Space Power Technological, Economic and Societal Issues in Space Systems Development Volume 7 Number 1 1988 —corfox— publi/hing company
SPACE POWER Published under the auspices of the SUNSAT Energy Council EDITOR Andrew Hall Cutler, Space Studies Institute ASSOCIATE EDITORS Eleanor A. Blakely, Lawrence Berkeley Laboratory William C. Brown, Raytheon Company Gerald P. Carr, University of Texas Lucien Deschamps, Electricite de France Philip K. Chapman, Space Energetics Inc Hubert P. Davis, Raytheon Company Gerald W. Driggers, Combustion Engineering Arthur M. Dula, Houston, Texas I. V. Franklin, British Aerospace, Dynamics Group Norman E. Gary, University of California, Davis Peter E. Glaser, Arthur D. Little, Inc Richard L. Kline, Grumman Aerospace Corporation Gregg Maryniak, Space Studies Institute Johannes Ortner, Austrian Solar and Space Agency Makoto Nagatomo, Institute of Space and Astronautical Science, Tokyo Klaus Schroeder, Rockwell International Harlan J. Smith, University of Texas Andrew R. Wolff, NASA Lewis Research Centre Gordon R. Woodcock, Boeing Aerospace Company Space Power is an international journal for the presentation, discussion and analysis of advanced concepts, initial treatments and ground-breaking basic research on the technical, economic and societal aspects of large-scale, space-based solar power, space resource utilization, space manufacturing, and other areas related to the development and use of space for the long-term benefit of humanity. Papers should be of general and lasting interest and should be written so as to make them accessible to technically educated professionals who may not have worked in the specific area discussed in the paper. Editorial and opinion pieces of approximately one journal page in length will occasionally be considered if they are well argued and pertinent to the content of the journal. Submissions should represent the original work of the authors and should not have appeared elsewhere in substantially the same form. Proposals for review papers are encouraged and will be considered by the Editor on an individual basis. Editorial Correspondence: Dr Andrew Hall Cutler can be reached by telephone at (619) 284-2779, and his address is 3030 Suncrest No. 214, San Diego, CA 92116, USA. Dr Cutler should be consulted to discuss the appropriateness of a given paper or topic for publication in the journal, or to submit papers to it. Questions and suggestions about editorial policy, scope and criteria should initially be directed to him, although they may be passed on to an Associate Editor. Details concerning the preparation and submission of manuscripts can be found on the inside back cover of each issue. Business correspondence, including orders and remittances to subscriptions, advertisements, back numbers and offprints, should be addressed to the publishers: Carfax Publishing Company, P.O. Box 25, Abingdon, Oxfordshire 0X14 3UE, United Kingdom. The journal is published four times a year, in March, June, September and December. These four issues constitute one volume. An annual index and title-page is bound in the December issue. ISSN 0951-5089 © 1988, SUNSAT Energy Council
SPACE POWER Volume 7 Number 1 1988 Andrew Hall Cutler. Editorial 3 Peter E. Glaser. The Emerging Opportunities for Solar Space Power 5 Frank P. Davidson. Perspectives on SPS 13 Yoshihiro Arakawa & Kimito Yoshikawa. Laser Propulsion with a Magnetic Nozzle 17 Gregg E. Maryniak & Brian Tillotson. Design of a Solar Power Satellite for Construction from Lunar Materials 27 William C. Brown. The SPS Transmitter Designed Around the Magnetron Directional Amplifier 37 Serge Flandrois, Claude Meschi & Pierre Delhaes. Prospects of Intercalated Graphite Fibre Use for Electrical Power Transmission in Solar Power Satellites 51 Christian Belouet. Silicon Ribbon for Space Solar Cells 57 John W. Freeman, Jr. A Polar Orbit Solar Power Satellite 69 Kyoichi Kuriki & Hiroaki Obara. An Energetics Experiment on a Space Platform 75 Gerry Webb. Implications of the Soviet Space Industrialization Programme 91 Space Power Titles and Abstracts 113
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Editorial ANDREW HALL CUTLER Space Solar Power Review became Space Power in 1986, and now has a change of Editor and publisher as well as title. The scope is new and enlarged, and the Editorial Board will be reinvigorated in the coming year. All this is being done to better serve you, our reader. This is your journal. Its success is your success, its failure your failure. So take your journal to heart and help us make it a success! Help us publish quality material on all aspects of advanced space activities. Help us understand what you want to see in your journal. Help us maintain a healthy subscription base by making sure appropriate libraries and individuals subscribe! As the new Editor-in-Chief, I need your help with a number of matters. I need nominations for Associate Editors, I need nominations for peer reviewers, I need submissions, and I need to know who might need a bit of encouragement from me to write that interesting paper that you all want to read. The newly revised scope statement is published on the inside front cover of this and every issue -1 encourage you to read it whether you intend to publish here or not. This scope statement says what the journal is all about. Let me know what you think about it. Let me know whether I should amend it, or if you like it just the way it is. With so much new at the journal, we are forced to be a little fast and loose in how we put together the next few issues. Be that as it may, we intend to run a peer- reviewed journal of the highest calibre. Let me call out a few of the ways in which we will depart from the usual practices followed by dry academic journals, and use the editorial process to your best advantage: • We will encourage referees to sign their reviews so the authors can get immediate feedback on comments and can understand the comments in the context of the reviewer's background. • When the reviewers make useful comments that the authors are not able to respond to in their paper we will publish the pertinent portions of the review along with the paper rather than inordinately delaying publication to get things straight. • We reserve the right to edit papers for style in order to spare our busy authors the burden of rewriting while offering the reader a clear and concise journal. • We will solicit papers as required to maintain a balanced and high quality journal. In order to keep the journal topical and full of new ideas, we will publish comments and notes on articles which have appeared herein. If you have a particularly interesting insight into some detail of an article, or if you have been able to calculate the value of a parameter that the author did not get around to, or if you have some means of showing the results in an interesting perspective please share it with the other readers of Space Power.
