Free-fall Phase Research Articles

TEMPUS-A microgravity electromagnetic levitation facility for parabolic flights.

During the ∼22s lasting free fall phase in an aircraft flying a parabola, the aboard installed electromagnetic levitation facility "TEMPUS" is used to investigate contactless and undisturbed of gravity induced convection thermophysical properties and microstructure formations of hot and highly reactive metal or semiconductor melts. The completely contactless handling and measurement of a liquid by the levitation technique keeps the melt free of contamination and enables the extension of the accessible sample temperature range far into the undercooled liquid state below the melting point. Additionally, the state of reduced weight during parabolic flights allows us to considerably decrease the strongly disturbing electromagnetic levitation forces acting in ground-based facilities on the suspended liquids. The present paper explains in detail the basic principle and the technical realization of the TEMPUS levitation facility and its attached measurement devices. Furthermore, it presents some typical experiments performed in TEMPUS, which also show the advantages resulting from the combination of reduced weight, electromagnetic levitation, and contactless measurement techniques. The control and data recording, as well as the planning, preparation, and operation of the TEMPUS experiments within the parabolic flight campaign, are another aspect outlined in the following.

Numerical Analysis of the Free-Fall Behavior of a Space Launcher Fragment

A numerical investigation of the behavior of stages or parts of launchers as a function of fundamental parameters, such as Mach number and incidence angle during the free-fall phase, is presented. The analysis is performed considering a cylindrical body as a representative model of a generic launcher fragment. Initially, a static analysis is conducted to determine the average equilibrium angle achieved for various positions of the center of gravity (c.g.). In the supersonic regime, because the center of pressure (c.p.) is at the center of the cylinder, the equilibrium position is 180 or 0 deg, excluding the condition in which the c.g. is in a tiny region around the cylinder center. In the subsonic and transonic regimes, the c.p. moves forward, approaching one-fourth of the chord, and the equilibrium position is related to the c.g. location. Then, a dynamic analysis is conducted to determine the dynamic stability of the cylinder at different equilibrium positions and oscillation amplitudes. It is found that the fragment is stable in all the aerodynamic regimes when the c.g. is far from the center of the body. In cases characterized by a c.g. location close to the center, a stable condition is observed only in the supersonic regime.

Design, analysis and method of foot trajectory generation for quadruped robots in swing-stance phase

PurposeLegged walking robots have numerous advantages over the wheel or tracked robots due to their strong operational ability and exposure to the complex environment. This paper aims to present details about the mechanical formation and a new conceptual elliptical trajectory generation discussed throughout the paper of the quadruped robot.Design/methodology/approachInitially, a realistic CAD model of the four-legged robot is developed in Solidwork-2019. The proposed model’s forward and inverse kinematics equations are deduced using Denavit–Hartenberg parameters. Based on geometry and kinematics, manipulability and obstacle avoidance are investigated. A method of galloping trajectory is proposed for aiming the increase of upright direction impulse, which is produced by ground reaction force at each step frequency. Furthermore, the locomotion equation of the ellipse trajectory is derived by setting transition angle polynomial of free-fall phase, stance phase and swing phase and the constraints.FindingsFinally, a successive simulation on a 2D sagittal plane is performed to check and verify the usefulness of the proposed trajectory. Before the development of the full quadruped, a single prototype leg is generated for experimental verification of the dynamic simulations.Originality/valueThe proposed trajectory is novel in that it uses force tracking control, which is intended to improve the quadruped robot’s robustness and stability.

Learning the High Bar Longswing:I. Task Dynamics and Emergence of the Coordination Pattern

ABSTRACT We studied novice gymnasts (N = 25) learning to form the longswing movement coordination pattern. The focus was the emerging behavioural organisation of centre of mass (CM) dynamics and the relative phase of the bar–CM angular motion. Seven novices learned a complete longswing by the end of the study, 8 novices showed no improvement in proportion of circle completed, and the remainder produced modest but persistent increments of final swing height without achieving a full circle. The radial angular velocity generated in the free fall phase and the circle location of bar-CM relative phase progressively and predominantly predicted circle completion. Bar-CM relative phase produced a consistent qualitative relation within- and between-subjects characteristic of a collective variable with the bar leading the CM on the initial downward antiphase segment and the CM leading on the upward antiphase segment. The ratio of these phases was related by the last practice session to degree of circle completion. The findings showed strong individual differences in the effect of practice on the early steps of learning the movement coordination where the progressive emergence of the longswing circle is driven by exploiting the positive- and minimising the negative-influence of gravity on the bar–CM coordination dynamics (candidate collective variable).

