ACRE: Autonomous Casting RovEr Final Technical Paper

ACRE: Autonomous Casting RovEr Final Technical Paper Northwestern University Team Members: Jaime Berkovich1a*†‡, Julian Rocher23b†*, Benjamin Taalman23a†*, Trevor Abbott3b*, Matthew Queen23b*, Tyler Nanoff3b*, Liam Warlick3a*, Daniel Lynch1b*, Margaret Gao3b*, Luke Fahrney3c*, Justin Chang Stauffer3b*, Liam O’Malley3d*, Xitlalli Casta˜ neda3b*, Bobby Cloninger3c*, Elaine Liu23b*, Daniel Guo3a*, Catherine Ott1a*, Sam Price1a*, Vishu Gupta1c+, Eliseu Antonio Kloster Filho3b+, Shane Grayson3d*, Rebecca Goldenson3b*, Camille Layden3a*, Fiona Ireland3b*, Zofia Muller-Hinnant3b*, Ian Falck3e* Space Grant Affiliation: Illinois Space Grant Consortium Space Grant Director Contact Information: Dr. Joshua Rovey, rov[email protected], (217) 300-7092 Faculty Advisor: Prof. Ian McCuea Industry Partners: Wearifi, Inc.; Rexnord Aerospace; Questek Innovations, LLC; Ansys, Inc. NUSTARS Sponsors: Boeing Co; Epsilon3, Inc.; Onshape, Inc.; The Murphy Society; Robert R. McCormick School of Engineering and Applied Science at Northwestern Northwestern University Space Technology and Rocketry Society (NUSTARS) McCormick School of Engineering and Applied Science October 23, 2023 1: PhD Candidate. 2: MS Candidate. 3: Undergraduate. a: Department of Materials Science and Engineering. b: Department of Mechanical Engineering. c: Department of Electrical and Computer Engineering. d: Department of Civil and Environmental Engineering. e: Department of Chemical Engineering. *: US Citizen. +: International Student. ‡: Team Lead. †: Co-Captain. Quad Chart 2 Contents 1 Executive Summary 6 1.1 OperationalScenario ...................................... 6 1.2 TechnologicalOverview .................................... 6 1.3 Verification Testing Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Problem Statement and Background 7 2.1 Introduction........................................... 7 2.2 ChallengeandApproach .................................... 7 3 Project Description 9 3.1 Technology Overview and Lifecycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 Constraints, Assumptions, and Technical Specifications . . . . . . . . . . . . . . . . . . . . 10 3.3 Subsystems and Engineered Components . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3.1 OWSPS......................................... 10 3.3.2 Crucible......................................... 11 3.3.3 Plow .......................................... 13 3.3.4 Feedstock........................................ 14 3.4 Stakeholders........................................... 14 4 Verification Testing on Earth 16 4.1 Overview ............................................ 16 4.2 SafetyStatement ........................................ 16 4.3 Optical Waveguide Solar Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3.1 Al-BoPET Mirror Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3.2 Fabrication of Mirror Test Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3.3 CalorimetricSetup ................................... 16 4.3.4 Calorimetric Characterization of Mirrors . . . . . . . . . . . . . . . . . . . . . . . 17 4.3.5 Other OWSPS Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.4 Crucible............................................. 18 4.4.1 PrototypeFabrication.................................. 18 4.4.2 PrototypeTesting.................................... 18 4.4.3 Ray Tracing Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.4.4 Transient Thermal Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.4.5 AdditionalWork .................................... 19 4.5 Furrowing (Plowing) and Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.5.1 Goals .......................................... 20 4.5.2 Soil Settlement Under Load in ABAQUS . . . . . . . . . . . . . . . . . . . . . . . 20 4.5.3 Empirical Characterization of Furrowing . . . . . . . . . . . . . . . . . . . . . . . 20 4.5.4 Dynamic Lunar Soil Simulations in Ansys LS-DYNA . . . . . . . . . . . . . . . . 21 4.5.5 Furrowed Soil Casting in Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.5.6 Furrowed Soil Casting in Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6 Materials and Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6.1 Regolith-Al Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.6.2 Al-Bi for Improved Wetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3 5 Results and Conclusions 23 5.1 OWSPS ............................................. 23 5.1.1 Parabolic Mirror Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1.2 Overall OWSPS Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1.3 Conclusions....................................... 23 5.2 Crucible............................................. 23 5.2.1 Ansys SPEOS Ray-Trace Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.2.2 Transient Thermal Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.3 Furrowing............................................ 24 5.3.1 Soil Settlement Under Load in ABAQUS . . . . . . . . . . . . . . . . . . . . . . . 24 5.3.2 EmpiricalFurrowing.................................. 24 5.3.3 LS-DYNA Simulations Under Earth Gravity . . . . . . . . . . . . . . . . . . . . . 24 5.3.4 Empirical Validation of Dynamic Simulations . . . . . . . . . . . . . . . . . . . . . 24 5.3.5 LS-DYNA Simulations Under Lunar Gravity . . . . . . . . . . . . . . . . . . . . . 25 5.3.6 Furrowed Soil Casting in Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.4 MaterialsandProcessing .................................... 25 5.4.1 Metal-Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.4.2 AluminumWicking................................... 25 5.4.3 Conclusions....................................... 25 6 Path-To-Flight 26 6.1 Overview ............................................ 26 6.2 OWSPS ............................................. 26 6.2.1 Al-BoPET Parabolic Mirror Fabrication Optimization . . . . . . . . . . . . . . . . 26 6.2.2 Optical Waveguide Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6.2.3 DustEjection...................................... 26 6.2.4 SunTracking...................................... 26 6.3 Crucible............................................. 26 6.3.1 Automated Feedstock Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6.3.2 IntelligentMelting ................................... 26 6.3.3 PouringActuation ................................... 26 6.3.4 Automated Crucible Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.4 Furrowing............................................ 27 6.4.1 RegolithStamping ................................... 27 6.4.2 Engineering Complex Plow Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.4.3 Plow Materials Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.5 Feedstock and Materials Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.5.1 Materials Characterization Post-Casting-Under-Vaccuum . . . . . . . . . . . . . . . 27 6.6 Exploration of Additional Use-cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.6.1 ReusableMold ..................................... 27 6.6.2 Direct Solar Regolith Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.6.3 Fusing Prefabricated Pavers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.7 General ............................................. 27 7 Detailed Timeline 28 4 8 Budget 29 8.1 Breakdown ........................................... 29 8.2 Sponsors & In-Kind Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 8.3 Summary ............................................ 29 A Acknowledgements 30 A-1 NUSTARS ........................................... 30 A-2 Facilities............................................. 30 A-3 Northwestern Faculty and Staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 A-4 Students............................................. 31 A-5 ACRESponsors/Partners.................................... 32 A-6 NUSTARSSponsors ...................................... 32 B Explanatory Figures 33 C Calculations 50 C-1 ParabolicMirrorSpotSize ................................... 50 C-2 RayTracingTotalAngle .................................... 50 C-3 Verification of FEM Model with 1D Analytical Solution . . . . . . . . . . . . . . . . . . . 51 D References 53 5 1 Executive Summary 1.1 Operational Scenario The establishment of a lasting human lunar presence necessitates the large-scale construction of roads, landing pads, and other critical infrastructure over vast areas of the lunar surface, creating a need for a manufacturing system with the capacity to produce substantial volumes of high-strength ISRU-derived material efficiently, reliably, and autonomously. We propose ACRE: Autonomous Casting RovEr to meet this need through the in-situ casting of ISRU-derived feedstock materials. ACRE functions by heating feedstock to a liquid state before depositing batches of the molten material directly into molds formed on the lunar surface by the rover’s plow as it drives along. Casting is uniquely suited to early-stage lunar manufacturing due to its innate versatility, robustness, and ability to produce high-quality components from a wide range of potentially available materials. Furthermore, casting most readily permits the creation of metal matrix composite (MMC) materials in which refined metal feedstocks are combined with raw lunar regolith for high-volume material production. ACRE packs its entire manufacturing process (energy collection, heat generation, feedstock storage, deposition, and mold forming) into the form factor of a self-sufficient and mobile rover, enabling it to autonomously operate for extended periods of time for fabrication across vast areas of the lunar surface without astronaut involvement. ACRE will require infrequent, periodic reloading of its feedstock hoppers and replacement of its high-capacity battery packs (functions which may also be achieved through other autonomous or manned rover systems). 