If the goal is a cold, reliable radiator with minimal area, pick a surface that keeps solar absorptance α low and infrared emittance ε high. The sweet spot is α/ε near 0.10. That is why silver shows up everywhere in thermal control stacks, either as silver-coated FEP film, often called Ag/FEP, or as optical solar reflector tiles, OSR, where a thin silver mirror sits behind fused-silica glass.
What to Use, with Numbers
You can size most trades with the values below. These are representative beginning-of-life numbers, always test your actual lot.
| Surface | α | ε | α/ε | Typical use |
|---|---|---|---|---|
| Ag/FEP film, 5–10 mil | 0.08–0.09 | 0.80–0.88 | ~0.10 | Flexible cold-side finish on deployables or panels |
| OSR glass tiles, Ag on fused silica | ~0.07 | ~0.80 | ~0.09 | Rigid, cleanable radiators and doors |
| White paint, Z93 | 0.19–0.23 | 0.88–0.92 | 0.21–0.26 | Low cost, tougher, runs warmer |
| Aluminized polymer, Kapton or FEP | 0.20–0.30 | 0.80–0.90 | 0.22–0.38 | MLI outer covers, not the coldest option |
Plain rule, replacing Z93 with Ag/FEP typically cuts required radiator area by about half for the same sink and load, because α/ε drops from roughly 0.23 to roughly 0.10.
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How much silver you actually need
The silver layer is a thin mirror, tens to a few hundred nanometers. Use silver density 10.49 g per cm³ and you get, at 100 nm thickness, about 1.05 g of silver per square meter.
- 50 nm, ~0.52 g per m²
- 100 nm, ~1.05 g per m²
- 200 nm, ~2.10 g per m²
Example, a 10 m² radiator with a 100 nm Ag layer carries roughly 10.5 g of silver. Milligrams of Ag buy you tens of watts of heat-rejection margin when α/ε drops.
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Pick quickly, know the trade-offs
- Need the smallest cold radiator, pick Ag/FEP, great α/ε near 0.10, light and flexible. Watch electrostatic discharge, add an ITO bleed layer when needed, manage contamination, consider atomic oxygen protection in LEO.
- Need a rugged panel you can wipe and inspect, pick OSR tiles, similar optics, easy to clean, confirm tile gap losses, handle carefully, accept higher mass.
- Need a budget outer blanket, pick aluminized Kapton or FEP, widely available, ε is fine, α is higher, the radiator will run warmer.
- Need a stray-light absorber, pick black paint like Z306, high α and ε, good for baffles, expect hot hardware, not for cold plates.
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Degradation in Orbit, Plan to End of Life
Surfaces age. Alpha usually creeps up, epsilon can drift. Budget the increase, qualify your stack.
- Atomic oxygen in LEO, erodes Kapton, roughens FEP, can undercut thin metals at edges, α rises.
- UV plus contamination, outgassing films darken under UV, housekeeping matters or you accept warmer end-of-life optics.
- Thermal cycling, large day-night swings can micro-crack stacks without proper overcoats and primers.
Design to end-of-life α and ε, not just brochure beginning-of-life values.
Worked example, quick back-of-envelope
Load is 40 W, target near 270 K, simple LEO view with margin for Earth IR and albedo.
- With Ag/FEP, α ≈ 0.08, ε ≈ 0.82, area lands around 0.4 to 0.6 m² at beginning of life.
- With Z93, same target and load, expect roughly 0.8 to 1.0 m².
- Silver mass check at 0.5 m² and 100 nm, about 0.5 g of Ag.
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Handy Material Constants for Calcs
Use these for quick thermal sizing, refine with vendor data at your temperature.
- Silver, k ~419 to 430 W per m·K, cₚ ~235 to 260 J per kg·K, density 10.49 g per cm³
- Aluminum, k ~205 W per m·K, cₚ ~900 J per kg·K, density 2.70 g per cm³
- FEP, k ~0.20 W per m·K, cₚ ~1170 J per kg·K, density 2.12 to 2.17 g per cm³
- Kapton HN, k ~0.12 to 0.20 W per m·K, cₚ ~1090 J per kg·K, density 1.42 g per cm³
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Aim for α/ε near 0.10, that is the cheat code for cool hardware. Ag/FEP and OSR give you that, with grams of silver across entire radiators. Lock the coating stack early, line up two vendors, test your lot at relevant temperatures and fluence.
| Item | Pg | α (solar) | ε (IR) | α/ε | Notes |
|---|---|---|---|---|---|
| Optical solar reflector (OSR), silvered fused silica (quartz) | 1 | 0.07 | 0.80 | 0.09 | Design value, second-surface mirror (OSR) |
| Optical solar reflector, silvered quartz, Helios program | 1 | 0.07 | 0.79 | 0.09 | Helios reference, second-surface mirror (OSR) |
| Teflon, silvered, 2 mil | 2 | 0.08 | 0.68 | 0.12 | Design value |
| Teflon, silvered, 5 mil | 2 | 0.08 | 0.80 | 0.10 | — |
| Teflon, silvered, 10 mil | 2 | 0.09 | 0.88 | 0.10 | Design value |
| Kapton, silvered, aluminum-oxide coated, 1 mil | 4 | 0.08 | 0.19 | 0.42 | Al₂O₃ protective coat |
| Kapton, silvered, aluminum-oxide coated, 1 mil, 2400 hours UV | 4 | 0.08 | 0.21 | 0.38 | 2,400 h UV exposure, Al₂O₃ coat |
| Chromeric 586 silver paint | 7 | 0.30 | 0.30 | 1.00 | — |
| DuPont 4817 silver paint | 7 | 0.43 | 0.49 | 0.88 | — |
| Silver, vapor-deposited, on glass, un-oxidized | 8 | 0.04 | 0.02 | 2.00 | — |
| Silver, polished, un-oxidized | 8 | 0.04 | 0.02 | 2.00 | Design value |
| Silver, oxidized | 8 | — | 0.03 | — | Design value, α not given |
| Silver, Denton vapor-deposited, with protective overcoat | 8 | 0.06 | 0.03 | 2.00 | Design value |
| Silver beryllium copper | 8 | 0.19 | 0.03 | 6.33 | — |
| Silver beryllium copper with Kapton overcoat | 10 | 0.31 | 0.57 | 0.54 | Kapton overcoat |
| Silver beryllium copper with Parylene C overcoat | 10 | 0.22 | 0.34 | 0.65 | Parylene C overcoat |
| Silver beryllium copper with Teflon overcoat | 11 | 0.12 | 0.38 | 0.32 | Teflon overcoat |
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