Landing on Mars: probabilistic modeling enables quantifying the last “six minutes of terror”

The last three phases of landing of a spacecraft on Mars, the parachuting phase (PP), the retro-rocket phase (RRP) and the freefall phase (FFP), are of major importance, as far as the spacecraft's safe landing is concerned. The characteristics of these phases are addressed considering inevitable uncertainties that affect their times, altitudes and landing velocities. A probabilistic predictive model, using physically meaningful Rayleigh distribution, is developed. The general concepts are illustrated by numerical examples. It is concluded that it might be impossible to assure a low enough probability that an acceptable landing velocity is exceeded and, because of that, cocooning the spacecraft in airbags to cushion the blow might be inevitable. In any event, landing on Mars will still remain a risky business for years to come.

Acoustic Overpressure Signals from Fully Confined and Vented Chemical Explosions

ABSTRACT We analyzed acoustic overpressure signals generated by overburied underground chemical explosions conducted in hard rock in New Hampshire in 2018. The explosions had comparable yields between 62.8 and 82.6 kg trinitrotoluene equivalent and were buried at depths between 12 and 13 m. Two explosions resulted in crater formation and gas venting, whereas the remaining explosions were fully confined and did not result in ground failure. Acoustic signals from the confined explosions were produced by the ground shock near ground zero. Acoustic signals from cratered explosions represent a combination of a ground shock signal and a time-delayed high-amplitude signal generated by gas venting. The cratering and venting occurred during the free-fall phase observed on the near-source accelerograms. We argue that the main reason for the cratering in this experiment is the low-rock porosity, preventing postexplosion pressure relief in the cavity and promoting long fracture formation during the unloading phase and subsequent containment failure. The ground-shock-induced signals were modeled using the Rayleigh integral of the near-source ground acceleration. The equations of nonlinear acoustics were used to model the observed gas venting signals produced by the gas flow from the explosion cavity to the surface. By comparing the near-source signals produced by venting to theoretical signals from surface blasts we have shown that the venting signals have significantly lower peak pressures and longer signal durations compared to surface blasts of the equivalent impulse. The observed amplitudes of the acoustic signals produced by the venting are significantly higher than the ground-shock related signals expected from overburied explosions. This is important to consider because higher acoustic amplitudes may potentially lead to errors in yield estimation.

Real-time Detection of Human Falls in Progress: Machine Learning Approach

The research reported in this study investigates the use of machine learning algorithms that receive real time sensory input from three-axis accelerometers and gyroscopes strategically placed on a human body to detect a fall-in-progress early enough and prior to the impact to facilitate inflation of a body airbag. A publicly available dataset that entails 30+ subjects and 4+ fall types including fall forward, backward, left and right was employed in the study. A set of 12 attributes were defined and provided as inputs to machine learning classifiers. Two machine learning classifiers were trained and tested with the dataset through three-fold cross validation to determine the feasibility of the proposed approach. Specifically, Random Forest and Support Vector Machine classification algorithms were employed through ensemble-in-time and their performances were profiled. Both classifiers offered promising performances as they were able to detect entry into the so-called the “free fall phase” for most combinations of the test subjects and fall types considered. In most cases the algorithms were able to deliver a fall detection decision with at least 100 milliseconds remaining before the impact, which is sufficient to inflate a body airbag.

A Fast and Self-Acting Release-Caging-Mechanism for Actively Driven Drop Tower Systems

Today’s and future scientific research programs ask for high quality microgravity conditions of 10−6 g on ground combined with high repetition rates of 100 flights per day or more. Accordingly, a new type of drop tower, the GraviTower Bremen, (GTB), has been suggested and is currently under development. As a first stage of development, a GTB-Prototype (GTB-Pro) has been designed which uses an active rope drive to accelerate a slider/drag shield and an experiment therein on a vertical parabola. During the free fall phase, the experiment is decoupled from the slider by a self-acting Release-Caging-Mechanism (RCM). Our prototype will provide 2.5 s of microgravity for experiments of up to 500 kg for at least 100 times per day. In this article, the final concept of the engineering of the active rope drive and the RCM are presented in detail. Based on extensive simulations aiming at an optimization of the whole system we developed a hydraulic rope drive system with minimized vibrational amplitude and low number of eigenfrequencies. The RCM achieves a very fast (≤ 0.1 s) self-acting release of the experiment from the slider by making use of the dynamics of the hydraulic rope drive. Furthermore, passive hydraulic stop dampers in the RCM build a passive and self-acting recoupling mechanism. This system is optimized for a fast decoupling to compensate for the time limitation posed by the chosen drive technology. The simulations included a comparison of different drive technologies, physical effects like the Coriolis force, and the dynamics of the RCM system itself.

