Acta Astronautica 00 (2023) 1–14 Acta As- tronautica Lunar Plume Surface Interaction Mitigation Challenge ` Alex Drago, Ioana El Kraye, Gabriele Zuin, Ricard Gonz ́ alez-Cinca UPC Escuela de Ingenier ́ ıa de Telecomunicaci ́ on y Aeroespacial de Castelldefels (EETAC), Carrer d’Esteve Terradas, 7, 08860 Castelldefels, Barcelona Abstract The complexity of lunar landings goes beyond limits of our imagination. The first Apollo mission were considered a massive success, but e ff ects recorded during the landings are not compatible with the long term plans that the European Space Agency has for the Moon. One of the main e ff ects is the plume surface interaction, an ejection of dust with very high speeds during the hovering phase. Due to the low gravity and the properties of lunar regolith, this ejection can be extreme on the Moon. Cratering mechanisms can lead to particles reaching the escape velocity and go on orbit. This is dangerous for any bases that will be constructed on the surface and for future orbiting spacecraft, such as the Gateway. For that reason, the study presented aims to improve our knowledge on this issue and an serious solution is proposed. Through methods of research, development and innovation, a trade-o ff analysis shows that a landing pad with a succession of radial layers of boulders was the best candidate to mitigate the plume e ff ects and the optimal landing spot should be placed in Mare Humorum. CAD models are presented and numerical simulations were performed to study the pressure and thermal interaction between the gas propelled by the nozzle with the obstacles. A CFD analysis of the solution indicates a reduction of the dust ejected velocity below the Moon’s escape speed, to under 800 m / s , and a reduction of the pressure under the nozzle by one order of magnitude. © 2022 Published by Elsevier Ltd. Keywords: Plume Surface Interaction, Regolith, Moon, Landing Pad, Computational Fluid Dynamics 1. Introduction Moon exploration is a global endeavor 50 years after the first human landing in 1969. After decades of technology development, humanity has set new objectives for this new era in aerospace, such as the landings in polar regions and the deployment of bases for research purposes. To achieve it, an increase in the number of non-intrusive and e ff ective lunar missions is required. Research teams have been studying the Apollo landings to appraise the di ffi culties that arose during the hovering processes. One source of concern was the dust ejected by the plume, as the crew was blinded during the final descent to the surface. For that reason, the aim of this project is to find a way to mitigate the e ff ects of Plume Surface Interaction when a space vehicle is landing on the lunar surface. In addition, studies of the terrain are going to vital to understand the constrains of this project. Also, simulations should be carried out to have an initial estimation of the e ffi ciency of our solution. By analyzing the damage sustained by previous lunar landers, it is possible to understand the e ff ects of plume impingement on the structure of the lander. The returned pieces of Lunar Surveyor III spacecraft showed that the 1 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 2 surface experienced sustainable cracking and holes formation caused by the lunar dust blown by Apollo 12 landing near the spacecraft. Even though the Surveyor spacecraft was not directly exposed to the dust, the hardware was severely damaged, and it can be used to understand how to protect future landers from dust blast. Curiosity rover wind sensor was damaged by the landing of MSL in 2012. During the descent stage, lunar dust was detected at an altitude of 63m due to surface plume interactions. As the vehicle continued to descend, it experienced more corrosion and when the hovering level was reached, large soil particles along with dust started recirculating and hitting the bottom of the rover causing permanent damage. As portrayed in these two examples, the final descent phase is very critical as the exhaust blast caused by the engine can form dust recirculation and dust clouds are abrasive. It is very important to identify the characteristics of this body to perform our solution. First, the Moon has an almost non-existent atmosphere, resulting in the pressure being close to vacuum and having a low solar radiation shield [1]. To understand the issue, it is also necessary to introduce the surface material. Regolith is the material of the upper layer of the lunar ground, with a depth of 4-15 m. They are the result of the disintegration of basaltic rocks due to the meteoric impacts and the continuous exposition to solar and interstellar charged particles. The density is 1 5 g / cm 3 and the bulk is formed by fine grains of less than 80 microns in size. The soil in the Sun-exposed side of the Moon emits electrons, so it is positively charged, according to [2]. The surface experiences a great thermal shock during a cycle, with a temperature with a temperature ranging from 120ºC to -180ºC. Studies made on geopolymers created on Earth state that the mechanical characteristics of regolith do not change a lot during the cycle, making them an optimal material in lots of applications [3]. The mechanical and structural properties of lunar regolith simulant based geopolymer under extreme temperature environment through experimental and simulation methods. The transfer of material from Earth would be very expensive and almost impossible, so many solutions were explored to use this material as the main source to create shapes and constructions. The possible techniques are many. Some of them simply melt it with microwaves rays and then use it in a 3D printing process. It could also use Sun or laser waves, but they are much more energetically expensive, and the microwave penetrates much deeper into the material [4]. There are also experimented solutions in which a machine sprays a saturated solution of salts in water directly on the zone of interest [5]. This technology would allow us to create walls or solid pavements in a short time with low expenses of space and energy. Regolith is typically brittle because its composition is oxide ceramics. For this reason, Aluminum was introduced as a possible binder to reduce this problem, with interesting results. This material could be used in a 3D printing process using a laser to melt and re-solidify it. Lunar regolith - AlSi 1 0Mg composite fabricated by selective laser melting [6]. Two types of regolith are considered: the highlands and the mare. The highlands are mountains made of lighter materials (anorthosite) and are formed by very slowly cooled lava, with thickness is of 10 to 15 m. The mare are easily identifiable flat dark areas that are formed by large lakes of ancient basaltic lava, with a thickness of 4 to 5 m. The regolith has been formed by constant meteoric bombardment and agglutination. Meteorite impacts may have fragmented the rocks, basalts, glass and other minerals. The constant bombardment could have caused the particles to agglomerate [1]. In addition to the regolith the Moon is covered by a cloud of dust. This lunar dust is generated by small particles from comets that hit the lunar surface. For the solution, determining a specific landing site was critical to optimise the research and the development of the logistics. To choose the best placement, an analysis was performed taking into account the following criteria. First, the site should be compatible with the idea of creating a lunar base. In order to do that, it is necessary to land somewhere with a regolith rich in components that are useful for ISRU construction. Those compounds should have a high percentage of Titanium and Iron ions, as well as other components such as basalt minerals. These components are specially rich on the lunar mare, the darker areas in the Moon. Secondly, it is preferred a smooth open terrain that allows good sunlight exposure for solar energy needs. Third, the landing site should be near the south pole, as the majority of the current missions are going there and having a certain proximity to those proves could have positive scientific retributions. Finally, the fourth criteria is the presence on the visible side on the Moon, to guarantee direct contact with Earth ground stations. Having into account the mentioned criteria, Mare Humorum was selected as the ideal candidate for our landing spot. Despite no landings have been performed on it and certain uncertainty on the exact composition of the regolith, spectrometry suggests a similar composition to the regolith found in the Apollo 17 landing. The most accurate simulant is JSC-2A for that region, which is studied above. This landing spot has an almost-circular shape and has an average diameter of 420 km and it is believed to have a 2 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 3 3 km thick layer of regolith rich in basalt. It is located at 24ºS39º W and it also o ff ers a geometrical advantage in the terrain. In the west and south of the region there are high craters that can help isolate the landing spot. To the north there is Copernicus and to the east is found Tycho, which can help mitigate dust ejection in those directions too. 2. Critical factors research 2.1. Soil simulant According to [7], JSC-1A is a basaltic ash, mined from a volcanic ash deposit located in San Francisco, with a composition typical of many terrestrial basalts. Lunar basalts don’t contain water and have low abundances of volatile oxides. Because it is no longer commercially available, JSC-2A is used today, although there is not much data on this simulant. This regolith represents mare basaltic regolith containing 50% silicon dioxide (SiO 2 ) and a low titanium (Ti) content (less than 4.5%). The JSC-2A simulant is a good terrestrial simulant as it has a similar chemical composition to the samples collected during the Apollo missions. 