We would also like to run a section of book reviews on books covering topics within the scope of the journal. I need your help to do this - you must make me aware of books which should be reviewed, and some of you must be willing to review them for me. I intend to follow a ‘voting' scheme in which I will solicit several reviews of each book in question and publish one most consistent with the consensus of the reviews and my editorial judgement. I hope that you will help me make your journal into the interesting, informative, topical and high-quality publication that it can be, and that you deserve.
The emerging opportunities for solar space power PETER E. GLASER Summary The expansion of space activities in an increasing number of countries and developing capabilities to pursue these activities are described in relation to emerging opportunities for solar space power. The evolution of a space industrial infrastructure based on the development of space stations and platforms that is opening up opportunities for space power projects is described. The need for a viable power supply to service the space infrastructure is emphasized. The space shuttle, space station, and the solar power satellite - and their relation to the development of a space infrastructure and the inherent commercial opportunities - are also described. The policy considerations to enable space mission planning and the development of systems and supporting technologies are cited. Finally, projections of space power markets in support of space activities are made, and a positive view of the achievable economic returns from space endeavours is emphasized. The bursting of terrestrial bonds and the dawn of the space age herald man's continuing evolution beyond the surface of the Earth. The first phase of space exploration reaped a rich harvest of scientific understanding and technological advancement beyond all expectations. Space activities have already enriched life on Earth by opening the doors to a huge source of new scientific knowledge about the Earth, the solar system, and the universe beyond. The scope of space activities is continuing to expand with an increasing number of countries developing capabilities to pursue these activities. It is only recently that space has been recognized as an arena for commercial activities that hold the key to future economic growth, with space activities providing the opportunity for industry to enter new markets, to develop new technologies, and to stimulate the innovation of materials and processes of value to society. No one today can accurately predict the impact of space activities over the coming decades. Nevertheless, much is known about the potential of these activities and the benefits that could be derived from them. There is a growing recognition that space offers inexhaustible energy and material resources and a unique environment. There is little doubt that the future exploitation of these resources will have a most profound effect on this civilization. Technology strongly controls the nature of industrial activities. The technical capabilities already demonstrated during the preceding three decades of space exploration have created a foundation for the evolutionary steps that are being taken to achieve the promise of the space frontier. Now that this new frontier has been opened, there is no turning back. Space activities are expected to influence the 21st century's Dr Peter E. Glaser, Vice President, Arthur D. Little, Inc., Cambridge, MA 02140, USA.
international, political, and commercial relationships as significantly as aeroplanes, electronics, computers and communications shaped the global economy of the 20th century. On Earth, an industrial infrastructure is taken for granted. The founders of new enterprises are concerned with business planning and financial affairs, but not with the existence of a transportation system to bring in raw materials and to ship products from their manufacturing facilities. Nor are they concerned with the required production equipment, the power to operate it, heat or air conditioning, light to create an atmosphere conducive to achieving high productivity by the work force, and all the other amenities and support systems taken for granted on Earth. The industrial infrastructure for space is beginning to evolve with the development of permanently manned space stations and free-flying platforms that are envisaged as evolving into ‘industrial parks' in orbit, equipped with suitable laboratories, production facilities, housing, hangars and supporting services, such as food and fuel supplies, storage, and power-generating plants. The space infrastructure will support a broad spectrum of activities to meet the requirements of scientific investigations, technology demonstration and development, and commercial endeavours. It will enable space operations to be carried out in Earth orbits, on the Moon, planets and asteroids. Growth of the space infrastructure will make it possible to engage in large-scale projects, such as the construction of solar power satellites [1] (SPS) with the appropriate mix of space crews, remotely controlled robots, and automated equipment. Space facilities that may be mannable or permanently manned in high-Earth orbits will most likely require the use of extraterrestrial resources, such as shielding materials and oxygen that can be derived from lunar materials during successive stages of the evolution of the space infrastructure and serve as a node for an integrated transportation system. Solar Space Power Development Phases A key requirement in the evolution of a space industrial infrastructure is the supply of power needed by the various facilities. Just as the industrial revolution was based on the availability of coal to fire boilers to generate steam, so solar energy can be the source of the power for industrial enterprises conducted in space. Extension of the analogy from a terrestrial to a space industrial infrastructure indicates that there will be a role for power generation in space similar to the generation of power on Earth. The opportunities to develop solar space power extend over a time frame into the middle of the next century. They include power generation for the space shuttle, free-flying platforms in low- and high-Earth orbits, mannable and manned-space stations in low- and high-Earth orbits, and the construction and assembly associated with projects such as SPSs, development of facilities in cis-lunar space at Lagrangian libration points, and resource recovery on the Moon and other solar system bodies. Space Shuttle A wide range of space shuttle missions, including Spacelab, are primarily devoted to activities associated with materials processing and science experiments. The on-board power available for these activities is about 3.5 kW, and it has to be shared among the various experiments. As several of the experiments move into the pilot plant and
