7.5 General Arrangement and Design Drivers

The main design drivers for the thermal control system are the individual component thermal requirements and the space environment. The space environment sets the tone for the thermal control system as the environment surrounds the spacecraft. The environment dictates if the spacecraft will run warm or cold. General blanket measures for warm environments include surface coatings that reflect light, which is the main source of energy and heat in space and insulation to protect components from getting too warm. General blanket measures for cold environments are surface coatings that absorb light, insulation to reduce heat loss, and bringing heat sources into space. Regardless of the environment, there are some components that can be uniquely designed to survive at lower or higher temperature ranges that should be selected per the space environment.

The thermal control system lead can use spatial placement within the spacecraft as the first method of controlling thermal ranges for individual components. Components near the centroid of the spacecraft stay warmer and experience less fluctuation, typically the computer and batteries. Components that are less sensitive to thermal fluctuation can be placed closer to the space environment. If components do not have the flexibility to be placed elsewhere, like the payload or ADCS sensors, the TCS specialist must design aspects of the mission or targeted thermal control. Here’s how the TCS specialist can negotiate with the other subsystems to regulate temperature:

The attitude determination, control, and sensing system can orient the spacecraft to preference surface area toward the sun. Constant rotation, like a rotisserie, will evenly warm a spacecraft, whereas preferential orientation can focus heat on one side and keep the other side cool.

• The communications subsystem typically generates a lot of heat, particularly with transmitting antennas which must produce a lot of power. The antennas are not entirely efficient so some of that power turns into heat. The antenna can also be used to reject heat through the large surface area [Perellon et al.].
• The command and data handling system is nearly always on and, thus, is always emitting heat. The onboard computer can almost be used as a stand-in heater.
• Likewise, the power system components will emit heat.
• The structures and mechanism system does not generate any heat but can be used to distribute heat through conduction and reject heat to the space environment. Primary, secondary, or heck even tertiary structures can be made of different more thermally conductive, or insulative materials as long as the structural integrity under critical loads holds.

Going through the different subsystems, you can see general trends of which subsystems contribute to heat and can be used to reject heat. You can also see that these various subsystems have thermal characteristics and we cannot think of the thermal subsystem as an isolated system. The thermal system is distributed and integrated through the spacecraft. The thermal lead must manage all the subsystem component characteristics and cleverly work with or around the other subsystem designs. I’ll make an analogy to chess, where great chess players initially start playing a game in which each move steers the game toward a configuration where they can win. In the beginning, there are many different configurations that could ultimately ensue. With more and more decisive movement and steering, the end of the game converges to a single result. There is certainly a lot of intuition and broad stroke decision making at the beginning then pointed decision making towards the end to close the spacecraft thermal design.