How Do Doors Keep Fire from Spreading?

Solution 1:

Level 2+ doors prevent fire from spreading in MOST cases. Otherwise; every (x) seconds, fire performs a check to see if it can spread. This check goes something like:

• Is there an open space nearby?
  • No: Do nothing;
  • Yes: Is there a level 2+ door between me and this space?
    • No: Spread to space;
    • Yes: Can I more easily spread to a different space?
      • No: 1/100(approx) chance to spread through door;
      • Yes: Spread to new space.

Fires will continue to spread until either; they run out of oxygen, or they are extinguished. With level 2 Blast Doors, most fires will burn themselves out, or be held off long enough for the player to air-lock them. Increasing the level of the doors will likely further increase effectiveness. Unlike actual fire, the fires in FTL do not run out of combustible material, meaning they can burn forever if left unattended.

Because of this, a smaller room is more 'at risk' of having a fire leave the room, because there are fewer spaces for the fire to occupy within in the room. However, since Oxygen is measured in units per space, this means the smaller room can run out of burnable oxygen sooner. Say each space has 100 units of air, burning 5 units per second with 2 unit-per-second recovery (O² subsystem), then with both squares on fire in a 2-square room, the fire would burn out in 33.33_ (rounded down) seconds.

Fires are most easily extinguished by opening a nearby airlock and draining all of the Oxygen from the room, but this can be hazardous if your Oxygen subsystem is damaged or destroyed in this time, or if they door Subsystem is destroyed, preventing you from closing the airlock.