Payload Requirements

As we have already mentioned in Chapter 7, certain payloads may have to be maintained within a strict temperature range to work properly, which will affect how the thermal control subsystem is designed (see Figure 7.1). To illustrate, we can use the example of a spacecraft that has a large imaging payload on board, such as an Earth-observation satellite or a space observatory. In both types of spacecraft the imaging equipment is made up of a variety of mirrors and lenses, and their job is to bring the light entering the payload to a focus in the right place so that the image can be recorded in a way similar to that done by a digital camera. To get the image in focus, the lenses, mirrors, and detectors all need to be kept at the right distance from one another. To do this, the optical components are all firmly mounted on a rigid framework, usually referred to as an optical bench, within the spacecraft. This framework needs to be a robust structure so that the imager still works after the rough ride to orbit on the launch vehicle (see Chapter 5). But another feature of importance is its thermal design. When in orbit, the spacecraft is exposed to extremes of temperature, and if unprotected from these, the optical bench framework will expand and contract in size in response to changes in temperature. From the point of view of the image quality, this small, relative movement among the mirrors, lenses, and detectors is clearly undesirable. Consequently, it is the job of the thermal control engineer to design the spacecraft so that the payload is isolated from these extremes of temperature, in order to ensure that it works. In large observatories, such as the Hubble Space Telescope (see Table 7.5 for details), this job can be challenging.

The In-Orbit Thermal Environment

To keep the temperature of the spacecraft and its components within the required range, the thermal control engineer has to consider the factors that heat the spacecraft and those that cool it. The engineer strives to achieve a balance, so that the spacecraft does not get too hot or too cold. If we focus on a spacecraft in Earth orbit, we can summarize its thermal environment in terms of heat inputs (factors that heat it) and heat outputs (factors that cool it).

Heat Inputs

The mechanisms that heat the spacecraft are shown in Figure 9.16. As we mentioned previously in Chapter 6, the major input is that due to direct solar radiation, that is, the direct electromagnetic radiation from the Sun. This amounts to about 1.4 kW of thermal power for every square meter of spacecraft area that is presented to the Sun. In addition, the spacecraft receives solar radiation that is reflected from Earth's surface. About one third of all the sunlight that falls on Earth is reflected back out into space, mostly from cloud and ocean surfaces. This is referred to as Earth albedo radiation, and this too heats the spacecraft. Another source of heat, referred to as Earth heat radiation, is the infrared radiation (see Figure 6.2) produced by Earth simply because it is a warm body. Sometimes we are very much aware of heat (infrared) radiation, such as when we sit in front of a glowing open fire and feel the heat on our faces. However, such heat radiation is produced by all objects to a greater or lesser extent depending on their temperature. This is true as long as the object's temperature is above absolute zero. On the Celsius temperature scale, absolute zero occurs at -273° and is so called because it is

Spacecraft heat radiation (output)

satellite satellite

Direct solar radiation (input)

Internal power^/"^ dissipation (input)

Internal power^/"^ dissipation (input)

Figure 9.16: A summary of the spacecraft's thermal environment, in terms of things heating it (inputs) and things cooling it (outputs).

Earth heat F f radiation (input)/ Earth albedo ____ / /radiation (input)

Figure 9.16: A summary of the spacecraft's thermal environment, in terms of things heating it (inputs) and things cooling it (outputs).

the lowest temperature that is physically possible. At -273°C all physical processes stop.

Objects such as Earth and people give off heat radiation simply because their temperature is above absolute zero. Since Earth is an object with a temperature of around 20°C (on average), it produces a heat radiation field that gently warms the spacecraft. Both of these Earth-generated effects, Earth albedo and Earth heat radiation, provide a heat input to the spacecraft that decreases with the altitude of the spacecraft's orbit in accord with an inverse square law (see Chapter 1). The final mechanism that heats the spacecraft is not an external one but rather an internal effect referred to as internal power dissipation. Typically a spacecraft is packed with electrical and electronic equipment, and most of it is not very efficient in that a significant percentage (between 10% and 50%) of the electrical power that is fed into the equipment to sustain its operation is wasted in producing heat. This is not just a feature of spacecraft components; the same thing happens in domestic electrical equipment. For example, if we leave the television set on for a couple of hours, and then just place our hand (carefully) by the air vents at the back of the set, we can feel that not all the electricity has been used to produce picture and sound. Some of it is being wastefully dissipated as heat. Internally dissipated power is another significant mechanism that acts to heat up the spacecraft.

Heat Outputs

Unless there is some way of getting rid of the heat, the spacecraft is going to become too hot, and the acceptable temperature ranges shown in Table 9.3 will be exceeded. However, as a warm body, the spacecraft itself will give off its own heat radiation, and the intensity of this radiation will increase as the spacecraft's temperature rises. This spacecraft heat radiation (see Figure 9.16) is the only effective thermal output acting to cool the spacecraft down.