2.2. Cratering mechanism and speed analysis The rocket exhaust plume coming out strikes the lunar surface. The shear stress of the plume is countered by the shear strength of the soil. When the plume shear stress is greater than the shear stress of the soil, the regolith will flow away. This leads to three mechanism of erosion. The first is the viscous erosion, which refers to the tangent shear stress of the plume gives speed and energy to the soil particles. The second is the bearing capacity failure as the pressure over the soil is too big and the soil compresses and spreads. And the third is cratering, which it occurs when the gasses enter into the pore spaces of the soil and come out at some other location or time bringing soil grains along with them. According to [8], measures in the Apollo landing videos showed that most of the ejected particles, with small diameter, had an ejection angle of less than 3º. According to [9], there is a dependence between the amount of spread regolith with the surface hardness (Vickers) and the hovering time of the landing. The hovering time has a big importance in the dust spreading phenomena. The area highly disturbed during the landing process is assumed to be in first approximation the base of a cone coming out from the nozzle. The hovering height plays a big role in the process, as the base of the cone increases. A greater hovering height means a bigger area of dust involved, but with a lower depth of disturbance. According to [8] there is a correlation between the hovering height, the grain size and the regolith velocity. On the surface, the average grain size of regolith is around 10 m and the general hovering height does not play a great role on the mean speed, which is around 3000 m / s. According to [10], the horizontal speed and altitude reached of dust particles has an almost linear tendency with the distance from the center of the landing area. Particles near the center have a higher vertical speed and reach higher altitudes. Those particles can be critical for the lander safety but not for the big scale spreading phenomena. On the other hand particles at a distance of 2 / 3 meters from the center of the landing site have a high horizontal speed. This zone is the most critical for the big scale spreading. The mean horizontal speed of the dust spread is around 3000 m / s , according to [10]. From this analysis it is concluded that the central zone of the landing site, with a radius under 2 meters, is the most critical factor for the lander safety. A landing pad is necessary to avoid the spreading of the dust in this region. The external area, for a radius higher than 2 m, has also an e ff ect which will be considered in our solution. Compacting the soil or increasing the spreading angle of the exhausted gasses can be a mitigation mechanism, according to [11] 2.3. Lunar atmosphere According to [12], the atmosphere of the Moon is similar to the upper portion of the Earth’s neutral atmosphere, the exosphere. The main components are either atomic species or simple molecules. The density of the gasses on the surface is very low, about 100 molecules per cubic centimeter. Detectors left by Apollo astronauts have detected argon- 40, helium-4, oxygen, methane, nitrogen, carbon monoxide, carbon dioxide and ,recently, traces of water vapor in the Chandrayaan-1 mission were found [13]. At the ground level, the pressure of this atmosphere is around 6 6 × 10 − 5 Pa and constant through the whole surface. 3 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 4 2.4. Blast zone It has been studied that the landing e ff ects follow a 2D polar symmetry on the Moon’s surface. In the centric part (around 12m in radius) there is a blast zone where most of the ejection is produced and it shrinks increasing its power when reducing the hovering altitude. The outer ring involves a dust ejection caused by unstable regolith structures (that remain together due to electrostatic e ff ects) that increase the dust ejection in an unpredictable pattern. In Figure 1, it is shown the dimensions on the extreme case. Figure 1. Shape of the blast zone and outer ring in the extreme case. 3. Solutions proposed 3.1. Sprayed landing pad As landing pads limits future lunar missions to a specific site