An analysis of the layered failure of coal: new insights into the flow process of outburst coal

Coal and gas outbursts are among the most destructive dynamic disasters. It is very important to study the failure process and mechanism of coal and gas outbursts in coal mines. Based on outburst cases and outburst experiments, the outburst process and failure types of coal are discussed, and the flow process of outburst coal is studied in depth. This paper concludes that the whole process of outburst generally involves four stages (preparation, trigger, development and termination), which include both time and spatial dimensions. There are two types of coal failure: shear failure and tensile failure. Under the effect of the gas pressure gradient, the coal exhibits tensile failure, and the coal ultimately fails in the form of multiple similar layers. After the layer is separated, the pressure difference between the two sides of the layer is found to be stable for a period of time, and a pressure difference coefficient is introduced to characterize the change of this pressure difference before and after the separation of the layer. Furthermore, the flow process of outburst coal is divided into a quasi-static phase, a constant acceleration phase, a freefall phase and a deceleration phase. Accordingly, a simplified model of the coal flow is established, and the model is calculated using the data of outburst experiments. Based on the layered failure of outburst coal, the flow velocity of outburst coal is obtained. The distribution features of outburst coal in both the experiments and the outburst cases are explained as well.

MEMS accelerometers for measuring dynamic penetration events in geotechnical centrifuge tests

Micro-electro mechanical system (MEMS) accelerometers are small, inexpensive sensors that have only recently been used in geotechnical centrifuge tests. This is unlike piezoelectric accelerometers, which are by comparison large and expensive but have been used extensively in geotechnical centrifuge tests over the past couple of decades. This paper examines the response of a single-axis ±500g MEMS accelerometer under both static and dynamic conditions in a centrifuge environment. The potential for MEMS accelerometers to be used to measure the depth of objects buried in soil is examined and the achievable resolution is discussed. Unlike piezoelectric accelerometers, which only measure changes in acceleration, MEMS accelerometers can measure both constant and changing accelerations. The merit of this feature is demonstrated through tests in which MEMS and piezoelectric accelerometers are embedded within a dynamically installed model anchor. The MEMS accelerometer is capable of measuring accelerations during both the free-fall phase and the soil embedment phase, whereas the piezoelectric accelerometer is only able to measure the changing accelerations that dominate during the soil embedment phase. Velocity profiles derived from numerical integration of the MEMS accelerations give mudline anchor velocities that agree with independent measurements and anchor embedment depths that agree with direct measurements.

On the operation of the chemothermal instability in primordial star-forming clouds

We investigate the operation of the chemothermal instability in primordial star-forming clouds with a suite of three-dimensional, moving-mesh simulations. In line with previous studies, we find that the gas at the centre of high-redshift minihaloes becomes chemothermally unstable as three-body reactions convert the atomic hydrogen into a fully molecular gas. The competition between the increasing rate at which the gas cools and the increasing optical depth to H2 line emission creates a characteristic dip in the cooling time over the free-fall time on a scale of 100 au. As a result, the free-fall time decreases to below the sound-crossing time, and the cloud may become gravitationally unstable and fragment on a scale of a few tens of au during the initial free-fall phase. In three of the nine haloes investigated, secondary clumps condense out of the parent cloud, which will likely collapse in their own right before they are accreted by the primary clump. In the other haloes, fragmentation at such an early stage is less likely. However, given that previous simulations have shown that the infall velocity decreases substantially once the gas becomes rotationally supported, the amount of time available for perturbations to develop may be much greater than is evident from the limited period of time simulated here.

Research of Person-Parachute System Motion in Free-Fall Phase

Firstly, the characteristics of person-parachute sys-tem (PPS) in free-fall phase are analyzed. Then, the particle motion equations of PPS are built up. The motion law of PPS in free-fall phase is found by computing. Fall synthesis velocity decreases firstly, then gradually increases, so there is a minimum value of synthesis velocity when is favorable parachute opening moment. Results and actual test show good agreement, which provides an evidence for determining parachute opening moment.

