Many modern liquid rocket engine powered space launchers also use quite a lot of helium for tank pressurisation in flight. Helium being inert with very low melt

Many modern liquid rocket engine powered space launchers also use quite a lot of helium for tank pressurisation in flight. Helium being inert with very low melting point is ideal for this use case, especially with cryogenic propelants. With the most modern generation of reusable space launchers such as SpaceX starship an alternative solution is being worked on, possible thanks to both fuel (methane) and oxidizer (oxygen) they use being cryogenic. Instead of pressurizing with helium they heat up a bit of the cryogenic liquids using the rocket engine heat, let it expand into gas and use it to pressurize the tanks, no helium required! One of the reasons is that for fully reusable space launchers the main cost is the propelant and helium in the amounts needed would be a big part of that!