possible, but difficult and expensive.3 Typical commercial launch vehicles (CLV) are able to put 2-30 metric tons of payload into low Earth orbit, at a cost of between $30 million and $500 million per launch. These costs must include the necessary infrastructure costs, such as ground support, tracking, and insurance. All the air, water, food, and equipment the crew needs during the mission must be brought up by launch. This manifest is in addition to the mass of the launch vehicle, including its structure, tankage, and avionics.
To achieve orbit, a payload must be launched along a carefully chosen trajectory with a rocket burn of precise magnitude and duration. It must be lifted above the atmosphere so that aerodynamic drag does not slow the payload down to roughly 100 kilometers above the Earth, a point called the Karman line, the boundary between air and space.4 It must be accelerated to a velocity of about 7.8 kilometers per second; at this speed, the distance traveled by the vehicle per unit time is greater than the magnitude of the curvature of the Earth. When this condition is achieved, the launched object will constantly circle the Earth—it is in orbit. At the altitudes of low Earth orbit (~200-300 km), traces of atmosphere occur, meaning that an orbit will eventually deteriorate over time. Because of atmospheric drag, a satellite in LEO eventually will reenter the atmosphere. To alleviate this problem, satellites carry small amounts of fuel that are burned in small rockets, thrusters fired in controlled bursts, to maintain its orbit.
To go beyond LEO to high geosynchronous orbit (36,000 kilometers above Earth), an L-point, or to the Moon or planets (see figure 6.1), additional velocity (positive delta-v) must be imparted to the spacecraft by a rocket burn in the current direction of travel. An engine burn requires propellant, but existing launch vehicles reach LEO with empty fuel tanks. The only way around this problem is to include the fuel needed for trans-LEO travel as part of the payload, which further reduces the remaining fraction available for useful payload, or refuel the upper stage from a stored supply already in LEO. The first method requires the development of something called a heavy-lift launch vehicle (HLV). Such a rocket’s specific size is not rigorously defined, but typically, an HLV is able to put 50 to 100 metric tons or more into LEO. An example of an HLV is the Saturn V of the Apollo era, which could launch 116 metric tons into space. The Saturn V was the biggest launch vehicle the United States ever built and was sized specifically for the requirements of a human mission to the Moon, which included the Saturn IV-B upper stage, the Lunar and Command-Service Modules, and the liquid hydrogen-oxygen fuel needed to send the entire structure to the Moon.
The alternative technique for travel beyond LEO is to store propellant at a depot in space, then refuel the departure stage from that source.5 The idea of propellant depots in low Earth orbit has drawn a lot of attention, especially from many armchair engineers who have never actually flown a mission beyond LEO. Although this sounds like a good idea—indeed, it is a spacefaring skill that we must eventually master—the hidden assumption of the depot concept is that we possess the capability to launch the propellant “cheaply” from Earth, usually via some magically inexpensive “commercial” source, and store it in orbit. This is a simplified account of the depot concept; many other complex variables must be considered such as propellant boil-off, transfer techniques, management of the arrivals and departures of the tankers, and manifesting the facilities and timings of each launch of propellant cargo. Propellant depots are something that we eventually will take advantage of, particularly when we are ready to export propellant from the lunar surface. For the moment, the use of depots is invoked primarily as a substitute for a heavy lift launch vehicle. In the future, once we begin to produce and export propellant from the Moon, depots will be essential for supplying the vehicles of cislunar and planetary spaceflight.
A benefit Earth provides is that we can decelerate returning spacecraft using atmospheric friction dissipated as heat for braking, thus eliminating the need for propellant to slow down a returning spacecraft, thus making practical spaceflight possible. All returning human missions to date have used this technique, called aerothermal entry. A variant of this concept is aerobraking, in which a vehicle does not actually land, but uses the atmosphere to slow down enough to enter orbit around a planet. Although not used yet on human missions, this approach has been used on some robotic spacecraft sent to orbit Venus and Mars.6 As part of a system that can be reused and expanded, aerobraking is another skill that must be mastered in order to develop a permanent space transportation system.
The rocket equation dictates that while travel to LEO is difficult, travel beyond it becomes increasingly more so. Although the actual numbers vary depending on the propulsion system and its fuel, putting a single kilogram in lunar orbit requires about five kilograms in LEO, while landing a single kilogram on the lunar surface requires about seven kilograms in LEO, most of which is propellant. A system that enables routine access to cislunar space—the volume of space between Earth and Moon, including the lunar surface—could be established by setting up staging areas where the intermediate travel segments to the varying levels of cislunar space might be launched. Examples of such staging areas include LEO—the ISS is one possibility; GEO, a useful location to access communications and weather satellites; the Lagrangian (libration) points, of which L-1 and L-2 are often mentioned; and lunar orbit, with a variety of possibilities. At these locations, different spacecraft and pieces may be assembled to travel to the next location; they would also be locales for the establishment of propellant depots. A network of transportation nodes will enable constant and routine flight throughout cislunar space.
