Fluids, Materials and Microgravity

Numerical Techniques and Insights into Physics

Marcello Lappa

• Publisher: Elsevier Science
• Year: 2004
• ISBN:
  9780080531731
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Each year, universities and research centres – most particularly the major space agencies such as NASA, ESA, and NASDA – devote a vast amount of time and money into the research of materials behaviour and production in microgravity. Recently, the possibility of creating special alloys, inorganic and organic crystals, as well as biological (living) tissues in this condition has been investigated.

Fluids, Materials and Microgravity provides a solid basis of established knowledge – through literature, fundamental studies, experimental methods, numerical (basic and sophisticated) techniques – as well as the latest in research advancements. Important for the prediction of material behaviour when exposed to the environment of space, this book explores the new knowledge provided by microgravity-based studies in producing unique inorganic, and organic materials on Earth (and in designing related new technological processes). A vital resource for any scientists interested in the understanding and modelling of the new important physical mechanisms disclosed by microgravity research, and in their possible effect on the production and behaviour of materials both in space and on Earth.

A vital resource for any scientists interested in the effect of microgravity on the production and behaviour of materials.

• Covers typical fluid-dynamic disturbances which can affect the behaviour and final quality of materials both in space and on Earth, and possible strategies to contain their effects
• Thorough attention is devoted to the most promising and innovative technological processes provided by microgravity experimentation
• Information is provided through application-based engineering models, as well as mathematical frameworks, to facilitate a deeper understanding of physical mechanisms

CHAPTER 1

Space research

1.1 What is microgravity?

Gravitational attraction is a fundamental property of matter that exists throughout the known universe. The presence of Earth creates a gravitational field that acts to attract objects with a force inversely proportional to the square of the distance between the center of the object and the center of Earth. When the acceleration of an object acted upon only by Earth’s gravity at the Earth’s surface is measured, it is commonly referred to as one-g (1g) or one Earth gravity. This acceleration is approximately 9.8 meters per second squared (m/s²). The weight of an object is the gravitational force exerted on it by Earth.

While the mass of an object is constant and the weight of an object is constant (ignoring differences in g at different locations on the Earth’s surface), the environment of an object may be changed in such a way that its apparent weight changes.

Although gravity is a universal force, there are times, in fact, when it is not desirable to conduct scientific research under its full influence. In these cases, scientists perform their experiments in “microgravity,” a condition in which the effects of gravity are greatly reduced, sometimes described as “weightlessness.” This description brings to mind images of astronauts and objects floating around inside an orbiting spacecraft, seemingly free of Earth’s gravitational field, but these images are misleading. The pull of Earth’s gravity actually extends far into space. To reach a point where the Earth’s gravity is reduced to one-millionth of that on Earth’s surface, one would have to be 6.37 million kilometers away from Earth (almost 17 times farther away than the Moon). Since spacecraft usually orbit only 200–450 km above Earth’s surface, there must be another explanation for the microgravity environment found aboard these vehicles (Rogers et al., 1997).

Astronauts floating in the Shuttle appear weightless, not because they have escaped Earth’s gravity, but because they are in a state of free-fall. Any object falling due only to the force of gravity (free-falling) is experiencing microgravity and appears to be weightless. For example, a scale taped to an apple falling from a tree would register zero because it is falling with the apple. A spacecraft in a circular orbit is actually in a continuous state of free-fall at an altitude and speed that cause its fall to match the curvature of the Earth (a circular orbit results when the centripetal acceleration of uniform circular motion, v²/r; v = velocity of the object, r = distance from the center of the object to the center of the Earth, is the same as that due to gravity). All objects carried by an orbiting spacecraft are also in a state of free-fall and so appear to be weightless.

As anticipated in the foregoing, the name given to such an environment is microgravity. The prefix micro- (μ) derives from the original Greek “mikros” meaning small. By this definition, a microgravity environment is one in which the apparent weight of a system is small compared to its actual weight due to gravity.

Quantitative systems of measurement, such as the metric system, commonly use micro to mean one part in a million. Using that definition, the acceleration experienced by an object in a microgravity environment would be one-millionth (10⁻⁶) of that experienced at the Earth’s surface. In practice, the microgravity environments used by scientific researchers range from about one per cent of Earth’s gravitational acceleration (aboard aircraft in parabolic flight) to better than one part in a million (for example, onboard Earth-orbiting research satellites).

1.2 Microgravity facilities and platforms

A microgravity environment provides a unique laboratory in which scientists can observe and explore physical events, phenomena, and processes that are normally masked by the effects of Earth’s gravity. For example, crystal growth from melt and solutions, capillary effects, multiphase flow, diffusive transport processes, can under normal gravity conditions be dominated by effects such as buoyancy, thermal and solutal convection and sedimentation.

Before embarking on the detailed coverage of these aspects, the following pages provide quite a broad overview of the different microgravity platforms and facilities to familiarize the reader with the microgravity environment and its peculiarities.

As discussed in Section 1.1, from a physical point of view, any object in free-fall experiences microgravity conditions, which occur when the object falls toward the Earth with an acceleration equal to that due to gravity alone.

Many factors, however, contribute to the effective experienced accelerations, i.e. the quality of the microgravity environment depends on the mechanism used to create it.

Very brief periods of microgravity can be achieved on Earth by dropping objects from tall structures. Longer periods are created through the use of airplanes, rockets, and spacecraft (see, e.g., Chen and Xiang, 1990). Research aircraft can expose experiments to a maximum of 30 s of low gravity while the aircraft approaches the top of a steep climb and begins a sharp descent. This parabolic curve is generally repeated many times during each flight, which is used primarily to perform experiments requiring short times for experimental equipment tests.

