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Abstract

     How do we begin to expand our civilization to the Moon? What are the technical issues that infrastructural engineers, in particular, must address? This paper has the goal of introducing this fascinating area of structural mechanics, design, and construction. Published work of the past several decades about lunar bases is summarized. Additional emphasis is placed on issues related to regolith mechanics and robotic construction. Although many hundreds of papers have been written on these subjects, and only a few tens of these have been referred to here, it is believed that a representative view has been created. This summary includes environmental issues, a classification of structural types being considered for the Moon, and some possible usage of in situ resources for lunar construction. An appendix provides, in tabular form, an overview of structural types and their lunar applications and technology drivers.

Introduction

     Concepts for lunar base structures have been proposed since long before the dawn of the space age.

     Unfortunately, by the mid-1990s, the political climate turned against a return to the Moon to stay and began to look at Mars as the ‘‘appropriate’’ destination, essentially skipping the Moon. The debate between ‘‘Moon First’’ and ‘‘Mars Direct’’ continues, although it is clear that the latter will do no more for the expansion of civilization into the solar system than did the Apollo program. It is also clear that we do not have the technology and experience to send people to Mars for an extended stay. Physiological and reliability issues are yet unresolved for a trip to Mars; the Moon is our best first goal.

     The emphasis below is on structures for human habitation, a technically challenging fraction of the total number of structures likely to comprise the lunar facility. The test for any proposed lunar base structure is how it meets certain basic as well as special requirements. On the lunar surface, numerous constraints, different from those for terrestrial structures, must be satisfied by all designs. A number of structural types have been proposed for lunar base structures. These include concrete structures, metal frame structures, pneumatic construction, and hybrid structures. In addition, options exist for subsurface architectures and the use of natural features such as lava tubes. Each of these approaches can in principle satisfy the various and numerous constraints, but differently.

     A post-Apollo evaluation of the need for a lunar base has been made with the following reasons given for such a base:

• Advancing lunar science and astronomy;
• Stimulus to space technology and test bed for technologies required to place humans on Mars and beyond;
• Utilization of lunar resources;
• Establishment of U.S. presence;
• Stimulation of interest of young Americans in science and engineering; and
• Beginning of long-range program to ensure survival of species.

     The potential for an astronomical observatory on the Moon is very great, and it could be serviced periodically in a reasonable fashion from a lunar base. Several bold proposals for astronomy from the Moon have been made. Nearly all of these proposals involve use of advanced materials and structural concepts to erect large long-life astronomy facilities on the Moon. These facilities will challenge structural designers, constructors, and logistics planners in the 21st century. One example is a 16 m diameter reflector with its supporting structure and foundation currently being investigated by NASA and several consortia.

     Selection of the proper site for a lunar astronomical facility, for example, involves many difficult decisions. Scientific advantages of a polar location for a lunar base are that half the sky is continuously visible for astronomy from each pole and that cryogenic instruments can readily be operated there due to the fact that there are shaded regions in perpetual darkness. Disadvantages arise from the fact that the sun will essentially trace the horizon, leaving the outside workspace in extreme contrast, and will pose practical problems regarding solar power and communications with Earth; relays will be required.

Environment

     The problem of designing a structure to build on the lunar surface is a difficult one, discussed here in a necessarily cursory way. Many issues are not discussed, but will need to be tackled eventually. Some important topics not discussed here, but necessary in a detailed study, include the following:

• Relationships between severe lunar temperature cycles and structural and material fatigue, a problem for exposed structures;
• Structural sensitivity to temperature differentials between different sections of the same component;
• Very-low-temperature effects and the possibility of brittle fractures;
• Outgassing for exposed steels and other effects of high vacuum on steel, alloys, and advanced materials;
• Factors of safety, originally developed to account for uncertainties in the Earth design and construction process, undoubtedly need adjustment for the lunar environment, either up or down, depending on one’s perspective and tolerance for risk;
• Reliability (and risk) must be major components of lunar structures, just as they are of significant Earth structures;
• Dead/live loads under lunar gravity;
• Buckling, stiffening, and bracing requirements for lunar structures, which will be internally pressurized; and
• Consideration of new failure modes such as those due to highvelocity micrometeorite impacts.

     In a light, flexible structural system in low gravity, light structural members (for example, composite cylinders that have a wall thickness of only a few 1/1,000^th of an inch) are sometimes designed to limit their load-carrying capacity by buckling when that limit is met. In turn, the load would have to be redistributed to other, less-loaded structural members. Such an approach offers possibilities for inflatable and other lunar surface structures where it would be simpler and less costly to include limit-state and sacrificial structural elements. Some of these discussions are under way, in particular regarding the design process for an extraterrestrial structure.

     Our purpose in this paper is to discuss the technical issues and provide some historical context. Important issues such as financing the return to the Moon, enhancing human physiological understanding, and many others are beyond the scope here. The focus for us, again, is to provide the reader with a brief glimpse of the structural and structural-related engineering issues for human habitation on the Moon.

     Important components in a design process are the creation of a detailed design and prototyping. For a structure in the lunar environment, such building and realistic testing cannot be performed on the Earth or even in orbit. It is not currently possible, for example, to experimentally assess the effect of suspended due to 1/6 g) lunar regolith lunar soil fines on lunar machinery. Apollo experience may be extrapolated, but only to a boundary beyond which new information is necessary.

     Another crucial aspect of a lunar structural design involves an evaluation of the total life cycle that is, taking a system from conception through retirement and disposition, or the recycling of the system and its components. Many factors affecting system life cannot be predicted due to the nature of the lunar environment and the inability to realistically assess the system before it is built and utilized.

     Finally, it appears that concurrent engineering will be a byword for lunar structural analysis, design, and erection. Concurrent engineering simultaneously considers system design, manufacturing, and construction, moving major items in the cycle to as early a stage as possible in order to anticipate potential problems. Here, another dimension is added to this definition. Given the extreme nature of the environment contemplated for the structure, concurrency must imply flexibility of design and construction. Parallelism in the design space must be maintained so that at each juncture alternate solutions exist that will permit continuation of construction, even in the face of completely unanticipated difficulties. This factor needs to be further addressed and its implications clearly explored. A discussion of lunar design codes has already started.

Loading, Environment, and Regolith Mechanics

     Any lunar structure will be designed for and built with the following prime considerations:

• Safety and reliability: Human safety and the minimization of risk to ‘‘acceptable’’ levels are always at the top of the list of considerations for any engineering project. The Moon offers new challenges to the engineering designer. Minimization of risk implies in particular structural redundancy and, when all else fails, easy escape for the inhabitants. The key word is ‘‘acceptable,’’ a subjective consideration deeply rooted in economic considerations. What is an acceptable level of safety and reliability for a lunar site, one that must be considered highly hazardous? Such questions go beyond engineering considerations and must include policy considerations: Can we afford to fail?
• 1/6 g gravity: A structure will have, in gross terms, six times the weight-bearing capacity on the Moon as on the Earth; or, to support a certain loading condition, one-sixth the loadbearing strength is required on the Moon as on the Earth. In order to maximize the utility of concepts developed for lunar structural design, mass rather than weight-based criteria should be the approach of lunar structural engineers. All of NASA’s calculations have been done in kgforce rather than newtons. Calculations are always without the gravity component; use kilogram feet per square centimeter as pressure, for example.

     In the area of foundation design, most classical analytical approaches are based on the limit-state condition, in which the design is based on the limit of loading on a wall or footing at the point when a total collapse occurs—that is, the plastic limit. Since many of the structures on the Moon require accurate pointing capabilities for astronomy, communication, and so on, a settlement-based design method would be more useful. Chua propose a nonlinear hyperbolic stress-strain model that can be used for the lunar regolith in a finite-element analysis. The paper also shows how the finite-element method can be used to predict settlement of the railway under a support point of a large telescope. Chua show how a large deformation-capable finite-element program can be used to predict the load-displacement characteristics of a circular spud-can footing, which was designed to support a large lunar optical telescope.

     A note against assuming that less gravity means a footing can support more load: if soil can be assumed to be linearly elastic, then the elastic modulus is not affected by gravity. However, the load-bearing capacity of a real soil depends on the confining stress around it. If the soil surrounding the point of interest were heavier because of larger gravity, the confining stress would be higher and the soil at the point of interest could support a higher load without collapsing.

     The area of lunar soil (regolith) mechanics was exhaustively explored in the 1970s. Much of the work was approached from interpretation based on classical soil mechanics. Newer work and development of nonlinear stress-strain models to describe the mechanics of the lunar regolith can be found in Johnson. Chua show how structure-regolith simulations can be done using the finite-element approach.

• Internal air pressurization: The lunar structure is in fact a life-supporting closed environment. It will be a pressurized enclosed volume with an internal pressure of nearly 15 psi. The enclosure structure must contain this pressure and must be designed to be ‘‘fail-safe’’ against catastrophic and other decompression caused by accidental and natural impacts.
• Shielding: A prime consideration in the design is that the structure be able to shield against the types of hazards found on the lunar surface: continuous solar/cosmic radiation, meteorite impacts, and extreme variations in temperature and radiation. In the likely situation that a layer of regolith ~lunar soil! is placed atop the structure for shielding, the added weight would partially (in the range of 10–20%) balance the forces on the structure caused by internal pressurization mentioned above.

     Shielding against micrometeorite impacts is done by providing dense and heavy materials, in this case compacted regolith, to absorb the kinetic energy. Lunar rocks would be more effective than regolith because the rocks have fracture toughness, but may be more difficult to obtain and much more difficult to place atop surface structures.

     Much effort in this country has been devoted to determining the damage effects on human beings and electronics resulting from nuclear weapon detonation, and little is being done to determine long-term, sustained low-level radiation effects such as those that would be encountered on the Moon. According to Silberberg, during the times of low solar activity, the annual dose-equivalent for humans on the exposed lunar surface may be about 30 rem (radiation equivalent man), and the dose-equivalent over an 11 year solar cycle is about 1,000 rem, with most of the particles arriving in one or two gigantic flares lasting 1 to 2 days. It appears that at least 2.5 m of regolith cover would be required to keep the annual dose of radiation at 5 rem, which is the allowable level for radiation workers (0.5 rem for the general public). A shallower cover may be inadequate to protect against the primary radiation, and a thicker cover may cause the secondary radiation (which consists of electrons and other radiation as a result of the primary radiation hitting atoms along its path).

     In recent years, there has been a move away from silicon-and germanium-based electronic components toward the use of gallium arsenide. Lower current and voltage demand and miniaturization of electronic components and machines would make devices more radiation hardened.

     Radiation transport codes can be used to simulate cosmic radiation effects, which is not possible in the laboratory. One such code that has been found to be effective is LAHET developed at the Los Alamos National Laboratory.

• Vacuum: A hard vacuum surrounds the Moon that will preclude the use of certain materials that might not be chemically or molecularly stable under such conditions. This is an issue for research.

     Construction in a vacuum has several problems. One would be the possibility of outgassing of oil, vapors, and lubricants from pneumatic systems. Hydraulic systems are not used in space for this reason. The outgassing is detrimental to astronomical mirrors, solar panels, and any other moving machine parts because these structures tend to cause dust particles to form pods. For more discussion of construction challenges in the extraterrestrial environment, see Chua. Another problem is that surface-to-surface contact becomes much more abrasive in the absence of an air layer. The increase in dynamic friction would cause fusion at the interfaces, for example, a drill bit fusing with the lunar rock. This is of course aggravated by the fact that the vacuum is a bad conductor of heat. The increase in abrasiveness at interfaces also increases wear and tear on any moving parts, such as railways and wheels.