1.2 Technological Overview The energy for batchwise heating of the rover’s feedstock is provided by the Optical Waveguide Solar Power System (OWSPS), which uses an array of parabolic mirrors to concentrate the powerful and nearly constant solar rays of the lunar south pole into Optical Waveguide (OW) cables. The OW cables channel concentrated solar rays through a conduction probe within a heavily insulated crucible. The crucible ensures that the heat provided by the OWSPS can be sufficiently accumulated to reach target casting temperatures. Natural wicking of deposited liquid feedstock combines with optimized mold shapes to securely anchor cast material into the lunar surface. Furthermore, the shape of the rover’s plow can be easily exchanged to optimize for the casting of a particular product. Feedstock materials may include a variety of potential metals mixed with raw lunar regolith for MMC creation. Aluminum (Al) is the preferred refined metal due to its lower melting temperature (for faster heating), proven compatibility for casting in regolith molds, and availability via the refining of lunar anorthite. The fraction of raw regolith in the MMC will be maximized to reduce the need for more resource-intensive refined metals, thus accelerating the rate and scale of lunar infrastructure development. The project did not develop the rover’s traversal system as previous NASA BIG Idea Challenges and NASA’s extensive rover development history previously address this need. 1.3 Verification Testing Summary Verification testing focused on individually validating ACRE’s three critical subsystems (the OWSPS, insulated crucible, and plow) and the properties of lunar-cast materials. The OWSPS team validated the fabrication of robust, high-efficiency, affordable parabolic mirrors that are well-suited to the challenging environments of lunar exploration and spaceflight. In particular, calorimetry tests with direct sunlight empirically assessed the power output of novel parabolic mirrors designs, especially an Al-BoPET-based parabolic reflectors. A combination of ray tracing simulations, transient thermal simulations, and empirical tests validated that the presented design of a novel YSZ-insulated crucible and tungsten (W) probe would enable the bulk liquification of metal and MMC feedstocks at rates suitable for the large-scale construction of lunar infrastructure via a 3000 W heat input from the OWSPS. Finally, a regolith bed was used to validate the formation of molds for in-situ casting, while agreement between experimental shapes and smoothed-particle hydrodynamics (SPH) simulations of the furrowing process validated the prediction of mold shapes prior to flight. Finally, casting into regolith molds validated the strength and wicking characteristics of representative lunar feedstock materials. 6 2 Problem Statement and Background 2.1 Introduction The establishment of a lasting human lunar presence necessitates the large-scale construction of roads, landing pads, and other critical infrastructure over vast areas of the lunar surface. Landing pads are essential to ensure that the propellant exhaust of landing vehicles does not accelerate loose lunar regolith to hazardous velocities. Furthermore, roads will be pivotal to enable safe and efficient traversal of the lunar surface to reach resources (especially water ice) that are distanced from the primary lunar settlements. The construction of such infrastructure requires a manufacturing system with the capacity to produce substantial volumes of high-strength ISRU-derived material efficiently, reliably, and autonomously. Additionally, this construction will occur over large surface areas that may be geographically distant from established settlements, requiring the efficient use of local resources and minimal astronaut labor. 2.2 Challenge and Approach Manufacturing resilient and reliable landing pads on the lunar surface is a uniquely difficult task as they must be large enough to prevent hazardous acceleration of lunar dust, have a high compressive strength, and withstand the high heat of propellant exhaust gasses. Transporting construction materials from earth would prove costly. Furthermore, alternative techniques like “instant” polymer landing pads1will not be implemented in large Artemis launch vehicles like the Starship Human Landing System and in-situ microwave sintering lacks proof-of-concept.2By directly casting into regolith with ISRU-derived materials, ACRE achieves both sufficient material strength and high technological readiness. Because casting completely liquifies working material, it does not have a sensitive dependence on the characteristics of its feedstock (unlike additive and sintering processes which require particular feedstock purities and grain size). Furthermore, casting processes are governed by a minimal number of well-understood and easily-controlled process parameters. This makes it the most promising method for the creation of high-quality products directly from unprocessed or inconsistent lunar feedstocks, particularly unprocessed lunar regolith. Casting of regolith simulant has been investigated and demonstrated for decades,1–7 with previous studies achieving ultimate compressive strengths of up to 120 MPa,5meeting the requirements for ”Ultra-High Performance Concrete”.8 Liquifying material for casting requires a reliable and high-magnitude source of thermal energy. NASA’s plans for a lunar fission reactor9may provide reliable energy for operations at fixed-location facilities, but it may not be sufficient for high-throughput manufacturing operations. Furthermore, large, static energy generators like these cannot be used to provide power for the mobile heating operations necessary for landing pad and road generation. Solar cells can be made more mobile, but suffer from large inefficiencies due to losses associated with solar-to-electrical conversion as well as energy storage. We propose the use of an Optical Waveguide Solar Power System (OWSPS)(see 3.3.1, 4.3, and 5.1.3) due to its ability to generate heat at high efficiencies from the abundant sunlight on the lunar surface.10 ACRE presents a solution for the efficient and safe creation of landing pads and other civil infrastructure on the lunar surface, capitalizing on the unique advantages of the lunar environment for on-site, in-situ construction. In addition to abundant solar energy, the hard lunar vacuum prevents convective heat transfer and increases heating efficiencies. Furthermore, the highly cohesive and thermally robust properties of lunar regolith enable it to serve as a casting mold. The desired casting shape will specifically be formed by a plow attached to the front of the rover as it drives. Its mobility, versatility, and autonomy make it uniquely wellsuited for the construction of offsite equipment and large-scale infrastructure such as roads and landing pads on the lunar surface. The system will be deployed on relatively flat patches of the lunar highlands near Shackleton crater, where it will receive solar power nearly 90%of the time.11 As it traverses the lunar surface, ACRE pushes a plow, creating an indentation in the lunar regolith. After a predetermined roving distance, ACRE will load its feedstock into the crucible from its hoppers, close the lid on its crucible, and heat the material to a molten 7 state by aligning the OWSPS with the sun. Once heated, the molten mixture will be deposited into the formed mold of the lunar surface. Driving ACRE in a path of straight segments allows it to form rectangular surfaces like roads, paramount for the development of supply lines for a new lunar base. Furthermore, driving ACRE in a spiral fashion will enable the construction of circular surfaces to be used as landing pads. Cast structures may be fused together or cast as individual bricks/tiles. At ACRE’s maximum material processing rate, it will deposit all of the contents of its hoppers (800 kg) of feedstock in approximately 9 days. Once depleted, these hoppers will need to be refilled by astronauts or other autonomous rovers. Additionally, most onboard systems (except for the heating system powered by solar energy) will be autonomously operated and powered by ACRE’s modular batteries. These batteries will have the capacity to power ACRE for more than the time required to deposit an entire batch of material. When astronauts come to refill ACRE’s hoppers, they will also exchange ACRE’s batteries with ones freshly charged from the lunar base’s central power source. This modality ensures that ACRE’s continuous operation rarely requires interaction, reducing risk to astronauts while providing exceptional manufacturing throughput. 