Granular column collapses down rough, inclined channels

We present experimental results for the collapse of rectangular columns of sand down rough, inclined, parallel-walled channels. Results for basal inclination θ varying between 4.2° and 25° are compared with previous results for horizontal channels. Shallow-water theory can be usefully combined with scaling relationships obtained by dimensional analysis to yield analytical functions of the maximum runout distance, the maximum deposit height and the time to reach the maximum runout. While the theory excellently predicts the maximum lengths of the deposit it generally overestimates the runout time. The inertial flows are characterized by a moving internal interface separating upper flowing and lower static regions of material. In an initial free-fall phase of collapse the deposited area (= volume per unit width) below the internal interface varies with the square-root of time, independent of the initial height of the column and channel inclination. In the subsequent, lateral spreading phase the deposition rate decreases with increasing basal inclination or with decreasing initial height. The local deposition rate at any fixed distance is a constant, dependent on the column aspect ratio, the channel inclination and the longitudinal position, but invariant with flow velocity and depth. In the lateral spreading phase, vertical velocity profile in the flowing layer take a universal form and are independent of flow depth and velocity. They can be characterized by a shear rate as a function of channel inclination and a length scale describing the fraction of the column involved in flow.

Influence of Falling Height on the Behavior of Skid- Launching Free-Fall Lifeboat in Regular Waves

This paper numerically investigated the influence of falling height on the behavior of the skid-launching free-fall lifeboat (FFLB) in regular waves. The boat has been treated as a rigid body when the differential equations of motion for the four falling phases, i.e., sliding or ramp phase, rotation phase, free-fall phase and water entry phase of the lifeboat were solved in the time domain. The hydrodynamic characteristics of the lifeboat has been studied for different falling heights such as H = 1.5m, 1.75m and 2.00m. Horizontal and vertical excursions and the rotation of the axis of the boat have been computed at different time along with its horizontal and vertical velocities. Hydrodynamic forces and accelerations at normal and axial directions have also been determined. At first the analysis has been done in still water and then in regular wave with amplitude of 0.5m and a period of 2.0 sec. In all of the cases, effects of regular wave are shown by comparing the results with those considering the falling of FFLB into calm water.

Transtibial Prosthesis Suspension Failure During Skydiving Freefall: A Case Report

ABSTRACT This report describes the unusual case of an everyday-use prosthesis suspension system failure during the freefall phase of a skydiving jump. The case individual was a 53-year-old male with a left transtibial amputation secondary to trauma. He used his everyday prosthesis, a transtibial endoskeleton with push-button, plunger-releasing, pin-locking silicon liner suction suspension and a neoprene knee suspension sleeve, for a standard recreational tandem skydive. Within seconds of exiting the plane, the suspension systems failed, resulting in the complete prosthesis floating away. Several factors may have led to suspension system failure, including an inadequate seal and material design of the knee suspension sleeve and liner, lack of auxiliary suspension mechanisms, and lack of a safety cover overlying the push-button release mechanism. This is the first report, to our knowledge, to discuss prosthetic issues specifically related to skydiving. While amputees are to be encouraged to participate in this extreme sport, special modifications to everyday components may be necessary to reduce the possibility of prosthesis failure during freefall, parachute deployment, and landing.

Static and flowing regions in granular collapses down channels

Through laboratory experiments we investigate inertial granular flows created by the instantaneous release of particulate columns into wide, rectangular channels. These flows are characterized by their unsteady motion, large changes of the free surface with time, and the propagation towards the free surface of an internal interface separating static and flowing regions. We present data for the time-dependent geometry of the internal interface and the upper, free surface for aspect ratios, a, in the range from 3 to 9.5 (where a=hi∕di is the ratio of the initial height to basal width of the column). The data were analyzed by two different approaches. First, by integrating under the entire internal interface we obtained data for the static area, AD, as a function of time for different a. Second, in order to characterize vertical deposition rates, we measured the thicknesses of the flowing region, hF(x,t), and the static region, hD(x,t), at fixed horizontal positions, x, and time, t, since the initiation of the experiment. We also determined detailed velocity profiles with depth at distances scaled to the final maximum runout distance to analyze the kinematic behavior of the flowing layer. In the initial free-fall phase, the temporal variation of the static area is independent of hi and scales as gdit. During the subsequent lateral spreading phase, AD(t) varies linearly with time and the nondimensional deposition rate (dAD∕dt)∕(gdi3)1∕2 is a linear function of a. The thickness of the interface hD(x,t) at constant x depends on a and varies linearly with time. The local deposition rate ∂hD∕∂t is not constant along the flow length. Data show that for the major part of the flow length ∂2hD∕∂t∂x is constant. In the lateral spreading phase, the velocity profiles are characteristically linear with a basal exponential region, a few grains in thickness, which separates static from moving regions. The shear rate is a constant dependent on a modified initial height h̃i as (g∕h̃i)1∕2, where h̃i is a characteristic length scale in the system describing the fraction of the granular column actually involved in the flowing region.