     Blasting in a vacuum is another interesting problem to consider. When the explosive in a blast hole is fired, it is transformed into a gas, the pressure of which may sometimes exceed 100,000 terrestrial atmospheres. How this would affect the area around the blast on the Moon and the impact of ejecta resulting from the blast is difficult to predict. Keeping in mind that a particle set in motion by the firing of a rocket from a lander could theoretically travel halfway around the Moon, the effects of surface blasting on the Moon would be something to be concerned about. Discussion of the tests involving explosives that were performed on the Moon can be found in Watson. Joachim discussed different candidate explosives for extraterrestrial use, and the Air Force Institute of Technology studied cratering at various gravities and/or in vacuum. Bernold presented experimental evidence from a study of blasting to loosen regolith for excavation.

• Dust: The lunar surface has a layer of fine particles that are easily disturbed and placed into suspension. These particles cling to all surfaces and pose serious challenges for the utility of construction equipment, air locks, and all exposed surfaces.

     Lunar dust consists of pulverized regolith and appears to be charged. The charge may be from the fractured crystalline structure of the material or may be of a surficial nature, for example, charged particles from the solar wind attaching themselves to the dust particles. Criswell reported that the dust particles levitated at the lunar terminator (line between lunar day and lunar night) and that this may be due to a change in polarity of the surficial materials. Johnson discuss the issue of lunar dust and its effects on operations on the Moon. Haljian studied the adhesive characteristics of regolith dust.

• Ease of construction: The remoteness of the lunar site, in conjunction with the high costs associated with launches from Earth, suggests that lunar structures be designed for ease of construction so that the extravehicular activity of the astronaut construction team is minimized. Construction components must be practical and, in a sense, modular in order to minimize local fabrication for initial structural outposts.

     Chua discuss guidelines and the developmental process for lunar-based structures. They present the governing criteria and also general misconceptions in designing space structures. For example, a device that is simple and conventional looking and has no moving parts is preferred over one that involves multiple degrees of freedom in an exotic configuration involving a yet-to-develop artificial intelligence control, if the former meets the functional requirements. Other misconceptions are that constructing on the Moon is simply a scaling of the effects of similar operations on Earth, and that theoretical predictive tools, especially those performed with computers, can accurately predict events. It is also a misconception that astronauts would have to work around the structure, rather than that, the structure would be designed as to make construction easy for the astronauts.

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• Use of local materials: This is to be viewed as extremely important in the long-term view of extraterrestrial habitation, but feasibility will have to wait until a minimal presence has been established on the Moon. Initial lunar structures will be transported for the most part in components from the Earth (Fig. 1).

     The use of local resources, normally referred to as ISRU (in situ resource utilization), is a topic that has been studied, more intensely now than ever, because of the possibility of actually establishing a human presence on the Moon, near-earth orbit, and Mars.

Possible Structural Concepts

     Various concepts have been proposed for lunar structures. In order to assess the overall efficiency of individual concepts, decision science and operations research tools have been proposed, used, and demonstrated. Along these lines, various concepts are compared using a points system for an extraterrestrial building system, including pneumatic, framed/ rigid foam, prefabricated, and hybrid (inflatable/rigid) concepts.

     In a very early lunar structural design study, Johnson presented the then-available information with the goal of furthering the development of criteria for the design of permanent lunar structures. In this work, the lunar environment is detailed, lunar soil from the perspective of foundation design is discussed, and excavation concepts are reviewed. An excellent review of the evolution of concepts for lunar bases up through the mid-1980s is available, as is a review of more recent work on lunar bases. Surface and subsurface concepts for lunar bases are surveyed with a recommendation that preliminary designs be considered that focus on specific applications. America’s future on the Moon is outlined as supporting scientific research, exploiting lunar resources for use in building a space infrastructure, and attaining self-sufficiency in the lunar environment as a first step in planetary settlement. The complexities and costs of building such a base will depend on the mission or missions for which such a base is to be built.

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     A complete Earth-Moon infrastructure uses proven technologies and the National Space Transportation System for early development of a lunar outpost (Fig. 2). Transfer vehicles and surface systems are developed so that the payload bay of the Space Shuttle can be utilized in transport. The lunar outpost structural scheme separates radiation protection from module support, allowing easy access, installation, and removal of elements attached to the shuttle trusses.

     Several types of structures have been proposed for lunar outposts. A preliminary design of a permanently manned lunar surface research base has been briefly studied by Hoffman, with criteria for the base design to include scientific objectives as well as the transportation requirements to establish and support its continued operations.

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Inflatables

     A pillow-shaped structure proposed by Vanderbilt as a possible concept for a permanent lunar base (Fig. 3) consists of quilted inflatable pressurized tensile structures using fiber composites. Shielding is provided by an overburden of regolith, with accommodation for sunlight ingress. These studies of the inflatable concept are continued by Nowak with consideration of the foundation problem and additional reliability concerns and analysis. This concept is a significant departure from numerous other inflatable concepts in that it shows an alternative to spheroidal inflatables and optimizes volume for habitation. Inflatable structural concepts for a lunar base are proposed as a means to simplify and speed up the process while lessening the costs. The inflatable structure is suggested as a generic test-bed structure for a variety of application needs for the Moon.

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     Another pressurized membrane structure, proposed by Chow for a permanent lunar base, is constructed of a double-skin membrane filled with structural foam. A pressurized torus-shaped substructure provides edge support, and shielding is provided by an overburden of regolith. Briefly, the construction procedure requires shaping the ground and spreading the uninflated structure upon it, after which the torus-shaped substructure is pressurized. Structural foam is then injected into the inflatable component, and the internal compartment is pressurized. The bottoms of both inflated structures are filled with compacted soil to provide stability and a flat interior floor surface. Backfilling is a difficult operation to carry out through an airlock. It will, of course, be crucial to ensure that the interior is dust-free (Fig. 4).

     A detailed architectural master plan is also proposed for a horizontal inflatable habitat. Finite-element simulations of inflatable structures are needed because it is very difficult to reproduce a hard vacuum and low-gravity condition on Earth. The finite-element modeling would have to be large-deformation capable and have membrane elements (which are essentially beam elements that are without bending stiffness) and axial tensile stiffness, but not axial compression stiffness. The program should also ideally be able to model regolith-structure interaction. GEOT2D is a program that has the capabilities needed to simulate inflatable structure-regolith interaction.

Erectables

     An expandable platform suggested by Mangan as a structure on the Moon consists of various geometrically configured 3D trussed octet or space frame elements used both as building blocks and as a platform for expansion of the structure. Examples of the shapes to be used include tetrahedral, hexahedral, octahedral, and so on. This effort is primarily qualitative.

     A concept proposed by King would use the liquid oxygen tank portions of the Space Shuttle external tank assembly for a basic lunar habitat. The modifications of the tank, to take place in low Earth orbit, will include the installation of living quarters, instrumentation, air locks, life-support systems, and environmental control systems. The habitat is then transported to the Moon for a soft landing. This idea, if proven economically feasible, may provide the most politically palatable path to the lunar surface, with the added advantage that many of the necessary technologies already exist and only need resurrection (similar to Fig. 2).

     A semiquantitative approach to lunar base structures provided by Kelso gives some attention to economic considerations, and the structural concepts included could be developed in the future. A modular approach to lunar base design and construction is suggested by Schroeder as a flexible approach to developing a variety of structures for the lunar surface. In a related vein, a membrane structure is suggested for an open structure that may be used for assembly on the lunar surface by Schroeder. A tensile-integrity structure has been suggested as a possible concept for larger surface structures by Benaroya (Fig. 4).

Concrete and Lunar Materials

     A structural analysis and preliminary design of a precast, prestressed concrete lunar base is reported by Lin. In order to maintain structural integrity, and thus air tightness, when differential settlement is possible, a floating foundation is proposed. All materials for such a lunar concrete structure, except possibly hydrogen for the making of water, may be derivable from lunar resources. Horiguchi study simulated lunar cement.

     The use of unprocessed or minimally processed lunar materials for base structures, as well as for shielding, may be made possible by adopting and extending terrestrial techniques developed in antiquity for harsh environments. A variety of materials and techniques discussed are candidates for unpressurized applications.

     The use of indigenous materials is considered by Happel for the design of a tied-arch structure. The study is extensive and detailed and also includes an exposition on lunar materials.

     Construction of layered embankments using regolith and filmy materials (geotextiles) is viewed as an option using robotic construction, as are fabric-confined soil structures.

     In order to avoid the difficulties of mixing concrete on the lunar surface due to lack of water, Gracia have suggested examining use of sulfur concrete because sulfur is readily available on the Moon.

Lava Tubes

     Ideas regarding the utility of constructing the first outposts under the lunar surface have been proposed. A preliminary assessment is provided by Daga of a lunar outpost situated in a lava tube. They conclude that an architectural solution is needed to the problems surrounding the development of a lunar outpost, but that lunar surface structures are not the best approach. Rather subselene development offers real evolutionary potential for settlement.

     In another structural approach, fused regolith structures are suggested by Cliffton. In this case, the structures are small and many and reside on the surface. A prime advantage offered for planning numerous smaller structures is safety and reliability. The premise of this work is to use the sun’s energy to fuse regolith into components.

Construction in New Environment

     Site plans and surface system architectures are forcefully presented as being fundamental to any development of structural concepts. One of the challenges to the extraterrestrial structures community is that of construction. Lunar construction techniques have differences from those on Earth; for example, the construction team will likely operate in pressure suits, motion is dominated by 1/6 g, solar and cosmic radiation are not shielded by an Earth-type atmosphere, and suspended dust exists in the construction site. Toups assesses various construction techniques for the classes of structures and their respective materials.

     Structural and architectural designs along with manufacturing plants and construction methods are discussed by Namba for a habitable structure on the Moon using concrete modules. The module can be disassembled into frame and panels.

     A qualitative study by Drake is made of the design and construction of a lunar outpost assembly facility. Such a facility would be used to construct structures too large for transport to the Moon in one piece. The assembly facility would also be used to support operation and maintenance operations during the functional life of the lunar outpost. A series of trade studies is suggested on the construction of such an assembly facility.

     Construction of a lunar base will at least partially rest on the capabilities of the Army Corps of Engineers. Preparations that are now under way are outlined by Simmerer and challenges discussed by Sargent.

     All the above are contingent on the ‘‘practical’’ aspects of building structures on the Moon. These aspects include the sort of machinery needed to move equipment and astronauts about the surface; the methods needed to construct in 1/6 g with an extremely fine regolith dust working its way into every interface and opening; and the determination of the appropriate layout of structures considering human safety and operations needs. Using harsh Earth environments such as the Antarctic as test beds for extraterrestrial operations is advocated by Bell.

     The performance of materials and equipment used for lunar construction needs to be examined in terms of the many constraints discussed so far. Structures that are unsuitable for Earth construction may be adequate for the reduced-gravity lunar environment. Several research efforts have been directed to producing construction materials, such as cement, concrete, and sulfur-based materials, from the elements available on the Moon.

     The appendix to this paper provides a long list of structures that require a study not only of the materials that could be used for construction, but also of the necessary tools/equipment, methods of operation/control, and most importantly, how to construct structures with and within the lunar environment (that is regolith, vacuum, 1/6 g). Because most of the construction methods developed since the beginning of mankind are adapted to fit and take advantage of terrestrial environments (that is, soil characteristics, atmosphere with oxygen, and 1 g gravity), technologies that are common on Earth either will not work on the Moon or are too costly or inefficient. The following sections will address some of the unique problems and circumstances that we face.

