A spacecraft fire is an extremely serious situation. Even if no serious damage is done to spacecraft systems, the cabin atmosphere is likely to be highly contaminated by toxic combustion products and possibly by whatever substance the extinguishers use.
For shuttle, a serious cabin fire would result in a early deorbit case if the cabin atmosphere could not be cleaned up in a reasonable amount of time.
I'll walk through the procedures for the case of a fire in a shuttle avionics bay and explain the rationale along the way. Please refer to this overview flowchart.
The crew was alerted to the presence of a fire when the built-in smoke detectors triggered. This caused a loud audible alarm, messages on the computer screens, and illumination of lights on panel L1.
The crew used the Fire/Smoke Cue Card to eliminate the possibility of a false alarm. If confirmed, the crew put on Quick-don Masks (QDMs) and opened the oxygen valves supplying them with O2. They then discharged a built-in fire bottle into the affected avionics bay using the switch and button on panel L1. They then split into two teams following parallel procedural paths as show in the flowchart. One team heads off to clean up the cabin atmosphere, the other to powerdown and reconfigure equipment in the affected avionics bay. Since this question is about cabin atmosphere cleanup, I will focus on the first procedural path.
Even aside from the combustion products, there are serious implications to the actions already taken.
- The QDMs are leaking significant amounts of O2 into the cabin atmosphere, causing it to approach dangerous flammability limits.
- The built-in fire bottles discharge Halon into the cabin atmosphere.
The crew first measured the amount of contaminants in the atmosphere using a handheld device called the Compound Specific Analyzer - Combustion Products (CSA-CP). This device measured the amount of O2 (%), Carbon Monoxide, Hydrogen Cyanide, and Hydrochloric Acid in the air (these last in parts-per-million). They are reported to the ground and recorded in a table in the flight data file.
The crew then installs a fresh Lithium Hydroxide canister in the atmosphere regeneration system, and a special Ambient Temperature Catalytic Oxidizer canister in the second slot. This canister works to remove Carbon Monoxide.
The shuttle toilet was activated to flow air through its charcoal filters to help remove other contaminants. Also, a purge of the affected avionics bay overboard through the vacuum vent line was set up.
The implications of staying on the QDMs then set in. To control the O2 concentration below the flammability limit and remove remaining contaminants, the cabin must be depressurized to 8 psi and a constant overboard purge set up. The purge is done through the airlock depressurization valve with the airlock inner hatch left open. Dropping the cabin pressure to 8 psi forces additional equipment powerdowns due to the reduced effectiveness of air-cooling at the lower density.
If the situation has gotten this far and the atmosphere can't be sufficiently cleaned up, the crew will have to prepare for an early deorbit. Depending on what the consumables that are being flowed overboard for the purge can support, an Emergency Landing Site (ELS) may have to be chosen. Every effort would be taken to make a Primary Landing Site (PLS) though - Kennedy Space Center or Edwards Airforce Base.
It was possible to back out of the procedures if the atmosphere was cleaned up sufficiently, but concerns about unknown damage to equipment in the avionics bay would likely still force an early landing.
- Contingency Deorbit 32001 Class materials (not online)
- Orbiter Caution & Warning System Training Manual (not online)
- Orbit Pocket Checklist