For spacecraft and their instruments, the engineering disciplines of mechanical and structural design work together, and both are founded on the study of the mechanics of materials. We create designs, then prove (by calculation and test) that they will work in the environments of rocket launch and flight. Mechanisms, by definition having relatively–moving parts, are not the whole of mechanical design; they are interesting enough to get a later chapter of this book (Chapter 5) to themselves. But any mechanism is itself a structure of some kind, since it sustains loads. We therefore define structures, which are more general than mechanisms, as ‘assemblies of materials which sustain loads’. All structures, whether blocks, boxes, beams, shells, frames or trusses of struts are thereby included.

Mechanical design should begin by considering the forces which load the structural parts. The twin objectives are to create a structure which is (i) strong enough not to collapse or break, and (ii) stiffly resistant against deforming too far. The importance of stiffness as a design goal will recur in this chapter. Forces may be static, or dynamic; if dynamic, changing slowly (quasi–static) or rapidly, as when due to vibration and shock. The dynamic forces are dominant in rocket flight, and vibrations are a harsh aspect both of the launch environment and of environmental testing, generating often large dynamic forces. To analyse each mechanical assembly as a structure, we shall need the concept of inertia force to represent the reaction to dynamic acceleration.

The challenge of space

The dawn of the space age in 1957 was as historic in world terms as the discovery of the Americas, the voyage of the Beagle or the first flight by the Wright brothers. Indeed, the space age contains elements of each of these events. It is an age of exploration of new places, an opportunity to acquire new knowledge and ideas and the start of a technological revolution whose future benefits can only be guessed at. For these reasons, and others, space travel has captured the public's imagination.

Unfortunately, access to the space environment is not cheap either in terms of money or in fractions of the working life of an engineer or scientist. The design of space instruments should therefore only be undertaken if the scientific or engineering need cannot be met by other means, or, as sometimes happens, if instruments in space are actually the cheapest way to proceed, despite their cost. Designing instruments, or spacecraft for that matter, to work in the space environment places exacting requirements on those involved. Three issues arise in this kind of activity which add to the difficulty, challenge and excitement of carrying out science and engineering in space. First, it is by no means straightforward to design highly sophisticated instruments to work in the very hostile physical environment experienced in orbit. Secondly, since the instruments will work remotely from the design team, the processes of design, build, test, calibrate, launch and operate, must have an extremely high probability of producing the performance required for the mission to be regarded as a success.

The background

At the end of the nineteen sixties satellites characteristically had masses in the range 150 to 300 kg. By the end of the nineteen nineties many, if not most, satellites are being designed to mass budgets between one and ten metric tonnes. Satellites have expanded to fill the launchers available would be one conclusion. In fact, many changes have taken place during the past thirty years and most of these have led to a growth in satellite masses. The scientific problems being solved from space have grown more and more exacting in terms of the equipment required. As more is discovered, seemingly, more is left to be discovered and ever more sensitive instruments are demanded. Greater sensitivity usually requires large collecting areas, cooled telescopes or massive detectors, all strong factors in determining the mass of the payload. In the fields of Astronomy and Earth Observation the scientific problems seem best tackled by ‘Observatory Class’ missions in which a payload of five or ten separate instruments is compiled to provide the varied individual measurements necessary to address the mission objectives. In some cases these instruments could be launched on separate platforms if their observations could be properly coordinated, in other cases the full set of measurements must be simultaneous in space and time. Launch vehicles which are principally designed to meet the growing needs of geostationary communications satellites are available with the lift capability of many tonnes and so there has been little pressure to identify missions which can achieve the highest quality science from small, and hence inexpensive, satellites.

One of the early motivations of space science was the opportunity for astronomy to use hitherto inaccessible wavelengths, from gamma–and X–rays to infrared, and visible–light astronomy, from orbiting telescopes, was allowed better seeing, free of atmospheric contamination. Meteorological observations and earth remote sensing required orbiting cameras and infrared radiometers. All needed applications of optics.

It will be clear from foregoing chapters that, for space instrumentation, optics should (i) have qualities of rugged mechanical design, (ii) be built of lightweight non–contaminating materials, and (iii) survive years of unattended use in orbit. This chapter offers an introductory account of materials and opto–mechanical design techniques which have been serving these ends. Optical design as such, and physics of sensors, are beyond our scope. The steady improvement of sensor and detector systems, often of great sensitivity, has fostered parallel development of computation and suppression of stray light. As in all space endeavours, pre–launch qualification testing should be carefully and thoroughly conducted. Operation and adjustment in space requires mechanisms whose life may be limited, but in-orbit repair or replacement is either impossible or very costly; hence trade–off decisions may be difficult to make.

Materials for optics

As remarked in paragraph 2.7.12, the concern with glasses and ceramics is their brittleness while exposed to the launch environment. The chart (Fig. 2.31) of fracture toughness versus strength for the diversity of materials shows optical glasses, as a class, to have high strength but low toughness.

General background

Preamble

The temperature of laboratories in which space experiments are assembled, calibrated and tested is nominally 20°C (293 K) and it is thus not surprising that in general this is a most desirable operating temperature for that same equipment in space. There is nothing unique about this temperature. It is, within a relatively small band, a typical temperature that is experienced anywhere on the Earth's surface and, as fossil records show, has remained remarkably stable over billions of years.

