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The Bioregenerative Life Support System (BLSS) is an indispensable part of extraterrestrial bases as it can provide sufficient food, fresh air, recycle waste, and create a safe environment for humans. However, it is still uncertain how the natural environment of extraterrestrial bodies would affect the function of an artificial ecosystem like BLSS. The Chinese Chang'e 4 lunar lander carried a micro-ecosystem called Biological Experiment Payload (BEP), developed by Chongqing University, and conducted the first biological experiments for humans on the back of the Moon to assess the effects of low gravity, intense radiation, and light on the lunar surface to living things. The payload weighs 2.608 kg, with 198 mm in height and 173 mm in diameter, offering a total inside space of 0.82L, containing 0.42L of bio-activity space. Cotton seeds, Potato seeds, Arabidopsis seeds, Rape seeds, Fly eggs, Yeast, and 18 ml of water were placed in the BEP, which was kept at a constant atmospheric pressure. Through a light guide tube, sunlight from the lunar surface can enter the payload's interior, allowing plants to photosynthesize. One remarkable feature of BEP was the gap in radiation protection design. On January 3, 2019, BEP arrived on the back of the Moon and conducted an experiment lasting eight days, 22 h, and 45 min (Earth Day) during the first lunar day after landing. After 22 h of watering, the CCD camera captured cotton seed germination, compared to 53 h after watering in the ground synchronization experiment. The cotton seedling on the lunar surface showed a faster germination rate than the simultaneous experiments on the ground. Also, the morphology of the cotton on the Moon was changed, with its stem growing curved, developing a long part close to the substrate and a short upright portion while forming fat-flowing nodes close to the substrate. Finally, the leaves were also curled and did not unfold. The payload was dormant from 12 to 31 January 2019, with its interior remaining dark and temperatures reaching as low as −52 °C. After the lunar night, the leaves of the terrestrial synchronous experiment were completely blackened and in pieces, in contrast to the Seedling on the Moon, which was upright and green. This experiment demonstrated that plants could grow on the lunar surface even in intense radiation, low gravity (1/6 g), and prolonged intense light. It was also the first time that a biological experiment was carried out by humans on the Moon using sunlight from space to power photosynthesis. • Confirmed the potential for plants to grow on the moon. • A detailed description of how plants grow in real gravity and light of the Moon. • Seedlings under 1/6 g gravity conditions quickly adapt to the Moon's night. • Original designs that may be useful for space cultivation experiments. • Prospects for future space agriculture and extraterrestrial base construction. [ABSTRACT FROM AUTHOR]

For humanity to complete its ambitious solar system exploration, it is crucial to comprehend how terrestrial life reacts to differing planet gravity. We followed the life trajectory of an earth cotton seed's germination, development, and ultimate fate after prolonged exposure to extremely low temperatures using the life-regeneration ecosystem carried by Chang'e 4 probe, which landed on the Moon on January 3rd, 2019, for the first time in human history. In a controlled environment with similar characteristics, such as temperature, humidity, air pressure, and nutrition, we compared this life trajectory on the moon to that on Earth, except for the differences in gravity, light, and radiation. We discovered that the 1/6 g moon gravity speeds up seed germination. Surprisingly, Moon seed-lings demonstrated rapid acclimatization to super-freezing (below minus 52 degrees Celsius) under 1/6 g lunar gravity, maintaining upright and green despite exposure to long-term extremely cold temperatures for 18–24 hours. Based on cellular and molecular reactions caused by moon-low gravity, we suggest probable mechanisms for cold resilience. These unique findings will enhance our understanding of how plants adapt to suboptimal environmental conditions in space. [ABSTRACT FROM AUTHOR]

To investigate the driving performance of the flexible metal wheel of the manned lunar rover in a low gravity environment, we established a 3D discrete element method (DEM)-finite element method (FEM) coupling simulation system. The wheel was modeled by FEM and the soil was modeled by DEM. Then, the direct shear tests were performed to verify the validity of the DEM contact parameters. Finally, the driving performance of the flexible metal wheels with different treads under low gravity conditions was studied, and the velocity field of soil particles under the wheels was observed. The results show that the sinkage of the flexible wheel is almost independent of gravity, while the torque and drawbar pull are significantly affected by gravity. Moreover, by comparing the driving performance of the wheel equipped with treads (WT wheel) and without tread (NT wheel), it is found that the tread have great effect on sinkage, torque, and drawbar pull to wheel weight ratio due to the differences in wheel-soil contact area. Also, through the observation of the particle velocity field under the wheel, it is found that the gravity and load also have significant effects on the movement of soil particles. This study provided a reliable method for studies on the interaction between the flexible wheel and soil in low gravity environments. [ABSTRACT FROM AUTHOR]