The tyranny of the rocket equation makes spaceflight difficult, and therefore expensive. It is possible to save some money by using clever engineering and some specialized tricks, but typically, such approaches only nibble around the margins and do not take big bites out of the core cost. This reality—the limiting arithmetic of spaceflight—cannot be addressed with finality as long as we haul everything we need up from the bottom of the deepest gravity well in the inner solar system. We will break loose from our tether once we learn how to create new capabilities by provisioning ourselves from what we find in space.
Launch Vehicle Options
After thirty years of service, the space shuttle was retired in 2011. Many observers regarded the shuttle as unsafe and inefficient, but although fourteen people died in two vehicle failures, 341 people safely made the trip to and from LEO, some taking multiple voyages. Moreover, the failure of two flights, Challenger and Columbia, out of a total of 135 flights, gives the space shuttle a 98.5 percent success rate, one of the best in the history of spaceflight. No one considers the loss of human life, even those who choose to challenge the limits, as anything but tragic, but each loss of vehicle and crew led to safer subsequent flights. At the end of the program, the shuttle was operating about as safely as any Earth-to-LEO transportation system could.
An enduring problem with the shuttle system was the amount of time and effort needed to refurbish it after each flight. Of the shuttle stack, only the external tank was discarded; all other pieces were recovered and reused. The solid rocket motor segments were simply refilled with propellant. However, the orbiter required many man-hours of work to prepare for launch, especially the silica tiles used to protect the vehicle during the searing heat of reentry. Copious labor-intensive work on the thermal protection system vacuumed up money (during its years of operation, shuttle operations took up the major fraction of the human spaceflight budget). For this reason, some critics consider the shuttle a policy failure, in that it did not make spaceflight to and from LEO “cheap,” even though that was never one of its design goals.7 However, a better way to look at the shuttle is that its goal as a vehicle was to make spaceflight “routine”—and it did. Moreover, the size and design of the shuttle gave it some unique capabilities, some not available on any planned future American spacecraft.
Now that the space shuttle is a historical relic, we are essentially in the beginning days of a new human spaceflight system. Currently under development are the remnants of Project Constellation: the Orion spacecraft and the Space Launch System (SLS).8 Orion can be configured to carry up to six passengers, four for cislunar flights, and has the capability to reside in space for about three weeks. This duration is adequate for almost all cislunar missions, but for missions beyond the Moon to, say, Mars or an asteroid, Orion will need additional modules for habitation, planetary landing, and other functions. In essence, Orion is only a single piece of a trans-LEO spacecraft system. Moreover, the design of the Orion command module is not conducive to satellite servicing, landing on a planetary object or extensive EVA; since it has no airlock, the entire spacecraft must be depressurized before crewmembers can egress.
The new rocket under development is a heavy lift vehicle (HLV), the Space Launch System (SLS). The SLS is built with pieces derived from the retired shuttle system, including its engines (modified shuttle main engines, burning LOX-hydrogen fuel), its solid rocket motors, and its central core tankage. In its basic form, the SLS can put about 70 metric tons into LEO; there are plans to increase that capacity, first to about 100 tons and ultimately to 130 tons. Depending upon the architecture, this core 70 ton payload capacity is adequate for most lunar missions. The largest variant of the SLS is scaled for human Mars missions staged completely from Earth. In such a case, eight to twelve separate launches are needed to assemble the 500+ ton Mars spacecraft in Earth orbit.
The principal advantage of an HLV is that the number of launches needed to conduct a mission is minimized. Each launch has a finite probability for failure, which is multiplied by the total number of launches. An architecture that uses a smaller LV has greater total risk, even though the impact of the loss of a single vehicle is lessened. Moreover, because ground infrastructure tends to be limited for most launch vehicle systems, the management of resources such as personnel, timing, and processing streams becomes a significant factor in conducting trans-LEO missions, depending on how far the mission is to go and how difficult it might be, lunar missions being easiest and Mars missions being the most demanding. Cost is also a consideration; HLVs tend to have greater economies of scale in terms of dollars per kilogram delivered, but they carry higher initial development and operating costs.
A return to the Moon can be accomplished using smaller launch vehicles and propellant depots. Several different architectures, with varying degrees of realism, have been developed to accomplish such a mission. In all cases, technical difficulties need to be solved before a viable transportation system is developed. The biggest unknowns are associated with the building and operation of propellant depots; delivery, storage, and transfer of propellant are technical issues that have yet to be demonstrated. These are particularly acute with cryogenic propellants (liquid oxygen and hydrogen) that have extremely low boiling points; some propellant will gradually be lost through sublimation regardless of how thermally well insulated the storage tanks are at the depot. One way to mitigate this loss is to store the propellant as water and crack it into its elemental form just prior to use. Such a strategy requires building a substantial infrastructure at the depot, including large solar arrays to generate high levels of electrical power to crack the water and processing facilities to capture and freeze the dissociated gases. This approach makes depots much more complicated facilities than simple storage tanks in orbit. It is possible to provision depots with storable propellants, that is, noncryogens that are much less susceptible to boil-off loss. But storables such as hydrazine and nitrogen tetroxide have much less specific impulse (total energy) when used, and the depots would not be configured to accept and use lunar-produced propellant in the future. The technical complexities associated with cryogenic oxygen-hydrogen depots make their development a protracted effort but after their establishment, one that provides the most extensibility, flexibility, and utility for spacefaring in the long run.
