Spacecraft Fire: Flame-Ball Stability Map (2D)

Cabin Environment

Cabin O₂ concentration 21.0%

Fuel vapor supply rate 1.00×

Ambient pressure 101.3 kPa

Flame State

STANDBY

Flame radius

0.0 mm

Flame temp.

— K

Damköhler no.

0.00

CO buildup

0 ppm

How this 2D model works

This is an independent 2D companion, not a flattened version of the 3D scene: it solves the same microgravity diffusion-flame physics but renders it as two purpose-built 2D views instead of a 3D flame shell. In freefall there is no buoyant convection, so oxidizer reaches the fuel source only by molecular diffusion — the flame relaxes into a spherically symmetric shell whose radius is set by a balance between diffusive fuel/oxidizer supply and radiative heat loss:

Radiative loss balance:  Q_rad ∝ σ·T⁴·A_flame
Damköhler number:        Da = τ_flow / τ_chem   (flame stable only if Da > Da_crit)
Diffusion supply:        m_dot ∝ D·(dC/dr) at the flame sheet

The left panel is a radial cross-section (r = 0 at the wick) showing fuel concentration falling and oxidizer concentration rising toward the flame sheet, with temperature peaking there and decaying outward by radiative and diffusive loss — the actual 1D profile shape NASA's FLEX/SoFIE experiments measured aboard the ISS. The right panel is a fuel-supply vs. cabin-O₂ flammability map: your current point is plotted live against a shaded region showing where a diffusion flame this size can sustain itself (Da above critical) versus where it quietly self-extinguishes or, at high O₂, becomes a hazardous self-sustaining flame even without any convective air replenishment.

Cabin O₂ — shifts the flammability boundary; below ~15% nothing ignites, above ~24% a microgravity diffusion flame no longer needs help to stay dangerous.
Fuel vapor supply — more fuel raises reaction intensity but also raises the radiating flame area, so both the numerator and part of the loss term in Da move together.
Ambient pressure — lower cabin pressure thins the diffusion boundary layer, changing the balance point without adding any convective airflow. Halving pressure roughly halves collision (reaction) rate, matching pressure-scaling data from NASA's microgravity flammability tests.

Extinction is the real hazard: below the critical Damköhler number the flame can't sustain itself on diffusion alone and self-extinguishes even though nothing visibly changed — the reason spacecraft cabin O₂ is held near 21% and any active ventilation near a fire is treated as a hazard, since it can push Da back above threshold and re-ignite a flame that looked dead.

Radial flame cross-sectionFuel × O₂ flammability map

Temperature Oxidizer conc. Fuel conc.