IN this and the next couple of chapters, we discuss how spacecraft are designed, and what physical factors influence (or drive) the design of the major elements that make up the vehicle. These major elements are referred to as subsystems. The process of spacecraft design is all about how these elements are designed and how they are integrated to produce a total spacecraft system capable of achieving the mission objectives and of surviving the damaging features of the space environment that we cited in the last chapter.
Orbit selection was discussed in Chapter 2, and the logic of the method used there is relevant to our discussion now. To review briefly, the process begins with the definition of the spacecraft's mission objective, the formulation of a precise statement defining the prime purpose of the spacecraft. This might be something like "the provision of high-resolution imagery of Earth with global coverage,'' for example. The next step is to choose the payload instruments or equipment required to achieve the objective; in this example it would be the cameras required to produce the images of the ground from orbit. The third step is the development of a payload operational plan: How does the payload hardware need to operate to best achieve the objective? In Chapter 2, when we discussed orbits, these requirements included where the payload needed to be physically located to maximize its effectiveness, which led naturally to the selection of an appropriate mission orbit—a near-polar low Earth orbit (LEO) in this case. However, this same logic also leads us to the requirements for the design of the subsystem elements of the spacecraft.
The payload is the most important part of the spacecraft; without it, the objectives of the mission cannot be achieved. The subsystems are there purely to support the payload in its operation. Thus the design of each subsystem is driven by what it needs to do and what resources it needs to provide to ensure that the payload does its job effectively. For example, the payload will need a certain amount of electrical power to operate, and so the
G. Swinerd, How Spacecraft Fly: Spaceflight Without Formulae, DOI: 10.1007/978-0-387-76572-3_7, © Praxis Publishing, Ltd. 2008
design of the electrical power subsystem—the size of the solar panels and batteries on board—is governed by this payload requirement. And this type of logic extends to define the design requirements for all the other subsystems as well.
We have mentioned the spacecraft's subsystems but we have not defined them and explained what they do. All spacecraft are comprised of these basic subsystem elements, and Table 7.1 lists the main ones and the functions they fulfill in supporting the payload in its operation.
Table 7.1: The main spacecraft subsystem elements and their function
Communications On-board data handling Thermal control Structure design
To fulfill the mission objective, using appropriate payload hardware (e.g., camera, telescope, communications equipment—depending upon the objective). To select the launch vehicle that will launch the spacecraft, to select the best orbit for the spacecraft to achieve the objectives of its mission, and to determine how the spacecraft will be transferred from launch pad to final orbital destination.
To achieve the spacecraft's pointing mission (e.g., to point a payload telescope at a distant galaxy, to point a solar panel to the Sun to raise electrical power, to point a communications dish at a ground station) To provide a capability to transfer the spacecraft between orbits, and to control the mission orbit (see Chapter 3) and the spacecraft attitude (see Chapter 8), using onboard rocket systems
To provide a source of electrical power to support payload and subsystem operation
To provide a communications link with the ground, to downlink payload data and telemetry, and to uplink commands to control the spacecraft To provide storage and processing of payload and other data, and to allow the exchange of data between subsystem elements
To provide an appropriate thermal environment on board, to ensure reliable operation of payload and subsystem elements
To provide structural support for all payload and subsystem hardware in all predicted environments (especially the harsh launch vehicle environment)
A few comments on Table 7.1: First, some engineers prefer not to classify the payload as a subsystem. They like to divide the spacecraft into two parts, and distinguish the payload from the spacecraft platform (or service module), the latter being the part of the vehicle containing all of the supporting subsystems. Second, mission analysis is sometimes not considered to be a subsystem, as there is no piece of hardware on board the vehicle that can be identified with this. However, I have included both payload and mission analysis in the table to reflect the structure of a typical spacecraft project design team (see next section). As we will see, all the areas in Table 7.1 are represented by design engineers in such a team, as they all have a profound influence on the overall design process. Third, telemetry is mentioned in the table in the communications subsystem section. This is essentially health-monitoring data, generated on board the spacecraft by sensors distributed around the vehicle. These sensors check the state of the spacecraft's components and issue a warning if problems occur. These data are downlinked as telemetry to the spacecraft operations room and are displayed on the operators' computer screens, so that action can be taken if trouble arises.
We can summarize the process described so far with the block diagram in Figure 7.1. Starting with the mission objective at the top and working down, we can decide what payload instruments we need, and how they are to be operated to achieve the objective. Then, once we have the payload, we can look at the resources it needs from the subsystems to operate successfully. Referring to Figure 7.1, the payload will require a particular amount of electrical power, for example, and this will lead to the design of the power subsystem (obviously, the subsystems will also need electrical power to operate, and so in this case the power subsystem design is not just dictated by the payload interface). If the payload requires pointing, such as a telescope or an Earth-observation imaging camera, then the accuracy and stability of pointing will govern the design of the attitude control subsystem. The payload will also generate data, perhaps in the form of pictures from an imaging payload. These data will either be stored on board or transferred directly to the communications subsystem for down-linking to the ground. The rate at which the payload generates the data, and the overall amount of data, will govern the design of the on-board data handling (OBDH) subsystem, which deals with these processes. The data rate generated by the payload then needs to be down-linked by the communications subsystem over large distances to a receiving ground station, and again this leads to the design requirements for the spacecraft's communications subsystem. Also, certain payloads may have to be maintained within a strict temperature range to work properly, and this will govern how the thermal control subsystem is designed.
The design of the subsystems is also influenced by other factors. In
Chapter 2 we saw how the payload operation led to the choice of mission orbit for the spacecraft, and this is indicated on the left-hand side of Figure 7.1. It is also the case that the orbit itself impacts on the design of the subsystems. For example, once the orbit is specified, the mission analysts can calculate the eclipse period for the spacecraft, which is the time that the spacecraft spends in darkness on each orbit revolution of the Earth. If the satellite is using solar panels to generate electricity during the sunlit part of the orbit, and batteries otherwise, then the design of these components of the power subsystem is greatly influenced by eclipse period of the mission orbit. Similarly, the eclipse period will dictate the amount of direct solar heating (and cooling while in darkness) the spacecraft will encounter on each orbit, which in turn will affect the thermal control subsystem design.
We saw in Chapter 5 how the harsh environment of the launch vehicle was the most influential input to the structure design of the spacecraft, and this is shown on the right-hand side of the Figure 7.1. The design method is not at all mysterious, and is basically applied common sense!
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