Retaining cryogenic propellant with minimal boil-off is an important issue, but in addition, transfer of supercold liquids in microgravity is a procedure that has yet to be attempted. Various complications of depot configuration are needed to enable the transfer of liquids in orbit, including the use of ullage by inert gases such as pressurized helium or by spinning the depot to generate small accelerations, causing liquids to move in a predictable direction. All of these systems need to be space-certified, meaning that moving parts must be designed for operation in extreme thermal and vacuum environments, which is costly. Presumably, much of the necessary operational work can be automated, but as we have not yet demonstrated the technologies needed for a space-based cryogenic depot, we cannot even begin to design the needed robotic systems. Human intervention and adjustment of the depot machinery probably will be necessary. Most likely, the earliest space-based propellant depots will be human-tended by technical necessity, not by programmatic requirement.
A variety of expendable launch vehicles are now or soon will be available that could implement a propellant depot-based architecture. The largest ELV available commercially is the Delta IV-Heavy,9 which can lift 26 metric tons to LEO. Use of such an LV could conduct a lunar surface mission with three launches. Smaller ELVs such as the Atlas 551 (21 tons) or Falcon 9 (11 tons) would require many more launches to stage such a mission. The proposed Falcon Heavy launch vehicle by SpaceX would consist of three strapped-together Falcon 9 vehicles with cross-fed engines.10 It remains to be seen whether this proposed rocket, with twenty-seven engines burning simultaneously on liftoff, will work and whether it will be fiscally viable as a commercial launch system. If the Falcon Heavy delivers as advertised, it could place about 50 metric tons into LEO, enough to conduct a lunar mission with two launches.
There are many ways to skin the cat of trans-LEO human spaceflight. NASA is currently building a heavy lift vehicle that will enable human missions to the lunar surface in its basic, core configuration (70 tons), so the establishment of propellant depots in LEO is not an immediate necessity. However, because one of the principal goals of a return to the Moon is to learn how to use its resources, establishing a cryogenic propellant depot is an essential piece of a complete system designed to use lunar propellant to fuel space transportation. We will have to address and solve these various technical problems sooner or later, so we might as well build and learn how to operate such a system now.
A Lunar Return Architecture: Leading with Robots
Several attempts to establish human presence on the Moon were abandoned after they foundered on fiscal and political shoals. While there are many reasons for this history, one of the principal ones is the continued and repeated attempt over the last thirty years to re-create the Apollo experience. Apollo, one of NASA’s finest accomplishments, took America from essentially zero spaceflight capability to the surface of the Moon in eight years. Unfortunately, this success led the agency to conclude that making leaps in technology and capability through the appropriation and expenditure of massive amounts of federal money was the only viable path to space success. Such an eventuality is extremely unlikely to reoccur. For the foreseeable future, the civil space program will probably be restricted to funding levels of less than one percent of the federal budget, and perhaps much less than that.
Given these restraints, is a trans-LEO human spaceflight program even possible? I believe it is, but we must design an approach that spends money carefully, invests in lasting infrastructure, and uses the resources of space to create new capability. Over the last decade, new data obtained for the Moon have shown that there is abundant water ice at the lunar poles. Moreover, this ice is proximate to locations that receive near-constant solar illumination. These two facts allow us to envision both a location and an activity in cislunar space where an off-planet foothold for humanity could be established. The development of an architecture that works under these constraints and achieves the objectives described above was a joint effort by myself and Tony Lavoie, an engineer from NASA-Marshall Space Flight Center with whom I worked closely on the Lunar Architecture Team in 2006.11 It should be emphasized that this plan is flexible; many aspects of it can be changed to accommodate evolving circumstances, resources, and prevailing societal and political conditions. It is offered as an example of what is possible and not as a detailed master plan that must be followed to the letter.
The mission statement of lunar return is “to learn how to live and work productively on another world.” We do this by using the material and energy resources of the lunar surface to create a sustained presence there. Specifically, our goal is to harvest the abundant water ice present at the lunar poles with the objective of making consumables for human residence on the lunar surface and propellant for access to and from the Moon and for eventual export to support activities in cislunar space. Initially, the architecture focuses on water production because propellant—in this case, hydrogen and oxygen—is by far the major fraction of vehicle mass and the most significant factor for the cost of human missions. The availability of lunar consumables and propellant allow us to routinely access all the levels of cislunar space, where our economic, national security, and scientific satellite assets reside.
