Cabin Fire Flammability Envelope

Cabin Atmosphere

Oxygen fraction 21%

Total cabin pressure 101 kPa

Test material

Readouts

Set conditions, then ignite

O2 partial pressure

21.2 kPa

LOI threshold (this P)

18.0%

Flammability margin

+3.0%

Spread rate

0.0 cm/s

How it works

A material only sustains a flame if the cabin's oxygen concentration is above its limiting oxygen index (LOI) — the minimum O2 volume fraction needed for a self-propagating flame. Raising total cabin pressure lowers that threshold (more O2 molecules reach the flame front per unit time), which is modelled here as a simplified engineering correlation:

LOI_eff(P) = LOI_ref x (101.3 / P_kPa)^0.35
margin      = O2% - LOI_eff(P)
spread rate = 0.09 x margin cm/s   (x0.55 in microgravity)

margin ≤ 0 — below the flammability boundary: the swatch chars at the ignition point and self-extinguishes.
margin > 0 — a sustained flame front advances along the swatch at the modelled spread rate.
Microgravity — without buoyant convection to resupply fresh O2, spread is slower and the flame sits closer to spherical rather than the teardrop shape buoyancy produces at 1 g.

This is why ISS and most modern spacecraft use a near sea-level 21% O2 / ~101 kPa mix rather than the pure, low-pressure oxygen atmosphere used in early capsules — the 1967 Apollo 1 cabin fire ignited in a 100%-O2, ~115 kPa ground-test atmosphere, one of the events that drove the switch. Numbers here are an educational approximation, not a certified NASA-STD-6001 flammability test.

Flame Unburnt swatch Burnt / charred

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