Liquid fuel rocket engines often require complex pumps and valves to properly move (and pressurize) the liquids from the fuel tank to the actual engine. These machines must work in extreme temperatures and pressures. Liquid oxygen is very cold (-223 �C) while the engine is very hot (3000 �C), and the pressure is oftentimes hundreds of times higher than the surrounding air pressure. Because of these conditions, liquid fuel rocket engines are often very complex, expensive, and require very specialised materials (metal alloys, ceramics, etc.).