The objective of lunar return defines our architecture: we stay in one place to build up capabilities and infrastructure in order to stay longer and create more. Thus, we build an outpost; we do not conduct sortie missions to a variety of landings sites all over the Moon. We go to the poles for three reasons: (1) near-permanent sunlight near the poles permits almost constant generation of electrical power from photovoltaics, obviating the need for a nuclear reactor to survive the fourteen-day lunar night; (2) these quasi-permanent lit zones are thermally benign compared to equatorial regions (Apollo sites), being illuminated at grazing solar incidence angles, and thus greatly reduce the passive thermal loading from the hot lunar surface; (3) the permanently dark areas near the poles contain significant quantities of volatile substances, including hundreds of millions of tons of water ice.
The return to the Moon is accomplished gradually and in stages, making use of existing assets both on Earth and in space. Early missions send robotic machines that are controlled by operators on the Earth. The short radio time-delay permits near instantaneous response to teleoperations, a virtue provided by the Moon’s proximity to Earth. An important attribute of this architecture is flexibility. We build infrastructure incrementally with small pieces on the Moon, operated as a single large, distributed system. The individual robotic machines have high-definition, stereo real-time imaging, anthropomorphic manipulation capabilities, and possess fingerlike end-effectors. The intent is to give the robotic teleoperators the sense of being physically present and working on the Moon. These surface facilities are emplaced and operated as opportunity and capability permit. Because there are many small pieces and segments involved in a distributed system, an incremental approach enables a broader participation in lunar return by international and commercial partners than was possible under previous architectures.
The advantage of using smaller units for robotic machines is that they can be either grouped together and launched on one large HLV or launched separately on multiple, smaller ELVs. Such flexibility allows us to create a foothold on the Moon irrespective of budgetary fluctuations. Commonality occurs at the component level, with common cryogenic engines, valves, avionics boxes, landing subsystems, filters, and connectors to allow maximum use and reuse of the assets that are landed on the surface. The goal is to create a remotely operated, robotic water mining station on the Moon. People arrive at the outpost late in the plan to cannibalize common parts, fix problems, conduct periodic maintenance, upgrade soft goods, seals, valve packing, inspect equipment for wear, and perform certain logistical and developmental functions that humans do best.
Phase I: Resource Prospecting. We first launch a series of small robotic spacecraft to: (1) emplace critical communications and navigational assets; (2) prospect the polar regions to identify suitable sites for resource mining and processing; and (3) demonstrate the steps necessary to find, extract, process and store water and its derivative products. The poles of the Moon have intermittent visibility with the Earth, which creates problems for operations that depend on constant, data-intensive communications between Earth and the Moon. Moreover, knowledge of precise locations on the Moon is difficult to determine and transit to and from specific points requires high-quality maps and navigational aids. To resolve both these needs, we establish a small constellation of satellites that serves as a communications relay system, providing near-constant contact between Earth and the various spacecraft around and on the Moon, as well as a lunar GPS system which provides detailed positional information, both on the lunar surface and in cislunar space. This system can be implemented with a constellation of small (~250 kg) satellites in polar orbits (apolune ~2,000 km) around the Moon. Such a system must be able to provide high bandwidth (several tens to hundreds of Mbps) for communications and positional accuracy (within 100 m) necessary to support transit and navigation around the lunar poles.
Two rovers will be sent to each lunar pole to explore the light and dark areas and to characterize the physical and chemical nature of the ice deposits. We must understand how polar ice varies in concentration horizontally and vertically, learn about the geotechnical properties of polar soils, and pinpoint location and access to mining prospects. The rovers will begin the long-term task of prospecting for lunar ice deposits so that we may select the outpost site near high-concentration deposits of water. In addition to polar ice, we must also understand the locations and variability of sunlit areas, as well as the dust, surface electrical-charging and plasma environment.
The rovers weigh about five hundred kilograms and carry instrumentation to measure the physical and chemical nature of the polar ice. In addition, they will excavate (via scoop, mole, and/or drill) and store small amounts of ice/soil feedstock for transport to resource demonstration experiments mounted on the fixed lander in the permanent sunlight. Because the rover must journey into and out of the permanent darkness repeatedly, it cannot rely solely on solar arrays to generate its electrical power. Power has to be provided by a continuously operating system, such as a radioisotope thermal generator (RTG).12 Possible nonnuclear alternatives include rechargeable batteries or a regenerative fuel cell (RFC).
During this phase, a propellant depot will be placed in a 400 km Earth orbit to fuel future spacecraft going to the Moon. Initially, the depot will be supplied by water delivered from Earth, but later from the Moon via space tugs. At the depot, water will be converted into gaseous hydrogen and oxygen and then will be liquefied and stored. This depot will fuel a robotic heavy lander with roughly eight metric tons of propellant and must be flexible enough to control its attitude in many configurations during both the absence and presence of docked vehicles. Using the depot to fuel a large lander increases our potential landed mass on the Moon by more than a factor of two. The depot will be supplied initially with water by commercial launch vendors, which can begin immediately after orbit emplacement and checkout. If no commercial providers emerge, separate NASA missions can supply the depot with water.
