L2-PTC-COAT ESSENTIAL THERMAL Level 2 · hardware

Thermal Coatings

Selective Thermal Surface Coatings System

Selective thermal surface coatings are material treatments, such as zinc oxide paints, second-surface mirror tiles, and anodized aluminum, applied to uninsulated radiator panels, antennas, and structural elements to regulate radiative heat exchange. By tailoring solar absorptance and infrared emittance ratios, these zero-power coatings control equilibrium temperatures, providing radiator absorptance below 0.15 and emittance above 0.85. Operating in the lunar environment requires surviving a 100-year lifetime where charged particle radiation and ultraviolet exposure degrade organic binders, driving a 1.5-fold increase in absorptance. Surface performance is further challenged by adhering lunar dust, which dramatically increases solar absorptance, and continuous thermal cycling that tests coating adhesion.

Selective thermal coatings applied to Ark surfaces to control solar absorptance and infrared emittance ratios, tuning radiative heat exchange with the environment.

Purpose

Tailors the radiative properties of exposed surfaces to achieve desired thermal balance -- low absorptance/high emittance for radiator surfaces, high absorptance for solar-heated surfaces, and specific ratios for temperature-sensitive areas.

Context

Applied to surfaces not covered by MLI (L2-PTC-MLI), particularly radiator panels, antenna surfaces, and structural elements, complementing the insulation system.

Principles

▸Solar absorptance (alpha-s) controls how much solar energy a surface absorbs
▸Infrared emittance (epsilon-IR) controls how effectively a surface radiates waste heat
▸Alpha-s/epsilon-IR ratio determines equilibrium temperature in sunlight
▸Coating degradation shifts these ratios over time, typically increasing absorptance

Typical implementations

▸White paints (AZ-93, S13G) for low alpha-s, high epsilon-IR radiator surfaces
▸Black paints (Aeroglaze Z306) for high-emittance internal cavity surfaces
▸Second-surface mirrors (OSR) for ultra-stable low alpha-s/high epsilon-IR
▸Gold or aluminum vapor-deposited coatings for low-emittance heat shields
▸Anodized aluminum for moderate emittance structural surfaces

Lunar considerations

▸Lunar dust adhesion dramatically increases solar absorptance
▸100-year UV exposure degrades organic paint binders
▸Atomic oxygen not present on Moon (unlike LEO), simplifying coating selection
▸Charged particle radiation darkens some white coatings over time
▸OSR mirrors offer best long-term stability but are fragile

Specifications

Functional

primary function	Control surface radiative properties through selective thermal coatings to manage solar heat absorption and infrared heat rejection
inputs	Solar radiation, Infrared radiation from lunar surface
outputs	Controlled radiative heat exchange at coated surfaces
radiator alpha s	<0.15
radiator epsilon ir	>0.85
end of life degradation factor	1.5x alpha-s increase over 100 years
adhesion	must survive 100 years of thermal cycling

Physical

materials	Zinc oxide white paint (AZ-93), Second-surface mirror OSR tiles, Vapor-deposited gold, Anodized aluminum
vacuum	True
radiation exposed	True
dust exposed	True

Operational

power consumption w	0
thermal range c	-173, 127
lifetime years	100

Interfaces

Provides

→Passive Thermal ControlSurface radiative controlessential
Provides controlled absorptance and emittance on exposed Ark surfaces
→Active Thermal Control SystemRadiator surface propertiesessential
Provides high-emittance surfaces for active thermal control radiator panels

Requires

←Primary StructureClean substrate surfacesessential
Requires prepared structural surfaces for coating adhesion
←Dust MitigationDust removal from thermal surfacesessential
Requires dust mitigation to maintain coating optical properties