Vacuum Radiative Cooling: In-Space Metal Forging

Environment

Workpiece

Initial temperature 1850 K

Emissivity ε (surface finish) 0.35

Radius (droplet / ingot) 18 mm

Live Readouts

env: vacuum · h_conv = 0 W/m²K

Surface temp

1850 K

Cooling rate

0 K/s

Elapsed

0.0 s

Solidified

How it works

A hot workpiece loses heat two ways: radiation, which needs no medium, and convection, which needs a fluid to carry heat away. On Earth, air convection usually dominates near-surface cooling of a hot part; in orbital vacuum there is no air, so convection is exactly zero and radiation is the only path — this is the real thermal-design problem for in-space forging, welding and metal 3D printing (electron-beam / wire-arc processes tested on the ISS and by companies like Redwire and RBC Signals).

Radiative loss (Stefan–Boltzmann):
  Q_rad = ε · σ · A · (T⁴ − T_env⁴)
  σ = 5.670374×10⁻⁸ W/m²K⁴  (Stefan-Boltzmann constant)

Convective loss (Newton's law of cooling, Earth only):
  Q_conv = h · A · (T − T_env)
  h ≈ 15–25 W/m²K for free convection in air

Lumped-capacitance energy balance:
  m·c·(dT/dt) = −(Q_rad + Q_conv)
  dT/dt = −[ε·σ·A·(T⁴−T_env⁴) + h·A·(T−T_env)] / (m·c)

Orbital Vacuum / Earth Atmosphere — toggles whether the Newton convection term is included; vacuum keeps only the T⁴ radiation term.
Emissivity ε — how efficiently the surface radiates (polished metal ≈0.05–0.2, oxidized/rough ≈0.6–0.9); it multiplies the radiative loss directly.
Radius — sets the surface-area-to-volume ratio A/V ∝ 1/r, so smaller parts cool (and solidify) proportionally faster in both environments.
Because T⁴ falls off steeply, vacuum radiative cooling starts fast at high temperature but slows sharply as the part approaches ambient — producing a distinctly different cooling curve shape (and hence a different solidification time and final grain structure) than Earth's steadier, convection-dominated curve at the same starting temperature.

Real-world relevance: this asymmetry is why in-space manufacturing hardware needs oversized radiators or reflective/high-emissivity coatings engineered specifically for vacuum — a process tuned for Earth's air-cooled forge will solidify far too slowly (or unevenly) once there is no air left to carry heat away.

Temperature vs time (K vs s)

Vacuum (radiation only) Earth (radiation + convection)

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