Cryogenic fuel, also known as supercooled fuel, is a type of fuel that is kept at extremely low temperatures in order to maintain its liquid form. Some synonyms for cryogenic fuel include liquid hydrogen, liquid oxygen, and liquefied natural gas. Liquid hydrogen is a commonly used cryogenic fuel in rockets and spacecraft due to its high energy density and ability to easily combine with other elements. Liquid oxygen is also commonly used in rockets, as it helps to ignite the fuel and propel the craft forward. Liquefied natural gas, or LNG, is used as a alternative fuel in vehicles and ships due to its high energy efficiency and lower emissions compared to traditional fossil fuels.