Low-cost sounding rockets, such as Space Processing Applications Rockets, also have parabolic flight paths – they ascend and then descend, rather than proceeding into orbit around the Earth. Sounding-rocket flights provide some minutes of low gravity. Although these periods of microgravity are brief, the test facilities are beneficial both for space-flight preparation and for some actual microgravity research.

The different facilities that can provide a microgravity environment for scientific research can be classified according to the time provided to perform the experiment (see Fig. 1.1).

Fig. 1.1 Microgravity facilities and related microgravity time.

1.2.1 Drop towers and tubes

These facilities are long shafts used for dropping experiments (see, e.g., Fig. 1.2). They essentially consist of a special tube, or pit, where vacuum can be realized and where a capsule with the payload is dropped. While the experiments drop, they are in free-fall, or microgravity conditions. Scientists make measurements and use video cameras to observe the experiments as they fall. The drop tubes are capable of accommodating small, uncontained material samples. Drop towers differ from drop tubes in their ability to accommodate large experimental packages, as opposed to the small uncontained material samples. A drop tower can use a drop shield, which contains the experimental package and isolates the experiment from aerodynamic drag during free-fall if air is present. In this way, it is possible to achieve few (3 or 4) seconds of microgravity by free-falling.

Fig. 1.2 Sketch of a drop tower.

1.2.2 Parabolic flights

In a parabolic flight, the aircraft is put into a suborbital trajectory that provides free-fall, or weightlessness. Each cycle begins by having the aircraft perform an aerobatic maneuver that starts from level flight, and pitches up to approximately 45° nose-high and wings level subjecting the passengers to a 2-g pull up lasting about 20 s. After that, the aircraft engines are powered back and the airplane is launched into the same parabolic trajectory that a ball would follow, providing everyone inside the airplane with around 20s of total weightlessness for experimentation purposes (Fig. 1.3). At the bottom of the parabola, the aircraft slowly pulls out of its dive and levels off for the next arc, restoring weight to the cabin. As many as 40 arcs can be flown on a typical flight so that scientists can conduct several experiments or can repeat short runs of one experiment many times. During the parabola, net accelerations drop as low as 1.5 × 10⁻² g for about 15–25s.

Figs. 1.3 (a) Aircraft used for parabolic flights, (b) sketch of suborbital parabolic trajectory.

1.2.3 Sounding rockets

Sounding rockets take their name from the nautical term “to sound,” which means to take measurements. These rockets are basically divided into two parts: a solid-fuel rocket motor and a payload. Many of the motors used in sounding-rocket programs are surplus military motors, which keep down the cost of the rocket. The payload is the section that carries the instruments to conduct the experiment and sends the data back to Earth (see, e.g., Monti et al., 1998a).

These rockets produce higher-quality microgravity conditions for longer periods than airplanes, or drop towers, and tubes. An experiment is placed on the rocket, which is launched and then allowed to free-fall back to Earth. A sounding rocket follows a parabolic arc, like the aircraft, but goes above the Earth’s atmosphere, where air drag does not disturb microgravity conditions. The typical flight profile (see Fig. 1.4a) of a sounding rocket is the following: subsequent to a launch and as the rocket motor uses up its propellants it separates from the vehicle; the payload continues into space after separation from the motor(s) and begins conducting the experiments; when the experiments are completed, the payload re-enters the atmosphere and a parachute is deployed, bringing the payload gently back to Earth; the payload is then retrieved (by retrieving the payload a considerable saving can be achieved because the payload or parts of the payload and experiments can be refurbished and flown again).

Figs. 1.4 (a) Typical flight profile of a sounding rocket, (b) NASA sounding-rocket family, (c) Texus, Maser and Maxus sounding rockets.

The main difference between a sounding rocket and an orbital launch vehicle is the velocity reached. In fact, a sounding rocket does not reach the velocity (in terms of (km/s)) needed to go into orbit, and after achieving the maximum altitude comes back to Earth.

The experiments experience several minutes of microgravity before the rocket re-enters the atmosphere. Acceleration levels are usually around 10⁻⁵ g.

Therefore, sounding rockets provide a reasonably economical means of conducting engineering tests for instruments and devices used on satellites and other spacecraft, prior to their use in more expensive activities. Also, because of their low cost and short mission lead time, they are valuable tools for undergraduate and graduate students conducting research in the microgravity environment.

NASA currently uses 15 different sounding rockets (Fig. 1.4b). The rockets come in a variety of sizes from the single-stage Super Arcas, which stands 7 ft (3 m) tall, to the four-stage Black Brant XII, which stands 65ft (20m) tall. These rockets can carry payloads of various weights to altitudes from 48 km to more than 1200 km.

Europe exploits three sounding-rocket families: Texus, Maser, and Maxus (Fig. 1.4c).

Although drop facilities, airplanes, and rockets can establish a reduced-gravity enviro...

Table of contents

• Cover image
• Title page
• Table of Contents
• Inside Front Cover
• Copyright
• Dedication
• Preface
• Acknowledgements
• Chapter 1: Space research
• Chapter 2: Fundamental concepts, mathematical models and scaling analysis for the microgravity environment
• Chapter 3: Dispersed droplets and metal alloys
• Chapter 4: Growth of semiconductors: the floating zone technique
• Chapter 5: Macromolecular crystal growth: surface kinetics and morphological studies
• Chapter 6: Macromolecular crystal growth at macroscopic length scales
• Chapter 7: The growth of biological tissues
• References
• Index