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SYSTEM DESIGN This chapter is organized into discussions of two subjects: user design requirements, and manned system design issues. For each subject, the recommendations of the Panel are summarized and illustrated. The results emphasize that the human role in space may be the most important system design issue affecting the development of a space station, and that an appropriate first step in the evolution of humans working in space should be the development of a "space service station" capability. USER DESIGN REQUIREMENTS Orbit Considerations A space station's orbit should be determined by user requirements and space transportation economics. Users performing onboard materials processing should have no strong orbit preference. Users performing earth sensing require orbits that meet ground coverage, ground track cycle, and local sun angle requirements. Users employing the space station for launch and retrieval of free-flying spacecrafts' will desire an orbit that closely matches their spacecraft's final altitude, orbit inclination, and ascending node location. The space station's orbit, on the other hand, will be constrained, since it must be compatible with resupply by the Space Shuttle. The approach to orbit selection should begin with a mission model that considers all potential users, and then trades off space station orbit and transfer stage performance in order to capture the maximum number of missions with a single (or few) space station(s). With this type of traffic analysis, a 28.5Â° inclination orbit 84
85 would be the choice for the initial space station. There is considerable traffic at higher-inclination orbits, but the traffic is distributed in a way that makes a single space station or space platform less attractive. Hence, the Panel recommends that a polar orbiting facility be part of the earth space station infrastructureâe.g. an unmanned space platform in a sun-synchronous low earth orbit. Space Station/Platform Modularity and Separability The many users of a space station require module isolation in order to enhance ownership, reduce environmental and other interactions, increase military utility, and promote international use. In addition, a modular approach can accommodate the differences between research activities and operational services. From a management standpoint, the use of dedicated modules or sections of the space station for different agencies, governments, or industries is highly desirable. Common use of general-purpose facilities by all users may seriously compound payload integration, design security, and resource allocations. The Panel recommends that the space station employ modularity and flexibility in the allocation of user space and resources. Information Systems Architecture Three principal issues that must be resolved in the design of information systems for a space station are collection and processing of payload data, management of vehicle housekeeping functions, and structuring of the communications network between the station, other space systems, and the ground. Advances in microcircuit technology, coupled with increased sophistication in sensor performance, have made onboard processing of sensor data both a feasible and an attractive alternative to transmission of large-bandwidth data streams to the ground. Moreover, information from several satellite systems will be available simultaneously, permitting data fusion and increasing data utility. The space station may be used as an intermediate data processing resource between space-borne systems and the ground. In addition, the space station processing center must provide access to both secure and nonsecure users if this capability is to serve both national defense and civil users.
86 In summary, the introduction of a space station as an intermediate data processing resource between satellites and the ground would provide the capability to reduce high data rate streams from payloads to low data rate streams relayed directly to users. It would provide the capability to support the fusion of information from several payloads. Key issues of data separation and security must be resolved in order to make a space station multiuser processing center practical. Other User Requirements Many uses of the space station involve the accommodation of science and applications payloads that have different sensitivities. Several areas demand special attention: Contamination controlâi.e., the protection of optical surfaces, detectors (particularly those cryogenically cooled), and thermal control coatingsâdemands careful system planning. Reaction control jets should produce clean, fully expanded exhaust gases with minimal impingement on sensitive areas; life support waste management should avoid overboard dumps; and all fluid connections must minimize leakage or spillage during servicing operations. Electromagnetic control (EMC) and radiofrequency interference (RFI) constitute another area where system planning will be required. Radiofrequency sensors used for microwave observation of the earth and for radio astronomy are examples of sensitive systems whose successful operation requires absence of interfering signals. But radiating experiments such as the synthetic-aperture radar can introduce interfering signals into space station systems and other payloads. Dynamic interactions between individual payloads and between payloads and the space station will require careful treatment. Various science experiments will require precision pointing and a highly stable platform. In some cases, the experiments may require greater pointing and stability capability than is practical to provide for the entire station; the accommodation can still be provided through the use of precision mounting points. The design of the space station must provide sufficiently high isolation
87 between the movements of the host vehicle appendages and the humans within the pressurized modules and those payloads requiring high pointing accuracy and stability. Power demands for support of payloads considered by NASA studies are as high as 75 kW (average). In addition, it will be necessary to deal with emergency station needs, eclipse effects, peak power demands of certain payloads, and demand conflicts. Furthermore, the power provided to users will need to be cleanâi.e., free of switching transients and high-frequency interference. MANNED SYSTEM DESIGN ISSUES The extent and nature of the human role in space is considered by the Panel to be a most fundamental system design issue affecting the development of the space station. Character of Human Presence in Space The majority of space activities to date have been conducted by remote control where, after delays of minutes or even days, decisions are made on the ground that are then implemented. However, astronauts have performed some experiments where the response was truly in real-time. Man's role in space should be considered in the context of three modalities: delayed response, telepresence,* and physical human presence. Man has unique capabilities of observation and manipulation that today's technology cannot reproduce. Although robotics and the technology of telepresence are progressing, some human capabilities will remain unique indefinitely. Although delayed response is an important mode, the primary space station choice is between physical presence and telepresence. Physical presence offers the potential advantages of timeliness, adaptability, dexterity, and autonomy; telepresence offers the potential advantages of efficiency, collaboration, and use of larger human resources. *Telepresence can be defined as the use of real-time communications, visual display, and remote control to provide an operator on the earth's surface the capability to carry out complex operations within the space station.
88 Functions for which data transmission delay is satisfactory are uncertain, but may include many human functions in space. However, the degree to which robots can perform all functions will increase only as the technology advances. Many data evaluation and target selection functions can be performed via telepresence, but considerable development will be required to produce a teleoperator with scanning and focusing eyes, facile arms, and pressure-sensitive fingers to perform complex on-orbit assembly. Technical Support for Expanding Human Capabilities in Space Man's effectiveness in space is limited by the performance of his spacesuits, tools, and maneuvering units. Without significant improvements in these areas, much of man's potential utility will be lost. The Panel recommends that technical advances in each of these areas should be an early objective and should precede establishment of large space stations housing many astronauts. CONCLUSIONS The human role in space may be the most important system design issue affecting the development of a space station. Both physical human presence in space and telepresence (i.e., the use of real-time communications to carry out complex activities) are important. The Panel recommends that the technologies for physical presence and telepresence be developed with comparable emphasis. The second Panel conclusion reflects the potential value of humans and the space station as an in-orbit service facility. Functions would include, but not be limited to, spacecraft and instrument repair and servicing, change or addition of payload instruments and spacecraft subsystems, and replenishment of expendables. The first step in the evolution of humans working in space should be the development of a "space service station" capable of tending all compatible spacecraft. As a third conclusion, the Panel recommends that NASA develop and promulgate design rules for a number of concepts. Specifically, there is need for rules concerning modular design; rules covering the introduction of new technology; and rules governing the definition of interfaces between major systems and between subsystems.
89 Finally, the Panel believes that a space station development program should make provision for technology advance or growth during the development period. Construction of a space station using off-the-shelf technology, although less costly initially, may not provide adequate capability to support requirements for onboard data processing, user needs, growth potential, or sufficient operational utility.