The Mass Spectrum of Metal‐free Stars Resulting from Photodissociation Feedback: A Scenario for the Formation of Low‐Mass Population III Stars

The initial mass function (IMF) of metal-free stars that form in the initial starburst of massive (virial temperatures >10^4K) metal-free protogalaxies is studied. In particular, we focus on the effect of H2 photodissociation by pre-existing stars on the fragmentation mass scale, presumedly determined by the Jeans mass at the end of the initial free-fall phase, i.e., at the so-called ``loitering phase,'' characterized by the local temperature minimum. Photodissociation diminishes the Jeans mass at the loitering phase, thereby reducing the fragmentation mass scale of primordial clouds. Thus, in a given cloud, far ultraviolet (FUV) radiation from the first star, which is supposedly very massive (about 10^3Msun), reduces the mass scale for subsequent fragmentation. Through a series of similar processes the IMF for metal-free stars is established. If FUV radiation exceeds a threshold level, the star-forming clumps collapse solely through atomic cooling. Correspondingly, the fragmentation scale drops discontinuously from a few time 10Msun to sub-solar scales. In compact clouds (>1.6kpc for clouds of gas mass 10^8Msun), this level of radiation field is attained, and sub-solar mass stars are formed even in a metal-free environment. Consequently, the IMF becomes bi-modal, with peaks at a few tenths of Msun and a few times 10Msun. The high-mass portion of the IMF is found to be a very steep function of the stellar mass, xi_high(m) being proportinal to m^{-5}. Therefore, the typical mass scale of metal-free stars is significantly smaller than that of the very first stars. Also we study the thermal instability in collapsing primordial prestellar clumps, and discuss why the thermal instability occuring during the three-body H2 formation does not appear to manifest itself in causing further fragmentation of such clumps.

Dynamics of retreating slabs: 2. Insights from three‐dimensional laboratory experiments

A laboratory analogue of a three‐layer linear viscous slab‐upper mantle‐lower mantle system is established in a silicone putty, honey and crystallized honey tank experiment. The same setup as in the numerical investigation (part 1) is used. We focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions. In our numerical experiments the lateral volume flux was set to zero. In interpreting the results relative to the real Earth, the base of the box is taken as the bottom of the mantle convection system, while the lateral boundaries may be associated with the presence of other nearby slabs. Dynamic force equilibrium, assessed on the basis of an analytical review of forces, is described for four different phases: (1) the subduction initiation instability, (2) the accelerating dynamic free fall phase of the slab, (3) the dynamic interaction with the 660‐km discontinuity, and (4) a final phase of steady state trench retreat. Phase 3 is an important feature not observed in the numerical experiments. This highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux. On the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux: one in which the lateral boundary can be considered open and the other class where it is “closed.” Closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab. Assuming a one‐to‐one relation between trench retreat and back arc spreading, enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations.

The Formation of the First Stars. I. The Primordial Star‐forming Cloud

To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of three-dimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ~ a few 100 K, and the density, n ~ 103-104 cm-3, characterizing the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is MJ ~ 103 M☉. The existence of these characteristic values has a robust explanation in the microphysics of H2 cooling, connected to the minimum temperature that can be reached with the H2 coolant, and to the critical density at which the transition takes place between levels being populated according to non-LTE (NLTE), and according to LTE. In all cases, the gas dissipatively settles into an irregular, central configuration that has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ~103 M☉, close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M* ≳ 100 M☉. After a gas element has undergone runaway collapse, and has reached densities in excess of 108 cm-3, a sink particle is created. This procedure allows us to follow the evolution of the overall system beyond the point where the first nonlinear region would otherwise force the calculation to a halt. These later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 104 M☉, is merging between clumps. Since the merging rate sensitively depends on the density of the gas, halos with the highest degree of central concentration are able to assemble the most massive clumps. Among these are halos with a low spin (λ ≃ 0.01), and with DM fluctuations imprinted according to a white-noise spectrum.

Freefall parachute jumping

Air resistance is essential to a safe parachute jump. Without air resistance the jumper's velocity would increase without bound. In this paper we present a simple differential equation model, which can be analysed using A-level calculus techniques, of the freefall phase of a parachute jump. Hie predictions of this model are compared with data taken from the parachuting literature. We show how the model can be modified to allow an investigation of the effect of the mass of the jumper on the distance fallen during the freefall phase