Reference
Flame guide & glossary
How to read a flame's colour and shape, and what the abbreviations in the papers mean.
Flame appearance guide
What colour and shape tell you about soot, temperature and oxygen supply.
Bright yellow teardrop
Normal gravity. Buoyancy pulls hot gas up; lots of soot glows yellow.
Dim blue sphere / hemisphere
Microgravity. Oxygen arrives only by diffusion; flame cooler than the ~1300 K soot threshold.
White (sooty)
PMMA at higher flow (> 1 cm/s): soot particles glow white.
Orange glow
Incandescent soot — vigorous burning, e.g. at Saffire ignition or bursting bubbles in PMMA.
Dark blue, nearly invisible
Close to the limiting flow — hard to see, but still alive. Danger zone.
Short and wide
Flow near the limit: minimum length, maximum diameter — highest chance of igniting nearby surfaces.
Oscillating edge
A near-asphyxiated flame oscillates (≈ 1 Hz) and grows unstable before it goes out.
Glossary
18 terms, each tied to the sentence that defines or uses it.
LOCLimiting Oxygen ConcentrationThe lowest oxygen level at which a material can sustain flame spread.
“The single-thickness fuel is barely flammable in air, with a limiting oxygen concentration for flame spread (LOC) of 21 vol% O2; the double-thickness fuel is non-flammable in air, with an LOC of 26% O2.”
MOCMaximum Oxygen ConcentrationThe highest O₂ at which a material still passes NASA-STD-6001 Test 1.
“Maximum oxygen concentration (MOC) is defined as the highest O₂ at which material passes Test 1.”
OMOSOxygen Margin of SafetyOMOS = MOC(0g) − MOC(1g). Negative means the material burns at lower O₂ in low gravity than on Earth.
“Data on right shows Oxygen Margin of Safety (negative means material burns at lower O₂ compared to normal gravity!).”
VlimLimiting flow velocityThe airflow speed below which a flame in microgravity dies from lack of oxygen.
“The test results identify a limiting air-flow velocity Vlim for each material, below which combustion ceases.”
BASS / BASS-IIBurning and Suppression of SolidsA small wind tunnel in the ISS Microgravity Science Glovebox for burning solid samples at 0–55 cm/s.
“Microgravity combustion tests were performed aboard the International Space Station using the BASS (Burning and Suppression of Solids) hardware.”
SaffireSpacecraft Fire ExperimentLarge-scale fires set on purpose inside an empty Cygnus cargo vehicle after it leaves the ISS.
“To address this concern, NASA has embarked on a research project, the Spacecraft Fire Experiment (Saffire), to carry out large-scale fire tests, or multiple small flame spread tests of varied materials in an un-manned spacecraft, the Cygnus spacecraft by Orbital ATK.”
Concurrent flowFlow-assisted spreadThe flame spreads in the same direction as the airflow — fast, generally more hazardous on Earth.
“Concurrent, or flow assisted, flame spread occurs when the spread of the flame and the gas flow are in the same direction (upward spread in buoyant flow).”
Opposed flowSpread against the flowThe flame spreads into the oncoming air. In microgravity it can grow more easily thanks to fresh oxygen.
“However, it has been shown that in a low-gravity environment opposed-flow flames can grow more easily than concurrent spreading flames because of a fresh supply of oxidizer, and sometimes burn under conditions where extinguishment would occur on Earth.”
PLFPressure Limit of FlammabilityThe total pressure below which a flame cannot survive.
“Under normal gravity, the flame-spread rate for successive reduced-total-pressure conditions diminishes slightly, and a pressure limit of flammability (PLF) is noted at 13% of the original total pressure.”
SEASpace Exploration AtmospherePlanned exploration cabin air: lower pressure (~56–60 kPa) and higher oxygen (~34 %).
“They include microgravity, low velocity flows, and potentially low pressure (~60 kPa) and elevated oxygen concentration (~34%).”
SIBALSolid Inflammability Boundary at Low-Speeds fabric75 % cotton / 25 % fiberglass test fabric, 18 mg/cm². Only the cotton burns.
“SIBAL Cloth: Solid Inflammability Boundary at Low-Speeds (SIBAL) composite material composed of 75 percent cotton and 25 percent fiberglass by mass, with a total area density of 18.05 ± 0.02 mg/cm² and thickness of 0.31 ± 0.02 mm. Only the cotton portion burns, leaving behind a glowing fiberglass skeleton structure. Releases about 3.2 times more heat per unit area than cheesecloth.”
PMMAPolymethyl methacrylate (acrylic)Clear plastic used as the standard thick solid fuel in most tests.
“Samples were 1-cm and 2-cm wide sheets of plastic (PMMA; polymethyl methacrylate). Thicknesses were between 0.1 and 0.4 mm.”
g-jitterAcceleration disturbancesSmall vibrations around the background microgravity level that can disturb flames.
“Acceleration disturbances or g-jitter about the background level of reduced gravity exist in all the microgravity facilities.”
HRRHeat Release RatePower released by the fire, in watts.
“However, as pressure is reduced from 100 kPa to 30 kPa, the chemistry starts to play an increasingly important role showing a lower HRR for both cases considered.”
BlowoffBlow-off extinctionA flame pushed off the fuel by too strong a flow.
“Flame nearing blow off extinction as the flow velocity crosses 40 cm/s.”
QuenchingLow-flow (radiative) extinctionA flame that dies because too little oxygen arrives and heat losses win.
“Sequence of top views showing radiative quenching when flow velocity goes below 2 cm/s.”
FDS / LCMFire Dynamics Simulator / Lumped Capacity ModelDetailed CFD vs single-zone models used to predict cabin fire effects.
“LCM more amenable to large-scale parametric studies. Can easily run hundreds of simulations over wide-ranging conditions such as vehicle volumes, fire sizes, relief valve sizes, etc.”
NASA-STD-6001 Test 1Upward flame propagation testA material passes if the flame self-extinguishes before spreading 15 cm.
“Materials “pass” this test if the flame self-extinguishes before it propagates 15 cm.”