John P. W. Stark, Graham G. Swinerd and Adrian R. L. Tatnall
Man has only had the ability to operate spacecraft successfully since 1957, when the Russian Sputnik I was launched into orbit. In a few decades technology has made great strides, to the extent that the Americans' manned expedition to the Moon and back is already history. In little more than four decades, unmanned explorer spacecraft have flown past all the major bodies of the solar system except for Pluto. Vehicles have landed on the Moon, Venus and Mars, and the Galileo spacecraft probe 'landed' on the gaseous 'surface' of Jupiter in 1995. A lander mission to Titan, one of Saturn's moons, is underway with the launch of the Cassini/Huygens spacecraft in October 1997. Minor bodies in the solar system have also received the attention of the mission planners. The first landing on such a body was executed by the Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, when it touched down on the Eros asteroid in February 2001. Similarly, a prime objective of the ambitious Rosetta programme is to place a lander on a cometary body in around 2012. Current manned space activity sees the ongoing construction in orbit of the International Space Station (ISS), and this represents a major step for both the technology and the politics of the space industry. The United States, Europe, Russia and Japan are all involved in this ambitious, long-term programme.
Many countries have the capability of putting spacecraft into orbit; satellites have now established a firm foothold as part of the infrastructure of society. There is every expectation that they have much more to offer in the future.
Before the twentieth century, space travel was largely a flight of fantasy. Most authors during that time failed to understand the nature of a spacecraft's motion, and this resulted in the idea of 'lighter-than-air' travel for most would-be space-farers. At the turn of the twentieth century, however, a Russian teacher, K. E. Tsiolkovsky, laid the foundation stone for rocketry by providing insight into the nature of propulsive motion. In 1903, he published a paper in the Moscow Technical Review deriving what we now term the rocket equation, or Tsiolkovsky's equation (equation 3.20). Owing to the small circulation of this journal, the results of his work were largely unknown in the West prior to the work of Hermann Oberth, which was published in 1923.
These analyses provided an understanding of propulsive requirements, but they did not provide the technology. This eventually came, following work by R. H. Goddard in America and Wernher von Braun in Germany. The Germans demonstrated their achievements with the V-2 rocket, which they used towards the end of World War II. Their rockets were the first reliable propulsive systems, and while they were not capable of placing a vehicle into orbit, they could deliver a warhead of approximately 1000 kg over a range of 300 km. It was largely the work of these same German engineers that led to the first successful flight of Sputnik 1 on 4 October 1957, closely followed by the first American satellite, Explorer 1, on 31 January 1958.
Four decades have seen major advances in space technology. It has not always been smooth, as evidenced by the major impact that the Challenger disaster had on the American space programme. Technological advances in many areas have, however, been achieved. Particularly notable are the developments in energy-conversion technologies, especially solar photovoltaics, fuel cells and batteries. Developments in heat-pipe technology have also occurred in the space arena, with ground-based application to the oil pipelines of Alaska as a spin-off. Perhaps the most notable developments in this period, however, have been in electronic computers and software. Although these have not necessarily been driven by space technology, the new capabilities that they afford have been rapidly assimilated, and they have revolutionized the flexibility of spacecraft. In some cases they have even turned a potential mission failure into a grand success, as evidenced by Voyager 2.
But the spacecraft has also presented a challenge to Man's ingenuity and understanding. Even something as fundamental as the unconstrained rotational motion of a body is now better understood as a consequence of placing a spacecraft's dynamics under close scrutiny. Man has had to devise designs for spacecraft that will withstand a hostile space environment, and he has come up with many solutions, not just one.
1.1 PAYLOADS AND MISSIONS
Payloads and missions for spacecraft are many and varied. Some have reached the stage of being economically viable, such as satellites for communications, weather and navigation purposes. Others monitor Earth for its resources, the health of its crops and pollution. Determination of the extent and nature of global warming is only possible using the global perspective provided by satellites, and the monitoring of ozone holes over both poles is of great importance to mankind. Other satellites serve the scientific community of today and perhaps the layman of tomorrow by adding to Man's knowledge of the Earth's environment, the solar system and the universe.
Each of these peaceful applications is paralleled by inevitable military ones. By means of global observations, the old 'superpowers' acquired knowledge of military activities on the surface of the planet and the deployment of aircraft. Communication satellites serve the military user, as do weather satellites. The Global Positioning System (GPS) navigational satellite constellation is now able to provide an infantryman, sailor or fighter pilot with his location to an accuracy of about a metre. These 'high ground' space technologies have become an integral part of military activity in the most recent terrestrial conflicts.
Table 1.1 presents a list of payloads/missions with an attempt at placing them into categories based upon the types of trajectory they may follow. The satellites may be categorized in a number of ways such as by orbit altitude, eccentricity or inclination.
It is important to note that the specific orbit adopted for a mission will have a strong impact on the design of the vehicle, as illustrated in the following paragraphs.
Consider geostationary (GEO) missions; these are characterized by the vehicle having a fixed position relative to the features of the Earth. The propulsive requirement to achieve such an orbit is large, and thus the 'dry mass' (exclusive of propellant) is only a modest fraction of the all-up 'wet mass' of the vehicle. With the cost per kilogram-in-orbit being as high as it currently is-of the order of $50 000 per kilogram in geostationary orbit-it usually becomes necessary to optimize the design to achieve minimum weight, and this leads to a large number of vehicle designs, each suitable only for a narrow range of payloads and missions.
Considering the communication between the vehicle and the ground, it is evident that the large distance involved means that the received power is many orders of magnitude less than the transmitted value. The vehicle is continuously visible at its ground control station, and this enables its health to be monitored continuously and reduces the need for it to be autonomous or to have a complex data handling/storage system.