Phase II: Resource Mining, Processing, and Production. The next phase moves from resource prospecting and exploration to water production. The initial processing approach will be to excavate ice-laden soil, heat it to vaporize the ice, collect the vapor, and store it for later use. It is possible that other, more efficient mining schemes, such as some type of in-place extraction, may emerge that do not require soil excavation. For now, the most conservative approach, one that we know will work, is to use heat to drive the water from the soil. The process of soil heating has the advantage of being able to use either electrical power or passive solar thermal energy to generate heat for the processing of the feedstock.
Figure 7.1. Artist’s rendering of robotic lander approaching surface. Previous lander has deployed solar arrays that rotate on vertical axis, to track Sun near pole. These robotic systems can begin the work of resource processing at the lunar poles. (Credit 7.1)
During this phase, we incrementally add excavators, dump haulers, soil processors, and storage tanks to obtain, haul, and store the water.13 Landers carrying large solar arrays generate electricity at the permanently illuminated (> 80%) sites; robotic equipment can periodically connect to these power stations to recharge their batteries (figure 7.1). Our immediate goals are to learn how to remotely operate these machines and begin to produce and store water for eventual use when people arrive. Processed water is easily stored in the permanently shadowed areas. During this phase we also land electrolysis units to begin cracking water into its component gases, making the cryogens, and storing the liquid propellant. Because we are developing an operational cadence as we go, it might take several months to get into a smooth rhythm that maximizes the rates of propellant production. Large unknowns that must be resolved include transit time between the mining and propellant production site, thermal profiles, power profiles, and the lifetime of machine parts. We make constant, steady progress, learning how to crawl before we try to walk.
Equipment used in this phase includes excavation rovers, processors, and power units, each on the order of 1,200-1,500 kilograms. Power stations are rolled solar arrays that when deployed, are gimbaled about a vertical axis to track the course of the Sun over a lunar day. Each array generates about 25 kW. Multiple power stations can be arranged and operated together to provide the power needs of the robotic equipment and, ultimately, the outpost. During this phase, we begin to investigate the making of roads and cleared work areas by microwave sintering of regolith. Many areas near the outpost site, particularly around the power stations, will get heavy repeat traffic and keeping scattered dust to a minimum is necessary for thermal control and to maximize the equipment lifetime.
Figure 7.2. Artist’s rendering of robotic bulldozers digging ice-laden regolith as feedstock for water extraction processing. The proximity of Earth allows us to teleoperate robotic machines on the Moon and begin resource processing prior to human arrival. (Credit 7.2)
Phase III: Outpost Infrastructure Emplacement and Assembly. The next phase will bring pieces of the outpost and prepare its site, emplace critical infrastructure for power generation and thermal control, and begin to construct the lunar surface transportation hub, which will receive and service the reusable robotic and human landers that make up our cislunar transportation system. Additional robotic assets are added, including upgrading the surface mining and processing equipment, replacing damaged items, and expanding the capacity of processing (figure 7.2). Our goal in this phase of development is to increase the output of water in order to support the arrival of human crews on the Moon.
Propellant is needed on the lunar surface to refuel the robotic and human landers that travel to and from the Moon. Returning cargo landers can carry the exported product as water or as propellant. Both options may be necessary, since propellant will be needed in the vicinity of the Moon to refuel transfer stages, but water delivered to low Earth orbit can be cracked and frozen there just as efficiently as on the lunar surface. Included in the power budget is the energy required for propellant liquefaction, which removes a large amount of heat from the fluid. Minimizing the boil-off of the volatile cryogens is a recognized technical challenge and will be addressed via selected technology development early in this campaign.
The first heavy cargo mission will bring the logistical pieces and power capability necessary to support human habitation for the initial stay on the lunar surface. Part of this cargo includes additional power generation capability to power the human habitat arriving later. The initial cargo complement would probably not include enough battery power to weather an eclipse, but it is expected that this capability would arrive by the third cargo mission. Part of this complement would be supplementary equipment needed to attach to the habitat or otherwise make it usable, such as leveling equipment, high priority spares, filters, thermal shields, various pieces of support equipment, lifting equipment, mobile pallets, EVA suit components, and logistics supplies, including a method to transfer the crew to the habitat in the form of a tunnel/airlock so that the mass of the human lander can be minimized. Included is a small, pressurized human rover (4.5 tons) to interface between the lander and the habitat to allow shirtsleeve ingress, as well as local mobility to access deployed equipment.
