Every conversation about industry in space (computing, refining, manufacturing) eventually becomes a
conversation about heat. Electricity is the easy part. A wire carries it across a spacecraft almost for free,
and when distance gets expensive you raise the voltage. Heat has no cheap equivalent. Left to itself it flows
downhill in temperature, every interface it crosses takes a toll, and in vacuum the only way out is to radiate
it, at a rate that scales with the fourth power of temperature. A heat pump can lift it to a hotter, smaller radiator, but
only by spending more electrical power; this model doesn't include one yet.
P = εσAT4
The Stefan–Boltzmann law:
power P radiated by area A at temperature T, with emissivity ε
and σ = 5.67 × 10−8 W m−2 K−4.
I built this explorer to make that intuition quantitative. It follows the energy from the solar array, through
the payload, down the temperature staircase, to the radiator, and shows which assumptions actually move the size
and mass of the hardware. It started as a question about orbital data centers. I want it to grow into a tool
for any energy-hungry work in cislunar space and, eventually, on the Moon.
It is a companion to Andrew McCalip's Space Datacenters,
which asks whether orbital compute pays. This one asks what the heat and power hardware has to look like. Every
number carries a source or a label (see Assumptions). Please check the math,
challenge the assumptions, and send me better numbers.
SpaceX's Starmind is a reference case because it is the most talked-about design, and SpaceX has shared more of
its numbers than anyone else: power, radiator and solar array areas, kW per ton, orbit. Credit where it's due.
Where this model and their figures disagree, that is a question about assumptions, not a verdict on their design.
I'd like cases for the others too: Starcloud, Sophia Space, Cowboy Space (formerly Aetherflux), Relativity Space,
Blue Origin's Project Sunrise, Google's Project Suncatcher, Axiom Space, and anyone else with published figures.
If you can source their numbers, please add a case.
Note: Built from publicly available information and first-principles physics. No proprietary
data. These are my personal views.
Try this first. In the Starmind case, lower the source
temperature limit and watch the radiator area grow. Then open the temperature staircase to
see where the temperature budget goes.
How the model fits together. Each part has its own inputs; three quantities carry its outputs to the
others, so a change in the heat path also moves the radiator area and the total mass. The page below follows the
same three parts, each with its inputs beside its charts. Charts 6–8 combine everything.
The same, as a table
mass budget, solar array : radiator area, sensitivity
Part 1
Heat path: from the source to the coolant
Everything between the heat source and the fluid that reaches the radiator. Its only output is the coolant temperature T, and every kelvin it spends shrinks what the radiator can do.
1. The first centimeter: the temperature staircase
Heat flows downhill in temperature: ΔT = Q × R, thermal Ohm's law. Every interface spends part of a small,
fixed budget, the source limit minus the radiator temperature. Most of it is spent within a centimeter of
the heat source: silicon, interface materials, lid, cold plate. Distance itself costs little once heat is in
a pumped loop.
Your chain, stage by stage
2. Why heat is harder to move than electricity
Each line is a 1 kW path from source to panel, scored as the share of ideal panel area doing useful work. A
wire loses almost nothing over distance, because voltage can be stepped up. Heat has no cheap transformer (a heat
pump costs power): a 10 × 10 cm solid copper bar cannot carry 1 kW much past a metre, and a pumped loop makes distance cheap but
still pays the fixed interface staircase.
Part 2
Radiator: rejecting heat to the sky
How much heat each square metre rejects at temperature T, given what the radiator sees. With the heat load Q, that sets the radiator area.
3. Radiator design curves: what a square metre buys
Net heat rejected per m² of two-sided panel, versus the radiating fluid temperature, in each environment. Flux
grows as T⁴, so hot waste heat is cheap to reject. A warm view (Earth, the lunar dayside, sunlit regolith)
cancels the low-temperature end of the curve. On the lunar equator at noon, a
vertical fin with the Starmind-mode surface properties absorbs more than it emits below about 335 K (curves
that drop off the log scale have gone net-negative).
4. Where you orbit: the planet-view penalty
A radiator near a body sees part of it instead of 3 K space and absorbs its infrared. Edge-on panels see less
than panels facing the body. The penalty fades with altitude, and is larger for colder radiators.
Part 3
Power supply: the solar array and the dark
The solar array and battery that deliver the load through light and dark. The location sets the sunlight; the heat load Q sets the power.
5. Why terminator orbits: eclipses and the β angle
The β angle between the orbit plane and the Sun sets how long each eclipse lasts. Above a critical β the
spacecraft never enters the planet's shadow: no eclipse-storage requirement, and no oversized solar array to recharge a battery. A
dawn–dusk sun-synchronous orbit flies along the terminator to stay there. The radiator barely notices, so the
case for terminator orbits rests on power. Not modelled here: eclipse thermal cycling of chips and radiators,
and losing compute in the dark.
Results
Putting it together
These charts combine all three parts, so they have no inputs of their own. Change anything above and they move.
6. Run hotter: areas versus source temperature
Same scenario, sweeping only the heat source's temperature limit. The solar array is fixed by the power
drawn. The radiator shrinks steeply with temperature. Chips are pinned near 350–400 K, so compute pays
nearly full price. For a process rejecting heat at 600 K or more, the radiator becomes small next to the
solar array.
7. Mass budget: power and thermal hardware only
Payload mass is excluded on purpose. Per m², radiators weigh several times more than solar arrays, so a 5:1
area ratio is not a 5:1 mass ratio. In darkness-heavy environments, energy storage can outweigh both. Storage for brief power peaks is not included.
Each reference case, plus your scenario. Each case uses its own hardware and location.
Your scenario moved to every environment (composition; totals at right)
8. Sensitivity slopes: what moves the mass
Each bar moves one parameter down or up by one step and holds everything else fixed. Steps are in natural
units: ±10 K on temperatures, ±0.05 on emissivity, ±20 % on areal densities. Hover a bar for the value tried.
Assumptions and provenance (current scenario)
Tags: [source] cited reference · ASSUMPTION engineering judgement · CALIBRATED tuned to
reproduce a published figure · SECONDARY widely quoted, not confirmed against a primary source.
Select a parameter's name (▸) to read what it means.