Thermal Equilibrium

From the above discussion about the thermal environment, we can see that at a particular temperature, the heat outputs will match the heat inputs, and a kind of thermal equilibrium will be reached where the temperature of the spacecraft remains more or less constant. This temperature is referred to as the equilibrium temperature. It is the job of the thermal control engineer to design the thermal control subsystem to ensure that when thermal equilibrium occurs, the equilibrium temperature is around room temperature. If this can be achieved, then there is a good chance that all of the on-board components will be able to operate reliably for the lifetime of the mission.

Thermal Control Design

How does the thermal control engineer achieve this objective? In the introduction to this section we said that the thermal control subsystem is what you see, and basically this is the clue to how it is done. First we need to discuss the thermal properties of materials, that is, why some surfaces get hot in the Sun, while others stay cool. If we walk barefoot on a beach on a hot summer's day, some surfaces, such as the sand or black tarmac on the seafront promenade, get hot and scold the soles of your feet, whereas others, such as the wooden steps down to the beach, feel quite comfortable.

Along the same theme, I have a friend who used to work as a spacecraft thermal control subsystem engineer, and he had a neighbor who owned a large motor boat. But there was a problem with the boat: parts of the decking were made of stainless steel, and when exposed to the Sun on a hot weekend the temperature of the decking would rise to a level that was hazardous. My friend did a few simple calculations and estimated that the temperature of the stainless steel decking could reach temperatures in excess of 100°C! The solution was an easy one; my friend recommended that the offending parts of the deck be painted white. As we will see below, a white-painted surface is poor at absorbing the Sun's heat but good at giving off heat radiation. As a consequence the temperature of the deck went down to a comfortable room temperature, and the boatman was happy. Obviously, boat builders can learn something from spacecraft engineers!

Some of the spacecraft's surfaces are good at absorbing the Sun's heat, but not good at giving off heat (infrared) radiation when they get hot. An example of this kind of surface is aluminium; it is quite good at absorbing the Sun's heat, but not good at radiating it. An aluminium surface exposed to the Sun in Earth orbit can reach temperatures of around 300° to 400°C. On the other hand, some surfaces are poor at absorbing the Sun's heat, but are good at giving off heat radiation, such as a white-painted surface, which stays cool even when exposed to direct sunlight. If we apply a coat of white paint to our orbiting aluminium surface, its temperature will drop to around, say, 20°C. Obviously, the precise numbers here depend on the orbit, which determines how long the spacecraft is in direct Sun, and how long it is in Earth's shadow. These so-called thermal properties of the surface materials of the spacecraft are used to good effect by the thermal control subsystem engineer to manage the balance between heat inputs and outputs to ensure acceptable local temperatures and an appropriate overall equilibrium temperature—around room temperature—for the spacecraft. This is why spacecraft look the way they do, with various types of surface material to manage this thermal balance.

In Figure 9.15 we can see a white-painted communications dish to ensure it stays cool in direct sun light. However, the main feature of the thermal design in this case is the extensive use of thermal blanket to insulate the spacecraft from the direct heating effect of solar radiation. This often gives the spacecraft its characteristic appearance of being wrapped in gold or silver foil. The blanket, sometimes referred to as multilayered insulation (MLI), is composed of multiple layers of a thin plastic film with a metallic coating of aluminium, silver, or gold. Each individual layer is similar to the survival blankets handed out at the end of marathons to keep the runners warm. Figure 9.17 shows a small section of MLI, which is made up of around 25 such layers. Each layer is perforated with tiny holes, to ensure that the air trapped in the blanket on the ground can escape easily when the spacecraft reaches the vacuum of orbit. Each layer of the MLI is separated from the next by a thin sheet of nylon bridal veil. In orbit, with a vacuum between each separate layer, the effectiveness of the insulation provided by the blanket is maximized.

However, if we were to wrap the spacecraft completely in multilayered insulation, the heat dissipated by the electrical equipment on board would not be able to escape easily, and the interior of the spacecraft would become hot. So sections of MLI in Figure 9.15 are cut away, and instead radiator surfaces are installed, which are usually mirror-like surfaces that are designed to be poor absorbers of the Sun's heat but good at giving off heat (infrared) radiation. As a result they remain cool, even in direct Sun, and encourage the escape of internally dissipated heat. Usually, the electrical

Figure 9.17: A section of multilayered insulation, with one layer folded back to reveal the nylon bridal veil spacer. Each layer is perforated with small holes, spaced at approximately 1-cm intervals, to allow trapped air to escape from the layers.

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