The Delta IV Heavy uses hydrogen fuel even for the first stage: https://en.wikipedia.org/wiki/Delta_IV_Heavy https://en.wikipedia.org/wiki/Delta_IV_Heavy Hydr

The Delta IV Heavy uses hydrogen fuel even for the first stage: https://en.wikipedia.org/wiki/Delta_IV_Heavy https://en.wikipedia.org/wiki/Delta_IV_Heavy Hydrogen offers excellent ISP but poor volumetric energy density, and it needs to stay much colder than oxygen to remain liquid. Fuel tanks for containing hydrogen tend to be more complicated and larger than those for other liquid rocket fuels. Upper stages see more relative benefit from the higher performance achievable with hydrogen fuel. The cost:benefit comparison has led to most rockets not using hydrogen for the first stage.