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: 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
• 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.

The 2-SDC tree that we can extract from the DSM above shows us that Satellite Data Communication (2SDC) is part of a larger abstraction of wireless duplex communication (1WDC), and that it requires the following key enabling technologies at the system level:

• 3SCP Satellite Communication Payload
• 3SGT Satellite Gateway Terminal
• 3USM User Satellite Modem
• 3UST User Satellite Terminal

In turn these require enabling technologies at level 4, the technology subsystem level:

• 4SRA Satellite Receive Antenna
• 4STA Satellite Transmit Antenna
• 4SCE Satellite Communication Electronics
• 4SGA Satellite Gateway Aperture
• 4GTE Gateway Transmit Electronics
• 4GRE Gateway Receive Electronics
• 4UMM User Modem Modulator
• 4UMD User Modem Demodulator
• 4USA User Satellite Aperture
• 4TSC Transmit Signal Conditioners
• 4RSC Receive Signal Conditioners

Note the DSM identifies the launch vehicle interface as a critical external dependency. While this is not in the scope of this roadmap, the availability and ultimately cost ($/kg) to launch the satellite to orbit is critical to the 2SDC roadmap and its profitability. Please reference Orbital Launch Vehicle Roadmap for more on that roadmap.

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