NASA has plans to send a man and woman to the lunar surface this decade, in what will be the first landing with humans since Apollo 17 in 1972.
In the last 20 years, astronauts have been making routine trips to and from the International Space Station (ISS).
But the Moon is nearly 1,000 times further than the ISS; getting astronauts there requires a monster rocket.
The SLS is the modern equivalent of the Saturn V, the huge launcher built during the Apollo era. Like the Saturn, it is split into segments, or stages, stacked on top of each other. But the rocket also incorporates technology from the space shuttle.
The SLS design, which was based on NASA technical studies, was unveiled in 2011. After work started, delays and cost overruns gave ammunition to critics, who thought Nasa should rely on rockets operated by commercial providers.
But without significant modifications, no existing boosters have sufficient power to send Orion, astronauts and large cargo to the Moon in one flight - as the SLS would have.
A recent oversight report says Nasa will have spent more than $17bn on the SLS by the end of the 2020 fiscal year.
But with the rocket's development phase now over, and a programme of evaluation known as the Green Run successfully completed, the first SLS rocket is now being processed at Florida's Kennedy Space Center in preparation for its maiden flight in late 2021.