8 3 Project Description 3.1 Technology Overview and Lifecycle The Autonomous Casting RovEr (ACRE) is a versatile manufacturing system designed for self-sufficient in-situ casting on the lunar surface. It functions by concentrating solar energy to melt Al, iron(Fe), refined metal oxides, and unrefined lunar regolith to cast rigid structures such as roads, landing pads, and girders. In addition to on-site casting, ACRE can act as a stationary general purpose casting module, allowing astronauts to quickly cast needed structures using only raw materials and sunlight. The system is designed to function autonomously, allowing it to operate on the lunar surface without the need for constant supervision or maintenance. ACRE’s systems can be broken down into four subcategories: (1) solar energy capture, (2) heating and deposition, (3) soil manipulation, and (4) vehicle systems. (1) Solar Energy Capture: ACRE concentrates solar energy using an array of parabolic mirrors, smaller secondary mirrors, conical microreflectors, optical fibers, and fused silica interface optics. These components act together to focus sunlight directly into a crucible. ACRE will also be equipped with electrodes on the underside of mirrors and near optical components to prevent lunar dust from covering them. Optical fiber cables will be protected from dust with space-grade polymer sheathing materials. (2) Heating and Deposition: This concentrated solar energy is used to heat the contents of an Alumina-YSZ-Alumina Crucible with an exterior Nickel (Ni) coating. ACRE is fitted with two 400-kg-capacity hoppers which will pass new material into the crucible during each operational cycle. The cap of the crucible will be enclosed by a carbon fiber ring and vertical slider system which permits autonomous opening of the crucible. A motorized screw conveyor is then used to release material from the hoppers into the crucible at precise proportions, enabling in-situ alloying or composite synthesis depending on material requirements and application. Once heating is completed, a set of robotic arms will clasp the exposed portion of the inner alumina crucible, lift it out of its surrounding insulation, and pour its contents into a pre-plowed regolith furrow (see 6.3.3). (3) Lunar Soil Manipulation (3.3.3): A specially-designed plow is mounted at the front of the rover to create furrows in the first few centimeters of lunar regolith. These furrows create the mold which molten material from the crucible flows into and hardens within. (4) Rover Systems: The rover will be equipped with modular battery packs, communication antennas, onboard computers, electric motors, and rugged-terrain wheels. Advancing these systems is outside of the scope of the project (see 3.2). Figure 1: ACRE Design Overview and Critical Components. 9 4 Verification Testing on Earth 4.1 Overview Verification testing prioritized validating ACRE’s three critical subsystems and lunar-cast material properties The OWSPS team validated the fabrication of lunar-suitable, high-efficiency, affordable parabolic mirrors. Calorimetry tests with direct sunlight assessed the power output of novel parabolic mirror designs, especially Al-BoPET parabolic reflectors. Ray tracing simulations, transient thermal simulations, and empirical tests validated the design of a YSZ-insulated crucible and W probe, which enables the bulk liquification of metal and MMC feedstocks at rates suitable for construction of lunar infrastructure. A regolith bed was used to validate the formation of molds for in-situ casting, while agreement between experimental shapes and smoothed-particle hydrodynamics (SPH) simulations of the furrowing process validated the prediction of mold shapes for lunar use. Finally, casting into regolith molds validated the strength and wicking characteristics of representative lunar feedstock materials. 4.2 Safety Statement Before performing verification experiments, the ACRE team drafted a 13-page safety plan, which was approved prior to the submission of the Mid-Project Report (MPR) by Northwestern University’s Office of Research Safety (NUORS) and was submitted to the BIG Idea Program team as an appendix to the MPR. The ACRE Safety plan detailed anticipated experiments, their physical risks, and specific risk mitigation steps, such as personal protective equipment (PPE) recommendations. The team stayed in contact with NUORS and other safety professionals (including facility managers and shop professionals) regarding safety risks. The team is glad to report no safety incidents occurred over the course of the project. 4.3 Optical Waveguide Solar Power System 4.3.1 Al-BoPET Mirror Fabrication Al-BoPET mirror manufacturing began by adhering an Al-BoPET sheet to a circular frame, by bending each overhanging edge around the outside of the frame using adhesives, and ensuring that the conductive side faced outward. An air tube was then attached through a hole to a rear PVC board. An airtight seal around the tube was created by applying adhesive. The PVC sheet was laid on top of the mirror base. Epoxy was spread on the PVC in a circle that matched the size of the circular frame. The Al-BoPET sheet was then positioned over the PVC board and epoxy applied on top of the Al-BoPET sheet. The circular frame was placed over the epoxy and left to dry. After the mirror’s framework was set, the Al-BoPET sheet was inflated to the desired curvature using an attached bike pump(Figure 7a). Fiberglass coatings were applied to the rear by layering epoxy and fiberglass (Figure 7b). After drying, the back of the mirror was polished. Greater rigidity was achieved through additional rear epoxy layers. 4.3.2 Fabrication of Mirror Test Rig The primary function of the mirror test rig was to mount and position the parabolic and flat mirrors in a manner that would reflect their usage on the rover. Furthermore, the rig allowed for data to be collected regarding the parabolic mirrors power outage and efficiency. First, 3 aluminium parabolic mirrors each of varying diameters (17 in, 8 in, and 4 in) were manufactured by Rexnord Aerospace via in-kind sponsorship. The mirrors were produced from cylindrical Al 6061 stock in one milling operation, one CNC lathe operation, and polished in-house. It is worth noting that Rexnord is not a mirror manufacturer and does not have the capability to produce optics-grade mirrors. The rest of the test rig, including the mounts for the mirrors and the flat secondary mirrors, were manufactured using Al feedstock, conventional Bridgeport Mills, and a manual lathe. A flat secondary mirror was also fabricated and polished. Additionally, a linear actuator was integrated into the test rig to test during different periods of daylight at different mirror angles. 4.3.3 Calorimetric Setup The calorimetry experiment (for assessing mirror heating efficiency) involved the test rig, a mirror, a ”black body” of known size and weight, a pyrometer, and direct sunlight. The ”black body” was a cylindrical 16 piece of aluminum (Al 6061; thickness = 0.775 in; diameter = 1.711 in;) coated in graphite spray paint (MG Chemicals Total Ground Carbon Conductive Paint). The cylinder with the sprayed-on coating had an emissivity approaching 1. The cylinder screwed into the test rig setup in place of the secondary mirror, enabling a direct test of the efficiency of the primary mirrors. The primary mirror was oriented and angled directly into the sun, justified by the cylinder shadowing the true center of the mirror (known by a drilled hole in each primary mirror). A pyrometer was used to record the initial temperature of the cylinder, and the environmental factors were recorded from local weather reports including the hourly solar flux in the area.22 The experiments were run for periods of 10-45 minutes, periodically adjusting the angle and direction of the mirror to maintain solar alignment. 4.3.4 Calorimetric Characterization of Mirrors The efficiency of the mirrors was evaluated by comparing the experimental temperature data with a transient thermal analysis created in Ansys Mechanical. Heat transfer losses incorporated free convection, forced convection, radiation to space, radiation to the mirror, radiation to the ground, and conduction through the screws. The ambient temperature (ground temperature assumed to be the same as air) and wind velocity were recorded for free convection and forced convection. The primary mirror was assumed to negligibly increase in temperature from initial. The forced convection was modeled as cross-flow over a cylinder at 3 MPH constant wind, and the heat transfer coefficient was calculated in the following manner: The Reynolds number was calculated for 5 temperatures from surface temperatures 212 K to 612 K and corresponding film temperatures 250 K to 450 K as well as resulting kinematic viscosity for air values, ν. ReD=V D ν Then, with the Prandtl Number, Pr, for each temperature, the Nusselt Number was calculated from the following standard empirical relation:23 NuD= 0.3 + 0.62Re 1 2 DPr 1 3 [1 + (0.4/Pr)2 3]1 4"1 + ReD 282,0005 8# 4 5 Finally, the temperature-dependent heat transfer coefficient, h, was calculated: h=NuDk D Where kis the thermal conductivity of Al and Dis the diameter of the cylinder.23 Ansys transient thermal simulations were then run with varying solar fluxes until the temperature increase matched that of the experiment. That solar flux was multiplied by the area of the parabolic surface, resulting in power output. The solar flux was also divided by the reported Chicago solar flux.22 See results in Section 5.1.1. 