A spacecraft bus balances three coupled subsystems: propulsion converts stored propellant into
delta-v, power generation (solar panels) must exceed the electrical + heat load, and a radiator
rejects excess heat to space by radiating infrared photons — the only way to lose heat in vacuum.
Δv = Isp·g₀·ln(m₀/m) (Tsiolkovsky rocket equation)
P_radiated = ε·σ·A·(T⁴ − T_space⁴) (Stefan–Boltzmann, ε≈0.85)
Equilibrium: P_radiated = P_heatLoad + P_thrusterWasteHeat
- Thruster throttle — fraction of max thrust firing; accumulates delta-v and adds waste heat.
- Solar panel deploy — how far the panels have unfolded; scales generated electrical power.
- Radiator area — larger radiator panels reject more heat per the Stefan–Boltzmann law, lowering equilibrium temperature.
- Internal heat load — heat from avionics/instruments the radiator must dump to space.
Real-world relevance: this exact propulsion/power/thermal balancing act is why real spacecraft
(from CubeSats to the ISS) carry deployable radiators and solar arrays sized specifically for
their worst-case internal heat and power draw.