Manufacturing in space?
Someday we may produce specialty materials in space for use here on Earth. For the moment, however, we are not certain that such processes can be profitable, or what those materials should be. We do not know exactly how to go about it, therefore a great deal of basic research must be done first.
Nevertheless, the possibilities are attractive. The United States, Western Europe, the Soviet Union, and Japan are all engaged in this research. What is there about materials processing in space (MPS) that is attracting such widespread interest? The answer is ''weightlessness.''
Materials processing often involves production of a solid from a fluid--that is, a gas or a liquid. When the temperature or composition of a fluid varies, its density also varies. Less dense portions rise through buoyancy, as hot air rises from a heated object. However buoyant rise requires not only a variation in density, but also a gravitational field. In the absence of gravity--that is, under the weightless conditions of orbital flight--buoyancy will not make the fluid move.
It is known that buoyancy-driven fluid motion can cause serious inhomogeneities in crystals grown from such a fluid. These inhomogeneities cause poor performance and yield problems in integrated circuits, lasers, infrared sensors, and other electronic devices made from such crystals. Thus, processing in space--where such buoyancy problems are unlikely to occur--may lead to more homogeneous materials, higher yields, and improved devices of various kinds.
Another aspect of buoyancy is the rise of bubbles in a liquid or the settling of heavy particles in a fluid. This will not occur in the absence of gravity. Again, this suggests that it should be possible to produce solids containing many small bubbles or particles distributed throughout their bulk and giving them new properties. It should also be easier to separate living cells for production of biological materials.
Yet another attractive possibility is the formation of glasses or solidification of melts without a container. Suitable containers do not exist for many high temperature melts. The melt may attack all possible containers or be contaminated by them. Some glasses that one would like to produce crystallize and become opaque when they contact any container during their formation.
In the absence of gravity one should be able to form a melt and allow it to cool without having to hold it in a container--it can float within the spacecraft. However some small residual forces remain in an orbiting spacecraft, so it is necessary to apply some restraining force to keep suspended melts from drifting to a wall. This can be done by an acoustic field, by an electromagnetic field, or by an electrostatic field. Acoustic and electromagnetic levitators have already been developed for the National Aeronautics and Space Administration (NASA).
There are many other aspects of weightlessness that can influence materials processing. No doubt, some have yet to be discovered. Virtually every MPS experiment on the 1973 Skylab mission produced unanticipated results, some of them still unexplained.
Some materials scientists have also proposed that the vacuum of space could be utilized for purification of metals and semiconductors and deposition of thin films.
In order to fully realize the potential of MPS, a great deal of experimental and theoretical research on Earth and in space must be done. Even if manufacturing in space is shown to be uneconomical, the results of the research will aid in production of better materials on Earth.
Several means are available to do weightless experiments on Earth. One may drop an object in a tall tower and obtain about 4 seconds without gravity. Ten to 20 seconds at 1 percent of Earth's gravity may be obtained in a research aircraft in a parabolic flight pattern--flying up, arcing over, diving, and finally pulling out of the dive. About five minutes at 0.01 percent of Earth's gravity is available during the coasting phase of a weather sounding rocket. For longer experiments, however, a laboratory in space is required.
The first manned MPS experiments were done during the Apollo 14, 16, and 17 flights to the moon. It was shown, for example, that mixtures shaken together do not settle out. This allows materials to be solidified with bobbles, particles, or fibers distributed through them like raisins in a cake.
It was also shown that liquid motion can be produced at a free liquid-gas interface if the temperature or concentration varies, even in the absence of gravity. This comes about because the surface tension of a liquid depends on temperature and composition. Regions of high surface tension ''pull'' regions of low surface tension. The underlying liquid is dragged along due to viscosity. Since the initial Apollo experiment, experimental and theoretical research sponsored by NASA and by the European Space Agency has shown the hitherto unsuspected importance of such flows for materials processing on the Earth.
Approximately two dozen materials were processed on the Skylab mission in 1973. The improved homogeneity of the semiconductors indium antimonide and germanium was demonstrated. Greater crystal perfection was obtained for the semiconductor alloy, indium gallium antimonide. An oil-water emulsion that separated in minutes on the ground was stable for 10 hours in space.
In 1975, another series of manned MPS experiments was performed during the joint US-USSR mission--the Apollo Soyuz Test Project (ASTP). Again improved properties resulted and unexpected phenomena were observed. And like the Skylab results, some of these phemomena have not yet been explained.
With the advent of the Space Shuttle Transportation System we now have the opportunity for more extensive flight experiments. In preparation, the European Space Agency has constructed the Spacelab to fly aboard the Shuttle for MPS experiments. The Europeans have invested about $1 billion of their own funds in this project. They have numerous experiments ready to fly. NASA is currently budgeted to spend about $23 million this year and next on MPS experiments, theoretical research, and flight-hardware development.
Our first MPS experiment was scheduled to fly in March on the third shuttle mission. It was designed to look at the phenomena involved in polymerization (linking together of chains of molecules) without gravity. We think this may allow us to produce tiny plastic spheres needed for calibrating electron microscopes and cell-counting devices. NASA has funded research to look at the influence of a spacecraft environment on the processing of semiconductors, infrared-sensor materials, new optical glasses, radiation-detector crystals, and so forth. These experiments will be flown over the next three years.
Meanwhile, our Soviet and Japanese competitors have taken notice of MPS. Last year, the Russians completed an extensive series of experiments, some of which were from Eastern Europe and from France. Informal contacts with Soviet scientists have revealed a keen interest, and so an expanded program is expected. The Japanese have purchased space on one Shuttle flight for MPS experiments of their own, about which they are saying nothing.
To encourage the commercialization of space in these early uncertain experiments, NASA has been authorized to negotiate joint-endeavor agreements with American industry. A corporation pays all the cost of the experiment while NASA flies it. Thus far, two companies have signed such joint-endeavor agreements. McDonnell-Douglas is planning an electrophoresis experiment for separation of living cells to be flown this summer. The GTI Corporation in California plans a furnace to produce many small materials samples for scientists to study. NASA has also reached agreements with John Deere and the International Nickel Company and with duPont to use their MPS ground-based facilities, including drop towers.