Considerations for Design Criteria for Lunar Structures

The development of design criteria for lunar structures must begin soon in order to establish adequate criteria. Some of the items that need consideration in such criteria are discussed. The categorization of the structures will provide designers with information on the purpose and level of complexity of the structure. Various construction materials and structure types that will be critical for the design of lunar structures, are considered. The environment of the moon and its possible effects on structures are presented and lead to the development of a few load cases that need to be considered in design. A probabilistic format for the criteria and design lifetimes are also discussed.     

Technical Requirements for Lunar Structures

The moon has recently regained the interest of many of the world’s space agencies. Lunar missions are the first steps in expanding manned and unmanned exploration inside our solar system. The moon represents various options; it can be used as a laboratory in low gravity, it is the closest and most accessible planetary object from the Earth, and it possesses many resources that humans could potentially exploit. This paper has two objectives: to review the current status of the knowledge of lunar environmental requirements for future lunar structures, and to attempt to classify different future lunar structures based on the current knowledge of the subject. The paper divides lunar development into three phases. The first phase is building shelters for equipment only; in the second phase, small temporary habitats will be built, and finally in the third phase, habitable lunar bases will be built with observatories, laboratories, or production plants. Initially, the main aspects of the lunar environment that will cause concerns will be lunar dust and meteoroids, and later will include effects due to the vacuum environment, lunar gravity, radiation, a rapid change of temperature, and the length of the lunar day. This paper presents a classification of technical requirements based on the current knowledge of these factors, and their importance in each of the phases of construction. It gives recommendations for future research in relation to the development of conceptual plans for lunar structures, and for the evolution of a lunar construction code to direct these structural designs. Some examples are presented along with the current status of the bibliography of the subject.     

Top-Level Considerations for Planning Lunar/Planetary Habitat Structures

This paper highlights important considerations to guide overall planning and element design of lunar/planetary surface habitat structures. Driving influences include stringent launch/landing payload limitations; high costs of human time for surface deployment and operational readiness; influences of the harsh environment on structures, devices and crews; and a paucity of equipment and human and consumable resources that necessitates extreme economies. General habitat concept options are proposed along with desired attributes for comparative assessments of figure of merit (FOM) rankings. Eight broad FOM categories are applied as a basis for top-level option evaluations: (1)?launch optimization features, (2)?landing optimization features, (3)?habitat capacity and functionality, (4)?environmental factors and features, (5)?deployment and operational readiness, (6)?reliability and maintainability, (7)?commonality with other surface systems, and (8)?pathways and potentials for growth. Much of the content of this paper draws on investigations conducted by the Sasakawa International Center for Space Architecture (SICSA) in support of separate National Aeronautics and Space Administration (NASA) contracts awarded to teams headed by Boeing and ILC-Dover for a “Minimum Functionality Habitation Systems Concept Study.” Comprehensive team study results were presented to NASA in February 2009 and have been publicly released to all interested parties.     

Mechanical Equipment Requirements for Inflatable Lunar Structures

Inflatable structures have been proposed by a number of authors. Several structural forms have been conceptually designed, including spherical, pillow‐shaped, semicylindrical, and domed saucer. Regardless of structural form, all inflatables require mechanical equipment to initiate and maintain inflation. This paper identifies the mechanical equipment and operations required to support an inflatable structure. A previously proposed semicylindrical structure is selected for this study, but the principal results are applicable to all inflatable structures. The results indicate that air for inflatable structures should be transported to the moon in a liquid (cryogenic) state. The liquefied air can be evaporated and heated to the proper temperature using solar energy and a conventional pumping system. Removing the air from the facility is an entirely different problem and requires different equipment. There are two alternatives: (1) Discharge the air to the moon; and (2) reclaim the air for reuse. The first alternative is not likely to be cost‐effective and might well be scientifically unacceptable. The second alternative presents numerous technical problems but appears technically feasible.     

