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I have seen the use of the term 'core stage' in the context of space launch vehicles. I am not able to clearly understand what it exactly means? I have learnt about launch vehicles having multiple stages: first, second and so on. Is the core stage just a different name for the first stage itself?

I tried to google it but couldn't get a clear meaning for it. I had a look at the SLS core stage fact sheet by NASA which says a few things about core stage.

  • It says that the core stage stores cryogenic liquid hydrogen and liquid oxygen. So, this makes me think that this might not be the same as the first stage because you don't generally carry a higher $I_{sp}$ fuel like liquid hydrogen in the first stage (however, I may be completely wrong and maybe it is used in first stage here) and so this must stay at least till the stage which actually uses this cryogenic liquid hydrogen separates.
  • It mentions that it houses the flight computers and much of the avionics needed to control rocket's flight. This again makes me think that the core stage is not same as first stage because the avionics and flight computers should be necessary for rocket after first stage separation too. Is it that the upper stages have their own flight computers and avionic system?

Thank you.

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The information you found is specific to the SLS.

In general, the term 'core stage' is used when the rocket also uses boosters. Boosters complicate things when you talk about stages. The term "First stage" can include the boosters, for instance.

So a term was needed for a rocket stage that is started on the launch pad, which excluded the boosters. This is what 'core stage' is used for.

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Here's another example, the Falcon Heavy. The boosters are the same size as the core stage, although the core stage has a stronger construction and uses a different burn profile, so it burns longer than the boosters.

In this case, the core stage and boosters both burn RP-1 and LOX.

The flight computers and avionics are usually installed in the last rocket stage, because then you only need one set of computers. When you install computers in the first stage or core stage, you have to hand over operations to another set of computers when the core stage separates.

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