• Scaling the density and gravity, the hovering flapping frequency on Mars is required to be 113.48 Hz, which is 4.93 times faster than on Earth. • Through CFD simulations, the hovering stability derivatives on Mars are computed to complete the longitudinal and lateral equations of motion. • Three flight modes including two subsidence modes, and one oscillation mode are identical in the hovering flights on Earth and Mars. • Although the hovering flapping-wing robots are both inherently unstable, the growth of flight disturbances on Mars is slower than on Earth. • Due to the slow growth of the disturbances, the KUBeetle is expected to be able to perform hovering flight on Mars by using the feedback controls. In efforts to search for life-form on a planet other than Earth, many projects have been launched to measure and analyze the surface and atmospheric conditions on Mars. To extend the exploration of Mars, scientists have attempted to perform flight in the extremely thin air environment of Mars. Inspired by the successful controlled flight of NASA's Helicopter on Mars, in this work, we investigate the six-degrees-of-freedom hovering flight dynamics of an insect-like flapping-wing robot, called KUBeetle. To hover the KUBeetle on Mars, using the scaling ratios of air density and gravity on Earth and Mars, we have found that the flapping frequency should be about 113.48 Hz, which is 4.93 times faster than that on Earth, and the required lift on Mars is about 38% of that on Earth. The computational fluid dynamics (CFD) analysis suggests that the lift produced by the flapping wings at the frequency on Mars is close to that predicted by the scaling ratios. A series of simulations are used to compute the stability derivatives to complete the equations of motion. The eigenvalue and eigenvector analyses have identified one fast subsidence mode, one slow subsidence mode, and one divergence oscillation mode in the longitudinal and lateral motions on Mars, which are identical to those on Earth. The dynamic responses of the robot under three initial disturbances in the translational speeds indicate that due to the 94% smaller real part of the complex roots in the divergence oscillation modes of the longitudinal and lateral motions on Mars, the growth of flight disturbances is much slower than on Earth. Therefore, the KUBeetle is capable of hovering flight on Mars, since it has enough time to use the feedback controls to stabilize the system. This work can be used to support the studies of the flight control of flapping wing robots in the Martian atmospheric condition, and to generalize such flight control for other non-terrestrial applications in the atmospheres of other planets and/or their moons. [ABSTRACT FROM AUTHOR]

Lunar in-situ resource utilization (ISRU) has been put on the agenda by many countries. Due to the special material nature and low gravity environment, the lunar regolith demonstrates significantly different behavior from terrestrial geomaterials. However, the systematic understanding of its geotechnical behavior is now seriously restricted by the scarcity of lunar regolith and the difficulty in simulating lunar gravity. A new lunar regolith simulant, termed as China University of Mining and Technology Number One (CUMT-1), has been developed to recover properties of the lunar regolith and simulate the lunar gravity by adopting the recently advanced geotechnical magnetic-similitude-gravity model testing (GMMT) method. The CUMT-1 simulant was prepared by reproducing the in-situ formation and fragmentation of the lunar matrix, which plays a key role in the irregular particle morphology. The mineralogical compositions, particle morphology and gradation, specific gravity, bulk density, void ratio, shear strength, and compressibility were determined. After quantifying the magnetization and magnetic-similitude-gravity characteristics, an application of the cone penetration resistance under low gravity was further given. The obtained results are compared to the values known for lunar regolith samples and other simulants, which demonstrates promising characteristics for use in geotechnical engineering-based and scientific-based applications, especially considering the influence of lunar gravity.

The future lunar missions of the space research organizations (SRO) such as the (US) National Aeronautics and Space Administration (NASA), the China National Space Administration (CNSA), the European Space Agency (ESA), the Indian Space Research Organisation (ISRO), the Japan Aerospace Exploration Agency (JAXA), and the Russian Federal Space Agency (RFSA or Roscosmos), etc., comprise extended stay on Moon, making the lunar surface a launchpad for the interplanetary missions and Moon habitation. These are the most challenging task for the space research organizations (SRO) and the researchers, which needs all engineering disciplines. Also, the distinct difference in the environment between the lunar surface (lunar gravity, moonquakes, high temperature, etc.) and the Earth forced the SROs and the researchers to study the geotechnical properties such as specific gravity, particle size distribution, density, shear strength, and bearing capacity of the lunar soils. In this, the lunar gravity (1/6g) significantly influences the bearing capacity of the lunar soil, which is the predominant property for the design and analysis of foundation systems of the lunar structures. Assessing the bearing capacity under the reduced lunar gravity will enhance the evaluation of design criteria for the lunar structures and their foundation systems. In this respect, this paper explains the low gravity effect on the bearing capacity of the new lunar highland soil simulant LSS-ISAC-1 developed to represent the geotechnical properties of the highland soils of the lunar surface. The bearing capacity of the simulant for both the lunar and Earth gravity was evaluated and compared with the lunar soils and lunar simulants. The plate load test and single wheel load tests were performed to determine the sinkage property (compression/settlement) of the LSS-ISAC-1. The slope stability and self-standing height of the LSS-ISAC-1 were also determined for a better understanding of stability. Overall, the comparison results state that the new simulant LSS-ISAC-1 is more than sufficient to support virtually any conceivable structures based on the results. [ABSTRACT FROM AUTHOR]