4.3.5 Other OWSPS Experiments Electroplating Parabolic Mirrors. As a third, potentially cost/weight-effective method of lunar mirror fabrication, electroplating was tried. Parabolic surfaces were 3D-printed (stereolithography; Anycubic Photon Mono X) and then spray painted with a conductive graphite coating (MG Chemicals Total Ground Carbon Conductive Paint). The 3D-prints were then sanded down with sandpaper of increasing grit (HSYMQ, 120 - 3000) and water to increase the electrical conductivity of the coated surface. A desk-top power supply, proving 1.5W (5.0 V, 0.3 A) of power was attached with alligator clips (and/or hot glue) to Cu sheet (anode) and the coated parabolic surface (cathode). The anode and cathode were submerged in a electroplating solution (Krohn; 0.1M H2SO4, 0.1M CuSO4) and left for several hours overnight to plate with copper as a prerequisite to silver plating (see Figure 11a). It was observed that while some copper successfully plated onto the graphite-coated surface, the surface’s charge quickly died off with increasing distance from the cathode lead, greatly hindering plating rates and rendering this method nonviable (Figure 11b). Application of a more electrically conductive silver adhesive (Henkel Loctite, 3888 Silver Filled Electrically Conductive 17 Adhesive) was also tried, with superior plating results, at the cost of inadequately high surface roughness (see Figures 12a, 12b, and 12c). Fiber Optics. Despite initial conversations with fiber optics manufacturers, delays in mirror fabrication and testing led to a timeline where ordering custom fiber optics became unfeasible given manufacturing lead times. Cheap fiber optics of diameters 50 µmand 1 mm were purchased and calorimetry was attempted. Tests were unsuccessful and hypothesized to be failures for a few reasons. Firstly, the fiber optics likely did not support much of range of wavelengths solar radiation produces. Second, the inlet surface of the fiber optics were not properly fabricated, as most were simply cut with scissors and likely saw significant losses across the boundary. Lastly, while the original mirror design was such to allow for a 2mm diameter single silica fiber optic per mirror, the off-the-shelf versions were bundles loosely packed together. Power was then lost through the gaps and around the edges. Ultimately, this method proved unsuccessful and required a pivot to effectively measure the power output of the mirrors. Electrostatic Dust Mitigation. To ensure a consistent power output from the mirror assembly, some form of dust mitigation must be employed. Given the project constraints, the development of an electrostatic cleaning system was considered the most promising option.24 Attempts at reproducing state of the art research papers on the topic were made, including the use of lunar simulant and vacuum chambers.25 However, to generate dielectrophoretic forces capable of displacing lunar dust particles, the voltage difference across electrodes would have to be an order of magnitude larger than what the team had initially planned.26 In the interest of prioritizing other essential components, these experiments were discontinued. 4.4 Crucible 4.4.1 Prototype Fabrication To validate the thermal performance of the crucible design, the team fabricated a full-scale crucible prototype using the selected YSZ fibrous ceramic insulation (Figure 13). The prototype insulation has the outer dimensions of a 9 in-cube with an insulation wall thickness of 1.5 in. Using a fine-toothed handsaw, boards of YSZ insulation were cut to size before being cured together with high-temperature YSZ cement. A 1L alumina crucible sourced from advaluetech.com was placed in the center of the box to contain feedstock. This cavity was covered by a custom cast alumina lid with a central 1” diameter hole and a 0.25” hole chiseled for a thermocouple feed through. All custom-cast alumina parts created were made from flexible silicone molds, which were cured around 3D-printed PLA structures. Due to oxidation at high temperatures of pure W in Earth’s atmosphere, a corrosion-resistant W alloy was fabricated in place of pure W. The alloy consists of 90% W, 6% Ni, and 4% Cu by weight. A disk-shaped base, hollow cylindrical shaft, and a conical tip were each ordered from Midwest Tungsten Service and thermal press fit together to form a probe. This probe was fed through the crucible lid when assembling the prototype as seen below. While the crucible design for lunar use would be axisymmetric, the team fabricated the prototype crucible into a cubic shape using standard-sized insulation boards from the manufacturer’s available inventory. This choice reduced the cost of the insulation to one-third of the equivalent cylindrical geometry and cut the manufacturer’s lead time in half. The thermal performance of the cubic prototype is still well-representative of the axisymmetric design, and thermal modeling accounts for this change in geometry (see section 5.2.2). 4.4.2 Prototype Testing The prototype was placed in a box furnace which controlled its outside temperature. The temperature was ramped from ambient (22°C ) to a maximum temperature of 900°C at a rate of 4°C per minute. The temperature was held at 900°C for three hours and allowed to cool to 517°C. During this thermal cycle, a thermocouple measured the temperature of the aluminum feedstock within the crucible prototype. To validate the thermal performance of the insulation, a transient thermal FEM model was developed with a time-varying temperature boundary condition applied to the outside surfaces of the crucible model according to the thermal program executed by the box furnace during the experiment. The experimental data from the thermocouple reading and the FEM results for the average temperature of the feedstock are well-agreed, with 18 a (magnitude) average percent difference of less then 2.5% (see Figure 15). The temperatures never differ by more than 11% across all times. This validates that the insulation performs as designed. Furthermore, close agreement between the experimental data and the thermal model confirms that we can develop accurate simulations for assessing crucible performance in the intended lunar environment (see 5.2.2). 4.4.3 Ray Tracing Simulations Ray tracing was performed to determine light absorbance inside of the probe and the ray power efficiency. The analysis was completed in Ansys SPEOS for the best visualization quality. The solar rays leaving the fused silica optics were modeled as a 3000W surface source black body at 5800K. The rays were set as a Gaussian distribution with a total angle of 24.74 degrees, and a Full Width Half Maximum angle of 24.74 degrees (values calculated from Snell’s Law and experimental data). Other material properties were selected from built-in modules. A study on total absorptivity and efficiency with varying reflectivity was conducted, and conclusions were drawn for optimal material choice (see 5.2.1). The results both validated the capability to trap and reabsorb reflected rays and served as boundary conditions for thermal simulations. 4.4.4 Transient Thermal Simulations The testing of the crucible prototype validated that the developed FEM thermal models will accurately represent the design’s thermal performance. To assess the maximum throughput of molten material that ACRE can deposit, the team applied the same FEM model with boundary conditions appropriate for lunar use. Some important notes of the model include: • Spatially-varying heat flux boundary conditions were applied to the W probe’s inside surfaces according to the results of the ray-tracing simulations in Ansys SPEOS. • The team’s FEM simulations use temperature-dependent thermal conductivity and specific heat values to attain more accurate results across the the entire range of considered temperatures. Temperaturedependent enthalpies are used for all feedstocks, ensuring that the models account for the feedstocks’ latent heat of fusion, illustrated by the temperature ”plateaus” in Figure 32. • The FEM simulations do not incorporate convection of the feedstock after melting for two reasons. First, the FEM simulations are intended to validate time required to liquefy the feedstock for casting, hence the feedstock is not in a liquid state until the final timesteps of the simulation. Secondly, the thermal diffusivity of the molten metal feedstocks dominates momentum diffusivity (i.e. the Prandtl number Pr << 1), so advection of the molten metal plays a negligible role in heat transfer through the feedstock compared to conduction, evidenced by the uniformity of the feedstock temperature at all times in the transient FEM simulation. See 4.5.5 for the results of the thermal FEM models applied in the lunar environment. 4.4.5 Additional Work Over the duration of the project, a variety of additional fabrication and experimental design was conducted that has not yet culminated in useful data. • The team purchased a Fresnel lens that concentrated columnated solar light to a circle of roughly 1” in diameter, which was positioned above a crucible. Calorimetry was attempted to characterize the energy output from the lens into a crucible (Figure 16), but it has not yet been successful. A new test crucible was built and completing calorimetry before the BIG Idea Conference is a priority. • An experimental plan for testing the prototype crucible using the Fresnel lens apparatus has also been completed. Given the power input calculated from calorimetry testing, the W probe’s heating capabilities will be tested. The steady state temperatures of the feedstock and outside walls of the crucible will be recorded while a constant power input from the fresnel lens is applied. 