Experiments on Molten Regolith Extrusion (MREx) Under Vacuum Conditions

76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 1 of 9 IAC-25,C2,IP,55,x101258 Experiments on Molten Regolith Extrusion (MREx) Under Vacuum Conditions Simon Stapperfend Chair of Space Technology, Technische Universität Berlin, Marchstr. 12-14, 10587 Berlin, Germany, [email protected] Abstract We evaluate the feasibility of additive manufacturing via Molten Regolith Extrusion (MREx) under vacuum conditions. A mare-type simulant (LX-M100) was melted in a vacuum chamber and extruded through an orifice for casting and deposition trials. Multiple experiments were conducted, of which three representative cases are discussed here. Compared with ambient-air trials—where rapid convective cooling produced brittle, glassy parts with cracking— vacuum operation slowed cooling, reduced thermal gradients, and improved visible integrity. We observed distinct outlet regimes (dripping, steady filament, pulsed flow) and demonstrated deposition on both stationary and moving substrates. When using cordierite as a substrate material strong outgassing behavior that resulted in the formation of large bubbles was as observed. The results, obtained within the scope of the 3D-LAVA project, indicate that MREx offers a mechanically simple and robust path toward scalable lunar construction. We provide a system overview, representative findings, and lessons learned to guide further process refinement. Keywords: ISRU; molten regolith; extrusion; vacuum; lunar construction; LX-M100 Acronyms/Abbreviations CTE – Coefficient of Thermal Expansion ISRU – In-Situ Resource Utilization MREx – Molten Regolith Extrusion PSD - particle-size distribution SoMR - Sintered or melted regolith 1. Introduction Establishing a sustained human-robotic presence on the Moon requires the local fabrication of critical infrastructure—landing pads for dust mitigation, berms and roads, radiation-shielding elements, storage, and habitat components. Shipping conventional building materials from Earth is massand cost-prohibitive [1,2]. Consequently, in-situ resource utilization (ISRU) is central to current mission architectures, especially at the lunar south pole, where permanently shadowed regions likely host water ice and near-continuous illumination at ridge crests supports power generation [3]. Two broad families of regolith-based construction approaches have emerged: • Binder-based methods (e.g., hydraulic/sulfur cements, geopolymers, polymer-bound mixes) can achieve rapid strength gain at relatively low temperatures but typically require imported reagents or water; some also rely on pressurized or humiditycontrolled environments to prevent sublimation or premature setting in vacuum, complicating early deployment [4–9] • Sintered or melted regolith (SoMR) methods consolidate regolith without additives using radiant furnaces, microwave heating, laser/solar concentration, casting, or additive routes. Comparative reviews spanning dozens of trials report promising strengths and scalability, but also emphasize that the majority of experiments were performed in air, with far fewer under vacuum and almost none in reduced gravity—leaving a translational gap to true lunar conditions [10–12]. Within melt-based additive manufacturing, Molten Regolith Extrusion (MREx) is compelling: regolith is fully melted in a crucible and extruded by gravity through an orifice, forming free-falling filaments or droplets, which can resolidify on a substrate and form a solid structire. Early work under ambient conditions demonstrated feasibility but suffered from cracking and brittleness due to rapid convective cooling [13]; trials conducted under vacuum showed slower cooling and reduced thermal gradients, suggesting that vacuum is not merely a constraint but can be an advantage for MREx. 2. Related Work / Background 2.1 Landscape and taxonomy Research on lunar construction from regolith is commonly organized into two broad families: binderbased materials and binderless thermal consolidation, the latter often summarized as Sintered or Melted Regolith (SoMR). Binder routes combine regolith with a chemical binder to form concrete-like materials at comparatively low temperatures. By contrast, SoMR consolidates 76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 2 of 9 regolith without additives—typically via radiant furnace heating, microwaves, lasers or solar concentration, casting into glass/glass-ceramics, or melt-based additive manufacturing (AM). Recent syntheses report dozens of SoMR trials across these sub-routes and discuss their suitability for early infrastructure such as landing pads, berms, and shielding [10–12]. 