landing spot, the cost of building multiple lunar pads to cover several regions could be excessive. This approach proposes to build the landing pad during the hovering phase by using spray technique. Thus, the landing pad is created directly under the lander. The proposal is based on the idea of changing the intrinsic properties of the material that is on the surface to make it denser and reduce the cratering. When regolith becomes denser, the solid particles won’t eject from the surface. Instead, they will fall back on ground. Moreover, as the landing pad is built directly, this technique can be applied to any lander size. Finally, this solution avoids transporting heavy loads to build the landing pad on the surface. The ejected materials could be water, wax or aluminum binders. [14] However, several factors should be taken into account, such as the melting point of the material and its possible evaporation or the increased chances of clogging the engine. 3.2. Deployable pad This solution aims to shoot a deployable landing pad on the lunar surface. It can be a optimized way to perform landings. The main idea of this solution would be to launch the pad from the module at a certain hovering altitude or in the previous cycle around the landing spot. Once the folded platform reaches the surface, the platform is deployed to receive the lander and absorb the kinetic energy from the gas ejected by the nozzle, avoiding cratering and plume surface interaction in the blast zone. To improve the e ff ectiveness of the solution, the pad should be have around 8 meters in diameter. Therefore, the landing pad should be folded inside the lander and deployed on the surface. Many geometries were considered, like a cubical shape or a circular one. The solution is constrained by the need for light materials that can sustain the thermal shock from the nozzle exhaustion. Also, precision on the shooting and the necessity to have a relatively smooth terrain limit its e ff ectiveness. 4 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 5 3.3. Cup of Co ff ee This solution proposes the use of walls with curved edges that surround the landing pad to deflect the regolith and avoid its escape. The wall that stops regolith from mitigating can be used for another purpose, that is collecting the regolith. The collected lunar dust can be later used as material for 3D printing. Several factors limit this solution such as the construction of an inclined wall on an uneven surface, the stability of the wall when subjected to high speed regolith tails and the large amount of material needed to construct the wall around the entire landing pad. 3.4. Flower power The aim of the project is to mitigate the dust speed created by the interaction with exhausted gasses coming from the nozzle. A possible solution could be to minimize this interaction with a pad that not only forms a flat homogeneous shape without dust, but also reduces the speed of the gasses, so the interaction with the external area sandy area would be lower. In this direction, walls and obstacles would be placed, reducing the speed of the exhausted gasses. 3.5. Trade of analysis To compare the di ff erent solutions a trade-o ff analysis has been developed. 11 key parameters have been featured in this study and the values are awarded from 1 to 5. Those are shown in Table 1 Parameters Meaning Performance E ffi ciency on dust mitigation. Applicability Application in di ff erent landing site. Reliability Level of security, redundancy and reaction to failure of the system. Operational conditions Physical properties required by the solution (i. e. temperature and pressure). Power consumption Power required to deploy or construct the solution. Geometry and structure System’s mass and volume: e ff ect on launch and transport. Cost Cost of manufacturing, development, operation and maintenance. TRL Technology Readiness Level. Operational life span The amount of time the solution will be used before it has to be changed or fixed. Operational timescale The amount of time it will be required to start using the solution. Table 1. Trade-o ff analysis parameters and their meaning. A weight parameter was also used and it is related with the importance of each parameter to decide which is the optimal solution. The results are presented in Table 2. Parameters W Sol. 1 Sol. 2 Sol. 3 Sol. 4 Performance 10 3 4 3 4 Applicability 7 4 5 2 5 Reliability 10 3 3 5 5 Conditions 5 4 4 3 3 Power 2 4 5 3 3 Structure 1 3 4 2 5 Cost 3 4 4 1 3 TRL 8 2 3 2 4 Life 8 3 1 2 5 Time-scale 3 1 1 2 5 Table 2. Trade-o ff analysis results. The results of the trade o ff have awarded the first solution with a 196 points, the second solution with 191, the third with a 165 and the forth with 247. Therefore, it is concluded that the solution that involves a central landing pad surrounded by obstacles is the best solution to be developed. 