Thermal Control System Calculator

The radiator's effective temperature is set to your target temperature plus 273 (Celsius to Kelvin) plus a fixed 20 K margin, and radiated power scales with that absolute temperature to the fourth power (Stefan-Boltzmann law). A higher target temperature raises the radiator's operating temperature too, so each square meter can shed more heat, letting the calculator size a slightly smaller radiator for the same total heat load.

Radiator area is heat load divided by the radiator's per-square-meter capacity, and that capacity itself depends only on the fixed radiator temperature (target temp + 20 K), not on the heat load. So for a fixed target temperature, radiator area actually does scale linearly with heat load in this model — what's nonlinear is how area changes if you instead vary the target temperature, since that shifts the per-m² capacity via the fourth-power term.

Crew heat is calculated at a fixed 100 W (0.1 kW) per person regardless of activity, while equipment heat is whatever electrical power figure you enter directly, since virtually all of it converts to waste heat indoors. For a typical 6-person crew with 15 kW of equipment, crew heat (0.6 kW) is a small fraction of the internal total compared to equipment power.

System mass has two components: 5 kg per kW of total heat rejected (covering pumps, plumbing, and coolant hardware that scales with heat volume) plus 3 kg per square meter of radiator area (the physical structure of the radiator panels themselves). Because radiator area and heat load don't move in lockstep once solar exposure and surface area change independently, combining both terms captures more of the real mass than either alone.