Creating Base Infrastructure

     The availability of an adequate infrastructure is key to the survival and growth of any society. ‘‘In all human societies, the quality of life depends first on the physical infrastructure that provides the basic necessities such as shelter, water, waste disposal, and transportation,’’ wrote Grigg. Today, and especially for the lunar base, we have to add communication and power as part of the physical infrastructure. All of these constructed facilities have one issue in common, namely the interaction with lunar surface materials: (1) rocks; (2) regolith; and (3) breccias. Lunar soil, referred to as regolith, differs from soil on Earth in several respects that are significant for construction. While the soil that establishes the top layers (10–20 cm) is loose and ‘‘powdery,’’ easily observable in Apollo movies, the regolith reaches the relative density of 90–100% below 30 cm. The grain size distribution of a common regolith, as well as its high density below the top layers, is hardly found in the terrestrial environment. This creates unique problems for excavating, trenching, backfilling, and compacting the soil. These operations, however, are needed to create (1) building foundations; (2) roadbeds; (3) launch pads; (4) buried utilities (power, communication); (5) shelters and covers; (6) open-pit mining; and (7) underground storage facilities.

Excavating ‘‘Hard’’ Lunar Soil

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     Bernold reported about efforts to study the unique problems related to digging and trenching on the Moon. All the common excavation technologies used on Earth depend on the effect of gravitational acceleration that turns mass into forces that are needed to cut, scoop, and move soil (Fig. 5).

     Because of the drastically reduced gravity, transporting the masses and material to the lunar surface would be prohibitively expensive. Dick presented the result of experimental work to study an alternative to traditional excavation of soil, namely the use of explosives to loosen the dense soil so it can be excavated with a limited amount of force. Fig. 6 presents images of the effect of a small amount of explosives on lunar simulants.

     Fig. 6(a) shows the direction and position of ejecta clumps 43.16 and 52.24 ms after detonation, while Fig. 6(b) presents an overview and 6(c) a cross section of the crater created by a small amount of explosives. Although the ejection of regolith would not be acceptable on the lunar surface, since the resulting dust would travel far, research showed that explosives buried deep enough would not create craters but loosen the soil very effectively. In fact, Fig. 5(b) demonstrates how a lightweight bucket pulled by a cable was slicing into the lunar soil simulant that had been loosened in this manner. Furthermore, the sensor-equipped lightweight backhoe excavator required a drastic reduction of energy to dig the loosened soil.

Building Transportation Infrastructure

     The creation of durable roads without using asphalt or concrete as a top requires the planning/cutting of the existing surface and the compaction of fill material. The main objectives of the road base and road surface are to distribute the point loads under the wheels/tracks to the maximum allowable bearing capacity, to provide stable traction resistance for the needed rim pulls (force at the rim of the wheel to allow motion) and breaking forces, and for abrasion resistance. Earthbound equipment that achieves these objectives depends on a large mass, gravitational force, and a sufficient power source. It is obvious that the size of each would make it cost-prohibitive to deploy on the Moon, even if the power source were switched from diesel engines to electric. In addition, Bernold showed that the compaction of lunar soil necessary for creating a stable roadbase would create unique problems. Preliminary research data indicated that the normal size distribution of soil particles would make it impossible to achieve needed density and strength using common methods of static or vibratory compaction.

     However, reducing the percentage of fines present in the regolith can increase the compacted strength of the dry and mainly cohesionless lunar soil. In addition, the surface has to be covered with larger-size stones that have to be crushed from rock, requiring additional equipment such as rock-drill and pyrotechnic equipment, loaders, rock crushers, vibrating screens, and conveyor feeders. It is apparent that the construction of trafficable and stable roads and/or pads on the Moon will require many different machines capable of pushing, loading, cutting, sizing, and compacting regolith as well as the crushing, transportation, spreading, and compaction of rock. The use of multipurpose equipment will certainly be desirable but, on the other hand, slow the operation. If one wants to rely on ‘‘Earth-proven’’ technologies, significant disadvantages will have to be overcome. The most significant handicap is the large reduction of gravitational acceleration that is the basis for the efficient operation of terrestrial roadbuilding equipment.

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     As an alternative to wheel-or water-based transportation, Bernold proposed a cable-based transportation system: ‘‘Lunar tramway systems can take advantage of the reduced gravity, which permits building wider spans and/or using smaller cable diameter for lifting and transporting heavy loads. The use of luffing masts and a unique semistable rigging platform provide many opportunities for reaching wide areas on the lunar surface and performing various tasks needed for handling material, construction, servicing and maintaining facilities needed on a lunar base.’’ Fig. 7 presents the basic elements of a cable-based transportation system that could cover large areas.

     As depicted in Fig. 7, the track cables are attached to two (or more) masts, thus being able to span long distances (for example, 3,000 m) because the lower gravity reduces not only the weight of the load to 1/6, but also the weight of the cable itself, leading to much smaller cable deflections than on Earth. Two electrowinches and cables at each mast provide the mechanism for luffing (sideways rotation around its base socket) the track cables. As indicated in Fig. 7(a), the luffing mechanism adds a significant capability in that it allows the transportation system to cover a rectangular area. In addition, the mast and cables can be lowered all the way to the ground during the final landing approach of a cargo ship. Fig. 7(b) presents the concept of a trolley-carriage that is being moved along the track by a haul cable. Attached to the carriage are either three or six lift cables that can be individually operated with winches. By combining these cable-based mechanisms, a spatially controlled platform can be established. While the system can be used to unload a lunar lander, it can also support construction and mining operations. Fig. 8 portrays a platform attachment capable of (1) excavating trenches to bury cables and pipes; (2) removing rock boulders; (3) collecting rocks for the crusher; (4) deploying soil or rock drills; and (5) mining open pits for processing.

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     As shown in Fig. 8, the same spatially constrained platform supporting a cargo-handling robot can be reconfigured to carry a shovel excavator capable of loading regolith and rock boulders into a pan mounted on top of the platform. For other operations, such as trenching, the same robot arm could reconfigure itself to work as a backhoe, drill boom, or other desirable end-effectors.

     A major problem that needs to be considered in the design of robotic systems for construction is the question of control. The complexities of working in a totally new environment will make it impossible to have the lunar construction equipment operate autonomously. Kemurdjian discuss several specific aspects of the problem. These include the effects of low gravity on traction, the amount of power to be consumed, and most importantly, the dynamics of such vehicles. At the same time, the problems associated with exposing astronauts to long prolonged extravehicular activities (EVA) and the cost of deploying human operators on the lunar surface make it unlikely that each piece of equipment will be steered by an operator who rides on it.

Robotic Control of Construction Equipment

     One of the main problems in robotic control of equipment is the time that signals need to travel through vacuum, atmosphere, or fiber-optic or other communication lines. The time it takes a signal to travel through a network is commonly referred to as latency. Nelson report about their work on the issue of latency: ‘‘Teleoperation is commonly used in the remote control of terrestrial mining equipment. Teleoperating mining equipment on the Moon from the Earth is attractive but involves a transmission loop time delay of 4 to 10 s. A human operator can handle time delays of about 1 second in simple teleoperation applications.’’ A variety of control schemes that help alleviate the problems caused by excessive signal delays have been developed. They range from teleautonomous, to predictive teleoperation, to semiautonomous operation. In this context, one effort by researchers is to equip mobile computerized equipment with the robustness and intelligence to react to the dynamics of the environment. To do this, it is critical ‘‘to build complete agents which operate in dynamic environments using real-time sensors. Internal world models which are complete representations of the external environment, besides being impossible to obtain, are not at all necessary for agents to act in a competent manner’’. Since work in construction always requires moving within, and interacting with, a complex environment while handling messy materials that have to be joined, layered on top, inserted, and so on, a distributed intelligence embodied in the site equipment and sensors and communicating via networks may serve as a uniquely qualified approach to creating a semiautonomous fleet of equipment.

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     Fig. 9 presents a partial model of layered control architecture for a teleoperated backhoe operation that integrates human control with intelligent control modules that work in parallel rather than in sequence. The layered control architecture for robotic excavation was first proposed by Huang. The key feature of this approach is the distribution of the control task to the most efficient module.

Issue of Water on Moon

In a recent development, it appears that there may be water-ice in some craters near the poles of the Moon. It was suggested that water-laden comets and asteroids may have deposited the water. If water does exist in those craters, it was conjectured by Chua and Johnson ~1998) that the moisture distribution may consist of water-ice mixing with the regolith to saturation or near saturation, and reducing outward according to the matric suction pressure ~which is influenced by the particle size distribution). Since the gravitation potential is relatively small compared to the matric suction potential, the water would have been drawn laterally or even upwards over some distance. ~Note: Since the regolith has no clays, unlike Earth, there would not be an osmotic suction component to influence moisture migration.) The extent of this unsaturated zone is primarily influenced by how fast the water vapor condensed at the bottom of the crater, which have temperatures as low as 2230°C. The Lunar Prospector Mission team indicated that the moisture content in the regolith at the bottom of the crater might be between 0.3 and 1%.

Issue of using Geosynthetics in Extraterrestrial Environment

     Some recent papers suggested using geosynthetics as soil reinforcement to construct earth structures such as berms, walls, and slopes. Several problems have to be considered in order for this to be a reality.

• Plastic materials are susceptible to degradation when subjected to radiation;
• The glass transition temperature of many if not all of the geosynthetics used on Earth is well above the cold temperatures encountered on candidate sites, including that on the Moon, which would make the plastics brittle, thus rendering them useless as reinforcing elements; and
• There is little experience on how geosynthetics fare in a hard vacuum and respond to the relatively more abrasive regolith.

Conclusion

     We have presented a summary of current thinking regarding some of the issues surrounding the engineering and construction of structures for long-term lunar human habitation. We close here with a NASA vision of how a lunar base may look (Fig. 10).

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What’s the most practical way to sustain a permanent Moon base through the two-week lunar night? In March of 2014, the Sacramento L5 Society (SL5S), a California chapter of the National Space Society, undertook the task of answering that question, eventually resulting in a detailed analysis of 20 different potential energy delivery systems. This article is a summary of the findings of the SL5S analysis to date. The detailed analysis itself and its accompanying spreadsheet, including a full description of the 20 systems the SL5S has studied to date, can be found on the SL5S website.

History of this analysis

In early 2014, two college students, Akhil Raj Kumar Kalapala and Krishna Bhavana Sivaraju of Rajiv Gandhi University in India, proposed beaming space-based solar energy to the Earth by way of a laser beam located in geosynchronous orbit. On March 14, 2014, an informal “brown bag” Moon Base Working Group (MBWG) started at NASA Ames Research Center in California “to develop a cost-effective plan for establishing and operating the NASA Moon Base that would be within 10% of the total NASA budget.” In March of 2014, Joseph Bland of the SL5S, one of the mentors for Akhil and Krishna, suggested to Michael Abramson, a member of both the SL5S and of the NASA Ames MBWG, that the group examine the possibility of powering a Moon base through the lunar night with a laser either at L1 or in lunar orbit.

Assumptions

Because it takes less energy to put a given mass into low Earth orbit (LEO) than into lunar orbit, and less energy to put a mass into lunar orbit than onto the lunar surface, it is useful to use a given lift capacity to determine relative masses of different systems in different locations. In the SL5S analysis, the lift capacity is defined in SpaceX Falcon Heavy (FH) units. One FH has a liftoff mass of 1,462,836 kilograms. It is assumed that one FH can put 53,000 kilograms into LEO, 17,216 kilograms into either a lunar or Earth-Moon L1 (EML1) orbit, and 5,739 kilograms onto the lunar surface.