small-scale production stage, power requirements will have to be increased to provide flexibility and extend on-orbit stay time. A survey of organizations that are engaged in these activities indicates that they will be increasingly constrained by power supply limitations. The power requirements in future space shuttle missions starting in 1989 are projected to be in the 10-kWe range, and they are expected to grow to the 30- and 40-kWc range by 1996. Associated with these increased power requirements is also the need to reject to space the heat generated by the power used by the various facilities. There is only a limited capability for heat rejection on the shuttle beyond the space radiators that utilize the cargo-bay doors for heat rejection purposes. These will be inadequate to meet the needs of significantly greater heat rejection requirements as power use increases. The limitations on shuttle power availability have been recognized, and the concept of a power extension package to generate up to 25 kWe has been considered, but not implemented. The space shuttle's ability both to perform various tasks in orbit requiring significant power and to reject heat is partly limited by landing weight constraints. In addition, launching a powerplant that is integral to the shuttle will increase costs, as the payload presented by the power plant would reduce the volume and mass available for other shuttle payloads. An alternative approach would be to utilize a free-flying powerplant, consisting of solar cell arrays, space radiators, storage batteries, power conditioning equipment, propulsion system and avionics, permanently placed in low-Earth orbit and capable of effecting a rendezvous with the space shuttle. Such a space powerplant (Powercraft) would dock with the space shuttle and supply industrial-type power to users and reject heat from space shuttle processes and equipment. The Powercraft could enhance the shuttle's ability to support in-orbit activities by providing: • increased availability of power to meet user requirements; • increased capability to reject heat to space; • extended mission duration by reducing the power demand on the fuel cells, and • productive use of on-board volume and mass for shuttle payloads. The Powercraft may not have to meet all the requirements imposed on man-rated systems pertaining to critical design, materials, and reliability criteria, resulting in decreased cost. Its design can be based on the technologies developed for a multimission spacecraft system that has already been demonstrated in the Solar-max and Landsat missions. The Powercraft could also supply supplemental power to free-flying platforms that would be placed in low-Earth orbits, such as the Industrial Space Facility. Space Station A permanently manned space station will permit a range of activities to be performed in a microgravity environment over an extended period of time-one that cannot be achieved with a space shuttle. In addition, a space station will provide routine, economical, and flexible access to Earth orbits by manned and robotic systems; permit routine checkout, refuelling, repairing, and upgrading of unmanned, and manned space platforms, space laboratories and various satellites; provide facilities for the assembly and construction of projects in space; and facilitate space debris removal when methods to reduce the hazards of such debris have been developed. The prerequisite for these activities will be the availability of adequate power in
space. The initial space station is projected to require about 75 kWe for its operation to meet science and commercial payload mission requirements. As the space station programme proceeds, projected power requirements will reach the 400-kWf range. At these power levels, it may not be possible to integrate the power generation system using solar arrays because of the drag in low-Earth orbit and the disturbing forces generated by the large area of solar arrays. One approach to meet the space power requirements is to separate the non- interruptible power requirements for life support and other mission-critical systems from the industrial-type power supply. An orbiting Powercraft could supply industrialtype power to the space station and co-orbiting platforms when placed in the same orbit as the space station. The Powercraft could occupy either a leading or trailing position, depending upon the magnitude of the drag exerted on the solar arrays. When the Powercraft drag exceeds the drag of the space station, a trailing position will be preferable to avoid potential collisions with a space station in case of failure of the attitude or station-keeping system. The Powercraft could be positioned as close as 500 metres from the space station to maintain its desired position. Power from this position could be transmitted to the space station either by a tether or by a beam. The tether would be designed to eliminate forces acting on the space station to a level specified by the microgravity environment in a laboratory or materials processing module. Microwave or laser beam power transmission could be employed to decouple the Powercraft completely from the space station. This would also make it possible to beam power to several coorbiting platforms where low levels of microgravity have to be maintained over extended periods. The Powercraft for the space station would provide an opportunity to demonstrate technological advances and construction and assemble techniques that would provide important information that would be applicable to the development of the SPS concept. SPS The SPS concept represents a long-term goal for the development of solar space power. The goal of the SPS is to provide an economically viable and environmentally and socially acceptable option for power generation on a scale substantial enough to meet a significant portion of future world energy demands. The realization of this goal will rely on increasing capabilities demonstrated in space power projects, such as Powercraft. As currently envisioned, the SPS would be placed in geosynchronous orbit, where solar cell arrays would convert energy from the sun directly into electricity and feed it to microwave generators forming a part of a transmitting antenna. The antenna would precisely direct a microwave beam of very low power density from the SPS to one or more receiving antennas at desired locations on Earth. At the receiving antennas, the microwave energy would be safely and efficiently reconverted into electricity and then transmitted to users. An SPS system could consist of many orbiting satellites, each beaming power to one or more receiving antennas. During the early 1970s when the SPS concept was being evolved by NASA, [2] the space technologies required were in an early stage of development. Since then, significant advances in a wide range of technologies have been achieved and are being successfully applied to expanding space activities. The resolution of issues associated with the implementation of the SPS, including electrical power demand, power network interfaces, load management, receiving antenna siting, availability of material
