The F-1 engine used for the first stage of the Saturn V is still the most powerful liquid fuel rocket engine. The development of the F-1 was started very early in 1955 by the US Air Force. In 1958 the Air Force eventually halted development of the F-1 because of a lack of requirement for such a large engine, but the recently created NASA decided to continue the development. Without the F-1 engine a manned Moon landing would have been impossible.
What was the planned usage for the F-1 engine at the start of development in 1955?
Some thrust numbers used for early manned spacecrafts:
F-1 engine thrust 6,770 kN used for Apollo to the Moon (5 engines)
Vostok 4,795 kN used for first orbital manned flight.
Mercury-Redstone 350 kN used for suborbital manned flight.
Mercury-Atlas 1,880 kN used for orbital manned flight.
Titan II 1,900 kN used for orbital manned flight with two astronauts.
(For the rockets total thrust of all engines running at launch.)