Spacecraft Fire: Microgravity Flame Ball Simulator

Cabin Environment

Gravity level 1.00 g

Cabin O₂ concentration 21%

Forced ventilation flow 0.10 m/s

Flame State

STANDBY

Flame radius

0.0 mm

Flame temp.

— K

Burn rate

0.0 mg/s

CO buildup

0 ppm

How it works

On Earth, hot combustion gas is buoyant: it rises, drags in fresh air behind it, and stretches the flame into the familiar tall yellow teardrop. Buoyant convection is a gravity effect — it needs a density gradient and a body force to sort by. In orbital freefall both the cabin and the flame are in continuous freefall, so buoyancy vanishes:

Buoyant velocity:  u_b ∝ √(g · L)
Grashof number:    Gr = g·β·ΔT·L³ / ν²   (→ 0 as g → 0)

With no buoyant flow, oxygen can only reach the flame by molecular diffusion, which is slow and radially symmetric — so the flame relaxes into a small, dim, nearly spherical shell centered on the fuel source, exactly as NASA's FLEX and SoFIE experiments recorded aboard the ISS. This model blends three real regimes with gravity level g:

Shape — flame stretch and asymmetry scale with √g (buoyant elongation); at g→0 it relaxes toward a sphere.
Peak temperature — lower without buoyant air replenishment, so soot production drops and the microgravity flame glows blue instead of yellow.
Burn rate — diffusion-limited fuel consumption scales roughly with the flame's diffusive supply area, so it falls sharply as g→0.
Extinction — below a critical Damköhler number the flame can't sustain itself on diffusion alone and quietly self-extinguishes, even though it looks like nothing is wrong — the real hazard behind spacecraft fire-safety protocols, which is why cabin O₂ is held near 21% and ventilation is engineered to actively resupply oxidizer instead of relying on convection.

Controls: Gravity sets the buoyancy regime from ISS freefall to full 1 g. O₂ concentration raises reaction intensity and burn rate but also raises fire risk — above ~24% even a diffusion-fed microgravity flame becomes self-sustaining and dangerous. Ventilation forces convective oxygen resupply mechanically, standing in for a cabin fan — enough of it can keep a flame burning even at zero gravity, which is exactly why uncontrolled airflow near a spacecraft fire is treated as a hazard, not a fix.

Fuel source (wick) Buoyant flame (1 g) Diffusion flame (0 g)

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