resources, and comparative assessments with other energy conversion methods, was already considered as part of the SPS Concept Development and Evaluation Program (CDEP), [3] and there is an existing framework for continuing these assessments. The studies supporting the CDEP examined an unprecedented variety of issues that might influence development of the SPS. An explicit objective was to involve public interest groups in discussions about the SPS so that future decisions concerning the project could be based on a broad consensus rather than on narrowly defined expert opinion. SPS designs ranging from 10-5000 MWe have been studied, indicating the wide interest in the power generation potential. One reason for the increasing confidence in the technical feasibility of the SPS is that alternative technologies have been identified for nearly all components of the system. Most studies have been concerned with the SPS reference system which was chosen during the CDEP to provide a common basis for assessments. This ‘reference system', based on assumed guidelines, was established by NASA to evaluate environmental effects, explore societal concerns, and perform comparative assessments. It is a design concept based on known technologies of the early 1970s; it does not represent a system that was expected to be actually constructed. An operational SPS would use some of the many alternative technologies that already have been identified for advanced SPS designs and would thus be quite different from the SPS reference system [4]. The SPS represents a fertile field for innovations. Few of the potentially interesting alternative technologies have been analyzed in detail. It would be premature to choose from among them because the consequences of these technologies cannot be evaluated without a vigorous system study of the impact of advanced technologies on SPS designs at the system and subsystem levels, and information obtained from projects such as Powercraft. The implicit assumption in the CDEP programme was that the SPS is a monolithic project requiring a massive commitment of funds over the next several decades. An approach can be devised for the development of the SPS that identifies the underlying generic technologies and their application to specific space projects, as shown in Fig. 1. The ‘terracing' of space projects would reduce the challenges typically associated with large-scale projects, including the control of the project, the effects of technical uncertainties, maintenance of investor confidence, reduction of environmental impacts, and the difficulties associated with termination of the project if warranted. The increasing capabilities needed for planned space projects-free-flying carriers, manned space stations, and space transportation systems of higher performance and lower cost-will contribute to the industrial infrastructure that could be the foundation for SPS development. As shown in Fig. 1, the SPS is only one potential application of space technology that could evolve from future space projects. However, projects such as the SPS are unlikely to be pursued until information from space projects at successive ‘terrace' levels can guide the evolution of the most appropriate design for the SPS. The designs that employ the most effective generic technologies can be developed, assessed, and analyzed, and the results shared with the participants in the SPS R&D programme. The assumption underlying the ‘terracing' approach is that advanced technologies will be developed in support of existing or planned national and international space projects. For example, some of the technologies that will be required for the SPS are being developed for near-term space applications, including telecommunications, remote sensing, materials processing, and space transportation. Specific technologies are being identified for future applications, such as Powercraft.
This approach will be judged successful when technical uncertainties and risks in the SPS program are greatly reduced, the industrial infrastructure is established, and substantial information is available on the technical feasibility, economic viability, and social and environmental acceptability of the SPS designs to the decision-makers. Policy Considerations It is only when long-range goals for a project, such as the SPS, are agreed upon that policies can be evolved to justify space missions and the development of systems and supporting technologies to meet both near- and long-term applications of solar space power. National and international institutions will have to evolve along with the space infrastructures so as not to retard SPS development. SPS development will occur within the context of the evolving space industrial infrastructure that will be essential for space commerce and in consonance with the political and economic interests and actions of governments and in accordance with international laws. Many policy issues raised by space activities, such as the supply of power in space or for use on Earth, will involve and affect many countries. These issues may not be resolved by only a few countries and may have to be handled in a multilateral framework to facilitate international cooperation for the peaceful uses of outer space. International cooperation leading to the development of the SPS has already been demonstrated in multilateral initiatives, such as satellite search and rescue systems, the proposal for an international satellite monitoring agency, and participation in a coordinated study of the biological, chemical and geological systems of the Earth to establish the impact of human activity on global habitability and global change. Conclusions To reap the widest possible benefits implied by the evolution of the space infrastructure, planning by the public and private sectors should ensure that the development of solar space power can proceed unhindered. Future economic benefits of solar space power can be projected in only broad outline. Assumptions about the development of
space activities have led to projections of substantial markets and potential revenues; however, even if annual revenues fail to increase as rapidly as projected, the market potential is so large that space industries could be among the fastest growing industrial activities in the next century. It is time to take a positive view of the achievable economic returns from space endeavours and to recognize the constructive and catalytic role that solar space power can play in sustaining the evolution of the space infrastructure. Strategic planning by the public and private sectors should begin now to ensure that space will play an increasingly important role in humanity's continuing evolution. As J. D. Bernal observed in his essay, The World, the Flesh, and the Devil [5], ‘We are on the point of being able to see the effects of our actions and their probable consequences in the future; we hold the future still timidly but perceive it for the first time as a function of our own action.' REFERENCES [1] Glaser, P.E. (1982) The Solar Power Satellite - Progress so Far, Interdisciplinary Science Reviews 7, pp. 14-29. [2] NASA, (1977) Lyndon B. Johnson Space Center, Solar Power Satellite Concept Evaluation, Activities Report, JSC-12973, Houston, TX 77058. [3] US Department of Energy, (1980) Program Assessment Report Statement of Findings, DOE/ER-0085, NTIS, Springfield, VA 22161, November. [4] Office of Technology Assessment, (1981) Solar Power Satellites, OTA-E-144, US Congress, Washington, DC 10510, August. [5] Bernal, J.D. (1969) The World, the Flesh and the Devil, Indiana University Press, Bloomington, Indiana.
