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The sign on the J-2 engine. It reads

J-2 Engine

A single engine powered the second stage of the Saturn IB launch vehicle used to place astronauts into Earth orbit for Apollo lunar program spacecraft tests, and for the three Skylab missions.

One re-startable engine powered the S-IVB third stage, and five engines powered the S-II second stage of the Saturn V launch vehicle during 11 Apollo manned flights to the Moon. Five engines on the S-II second stage also propelled the Skylab orbiting laboratory into Earth orbit.

The five S-II engines burned for about 6 1/2 minutes.