Variable Emissivity Materials For Spacecraft
Technology Roadmap Sections and Deliverables
- 4VEM - Variable Emissivity Materials for Spacecraft
Thermochromic and electrochromic 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.
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
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