Safety & operations
From research to crew procedures
Choose materials and ventilation so any fire stays tiny.
Smoke can lag the toxic gas — watch CO / CO₂ too.
Cut the airflow, then a suitable agent; keep watching hot surfaces.
“Thus, the fire safety strategy in a spacecraft is (1) to keep any fire as small as possible, (2) to detect any fire as early as possible, and (3) to extinguish any fire as quickly as possible.”
Extinguishment methods and their results
Mir: extinction 5–20 s after shutoff; <13 s in µg vs 55 s in 1g. Saffire I: immediate.
More effective than CO₂ per mole at µg and ~11× per mass; no physiological activity; less bubble risk on depressurization.
Raises the oxygen needed to burn (21 → ~35 % as diluent), but in µg can radiate heat to fuel; no clear extinction boundary in 142 points.
A 500 cc/min N₂ jet weakened but did not extinguish a PMMA flame; flushing with N₂ was used to end ground tests.
Skylab: vacuum extinguishes when O₂ drops enough; µg flames are much more pressure-sensitive (PLF 78 % vs 13 %).
Skylab showed water extinguishment is possible with controlled, adequate application.
The detection gap
Smoke was seen late; the dangerous gases came first.
Saffire VI-3 event timeline
Gas released vs exposure limits
Fire growth drives hardware sizing
Growth rate is the input for relief valves, extinguishers, cleanup and crew response.
Pressure rise scales with the mass of material consumed and its rate.
“Safety-critical parameters such as temperature and pressure scale with mass consumed and rate of mass consumption.”
Fire growth rate sets how big a fire is when the crew responds.
“Pressure relief valve sizing; extinguisher size; consumables for cabin cleanup; crew response times (fight-or-flee decisions).”
CO₂ scales with mass burned; CO and acid gases must be scrubbed after the fire.
“CO₂ concentration approximately scales with mass of material consumed.”
Growth rate drives detection time, fire size, collateral damage and emissions.
“The paper addresses how rapidly a fire can spread in low-g and identifies the rate of fire growth as a factor in spacecraft fire safety strategy. The rate of fire growth impacts time to detect, size of fire, heat release rate, fire spread to surrounding materials, collateral damage, emission of combustion products, post-fire cleanup strategy and consumables.”
Fight-or-flee decisions depend on how fast the fire grows.
“Pressure relief valve sizing; extinguisher size; consumables for cabin cleanup; crew response times (fight-or-flee decisions).”
A heat exchanger above the fire cut chamber pressure rise ~7×.
“Heat Exchangers and Quench Bars: Mounting a heat exchanger directly above the burning sample in the path of the plume proved an effective means for chamber overpressure mitigation, especially for tests producing the most total heat release. L2 (5208 g) reduced upward 3-sample plain Delta P from 1.369 psi down to 0.202–0.205 psi (and single sample from ~0.43 psi to 0.097 psi). However, L2 had an undisclosed protective coating that generated excess smoke and ignited during a full-size SIBAL test (SUP 12-18, Delta P = 3.527 psi, popping the chamber relief). Quench bars slightly reduced Delta P (e.g., 3-sample upward from 1.369 to 0.859 psi sequential; 3-sample downward simultaneous from 0.337/0.294 to 0.275 psi).”
Pressure rise in a sealed 6.35 m³ chamber
Heat exchangers and slower burns cut the pressure spike several-fold.
Key operational findings
Each with a confidence meter: High = real microgravity + measured value; Medium = real µg qualitative or ground measurement; Low = model / overview.
“It has been demonstrated that at flow shutoff the extinction in microgravity occurs in 5 to 20 s, which is significantly lower than the extinction time obtained during the ground testing.”
“The space experiment indicated that the fire extinguishes faster in microgravity (1:1< 13 sec) at flow shutoff than in normal gravity 0:1 = 55 sec).”
“Extinguishment [9]. One sided PMMA, 300 seconds of low air flow (5 cm/s). In some circumstances, reducing air flow to a small value rather than quiescent conditions may be useful to disperse fuel vapor and help cool the material to enable extinction. Plot: Flow Speed (cm/s) and Incident Flux (mW/cm^2) vs Time (s), Radiometer 4.”
“Premature cessation of agent application or attempts to cool with forced air flow may instead provide fresh oxidizer to the fire-heated surface to re-establish the fire, particularly in untended periods when the affected zone cannot be observed.”
“Since heat transfer by natural convection in low gravity is negligible, fire-heated surfaces cool slowly.”
“This has implications for spacecraft fire safety since a tiny flame might be undetected for a long time.”
“Particulate size and transport changes: Terrestrial standards for detection not necessarily applicable”
“Preliminary data on smoke emissions in microgravity indicate that soot-aggregate size may be typically larger than those in normal gravity.”
“Smoke was readily detected at 981 seconds, after most of the hazardous gas release had occurred.”
“In microgravity, the burning drops of melt do not separate from the material in the air flow with velocities below 10 cm/s.”