Interestingly an Earth satellite is in a similar thermal environment, modified of course by the presence of the Earth. It is instructive therefore to consider what each of these thermal environments are and in what subtle ways they differ.

The temperature of the Earth

Essentially the Earth's surface temperature of about 290K results from the fact that the Earth orbits the Sun, which has a luminosity of 3.9 × 1026 W, and is at a mean distance of 1 Astronomical Unit (AU) from it, that is 1.5 × 1011 m. The emitted solar power crosses the surfaces of a succession of concentric, imaginary spheres centered on the Sun. The sphere which intercepts the Earth has a radius equal to the Astronomical Unit, so it is a simple calculation to show that the energy flux density at the distance of the Earth is 1.37 kWm−2. Thus the power equivalent to a one bar electric fire is received across every square metre of the Earth's projected area.

Preamble

The selection and operation of an appropriate and efficient project management scheme is as necessary to the success of a space project as the selection of the correct electronic components or the execution of a competent thermal design. Unlike pure engineering tasks, project management concerns the engineering of complex systems, the components of which are individual human beings. These sometimes exceed their specifications and sometimes fail to meet them, but they are always different. The presentation here of a structured approach to the creation of a project management scheme does not imply that one structure will suit all projects or all individuals. Considerable effort and care is needed to ensure that the management plan is efficient, appropriate and agreeable to all parties. The task of designing a project management plan can be seen as a method of ensuring that the project meets its time and cost budgets, in the same way as the electronic and mechanical engineering described previously in the book seek to meet power and mass budgets.

Introduction

Management is a very common word in everyday life but an accurate definition of it is difficult to get agreement on amongst practitioners. One definition that can be considered is:

Although there are some unfortunate overtones in the last part of the definition, this description is a good one for endeavors like space projects which require a team of people with a mix of skills.

Scientific observations from space require instruments which can operate in the orbital environment. The skills needed to design such special instruments span many disciplines. This book aims to bring together the elements of the design process. It is, first, a manual for the newly graduated engineer or physicist involved with the design of instruments for a space project. Secondly the book is a text to support the increasing number of undergraduate and MSc courses which offer, as part of a degree in space science and technology, lecture courses in space engineering and management. To these ends, the book demands no more than the usual educational background required for such students.

Following their diverse experience, the authors outline a wide range of topics from space environment physics and system design, to mechanisms, some space optics, project management and finally small science spacecraft. Problems frequently met in design and verification are addressed. The treatment of electronics and mechanical design is based on taught courses wide enough for students with a minimum background in these subjects, but in a book of this length and cost, we have been unable to cover all aspects of spacecraft design. Hence topics such as the study of attitude control and spacecraft propulsion for inflight manœuvres, with which most instrument designers would not be directly involved, must be found elsewhere.

The authors are all associated with University groups having a long tradition of space hardware construction, and between them, they possess over a century of personal experience in this relatively young discipline.

Initial design

In the beginning

Designing electronic subsystems for space vehicles can be considered in two overlapping phases. The circuitry has to carry out the required signal processing functions but also has to be capable of overcoming the particular problems associated with the subsystem existing and operating in the environment associated with the spacecraft.

In the early stages of a design, estimates have to be made of mass, volume and power consumption to determine what is feasible, within the constraints imposed by the spacecraft. Estimates are also required for cost, time and manpower to ensure that the flight hardware can be realistically produced by the required delivery date. Thus it is important to consider as soon as possible what problems associated with the space environment are seriously going to effect these estimates compared to a ground – based design.

A long life mission will have a significant impact on costs due to the requirement for increased reliability of components and manufacturing techniques, and for the introduction of component or system redundancy.

Apart from the requirements of telemetry transmitter power and type of antenna, the orbit can have a very significant effect on cost if it is associated with a high radiation environment. This may require the use of highly specialized, radiation tolerant components which may be difficult to procure. A high radiation environment can also have a major impact on mass where the wall thickness of the structure is no longer defined by structural and electrostatic screening requirements but by its ability to absorb radiation.

Most of the simple methods described in Part III have almost no potential for practical applications, due mainly to stability problems. This fourth part of the book concerns methods at the next higher level of sophistication, here referred to as the first generation of numerical methods for computational gasdynamics. However sophisticated they may be in other ways, by definition, first-generation methods do not use flux averaging, slope averaging, or other forms of solution sensitivity, except possibly upwinding. As a result, first-generation methods experience a sharp trade-off between accuracy and stability: They can model shocks well but then experience low accuracy in smooth regions; or they can model smooth regions well but then experience poor stability near shocks in the form of spurious oscillations and overshoots. The next part of the book, Part V, describes solution-sensitive second- and third-generation numerical methods, which reduce this trade-off by doing one thing in smooth regions and another at shocks. First-generation methods prove useful in undemanding applications and, more importantly, are the basic building blocks of second- and third-generation methods.

Chapter 17 describes numerical methods for scalar conservation laws. Chapter 18 extends those methods to the Euler equations using flux vector splitting and Riemann solvers. Chapter 19 concerns solid and far-field boundary treatments, a crucial topic avoided until now by using periodic boundaries as in Chapter 15 or infinite boundaries as in Chapter 16.