Aerojet M-1 engine.

[Galane]

A while back, someone made a Rocketdyne F-1 engine for KSP. For a long time that was the most powerful rocket engine ever built. IIRC it's still the most powerful single chamber engine actually used on a rocket.

But back in the 1960's an even larger engine was on the drawing boards and prototype pieces were being built for the Aerojet M-1, designed to be run at 1.2 to 2 million pounds thrust. The turbopumps alone produced 75,000 and 27,000 horsepower for the hydrogen and oxygen, respectively. The turbopump exhaust was used to cool the lower section of the bell, exiting through nozzles around the bottom and contributing 28,000 pounds of thrust. Who needs closed cycle when the exhaust of the fuel pumps is *that frigging powerful* on its own?

http://en.wikipedia.org/wiki/M-1_(rocket_engine)

But alas, the Saturn rocket won out over the Nova for the race to the Moon and funding for the M-1 got diverted to Saturn and other projects.