Geometric Modeling of Inflatable Structures for Lunar Base

A modular inflatable structure consisting of thin, composite membranes is presented for use in a lunar base. Results from a linear elastic analysis of the structure indicate that it is feasible in the lunar environment. Further analysis requires solving nonlinear equations and accurately specifying the geometries of the structural members. A computerized geometric modeling technique, using bicubic Bezier surfaces to generate the geometries of the inflatable structure, was conducted. Simulated results are used to create three‐dimensional wire frames and solid renderings of the individual components of the inflatable structure. The component geometries are connected into modules, which are then assembled based upon the desired architecture of the structure.     

Structural Design of a Lunar Habitat

A lunar base is an essential part of all the new space exploration programs because the Moon is the most logical first destination in space. Its hazardous environment will pose challenges for all engineering disciplines involved. A structural engineer’s approach is outlined in this paper, discussing possible materials and structural concepts for second-generation construction on the Moon. Several different concepts are evaluated and the most reasonable is chosen for a detailed design. During the design process, different solutions—for example, for the connections—were found. Although lunar construction is difficult, the proposed design offers a relatively simple structural frame for erection. A habitat on the Moon can be built with a reasonable factor of safety and existing technology. Even so, we recognize the very significant difficulties that await our return to the Moon.     

Lunar Astronomical Observatories: Design Studies

The best location in the inner solar system for the grand observatories of the 21st century may be the Moon. A multidisciplinary team including university students and faculty in engineering, astronomy, physics, and geology, and engineers from industry is investigating the Moon as a site for astronomical observatories and is doing conceptual and preliminary designs for these future observatories. Studies encompass lunar facilities for radio astronomy and astronomy at optical, ultraviolet, and infrared wavelengths of the electromagnetic spectrum. Although there are significant engineering challenges in design and construction on the Moon, the rewards for astronomy can be great, such as detection and study of Earth‐like planets orbiting nearby stars, and the task for engineers promises to stimulate advances in analysis and design, materials and structures, automation and robotics, foundations, and controls. Fabricating structures in the reduced‐gravity environment of the Moon will be easier than in the zero‐gravity environment of Earth orbit, as Apollo and space‐shuttle missions have revealed. Construction of observatories on the Moon can be adapted from techniques developed on the Earth, with the advantage that the Moon's weaker gravitational pull makes it possible to build larger devices than are practical on Earth.     

Overview of Existing Lunar Base Structural Concepts

Various structural concepts for lunar bases have been proposed and considered during the past three decades, each balancing the multitude of lunar constraints and anticipated base functions with a different distribution of weighting factors. This paper provides a brief review of the study of lunar base structures as well as an overview of current ideas. Lunar environmental characteristics are highlighted, and questions of structural functionality are categorized. In an appendix to the overview report, building systems proposed for lunar applications are categorized according to applications, application requirements, types of structures, material considerations, structures technology drivers, and requirement definitions. Another appendix presents a short list of the most important references related to these issues, as ranked by the task committee. This document contains a brief overview rather than an exhaustive review of concepts proposed for lunar outposts. The original documents, including those cited as references, are not replaced by this paper. However, this review is intended to be reasonably complete. There has been no conscious intention to ignore any work in this area of extensive ongoing activity.     

Engineering Issues for Early Lunar‐Based Telescopes

A telescope on the Moon is needed for astronomy and can be constructed in this decade or early in the next century. Design for this telescope will be fundamentally different from the design of free‐flying telescopes. Its design will be more like the new Keck telescope being completed on a mountaintop in Hawaii than the Hubble Space Telescope, in low Earth orbit. Success of the lunar‐based telescope will depend on an appropriately engineered structure, a suitable interface (foundation) in the lunar soil, and a carefully thought out construction process. Participation of engineers in identifying and resolving issues for this extraterrestrial engineering and construction project is a natural extension of the traditional engineering role, and will prepare the engineering and construction communities for the subsequent greater challenges associated with basing on the Moon. These communities need to document now the types of data and information that NASA should obtain in the next early lunar missions so that construction on the Moon will be facilitated.     