Low Earth orbit (LEO) missions are altogether different. Communication with such craft is more complex as a result of the intermittent nature of ground station passes. This resulted in the development, in the early 1980s, of a new type of spacecraft-the tracking and data relay satellite system (TDRSS)-operating in GEO to provide a link between craft in LEO and a ground centre. This development was particularly important because the Shuttle in LEO required a continuous link with the ground. More generally, the proximity of LEO satellites to the ground does make them an attractive solution for the provision of mobile communications. The power can be reduced and the time delay caused by the finite speed of electromagnetic radiation does not produce the latency problems encountered using a geostationary satellite.
The power subsystem is also notably different when comparing LEO and GEO satellites. A dominant feature is the relative period spent in sunlight and eclipse in these orbits. LEO is characterized by a high fraction of the orbit being spent in eclipse, and hence a need for substantial oversizing of the solar array to meet battery-charging requirements. In GEO, on the other hand, a long time (up to 70 min) spent in eclipse at certain times of the year leads to deep discharge requirements on the battery, although the eclipse itself is only a small fraction of the total orbit period. Additional differences in the power system are also partly due to the changing solar aspect angle to the orbit plane during the course of the year. This may be offset, however, in the case of the sun-synchronous orbit (see Section 5.4 of Chapter 5), which maintains a near-constant aspect angle-this is not normally done for the benefit of the spacecraft bus designer, but rather because it enables instruments viewing the ground to make measurements at the same local time each day.
It soon becomes clear that changes of mission parameters of almost any type have potentially large effects upon the specifications for the subsystems that comprise and support a spacecraft.
1.2 A SYSTEM VIEW OF SPACECRAFT
This book is concerned with spacecraft systems. The variety of types and shapes of these systems is extremely wide. When considering spacecraft, it is convenient to subdivide them into functional elements or subsystems. But it is also important to recognize that the satellite itself is only an element within a larger system. There must be a supporting ground control system (Figure 1.1) that enables commands to be sent up to the vehicle and status and payload information to be returned to the ground. There must also be a launcher system that sets the vehicle on its way to its final orbit. Each of the elements of the overall system must interact with the other elements, and it is the job of the system designer to achieve an overall optimum in which the mission objectives are realized efficiently. It is, for example, usual for the final orbit of a geostationary satellite to be achieved by a combination of a launch vehicle and the boost motor of the satellite itself.
This starts us towards the overall process of systems engineering, which will be treated in detail in the final chapter of this book. Figure 1.1 shows the breakdown of the elements needed to form a satellite mission. Each of these may be considered to perform functions that will have functional requirements associated with them. We can thus have an overriding set of mission requirements that will arise from the objectives of the mission itself. In the process of systems engineering, we are addressing the way in which these functional requirements can best be met, in a methodical manner.
Chambers Science and Technology Dictionary provides the following very apt definition of the term 'system engineering' as used in the space field:
'A logical process of activities that transforms a set of requirements arising from a specific mission objective into a full description of a system which fulfils the objective in an optimum way. It ensures that all aspects of a project have been considered and integrated into a consistent whole.'
The 'system' in question here could comprise all the elements within both the space and the ground segments of a spacecraft project, including the interfaces between the major elements, as illustrated in Figure 1.1. Alternatively, the system approach could be applied on a more limited basis to an assembly within the space segment, such as an instrument within the payload. In the case of an instrument, the system breakdown would include antenna elements or optics and detectors as appropriate, and the instrument's mechanical and electrical subsystems.
The mission objectives are imposed on the system by the customer, or user of the data. They are statements of the aims of the mission, are qualitative in nature and should be general enough to remain virtually unchanged during the design process. It is these fundamental objectives that must be fulfilled as the design evolves.
For example, the mission objectives might be to provide secure and robust three-dimensional position and velocity determination to surface and airborne military users. The Global Positioning System (GPS) is a method adopted to meet these objectives.
An illustration of the range of methods and the subsequent requirements that can stem from mission objectives is given by the large number of different concepts that have been proposed to meet the objective of providing a worldwide mobile communication system. They range from an extension of the existing Inmarsat spacecraft system to schemes using highly eccentric and tundra orbits (see Chapter 5 for the definitions of these), to a variety of concepts based around a network of LEO satellites, such as Iridium, which was originally conceived as a constellation of 77 satellites (hence the name) and subsequently became an operational constellation of 66 satellites with additional in-orbit spares.
This example demonstrates an underlying principle of system engineering, that is, that there is never only one solution to meet the objectives. There will be a diverse range of solutions, some better and some worse, based on an objective discriminating parameter such as cost, or mass or some measure of system performance. The problem for the system engineer is to balance all these disparate assessments into a single solution.
The process that the system engineer first undertakes is to define, as a result of the mission objectives, the mission requirements. The subsequent requirements on the system and subsystems evolve from these initial objectives through the design process. This is illustrated in Figure 1.2, which shows how a hierarchy of requirements is established. In Chapter 19 this hierarchy is further explained and illustrated by considering a number of specific spacecraft in detail. At this point, however, it is important to note the double-headed arrows in Figure 1.2. These indicate the feedback and iterative nature of system engineering.
We turn now to the spacecraft system itself. This may be divided conveniently into two principal elements, the payload and the bus. It is of course the payload that is the motivation for the mission itself. In order that this may function it requires certain resources that will be provided by the bus. In particular, it is possible to identify the following functional requirements:
1. The payload must be pointed in the correct direction.
2. The payload must be operable.
Excerpted from Spacecraft Systems Engineering Copyright © 2003 by John Wiley & Sons Ltd. Excerpted by permission.
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