19 • A machinable and inexpensive crucible design was investigated during Phase II. It consisted of unpacked zirconia powder encased in alumina (Figure 17). It was posited that the desired insulation could be achieved without expensive, custom designed insulation. A miniature scale crucible was fabricated using this insulating technique, which utilized exceptionally thin layers of alumina for the inner and outer walls. The concept relies on the high insulating capabilities in vacuums of powdered insulation, but lack of information on the thermal capabilities of the powder in vacuum required the team to focus on the YSZ prototype. However, prior to the BIG Idea Conference, the prototype will be thermally validated in a VIF. • Oxidation of the W probe was considered a potential failure point entering Phase II. However, given W heating would occur in the oxygen-free lunar atmosphere, with only trace amounts of oxides present in the feedstock, concerns around W oxidation decreased significantly. 4.5 Furrowing (Plowing) and Casting 4.5.1 Goals Several experiments and simulations were performed to verify the feasibility of in-situ furrowing and casting into lunar (highlands) soil. The verification experiments sought to inform specific design choices and ensure reproducible casting of roads and landing pads on the lunar surface. 4.5.2 Soil Settlement Under Load in ABAQUS ABAQUS finite element analysis was conducted to determine the ideal brick geometry for cast material and to study settlement characteristics of the roadway structure. Lunar soil was modeled using a MohrCoulomb model. Physical and mechanical properties were taken from the Lunar Highlands Simulant Data Sheet27 and from Slyuta.28 The Mohr-Coulomb model requires the use of a stress hardening function, which relates the soil cohesion and the plastic strain. A normal distribution approximated the variable cohesion of the soil as loads were applied, with cohesion decreasing as plastic strain increased.29 Plastic strain was allowed to increase from 0to 0.01 as plastic strains of that magnitude are characteristic of soils. Bending analysis of the roadway was conducted to determine the minimum brick height. Having determined the height, the width and length were varied to produce a brick of reasonable dimensions and a volume of one liter or less. Four brick shapes were tested to determine settlement under lunar gravity and lateral displacement performance. A 3m-by-3m section of the roadway was also tested to determine expected settlement of the full roadway structure (see 5.3.1). 4.5.3 Empirical Characterization of Furrowing Design and Fabrication of an XYZ Gantry System and Regolith Simulant Test Bed. Repeatable manipulation of lunar regolith simulant was achieved through the fabrication of a robust testing bed (0.5 m (width) by 0.5 m (length) by 0.3 m (depth)), designed for adaptability and safety. Its frame was constructed from Al T-slot extrusions sourced from 80/20. To enhance the structural integrity of the framing, steel brackets were cut and used for reinforcement. These served as a joining mechanism that provided greater strength than the Al alternatives, safely holding upwards of 80 kg of regolith without failure. Steel panels were inserted between the framing sections which functioned as a thermal protection between the regolith and molten material. This precautionary feature counteracted any potential failure in the insulation layer. To withstand the extreme temperatures encountered during casting Al, the bed’s interior was lined with ceramic fiber insulation. The bed was ultimately placed in a temperature and humidity controlled negative pressure chamber ensuring safety and experimental consistency. The purpose of the XYZ gantry system is to enable precise, repeatable motion for the testing of fabricated plow designs. The components were sourced from OpenBuilds, ensuring compatibility between the electronics and hardware. Al V-slot extrusions served as tracks, enabling the gantry carts to move smoothly along each axis. Connecting all three carts to each axis gave the final cart a full range of XYZ motion for the quick-connect system for the plow. Stepper motors controlled the motion along the XYZ axis. Idle 20 pulleys were positioned opposite the stepper motors to keep the timing belt in its correct orientation. Slight dimensional incompatibilities between the OpenBuilds and 80/20 parts were mitigated with acrylic panels fixed atop the X-axis carts, allowing the motors to sit without interference. The plow’s quick-connect system fastens onto the Z-axis cart and allows operation at three height levels. This system is designed for swift attachment and detachment, requiring two screws to secure the plow. Experimental Plan. To experimentally simulate lunar south pole conditions for soil furrowing, the test bed, was filled about 80% full with lunar highlands regolith simulant (Exolith Lab, LHS-1, 70kg) and the gantry system was placed on top. The bed was reset using a flat rake before every plow. Plows were made from Polylactic Acid (PLA) using Fused Filament Fabrication (FFF) and had varying shapes and teeth (to create grooves in the soil). Once a plow was attached to the gantry, stepper motors controlled the depth, angle, and speed at which the simulant was plowed. LIDAR scans (Apple iPhone 13 Pro Max) performed with freely available software (Polycam) were taken to create 3D models of the plowed regolith surface. These models were analyzed to extract the cross-sectional 2D geometry (x and y axes corresponding to width and depth respectively) of the soil. For analysis of the cross-sectional area, see section 5.3.4. Lastly, the lunar simulant bed was used to cast materials to determine wicking properties and allow shape analysis. The bed was insulated with ceramic fiber and made from heat-resistant low-carbon steel, making it suitable for future larger-scale experiments. For experimental casting results, see materials conclusion. 4.5.4 Dynamic Lunar Soil Simulations in Ansys LS-DYNA The erosion behavior and soil properties of regolith are heavily dependent on the gravity where tests are performed. To verify the behavior of soil furrowing in low gravity, LS-DYNA simulations using Smooth Particle Hydrodynamics (SPH) were performed. Briefly, a rigid plow was simulated moving through a bed (0.2 m by 0.2 m by 1 m) of spherical soil particles (walls fixed) at a constant rate (0.05 m s). The crosssection of the plow was informed by brick settlement simulations and experimental furrowing (see 5.3.1 and 5.3.2). The behavior of the soil was dictated by the model with the parameters listed in Table 1.30 These material model parameters were used to describe the behavior of lunar regolith during cube-sat impact on the moon. Furrowing was simulated in earth gravity (see 5.3.3) and cross-sections were compared to LIDAR scans of experimental results to ensure the simulated model’s accuracy (see 5.3.4). For precise comparison, the simulated results are scaled uniformly to the test plow size. As the simulations are scale invariant, the scaling has no significant effect. Subsequently, simulations were performed under moon gravity to assess its impact on the geometry of furrowed soil and the reaction forces produced by the soil (see 5.3.5). Table 1: Mat 147 FHWA Soil Parameters (Kg-N-m-s)* Parameter Value Parameter Value ρ1500.0 AHYP 100.0 SPGRAV 3.1 ET 10.0 ρWat 1000.0 COH 1000.0 VN 1.1 ECCEN 0.07 INTERMX 1.0 DAMLEV 1.0 K 5.691E9 εMax 0.03 G 8.3E7 DINT 5.0E-4 ϕMax 1.1 VDFM 1.0E-7 ∗All parameters with value 0 are excluded 4.5.5 Furrowed Soil Casting in Air Interaction between furrowed regolith simulant and molten material was verified by casting in air. In order to replicate the surface energy and wicking behavior of Al in a vacuum, a proprietary Al-Bi alloy was cast (see 4.6.2). An optimized plow geometry (see 5.3.1) was used to furrow smoothed soil. Material was heated to 900 C and held for 20 minutes before directly pouring molten material into the furrowed soil. The shape and microstructure of the material were characterized after cooling. 21 4.5.6 Furrowed Soil Casting in Vacuum A vacuum induction furnace (model# VIF2000, Across International) was ordered in August and set up in October for verification of casting molten material into regolith under vacuum conditions. This furnace is capable of melting material via induction and pouring the material into a bed of regolith, all under vacuum (10−5Torr). Although this system was ordered early in Phase II, electricians were not able to install power requirements until the end of Phase II. This system will be used to create full bricks under lunar conditions. 