The second heavy cargo mission brings the human habitat to the Moon. While it is envisioned that the habitable areas at the outpost ultimately will be significantly larger than a single 12-ton module, initial needs are to have sufficient habitable volume to support two to four crewmembers for a month. Included in either this or the previous mission payload are radiators and heat rejection equipment, as well as a fully operational environmental control and life support system.
Phase IV: Human Lunar Return. During this phase, we prepare the site, emplace the elements, and connect all the pieces to create a ready-to-use outpost. Those pieces include power and thermal control systems, habitats, workshops, landing pads, roads, and other facilities. Remotely operated robotic machines assemble this entire complex before people arrive. The outpost is “human-tended” and supports a crew of four for biannual visits of several weeks duration. During these periods, the crew repairs, services and operates the previously emplaced robotic assets. In addition, some of the crew will conduct local geological exploration and other science-related tasks. By the time the first crew arrives, the outpost will be producing about 150 tons of water per year, enough to completely supply the lunar transportation system with propellant.
The lander for these human missions is a smaller, LM-class vehicle (~30 ton) rather than a lander similar to the Constellation Altair vehicle (~50 ton). Its primary mission is to transport crew to and from the lunar surface and does not contain significant life-support capability, since the crew will live in previously emplaced surface habitats while they are on the Moon. This lunar taxi becomes a permanent part of the cislunar transportation system. It is reusable and refuelable with lunar-produced propellant and can be stored either on the lunar surface or at the cislunar transport node. Because of its similarity in size and functionality to the robotic landers, common components are used so that the parts count for lunar surface maintenance can be minimized. Specifically, both landers use a common reusable cryogenic engine developed in part (or totally) for use by the robotic heavy lander, with both vehicles using a multiple engine complement for reliability and redundancy, as well as cost. Engines will be designed to be serviced or changed out on the Moon, thus maximizing the lifetime of the vehicles in which they reside.
With refueling at the LEO depot, a cargo variant of the human lander launched on a HLV can deliver 12 tons of payload to the lunar surface. Once on the Moon, it will be cannibalized and used for parts. The lander has a dry mass of 8,300 kg and is launched from the LEO station using a Cislunar Transfer Stage (CTS), which requires about 60,000 kg of cryogenic propellant to take the lander to the Moon. Initially, the CTS will be used and discarded, but once lunar propellant production is up and running, we can reuse this element by rendezvousing in low lunar orbit with the cislunar depot. This architecture does not presume full success with extracting lunar resources, except for refueling for human Earth return. As the concept matures and our understanding of the logistics, cost, and sustainability of this approach solidifies, the lunar refueling process can expand significantly, as much as the demand will allow, to include the incorporation of the cargo landers.
Phase V and beyond: Human Habitation of the Moon. Once the outpost has been established, initial human occupancy will consist of periodic visits designed to explore the local site and to maintain and assure the proper operation of the mining and production equipment. These visits will be interspersed with the landing of additional robotic assets to continually increase the level of production, with the aim of exporting surplus water to cislunar space. Initially, the crew will validate and ensure the propellant and water production chain, including periodic maintenance and optimization of the operations concepts and timelines. With subsequent cargo deliveries, the crew will evaluate production techniques, procedures, technologies, and tools that allow expansion to the next step in utilization (figure 7.3).
Figure 7.3. Example of resource processing at a lunar outpost during early phases of operation. A vacuum induction furnace takes metal obtained from the regolith, melts it and pours the liquid into molds to make metallic members for construction. A crane at right is unloading a payload from a robotic lander. Electrical power is provided by solar array landers in the distance at top left. (Credit 7.3)
Although the concept of lunar resource utilization has been studied for years, many unknowns need to be addressed, starting with basic technologies and technology applications in the lunar environment. Techniques, tools, and extensive physical and metallurgical analysis of the properties of the final products need to be examined to obtain the best products for as yet undefined applications. Research in this technology is vitally important to extending human reach in space, although habitat upkeep and propellant supply chain management has higher priority. A broad ISRU material investigation lends itself well to both international participation and commercial development. Because no single strategy or technology or method works for every application, research can be divided into discrete investigations. Toward that end, on one of the cargo missions, a materials processing laboratory is delivered. Next in priority for crew time is data on biological interaction and plant growth in lunar gravity. These investigations will examine the vitality, reaction, and long-term logistical needs for developing local food production to sustain human habitation of the Moon.
At this stage, we may begin to recoup our investment in the outpost. Several possible models for the privatization of water processing may be viable. We anticipate that the federal government will be an early and repeat customer for lunar water, not only for future NASA missions beyond the Earth-Moon system but also for the cislunar missions of other agencies, such as the Department of Defense. Additionally, international customers will likely emerge. Whether the production facilities become commercialized before or after these markets emerge cannot be easily foreseen at this stage and in fact, is unimportant. The critical point is that we will be in a position to industrialize the Moon and cislunar space, a cornerstone in making space part of our economic sphere. We can openly share the technology developments as well as any undesirable outcomes and pitfalls from our experience, so that others can leverage what we have learned. This will enable the commercial sector to take over many lunar activities and services.
