RS-84

The RS-84 was a liquid-fuelled rocket engine that Rocketdyne developed a plan for as part of NASA's Space Launch Initiative in the early 2000s. It was designed to burn kerosene with liquid oxygen in a staged combustion cycle, and it would have been the first reusable engine of that kind, using a hydrocarbon fuel, ever produced in the United States. The Soviet Union had reached that point earlier with the RD-170 on the Energia rocket in the 1980s, and the RS-84 was in effect the American attempt to catch up. It was conceived as a large booster engine of about one million pounds of thrust, with reusability, safety and quick turnaround between flights as its main aims. The engine passed a preliminary design milestone, but NASA cancelled further development in 2004, so the RS-84 remains a design study and never became a flying engine.