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Perspectives on SPS FRANK P. DAVIDSON Summary The Solar Power Satellite (SPS) poses both technical and organizational challenges. Thanks to recent improvements in subsystems and their components it is becoming realistic to think in terms of a pilot project. Such a project could be designed by an international study group. In this manner, official and private agencies can cooperate with a minimum of formality, somewhat along the lines of the original, highly efficacious Channel Tunnel Study Group. Macro-engineering, generally defined as the study, preparation and management of the very largest technical enterprises that can be built at any given stage of history, may offer a perspective of some relevance to the launching of a Solar Power Satellite programme. In the very country where the concept of ‘Les Grands Travaux' first took root, it could appear that to encourage such an approach is tantamount to ‘gilding the lily'. But our problem, very precisely, is to accommodate diverse procedures and even diverse cultures of organization so that a combined effort will be able to offer, at minimum risk, the human and environmental benefits of what could be a virtually limitless and - hopefully - a benign source of power. History is replete with examples of great conceptions which took many decades, or even many centuries, to accomplish. According to Herodotus, the Pharaoh Necho, who ruled over Egypt from about 609 BC to perhaps 593 BC began to build a canal between the Nile Delta and the Isthmus of Suez. The modern Suez Canal was completed in 1869. The Panama Canal was first proposed by Alvarado, a cousin of Cortez, in the early years of the 16th century. It was nearly four centuries later, in 1913, that the canal was finally opened to traffic. Important ideas are not self-executing. If competent opinion decides that there is more than a reasonable hope that the SPS will be of substantial benefit to humanity, then the stewards of the technology and of the ‘caisses des depots' capable of underwriting it, must take the organizational problem seriously: just as there is a mingling of doctrine and experience in matters of engineering, there can be useful recourse to precedents and guidelines when new institutional agencies are to be devised and sustained. If the SPS is to be launched as a purely national undertaking, then the modalities will follow the institutional habits and procedures of the sponsoring state or its component entities. For our purposes here, I assume that there will be strong elements of international teamwork in a foreseeable SPS initiative. Surely there will be little Frank P. Davidson, Lecturer and Program Coordinator, Macro-Engineering Research Group, Massachusetts Institute of Technology, Memorial Drive, Cambridge, MA 02139, USA.
argument with the proposition that the entire world community stands in urgent need of a non-depletable, non-threatening source of electrical energy. It is also self-evident that theoretical discussion, even by learned specialists, cannot be expected to result in accurate and reliable forecasts of the behaviour of new man-machine systems. As G. K. Chesterton reminded us, in The Ballad of the White Horse, ‘There is always a thing forgotten'. Therefore, prudent people will wish to see the SPS tested by a pilot experience. And as a non-expert I can only report to you that I have been told that a meaningful pilot plant might cost in the neighborhood of $10 billion. If we can accept this figure as (temporarily) acceptable for discussion purposes, then we may take comfort from the fact that comparable sums of money have been raised for new macro-systems within the private sector: the Alaska Pipeline will serve as an example. Other systems on our own planet have been far more costly: highway networks, railroads, long-distance water transport, hydroelectric facilities and a host of other infrastructure improvements have involved the investment, from the public and/or private sector, of the equivalent of many billions of dollars. The question before us, then, is not one of magnitude but of organization, of the allocation of risks and responsibilities. SPS is not a monument. It is a new means of producing a marketable commodity, electricity. To the extent that the market for a pilot project can be identified and placed under contract, the question of finance will be facilitated. If a pilot system can serve two or three cities in different countries, their willingness to sign purchase contracts will alleviate the uncertainties of the planners and investors. With customers ‘signed up', it might be possible to argue in favour of some form of guarantee of securities issued to the public. The World Bank (The International Bank for Reconstruction and Development), for instance, has a statutory guaranty power which is seldom used. I am not a professional banker and I would not pretend that I am capable of proposing a proper financial plan. But experience tells me that a study group which seriously intends to launch a pilot project will wish to appoint very senior and competent financial advisors: the choice of banking experts is, in its way, just as important as the selection of the most competent engineers and scientists. We live in a world of ‘trivial pursuits', but there is nothing trivial in the wise identification of the resources without which engineers must remain dreamers. The idea of a ‘Global Infrastructure Fund', a sort of Marshall Plan to develop the infrastructure of countries which regard themselves as deficient in modern industrial equipment, has been put forward by Dr. Masaki Nakajima of the Mitsubishi Research Institute. The concept itself is nearly ten years old; its future cannot be predicted. But surely those who consider such matters in the corridors of power will wish to inform themselves of the potential of Solar Power Satellites for serving the needs of the entire planet. Nor is this a matter for industrialized nations alone. Sir Robert G. A. Jackson, Senior Advisor to the Secretary-General of the United Nations, recently chaired a technical meeting on the recurrent droughts of sub-Saharan Africa. What emerged was the indispensable requirement of additional sources of electrical energy, if the international community is to render realistic assistance in the long-distance transfer of water supplies from areas of abundance to areas of deficit. The pumping of water requires the availability of electricity. It is as simple as that. The world community may choose, through inaction and inertia, to spend yet another decade in calculations and forecasts. We have learned, however, that novel macro-systems involve a ‘learning curve', that, at some point, it is necessary to move from blueprints to experience. Risks can be evaluated and reduced; but this is not, and
