Heat Transfer (cont’d)

Heat Transfer (cont’d) • Radiation is the means of transferring energy (such as heat) through space. • Radiating heat transfer occurs through electromagnetic (EM) radiation. • EM radiation is in the form of waves (or particles) emitted from an energy source. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • Stefan-Boltzmann’s Law describes the heat-power transfer by radiation. • This relationship tells us that, the energy emitted by an object depends on its temperature and its basic ability to store or give off heat. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • In space, conduction allows us to move heat from one place to another in a spacecraft. • Convection works onboard the spacecraft if the molecules and moving force are provided. • But the only way we can remove heat from a spacecraft is through radiation. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • Finally, you’ll note that the efficiency of the Stefan-Boltzmann relationship depends heavily on emissivity—a surface property. Let’s look at key surface properties. • Assume we have a surface as shown in Figure 11-8, with some incoming radiation hitting the surface. The incoming radiation can be reflected, absorbed, or transmitted through the material. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • Reflected radiation is the same as reflected light from a mirror—the radiation bounces off the surface. • Reflectivity is the percent of incoming radiation that is reflected off a material. • It could range from zero to 100 percent depending on the material. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • Absorbed radiation is energy a surface captures, just as a sponge soaks up water. • Absorbed radiation eventually causes the surface temperature to rise. • Absortivity is the percent of incoming radiation that is absorbed by a material. • It could range from zero to 100 percent depending on the material. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d) • Transmitted radiation is energy that passes right through the material, like light passing through a piece of glass. • Transmissivity is the percent of incoming radiation that passes through a material. • It could range from zero to 100 percent depending on the material. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Emissivity • Emissivity is a material’s ability to emit heat. • An object with a temperature above 0 Kelvin emits EM radiation per the Stephan-Boltzmann Law. • The greater the emissivity of a material, the more energy it emits at a given temperature. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Methods for Spacecraft Thermal Control • Methods for spacecraft thermal control divide into passive (open-loop) and active (closed-loop) control techniques. • We also can think about external and internal thermal-control techniques. • The Upper Atmosphere Research Satellite (UARS) in Figure 11-10 shows MLI used for thermal control. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Methods for Spacecraft Thermal Control (cont’d) • Passive thermal control manages the spacecraft’s temperature by carefully designing the entire system to regulate heat input and output and creating convenient heat-conduction paths. • Active thermal control manages the spacecraft’s temperature by employing working fluids, heaters, pumps, and other devices to move and eject heat. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

External Thermal Control • The spacecraft’s external surface is the first place to start controlling its temperature. • By selecting the right surface coatings, we can limit the amount of heat coming in and manage the amount of heat emitted. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

External Thermal Control (cont’d) • When we go out on a cold winter day, we often wear dark clothes that absorb heat. • On the other hand, when we go out on a very hot summer day, we may wear light-colored clothing to reflect much of the heat away. • We do the same thing for spacecraft: select the proper color of coatings with the right absorptivity, reflectivity, and emissivity. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

External Thermal Control (cont’d) • Instead of actually painting a spacecraft’s surface, we may cover it with materials that have the proper absorptivity, reflectivity, and emissivity. These materials are called multi-layer insulation (MLI). • We also may rotate the spacecraft, sometimes referred to as the barbeque mode, to keep the surface temperature as even as possible. • We are trying to control the amount of heat coming in and going out of the spacecraft. But so far, we’ve discussed mainly those items that control heat entering the spacecraft. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

External Thermal Control (cont’d) • A flash evaporator is a device used to remove excess heat. • Radiators are the main thing we use to rid a spacecraft of heat. • Radiators are like “heat windows” that allow hot components on the inside of a spacecraft to radiate their heat into the cold of space. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

External Thermal Control (cont’d) • In Figure 11-11 you can see two of the radiator panels on the inside of the Space Shuttle’s payload bay doors. • These radiators have the right properties to emit the necessary heat. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Internal Thermal Control • On the inside of the spacecraft, we try to control the amount of heat available in different places. • Electronics and people need to be at room temperature. • Batteries need to be cooler than room temperature to be most efficient. • Propellants need to be warmer than room temperature in some cases. • Some devices need to be very cold. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Internal Thermal Control (cont’d) • So the challenge inside the spacecraft is to provide the right amount of heat to certain locations. • We can do this using conduction and convection. • A spacecraft’s metal structure provides good heat-conduction paths throughout. • Sometimes, the metal structural members can’t move enough heat around, so we have to go with some more efficient means of heat transfer. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Internal Thermal Control (cont’d) • Heat pipes, such as those in Figure 11-12, are closed tubes filled with a working fluid (like ammonia). • When one end of the pipe heats, the fluid absorbs this heat and vaporizes. • Gas pressure forces the heated vapor to the cold end of the pipe where the heat passes out of the pipe by conduction. • As the vapor loses its heat, it re-condenses as a liquid. Then it flows back to the hot end through a wick. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Latent Heat of Vaporization • Latent Heat of vaporization is the principle of storing extra heat in a liquid as it vaporizes. • Figure 11-13 shows a graph of energy input versus temperature for water. • As the fluid in a heat pipe vaporizes it absorbs a large amount of the heat. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Latent Heat of Fusion • Latent heat of fusion is the same basic idea as latent heat of vaporization, but uses melting instead of boiling. Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Internal Thermal Control • Some spacecraft, such as the Defense Support Program Satellite, have very sensitive infrared sensors onboard that have to be kept near –208° Celsius to work properly. DSP Spacecraft Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Internal Thermal Control (cont’d) • This poses a significant challenge for thermal control. • The spacecraft must carry large refrigerators to keep the detectors cool. Cryo-cooler Unit 3, Chapter 11, Lesson 11: Environmental Control and Life-support Subsystem

Heat Transfer (cont’d)