4.6 Materials and Metal Matrix Composites 4.6.1 Regolith-Al Composites The feasibility of creating composites with Al and regolith for improved strength and reduced cost was evaluated. First, a mixture of molten Al and regolith was stirred in a crucible; however, these two materials would not wet each other well in Earth’s atmosphere. To overcome this poor wetting, Al and regolith were tumbled to ensure full mixing and pressed into a cylinder under 1 ton of force (see 23a). These pellets were sintered at temperatures under 1000 °C and could only melt at much higher temperatures in an argon atmosphere. This behavior is likely due to the oxide layer on the Al powder, which is known to impair melting.31 Mechanical tests were performed on the pressed Al-regolith composites to validate viability for road structures. Sintered samples both in air and an argon atmosphere were compressed using ASTM E909 standards (see Figure 23a).32 Geometric non-uniformity limited characterization of melted structures to Vicker’s hardness measurements (see Figure 23b). Another experiment attempted to create a regolith MMC by sandwiching one 20-gram layer of regolith simulant (Exolith Lab, LHS-1) in between two 50-gram layers of a low surface energy Al80-Bi20 wt% (see 4.6.2) alloy which was heated to 1000 °C under argon for 3 hours. This alloy wicked through all of the regolith and through the slightly porous alumina crucible. The team cross-sectioned and imaged this sample under SEM/EDS to confirm that the metal wicked through both the regolith and the porous crucible. The feasibility of a redox reaction between Al and regolith or ilmenite for the creation of unique alloys of Al and Fe, Ti, or Si was explored with differential scanning caloimetry (DSC) to 1200 C. No reaction was observed in air. Despite the results, the redox reaction may still be viable without an Al oxide skin in lunar UHV. 4.6.2 Al-Bi for Improved Wetting Background Earth’s oxygen-rich atmosphere oxidizes Al where it develops a surface layer of aluminum oxide. On the moon, the vacuum of the exosphere prevents this layer from forming and causes it to react and sublimate at high temperature.33 The layer of oxide present in Al at earth’s oxygen pressures increases the surface energy of the metal, making it less likely to wet surfaces, presenting a key challenge to verification of aluminum MMC fabrication.34 To address this issue, Al-Bi alloys were explored to lower surface energy to anticipated levels. Contact Angles. To ensure comparable surface energies between Al-Bi alloys and Al in UHV, contact angle were measured for a range of compositions. Small cube shapes of Al-Bi were cut, placed on a nonporous alumina crucible, and heated at 1100 °C for 8 hours under an argon atmosphere. After cooling, the samples were then removed from the crucible and placed on a stage to be imaged with a goniometer (ram` e-hart, 90-U3-PRO)(Figure 25). In cases where the bottom of the sample was not flat enough, contact angles were manually measured using ImageJ software. Contact angles were matched closest to reported values for Al in high vacuum when a 50/50 wt% Al-Bi alloy was used (Figure 26).35 Wicking After the 50/50 wt% Al-Bi was cast into lunar regolith, it was imaged to determine the degree of wicking. Significant wicking into the lunar regolith would increase adhesion into the soil and this has the potential to increase resistance to lateral displacements by road and landing pad bricks. Additionally, wicking would be valuable for creation of MMC via sandwiching. 22 5 Results and Conclusions 5.1 OWSPS 5.1.1 Parabolic Mirror Efficiencies Data from the outdoor solar concentration tests is listed in Table 2 below. The 8 inch parabolic mirror had an efficiency of 63.4% while the 17 inch mirror reached 77.5% and temperatures were matched by simulation in Ansys. Further details on the nature of temperature matching through simulation are outlined in 4.3.4. Table 2: Mirror Calorimetry Test Results Mirror Starting Temp. (◦C) Final Temp. (◦C) Time Elapsed (s) 8 inch Al 16.4 68.0 1800 17 inch Al 14.0 233.0 600 27 inch Al-BoPET 20.0 29.8 60 Initial calorimetry tests yielded an efficiency of 10% for the Al-BoPET mirror. While this value is low compared to the machined parabolic mirrors, there are two reasons to continue testing and development of the Al-BoPET mirror. First, the surface geometry of the mirror is heavily influenced by the surface roughness of the rear fiberglass mesh, resulting in peaks and craters where rays would not behave in accordance with a parabola. If the AlBoPET was instead shaped by a process such as vacuum forming and adhesion of the film to a rigid backing, the surface roughness could improve drastically. Second, the test rig was not optimized for calorimetry with a large diameter mirror. Tests were done by aligning the mirror to the sun by hand and were accordingly inaccurate. A proper test setup would significantly increase confidence in the results. 5.1.2 Overall OWSPS Efficiencies Given the fabricated parabolic mirrors and their resulting experimental efficiencies, one can return to expected efficiencies of the rest of the OWSPS system based on experimental values from Nakamura et al. to calculate an expected efficiency and mirror array size (17 inch mirrors) for the entire OWSPS system.10 The efficiencies of subsystems are as follows: parabolic mirror, 77.5%; secondary mirror, 94%; inlet of optics, 94%; and fiber optic cables, 86%. The resulting overall efficiency (58.9%), combined with the crucible system requirement of 3000W input power, results in the need for a minimum array of 27 17-inch parabolic mirrors for ACRE to function. 5.1.3 Conclusions The power output of the cheaply fabricated mirrors is above 60% for the 8-inch and and 75% for the 17inch. The mirrors were produced in a single day using one mill and one CNC lathe operation. The surface was not of optical quality, yet the 17-inch mirror saw efficiencies within 20% of optical quality Al. While the Al-BoPET mirror was in early prototyping stages, the 10% efficiency recorded is likely well below its potential. While the goal of experimenting with incorporating fiber optics was ultimately not attained, the methods for producing cheaper and lighter solar concentrators showed promise. Continued effort and time spent prototyping higher-quality Al-BoPET mirrors may ultimately yield a true ceiling for efficiency. 5.2 Crucible 5.2.1 Ansys SPEOS Ray-Trace Simulations Graphing the resultant system power efficiency and power absorbance at the tip of the probe as functions of material reflectance yielded an optimization problem (Figure 27). Maximizing both efficiency and power absorbance through an objective function resulted in the optimum 56.2% reflectance. This correlates well with W’s typical reflectance range of 50-65%. At this optimum, power efficiency is 94.46% and maximum absorbance is 1.37 E06 W/m2. Near the optimum material reflectance, there is a consistent absorbance distribution along the length of the probe. A lower reflectance (i.e. 20%) has a high power density along the length of the cylinder furthest away from the probe; at a higher reflectance (i.e. 80%), there is more power distributed on the 23 probe’s bottom, but with significant lost energy due to reflectance out of the probe’s top. See Figure 27 for illustration. The total system energy at each reflectivity can be determined through integration of each line. The peak for the 80% at position 0mm can be attributed to our novel geometry, which works to trap and reabsorb light as it is reflected within the probe. 5.2.2 Transient Thermal Simulations After verification that the previously-discussed FEM thermal model accurately represented the prototype crucible’s thermal performance, the model was used to assess performance in the lunar environment (sec 4.4.4). The model was applied for two different full 1.3 L feedstocks: Al and Fe. Using the 3000 W input from the OWSPS, 1.3 L of Al feedstock can be heated to a target casting temperature of 980 K in approximately 1520 seconds (25.3 minutes) while Fe can be heated to a target casting temperature of 1900 K in approximately 6870 seconds (114.5 minutes). Al’s lower melting temperature enables faster casting cycles and would be well suited for large-scale applications (such as the fabrication of Al-regolith MMC roads) while Fe is valuable for applications requiring elevated temperatures and mechanical loading, especially lunar landing pads. The FEM model was verified via 1D analytical solution (see Appendix C-3). 5.3 Furrowing 5.3.1 Soil Settlement Under Load in ABAQUS Four brick geometries were tested for settlement under gravity loads and lateral displacement under a 1 MPa lateral stress applied to the side of the bricks. The results of these simulations can be seen in Table 3 Table 3: Brick Settlement and Displacement Results Geometry Dimensions (wxhxd) [cm] Tooth Height [cm] Settlement [mm] Lateral Displacement [mm] Toothed 20 x 10 x 6.66 5 12 15 Flat 12 x 4 x 20 N/A 18 39 Tall 4 x 12 x 20 N/A 19 120* Cubed 10 x 10 x 10 N/A 19 48 ∗Separated from test bed during lateral loading simulation The brick that demonstrated the best performance was a rectangular brick with three “teeth” on the bottom as shown in Figure 33. The teeth work to combat lateral displacement of the brick. The overall roadway structure as shown in Figure 34 performed well under settlement analysis. The roadway settled uniformly, with a magnitude between 50 and 70 mm. No concerning soil behaviors, such as upwelling or shear failures, were observed during the simulation. The simulation results indicate that settlement and soil strength are within acceptable limits for the proposed roadway construction. 