How do heat pipes work for spacecraft thermal control?

Heat pipes are passive thermal devices that transfer heat efficiently using phase change. Operation: Working fluid evaporates at hot end (evaporator), Vapor travels to cold end (condenser), Heat released as vapor condenses, Liquid returns via capillary wick or gravity. Advantages: High thermal conductivity (100x copper), Passive (no moving parts, no power), and Reliable with long life. Types: Constant conductance (fixed capacity), Variable conductance (gas-loaded for temperature control), and Loop heat pipes (longer distances). Applications: Electronics cooling, Radiator integration, and Isothermalizing structures. Working fluids: Ammonia (-60 to +100C), water (0 to +200C), and others for extreme temperatures. Spacecraft use heat pipes extensively to spread heat from electronics to radiators, maintaining component temperatures within limits.