Weighing in on Microgravity?

Abstract

Perfect weightlessness is an idealized state that is practically impossible to realize in a free-falling vehicle, because of various perturbations. Therefore, one prefers to talk about microweightiness or, as commonly said, microgravity (although this being a language abuse).

In this chapter, microgravity is first introduced. The methods and means used to create microgravity, according to various free-fall trajectories, are introduced and compared to each other. Advantages of this research environment are presented and some examples are given in material sciences and fluid physics. Perturbing forces, and means of simulating microgravity based on principles other than free-fall, are addressed in the appendices. Finally, an application of microgravity research in the automotive industry is given in the last appendix.

Appendices

Appendix 10: Perturbing Forces

Perturbing forces acting on a free-falling vehicle can be divided in two groups: “external” non-gravitational forces and forces “internal” to the vehicle.

1.1 1 External Forces

Non-gravitational forces in this case are not totally compensated. One distinguishes on one hand forces caused by phenomena external to the vehicle and thwarting its free fall movement, and on the other hand, the forces induced by the vehicle or its control systems to ensure its desired trajectory and attitude. These forces are expressed in the non-inertial reference frame attached to the vehicle inertia centre.

1.1.1 1.1 External Forces Caused by External Phenomena

• Atmospheric Drag Force

For a vehicle of mass m moving at a velocity $\overrightarrow {v}$ in an atmosphere of volumetric mass density (i.e. mass per unit volume) ρ and offering a section S projected parallelly to the movement direction, the aerodynamic drag force or the resistance of the air friction reads

$$ \overrightarrow {F}_{D} = - \frac{1}{2}C_{D} .\,\rho .\,S\,.\,v^{2} .\left( {\frac{{\,\overrightarrow {v} }}{v}} \right) $$

(A10.1)

where C_D is a dimensionless coefficient whose value generally varies between 1 and 3.5, depending on the vehicle geometry and the kind of interaction between the vehicle surface and the atmospheric gas particles. The drag force direction is opposite to the velocity vector $\overrightarrow {v}$ and yields a deceleration

$$ a_{D} \, = \,\,\,\frac{1}{2}C_{D} .\,\rho .\,\frac{S}{m}\,.\,v^{2} $$

(A10.2)

to the vehicle. The main parameter varying with altitude is the volumetric mass ρ. For a space vehicle, like the NASA Space Shuttle, or the Russian Soyuz or the Chinese Shenzhou or automatic research satellite, this deceleration can vary between 10⁻⁵ and 10⁻⁶ g at 200 km altitude depending on attitude, and become less than 10⁻⁸ g at 500 km altitude, slightly above the average altitude of the International Space Station (typically 400–450 km) or of the future Chinese Space Station.

• Radiation Pressure Force

This force is due to the pressure exerted by electromagnetic radiations, or by the transfer of momentum from a photon flux on the vehicle surface. For a vehicle orbiting Earth, the main source is solar radiation inducing a force

$$ \overrightarrow {F}_{sr} \, = \,\,\frac{S\,.\,A}{c}\,.\,\overrightarrow {E} $$

(A10.3)

where S is the cross section on which radiations are projected (and that can be different form the section considered for the atmospheric drag), A a dimensionless coefficient varying between 1 and 2, depending on the surface thermo-optical properties, c light speed and $\,\overrightarrow {E}$ the total light energy flux (i.e. rate of energy transfer per unit area), whose magnitude can be correctly approximated by the solar constant (the mean solar electromagnetic radiation per unit area at a distance of 1 AU), approximately 1360 W/m² close to Earth. This force acts in the direction of the energy flux and yields an acceleration

$$ a_{sr} \, = \,\,\frac{S\,.\,A}{m\,.\,c}\,.\,E $$

(A10.4)

in the order of 10⁻⁸ g, independently from altitude.

• Earth Infrared and Albedo Radiation Pressure Forces

Radiation pressure forces due to Earth’s own radiation, mainly in the infrared domain, and to Sun radiation reemitted by Earth, called albedo and whose effect is only sensed in the illuminated part of an orbit. These forces read as in (A10.3).

Induced accelerations are smaller by an order of magnitude than that due the direct Sun radiation (A10.4). For an orbital vehicle, these external forces slowly vary with a period close to that of the orbital revolution.

1.1.2 1.2 External Forces Caused by the Vehicle Control Systems

In this second category, external forces are related to vehicle manoeuvres and their variations are characterized by much shorter durations. Generally speaking, the trajectory of a vehicle inertia centre is controlled by a propulsion system and vehicle movements around its inertia centre is controlled by the attitude control system.