Mass doesn’t necessarily have to be lifted directly from Earth to its final destination. For some SL5S calculations, it is assumed that the propellant and electric propulsion (EP) drive used to move a mass from LEO to either lunar orbit or EML1 will equal 30 percent of the transported mass. An EP system can also be used for orbital station keeping, which can be broadly defined as maintaining an object in space in a preferred position or orbit

The SL5S analysis examined energy storage by flywheel, electric battery, chemical, and thermal battery systems. It was concluded that lithium-sulfur (Li-S) batteries presently appeared to have the best specific energy (measured in kilowatt-hours per kilogram), but that other systems would benefit greatly from in situ resource utilization (ISRU) and would become competitive fairly rapidly once manufacturing on the lunar surface began. A specific energy of 0.5 kilowatt-hours per kilogram for Li-S has been used in the SL5S analysis as the basis for energy storage mass calculations for all systems.

As originally suggested by SL5S member Roger Arnold, aggressive collimation of a laser beam with a Fresnel optical lens could be used to dramatically reduce the diameter of a laser beam over a long distance since, for a given light wavelength and distance to target, spot diameter is inversely proportional to aperture diameter. Aggressive laser collimating may be especially practical in a weightless, weather-less environment. Because objects in space are weightlessness, and because space has no atmosphere, a space-based Fresnel laser collimating lens might only be a few mils thick. In this analysis, the mass of a Fresnel lens, including the mounting framework, is assumed to be 0.25 kilograms per square meter, with most of that mass assumed to be in the mounting framework.

Two different different approaches to powering a laser were examined. One uses electricity to power the laser. The system utilizes photovoltaic cells, although the electricity can also be created by a heat engine driven by concentrated solar energy. Another type of system is possible using solar-pumped lasers. In such a system, the solar insolation is concentrated directly on the laser, bypassing the electrical conversion system. Efficiencies for the two systems are expected to eventually be about the same, but the solar-pumped one appears to have a higher specific power even at present efficiencies.

A satellite that is not in sun-synchronous lunar orbit will move regularly into the Moon’s shadow. Adding an energy storage system permits an orbiting laser system to continue beaming energy even when this occurs. This energy storage system can also be used with a sun-synchronous satellite to store energy until the laser system can regain direct contact with the lunar base. Finally, by leaving parts of the energy storage system in orbit rather than moving it to the lunar surface, the overall project mass can be reduced, since rocket motors, fuel, and so on can be scaled back.

A deflecting system, as the term implies, permits a laser beam to be deflected. In certain circumstances, such a system in lunar orbit may permit nearly uninterrupted laser beaming from an orbiting solar-powered laser system to a given location, thus obviating the need for energy storage either in orbit or on the lunar surface.

A deflecting can also find use in other ways. It is possible to envisage a series of non-orbiting deflection modules placed directly on the lunar surface to transfer beamed energy to other locations. Finally, a surface-based solar powered laser system at the lunar poles may beam solar-powered laser energy to an orbiting deflecting system, distributing lunar pole-generated laser energy to other locations on the Moon (see Lunar Polar Multi-array System below).

The 2009 NASA concept study

In January of 2015, the SL5S analysis was essentially complete when the organization discovered a presentation titled “NASA JSC Lunar Surface Concept Study.” Since January, SL5S conducted an extensive rewrite of its analysis to actively compare the earlier analysis with the 2009 NASA study.

The preferred system recommended in the 2009 NASA study was a photovolatic solar array-powered cryogenic storage regenerating fuel cell system. NASA calculated that a five-kilowatt continuous delivery system would store 2,000 kilowatt-hours with a system energy density of 1.15 kilowatt-hours per kilogram. The study’s alternate preferred system was a fixed orbit laser system, with a 16.1-hour orbit period that required a surface receiver installation with 525 kilowatt-hours of energy storage. The laser was powered and fired when it was both in direct sunlight and in direct line-of-sight with the Moon base.

Other approaches

One concept that was not explored in the 2009 NASA study is the use of several separate solar collectors situated on high lunar mountain peaks, or so-called “peaks of eternal light,” each connected directly to the base via electric cables, lasers, or reflected solar beams. This concept was suggested by SL5S member Michael Abramson in early February of this year. Such a system would approach a continuously-powered polar Moon base. Periods of darkness as long as 36 hours may still be likely, requiring additional energy storage capacity, estimated at 540 kilowatt-hours.

Yet another approach to continuously powering a Moon base was looked at and rejected by NASA as impractical. That approach involved parking a solar-powered laser system at the Earth-Moon L1 or L2 points. However, in making its determination, NASA did not consider the possibility of using aggressive laser collimating.

For any two bodies in space rotating about a common center, there are five points of special interest called Lagrange points. For the Earth-Moon system, they’re called EML1, EML2, and so on.

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In the case of EML1, this point is between the Earth and the Moon and along a line between their centers of gravity. In the case of EML2, this point is along the same line but beyond the far side of the Moon. The Lagrange points mark positions where the combined gravitational pull of the two bodies precisely matches the centripetal force required for the points to orbit with those bodies. For example, an object placed at EML1 orbits at the same rate as the Moon even though it is much closer to the Earth.

Lagrange points L1, L2 and L3 are said to be unstable. That is, it requires a force to maintain an object’s position in their vicinity. One type of orbit, called a “halo orbit,” while also unstable, is much more stable than an object placed exactly at one of these Lagrange points. A halo orbit is a three-dimensional somewhat circular orbit approximately perpendicular to the center line connecting the Earth and the Moon at the approximate location of the Lagrange point along that line.

The oscillating Lagrange point orbit

Presently, station keeping in the vicinity of EML1 or EML2 is usually accomplished by a three-dimensional halo orbit. An alternative approach known as the oscillating Lagrange point orbit (OLO), is largely a two-dimensional orbit in the two-body orbital plane. The figure below generally illustrates the theorized course of an OLO over the roughly four weeks of a lunar rotation, as mapped along the approximately 6,400-kilometer line of travel of L1 relative to the Moon between perigee and apogee.

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Looked at from above or below the Earth-Moon plane, the OLO approximates a figure eight, orbiting alternatively along the lobe on one side of the line connecting the center of the Earth and the center of the Moon and then along the lobe on the other side of that line. Note that the center line is relative to Earth and Moon and is described by the elliptical movement of the Moon as it orbits around the Earth/Moon barycenter. SL5S member Roger Arnold, originator of this concept, put it this way:

“The metastable neutral point that the station should track is not the L1 point, wherever it happens to be at any given moment. Rather, it’s a point in the 6D phase space for the system (x, x-dot, y, y-dot, and z, z-dot). I.e., both position and velocity vectors must be right. When they are, the station describes an oscillating “orbit” that passes through the L1 point twice per month. Once the station achieves the neutral point, I believe the only station keeping that’s needed is to correct for drift due to measurement errors and unaccounted solar radiation pressure. The neutral point that the station follows will oscillate with the L1 point, but with lesser amplitude and lagging in phase. The proper analogy is to balancing a weight atop a pole whose base is oscillating back and forth. The weight does not precisely track the base and remain positioned exactly above it.”

The OLO appears to be able to operate in much closer proximity to the Lagrangian point with which it is associated than a halo orbit. This is important for beaming energy to the lunar surface from EML1 or EML2. Work is presently being carried out by the SL5S to roughly estimate the extents of a typical OLO at EML1.

Solar sail propulsion and the gravity winch

In certain circumstances, a solar sail arrangement can be used to enhance or even replace an electric propulsion system for OLO station keeping purposes at EML1/EML2. This system has an advantage because of its ability to modify a spacecraft’s position without using fuel. A related idea is the use of reels to pull in or let out either solar sails or “gravity anchors” relative to a space-based platform. This constitutes what might be called the concept of a “gravity winch.” A gravity winch is basically a reeled tether that’s dropped down gravity wells from a “neutral” gravity point, such as a Lagrange point. In the case of a platform at EML1, a tether can be dropped from the platform down the gravity wells of both the Moon and Earth. Shifting the gravity anchors from one side of the platform to the other allows the EML1 platform to “balance” between the two gravity wells, similar to the way a pole helps a tightrope walker balance. This technique may also be useful for putting in place a “lunar elevator” extending from EML1 to the lunar surface.

Results and conclusions

The table below collates the results of our analysis. The Cryogenic-storage Regenerating Fuel Cell (CRFC) and Fixed Orbit Laser System (FOLS) systems are included for reference purposes only. This presumes that any advances in battery technology will be applied across the board to all systems. To aid in comparisons, a CRFC-type system using Li-S energy storage is included as the Lunar Non-polar Surface Mounted System (LNSMS). Also, a FOLS-type system using Li-S energy storage and aggressive laser collimating is included as the Lunar Orbiting Photovoltaic-powered Laser System (LOPVLS).

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In the table, the 15-kilowatt continuous systems are shown ranked from low cost to high cost by the column “Tot FH $ without EP” (Total Falcon Heavy cost without electric propulsion). It is assumed that, for an initial Moon base, electric propulsion will not be used to deliver the payloads to their ultimate destinations. FH dollars are calculated based on a price of $1,200 per kilogram in accordance with the statement by SpaceX CEO Elon Musk that, “Ultimately, I believe $500 per pound or less is very achievable.” It’s important to note that FH dollars do not include any costs associated with developing the various systems shown in the table.

The findings of the SL5S analysis are very much first order approximations. In addition, the analysis is still a work in progress. However, in light of the dramatic nature of those findings, it is felt that the systems in question merit a far more in-depth analysis than the S5LS is capable of delivering. It is hoped that this article will inspire the undertaking of such an in-depth analysis by NASA or some other interested party.

Abstract

We propose using the temperature gradients between the Moon’s surface and the soil at a certain depth to power an Organic Rankine Cycle that could supply a permanent installation, particularly at night, when solar power is not available. Our theoretical and engineering considerations show that, with existing working fluids and quite feasible technical requirements, it is possible to continuously yield 25kW to sustain a 3 member crew.

1. Introduction

     Indeed, the Moon can unravel unique opportunities for science, engineering and resource exploitation. As an example of scientific and engineering goals, we could mention the advantageous opportunities for further lunar laser ranging, astronomy, due to the inexistence of an atmosphere, and radio-astronomy. It is also well know that Moon’s soil is particularly rich in He³, the fuel of the future fusion nuclear reactors. Furthermore, the impact of comets and asteroids on the Moon are important sources of metals, ice and compounds that can supply humankind for many centuries.

     In fact, the colonization of space presents a number of technical challenges, most of which are yet to be overcome. One of which is how to ensure a sustained power supply given the specific conditions of the Moon.

     The power requirements of an initial lunar base camp with 3 crew members have been estimated at 25kW. However an advanced base with an industrial or mining operation could need over 1MW.

     A key feature of the lunar environment when considering long term settlement is the length of the day-night cycles. The moon has a rotation period of approximately 27 days, which is tidally coupled with its orbital period around Earth. This means each day and night on the Moon lasts for approximately 14 Earth days. This poses a problem regarding a continuous power supply.

     The thermal amplitudes on the Moon are extreme, oscillating between day and night mean temperatures of approximately 380K and 120K, respectively.

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     Owing to these sharp temperature amplitudes, the lunar soil has significant temperature gradients with depth. Measurements conducted during the Apollo lunar landings show that at depths below 80 cm in the lunar regolith the day-night temperature variations are no longer present. Even below 50 cm the temperature fluctuations are on the order of only ±1 K. The mean soil temperatures in the first few meters where the measurements were taken are on the order of 250 K and there is a temperature gradient of the order of 1 to 3 K/m (ct. Figure 1). The gradient arises from the heat flow from the Moon’s crust.

     The default option for supplying power to a lunar colony has been to use photovoltaic cells in solar panels, similarly to most power systems in space. The use of solar panels as the exclusive power source, requires some kind of energy storage to cover for periods in the shadow. However, current battery technology is far from being able to cover for 14 days without sunlight, unless massive battery farms were to be used.