5.3.2 Empirical Furrowing After furrowing, the soil held its grooved shape well. Rendered LIDAR scans in Figure 36 demonstrate the geometry of the furrowed soil. 5.3.3 LS-DYNA Simulations Under Earth Gravity During simulated furrowing under earth gravity, soil particles were plowed out in front of moving plow forming well-defined grooves (Figure 37). Grooves remained distinct without any significant erosion. Upon initial contact between the plow and soil, some particles were ejected from the sample at high velocity. This effect may be a result of unstable volumetric crushing of the soil during initial contact within the fixed boundary conditions of the soil section. The velocity of the plow also causes some ejection of particles throughout the simulation, suggesting dust mitigation should be considered more deeply. 5.3.4 Empirical Validation of Dynamic Simulations To ensure LS-DYNA simulations accurately model the behavior of lunar regolith, empirical furrowing tests and simulation cross-sections were compared (see Figure 38). The furrowed cross-sections are almost 24 identical. The tests show minimal erosion of the furrowed grooves, and equal groove depths and angles, confirming the accuracy of the LS-DYNA SPH simulations in capturing the behavior of furrowing. 5.3.5 LS-DYNA Simulations Under Lunar Gravity After ensuring accuracy of earth gravity simulations with experimental results, the same simulations were run under lunar gravity. The geometry of plowed grooves was consistent in lunar gravity, showing even sharper peaks between the teeth of the plow. Notably, significantly more SPH particle ejection occurred in low gravity tests, emphasizing the need further dust mitigation strategies moving forward. 5.3.6 Furrowed Soil Casting in Air Al-Bi alloy was successfully cast into furrowed grooves in lunar regolith simulant. The molten material filled the groove it was cast into and maintained the shape of the regolith mold (Figure 39). 5.4 Materials and Processing 5.4.1 Metal-Matrix Composites Al MMCs with regolith inclusions were synthesized. Samples were sintered in ambient air and showed low ultimate strength and brittle fracture. It was hypothesized that oxygen in the atmosphere caused oxidation of the samples. Samples sintered under argon demonstrated higher yield strength than those sintered under Earth’s atmosphere(Figure 40). The 10 wt% regolith sample heated in argon had an ultimate strength approximately 3 times larger than the highest ultimate strength air-heated sample. Optical microscopy displayed limited particle fusion for the air-sintered samples and significant fusion for the argon-sintered samples (Figure 41 and 42). Three distinct phases were identified in the argon-heated samples: the metallic, a glassy ceramic, and mixed metal-ceramic phases. Another pressed cylindrical sample, heated to 1500 °C, showed the best cohesion of the Al and regolith, containing Fe-rich precipitates indicative of a redox reaction (Figure 43). Regolith particles in this sample were majority alumina and smaller than expected, indicating a possible reaction between Al and silica (Figure 44). Vicker’s hardness testing was performed. The metal and mixed phases demonstrated low mean hardness of 42.31 and 49.25 HV0.2, respectively. The pure ceramic phase demonstrated the highest mean hardness of 835.60 HV0.2. The similarity between the metal and mixed phases suggests that the mechanical properties of the mixed phase. MMC formation was verified by placing a layer of regolith between two layers of Al80-Bi20. SEM/EDS maps confirm the presence of Al and Bismuth (Bi) in the porous alumina crucible but not the remaining regolith (Figure 45). This evidence shows that the Al80-Bi20 alloy wicked through the loose regolith and into the porous alumina crucible, with much of the Al-Bi wicking beyond the crucible (Figure 24). 5.4.2 Aluminum Wicking Optical micrographs were taken from both the top and bottom of a brick of 50/50 wt% Al-Bi (Figure 46). Sporadic particles were seen contained in the top and bottom of the Al matrix; however, these particles did not appear the same as the regolith particles from other experiments. Moderate porosity could also be observed, but this is expected to be less of a concern when in vacuum. Although significant wicking was not observed in this case, it is difficult to draw a conclusion without direct testing of Al in high vacuum. 5.4.3 Conclusions The results from the MMC and wicking experiments show promise of a potential new manufacturing method for large scale MMCs. The fact that wicking was observed in loose regolith but not compacted, plowed regolith could enable the creation of simple sandwich structures with defined shape. The plow will be used to make the grooves in the lunar surface as planned and ACRE will deposit a layer of loose regolith. Then, ACRE will cast a layer of Al over the thin layer of regolith which will wick through until it reaches the plowed layer. In theory, this should create an MMC retaining the same brick geometry, and this will be tested in the test bed with Al50/Bi50 and with the VIF in the coming weeks. 25 A-5 ACRE Sponsors/ Partners The team would like to recognize several private corporations for providing in-kind donations, access to software, and advising from industry experts. •Rexnord Aerospace. The team is grateful to Rexnord Aerospace for supplying Al feedstock and machining services in order to fabricate three parabolic mirrors. Special thanks to Jim DeVries - Plant Manager in the Aerospace Division, Brad Fields - Senior CNC Programmer/Tooling Engineer, Marek Hreska - SEALS CNC Programmer, and Jorge Casta˜ neda - team lead of SEALS production at the Regal Rexnord Corporation. •Questek Innovations. The team acknowledges Questek Innovations for providing useful feedback and guidance on our MMC processing strategies and assistance with the use of ThermoCalc software. Special thanks to Amit Behera, PhD - Manager of Design Group, Jeff Grabowski - Business Development Director, and Jason Sebastian - President at Questek Innovations, LLC. •Wearifi. The team thanks Wearifi providing access to several pieces of fabrication equipment as well as supplies, including 3D printers, photo-polymer resin, FDM filament, CNC mills, and fast-curing silicone. Special thanks to Anthony Banks - CEO, Jan-Kai Chang - CTO, and Prof. John Rogers - COO at Wearifi, Inc. •Ansys. The team thanks Ansys for providing access to several key software licenses: –Ansys Academic Research EM (5 tasks) –Ansys Academic Research HF (5 tasks) –Ansys Academic Research HPC (per core) –Ansys Academic Research Mechanical and CFD (25 tasks) –Ansys GRANTA Research Selector (5 tasks) –5x Ansys Academic Research Optics (1 task) These licenses were crucible to generate advanced simulations, adding credibility to the team’s validation efforts. Special thanks to Vishal Ganore - Academic Project Manager at Ansys, Inc. A-6 NUSTARS Sponsors OnShape. The team benefited from OnShape’s online CAD software package, allowing for fast multiuser editing of CAD files, and convenient cloud storage and organization capabilities. 32 B Explanatory Figures Figure 5: A materials selection diagram (Ashby Plot) for crucible insulation material. Ansys Granta Selector was used to filter materials and prepare representative materials selection diagrams. Based on acclaimed metallurgist Dr. Michael Ashby’s Materials Selection in Mechanical Design,40 minimization of total energy consumed during thermal cycling requires maximization of the material index M= (λρC)1 2, where λis the thermal conductivity, ρis density, and Cis specific heat. Figure 6: Cross-sectional diagram of the process of casting material inside a regolith furrow formed by a previous iteration of ACRE’s plow (not to scale). 33 (a) The aluminized mylar (Al-BoPET) is inflated while adhered to a PVC board. (b) The back of the mirror is coated in layers of epoxy resin and fiberglass mesh fabric for structural stability. Figure 7: Al-BoPET Mirror Fabrication. Figure 8: Outdoor Testing Setup with 8 inch Mirror, ”Black Body” Attached Figure 9: Ansys Thermal Transient Setup Displaying Modes of Heat Transfer to Black Body 34 Figure 10: Ansys Thermal Transient - temperature matching result for a 17-inch diameter parabolic mirror. (a) Copper electroplating setup within a chemical fume hood. A power supply, a graphite-coated parabolic object with tentacle-like leads, anode and cathode wires, and blue copper electroplating solution are pictured. (b) A large copper lump is observed to have grown due to electroplating solution coming in contact with cathode leads. Figure 11: First iteration mirror electroplating setup. (a) Copper electroplating setup within a chemical fume hood. (b) Parabolic surface coated in silver adhesive removed from plating solution mid-process. (c) Parabolic surface coated in silver adhesive after a completed copper plating process. Figure 12: Second iteration mirror electroplating setup. 35 Figure 13: Uncovered YSZ-based crucible prototype with alumina inner crucible, W probe, and Al feedstock. Figure 14: Temperature in Kelvin of the FEM thermal model for the crucible prototype experiment. The geometry is twice planarsymmetric, so a one-fourth slice of the prototype crucible geometry is pictured. This image shows the temperature distribution at the start of the 900 C holding period (3.71 hours from the start at the start of the experiment). 36 Figure 15: Comparison of the feedstock temperature from experimental data and FEM thermal simulation. 