In case of an atmospheric vehicle (capsule in a drop tower, aircraft, rocket) in ballistic flight, propulsion force is not completely nullified but strongly reduced and can be adjusted to balance the atmospheric friction force. Attitude control is made passively by the vehicle profile (e.g. aileron or wing tip) or by differentially adjusting the propulsion system.

For an orbital vehicle, trajectory and attitude are not generally continuously controlled but sporadically, either by gas ejection of attitude engines, or by activating reaction wheels. Induced accelerations can be very important, but generally of short duration. Usually, experiments are not planned at the same time as trajectory or attitude correction manoeuvres.

1.2 2 Internal Forces

Internal forces include inertia forces that do not compensate exactly gravity. Three types of internal forces are distinguished.

1.2.1 2.1 Forces Due to Distance from the Inertia Centre

These forces are felt by an object that is not exactly located at the vehicle inertia centre (see Appendix 8).

1.2.2 2.2 Inertia Forces Due to Movement Inside the Vehicle

These forces are caused by the movement of an object in the non-inertial reference frame attached to the vehicle inertia centre. The most frequent case is the one of a satellite rotating around its inertia centre and in which an object is moving. This movement causes two inertia forces to appear, typically centrifugal and Coriolis forces (see Appendix 3 in Chapter 1).

1.2.3 2.3 Forces Created by the Displacement of the Inertia Centre

Let us recall that it is the centre of inertia of the entire vehicle and its content that describes the free fall trajectory, either an Earth’s orbit for a space vehicle, or a parabolic ballistic trajectory for an atmospheric vehicle. The position of the inertia centre may vary in function of the variation of the mass distribution in the vehicle. This variation can be spatial (case of motion of mechanical parts) or temporal (case of fuel use). Furthermore, this variation can be slow and continuous (case of slowly moving mechanical parts) or brusque and discontinuous (case of an astronaut moving in a space station or of docking of two space vehicles).

Appendix 11: Methods of Microgravity Simulation

To the contrary of the means creating microgravity, simulation methods do not allow to really create microgravity. The simulation means allow to obtain experimental configurations in which certain aspects of phenomena can be studied in a way similar to what could be observed in microgravity but without being in weightlessness.

Therefore, these methods have important limitations that reduce their scientific interest to the investigations of some very specific cases. Results obtained by these simulation methods generally complete those obtained in real microgravity. In none of the three following configurations is microgravity really created as there is no free fall.

The first simulation method was used at the end of the 19th century by a Belgian physicist, Joseph Plateau, who gave his name to this method. The principle is simple: it consists to immerse a liquid in another immiscible liquid matrix having the same volumetric mass. By Archimedes principle, the buoyancy exerted by the liquid matrix of volumetric mass ρ₁ on a volume V of a liquid of volumetric mass ρ₂ is directed along the gravity acceleration vector and reads

$$ \overrightarrow {F}_{b} \, = \,V.\,\left( {\rho_{1} \, - \,\rho_{2} } \right)\,.\,\overrightarrow {g} $$

(A11.1)

This force becomes null for $\rho_{1} \, = \,\rho_{2}$, yielding results similar to what could be obtained in weightlessness when $g = 0$. In the Plateau configuration, the gravity force is not balanced by inertia forces but by a buoyancy force.

Only static configurations are truly well simulated with this method, e.g. configurations of static equilibrium of liquid zones. Dynamic situations of fluid movements or of thermal phenomena cannot be correctly simulated as the viscosity of liquid matrix greatly limits movement of the second liquid, and that furthermore border conditions at the two liquid interface are very different from those of free floating liquid zone in a gaseous atmosphere, due e.g. to thermal conduction at the interface.

The second simulation method is less known. It consists in balancing locally the force of gravity acting on a body by a magnetic or electrostatic force acting in the other direction. The effects of two fields, the gravitational field and a magnetic or electrostatic field, have to be locally balanced. One sees immediately the limitation of this configuration that would work only for bodies sensitive to magnetic induction or electrically charged. Furthermore, the power needed to maintain these fields is quite important and limits the size of observed configurations. Nevertheless, this method is used sometime to investigate magnetohydrodynamics problems in absence of gravity. This method is more used by Russian or eastern European researchers than by their westerner colleagues.

The third simulation method is what is called the dimensionless reduction. This method mainly applies to fluid research for which scientists use a series of dimensionless numbers describing the ratios of different forces acting on fluids. Reducing physical dimensions of an experimental liquid zone greatly diminishes effects caused by gravity in comparison to other forces acting on fluids, e.g. superficial tension force or capillarity forces. One manages to build floating liquid zone of a few millimetres size that allow to study certain phenomena. Main limitations of this method are linked to reduced sizes: firstly, they make it difficult to install precise means of observation and measurement; secondly, they reduce the field of investigation to limited ranges of values of other effects specific to fluids.