With advancements in space exploration, operations in low-gravity environments such as the lunar surface are expected to be conducted. Geotechnical engineering problems such as those associated with the foundation bearing capacity considerably influence the feasibility and safety of these operations. In this study, the findings of experiments conducted by Japan Aerospace Exploration Agency (JAXA) scientists on parabolic flight, aimed at measuring the ultimate bearing capacity of shallow foundations under 1/6 g to 2 g of gravity, are analysed. Specifically, the results are analysed to calculate the ultimate friction angle based on the classical Terzaghi limiting equilibrium solution in soil mechanics. The friction angle of foundation sand increases as the gravity level decreases. This finding is verified through advanced arbitrary Lagrangian–Eulerian (ALE) finite element simulations based on a simple Mohr–Coulomb model. Moreover, the underlying mechanism for this phenomenon is examined considering an ALE finite element simulation based on a newly developed rheological model known as the Tsinghua–MiDi sand model. The pressure-sensitive and rate-dependency constitutive behaviour of sand is clarified. Notably, this phenomenon increases the viscous shear stress and ultimate friction angles in low-gravity conditions. The coupled effects of the loading rate and low-gravity level on the bearing capacity of foundation sand are predicted. The findings can provide a novel theoretical prospective for geotechnical studies in space exploration engineering in low-gravity conditions. [ABSTRACT FROM AUTHOR]

New Findings: What is the topic of this review?This narrative review explores past and recent findings on the mechanical determinants of energy cost during human locomotion, obtained by using a mechanical approach based on König's theorem (Fenn's approach).What advances does it highlight?Developments in analytical methods and their applications allow a better understanding of the mechanical–bioenergetic interaction. Recent advances include the determination of 'frictional' internal work; the association between tendon work and apparent efficiency; a better understanding of the role of energy recovery and internal work in pathological gait (amputees, stroke and obesity); and a comprehensive analysis of human locomotion in (simulated) low gravity conditions. During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way), and energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on König's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e.g., the pendulum‐like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the past 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g., low gravity, unsteady speed). This narrative review aims to summarize current knowledge of the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in unsteady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g., frictional internal work and elastic contribution) deserves future research that would improve our knowledge of the mechanical–bioenergetic interaction during human locomotion, as well as in sport science and space exploration. [ABSTRACT FROM AUTHOR]

Reduced paraspinal muscle size and flattening of spinal curvatures have been documented after spaceflight. Assessment of trunk adaptations to hypogravity can contribute to development of specific countermeasures. In this study, parabolic flights were used to investigate spinal curvature and muscle responses to hypogravity. Data from five trials at 0.25 g, 0.50 g, and 0.75 g were recorded from six participants positioned in a kneeling-seated position. During the first two trials, participants maintained a normal, upright posture. In the last three trials, small-amplitude perturbations were delivered in the anterior direction at the T10 level. Spinal curvature was estimated with motion capture cameras. Trunk displacement and contact force between the actuator and participant were recorded. Muscle activity responses were collected by intramuscular electromyography (iEMG) of the deep and superficial lumbar multifidus, iliocostalis lumborum, longissimus thoracis, quadratus lumborum, transversus abdominis, obliquus internus, and obliquus externus muscles. The root mean square iEMG and the average spinal angles were calculated. Trunk admittance and muscle responses to perturbations were calculated as closed-loop frequency-response functions. Compared with 0.75 g, 0.25 g resulted in lower activation of the longissimus thoracis (P = 0.002); lower responses of the superficial multifidus at low frequencies (P =0.043); lower responses of the superficial multifidus (P = 0.029) and iliocostalis lumborum (P = 0.043); lower trunk admittance (P = 0.037) at intermediate frequencies; and stronger responses of the transversus abdominis at higher frequencies (P = 0.032). These findings indicate that exposure to hypogravity reduces trunk admittance, partially compensated by weaker stabilizing contributions of the paraspinal muscles and coinciding with an apparent increase of deep abdominal muscle activity. NEW & NOTEWORTHY This study presents for the first time novel insights into the adaptations to hypogravity of spinal curvatures, trunk stiffness, and paraspinal muscle activity. We showed that exposure to hypogravity reduces the displacement of the trunk by an applied perturbation, partially compensated by weaker stabilizing contributions of the paraspinal muscles and concomitant increase in abdominal muscle responses. These findings may have relevance for future recommendations for planetary surface explorations. [ABSTRACT FROM AUTHOR]