37 Figure 16: Fresnel lens frame for solar heating Figure 17: Scaled-down prototype alumina-zirconia-alumina with maximum diameter of 3 in and height of 4.3 in. 38 Figure 18: A) A labelled 3D-visualization of the XYZ gantry system and test bed used to perform plowing and casting experiments. Figure 19: A) View of the XYZ Gantry and Regolith Simulant Test bed used for Plow Testing. Figure 20: A) Photograph of the Gantry Test Bed System with Plow attached prior to furrowing. 39 Figure 21: A) Picture of the Gantry Test Bed System with Plow Attached After Plowing. Figure 22: Vacuum Induction Furnace 40 (a) Two Al-(LHS-1) pressed-powder pellets and their corresponding die press components. The numbered units on the ruler are inches. (b) An Al-(LHS-1) pellet cut in half after undergoing melting in an Ar gas environment via induction. Dispersed regolith simulant is seen with the naked eye within a metal matrix. Figure 23 Figure 24: Aftermath of the Al80-Bi20 sandwich experiment. The metal wicked entirely through the regolith and alumina crucible. At the bottom is broken quartz from the furnace welded to the bottom of the crucible from the metal. Figure 25: Images taken with goniometer to measure contact angles of Al-Bi alloys at various concentrations. A) 2wt% Bi B) 50wt% Bi C) pure Bi 41 Figure 41: 20x optical microscope image of air-sintered aluminum regolith pellet microstructure Figure 42: 20x optical microscope image of argon-sintered aluminum regolith pellet microstructure Figure 43: A) Backscattered electron micrograph taken by a Hitachi s3400 SEM; the brighter pixels indicate a higher molecular weight region than the darker surrounding matrix. B) energy dispersive X-ray spectroscopy (EDS) map of a precipitate within an Al-regolith MMC sample showing that it is of higher Fe and silicon content than the matrix; no appreciable difference in oxygen content is observed. 48 Figure 44: A) SE image of aluminum-regolith composite made in induction furnace. B) Results from measurements of small particles in microns. C) SE image of LHS-1 simulant provided by Exolith Labs.27 D) Expected particle distribution of LHS-1 simulant Figure 45: SEM EDS of porous alumina crucible and regolith powder which the Al80-Bi20 wicked through. A) SE image of the porous alumina crucible. B) EDS map of the porous crucible with signals from both Al and Bi proving the alloy coated the crucible. C) SE image of the remaining regolith powder atop the porous crucible. D) EDS map of the regolith showing only metal and oxygen images suggesting Al80-Bi20 completely wicked through the regolith 49 Figure 46: Optical micrograph at 20x zoom of the A) top of the casted Al-Bi brick B) bottom of the casted Al-Bi Brick. No significant wicking or difference in microstructure is apparent between the two images. C Calculations C-1 Parabolic Mirror Spot Size While perfect parabolic reflectors can concentrate collimated light to an infinitesimally small point, sunlight is not perfectly collimated because the sun is not a point source. Using Figure 47, it can be geometrically derived that Dg= 2fθs, where Dgis the minimum focal spot size diameter, fis the focal length of the parabolic concentrator, and θsis the characteristic solar half angle (4.66 mrad).10 The equation for the focal length of a parabolic reflector, f, is f=Dp2 16hwhere Dpis the diameter of the parabolic dish, and his the height of the parabola (measured as the distance of the vector normal to the tangent plane at the dish’s center, which begins at this same plane and ends at a second plane which is tangent to the circular edge/rim of the dish). The height relates to the desired half angle of focused light (θhalf ) via the relation tan(θhalf ) = D/2 f−h. Solving the system of equations, it was found that θhalf =20◦, it was found that the diameter of the parabolic mirror, Dpmust be less than 17 inches to satisfy Dg≤2mm. Figure 47: Illustration of a parabolic concentrator from Nakamura et al.10 C-2 Ray Tracing Total Angle Snell’s Law was utilized to calculate the total angle emitted from the source in Ray Trace simulations. In the actual optical system, incident light emitted from optical cables refracts through Silica, then Quartz before being exposed to the heating element. Gathering that maximum incident light from the optical cable has an angle of 19.43 degrees, Snell’s law can be used to calculate the maximum angle of light emitted from the Quartz. Refractive and incident indexes: nAir = 1 nSilica = 1.45 50 nQuartz = 1.553 n1∗sin(theta1) = n2∗sin(theta2) nair ∗sin(19.43) = nSilica ∗sin(thetaSilica) thetasilica = 13.26 degrees nSilica ∗sin(13.26) = nQuartz ∗sin(thetaQuartz) thetaQuartz = 12.37 degrees Total angle is the multiple of the maximum refractive angle. thetatotal = 24.74 degrees C-3 Verification of FEM Model with 1D Analytical Solution Previously described FEM thermal modeling (Section 5.2.2) provides an analysis of the time required to melt aluminum and iron feedstocks for in-situ casting. To verify the accuracy of the FEM model, we will assess the temperature profile within the YSZ insulation (where the temperature gradients are the highest) using an analytical solution for the conduction equation. This model is very similar to one incorporated within ACRE’s original proposal; however, this revised model serves the new purpose of verifying the FEM model described in the final report, rather than to assess ACRE’s feasibility. Furthermore, this new model will utilize cylindrical (rather than planar) geometry. At its midplane, the crucible can be well-represented by a cylindrical geometry that is approximately uniform with regards to height and azimuthal angle. Furthermore, the FEM model is solved as a transient analysis, hence we begin with the one-dimensional transient heat conduction equation for radial dependence: 1 r ∂ ∂r(rk(T)∂T ∂r ) = ρc∂T ∂t (1) We note that the wide variation in the YSZ insulation temperature results in significant changes in the insulation’s thermal conductivity through the domain of interest. Hence, we use a temperature-dependent thermal conductivity, k(T). To compare the temperature profile in the insulation between the two models, we must select a particular time. For ease, we select the steady state time t→infinity, in which case the equation no longer varies in t. Hence the differential equation becomes ordinary: 1 r d dr(rk(T)dT dr ) = 0 (2) Our 1D analytical model will be compared to the FEM model for the melting of iron to steady state. Additionally, this simulation will utilize one-fourth of the typical solar heat flux provided by the OWSPS to obtain a steady state solution that does not contain excessively high feedstock temperatures. The temperature distribution of this simulation is shown below. The manufacturer Zircar Zirconia, Inc. publishes data on the thermal conductivity of the YSZ insulation at several temperatures.19 To determine a continuous function for the conductivity of the YSZ material, we fit these empirical data to produce the following quadratic polynomial for k(T), where a= 7.111 ∗10−8 W/(m·K3), b=−5.759∗10−5W/(m·K2), and c=0.08731 W/(m·K). The fit has a Sum of Squares Error (SSE) of 1.228 ∗10−8. k(T) = aT2+bT +c(3) 51 Figure 48: Steady state temperature distribution (in Kelvin) for the FEM model, considering the melting of iron feedstock with only 750 W of solar input (rather than the typical 3000 W provided by the OWSPS). With k(T) known and dk/dT easily derived, we can expand the heat conduction equation from before with all terms known in terms of T and its derivatives: d2T dr2+1 r dT dr +1 k(T) dk dT (dT dr )2= 0 (4) Making a substitution with the variable J=dT/dr, we can rewrite the above equation as a system of first-order nonlinear ODEs and solve using standard ODE numerical techniques. dT dr =J(5) dJ dr =−J r−J2 k(T) dk dT (6) T(r= 114.3mm) = 608.47 KJ(r= 114.3mm) = q”loss k(Touter)=−4082.6 (W/m2) k(Touter)(7) Our domain is set from r=114.3 mm (the outer radius of the YSZ) to r=63.5 mm (the inner surface of the YSZ). We set the boundary conditions for our 1D model according to the temperature and heat flux given by the FEM model at the outer radius of the YSZ: If the FEM model follows the physics we expect, then setting the boundary condition of the 1D analytical model as specified should produce a temperature distribution that closely matches that of the FEM model. Indeed, upon solving the 1D analytical model with a nonlinear numerical solver, we observe that the two plots are well-agreed (see below), thus verifying that the FEM model follows expectations for thermal distribution as influenced by the insulation’s temperature-dependent thermal conductivity. In particular, we note that the 1D analytical solution predicts the temperature of the interior surface of the insulation within 5%, and the divergence between the two models is likely attributed to the fact that the crucible geometry is not truly one-dimensional, as assumed in the 1D analytical model. 52 Figure 49: Comparison of 2D FEM Model and 1D Analytical Solution D References References [1] Mitigating lunar dust: Masten completes FAST landing pad study. 2021; https://masten.aero/blog/mitigating-lunar-dust-masten-completes-fast-landing-pad-study/. [2] Farries, K. W.; Visintin, P.; Smith, S. T.; van Eyk, P. Constr. Build. Mater. 2021,296, 123627. [3] Mackenzie, J.; Claridge, R. 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