never has been, a risk-free world. If SPS has a reasonable potential for meeting the world's increasing demand for energy and without the hazards of other systems even though they may yet prove beneficial as engineering knowledge improves and becomes more widespread, then there is a case, and a very good case too, for a prompt start on a pilot project. The learning curve is with us ‘for the duration'. But without the nuts and bolts of actual experience, how can true learning be assured? The Challenger accident of course provided sombre reminders that even admirable technical ideas are prone to error and failure. I would like, therefore, to add a plea for an approach that goes far beyond the scope of even so ambitious a programme as the SPS. When I was in college, professors were fond of quoting with approval the old adage, ‘Experts on tap, not on top'. We have now learned anew that management cannot be ‘scientific' if the manager does not have a thorough understanding of what is being managed. Industry and government, faced with the dramatic enlargement of the size of engineering enterprises in the wake of World War II, developed cadres of ‘engineer-managers' who proved, in the field, that they could handle the complex tasks implicit in the administration of vast new engineering systems. Proper training of top management is a sine qua non, or we shall all suffer the consequences. I wonder whether it is not in the deeper interest of the world community to initiate one graduate academy-or perhaps several-for the recruitment and training of engineer-managers of the highest calibre. SPS is but one of a whole host of technologies whose time has come. But we need a reliable, astutely contrived school and career path for the men and women who will guide such enterprises to safe harbours. Nor is this a matter for rich countries alone. Kathleen Murphy's statistical study, Macroproject Development in the Third World (Westview Press, Boulder, Colorado, 1982) indicated that in the single decade of the 1970s, the equivalent of a trillion dollars was invested in macro-engineering ventures costing upwards of $100 million each. As micro-technology advances, the range, impact and complexity of engineering systems must increase. Macro-engineering can improve the quality of life, but vigilance is called for to monitor the competence and character of those to whom new and powerful technical systems are entrusted. Ad Astra, yes! And inevitably, Per Ardua.
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Laser Propulsion with a Magnetic Nozzle YOSHIHIRO ARAKAWA & KIMITO YOSHIKAWA Summary In laser propulsion devices, propellant is irradiated by a high-energy density laser beam, producing an ionized high temperature, high density plasma. Laser energy is absorbed in the plasma through a process of inverse Bremsstrahlung radiation, producing an explosive expansion which can be used for propulsion. Laser propulsion can be divided into two categories: (i) continuous wave (CW) devices; and (ii) repetitively pulsed (RP) devices. This paper deals with RP devices. The question of which propellant and what type of propellant feed system to use has not yet been answered. In these experiments metal pellets were utilized as propellant and a new propellant feed system was developed for RP laser propulsion experiments. Also, in order to improve the performance of laser propulsion devices, the plasma produced must have a higher temperature and a higher density than present experimental devices allow. With this in mind, methods for protecting the combustion chamber and expansion nozzle surfaces, and obtaining a higher efficiency in the conversion of thermal to propulsive energy were investigated. It is thought that a magnetic nozzle can help to solve these problems. Using the experimental system described, fundamental experiments were performed investigating plasma expansion processes inside a magnetic nozzle. Introduction At present there are several types of advanced space propulsion systems. Electric propulsion systems [1], such as ion engines and Magnetoplasmadynamic (MPD) thrusters, produce high specific impulse and high thrust efficiency. However, they have a common drawback, as their power source must be carried onboard the space vehicle. This results in poor thrust-to-weight ratios and, because of limited power, leads to low thrust levels. By contrast, laser propulsion systems eliminate the need for a heavy onboard power source. The idea of laser propulsion, which is to use a high-power laser as a remote power source, was first proposed by Kantrowitz [2]. Instead of using energy from an onboard power source, energy is supplied by a remote laser station and is used to heat propellant to form a dense plasma, thus providing thrust [3, 4, 5, 6, 7]. Laser propulsion can be divided into two categories: (i) continuous wave (CW) devices; and (ii) repetitively pulsed (RP) devices [8]. This paper deals with RP devices. Experiments so far seem to verify that relatively high specific impulse can be achieved with good efficiency. A major uncertainty, however, is what will happen when Yoshihiro Arakawa is Associate Professor, Department of Aeronautics, University of Tokyo, Japan; and Kimito Yoshikawa is an Aerospace Engineer with Mitsubishi Heavy Industries, Tokyo, Japan. A version of this paper was presented at the Sixth ISAS Space Energy Symposium, 12-13 March 1987.