Numerical simulation of 2D Cryogenic Pulsating Heat Pipe (CPHP) is carried out with near critical diameter. Liquid nitrogen is used as working fluid for targeting cryogenic applications in the range from 77 to 123 K. The volume of fluid (VOF) model is considered for two phase simulations. The evaporator and condenser section are kept at constant wall temperature condition while adiabatic section is considered insulated. The evaporator temperature varied from 85 to 110 K and condenser section kept at 75 K. The filling ratio (FR) of working fluid is varied as 50, 60 and 70%. CPHP model is first tested with on ground gravity condition of 9.81 m/s2. It is then tested with low gravity conditions (0.981 m/s2) and milli-gravity condition (0.01 m/s2). In low gravity condition, surface tension force is observed more dominant than body force which significantly altered the performance of a CPHP. In comparisons with ground level condition, more stable flow patterns are observed which led to the improved heat transfer performance of a CPHP in case of low gravity. [ABSTRACT FROM AUTHOR]

Collisions are ubiquitous phenomena on the surface of solar system objects. When secondary craters form on the surface of small bodies, such as asteroids and comets, the impactors have very low velocities of several meters per second or less. These secondary collisions can affect the surface topography of small bodies. Therefore, it is important to investigate low‐velocity impacts on a target simulating regolith to understand the effects of secondary collisions on the surface of small bodies. We conducted impact experiments into granular materials with a velocity range of 1 to 4.6 m/s, under a gravity range of 0.20 to 1 G, to investigate the effects of gravity on crater diameter. The gravitational dependence obtained under the reduced ambient pressure almost agreed with those obtained for high‐velocity impacts in previous studies. Gravitational dependences obtained under the standard atmospheric condition roughly agreed with the dependence obtained under the reduced pressure condition when a term of nondimensional atmospheric pressure was added to the scaling relationship. It was found that the crater‐size scaling law obtained for high‐velocity impacts could be applied to low‐velocity impacts of several meters per second under the reduced ambient pressure when the density ratio of the projectile to the target was close to unity. Finally, we applied the experimental results to estimate the diameters of secondary craters on asteroid 25143 Itokawa and compared the diameters with the dimple topography on Itokawa. The results support the hypothesis that dimples are formed by low‐velocity impacts of meter‐sized boulders. Key Points: We investigated the effects of gravity on crater diameter in low‐velocity impact experiments into granular materialsWe found the crater size scaling law of high‐velocity impact can be applied to the low‐velocity of meter per second, under reduced ambient pressureWe estimated secondary crater diameter on asteroid Itokawa based on the results of our experiments [ABSTRACT FROM AUTHOR]

Highlights • A two-dimensional axisymmetric model is developed for subcooling pool boiling under low gravity. • The superheated layer and the thermocapillary convection are considered. • The simulations are validated against the experiments in the literatures. • Bubble dynamics, critical subcooling and heat transfer are analyzed in details. Abstract The numerical simulation of a single vapor bubble growth in subcooling liquid under different gravity conditions has been carried out. In the numerical model, a thin superheated layer and the thermocapillary convection caused by the surface tension variation along the surface are considered. The continuity equation and energy equation are modified to allow for the phase change. In addition, the thermocapillary convection effect has been included in the momentum equation. The vapor-liquid interface is captured by the phase field method. The results show that the bubble behavior in the numerical model agrees well with previous experiments conducted in high subcooling liquid under microgravity. The effects of gravity level, contact angle and wall superheat on the bubble growth, critical subcooling (the liquid subcooling under the condition that the evaporation rate of a bubble is equal to its condensation rate), together with heat transfer have been investigated. The growth period and departure radius both reduce with the increase in gravity level, while the critical subcooling increases slightly. Large contact angle at the three-phase contact line augments the departure radius. However, the critical subcooling decreases as contact angle increases. With the wall superheat increasing, the growth period reduces rapidly, while the departure radius and the critical subcooling increase. What's more, the non-departing bubble adhering to the surface would prevent heat transfer with a dry spot, which may damage the heating element in application. [ABSTRACT FROM AUTHOR]

In pool boiling under low gravity conditions, the vapor bubbles remain attached to the heating surface, which deteriorates the heat transfer rate. The present work demonstrates the integration of the electrowetting (EW) technique to resolve the bubble departure problem in pool boiling at low gravity conditions. A phase-field-based numerical method with a dynamic contact angle model has been implemented in COMSOL Multiphysics (V5.3) to investigate the problem. The bubble dynamics in the electrowetting integrated pool boiling for low gravity conditions are analyzed at various frequencies for different voltage waveforms, i.e., square, sinusoidal, and triangle. The EW technique increases the departure frequency of the bubble and the heat transfer rate compared to the conventional pool boiling. The sinusoidal voltage waveform shows the best performance among different voltage waveforms, producing 119.3% higher bubble departure frequency and 74.3% heat transfer enhancement than the conventional pool boiling. [ABSTRACT FROM AUTHOR]

