Image copyright NASA Image caption The massive core section of the SLS has shuttle heritage in it

Nasa has finished assembling the main structural components for its largest rocket since the Apollo-era Saturn V.

Engineers at the agency's Michoud Assembly Facility (MAF) in New Orleans connected the last of five sections that make up the core of the Space Launch System (SLS).

The rocket will be used to send an uncrewed Orion craft to the Moon, in a flight expected to launch in 2021.

This will pave the way for crewed missions, with a landing in 2024.

The last piece of the SLS' 64m (212ft) -tall core stage was the complicated engine section. This will serve as the attachment point for the four powerful RS-25 engines, which are capable of producing two million pounds of thrust (9 meganewtons).

The RS-25 engines, built by Sacramento, California-based Aerojet Rocketdyne, are the same ones that powered the now-retired space shuttle orbiter.

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Julie Bassler, Nasa's SLS stages manager, said: "Now, to complete the stage, Nasa will add the four RS-25 engines and complete the final integrated avionics (electronics) and propulsion functional tests.

"This is an exciting time as we finish the first-time production of the complex core stage that will provide the power to send the Artemis 1 mission to the Moon."

It marks a significant milestone for the SLS rocket, which has experienced delays and cost overruns since being announced in 2010.

Nasa wants to send the first woman and the next man to the lunar South Pole by 2024. Artemis 1 will be followed by the first crewed mission, planned for launch in 2022, which will send astronauts on a loop around the Moon without landing. The first lunar landing since Apollo 17 in 1972 will occur on Artemis 3.

Image copyright NASA Image caption The first flight of the SLS will carry an uncrewed Orion capsule

Over the autumn, SLS engineers will work to attach the four RS-25s and connect them to the main propulsion systems inside the engine section.

The SLS consists of a core stage atop which the Orion spacecraft will sit, and two solid rocket boosters (SRBs) strapped to either side of the core. Orion is America's next-generation crewed spacecraft, capable of journeys to the Moon, asteroids and other targets in deep space. It could also be part of a transfer vehicle for human missions to Mars.

Eventually, Nasa plans to build a space station in lunar orbit called Gateway. The full station won't be ready by the 2024 landing, but astronauts on this mission could dock with a "lite" version of Gateway, consisting of just a power and propulsion module and a small habitat. They would then take a lander down to the surface.

In June, John Shannon, programme manager for the SLS at Boeing, the prime contractor building the rocket for Nasa, told me: "[The SLS is] built to carry crew and it's got the performance to take that crew where you want to take them.

"There are other rockets on the drawing board right now. But as far as a human-rated vehicle that's purpose-built to take Orion and parts of the Gateway station or landers to the Moon, SLS is really the only one."

Image copyright NASA Image caption The powerful quartet of RS-25 engines will power the core stage

Jennifer Boland-Masterson is the MAF operations director for Boeing. She said: "Boeing expects to complete final assembly of the Artemis 1 core stage in December."

She added: "After we deliver the stage, Nasa will transport it on the agency's Pegasus barge from Michoud to Nasa's Stennis Space Center near Bay St Louis, Mississippi, for Green Run testing."

The Green Run will involve test firing all four RS-25 engines for eight minutes - the full duration of the rocket's first flight into space. It will stay on the ground while the engines are fired at the B-2 test stand at Nasa Stennis. The Green Run will provide critical performance data needed to demonstrate the core stage design is flightworthy and ready for launch.

The SLS core stage incorporates state-of-the-art avionics, miles of cables and two huge liquid propellant tanks that collectively hold 733,000 gallons (2.7 million litres) of liquid oxygen and liquid hydrogen to power the four RS-25 engines.

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