laser beams are increased in power intensity. If highly energetic plasmas are produced by laser energy, damage to a solid nozzle will be unavoidable. In this case it will be necessary to use a magnetic nozzle in the expanding plasma region to protect the wall surface of a solid nozzle. Another problem in RP laser propulsion is how efficiently propellant is utilized. Low propellant utilization efficiency not only leads to low specific impulse but also degrades overall efficiency. It is thought that the use of a pellet-shaped propellant with a magnetic nozzle can help to solve the above-mentioned problems. For this reason, we have constructed a fundamental experimental device for RP laser propulsion research and investigated plasma expansion processes in a magnetic nozzle. Experimental Apparatus A schematic of the experimental set-up is shown in Fig. 1. This set-up is composed of vacuum chamber (~1.5 mPa), propellant feed and photo-detection systems, a ruby laser with kryptocyanine Q switching, magnetic coils, and plasma diagnostic instruments. All of them are controlled by a microcomputer. The output pulse of the beam has a duration of 0.1 to 2 //sec half-width, is focused by a simple 30 cm lens and irradiates the target with a spot diameter of approximately 0.5 mm. Alignment of the target at the focal point of the lens on the optical axis was accomplished by means of a 500 mW CW He-Ne laser. The targets - spherical pellets made of copper with a diameter of 0.5 mm-are successively injected towards the focal point of the laser beam by the pellet feed system as shown in Fig. 2. Several hundred pellets are stocked in the reservoir and extracted one by one through the pinhole of a thin plate driven by a computer-controlled stepping motor. They free-fall approximately 30 cm to the focal point of the laser beam. The time
required for a pellet to reach the focal point of the laser beam is determined by using two photosensors to measure the pellet's position and velocity while falling. This is done to ensure that the magnetic nozzle and laser pulse are generated synchronously with the passage of the pellet through the focal point of the laser beam. The magnetic nozzle is an axisymmetric magnetic mirror produced by two solenoid coils as shown in Fig. 3. The magnetic field strength produced by the forward coil is higher than that produced by the rear coil. This results in a thrust production since laser-induced plasma flows downstream through the rear coil. By controlling the entire experimental system by microcomputer, comparatively high precision and flexibility in changing experimental conditions were realized. The characteristics of plasmas and expansion processes in the magnetic nozzle were investigated using Langmuir probes (this method is applicable for streaming plasma produced by laser pulses [9]). Results and Discussion Fig. 4 shows typical wave forms of the pulsed laser beam detected by a fast photodiode and ion saturation current measured by a Langmuir probe located 50 mm downstream from the focal point. The wave forms were obtained without applied field and the halfwidth of the laser pulse is about 2 //sec. As seen in this figure, the ion saturation
current has two peaks in its shape. This wave form appears frequently when a high- intensity laser pulse irradiates the pellet surface [10]. The propagation speeds of the first and second peaks, which were derived using the time-of-flight method, were estimated to be approximately 104 km/sec and 20 km/sec, respectively. This indicates the existence of two different plasma groups with different plasma generation and acceleration processes [11], However, the contribution of the first peak to thrust production is considered to be small since the amount of plasma generated in the first peak is much less than that in the second peak. For this reason, hereafter we examine the effect of the magnetic nozzle on only the second peak plasma. The effect of the magnetic nozzle configuration on the expansion process of laser- induced plasma was examined using two types of magnetic nozzles as characterized by Figs. 5 and 6. The phenomena of the plasma expansion in these magnetic nozzles are shown in Figs. 7 and 8. In both magnetic nozzles A and B, the probe output (ion saturation current) in the direction 0=90° decreases monotonically with the applied field, indicating that radial expansion is more intensely restrained towards the centre axis of the magnetic nozzle. In magnetic nozzle B, the probe output in the direction 0= 10° increases with increasing field strength. This also means that the laser-induced plasma is concentrated
on the centre axis and expands downstream along the field lines. In magnetic nozzle A, from a wave form observation, the pulse-width in the direction 0=90° increases with the applied field although the peak value decreases. Also, we often noticed oscillations in the tail of the probe output. This suggests that the laser-induced plasma expands in the magnetic nozzle in the same manner as a plasma contained in a magnetic mirror [12].