Including plants in bioregenerative life-support systems enables simultaneous food production and water and air recycling, while closing cycles for water, oxygen, nitrogen, and carbon. To understand and predict higher plant behavior for a wide range of environmental conditions, including reduced gravity levels, a mechanistic physical model is being developed. The emphasis is set on the influence of gravity levels and forced convection on higher plant leaf gas exchanges, which are altered by reduction of free convection in lower gravity environments, such as microgravity or martian and lunar gravities. This study highlights the significance of understanding leaf boundary layer limitations and ultimately will lead to complete mechanistic modeling of mass and energy balances on plant growth in reduced gravity environments. [ABSTRACT FROM AUTHOR]

A novel hydrophobic membrane-based dehumidification technology with chilled water is proposed to substitute the traditional dew point method for humidity control in spacecraft, which fulfils the tough tasks of air/water separation and water reuse under low gravity at the same time. A prototype hydrophobic membrane module was designed and fabricated by packing 853 hollow fibres in the shell-and-tube configuration, which could prevent the leakage of the tube-side chilled water to the air side. Theoretical models concerning conjugated heat and mass transfer have been established for the explanation and prediction of the moisture migration across the membranes. Experiments were conducted to validate the theoretical models and the feasibility of water vapour removal under various operating conditions. Although undesirable dewing occurred on the outer membrane surface due to the slow mass transfer relative to heat transfer across the membranes, it was verified by the water increase in graduated cylinder that some water vapour actually transferred through the membrane pores under partial pressure difference. A maximum net dehumidification rate (excluding dewing) of 45 g/h has been achieved by the experiment. [ABSTRACT FROM AUTHOR]

Previous literature on the effects of low gravity on the mechanics and energetics of human locomotion already dealt with walking, running, and skipping. The aim of the present study is to obtain a comprehensive view on that subject by including measurements of human hopping in simulated low gravity, a gait often adopted in many Apollo Missions and documented in NASA footage. Six subjects hopped at different speeds at terrestrial, Martian, and Lunar gravity on a treadmill while oxygen consumption and 3D body kinematic were sampled. Results clearly indicate that hopping is too metabolically expensive to be a sustainable locomotion on Earth but, similarly to skipping (and running), its economy greatly (more than ×10) increases at lower gravity. On the Moon, the metabolic cost of hopping becomes even lower than that of walking, skipping, and running, but the general finding is that gaits with very different economy on Earth share almost the same economy on the Moon. The mechanical reasons for such a decrease in cost are discussed in the paper. The present data, together with previous findings, will allow also to predict the aerobic traverse range/duration of astronauts when getting far from their base station on low gravity planets. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to present an investigation of the effect of different gravity conditions on the penetration mechanism using the two-dimensional Distinct Element Method (DEM), which ranges from high gravity used in centrifuge model tests to low gravity incurred by serial parabolic flight, with the aim of efficiently analyzing cone penetration tests on the lunar surface. Design/methodology/approach – Seven penetration tests were numerically simulated on loose granular ground under different gravity conditions, i.e. one-sixth, one-half, one, five, ten, 15 and 20 terrestrial gravities. The effect of gravity on the mechanisms is examined with aspect to the tip resistance, deformation pattern, displacement paths, stress fields, stress paths, strain and rotation paths, and velocity fields during the penetration process. Findings – First, under both low and high gravities, the penetration leads to high gradients of the value and direction of stresses in addition to high gradients in the velocity field near the penetrometer. In addition, the soil near the penetrometer undergoes large rotations of the principal stresses. Second, high gravity leads to a larger rotation of principal stresses and more downward particle motions than low gravity. Third, the tip resistance increases with penetration depth and gravity. Both the maximum (steady) normalized cone tip resistance and the maximum normalized mean (deviatoric) stress can be uniquely expressed by a linear equation in terms of the reciprocal of gravity. Originality/value – This study investigates the effect of different gravity conditions on penetration mechanisms by using DEM. [ABSTRACT FROM AUTHOR]