     There have been proposals to store thermal energy in a heat mass made of processed lunar regolith. In this concept, a solar power concentrator heats a mass of compacted lunar regolith to high temperatures. The stored heat is then used during the night to power a Stirling engine that produces electricity.

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     The presence of the aforementioned temperature gradients in the Moon’s soil presents an opportunity to build a thermodynamic power system that can ensure the long-term continuity of power supply on the Moon. Our proposal is to use these temperature gradients to power a thermal engine to supply an installation with uninterrupted power. The main issue when considering classic thermal power systems, usually based on water or air as working fluid, is that they require very high temperatures only achievable either by burning some kind of fuel or through concentration of solar power.

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     Recently, however, there has been an increasing number of proposals on the application of what is usually called an Organic Rankine Cycle (ORC), which has been proposed as an alternative for power generation from low-temperature heat sources such as solar heat, waste heat or geothermal energy. This cycle is characterized by the use of an organic working fluid instead of water, allowing for heat conversion in low-temperature sources. The organic working fluids typically have much lower melting and boiling temperatures than water, allowing the engine to work at lower heat source temperatures. Many of the working fluids employed are the ones commonly used in refrigeration cycles. Current applications of ORC on Earth include cogeneration facilities, Ocean Thermal Energy Conversion and low-grade geothermal heat sources.

In this paper, we propose that an ORC can be used effectively as the basis of a thermal power system using the temperature gradients in the Moon’s soil, particularly, during the long lunar night. As we explore a set of possible working fluids, we examine the feasibility of an ORC to power a lunar colony during its long nights and estimate its performance.

4.5 Asteroid Base

The mission assumes an asteroid mining operation with a 5000 person habitat. The complex transportation scenario for this advanced mission involves four different vehicles and three separate space bases (refs. 86 and 91).

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c. The GEO base serves as the final assembly area for the large fusion rocket system used to propel payloads out to the asteroids. Cargo and propellant are unloaded from electric-powered transfer vehicles sent up from the LEO base. The enlarged OTV used to transfer personnel and priority cargo is designed to transport 441,000 lb (200,000 kg) from LEO to GEO. The complex fusion propulsion system is assembled at the base with the fusion power core, propellant tanks, large thermal radiators, and the personnnel and priority cargo modules. The resulting vehicle, shown in figure 4-11, can transport 1250 passengers and 150 metric tons of priority cargo to the asteroids.

The gross start mass for the resupply mission would be 10,000 metric tons, of which power plant comprises 2000 tons; hydrogen propellant, 4000 tons; and payload, 4000 tons (1250-person habitat plus consumables and priority cargo). The power plant consists of two 6 GW fusion reactors utilizing the deuterium-deuterium fusion reaction. The total power plan provides 4.8 GW of thrust power while radiating almost 2.8 GW of waste heat and 4.4 GW of high energy neutrons.

d. There are two methods the fusion rocket will use to propel vehicles to the asteroid base: fast transfer for personnel and priority cargo, and slow transfer for nonpriority cargo. The manned resupply mission is a fast hyperbolic transfer orbit consisting of an 11-day thrust period to achieve hyperbolic velocity, followed by a 226-day coasting, and a 13-day deceleration to match velocity with the asteroid base. The return mission leaves the asteroid approximately 113 days later for a reverse of the ascent mission.

The second method is used to accelerate unmanned cargo pods on a slow elliptical (Hohmann) transfer orbit out to the asteroid base. Figure 4-12 illustrates the different trajectories. The slower trip takes 130 days longer but costs less than half of what the fast, hyperbolic trip costs. All nonpriority cargo is brought to the asteroid facility in this manner. Empty cargo pods are not returned to Earth, they may be discarded or used in a variety of ways as storage modules or Closed Ecological Life Support System (CELSS) modules.

e. A fleet of two fusion rockets is envisioned. They each make one round trip per asteroid orbit (synodic cycle) to the asteroid mining facility and leave a few days apart. Because of the synodic cycle, the fusion rocket vehicles are delayed at the asteroid base for approximately 113 days, at the GEO location they are delayed approximately 288 days. During these delays the fusion rockets are used to decelerate unmanned cargo pods at the asteroid base and to accelerate the pods at GEO. Cargo pod launches are timed to arrive at the asteroid base shortly after the manned resupply vehicles so that the fusion rockets can decelerate the cargo pods. The rendezvous opportunity (synodic cycle) repeats itself every 928 days. This transportation system allows half of the total crew to be rotated each cycle.

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86. Advanced Propulsion Systems Concept for Orbital Transfer. 1981. NAS8-33935.

91. Technology Requirements for Future Earth-to-Geosynchronous Orbit Transportation Systems. June 1980. NAS1-15301.

William Black:

This is still a work in progress, and more or less I’m attempting Winchell’s stone soup method. I’ve previously come up with a model for an asteroid mining base, I haven’t plugged in numbers for every item yet, I’m looking up references and doing research, this is what I have so far. This is all preliminary, so feel free to make suggestions.

In this case the report suggests these 1,250 persons are passengers en route an asteroid mining base.

With extensive automation (at the base) crew size would be determined by maintenance and servicing needs. Total crew size depends on the scale of the operation. In my experience many people suffer from a lack of perspective when it comes to industrial scale commercial operations. It’s very easy to think; “well this will all be automated, so there hardly need be any personnel.” Well, the equipment has to run 24/7 and when its down the operation is not making money. So even if your mining equipment is largely automated or robotic, it has to be monitored, maintained, and repaired in the field, or even recovered and brought into the shop situated at the main base for repair.

I’ve prepared a rough breakdown based partly on my own work for my own future history setting but adapted to this scenario.

The report specifies an asteroid mining base, specifically a commercial operation, with a base population of 5000. The report is not specific on what is being mined, but the example given is Ceres. We might speculate this is an ice mining operation recovering water ice to feed electrolysis plants separating hydrogen and oxygen on an industrial scale, perhaps in support of other large scale mining operations spread across the asteroid belt. Perhaps providing material for transit to propellant depots located elsewhere. There are similar operations in my future history setting and I’ve done some research on this previously.

Fusion powered Transport

The fusion powered transport might need as few as 3 to 4 flight crew I would think. Pilot, co pilot, navigation/communications officer, power-systems specialist. 4 crew per watch, 4 watches per 24 hours in-flight. So a flight crew of 16.

In regards the asteroid mining base:

Assume commercial operations running four shifts 24/7.

Operations management follows a mission control model managing one hundred remote mining sites scattered over a relatively large area, each with autonomous robotic mining/excavating vehicles and/or drilling rigs. Autonomous loaders, conveyors, for feeding material into separators for processing. Autonomous vehicles to remove and cart away waste material for disposal. Electrolysis plants for separating water ice to hydrogen/oxygen and a ice-melt and filtration plant producing fresh water for base consumption.

Mission Control

Mission control is situated at the main base. Operators remotely monitor operations at each site and assign a trouble shooting tickets for breakdowns and malfunctions.

Each mission control operator handles two remote operations sites, so mission control needs 50 men per shift x 4 shifts: 200 mission control operators.

Trouble shooting maintenance/repair teams for each operational site:

Software/hardware computer team

2 technicians each shift for each site. 200 technicians per shift x 4 shifts: 800 technicians.

Responsible for remote autonomous vehicles primary control systems including sensors and switches, data uplinks, and other operational mining site-to-mission control communications systems, remote cameras and the like.

Mechanical team 1

Stationed at central maintenance hub.

Eight crew for each operational site, per shift, x 4 shifts. 3200 crew

Responsible for

Hardware: remote manipulator systems
Hardware: conveyor systems
Hardware: motors/drive systems
Hardware: pneumatics/hydraulics
Hardware: thermal management, heaters and heat rejection systems
Hardware: electronics

Mechanical team 2, Surface EVA

Four recovery/maintenance teams for each shift, with a five man crew for each. 20 men per shift x 4 shifts: 80 man surface recovery/on-site maintenance crew.

Manages remote autonomous vehicle recovery and/or maintenance in-field and transportation of units back to central maintenance hub.

Cryogenics team.

Manages handling, storage, and transfer of cryogenic materials.
Maintains refueling station for cargo and personnel surface to orbit transfer vehicles and surface-to-surface suborbital transports.
Manages maintenance and repair of cryogenic systems.

Base Hospital

Staff 200
Emergency trauma team on duty 24/7.
Surgical suite staffed 24/7
General care services staffed daily, say 9-5.
Hospital/recovery services, nursing staff, on duty 24/7.

EMT EVA Team

Two five man teams per shift x 4 shifts: 40 crew.

This might just be a meat wagon. A suit failure on the surface of Ceres say fifty kilometers from the base, or a vehicle crash wouldn’t be likely to leave survivors, but it is possible crew may survive such incidents and require rescue.

Base Stores

Warehouse/supplies management & cargo/freight handling crew, warehousing would likely be largely automated such as large scale commercial systems commonly used in the air freight industry today, still a certain number of human operators are required and inventory control needs a certain level of human intervention.

Base Police Force

Staff on duty 24/7.

I took a look at personnel per population studies and most specify 1.8 to 2.3 police officers per 1,000 residents. So, ten officers per shift x 4 shifts: 40 officers. Call center/dispatch staff of 2 per shift x 4 shifts: 8 dispatch staff.

Base Systems

Life support systems, plumbing, air circulation, electrical, computer network, power systems management crew, staff on duty 24/7. 50 man crew on duty each shift x 4 shifts: 200 man crew.

Base Command

Watch officers, office staff, manages book keeping and accounting, documentation of all issues, and maintains base database and records. Watch officer staff on duty 24/7.

Communications Center staff

?

Flight Team

48 dedicated pilots

Flight Operations Control Center

For my theoretical commercial operation I’ve assumed all propulsive vehicles rather than surface crawlers.

Pilots control all suborbital point-to-point vehicles, equipment retrieval vehicles, cargo tugs, EMT EVA transports. Base flight crew is supplemented by crew from fusion transport flight crew who are stationed on the base for 113 days between arrival and departure. The fusion powered transport is used to decelerate unmanned cargo pod shots between arrival and departure. Since the fusion drive system cannot be fired in close proximity to the base there need be a number of cargo tugs to load and off-load cargo pods and transfer personnel between base and the fusion powered transport.

Number of and types of vehicles:

Equipment recovery and on-site maintenance vehicles: 4
EMT EVA Rescue vehicles: 2
Cargo Tugs: 4
Fleet Size: 10 vehicles.

These might be differently equipped vehicles of a single common type. For example the Equipment recovery and on-site maintenance vehicle might carry a tool crib and spare parts storage bay with an on-board work station where the EMT EVA Rescue vehicle would carry a medical pod with an on-board trauma suite. Personnel transfer craft might be cargo tugs with a passenger module rather than a cargo pod.

Flight Center Vehicle Maintenance Crew

100 vehicle specialists, 25 on duty per shift.

Additional Base Personnel

Cafeteria
Food Preparation & service staff

With a base of this scale there are likely to be attached concessions, a convenience store, fast food outlets, or even an upscale restaurant or two, a bar/night club, a theater perhaps. Likely the base has its own on-site local internet, television and movie channels playing recorded material.

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In my model I’ve assigned duty stations/shifts to 4,676 Base personnel leaving 324 base personnel split among the various positions with uncounted crew.