The transition to commercial activity may occur early or late in outpost development. Part of NASA’s ultimate purpose is to expand and enhance the nation’s commercial and industrial base and this activity is to be encouraged where possible. However, in contrast to NASA’s obsession with devising an “exit strategy” for the Moon, we should instead plan to participate in lunar development for at least as long as deemed necessary for fully commercial (that is, not government subsidized) providers to emerge. Because the capabilities we are developing have critical national strategic importance, the involvement of the federal government is important to ensure continuing access to lunar resources and the capabilities they provide.
Establishing a permanent foothold on the Moon opens the space frontier to many different uses. By creating a reusable, extensible cislunar spacefaring system, we build a “transcontinental railroad” in space, connecting two worlds, Earth and Moon, as well as enabling access to all the points in between. We will have a system that can access the entire Moon, but more importantly, we can also routinely access all of our assets within cislunar space: communications, GPS, weather, remote sensing, and strategic monitoring satellites. These satellites can be serviced, maintained, and replaced as they age.
I have concentrated on water production at a lunar outpost because such activity provides the highest leverage through the making of rocket propellant. However, there are other possibilities to explore, including a paradigm-shifting culture to eventually design all structural elements of space hardware using lunar resources. These activities will spur new commercial space interest, innovation, and investment. This further reduces the mass needed from Earth’s logistics train and helps extend human reach deeper into space, along a trajectory that is incremental, methodical, and sustainable within projected budget expectations. Instead of the current design-build-launch-discard paradigm of space operations, we can build extensible, distributed space systems with capabilities much greater than currently possible. Both the space shuttle and the ISS experience demonstrated the value of human construction and servicing of orbital systems. What we have lacked is the ability to access the various systems that orbit the Earth at altitudes much greater than LEO—MEO, GEO, and other locations in cislunar space.
A transportation system that can access cislunar space can also take us to the planets. The assembly and fueling of interplanetary missions is possible using the resources of the Moon. Water produced at the lunar poles can fuel human missions beyond the Earth-Moon system, as well as provide radiation shielding for the crew, thereby greatly reducing the amount of mass launched from the Earth’s surface. To give some idea of the leverage this provides, it has been estimated that a chemically propelled Mars mission requires at least roughly one million pounds (about 500 tons) in Earth orbit. Of this mass, more than 80 percent is propellant. Launching such propellant from Earth requires eight to twelve HLV launches at a cost of almost $2 billion each. Such an approach does not establish a true exploration capability. A Mars mission staged from the facilities of a cislunar transport system can use propellant from the Moon to reduce the mass launched from the Earth by a factor of five.
The modular, incremental nature of this architecture facilitates international and commercial participation by allowing their contributions to be easily and seamlessly integrated into the lunar development scenario. Because the outpost is built around the addition of capabilities through the use of small, robotically teleoperated assets, other parties can bring their own pieces to the table as time, availability and capability permit. International partners will be able to contemplate their own human missions to the Moon without the need to develop a heavy-lift vehicle by purchasing lunar fuel for a return trip. Flexibility and the use of incremental pieces make international participation and commercialization in this architecture easier than under the Project Constellation architecture.
These are only the initial steps of a lunar return based on resource utilization. Water is both the easiest and most useful substance that we can extract from the Moon and use to establish a cislunar spacefaring transportation infrastructure. Once established, many different possibilities for the lunar outpost may emerge. It may evolve into a commercial facility that manufactures water, propellant, and other commodities for sale in cislunar space. It could remain a government laboratory, exploring the trade space of resource utilization by experimenting with new processes and products. Alternatively, it might become a scientific research station, supporting detailed surface investigations to understand the planetary and solar history recorded on the Moon. We may decide to internationalize the outpost, creating a common use facility for science, exploration, research, and commercial activity by many countries. By emphasizing resource extraction early, we create opportunities for flexible growth and for the evolution of a wide variety of spaceflight activities.
Schedules, Budgets, Politics, and “Sustainability”: Is Any of This Possible?
It is an article of faith in the space community that the US civil space program is woefully under-funded and should receive much more money; some advocate for at least doubling the current NASA budget. Is it really true that the space program does not receive enough money? Certainly, the space program is now funded at a much lower fraction of the federal budget (about 0.3 percent) than was appropriated at the height of the Apollo program (about 4 percent).14 But at that time (1961-68), NASA had virtually no infrastructure, including laboratories, offices, test stands, launch complexes, and supporting facilities, and little off-the-shelf technology to draw on. Much of the Apollo spending went to these ends and created a supporting network and organizational base that the agency has used and drawn upon for all of its many programs ever since.