Simulations of the bearing capacity and shear strength of regolith under Earth’s gravity produce different results from those under low gravity. A low-gravity simulation device was developed in this study, and an internal stress model of regolith simulant was established to correct the errors. The model revealed additional force on both shear plane in the shear test and the press plate area in the pressure–sinkage test. The sinkage and shear test results showed that low gravity decreased the deformable index n , frictional modulus k φ and cohesion c , whereas there were no obvious changes to the cohesive modulus k c and internal friction angle φ . The sinkage generally increased as the gravity decreased under a consistent normal load larger than 50 N, but when the wheel load was lower than 50 N, the sinkage of the TYII-1 simulant was larger under 1 G than 1/6 G. Gravity had little effect on the shear strength of the regolith. However, the tractive thrust of the TYII-1 simulant was lower under 1/6 G than 1 G. The smaller difference was due to differences in the way the soils responded to changes in the gravity level for the TYII-2 simulant. [ABSTRACT FROM AUTHOR]

Hypogravity challenges bipedal locomotion in its common forms. However, as previously theoretically and empirically suggested, humans can rely on "skipping," a less common gait available as a functional analog (perhaps a vestigium) of quadrupedal gallop, to confidently move when gravity is much lower than on Earth. We set up a 17-m-tall cavaedium (skylight shaft) with a bungee rubber body- suspension system and a treadmill to investigate the metabolic cost and the biomechanics of low-gravity (Mars, Moon) locomotion. Although skipping is never more metabolically economical than running, the difference becomes marginal at lunar gravities, with both bouncing gaits approaching values of walking on Earth (cost ≈2 J⋅kg-1⋅m-1). Nonmetabolic factors may thus be allowed to dominate the choice of skipping on the Moon. On the basis of center of pressure measurements and body segments kinetics, we can speculate that these factors may include a further reduction of mechanical work to move the limbs when wearing space suits and a more effective motor control during the ground (regoliths)-boot interaction. [ABSTRACT FROM AUTHOR]

Abstract: Walking and running in low gravity cannot be used at useful speeds, while ‘skipping’, a gait displayed by kids and spontaneously adopted by astronauts of Apollo missions, proved to have the potential to become the gait of choice in that condition. In this paper the previous biomechanical and metabolic analysis of level skipping is extended to positive and negative gradients, in normal gravity. The results confirm at all gradients the higher (average) ground reaction force during the contact phase, with respect to running at the same speed, which would allow confidently facing the Lunar surface where the dust and regoliths affect, in addition to a lower gravity, the locomotion dynamics. Metabolic data, other gait variables related to the mechanical work done and the locomotor/respiratory coupling have also been investigated. [Copyright &y& Elsevier]

Abstract: One of the most important issues in a liquid propellant rocket is to measure the amount of remaining liquid propellant under low gravity environment during space mission. This paper presents the results of experiment and analysis of a pressure–volume–temperature (PVT) method which is a gauging method for low gravity environment. The experiment is conducted using 7.4l tank for liquid nitrogen with various liquid-fill levels. To maximize the accuracy of a PVT method with minimum hardware, the technique of a helium injection with low mass flow rate is applied to maintain stable temperature profile in the ullage volume. The PVT analysis considering both pressurant and cryogen as a binary mixture is suggested. At high liquid-fill levels of 72–80%, the accuracy from the conventional PVT analysis is within 4.6%. At low fill levels of 27–30%, the gauging error is within 3.4% by mixture analysis of a PVT method with specific low mass flow rate of a helium injection. It is concluded that the proper mass flow rate of a helium injection and PVT analyses are crucial to enhance the accuracy of the PVT method with regard to various liquid-fill levels. [Copyright &y& Elsevier]

Abstract: An axisymmetric phase field model is developed and used to model surface tension forces on liquid jets in microgravity. The previous work in this area is reviewed and a baseline drop tower experiment selected for model comparison. The model is solved numerically with a compact fourth order stencil on an equally spaced axisymmetric grid. After grid convergence studies, a grid is selected and all drop tower tests modeled. Agreement was assessed by comparing predicted and measured free surface rise. Trend wise agreement is good but agreement in magnitude is only fair. Suspected sources of disagreement are the simple turbulence model and the existence of slosh baffles in the experiment that were not included in the model. Parametric investigation was conducted to study the influence of key parameters on the geysers formed by jets in microgravity. Investigation of the contact angle showed the expected trend of increasing contact angle increasing geyser height. Investigation of the tank radius showed some interesting effects and demonstrated the zone of free surface deformation is quite large. Variation of the surface tension with a laminar jet showed clearly the evolution of free surface shape with Weber number. A breakthrough Weber number of 1 was predicted by the variation of the surface tension model which is close to the experimentally measured Weber number of 1.5 found in prior experimental work. [Copyright &y& Elsevier]

The dynamic characteristics of liquid sloshing in a transversely vibrating spherical tank with spacer under low gravity were investigated. By expanding the characteristic functions, the frequencies and velocity potentials of liquid free-sloshing were obtained. The dynamic equations and boundary conditions of liquid sloshing in the traverse vibrational spherical tank with spacer under low gravity were derived. By modifying the velocity potentials of liquid free-sloshing, the velocity potentials of liquid sloshing in the traverse vibrating spherical tank with a spacer were obtained. Furthermore, the forces and the moments acting on tank wall were given. The numerical results show when a spacer is inserted in the tank, the sloshing frequencies of liquid and the forces acting on tank wall will decrease, but the moment of force to the centre of the tank which is caused by the force of liquid acting on the spacer will occur. [ABSTRACT FROM AUTHOR]