2.2 Binder-based materials Binder-based routes are attractive because they can deliver useful early strengths at modest processing temperatures and use familiar forming and casting workflows. Early “lunar concrete” studies examined hydraulic binders and aggregate gradations adapted to the angular, fine-grained morphology of lunar soil analogues, and they explored hydration and strength development under reduced-pressure or vacuum conditions [4–6]. Those investigations consistently highlighted practical challenges for cementitious systems in vacuum, including water management, outgassing, and the need to control curing environments to avoid premature moisture loss. Sulfur-bound regolith has been investigated as a water-free alternative that sets rapidly at comparatively low temperatures. Nonetheless, experiments report risks related to sulfur sublimation or volatilization in vacuum, time-dependent deformation (creep) and thermal softening at elevated temperatures, and crack formation under large lunar-like diurnal temperature swings. [14,15] Geopolymers—understood here as alkali-activated aluminosilicate binders—offer competitive compressive strengths and can be cast or extruded. Several studies demonstrate low-water or near-zerowater mix designs and examine the use of urea as a plasticizer to reduce water demand and improve buildability. At the same time, exposure to vacuum tends to accelerate water loss and disrupt early microstructural development, which implies that geopolymer systems benefit from controlled humidity/pressure during curing to achieve their target properties. [16,17] Polymer-bound regolith composites and polymerbased surface stabilizers have also been demonstrated at laboratory and limited field scales. Their applicability to large-scale lunar construction is constrained by the logistics of importing binder mass, potential outgassing and volatile release in vacuum, and limited thermal margins across the lunar diurnal cycle. [18,7] 2.3 Binderless thermal consolidation (SoMR) Radiant-furnace sintering. Heating compacted regolith close to its liquidus can achieve substantial densification by viscous flow of glassy phases and solution–precipitation in crystalline phases. Strength generally tracks final density, so the thermal profile, particle-size distribution (PSD), and dwell times are critical to avoid entrapped porosity and shrinkage cracking. Much of the historical data was generated in air; comparative studies and recent work increasingly highlight that vacuum alters oxidation, pore evolution, and densification behavior, which in turn shifts attainable properties and cycle design. [19–22] Microwave sintering. Fe/Ti-bearing mare analogs couple efficiently to microwaves, enabling rapid volumetric heating that has produced dense, strong parts at laboratory scale. Current research addresses uniformity in larger geometries, atmosphere control (including high-vacuum operation), and the trade-off between energy efficiency and microstructural control during thermal transients. [23–26] Laser/solar sintering or melting. Rastered lasers and concentrated sunlight have been used to sinter and even fully melt regolith surfaces with an eye toward landing-pad and roadway throughput. Key challenges include managing thermal stresses (e.g., scan strategy and cool-down pauses), avoiding crack initiation at track overlaps, and maintaining consistent coupling as surface reflectance/absorptance evolves during processing. [27– 31] Molten Regolith Extrusion (MREx). In Molten Regolith Extrusion (MREx), fully molten regolith is discharged by gravity through an orifice, forming freefall filaments or droplets that resolidify on a substrate— without additives, binders, or pressurized feed. Ambientair trials showed feasibility but frequent fracture due to rapid convective cooling; vacuum operation slows cooling and reduces thermal gradients, which has been observed to lower crack incidence and improve visible integrity [13,32]. MREx can be utilized for a variety of technologies, like casting or fiber drawing. Full melting allows casting of glass/glass-ceramics and the drawing of basalt-like fibers, but annealing schedules are essential to relieve residual stresses and control devitrification— particularly in vacuum, where cooling kinetics differ from ambient. Recent studies demonstrate fiber drawing and composition–property links for mare-basalt analogs, underscoring the importance of chemistry for process windows. [33–36] Across SoMR methods, comprehensive reviews conclude that the majority of published trials were conducted in air, with comparatively few in vacuum and almost none in reduced gravity—leaving a fidelity gap to lunar conditions that is particularly consequential for melt-based processes, where cooling, wetting, and stress development are strongly environment-dependent. 