As we have seen, previous efforts to return to the Moon were cut short by budgetary shortfalls. In Washington, the estimates for the cost of new programs have a long history of running significantly lower than what things actually and eventually cost. Nonetheless, one problem with talking about money is that the cumulative costs for a multiyear or multidecadal program seem horrendously high.15 As implemented by the 90-Day Study to support President George H. W. Bush’s 1989 Space Exploration Initiative (SEI), the estimated cost was $600 billion; at the time, the agency’s yearly budget was a bit more than $10 billion. But that $600 billion number was the total cost of a thirty-year program and included all of the ancillary costs of facilities and overhead. Even though few federal programs could withstand such accounting scrutiny, critics used the $600 billion number as a cudgel to beat the SEI to death. One might stop and consider than in the twenty-five years since SEI was unveiled, the agency has spent about $498 billion (FY 2014) dollars, almost the same gasp-inducing number as that of the 90-Day Study. One might pause and reflect on what that sum has bought us in terms of spacefaring capability over the last two and a half decades.
Rushing in where budgetary angels fear to tread, I now present, in table 7.1, our estimate for the cost of lunar return via the scenario described in this chapter.16 Tony Lavoie and I assumed federal budget austerity for the indefinite future and used the budget guidelines for the agency assumed by the 2009 Augustine committee as a cost cap; in effect, a maximum of $7 billion (FY2011) constant dollars per year is to be spent on “exploration systems.”17 The Augustine committee concluded that NASA could not return to the Moon under these fiscal constraints and suggested that an additional $3 billion per year would be needed to fulfill the VSE goals. We simply did not believe that conclusion and that disagreement was in part the motivation for writing our paper. We found that by carefully defining our mission objectives up front and using remotely controlled robotic systems on the Moon in the early stages of the program, we could create a permanent resource-processing outpost at one of the poles under fairly tight fiscal restrictions. Our plan costs an aggregate total of $88 billion (FY 2011) constant dollars over the course of about sixteen years. That amount includes the cost of the development of the robotic infrastructure, propellant depots, reusable lunar lander, the CEV, and a medium HLV (70 ton class). It also includes all of the commercial ELV launch costs at the then-quoted rates. At the end of this nominal program, we have an operating, human-tended polar outpost on the Moon that produces 150 tons of water per year.
Table 7.1. Cost Data for Robotic Lunar Architecture
All costs are in millions of US FY 2011 dollars. Cost for each mission and/or mission element shown in “Total” column at far right; yearly costs shown across bottom row; total program cost at bottom far right. Human mission costs shown in bold italic.
Costs include two versions of Orion crew exploration vehicle (CEV), medium-class heavy lift vehicle (HLV, 70 metric ton), technology development funds, and operations costs shown at bottom.
A critical aspect related to cost is program performance. Any human spaceflight program must show continual progress in order to maintain its level of funding. The best way to accomplish this is to attain significant and recognizable intermediate milestones on a continuing and regular basis. A manager has much more credibility when he can report program accomplishments as he asks for the next increments of funding. Part of the problem with Project Constellation was that its intermediate milestones were too few and far between. In the five years that program ran, the only significant milestone was a launch test in 2009 of the Ares-X, basically a four-segment shuttle solid-rocket booster with a dummy upper stage. When the program was cancelled in 2010, flight tests of Orion into orbit were not scheduled to begin until 2015. Lunar return was over a decade away; the Augustine Committee claimed that it would not occur until after 2030, a completely undocumented assertion but one embraced by the opponents of the VSE, who were eager to terminate the whole effort. The Constellation program’s own lack of near-term milestones, accomplished on a regular cadence, allowed this assertion to go unchallenged.
By crafting an incremental program using smaller spacecraft, flight rates are dramatically increased and consequently, many intermediate milestones are achieved early and often. Yet, no capability is lost because the small pieces are operated together as a single, large “system of systems.” In addition, a program that is divided into small pieces is more robust in that it can survive budgetary storms with more resilience. Less progress is made during lean times, but some progress is still made. It is also easier to take advantage of technical breakthroughs and incorporate them into the program because system and vehicle designs are not frozen in place decades ahead of time. As mentioned, an incremental program also facilitates the integration of commercial and international partners, with more “on-ramps” and a lower bar to program entry. Moreover, the possible failure or poor performance of an individual partner has less impact on program progress and viability.
It is difficult to sustain large-scale technological projects over periods of more than a few years. In the history of America, only a few such programs have succeeded and almost all were somehow related to national security concerns. As we shall see, the program to develop a permanent cislunar transportation system is no exception. Although I have described this program as a return to the Moon, it is also a step toward the creation of a permanent spacefaring capability. By building this system, we access on a routine basis, not only the lunar surface, but also all of the other points within cislunar space, where our national scientific, economic, and security assets reside. Other nations are well aware of the security dimensions of this capability, and some, such as China, are actively pursuing the means to possess freedom of access to this theater of operations. A program to create true spacefaring capability has many critical national benefits that transcend politics. A national bipartisan consensus has defended this nation on land, at sea and in the air for more than two hundred years. Can we afford to do less on the new ocean of space?