Abstract: This work investigates how simultaneous bubbles desorb from water and n-heptane when these liquids become supersaturated with dissolved . Supersaturation is imposed locally by a miniature heater so bubbles grow at adjacent sites across the heater. To suppress buoyancy, experiments are performed at low gravity conditions. The number of nucleation sites and nucleation time delay depend on liquid properties and heating power. Simultaneous bubbles do not grow at exactly the same rate and calculations show that this corresponds to slightly different bubble temperatures. Moreover, they exhibit smaller growth rates than single bubbles at similar conditions, an indication that they compete for dissolved . As bubbles expand into the surrounding cold liquid, the heater''s temperature decreases in a fashion implying the inception of Marangoni convection. Simultaneous bubbles detach due to g-jitters but following different ways in the two liquids. However, they always detach together and at smaller sizes than single bubbles do. A temperature triggered destabilization of contact lines is deemed responsible for this. [Copyright &y& Elsevier]

Under the condition of low gravity the characteristics of liquid forced sloshing in a turning spherical tank with a spacer were investigated. The static shape of the liquid surface was analyzed. By expanding the characteristic functions, the frequencies and velocity potential of liquid free-sloshing were derived. The governing equations and boundary conditions for the forced sloshing of liquid under the tank turning were established. The transverse force of liquid acting on the tank and the moment of force to the centre of the tank which is caused by the force of liquid acting on the spacer were given. Numerical results were compared with the ones of the spherical tank without a spacer. The results show that when a spacer is inserted in the tank, the sloshing frequency of liquid and the transverse force of liquid acting on the tank will decrease, but the moment of force to the centre of the tank which is caused by the force of liquid acting on the spacer will occur. [ABSTRACT FROM AUTHOR]

We document some basic facts about how liquids behave in the low gravity of outer space and how this poses challenges to space scientists and engineers who need to design and test systems that handle fluids in outer space while working in the 1-g environment on Earth. [ABSTRACT FROM AUTHOR]

Abstract: This work reviews the up to date state of understanding of dynamic phenomena occurring when gas bubbles grow over submerged heated surfaces. Gas bubbles are produced on hot surfaces because the adjacent liquid layers become superheated causing local desorption of dissolved gases while the liquid far afield remains at low temperatures. Non-isothermal degassing is a very complex process combining heat and mass transport coupled with momentum exchange between the two phases. Difficulties due to buoyancy effects on gas bubbles as well as natural convection of hot liquid layers hindered its thorough investigation in terrestrial conditions and only recent microgravity data allowed serious progress to be made. To reduce the complexity, gas bubble growth on a heated wall was studied here separately from bubble lateral motion and coalescence. A complete mathematical formulation was provided but given the inability to solve the problem numerically with the present resources, a series of approximate solutions were attempted. The comparison between experimental observations and theoretical predictions revealed useful information regarding the governing mechanisms of bubble growth. [Copyright &y& Elsevier]

In this study the first mode of axisymmetric surface oscillations of a free liquid interface in a partly filled right circular cylinder under compensated gravity is investigated numerically. The situation is similar to a spacecraft that enters a ballistic flight at the end of thrust. A reorientation of the liquid of the free liquid surface toward the new equilibrium position takes place. The characteristics of this flow, such as the frequency and the damping, are governed by the behavior of the contact line. The investigations focus on the dependence on the contact line boundary condition, including contact angle variation, and on viscous effects. The effect on the natural frequency and the damping is considered. An analytical approximation for the frequency behavior for a frictionless liquid for a free and a fixed contact line condition is given. Further analytical approximations for the damping behavior in dependence on the contact angle and the Ohnesorge number are determined. The numerical results show very good agreement with experimental data. [ABSTRACT FROM AUTHOR]

This paper reports the experimental and theoretical investigations of forced liquid flows through open capillary channels under reduced gravity conditions. An open capillary channel is a structure that establishes a liquid flow path at low Bond numbers, when the capillary pressure caused by the surface tension force dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. In case of steady flow through the channel, the capillary pressure of the free surface balances the pressure difference between the liquid and the surrounding constant-pressure gas phase. Because of convective and viscous momentum transport, the pressure along the flow path decreases and causes the free surface to bend inward. The maximum flow rate is achieved when the free surface collapses and gas ingestion occurs at the outlet. This critical flow rate depends on the geometry of the channel and the properties of the liquid. In this paper we present a comparison of the theoretical and experimental critical flow rates and surface profiles for convective dominated flows. For the prediction of the critical flow rate a one-dimensional theoretical model taking into account the entrance pressure loss and the frictional pressure loss in the channel is developed. [ABSTRACT FROM AUTHOR]