5 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 6 4. Solution development Figure 2. Preliminary design of the pad structure without the boulders 4.1. Description The solution adopted is composed of three di ff erent parts, each one designed to solve a specific issue. As it can be seen from the previous speed analysis, the central area is the most critic for the lander, because here regolith spread with a high vertical velocity. To overcome this problem, a small compact hill made of melted regolith is designed. A hill is also useful to better directionate the outgoing gasses from the nozzle. This central region has a diameter of 9m and a height of 0.5m. Around the hill, a flat area is created and it is composed by three layers of stones made by melted regolith or natural rocks found in the area. The stones will reduce the spreading of the dust and would fix the ground below. This area will have the diameter of 20m. Finally, to isolate the external area, a semi-circular bank is designed, which encloses the entire pad. This semi- circular shape will have the diameter of 3m. It will be useful to catch the spread sand from the central area and also will rise the angle of the outgoing flow, reducing its e ff ect on the area around. 4.2. Benefits Building this landing pad the total ground e ff ected by the PSI phenomena is restricted to a area of 26m of diameter. The dust around is almost untouched by the flow field of the outgoing gasses. This pad keeps the lander totally safe, due to the central hill of compact soil. This solution is quite simple to build and does not require materials from earth. It is designed to be reusable, to be a permanent spot for many landings in the same place. It can be built in steps. The first landing can be done with only the central layer of stones covering all the area and then the solution can be upgraded in further missions. 4.3. Realization Microwave technology is chosen for the construction of the parts. Regolith melting though microwave heating is a technology already developed and tested. To realise the pad there will be necessary a previous mission to bring the microwave machinery on the mood surface. This machinery can be assembled on the surface and requires very low energy to work compared with other possible solutions; in fact microwave heating only requires 23% of the energy compared to that for laser sintering [5]. 6 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 7 5. Map of the lunar surface The diameter across the surface of Mare Humorum is 419.669Km. The soil’s composition and terrain character- istics vary in the mare. By setting the desired terrain and soil profiles, several spots in the mare are discarded. The landing site is delimited by a rectangular area of the size of 80x90m. This allows a better analysis of the landing pad site because the data range displayed by the layer can be reduced, thus, the depicted surface portrays a more specific representation of the place. Figure 3. Landing site in blue as seen from 5km altitude. 5.1. Terrain and soil characteristics The purpose of this section is to analyze the lunar surface and select a specific landing site in Mare Humorum. Quickmap, a highly customizable web-based mapping and analysis tool, is used for this project. This tool allows the user to view the lunar surface and access the data. For the selection of the landing site in Mare Humorum, three criteria were taken into account: the terrain’s slope and height, the abundance of rocks near the site and a similar chemical composition to regolith simulant JSC-2A. 5.1.1. Slope and height variation As the aim of the project is to construct durable and resilient landing pad structure, the landing pad base should be flat with minimal slopes. This facilitates the construction phase and ensures more stability to the pad. The terrain height on the lunar surface varies drastically from a spot to another. The terrain height layer was assembled by ACT, by combining a group of regional and global topographic models with highest resolution on top. It can be seen in Figure 4, that the height changes significantly outside the selected area. Nonetheless, inside the area the highest altitude is -2957.894m and the lowest is -2958.184m. Another way to represent terrain’s height variation is using terrain slope layer. This layer was derived from elevation data. In this delimited area the slopes are small, the range varies from 0.39deg to 0.47deg. The data outside this range is masked, which shows that the slopes outside the rectangular area is are higher. Therefore, this potential landing site is a good choice because the slopes and terrain are acceptable for a stable structure. 