76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 3 of 9 2.4 Regolith simulants Because access to returned lunar material is extremely limited, terrestrial regolith simulants have become the workhorse for systematic ISRU studies [37]. Over the past decades, NASA, ESA, and several institutions and companies have developed families of simulants engineered to reproduce the properties most relevant to processing—bulk oxide chemistry, mineral assemblages, and particle-size distributions (and, where feasible, glass/agglutinate content) [38–41]. By geological origin, lunar materials are typically classified into two groups: anorthositic highlands and basaltic mare. For all trials, we selected LX-M100—the mare simulant within the LX modular simulant system. Comprehensive mineralogical and chemical characterizations of the LX-simulants are reported by Patzwald et al. (2025) [42], providing a consistent baseline for comparative process evaluation. This work uses LX-M100, a mare-type simulant, whose oxide chemistry is shown in Table 1. Table 1: Bulk rock chemistry of representative lunar basalts compared to LX-M100. Values are giving in wt%. Data taken from [37,42]. Olivine basalt (Apollo 12) Quartz basalt (Apollo 15) LX-M100 SiO2 45.00 48.80 49.96 TiO2 2.90 1.46 2.33 Al2O3 8.59 9.30 13.98 FeO(T) 21.00 18.60 10.23 MgO 11.60 9.46 8.18 CaO 9.42 10.80 7.88 Na2O 0.23 0.26 3.41 K2O 0.06 0.03 1.85 MnO 0.28 0.27 0.20 Cr2O3 0.55 0.66 0.04 P2O5 0.07 0.03 0.56 LOI n/a n/a 0.74 Sum 99.70 99.67 100.14 3. Experimental Setup The experiments were carried out in the 3D-LAVA test setup at the Chair of Space Technology at Technische Universität Berlin. Vacuum chamber. Trials ran inside a cubic vacuum chamber (outer dimension ~790 mm), hosting the printhead, a traversing table, and several sensors. The moving substrate can easily be changed, for example to a regolith bed inside a 380 mm × 380 mm stainless-steel pan. The vacuum chamber and some subsystems are shown in Figure 1. Figure 1: Rendering of the experimental setup. Printhead & crucible. The printhead (see Figure 2) is a custom vertical tube furnace (HTM Reetz) with two independently regulated heating zones: an upper melt zone for the crucible and a lower orifice zone that stabilizes outflow and mitigates freeze-off. Heating is perfromed via molybdenum wire elements around an alumina tube, insulated by a porous ceramic stack; the flange mount to the vacuum chamber is water-cooled to protect seals. Temperatures are measured by Type-C thermocouples and regulated by Eurotherm controllers. The crucible is made of Pt90-Rh10; its orifice was machined by spark erosion (~6 mm) and then widened/tapered (~10 mm) to discourage external wetting/creep and keep the jet centered in free fall. Figure 2: Cross-section of the high-temperature printhead. Sensing & control. Process monitoring is done using two Basler a2A1920-160ucPRO cameras (different lenses/positions), a pyrometer (Sensortherm MY51, 5.14 µm) aimed just below the orifice (outside the chamber, viewing through an uncoated CaF₂ viewport), and a thermal camera inside the chamber (Optris PI 640 i G7, 7.9 µm) to track cooling of deposited melt. The CaF₂ transmission (~0.93 at 5.14 µm) and a literature-based glass emissivity were used to set the effective emissivity 76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 4 of 9 for the pyrometer; the thermal camera was operated with a high-emissivity setting per the draft’s glass reference. Motion control for the traversing table was G-code executed on a NI cRIO; the PKR361 gauge provided vacuum chamber pressure to the same controller/software stack. Deposition modes. We evaluate stationary (rigid hotcapable plate) and moving substrate modes at a fixed inplane speed per condition. The free-fall distance from orifice to substrate is set a priori and held constant within a run, as it governs filament thinning and coiling behavior in vacuum. 4. Results and Discussion Some initial tests with the new experimental setup have been performed already. Some selected experiments with different parameters of the traversing table, substrate material and furnace temperature will be discussed in the following: 4.1 First Experiment In one selected trial, 55.15 g of coarse (1 – 2 mm size) pre-dried LX-M100 regolith simulant was loaded into the crucible. The lunar regolith simulant LX-M100 with the normal PSD was used as substrate material, which was compacted by hand to achieve a flat surface. After reaching a vacuum level of 4⋅10-4 mbar, the furnace setpoint temperature of both heating zones was set to 1,350 °C with a heating ramp of 5 K/min. The maximum temperature was held until the extrusion of the molten regolith was finished. The traversing table was set to move in a linear interpolated circular movement of a radius of 40 mm with a velocity of 1,000 mm/min. Figure 3: Time sequence of the extrusion process, as seen by a RGB camera (top) and the thermal camera (bottom). Thermal image is taken