Abstract: Heat transfer performance in nucleate boiling crucially depends on a circular thin film area near the foot of a vapour bubble where high heat fluxes and thus high local evaporation rates occur. The corresponding wall temperature drop close to the tiny thin film area is computed using an existing nucleate boiling model. To verify the predicted temperature distribution, an experiment is designed with a thin electrically heated wall featuring two-dimensional, high resolution temperature measurement using unencapsulated thermochromic liquid crystals. By means of temperature measurements during a parabolic flight under low-g conditions, the validity of the model used to calculate the temperature distribution in the tiny thin film area could be confirmed. [Copyright &y& Elsevier]

Abstract: This paper reviews the history of cryogenic fluid management technology development and infusion into both the Saturn and Centaur vehicles. Ground testing and analysis proved inadequate to demonstrate full scale performance. As a consequence flight demonstration with full scale vehicle was required by both the Saturn and Centaur programs to build confidence that problems were addressed. However; the flight vehicles were highly limited on flight instrumentation and the flight demonstration “locked-in” the design without challenging the function of design elements. Projects reviewed include: the Aerobee Sounding Rocket Cryogenic Fluid Management (CFM) tests which served as a valuable stepping stone to flight demonstration and built confidence in the ability to handle hydrogen in low gravity; the Saturn IVB Fluid Management Qualification flight test; the Atlas Centaur demonstration flights to develop two burn capability; and finally the Titan Centaur two post mission flight tests. [Copyright &y& Elsevier]

Abstract: A gas bubble growing in a saturated liquid under the action of an internal heat source is studied theoretically as a model problem for the growth of a bubble around a miniature spherical heater submerged in the liquid. In general, this problem does not admit a self-similar solution but it is shown that for a heat source with strength increasing in proportion to the square root of time, a self-similar solution exists even under temperature-dependent transport properties. In that case the bubble radius is also proportional to the square root of time with a proportionality constant which can be found from the solution of a boundary value problem. The effect of temperature dependence of gas-in-liquid diffusivity and liquid and gas thermal conductivities on bubble growth is examined in detail. Finally, approximating explicit procedures are proposed for the computation of bubble growth rate without resorting to the solution of the boundary value problem. [Copyright &y& Elsevier]

This work investigates bubbles growing in a liquid saturated with a dissolved gas when its temperature is locally raised above its saturation value, yet below boiling. Local supersaturation is realized by giving heat pulses of variable power and duration to a miniature heater. The experiments are conducted in a low-gravity environment to decouple bubble growth from buoyancy effects. A diffusion model is formulated considering the spherical growth of an internally heated bubble inside a uniformly subcooled liquid. The simple case of an isothermal bubble growth is solved analytically and compared to measurements to gain some physical insight. Two distinct regions are identified in the measured bubble growth curves: the initial stages are satisfactorily described by a parabolic (diffusion) law, whereas the later stages are approximately linear. [ABSTRACT FROM AUTHOR]

Hydrodynamic measurements are presented for two-phase flows in Mars and Moon gravity conditions. High accuracy pressure drop and flow rate data were obtained using dichlorodifluoromethane (i.e., R-12) as the working fluid flowing in a nominally 11.1 mm inner diameter tube. Measurements were made at Mars gravity, approximately 0.38-g, and Moon gravity, approximately 0.17-g, using NASA''s KC-135 aircraft. A simplified scaling approach was developed using dimensional analysis and can be used to design an Earth-based test bed to simulate a Mars or Moon gravity prototype. For a specific geometry, a selected working fluid at a fixed temperature and pressure, and a particular flow regime condition, the pressure drop functional scaling equation is a simple, power-law relationship for the Euler number as a function of only the Froude number. The research completed supports the use of Earth-g tests to predict the behavior of two-phase systems for Moon-g and Mars-g applications. [Copyright &y& Elsevier]

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation. [ABSTRACT FROM AUTHOR]

A study is presented concerning an experiment under weightlessness. A pure fluid (SF6) is heated near and below its critical point, where liquid coexists with vapor. After the temperature rise, the vapor phase passes well beyond the temperature of the heating walls. This surprising finding is discussed in the light of an adiabatic heat transfer or “Piston effect” and the special geometry of the liquid and vapor. In addition, the shape of the gas–liquid interface is distorted near the solid wall by the thrust of vapor production (“recoil” force), a precursor to the well-known boiling crisis in heat exchanger. [Copyright &y& Elsevier]

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF6 in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase 3He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor. [ABSTRACT FROM AUTHOR]