Let us also add that, for medical and physiological research on adaptation of the human body to weightlessness, researchers use two simulation techniques that allow within certain limits to recreate the effects of microgravity on the human body. It consists firstly of immobilization (or hypokinesia) in a horizontal position or slightly inclined (head down), that simulates the shift of body fluids, mainly blood, toward the upper part of the body like in weightlessness. The second technique is water immersion. As the human body is mainly made of water, buoyancy induces conditions partially similar to microgravity acting on the human body, somewhat akin to Plateau’s configuration.

These microgravity simulation methods are complementary to means used to create microgravity, allowing to study in the laboratory certain aspects of phenomena appearing in weightlessness.

Appendix 12: An Example of Application of Microgravity Research

When two phases coexist (liquid/liquid or liquid/gas), fluids move along the interface by convection caused by interfacial tension gradients due to differences of temperature (or concentration) existing in the liquid matrix. This effect, called Marangoni convection, usually small on Earth in front of the gravity induced “natural” convection, depends on the temperature (or concentration) differences and is negligible in most of cases when these differences are small. For differences sufficiently large, this effect can no longer be neglected, even on Earth. Many experiments were performed in microgravity first during Spacelab missions and now on board the International Space Station to study this Marangoni effect on different liquids (solutions, semi-conductors and melted alloys, …).

Monotectic alloys like Al–Bi (aluminium-bismuth) for example, are non-miscible in liquid phase. Heated above a certain temperature during sufficiently long time, the components of these alloys form a homogeneous melt. During cooling and solidification, components separate and form liquid inclusions of a component (Bi) in the other one (Al). Sedimentation and buoyancy yield eventually a complete phase separation on Earth.

It was thought that in absence of sedimentation in microgravity, this experiment on monotectic alloys would have allowed to keep the inclusions of a component (Bi) dispersed during solidification, allowing to obtain new dispersion alloys. However, it did not happen. Looking for the reasons of this failure, one realized that under the influence of temperature gradient during cooling in microgravity, liquid drops of inclusion (Bi) appearing in the melt are pushed by Marangoni effect toward hotter zones of the liquid matrix (Al). This mechanism of drop movements toward higher temperature areas due to Marangoni effect can be used on Earth to counteract sedimentation of inclusion drops caused by gravity.

Based on this principle, a new continuous casting process was conceived, employing steep temperature gradients to counteract sedimentation of droplet inclusions, with the objective of producing Al–Pb and Al–Bi monotectic alloys for self-lubricating bearings for the commercial market. The melt is poured into a mould which is cooled laterally, thus cooling the melt from the outside during solidification. The drops of lead or bismuth are propelled upwards and towards the centre by Marangoni flow, while at the same time sedimenting due to gravity. The net displacement of the drops is thus towards the centre. Homogeneous dispersions of lead or bismuth in aluminium alloys may thus be obtained, and these alloys are highly promising for applications in self-lubricating bearings.

Self-lubricating bearings are employed in all automotive engines, specifically crankshaft and camshaft bearings, where ball or roller bearings cannot be used because of the high forces that are exerted. Three regimes of lubrication are usually considered. In the first regime, during normal operation, the surface of the bearing is separated from the surface of the shaft by a film of lubricant. In the second regime, during the engine idling, the lubricating film is marginally thin, and direct point contacts between bearing and shaft material cause increased friction and wear. In the third regime, during the engine starting, the lubricant in the bearings has drained out, and bearing and shaft are in direct contact. In the second and third regimes, the self-lubricating properties of the bearing alloy prevent blocking and allow for operation at acceptable friction and wear. This is accomplished by imbedding soft metal particles, such as lead, bismuth or tin in a matrix which is sufficiently strong mechanically, as for example aluminium alloys. Aluminium alloys with dispersions of bismuth or lead had been identified as the ones with the most promising properties for self-lubricating bearings, but one has only succeeded a few years ago in producing dispersions of these alloys employing the above process.

Modern automotive engines operate at increasingly high compression and temperature, in order to increase fuel economy and reduce pollution. New materials need to be developed to meet the new requirements, and the new alloys are of this category. A comparison between the laboratory test results of standard bearing alloys and dispersion alloys obtained with this Marangoni transport effect, shows that friction is halved, and wear is reduced by a factor ten with the new alloys. The market for these new alloys is large. Several millions of cars are sold per year worldwide alone, and each four-cylinder engine needs at least eight bearings of this type.

This example illustrates the fact that fundamental microgravity research can lead to important practical applications in ways which could not have been foreseen originally. It shows the importance of maintaining fundamental research and to bring it close to applied research and engineering. More detailed information can be found in Refs. 35 and 36.