from the other side of the furnace and rotated for clarity. The extrusion of the molten regolith (Figure 3) was mostly homogeneous for about 28 seconds, after which single droplets were released from the printhead until roughly 121 seconds after the first extrusion. A total of 6.37 g of regolith remained in the crucible, meaning that 48.74 g of feedstock was extruded. The molten regolith resolidified into a main black, glass-like piece that cracked in one spot, along with several beads carrying fibers up to 20 cm in length. Some of the droplets landed on the still-hot main body and fused with it. The crack in the main body appeared once the surface temperature reached 300 °C (thermal camera reading), approximately 10 minutes after the beginning of the extrusion. The sample is displayed in Figure 4. Figure 4: Resolidified sample on regolith substrate (left), Bottom of main body of the sample (right). The sample appears glassy with no visible pores. On the side that was in contact with the lunar regolith simulant, grains of regolith have adhered and become embedded in the glassy main body. A particularly striking feature of this sample is its curvature. Since the regolith was dried beforehand and no gas bubbles were visible in the extruded molten regolith stream, a trapped gas bubble is unlikely to be the cause. A more plausible explanation can be found in the interplay between the coefficient of thermal expansion (CTE) and surface tension. As the outer surface of the melt cools rapidly in contact with the regolith, it solidifies first and contracts. The core, however, remains hotter for longer, with lower viscosity and thus more ability to flow. The inward contraction of the solidifying shell, combined with surface tension forces, displaces the low-viscosity interior and pushes it upward, producing the observed curvature. Although the CTE of lunar glass is quite low (about 8⋅10-6 1/K) [43], it should still be taken into account when using large amounts of glass. Due to the process parameters, no practical 3D-print could be achieved. The viscosity of the melt was too low, resulting in excessive flowability, and the movement pattern of the traversing table was too narrow, producing a thick blob rather than well-defined structures. 76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 5 of 9 4.2 Second Experiment In this trial, 105 g of coarse pre-dried (1 -2 mm size) LX-M100 regolith simulant was loaded into the crucible. To improve the visibility of the extrusion process, some insulation material below the nozzle was removed, allowing for better observation angles. This modification increased radiative heat losses, since the bottom of the crucible was left exposed and free to emit heat. The heating procedure started with a furnace setpoint of 1,250 °C, which was held for 10 minutes. The setpoint was then increased in steps of 10 K until extrusion began, ultimately reaching 1,430 °C. At the end of the process, 4.6 g of regolith remained in the crucible, meaning that 100.4 g of feedstock was extruded. The traversing table was programmed to follow a square path with a side length of 50 mm and a velocity of 2,000 mm/min. The substrate material used was an as-purchased cordierite plate. Cordierite was chosen because it withstands thermal shocks well, tolerates high temperatures, and provides a solid surface, avoiding the handling of loose powders inside the vacuum chamber. During the extrusion, the molten regolith impacted the cordierite substrate as it moved along the programmed square path. After deposition, the melt reacted with the substrate and released gases, leading to the formation of large bubbles and localized foaming (see Figure 5). Ideally, cordierite should not contain volatiles; however, since the substrate was not heat-treated prior to the trial, gases were released. This outgassing behavior of cordierite has been reported previously, for example in the dissertation of Dr. Rahmoun (2006) [44]. Figure 5: RGB Camera stream of the melt extrusion process onto the cordierite plate. Small cavities formed after the melt touched the substrate material (left), which resulted in the formation of large bubbles of up to 100 mm size (right). Interestingly, such foaming and large bubble formation had not been observed in earlier experiments—neither on the substrate nor inside the crucible. This indicates that the substrate material itself was the main source of the gas release. 