Considering the above, it seems that magnetic nozzle B is more appropriate for laser propulsion. As seen in Fig. 8, the ion saturation current becomes saturated at a field strength of 0.4 Tesla. If laser-produced ions are assumed to be singly charged and at the same temperature as the electrons, the ion Larmor radius at a field strength of 0.4 Tesla is 3 mm - comparable with, or less than, the characteristic length of the magnetic nozzle. Consequently, the application of a stronger field has little influence on the laser-induced plasma in our experiment. Characteristics of the expanding plasma in magnetic nozzle B were investigated using Langmuir probes which were set at positions 50 mm and 100 mm apart from the focal point with several different angles. Figs. 9 and 10 show the angular distribution of the velocity and the peak density of the expanding plasma, respectively. The expansion velocity is in the range of 20 to 30 km/sec and is largely insensitive both to angle and to the presence of an applied field. On the other hand, a large angular
dependence is seen in the ion saturation current distribution. The laser plasma expands in the narrow region near the centre axis even if no field is applied. This phenomenon is one of the characteristics of laser-induced plasmas. Once a laser pulse irradiates the target, a surface layer of the target is ionized to form a dense plasma which absorbs most of the laser energy. Then the pressure at the target surface
is raised. As a result the high surface pressure and laser pulse cause the plasma to move predominantly toward the laser [11], Such a sharp angular distribution is enhanced by an applied field. Expansion of the laser-induced plasma is restricted, for the most part, to the centre axis. That is, radial momentum is effectively converted into axial momentum in the magnetic nozzle. The electron temperature obtained from the probe characteristic is shown in Fig. 11. The measured electron temperature ranges from 0.5 to 2 eV and decreases slightly with applied field. This may be explained by increased conversion of thermal energy into axial energy through expansion in the magnetic nozzle. From probe measurements the axial momentum obtained in magnetic nozzle B was calculated to be about 1 /zN- sec without the applied field and increases up to 4 /zN-sec with the applied field. This result indicates that the application of the magnetic nozzle is effective for pulsed laser propulsion. Conclusion We have constructed a relatively simple and low-cost experimental set-up for laser propulsion research. In a vacuum chamber, metal pellets were injected one by one, their position and velocity were detected by photosensors, and a laser-induced plasma was generated. This system enables us to obtain high accuracy, reproducibility and flexibility under various experimental conditions. Moreover, with this device, laser- induced plasmas were seen to expand in a magnetic nozzle in which the conversion of radial to axial momentum was confirmed from probe measurements. REFERENCES [1] Jahn, R.G. (1968) Physics of Electric Propulsion (New York, McGraw-Hill). [2] Kantrowitz, A. (1972) Propulsion to orbit by ground-based lasers, Astronautics and Aeronautics, 10, pp. 74-76.
[3] Weiss, R.F., Pirri, A.N. & Kemp, N.H. (1979) Laser propulsion, Astronautics and Aeronautics, 17, pp. 50-58. [4] Jones, L.W. (1980) Laser propulsion - 1980, AIAA Paper 80-1264. [5] Nebolsine, P.E. et al., (1981) Pulsed laser propulsion, AIAA Journal, 19, pp. 127-128. [6] Gulati A. & Merkle, C.L. (1984) Absorption of electromagnetic radiation in an advanced propulsion system, Journal of Spacecraft and Rockets, 21, pp. 101-107. [7] Kuriki, K. & Kitora, Y. (1977) Momentum transfer to target from laser- produced plasma, Applied Physics Letters, 30, pp. 443-446. [8] Glumb, R.J. & Krier, H. (1984) Concepts and status of laser-supported rocket propulsion, Journal of Spacecraft and Rockets, 21, pp. 70-79. [9] Koopman, D.W. (1971) Langmuir probe and microwave measurements of the properties of streaming plasmas generated by focused laser pulses, Physics of Fluids, 14, pp. 1707-1716. [10] Valeo E.J. & Bernstein, LB. (1976) Fast-ion generation in laser-plasma interaction, Physics of Fluids, 19, pp. 1348-1353. [11] Sucov, E.W. et al. (1967) Plasma production by a high-power Q-switched laser, Physics of Fluids, 10, pp. 2035-2048. [12] Tuckfield, R.G. & Schwirzke, F. (1969) Dynamics of a laser created plasma expanding in a magnetic field. Plasma Physics, 11, pp. 11-18.
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Design of a Solar Power Satellite for Construction from Lunar Materials GREGG E. MARYNIAK & BRIAN TILLOTSON Summary Solar Power Satellites may be constructed from materials mined on the Moon and transported into free space by means of an electromagnetic catapult called a mass-driver. Both the mass-driver and the chemical processing techniques required to obtain construction materials from lunar soil have been demonstrated in the laboratory. A Solar Power Satellite has been designed for construction from lunar materials. This design requires only 1% of its mass from the Earth. Rationale for Use of Non-Terrestrial Materials The principal barrier to space operations is currently the high cost of launching material into space from the Earth's surface. One way to improve the economics of space operations is to use material already in space for space construction. The concept of using local sources of supply for construction is found throughout history. Early settlers of new lands did not carry building materials but instead brought only their tools, and used locally available resources for construction and supply. One local source of supply in space is the Moon. Lunar soil contains oxygen, silicon, iron, aluminium, magnesium, titanium and other useful materials. These materials can be launched into free space for about 1/22 the amount of energy required for Earth escape. The lack of atmosphere on the Moon also makes possible delivery of materials into space without expensive rocketry. Packets of lunar soil may be catapulted into free space using an electromagnetic launcher called a mass-driver. The stream of packets is captured near the Moon and can be processed into the feedstock for space construction and industry. Mass-drivers with accelerations approaching 1800 gravities have been demonstrated by the Space Studies Institute (SSI) in laboratories at Princeton University. In addition, the chemical processing of lunar materials has been carried out under an SSI research programme. During the late 1970s the Convair division of General Dynamics looked at the possibility of producing Solar Power Satellites (SPS) from lunar materials. Although the terms of the Convair study allowed only minor changes to an earlier Boeing design (the Earth baseline design), the study concluded that about 90% of such an SPS could be constructed from lunar materials. Gregg E. Maryniak, Executive Vice President, Space Studies Institute, Box 82, Princeton, NJ 08540, USA; Brian Tillotson, Director of Research, Space Research Associates, 22907 NE 15th Place, Redmon, WA 98053, USA.
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