5.1.2. Comparison to JSC-2A The representation of FeO abundance in mosaic shape was created from the correction of the Mineral Mapper reflectance data collected by JAXA SELENE / Kaguya mission. The unit is given in elemental weight percent(wt%). To obtain FeO mosaic layer, according to [15], Hapke’s radiative transfer equations along with defined optical constants were used to compare each pixel to the a reference table of spectra and generated the chemical composition of the surface by using correlation method. As it can be observed in Figure 5, the selected location shows di ff erent abundance of FeO. Nonetheless, the variation is minimal, from 15.072 wt% to 15.345 wt%.The abundance of FeO (wt%) in this site is very close to JSC-2A simulant, JSC-2A FeO concentration is 15.2 wt%, whereas the soil in the landing site is about 15.2 wt% as average. 7 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 8 Figure 4. (a) Terrain height variation, (b) Terrain slope variation. The abundance values of Titanium Ti were derived from the Lunar prospector gamma ray spectrometer measure- ments acquired during di ff erent altitudes of the prospector’s mission. The data from the map shows an abundance of 2.549 wt%, compared to JSC-2A with a value of 2.32 wt%. The abundance of plagioclase ( Na , Ca )( Al , S i ) 4 O 8 is depicted as mosaic surface layer in Figure 5, that was created using the data from SELENE / Kaguya mission. The abundance of this element in the landing site varies from 35.76 wt% in the top right quadrant, to 46.706 wt% in the bottom right corner. The abundance of plagioclase mineral in JSC- 2A is 45.7 wt%. [16]. As plagioclase representation, Orthopyroxene abundance representation shows a significant change from top right and bottom right quadrants, from 25.490 wt% to 15.882 wt%, Figure 5. Figure 5. (a) FeO Abundance (wt%), (b) Plagioclase Abundance (wt%), (c) Orthopyroxene Abundance (wt%). 5.1.3. Rocks abundance As the proposed landing pad design is based on the placement of gravel stones in the peripheral area of the central cone, high rock abundance can be an advantage, because it is not necessary to transport gravel stones from Earth or from other places on the moon. Additionally, a manufacturing step can be skipped. For this representation, infrared data from the Lunar Reconnaissance Orbiter LRO was used to globally map the thermophysical properties of the Moon’s regolith fine layer. By using the data acquired by the Diviner multi-spectral brightness temperature instrument, it was possible to separate the thermal behavior of the regolith from that of larger rocks. In Figure 6, rocks and regolith representation is based on day to night temperature comparison. During lunar 8 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 9 nights, lunar rocks are relatively warmer than regolith or lunar soil, which enables the distinction between the two. Areas with high rock abundance are shown in red in 6, whereas areas covered with regolith are portrayed in blue. Figure 6. Rocks abundance. 6. Simulations and numerical analysis A COMSOL Multiphysics model has been developed to study how the boulders interact with ejected dust in order to predict which geometries should be adapted and which is the optimal size of the landing pad. Stationary studies have been conducted for a hovering altitude of 1.5m, 2.5m, 5m and 7.5 and the model has been validated following [8]. Despite using a di ff erent software, the equations of the physics governing in the boundary conditions were compatible. It has been studied how plain and circular surfaces modify the direction and velocity of the PSI, the pressure on those boundaries and the temperature that should be expected. This study has provided us with extra information on the behaviour of the PSI, which led to changes in geometry of the final solution. It should be highlighted that the simulations, despite providing a good visual idea of what is happening, don’t tell the full story and many simplifications have been assumed (in this hostile environment the conditions are much more unpredictable and unstable). The geometry used is presented in Figure 7. Figure 7. Symmetry used for each study. Left: 2D study, in green is presented the pressure ( P ) studied at the boundary layer. Right: 2D axisymmetric study, in green and red are presented the pressure ( P ) studies at the two interesting boundary layers on the surface and in blue the velocity field ( v ) study at the boundary layer with the lunar atmosphere. 