Its not that simple. You can't just do it, but the idea exist, ULA is planning something along those lines for 2024. See: https://en.wikipedia.org/wiki/Advance

Its not that simple. You can't just do it, but the idea exist, ULA is planning something along those lines for 2024. See: https://en.wikipedia.org/wiki/Advanced_Cryogenic_Evolved_Stage https://en.wikipedia.org/wiki/Advanced_Cryogenic_Evolved_Sta...