     It was at least two years since Lawrence had been inside an igloo. There was a time, when he had been a junior engineer out on construction projects, when he had lived in one for weeks on end, and had forgotten what it was like to be surrounded by rigid walls. Since those days, of course, there had been many improvements in design; it was now no particular hardship to live in a home that would fold up into a small trunk.
     This was one of the latest models—a Goodyear Mark XX—and it could sustain six men for an indefinite period, as long as they were supplied with power, water, food, and oxygen. The igloo could provide everything else-even entertainment, for it had a built-in microlibrary of books, music, and video. This was no extravagant luxury, though the auditors queried it with great regularity. In space, boredom could be a killer. It might take longer than, say, a leak in an air line, but it could be just as effective, and was sometimes much messier.
     Lawrence stooped slightly to enter the air lock. In some of the old models, he remembered, you practically had to go down on hands and knees. He waited for the “pressure equalized” signal, then stepped into the hemispherical main chamber.
     It was like being inside a balloon; indeed, that was exactly where he was. He could see only part of the interior, for it had been divided into several compartments by movable screens. (Another modern refinement; in his day, the only privacy was that given by the curtain across the toilet.) Overhead, three meters above the floor, were the lights and the air-conditioning grille, suspended from the ceiling by elastic webbing. Against the curved wall stood collapsible metal racks, only partly erected. From the other side of the nearest screen came the sound of a voice reading from an inventory, while every few seconds another interjected, “Check.”
     Lawrence stepped around the screen and found himself in the dormitory section of the igloo. Like the wall racks, the double bunks had not been fully erected; it was merely necessary to see that all the bits and pieces were in their place, for as soon as the inventory was completed everything would be packed and rushed to the site.
     Lawrence did not interrupt the two storemen as they continued their careful stock-taking. This was one of those unexciting but vital jobs—of which there were so many on the Moon—upon which lives could depend. A mistake here could be a sentence of death for someone, sometime in the future.
     When the checkers had come to the end of a sheet, Lawrence said, “Is this the largest model you have in stock?”
     “The largest that’s serviceable” was the answer. “We have a twelve-man Mark Nineteen, but there’s a slow leak in the outer envelope that has to be fixed.”
     “How long will that take?”
     “Only a few minutes. But then there’s a twelve-hour inflation test before we’re allowed to check it out.”
     This was one of those times when the man who made the rules had to break them.
     “We can’t wait to make the full test. Put on a double patch and take a leak reading; if it’s inside the standard tolerance, get the igloo checked out right away. I’ll authorize the clearance.”

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“The Challenge” given to the JSC team was to further explore and refine the concept of the “wet” habitat, namely using the spent propellant tanks as habitable volume after landing on the surface. The Lunar Lander Preparatory Study Team was formed to explore the HabiTank, as it came to be called, its design, its features, capabilities, derived requirements, and interface requirements, and how it might affect the design of the CEV.

“The Vision” was to develop a multifunctional and well-integrated lunar lander system that allowed sustainability and flexibility for landing crew and cargo to explore and establish a human presence on the Moon circa 2020.

The HabiTank Vehicle Concept

During the lunar Apollo missions, living spaces were small, there was no privacy, and the lunar surface stays were very brief.

The novel feature of the HabiTank Lander concept was the ability to dual-use the lander’s two large hydrogen tanks as habitable volumes for a lunar outpost. The use of these tanks offered the capability for long-term human presence to be developed very quickly, and they also offered flexibility to build alternate outpost sites compared to deployment of an entirely new habitat and infrastructure. They also provided easy expansion of the habitable space of the outpost, and maximum utilization of all lander elements.

The team worked out many of the outfitting details of converting the tank components from cryogenic propellant tanks to fully outfitted living and working habitation spaces.

The team first decided that the HabiTanks should be designed and built with their end-state in mind. Rather than optimize the tanks for hydrogen storage, which represented only a small portion of their operational lifetime, they were optimized for their use as habitable elements. This was reflected first in their dimensions. Instead of an optimized cylindrical cross-section, the HabiTanks had a rounded rectangular cross-section, with a width of 2.6 m by a height of 2.9 m by 4.6 m long – each tank providing ~ 25 m³ of volume. For the design to work, an airlock was required to be pre-connected and nestled in between the two large hydrogen tanks. Due to the time required to retrofit a tank into a habitable unit, it was deemed necessary to provide a “Sortie” Hab for missions of 3 to 7 surface days (meaning that the Sortie hab gives the crew a place to live in while they convert the HabiTanks into something habitable).

Each tank had a closed hatch on its aft face that was later used to interconnect the tanks together. They were also hard mated to an airlock with hatchways that were sealed while the tanks contained propellant. The Sortie Habitat that contained much of the equipment was later used to outfit the HabiTanks. The ascent stage of the vehicle was a minimalist design and provided crew access to the Sortie Habitat through a hatchway that had a pyrotechnic separation capability that was activated when the ascent stage departed at end of mission. Up to four crew members could be carried to the surface of the Moon in this lander. Power was provided by fuel cells and solar collection.

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Preparing the HabiTank for Human Entry

The design team studied the process required for the conversion of these vessels from cryogenic propellant tanks to fully outfitted living and working quarters. These steps included removing any residual propellant from the tanks, warming up the tanks, pressurizing the tanks with breathable atmosphere, verifying the pressure seal in the airlock/crew quarters vestibule, installing lighting, power and air circulation, disassembling the vertical baffles, installing the environmental monitoring system, alarm system, and communications systems, and activating airflow to the HabiTank.

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This process was repeated for each HabiTank with minor variations, driven by the specific items to be outfitted in each one. The concept was to convert each HabiTank into crew quarters, a galley and wardroom area, or other surface support areas such as extravehicular activity and mission operations.

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Once the lander had reached the lunar surface, crew or Mission Control personnel could begin to initiate the following steps to make the HabiTanks safe for human entry. First, the propellant tanks were closed off (deadfaced) and sealed from the propellant feed-lines, tank quantity sensors, repress lines, etc. Then the ground initiated pump-out of residual propellant. Next, the ground or crew was to initiate purge of residual propellant vapor. There was to be a time delay of ~13 hours for tanks to thermally condition, after which the internal atmosphere was sampled. Then, N₂ was bled into the HabiTanks to establish the correct PP_N2. Once this step was complete, O₂ was to be introduced into the HabiTanks to establish the proper O₂/N₂ mix. Once the HabiTank was thermally stabilized and contained a breathable atmosphere, it was possible to begin the transformation from tank structure to outpost Habitation Module. The Pressurized Logistics Module (PLM) was mated to the HabiTanks with extendable inflatable tunnels. (the PLMs were delivered previously to the Lunar surface by an uncrewed cargo flight)

The crew would perform several steps to prepare the HabiTank for habitation beginning with Crew Quarters Module 1. First, the crew would verify the pressure seal in airlock/crew quarters vestibule. Then they would open the equalization valve to equalize pressure between HabiTank and vestibule. When complete, they could open the hatch in the lander airlock, giving access to the airlock/crew quarters vestibule. The crew would then remove the thermal insulation from the vestibule, followed by the HabiTank cover plate. The passageway from lander airlock to HabiTank was then opened.

The crew members then were to install temporary lighting, power, and air circulation fans in the HabiTank so they could work intravehicular activity inside the tank to complete its transformation. The next step was to disassemble the removable sections of vertical baffle 1 and store the baffle plates temporarily in the airlock. They likewise disassembled removable sections of vertical baffles 2 and 3, and also temporarily stored those baffle plates in the airlock. Once the baffles had been removed, the crew needed to verify the pressure seal in the crew quarters to PLM vestibule, after which the crew could open the equalization valve to equalize pressure between the HabiTank and the vestibule. Next, the crew members were to remove the HabiTank cover plate and open the equalization valve to equalize pressure between the HabiTank and the PLM tunnel so that the hatch to the PLM could be opened.

Once the passageway tunnel from the HabiTank to PLM was open, the crew could begin to install the permanent utilities trunk (i.e., power, data, communications, air) in the HabiTank ceiling. Then, they could install the environmental monitoring, alarm, and communications systems, and the PLM airflow could be activated to the HabiTank. Any temporary air circulation fans could be removed. They would then configure the internal power and data cabling, and remove and stow any temporary power.

Next, the crew was to install Ortho-grid outfitting connectors and the wall liners (if applicable – some functional areas may have left the exposed aluminum skin). They could install and activate the permanent ambient lightemitting diode (LED) lighting. Any temporary lighting was removed and set up as task lighting. Next, the crew was to assemble the HabiTank furniture (chair, bed, desk, stowage, counter, table, etc., as appropriate) from the baffle plates temporarily stored in the airlock and fabric and padding stored in the PLM. The crew could position the furniture in designated locations in the HabiTank. Support cables and additional fabric partitions could be installed, and the remaining crew accommodations equipment (e.g., laptops, displays, crew personal items, etc.) were to be unpacked from the PLM and stowed in the HabiTank.

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All the system utilities were then checked and verified operational prior to connection of electronic equipment. Upon a successful systems check, the HabiTank crew quarters were ready for occupation. This process was repeated for each HabiTank with minor variations driven by the specific items to be outfitted in each one. The Galley, EVA Maintenance, Geo Lab, Life Science, and Medical HabiTanks would have all also included a water line in their utilities trunks.

To refine the concepts and designs, and the outfitting process, the team built a full-scale mock-up of the HabiTank. By evaluating the mock-up, the team had a better understanding of the various issues that would impact the conversion process from a hydrogen tank to a habitable volume.

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Working Through All the Mission Phases

The team also investigated the features of the ascent and descent stages. Using a full-scale mock-up of the ascent vehicle, the team was able to identify requirements and issues relating to crew accommodations, flight communications, ground communications, and mission operations.

The ascent stage avionics hardware and software provided Guidance, Navigation & Control (GN&C) information to enable the crew to land the combined ascent and descent stages on the Moon and support the later rendezvous of the ascent stage with the CEV. Displays and Controls provided the data necessary to control and manage the vehicle through all mission phases. Communications and Tracking equipment allowed the crew to talk to each other and to the ground mission control center.

Strategizing the Outpost Development and Lander Design

The HabiTanks from the spent lander descent stages formed Habitation Modules that could be connected together with the PLM to form a larger lunar outpost. The PLM was pre-integrated with inflatable tunnels and portals to allow connections to the opposite ends for the HabiTank/airlock system. The PLM or utility module served as a logistics module to transport the HabiTank outfitting and was also where the habitation subsystems were integrated and operated. This strategy allowed the outpost to be quickly assembled and become operational earlier. The HabiTanks and airlocks were planned to remain intact on the lander super structure to ensure structural integrity while transporting the entire lander stage to the final outpost location. Four descent stages and two PLMs were to be used to create the required functionality and volume for the outpost to support four crew members during their 180- to 210-day surface operations stay.

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The Lander HabiTank concept offered a number of advantages for lunar outpost development and lander design. These advantages included the following:

• Capability for long-term human presence developed very quickly.
• If the HabiTank failed on one flight (e.g., could not relocate, or could not be accessed), subsequent flights could provide it with little overall manifest impact.
• More useful volume (e.g., tanks, airlock, Sortie Habitat, etc.) came with every flight.
• The HabiTank was not locked into one outpost location – Sortie Landers with keep-alive capability could be deployed at different sites on the lunar surface to support an extended sortie strategy of exploration (“Cabin in the Woods”).