Design and Construction Considerations for Lunar Outpost

Engineers utilize various codes in the process of design, whether structural, mechanical, or otherwise. Reliance on a code for design is based on the knowledge that a tremendous amount of time and effort was spent by experienced engineers to codify theories and good practice in a particular design discipline. Good practice in structural design implies cognizance of materials, structural behavior, environmental loadings, assumptions made in analysis and behavior, and the uncertainties inherent in all of these. The American Institute of Steel Construction's (AISC) Manual of Steel Construction is such a codification for the design and construction of steel structures. It includes information, some tabular and the rest in the form of specifications and commentaries, necessary to design and provide for the safe erection of steel‐framed structures. The design equations are generally semiempirical, that is, they are based on a mix of theoretical analysis, experimental data, and factors of safety. Each of these components has associated implicit assumptions. Some of these assumptions were explored to understand how and if the Earth‐based design code could be used for the design of a lunar outpost. Topics discussed come from the AISC Code of Standard Practice and the commentaries, and issues such as scaling of loads and strength in the 1∕6 g lunar environment, thermal cycling effects and fatigue, stiffening and buckling are briefly discussed. Important topics for further detailed study include: (1) The relationships between severe lunar temperature cycles and fatigue; (2) very low temperature effects and the possibility of brittle fractures; (3) outgassing for exposed steels and other effects of high vacuum on steel∕alloys; (4) factors of safety originally developed to account for uncertainties in the Earth design∕construction process undoubtedly need adjustment for the lunar environment; (5) dead loads∕live loads under lunar gravity; (6) buckling∕stiffening and bracing requirements for lunar structures that will be internally pressurized; and (7) consideration of new failure modes such as high‐velocity micrometeorite impacts.     

Design and Construction of Shielded Lunar Outpost

The construction of an outpost on the Moon in which humans can live and work for periods exceeding six months will require special countermeasures to adapt to the hostile environment present at the lunar surface. Various inherent dangers such as meteoroids, galactic cosmic radiation, solar proton events, and large thermal extremes will drive the design configuration of the outpost. Other considerations such as lunar soil mechanics, equipment performance, mass delivery, risk, reliability, and tele‐operability act strongly as constraints that shape and control the design alternatives. Analysis of these fundamental relationships have resulted in lunar civil engineering guidelines, which are unique to this domain, and these in turn have pointed to research areas needing further attention. A preliminary design is presented for a lunar outpost shelter. Additionally, the design methodology is explored, and early enabling technologies are identified to facilitate an understanding of lunar shelter designs from an integrated system standpoint.     

Engineering, Design and Construction of Lunar Bases

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.     

Preparing to Bridge the Lunar Gap

The United States is committed to the exploration of and the expansion into space. A manned earth‐orbiting space station is planned for the next decade and studies continue looking at manned lunar bases. Appropriate planning should be initiated for such a mission now as a high national priority. Many systems must be examined and technologies developed as soon as possible. Some of these include types of power sources, life support systems, construction equipment and techniques, construction methods, lunar mapping, and logistical constraints.     

Cable Structures and Lunar Environment

Lunar environmental characteristics, such as the lack of atmosphere, the smaller gravitational acceleration, and the weaker regolith, place different requirements on structural systems than the earth environment does. Some of these requirements are the internal pressurization of structures, emphasis on details, and careful design of foundation systems. Popular structural systems on the Earth environment, such as steel and reinforced concrete frames and trusses with traditional rigid connections may be inefficient for the lunar environment. Cable structures can be shown to meet the different and sometimes conflicting requirements of the lunar environment. The behavior of three different groups of cable structures in the lunar environment (differentiated by their small, medium and long spans) are studied in this paper. The structural systems can be designed to meet the main requirements in an efficient way. Foundation uplift problem is of particular interest, especially in the early lunar colonization stage. It was shown that with a slight modification in the cable system, the uplift problem can be solved, thus saving manpower and costs, while improving the overall system behavior.     

