Complex sentence: The Rocketdyne RS-68 (Rocket System 68) is a liquid hydrogen/liquid oxygen engine developed starting in the 1990s with the goal of producing a simpler, less-costly heavy-lift rocket engine for the Delta IV rocket.
Split into simpler sentences: The Rocketdyne RS-68 (Rocket System 68) is a liquid-fuel rocket engine that burns LH2 with LOX. Development of the engine started in the 1990s with the goal of producing a simpler, less-costly heavy-lift engine for the Delta IV rocket.