A hydrogen based combined steam cycle apparatus having an irreversible isobaric Rankine steam cycle portion, an irreversible isobaric Carnot steam cycle portion and a reversible isobaric Rankine steam cycle portion, all three portions of which operate simultaneously. The apparatus includes a source of liquid oxygen, a source of liquid hydrogen, a combustion chamber, a first pump, a second pump, a pressure vessel, a steam turbine, a superheater, and a condenser.A plurality of valves and a computer are used to control the flow of working fluid in the hydrogen based combined steam cycle apparatus.