Variable Emissivity Materials For Spacecraft

Technology Roadmap Sections and Deliverables

• 4VEM - Variable Emissivity Materials for Spacecraft

Thermochromic variable emissivity materials (VEMs) can be used for a wide range of applications, from spacecraft radiators to windows used on Earth. For this technology roadmap, the use of VEMs for spacecraft radiators will be the focus.

Roadmap Overview

The only way that orbiting spacecraft can reject heat is through radiation. Because of this, the thermo-optical properties of spacecraft radiators are important. The thermo-optical properties of radiators, such as emissivity, determine how much heat is radiated away.

There is a way to calculate how much heat is radiated from a surface, and it is shown in the equation below. Q is heat being radiated away from a surface, A is the area of the radiating surface, σ is the Stefan-Boltzmann constant, ε is the emissivity of the surface material, and T is the temperature of the surface.

Q=A\sigma\varepsilon T^4

The emissivity of a material is directly proportional to the heat radiated away. So, when a material has high emissivity, more heat is radiated away. Most materials have a constant emissivity, but there are some materials whose emissivity can change based on environmental conditions or whether they are powered. These materials are called variable emissivity materials (VEMs).

There are active (electrochromic) and passive (thermochromic) VEMs. Electrochromic VEMs require power input to change emissivity, unlike thermochromic VEMs which change their emissivity based on their temperature. The technology of thermochromic VEMs whose emissivity is lower at low temperatures and higher at high temperatures is expected to be widely used in radiators for spacecraft, because compared to constant-emissivity radiators, it reduces spacecraft heater power requirements and temperature swings, all without power or human input. Figure 1 shows how a thermochromic VEM can help spacecraft more efficiently manage their temperature.

Figure 1: Diagram of how VEMs affect spacecraft, from [1]

This roadmap will explore how thermochromic VEMs have evolved, their Figures of Merit, and what is expected in the future.

Design Structure Matrix (DSM) Allocation

Figure 2 below shows the interactions between various technologies and the 4VEM variable emissivity technology. The x's in the matrix signify interaction.

Figure 2: DSM Matrix

VEMs are part of a larger technology tree, with the 1st level being the technology of a general spacecraft:

• 1SPC Spacecraft

The 1SPC Spacecraft technology has a variety of subsystem technologies; all of the subsystems of a spacecraft interact with each other and make the spacecraft work, and they make up the second level:

• 2STR Structures Subsystem
• 2POW Power Subsystem
• 2CDH Command and Data Handling Subsystem
• 2ADC Attitude Determination and Control Subsystem
• 2PAY Payload Subsystem
• 2PRO Propulsion Subsystem
• 2COM Communication Subsystem
• 2THE Thermal Subsystem.

VEMs are part of the 2THE thermal subsystem of a spacecraft. There are many technologies that are used for spacecraft thermal subsystems, and they are in the 3rd level:

• 3HEP Heat Pipes
• 3LOU Louvers
• 3HEA Heaters
• 3CRY Cryocoolers
• 3PCM Phase-change materials
• 3TIM Thermal interface materials
• 3RAD Radiators

VEMs are used in the 3RAD Radiators technology. The 3RAD technology can be split into two groups as well, which comprise the 4th level:

• 4CEM Constant emissivity materials used for radiators
• 4VEM Variable emissivity materials used for radiators.

So, VEMs are a small part of the whole spacecraft technology. Figure 3 below shows where the 4VEM technology is inside a tree.

Figure 3: 4VEM Technology Tree Placement

Roadmap Model using OPM

The Object-Process Diagram (OPD) and Object-Process Language (OPL) represented in Figure 4 and Figure 5, respectively, were generated by OPCloud. This model discribes the processes and Figures of Merit (FOM) involved with passive spacecraft VEMs, as well as their object interactions. It highlights the thermochromic VEMs discussed in this roadmap, which require no electrical energy to operate.

Figure 4: VEM OPD

Figure 5: VEM OPL

Figures of Merit (FOM)

Figure of Merit	Units	Description
Temperature Hysteresis (\Delta T_{hys} )	[K]	This describes how large hysteresis is for the VEM, and it is a difference of temperatures. The smaller the quantity, the better. The nominal value is 0 Kelvin (or 0 degrees Celsius).
Minimum Emissivity (\epsilon_L )	[-]	This is the minimum emissivity of a VEM. The nominal value is 0.
Maximum Emissivity (\epsilon_H )	[-]	This is the maximum emissivity of a VEM. The nominal value is 1.
Emissivity Range (\Delta\epsilon_H )	[-]	This is the difference between \epsilon_H and \epsilon_L
Lifetime Energy	[MWhr]	Reactor net electrical power output multiplied by its lifetime in hours

Sources

[1] I. Foster, "Variable Emissivity Materials for Thermal Radiators: Introduction to Characterizing Thermochromic Infrared Surfaces in Space," in AIAA SciTech 2024 Forum, 2024. https://arc-aiaa-org.libproxy.mit.edu/doi/abs/10.2514/6.2024-1295