What element is most commonly used as rocket fuel?

The most commonly used element as rocket fuel is liquid hydrogen. This choice stands out because liquid hydrogen serves primarily as a propellant due to its high energy content and efficiency when burned with an oxidizer, making it ideal for propulsion systems in space exploration.

When liquid hydrogen is combined with an oxidizer, such as liquid oxygen, it creates a powerful propulsion system that is capable of producing significant thrust necessary for launching spacecraft. This combination is frequently seen in many prominent rocket engines, including those used in the Space Shuttle and various other launch vehicles.

While liquid oxygen is essential as an oxidizer in many rocket engines, it does not function as a fuel on its own. Therefore, while it plays a critical role in the combustion process, it is not accurate to classify it as the fuel element in this context.

Liquid nitrogen and liquid helium are not typically used as propellants in rocketry. Liquid nitrogen mainly serves as a coolant or for inert gas applications, while liquid helium is mostly utilized in cryogenic cooling applications, particularly in maintaining temperatures necessary for certain scientific instruments or equipment rather than as a fuel for launching rockets.