6.1. Boulder interaction Two di ff erent types of boulder were studied in a 2D geometry for a stationary study. The first one involved rectangular shaped obstacle and the second one had a spherical shape. Results on the rectangular boulder showed a 9 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 10 massive change on the shape of velocity profile, but it was discarded due to high pressure di ff erences between the top and bottom part of the structure, which could lead to torque and other instabilities. Regarding the spherical boulder, the visual results are presented in Figure 8. In this case the flow of gas was more smooth and the pressure on the surface was around one order of magnitude lower than in the previous case. Figure 8. Interaction of the spherical boulders with the gas ejected from the nozzle on the lunar surface at a hovering altitude of 1.5m. Top: Pressure profile Middle: Temperature profile Bottom: Velocity profile. 6.2. Pad interaction The interaction of the pad with the ejected gas from the nozzle is presented as a 2D axisymmetric geometry for a stationary study of the gas behaviour. The geometry has been adapted to a semi-ellipse of 0.75m of semi-minor axis 10 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 11 to allow the gas to flow smoothly and avoid extra turbulence, which can lead to more PSI around the pad. In Figure 9, it can be observed the thermal profile of the gas ejected from the nozzle at a 5m. In this Figure, the radial rebound of the gas is clearly shown, which suggest a reduction on the noise and its e ff ects on the lander. Figure 9. Thermal study of the interaction of the pad with the nozzle ejected gas. 6.3. Results and discussion To analyse the boundary interaction between the gas ejection and the lunar surface with the obstacles presented in this paper,di ff erent data acquisition boundaries for the two studies were set. The boundary layers selected are presented in Figure 7. In Figure 10 it is presented as the pressure profile on the lunar surface in the 2D axisymmetric study. As expected, there is a massive maximum of pressure at zero radial distance, where the maximum elevation of the pad is found with a value of 28000 Pa. The blue line indicates a hovering altitude of 1.5m, the green indicates a hovering altitude of 2.5 meters, the red indicates a hovering altitude of 5 meters and the orange one indicates a hovering altitude of 7.5 meters. The pressure grows as the hovering altitude decreases and the same pressure decreases radially as well. However, it does so in an exponential way, with a surprising dip at around 1 m for low hovering altitudes. At 3.5 meters, the pad finishes, but another increase is found at around 4 meters. This is a trend that is followed by all the hovering altitudes to some extent. For that reason, the pad will be extend radially on the ground at least 0.5 meters, to prevent an ejection of dust just after the pad (as this could lead to instability and unexpected behavior of the dust). Also, the pressure is reduced one order of magnitude compared the case without any pad, which shows the e ff ectivity of this shape for the pad. The pressure profile on the lunar surface from the 2D geometry study is presented in the right image of Figure 11 on the boundary highlighted in green in the right side of Figure 7, which is the area between the first row of boulders. As it was expected, the maximum of pressure is located in the same x-axis position as the nozzle. However, the maximum pressure here is much greater than in the case with the pad, with a value of 140 kPa. This could be explained by the geometry of the pad, which deflects the gas radially and reduces the acoustic rebound of the wave from the surface to the nozzle. From this graphic, notice the increase of pressure just before the nozzle. This could result in an increase of the PSI in that area or even long term problems due to dust being ejected from that area and possible cavitation. Further research should focus on how to prevent this to happen through processes of dust covering, regolith sintering or binder solutions. The pressure profile on the boulder from the 2D geometry study is presented in the left image of Figure 11. There is a common maximum at a certain angle of the boulder, where the flow of dust collides directly with the resistance. In this case, the di ff erence between 2.5 meters in hovering altitude to 1.5 meters is more exaggerated than in other pressure studies. This could be explained by the amount of radial velocity of the ejected gas due to how close the lunar surface is. It is important to also notice that after the crest of the boulder (at around 2m), the amount of pressure 11 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 12 Figure 10. Pressure profile on the lunar surface inside the first layer of boulders and without the pad. On the y axis