Additional advantages of this lander configuration were:

• Single descent engine configuration met reliability and redundancy requirements.
• Spent descent stages did not create a field of impacted debris.
• Packaging brought the airlock porch close to surface.
• Good visibility for crew during landing.
• Propellant feed problems of large tanks mitigated by use of sump tank.
• Used a minimal ascent stage.
• Airlock was not thrown away with each flight.
• Only one common descent style was needed to perform a variety of missions:
  • Crew plus cargo (airlock, Sortie Habitat, HabiTanks)
  • Cargo only (but could carry airlock and HabiTanks, or not, as required)
  • Crew exchange (ascent stage only, no airlock or Sortie Habitat)

The “HabiTank” concept converts two of the descent propulsion hydrogen tanks into surface habitation units by pre-integrating an airlock between the two tanks. These tanks are fitted with access ports at each end of the tank that are covered by sealing plates for tunnel attachment during the outpost assembly phase. The overall description of the HabiTank’ lander is a two stage lander with an ascent stage and descent stage (fig 2). The ascent stage is a cylindrical aluminum shell cylinder sized to fit four suited crew members—donning a Mark 3 suit—during the descent landing and ascent to the CEV. It is sized for minimum occupancy duration of a minimum of three hours as dictated by the Constellation Program Office (CxPO). However, this design study feels the ascent stage should have a contingency operations timeline built into the design for up to a range of 8-12 hours pending further studies of abort and rendezvous scenarios. Figure 4 depicts the habitable volumes of the lander concept.

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The Ascent Stage (A.S.) is a single engine pressure fed propulsion system with a Low Impact Docking System (LIDS) compatible with CEV (figure 10). The A.S. has three windows (two for landing operations and one for CEV rendezvous proximity operations). The A.S. also has two dual common wall tanks for the liquid oxygen and methane. It has the capacity to carry crew provisions, stowage, flight systems, subsystems, and return the required 100 kg of lunar samples.

The Descent Stage (D.S.) is comprised of the main single dual pump fed liquid oxygen and hydrogen engine and RCS system. It has four LO₂ tanks and three LH₂ tanks. Two of the tanks are the 2.59 m x 2.89 m x 4.57 m “HabiTanks” while the other tank is the vertically oriented sump tank. The D.S. also consist of the main base frame system, airlock, sortie habitat, two deployable & retractable solar disks, two deployable & retractable radiators, and the landing leg system (Figures 5-9). The Airlock has a deployable dust porch and ramp with railings that provide surface access for the EVA crew. The dust porch will be approximately three feet to the surface.

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The A.S. is connected to the Sortie Hab via a pressurized tunnel that is capable of separation before the A.S. departs from the lunar surface (Figures 8 & 9). The separation technique is based on the Shuttle “Super-Zip” concept that initiates the pre-integrate zip cord to expand and break the metallic structural tunnel ring. This separation event occurs prior to the crew’s departure after the hatches on both sides have been sealed shut. One of the concerns during ascent is the damage or destruction of the HabiTanks and descent stage system. For this reason a plume impingement analysis was performed to preliminarily understand the plume interactions with the descent systems. The preliminary analysis indicated by the temperature and pressure impingements that a plume impingement shield is required. The plume shield concept is a series of titanium plates attached to the structural frame and tanks for protection. In addition to the side wall shields a conical shield was placed over the descent engine. This helps direct the flow down and out the bottom of the descent stage. It also protects the engine from creating potentially damaging debris to the tanks and back flow to the ascent engine.

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The positioning of the four crewmembers in the A.S. is shown in Figure 16 showing an oblique top-view cutaway of the ascent vehicle cabin and in Fig 16. The two crewmembers in front have windows in front for viewing the lunar surface, and at the top of the cabin for viewing the CEV during docking.

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For the transitioning of the IVA suits to EVA suits, the crew and the suits must descend from the ascent vehicle through the connecting tunnel to the Sortie Hab, and finally to the Airlock, as depicted in Figure 18. A pressure sealable hatch is opened at the connecting tunnel to allow the crew access to the crew modules below. A discussion on the suit re-configuration required is presented in the descent vehicle section.

For transitioning the EVA suits back to IVA suits for the lunar ascent and return, the paths are reversed. A pressure sealable hatch is opened at the connecting tunnel to allow the crew access to the ascent module from the airlock and sortie Hab modules below. The hatch is closed and secured before ascent, and separation of the ascent vehicle from the sortie Hab occurs at the tunnel interface. The location of the crew in IVA suits in the departing ascent vehicle is shown in Figure 19.

The lander concept is designed with the outpost end-state in-mind. To design a lander for only a sortie mission only and then try to deliver outpost payloads on it is short sighted. One needs to consider the functional uses, requirements and operations to fully define the lander system. The HabiTank Lander concept has two primary configurations; the crewed lander configuration and the cargo lander configuration. The primary focus of the design effort was on the crewed lander. Figure 20 shows the crewed lander and mass properties summary. Figure 21 shows the cargo lander and mass properties summary.

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The lander-packaged configuration was designed to fit in the Cargo Launch Vehicle 8.5 m (27.88 ft) diameter shroud, which has an internal working diameter of 7.5 m (24.6 ft), fig 22. The launch support structure and interface to the Earth Departure Stage and shroud was not designed. The mass of this structural interface will be a mass threat to the overall mass budget. The final report covers the details of the LEO MM/OD risk and EDS shared resources while loitering in LEO for 95- days.

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The design team performed mission mode analyses to understand the functional capabilities and shared resources of power, thermal, ECLS, crew space, and avionics during the Trans-Lunar coast (fig 23) and Low Lunar Orbit rendezvous (fig 24). Further analyses will be required to flush out the design impacts on the lander from the Constellation system requirements review, Constellation Architecture Requirements Document (CARD) and CEV system requirements document.

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The spent descent stages are reused to assemble an outpost based on the nestled airlock and two HabiTanks to form habitation pods that when connected together with the pressurized logistics module (PLM) forms a larger habitation facility. Based on historical volume trends for long duration space missions and the standards for optimal habitable volume per crewmember, a total pressurized outpost volume may range from 60 – 80 cubic meters per crewmember. The PLM is pre-integrated with inflatable tunnels and portals to allow connections to the opposite ends for the HabiTank/Airlock system. The PLM—or utility module— serves as a logistics module to transport the HabiTank outfitting via a cargo flight and is also where the habitation subsystems are integrated and operate. This strategy allowed the outpost to quickly be assembled and become operational earlier than having to install utility systems in each tank. The HabiTanks and airlocks remain intact on the lander super structure to ensure structural integrity while transporting the entire lander stage to the final outpost location. Four descent stages and two PLMs are used to create the required functionality and volume for the outpost to support four crewmembers during their 180 – 210 day surface operations stay (fig. 25). Environmental protection (radiation, thermal, and micrometeoroid) is provided by an air-beam deployable-rigidized Quonset-style concept. Notionally, the shelter is laid out on the surface, regolith pushed on top, and then the shelter inflated—lifting the shield (with regolith) in place. The shield has “pockets” to keep the regolith from sliding off during inflation. The open-ends are “radiation-closed” by berming regolith at the ends to eliminate “line-or-sight” radiation influx. This would be performed robotically prior to moving the HabiTanks into outpost position. The descent stage landers with tanks intact are moved in their entirety. There are a number of options that were explored—including deployable wheels, use of the JPL Athlete concept, and an independent ARC “MULE” concept that attaches to each leg (requiring for units). This study did not baseline the specific concept to achieve lander movement to the outpost since the intent of the study was to focus on feasibility of retrofitting the tanks into habitable volumes for an outpost. The crew performs the change over from a propulsion tank to Habitat element once the lander is on the surface. The two tanks are converted by removing the excess liquid hydrogen from the tanks into the purge tank and then purging the residual gas by venting it into other tanks or overboard. Once the tanks have been warmed to the appropriate internal temperature and breathable air introduced into the tanks, the crew can begin outfitting and surface operations.

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The Sortie Habitat is used as the primary living quarters and as a 2-person sleep quarters during the 7- day sortie mission. A deployable loft area is used to sleep two crewmembers in the upper section of the Sortie Habitat (fig 26). When configured for day operations (fig 27), the Sortie Habitat provides access to the airlock, computer interface to vehicle systems, hygiene and waste collection facilities (with a privacy curtain), meal prep, and stowage access. The Sortie Habitat is extremely limited in internal volume and is not designed for extensive habitation. The purpose of the sortie mission is primarily EVA driven and it is thus expected that the majority of the crew’s waking time will be spent on the lunar surface, outside the habitat.

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The HabiTanks are retrofitted into various functional spaces. The concept is to outfit each HabiTank into crew quarters, a galley and wardroom area, and other surface support areas such as EVA and Mission operations. Figure 28 and 29 show conceptual ideas of how the tanks could be retrofitted using acoustic fabric panels and deployable systems.

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HABITANK OUTFITTING

The central feature of the HabiTank Lander concept is the ability to use the lander’s two large hydrogen tanks as habitable volumes in the Lunar Outpost. The design team studied the process required for the conversion of these vessels from cryogenic propellant tank to fully outfitted living and working quarters. The HabiTanks are configured from their initial design with their end state in mind. Rather than optimize them for hydrogen storage, which represents only a small portion of their operational lifetime, they are optimized for their use as habitable elements. This is reflected first in their dimensions. Rather than having a cylindrical cross section, the HabiTanks have a rounded rectangular cross section, with a width of 2.59 m and a height of 2.89 m.

Once the lander has reached the lunar surface, crew or Mission Control personnel can begin to initiate the following steps to make the HabiTanks safe for human entry. First, the propellant tanks are closed-off (deadfaced) and sealed from the propellant feed-lines, tanks quantity sensors, repress lines, etc. Then the ground initiates pump out of residual propellant. Next, the ground or crew initiates purge of residual propellant vapor. There is a time delay for ~13 hours for tanks to thermally condition. After which, the internal atmosphere is sampled. Then we bleed N₂ into the HabiTanks to establish the correct PP_N2. Once this step is complete then we bleed O₂ into the HabiTanks to establish proper O₂/N₂ mix. Once the HabiTank is thermally stabilized, contains a breathable atmosphere, and is mated to a Pressurized Logistics Module (PLM), it is possible to enter the HabiTank and begin the transformation from tank structure to Outpost habitation module. The PLM is mated to the HabiTanks with extendable inflatable tunnels as depicted in figure 30 and 31. This connection process starts with the Crew Quarters unit.

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Beginning with Crew Quarters Module 1, the crew will perform the several steps to prepare the HabiTank for habitation. First the crew will verify the pressure seal in Airlock/Crew Quarters vestibule. Then they will open the equalization valve to equalize pressure between HabiTank and Vestibule. When that is complete, they will open the hatch in the lander Airlock giving access to the Airlock/Crew Quarters vestibule. The crew will then remove the thermal insulation from the vestibule. Once the vestibule is clear, the crew will remove the HabiTank cover plate (fig 32). The passageway from Lander Airlock to HabiTank is now open.

The crew will install temporary lighting, power, and air circulation fans in HabiTank. This will allow the crew to work IVA inside the tank to complete the transformation. The next step is to disassemble the removable sections of vertical baffle 1 and store baffle plates temporarily in Airlock. This step is repeated for the other two baffles (fig 33). Once the baffles have been removed, the crew will verify the pressure seal in Crew Quarters to Pressurized Logistics Module vestibule. After which, the crew will open the equalization valve to equalize pressure between HabiTank and vestibule. Next, the crew will remove the HabiTank cover plate and store it temporarily in Airlock. Then they will open the equalization valve to equalize pressure between the HabiTank and Pressurized Logistics Module tunnel.