Lunar Dust Levitation

Observations of a lunar “horizon glow” by several Surveyor spacecraft on the lunar surface in the 1960s and detections of dust particle impacts by the Apollo 17 Lunar Ejecta and Meteoroid Experiment have been explained as the result of micron-sized charged particles lifting off the surface. The surface of the Moon is exposed to the solar wind and solar UV radiation causing photoemission, so it develops a surface charge and an electric field near the surface. Dust particles injected into this plasma from the lunar regolith, whether from human and mechanical activity or from meteoroid impacts or electrostatic forces, may be stably levitated above the surface and may undergo preferential deposition onto areas of the lunar surface (or equipment) with different electrical properties. This can lead to a net transport as well as contamination of sensitive equipment. This paper reports on new experimental measurements and numerical simulations of the plasma environment above the lunar surface and the related behavior of charged dust.     

Concepts for Lunar Outpost Development

This paper summarizes the results of a qualitative investigation to identify concepts for design and construction of near‐term lunar facilities. Accomplishing such construction will require an adaptation or transfer of current terrestrial technology and methods. Discussions on modularization, geosynthetic materials, aluminum materials, static load analysis, and dynamic load analysis provide illustrative examples of how terrestrial technologies can be adapted to lunar applications. These discussions provide support for the development of a phased lunar construction strategy. The initial stage of construction is characterized by small self‐supporting accomodation and laboratory modules. The assembly facility stage is characterized by the construction of a large pressurized module‐assembly facility. The module production stage is characterized by the fitting together of terrestrial or low earth‐orbit subassemblies into completed modules within the module assembly facility. The completed modules are also tested and moved to their final location in this stage. The lunar materials stage is characterized by the construction of facilities with maximum use of lunar materials.     

Indigenous Resource Utilization in Design of Advanced Lunar Facility

The most important consideration in the establishment and support of a permanently manned lunar base will be resource utilization. Seven potential lunar construction materials were analyzed with respect to their physical properties, processes, energy requirements, and resource efficiency. Reviewing the advantages and disadvantages of each material led to the selection of basalt as the primary construction material for initial use on a lunar base. The team conceptualized a construction system that combines lunar regolith sintering and casting to make pressurized structures. The design uses a machine that simultaneously excavates and sinters the lunar regolith to create a cylindrical hole. The hole is then enclosed with cast basalt slabs, allowing the volume to be pressurized for use as a living or work environment. Cylinder depths up to 4–6 m in the lunar mare and 10–12 m in the lunar highlands can be achieved. Advantages identified in the construction system include maximum resource utilization, relatively large habitable volumes, interior flexibility, and minimal construction equipment needs. The conclusions of this study indicate that there is significant potential for the use of basalt as a low‐cost alternative to Earth‐based materials. It remains to be determined, during lunar base phasing, whether this construction method should be implemented.     

Geotechnical Properties of NT-LHT-2M Lunar Highland Simulant

Future lunar explorations require a thorough understanding of the geotechnical properties of lunar soils. However, the small amount of lunar soil that was brought back to earth cannot satisfy the needs. A new lunar soil simulant, NU-LHT-2M, has been developed to simulate lunar regolith in the lunar highlands region. It is characterized to help the development of regolith-moving machines and vehicles that will be used in future missions to the moon. The simulant’s particle size distribution, specific gravity, maximum and minimum densities, compaction characteristics, shear strength parameters and compressibility have been studied; and the results are compared with the information about lunar regolith provided in the Lunar Sourcebook.     

Lunar‐Base Construction Equipment and Methods Evaluation

A process for evaluating lunar‐base construction equipment and methods concepts is presented. The process is driven by the need for more quantitative, systematic, and logical methods for assessing further research and development requirements in an area where uncertainties are high, dependence upon terrestrial heuristics is questionable, and quantitative methods are seldom applied. Decision theory concepts are used in determining the value of accurate information and the process is structured as a construction‐equipment‐and‐methods selection methodology. Total construction‐related, earth‐launch mass is the measure of merit chosen for mathematical modeling purposes. The work is based upon the scope of the lunar base as described in the National Aeronautics and Space Administration's Office of Exploration's “Exploration Studies Technical Report, FY 1989 Status.” Nine sets of conceptually designed construction equipment are selected as alternative concepts. It is concluded that the evaluation process is well suited for assisting in the establishment of research agendas in an approach that is first broad, with a low level of detail, followed by more‐detailed investigations into areas that are identified as critical due to high degrees of uncertainty and sensitivity.