Space Hulks are littered with motor-controlled iris-style airlock doors. Closed doors block LoS, prohibit movement, block flames, and stop the lesser-minded of the Bug variants from passing through. All doors start the game closed. They may be opened or closed for 1 AP by an adjacent unit who is facing the door in his front three squares. Since a Blip has no facing, it only needs to be adjacent. A door can not be closed if a unit or object occupies its space. If a door is opened, remove it from the square and set it beside the square. This differentiates whether the door was destroyed or merely opened. Destroyed doors are removed from the game entirely.
Chainfists can automatically destroy closed doors for 1 AP. Techmarines can destroy open doors for 1 AP. Regular weapons also work. Doors are destroyed in the same way any other unit would be destroyed, based on the ranged or melee attack. So an assault cannon destroys a door on a 5 or 6 on 3d6. Sustained fire also works on doors. Open doors can not be destroyed except by Techmarines.
Opening, closing, or destroying a door does not trigger any overwatch actions unless the unit that initiated the act of opening/closing/destroying is still visible after the door changes states. A door changing states in LoS does count as an action within LoS though, so command points could be spent as a reaction. So if a Genestealer spent an action point to close a door that then blocked LoS to that Genestealer, a Terminator would not fire as part of overwatch, but could break overwatch, spend a command point, and shoot the door.
Bulkheads follow the same rules for doors except that they can not be destroyed as normal. Bulkheads often have custom rules for each scenario, so make sure to know how they operate beforehand. Here are some of the common bulkhead rules that could be used:
Certain neutral objects are sometimes part of the game setup, such as Flamer Reloads, Krak Missiles (Cyclone Missile Launcher Reloads), or a mission-based objects. For an object to qualify for these rules, they are considered to be large enough to require two hands to manipulate. Techmarines ignore parts of these rules, see their description. Bug players can never manipulate objects.