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) pros
pect 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 equip
ment, 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 Value of the Moon Page 17