is presented the pressure in [kPa] and on the x axis is presented the horizontal distance in [m]. Blue (1.5m), Green (2.5m), Red (5m) and Orange(7.5m) represent di ff erent hovering altitudes. Figure 11. Left: Pressure profile on the boundary layer between the boulder and the lunar atmosphere. On the y axis is presented the pressure in [kPa] and on the x axis is presented the horizontal distance of the boulder’s surface in meters, defined by x = R cos θ , with 0º < θ < 180º. Blue (1.5m), Green (2.5m), Red (5m) and Orange(7.5m) represent di ff erent hovering altitudes. Right: Pressure profile on the lunar surface inside the first layer of boulders and without the pad. On the y axis is presented the pressure in [kPa] and on the x axis is presented the horizontal distance in meters. Blue (1.5m), Green (2.5m), Red (5m) and Orange(7.5m) represent di ff erent hovering altitudes. is despicable. This can be explained by the absence of pressure from the lunar atmosphere, which leads to absence of eddies behind the resistance. This e ff ect can allow us to play with the geometry of the boulder on the free-pressure zone and even utilize that di ff erence of pressure to generate torque for electric production purposes. In Figure 12 it is presented the velocity profile of the gas exhaustion from the nozzle at 10 meters from the landing spot. These results are gathered from the simulations at di ff erent hovering altitudes and the geometry and position 12 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 13 of the boulders is the same for each case. The results are surprising in some cases, but can be explained due to the fact that the study is stationary, rather than time dependent. In all cases, the velocity was reduced below the escape Moon’s velocity. For the blue line, at a hovering altitude of 1.5 meters, the maximum velocity is found at a very high altitude, at around 9 meters in vertical length. This is much higher than the other cases and seems to align well with the results found from Figure 12, which suggested a very singular behavior of the fluid when the nozzle was so close to the surface. For hovering altitudes of 2.5 m, 5m and 7.5m similar heights are found for a small peak in velocity at 9 meters. However, the maximum velocity is found at around 7 meters, but with di ff erent values. Contrary to what is expected, the velocity is higher for higher hovering altitudes. This could be explained by the gas coming directly from the nozzle without interacting with the surface, which would add up to create higher velocities at that point. It is also important to notice that at low hovering altitudes, the velocity near the surface is not zero. This e ff ect justifies the need for extra layers of boulders, rather than a single circular one. Multiple layers will gradually reduce the velocity to the point that the plume surface interaction with the dust can be small enough to not be a problem for the rest of the lunar surface. Figure 12. Velocity profile on a vertical atmospheric layer at 10 meters from the landing spot. On the y axis is presented the velocity in [m / s] and on the x axis is presented the vertical distance from the surface. Blue (1.5m), Green (2.5m), Red (5m) and Orange(7.5m) represent di ff erent hovering altitudes. 7. Conclusion In conclusion, this study aimed to improve our knowledge on the issues associated with the Plume Surface Inter- action and to propose an optimal solution. Through the research stage of this work, a compilation of information regarding the regolith and its interaction with the environment was achieved. Extense studies on the Mare Humorum’s terrain show the potential of this location for a lunar base. Through the innovation stage, a trade-o ff analysis for the di ff erent proposed solutions was performed. It was determined that the landing pad with a succession of radial layers of boulders was the best candidate to mitigate the plume e ff ects. 13 I. El Kraye, ` A. Drago, G. Zuin, M. Ros / Acta Astronautica 00 (2023) 1–14 14 Through the development phase, CAD models were developed to optimize the geometry of the structure and COMSOL simulations were performed to study the pressure and thermal e ff ects. The CFD analysis of the solution indicates a reduction of in the velocity of the dust ejected below the escape velocity of the Moon, to under 800 m / s and a reduction of the pressure under the nozzle of one order of magnitude. Further research will need to be performed in order to study the necessary characteristics of the materials that can sustain the thermal and pressure conditions found for the solution. Also, ways to build this structures have to be found through ISRU methods. 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