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Now that the passageway tunnel from HabiTank to Pressurized Logistics Module is open, the crew can begin to install the permanent utilities trunk (power, data, comm., air) in the HabiTank ceiling (fig 34). Then they will install the environmental monitoring, alarm, communications systems. The Pressurized Logistics Module airflow is activated to the HabiTank. The crew removes the temporary air circulation fans. They then configure the internal power and data cabling, then they remove the temporary power and stow them. Next the crew installs Install Ortho-grid outfitting connectors and the wall liners (if applicable—some functional areas may leave the exposed aluminum skin). Next they install the permanent ambient LED lighting and activate. The temporary lighting is removed and set up as task lighting. The crew then assembles the HabiTank furniture (chair, bed, desk, stowage, counter, table, etc. as appropriate) from baffle plates temporarily stored in the Airlock and fabric and padding stored in the Pressurized Logistics Module. The crew positions the HabiTank furniture in designated locations in the HabiTank (fig 35). Support cables and additional fabric partitions are installed. The crew then unpacks the remaining crew accommodations equipment (e.g. laptops, displays, crew personal items, etc.) from Pressurized Logistics Module and stow in HabiTank. All the system utilities are checked and verified operational prior to connection of electronic equipment. Then they connect the power and data lines to the electronic equipment and conduct HabiTank systems activation and check out. Upon a successful systems check, the HabiTank crew quarters are ready for occupation (fig 36). This process is repeated for each HabiTank with minor variations driven by the specific items to be outfitted in each HabiTank. The Galley, EVA Maintenance, Geo Lab, Life Science, and Medical HabiTanks will all also include a water line in their utilities trunks.

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TECHNOLOGY NEEDS AND DESIGN CHALLENGES

The Lander team identified a number of technology needs and design challenges. The team divided the technology needs into lander and outpost specific needs.

The Lander HabiTank technology needs are the ascent stage main engine development; robotic systems for surface ops; payload unloading & surface transportation; autonomous hazard avoidance and landing; a full-up integrated HabiTank/Airlock/PLM interface testing (seal performance, accessing and conversion of HabiTank, etc.); ascent Stage tunnel separation; and vacuum harden displays (which can piggy back on CEV work).

The Lunar Outpost technology needs are dust mitigation (including dust mitigation development needs to include both ways to prevent dust from entering the habitable environments, ways to manage and remove dust that enter the environment, and dust effects on solar arrays and radiators); radiation protection and the use of unpressurized deployable shelters; medical systems to treat patients in-situ at the outpost; power generation and management systems (specifically regenerative fuel cells, development of ultra-lightweight, thin-film solar arrays, and nuclear power); robotic systems for surface ops (including payload unloading & transportation, and surveying, leveling & surface preparation); and long duration life support (including closed-loop life-support with waste recycling from water, solid waste, or air systems). These are known technology drivers and are technology needs for longterm Outpost operations. Specifically for this concept, developing technologies that are focused on being modular or upgradeable to reuse as much delivered mass as possible from the short duration missions to Outpost class missions).

The sortie habitat life support has a duration similar to previous spacecraft experience, but must be evolvable, must operate in gravity and potentially also in space, and must handle the challenges of lunar regolith contamination and frequent EVA. Dust mitigation is a major design challenge that needs to be addressed with new technology. Waste Collection System design from previous flight capabilities may not be sufficient since the wastes are likely to be left on the lunar surface and the toilet functions will be in a volume eventually incorporated in the Outpost. To avoid duplication of the toilet in the Outpost, some sort of modular or upgradeable design for resource recovery would be necessary.

The lunar Outpost must provide resource recycling capability in the PLMs, but minimize the complexity of interfaces while reusing hardware as much as possible. Closed loop life support is an area where major technology development is still required to mature concepts into technologies that are ready for service in the lunar Outpost. Upgradeable or modular technology that would make best use of the lunar lander components in the Outpost would impose new requirements on the existing concepts.

A single LO₂/LH₂ descent stage main engine offers benefits and reduces risks by reducing debris damage risks caused by the plume, reducing dry mass and total mass, reducing complexity, and reduced throttling requirements, as compared to 4 engines. A four engine plume causes significant debris to travel up the center of the stage. The main risk for a single engine is the reliability. Advanced technology is required to achieve the necessary reliability for a single engine. The approach could be using redundant pumps as shown or using highly reliable pumps that have significant margin. Technology development is required in this area.

The storage of LH2 on the descent stage during the 95 day LEO loiter requires a vapor cooled shield on the LO₂ and LH₂ tanks. This vapor cooled shield can be cooled using LH₂ and LO₂ boil-off that is vented from the EDS. The alternate technology is a GHe loop cooled shield that uses a cryo-cooler.

The Descent stage RCS uses pressure-fed LO₂/LCH₄ to avoid plume contamination of the surface and to allow simple safing of the system. The alternate technology, MMH/NTO, would remain in the lines after landing and would require heaters to avoid freeze/thaw rupture of the lines. The technologies required for this cryogenic RCS are similar to the ascent stage, and requires developing an engine that provides repeatable pulses and reliable ignition, and developing a cryogenic feed-system using a thermodynamic vent system.

The ascent stage is an integrated main engine and RCS, such that the RCS provides back-up to the main engine in the later part of the ascent burn. The propellants are LO₂/LCH₄ to enable a pressure-fed system, higher performance, and non-toxic plume. Zero boil-off is achieved by using the sub-cooled property of the propellant at high pressures with a GHe pressurant. The advanced technologies required are a composite over-wrapped aluminum lined pressure vessels, long duration passive cryogenic storage and acquisition, a cold GHe pressurization system, a high performance 1587.6–3402 kg-f LO₂/LCH₄ main ascent engine with fast start and redundant ignition system, electro-mechanically actuated valves, and a cryogenic RCS that is similar to the descent stage RCS. The LO₂/LCH₄ RCS requires a similar technology development and the descent RCS. The highly reliable main engine ignition is achieved by redundant igniters that are of different type, such as spark, pyrotechnic, glow-plug, etc. The main engine thrust of 3402 kg-f is only required if commonality with CEV SM is maintained, otherwise a minimum of 1587.6 kg-f will work.

SUMMARY AND CONCLUSIONS

The Lunar Lander study team designed the corresponding subsystems and developed a number of products during this study. The details of these subsystems and products are documented in the ‘Lunar “HabiTank” Lander Element Conceptual Design Report,’ JSC-63600.

The Lander HabiTank concept offers a number of advantages to the strategy of an outpost development and lander design. These advantages are four, or more, fewer heavy-lift launches needed to get to same Outpost end state. This is approximately 38,555 kg less accumulated mass taken to lunar surface to get to Outpost end state. Therefore, the capability for long-term human presence is developed quickly and the flexibility of creation of alternate Outpost sites compared to deployment of an entirely new habitat and infrastructure. The outpost layout with closed loops of pressurized tanks is failure tolerant; is flexible to easy expansion of habitable space of the Outpost; and makes maximum utility of all lander elements. With this approach much less hardware is discarded—thus a smaller lunar junk yard. In addition, it makes maximum utility of airlocks, has good redundancy, and does not throw away hardware with each flight. If a HabiTank has a mission failure on a flight (e.g., cannot relocate, or cannot access) subsequent flights can provide the replacement with little overall manifest impact. Another advantage is that more useful volume comes with every flight (tanks, airlock, sortie habs etc.) and is not locked into one Exploration or Outpost concept. Sortie Landers with keep a live capability can be deployed at different geographical sites on the lunar surface to support an extended sortie strategy of exploration (“Cabin in the Woods”).

A specific advantage to this lander configuration is a single descent engine configuration meets reliability and redundancy requirements; spent descent stages do not create a field of impacted debris; packaging brings airlock porch close to surface; it has good visibility for crew during landing; propellant feed problems of large tanks are mitigated with sump tank strategy; it uses a minimally-sized approach Ascent Stage (separation of A.S and airlock, so the airlock is not thrown away with each flight); only one lander platform/vehicle is needed to achieve a variety of missions; the outpost is achieved with crew and cargo lander volumes (Airlock, sortie Hab, HabiTanks).

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When astronauts set foot on Mars, they may stay for months rather than days as they did during Apollo missions to the moon. The surface of Mars has extreme temperatures and the atmosphere does not provide adequate protection from high-energy radiation. These explorers will need shelters to effectively protect them from the harsh Martian environment and provide a safe place to call home.

For researchers at NASA’s Langley Research Center in Hampton, Virginia, the best building material for a new home on Mars may lie in an unexpected material: ice.

Starting with a proposed concept called “Mars Ice Dome,” a group of NASA experts and passionate designers and architects from industry and academia came together at Langley’s Engineering Design Studio. The project was competitively selected through the Space Technology Mission Directorate’s (STMD) Center Innovation Fund to encourage creativity and innovation within the NASA Centers in addressing technology needs. This is just one of many potential concepts for sustainable habitation on the Red Planet in support of the agency’s journey to Mars.

“After a day dedicated to identifying needs, goals and constraints we rapidly assessed many crazy, out of the box ideas and finally converged on the current Ice Home design, which provides a sound engineering solution,” said Langley senior systems engineer Kevin Vipavetz, facilitator for the design session.

The team at Langley had assistance in their concept study, as a collaborative team from Space Exploration Architecture and the Clouds Architecture Office that produced a first-prize winning entry for the NASA Centennial Challenge for a 3D-printed habitat (Mars Ice House) played a key role in the design session.

The “Mars Ice Home” is a large inflatable torus, a shape similar to an inner tube, that is surrounded by a shell of water ice. The Mars Ice Home design has several advantages that make it an appealing concept. It is lightweight and can be transported and deployed with simple robotics, then filled with water before the crew arrives. It incorporates materials extracted from Mars, and because water in the Ice Home could potentially be converted to rocket fuel for the Mars Ascent Vehicle, the structure itself doubles as a storage tank that can be refilled for the next crew.

Another critical benefit is that water, a hydrogen-rich material, is an excellent shielding material for galactic cosmic rays – and many areas of Mars have abundant water ice just below the surface. Galactic cosmic rays are one of the biggest risks of long stays on Mars. This high-energy radiation can pass right through the skin, damaging cells or DNA along the way that can mean an increased risk for cancer later in life or, at its worst, acute radiation sickness.

Space radiation is also a significant challenge for those designing potential Mars outposts. For example, one approach would envision habitats buried underneath the Martian surface to provide radiation shielding. However, burying the habitats before the crews arrive would require heavy robotic equipment that would need to be transported from Earth.

The Ice Home concept balances the need to provide protection from radiation, without the drawbacks of an underground habitat. The design maximizes the thickness of ice above the crew quarters to reduce radiation exposure while also still allowing light to pass through ice and surrounding materials.

“All of the materials we’ve selected are translucent, so some outside daylight can pass through and make it feel like you’re in a home and not a cave,” Langley Mars Ice Home principal investigator Kevin Kempton said.

Selecting materials that would accomplish these goals was a challenge for materials experts.

“The materials that make up the Ice Home will have to withstand many years of use in the harsh Martian environment, including ultraviolet radiation, charged-particle radiation, possibly some atomic oxygen, perchlorates, as well as dust storms – although not as fierce as in the movie ‘The Martian’,” said Langley researcher Sheila Ann Thibeault.

In addition to identifying potential materials, a key constraint for the team was the amount of water that could be reasonably extracted from Mars. Experts who develop systems for extracting resources on Mars indicated that it would be possible to fill the habitat at a rate of one cubic meter, or 35.3 cubic feet, per day. This rate would allow the Ice Home design to be completely filled in 400 days. The design could be scaled up if water could be extracted at higher rates.

Additional design considerations include a large amount of flexible workspace so that crews would have a place to service robotic equipment indoors without the need to wear a pressure suit. To manage temperatures inside the Ice Home, a layer of carbon dioxide gas would be used as in insulation between the living space and the thick shielding layer of ice. And, like water, carbon dioxide is available on Mars.

It’s important, Kempton said, for astronauts to have something to look forward to when they arrive on the Red Planet.

“After months of travel in space, when you first arrive at Mars and your new home is ready for you to move in, it will be a great day,” he said.

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