4.3 Third Experiment In this trial, 105 g of LX-M100 regolith simulant (normal PSD) was pre-heated together with the crucible in a furnace in air to avoid direct powder handling. In this preparation step, the material reached a partially sintered state, i.e. in between sintering and melting, but did not flow out of the crucible. For the extrusion experiment, additional insulation was added below the crucible to reduce radiative heat losses and thus enable extrusion at lower furnace setpoint temperatures. A setpoint of 1,250 °C was sufficient to initiate extrusion at a pressure of 8⋅106 mbar. The substrate material was an as-purchased cordierite plate. The traversing table was programmed to follow a meandering pattern with a total size of 180 mm × 180 mm, consisting of five meanders at a velocity of 2,000 mm/min (Figure 6). Figure 6: Deposited droplets from the meandering extrusion pattern (180 mm × 180 mm, 5 meanders). Based on the results of the previous trial, extrusion was not expected to occur at such a low temperature. Therefore, no cameras were set up to record the process. The deposited samples on the substrate exhibited a rough bottom side, with the texture of the cordierite plate clearly imprinted. Several of the extruded droplets showed evidence of small and large pores originating from the bottom, suggesting gas reactions similar to those observed in the previous trial. Cross-sections revealed additional small pores within the droplets themselves, rather than only at the interface with the substrate. This indicates that gases were trapped inside the extruded droplets. Observing the process through the viewport on top of the printhead allowed a direct view into the 76th International Astronautical Congress (IAC 2025), Sydney, Australia, 29 Sep-3 Oct 2025. Copyright ©2025 by the International Astronautical Federation (IAF). All rights reserved. IAC-25,C2,IP,55,x101258 Page 6 of 9 crucible. There, during the formation of a droplet at the nozzle, the commonly observed small bubbles were not eliminated by thinning of the molten stream during extrusion, but rather reappear after the droplet has been extruded (Figure 7). Figure 7: Bubble dynamic during the extrusion of a drop. The bubbles get sucked out of the crucible by the formation of the droplet at the nozzle (a, b), reappear (c) and then pop. Earlier in the extrusion process, the formation of large bubbles was also clearly visible (Figure 8). This behavior is likely linked to the feedstock preparation method: during the pre-heating step, the regolith was not fully molten, and some closed pores remained in the partially sintered body. At high viscosities during heating, these pores could not escape, instead accumulating in the melt and finally being released once the viscosity decreased. This phenomenon caused heavy bubbling, with melt spilling over the crucible edge and penetrating the insulation, ultimately damaging the furnace. Figure 8: Formation process of large bubbles in the crucible: Small bubbles arising (a) and fuse into larger bubbles (b, c) until they fill the whole area (d). The addition of the insulation below the crucible effectively reduced the extrusion temperature by approximately 180 °C. 5. Conclusions In this study, multiple extrusion trials with molten lunar regolith simulant LX-M100 were carried out under high vacuum conditions, of which three representative experiments were presented in detail. The results demonstrate that continuous extrusion of regolith is feasible and that the material solidifies into glassy bodies with fibers, droplets, and deposition patterns following the programmed traversing paths. Modifications to the furnace, such as changes in insulation, were shown to directly influence the effective extrusion temperature, reducing the required setpoint by up to 180 °C. At the same time, several challenges were identified. The low viscosity of the melt resulted in poor shape fidelity and prevented the formation of defined 3Dprinted structures. The use of cordierite as a substrate was not intended as a candidate material for lunar applications, but rather as a convenient alternative to regolith powder handling due to its thermal stability and robustness. Its use revealed significant outgassing and bubble formation, which compromised the integrity of the deposited material. Furthermore, short high-temperature preheating the regolith feedstock to avoid powder handling introduced closed pores into the crucible charge, which were later released as large bubbles during melting. This not only affected extrusion stability but also caused severe furnace damage due to overflowing molten material and infiltration into the insulation. Overall, the experiments provide valuable insights into the key factors governing molten regolith extrusion under vacuum. While extrusion was successfully demonstrated, further work is required to improve process stability and print fidelity. In particular, optimized preparation of the regolith feedstock, better substrate strategies, and furnace designs that minimize heat loss and safely accommodate outgassing are critical steps toward reliable ISRU–based additive manufacturing on the Moon. 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