Comparison tools
Side by side
Two papers
Combustion of Solids in Microgravity: Results from the BASS-II Experiment
- Year
- 2014
- Evidence
- Real microgravity
- Platform
- ISS
- Fuel
- PMMA, SIBAL
- O₂
- 13.6–21 %
- Pressure
- 1 atm
- Airflow
- 0–55 cm/s
- Findings
- 38 quoted statements
- Key finding
- PMMA burned steadily at 13.6 % O₂ in orbit while the 1g limit is 16 % — a negative oxygen margin of safety of 2.4 %.
Preliminary Results from the Saffire VI Experiment
- Year
- 2024
- Evidence
- Real microgravity
- Platform
- Cygnus
- Fuel
- SIBAL, Cotton, PMMA
- O₂
- 28.8–31 %
- Pressure
- 54.1–55.2 kPa
- Airflow
- 20 cm/s
- Findings
- 27 quoted statements
- Key finding
- 1.5–3.2 kW fires in a 17–19 m³ cabin: CO and CO₂ passed crew limits, while smoke was detected only at 981 s — after most toxic gas was released.
Two materials
Flammability facts stated in the sources.
PMMA (acrylic)
- O₂ limit, 1g
- 16 % (¼" rod blowoff)
- O₂ limit, µg
- 13.6 % (steady spread)
- Margin of safety
- −2.4 % O₂ (burns more easily in µg)
- Limiting flow Vlim
- 0.5 cm/s at 23.6 % O₂
- Saffire outcome
- VI-3/VI-4: CO and CO₂ above crew limits; burned up to 1200 s
Nomex HT90-40
- O₂ limit, 1g
- ≈ 24–25 % O₂
- In µg (ISS)
- Did not burn at 21–22 % O₂, ~15 cm/s
- Saffire outcome
- Not ignited by a 460 W PMMA pilot flame
- Note
- Nomex-III (a different grade) did burn on ISS
Normal vs microgravity flame
Same PMMA, same 21 % O₂. Left: Earth. Right: ISS with a gentle 3 cm/s airflow.
| Aspect | Normal gravity | Microgravity | Source |
|---|---|---|---|
| Shape | Teardrop, tall | Spherical / hemispherical, shorter and wider | |
| Colour | Bright yellow (soot) | Dim blue, extended blue zone | |
| Base | Concave base, extended soot zone | Convex base | |
| Behaviour | Growing, flickering, pulsating spread | Steady size and spread | |
| Distance from fuel | Close to surface | Farther from surface, dimmer, less soot | |
| Oxygen supply | Buoyant convection | Diffusion + ventilation only |
Extinguishing options
What each method achieved in the experiments, with the evidence behind it.
Vent / flow shutoff
Effective for thin fuels in air
Mir: extinction 5–20 s after shutoff; <13 s in µg vs 55 s in 1g. Saffire I: immediate.
Caveat: Thick or hot fuels and enriched O₂ can survive no-flow periods.
Evidence (3)
“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 [8]. Extinguishment of bubbling polymers cannot be assumed to occur rapidly in quiescent conditions. Flame shrank during the reduced flow interval of 65 seconds but survived the 15 second no-flow interval. Flame extinguished below the sample for 6.4 seconds but reignited. Plot axis: Incident Flux (mW/cm^2).”
Helium
Promising (drop-tower only)
More effective than CO₂ per mole at µg and ~11× per mass; no physiological activity; less bubble risk on depressurization.
Caveat: Not yet tested on long-duration platforms.
Evidence (2)
“First, helium is more effective than CO2 on a mole basis (thus pressure times storage volume basis) at µg, meaning that the size and weight of storage bottles would be smaller for the same fire-fighting capability.”
“Third; helium has no physiological activity, unlike CO2 that affects human respiration.”
CO₂
Mixed — ISS standard, but weaker in µg
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.
Caveat: Affects crew respiration; can cause unsteady spread.
Evidence (3)
“Results show, first, that the substitution of carbon dioxide as the atmospheric diluent for nitrogen greatly diminishes the flammable range; the limiting oxygen concentration for combustion is increased from 21 vol% to near 35 %.”
“CO2 may not be as effective as an extinguishing agent at µg as it is at earth gravity in some conditions because of the differences in spread mechanisms between the two cases.”
“On this plot, there is no discernable boundary between flames that were extinguished by suppressant and those that were not.”
Nitrogen
Weak on hot thick fuels
A 500 cc/min N₂ jet weakened but did not extinguish a PMMA flame; flushing with N₂ was used to end ground tests.
Caveat: Warm solids keep re-igniting.
Evidence (2)
“Nitrogen flow of 500 cc/min weakens but does not extinguish the flame”
“The flames were extinguished by flushing the system with nitrogen after they had spread over the entire sample region.”
Venting to vacuum
Works, with a flare-up
Skylab: vacuum extinguishes when O₂ drops enough; µg flames are much more pressure-sensitive (PLF 78 % vs 13 %).
Caveat: Induced flow briefly intensifies the flame.
Evidence (2)
“Extinguishment by vacuum is effective, and the flame is extinguished when the available oxygen decreases sufficiently.”
“The observation of venting in Skylab indicated that the depressurization-induced flow causes a flame to flare up, and presumably spread faster, before it succumbs to the reduced total-pressure environment.”
Water
Possible if controlled
Skylab showed water extinguishment is possible with controlled, adequate application.
Caveat: Very little data.
Evidence (1)
“In addition to the flame-spread data, the Skylab experiments provided two observations of fire extinguishment that are still of interest for current applications: Extinguishment by water is possible, provided the application is controlled and adequate.”