The stage lifts the rocket to an altitude of approximately 162 km (531,380 ft) before separating, reentering the atmosphere over the Pacific Ocean.

The CAPU is a turbine which is spun by pressurized gaseous helium during vehicle startup, then by hydrogen gas, as opposed to its Space Shuttle usage when it was powered by the flow of hydrazine.

The main propulsion system works to reduce the risk of fire in the engine section: while staged for work and servicing, the engine section is purged with clean air; on the launch pad, during flight preparations, the space is filled with nitrogen gas supplied from ground support equipment to mitigate the buildup of hazardous gases like hydrogen or oxygen.

Before flight, the core stage also receives all of its supplies through the MPS, with propellants and helium pressurant flowing through quick-disconnect connections of the tail service mast umbilical, interfacing with the vehicle on a plate located on the engine section.

[11] The core stage is also made from a different, harder aluminum alloy than that in the definitive version of the external tank, which was lighter but more difficult to work with.

[15] These tools are generally designed to enable the friction-stir welding, both self-reacting and conventional, circumferential and linear, of the 2219 aluminium-copper alloy of much of the core stage.

[17] The core stage's 2 propellant tanks – the "wet' structures – are each built up from a number of barrels, 2 rings, and 2 domes.

[18] Substantial technology development was required to enable the self-reacting friction stir welding of the core stage's propellant tanks, as the metal making up their walls was thicker than any previously joined using the technique.

Accordingly, it features external stiffener ribs and thicker structure,[20] which prohibit the use of welding for assembly; therefore, the intertank is bolted together from eight panels using more than 7500 fasteners.

Considered the least complicated and complex of the "dry" structures, its assembly is more straightforward and takes less time than any of the other main elements of the core stage.

[16] For engine sections integrated at Michoud Assembly Facility, small enclosures with airlocks were set up surrounding the article, torn down and rebuilt as needed.

[23] The tanks then are given their spray-on foam thermal protection system, which gives the stage its distinctive orange appearance.

[25] This insulation itself is made up of isocyanates and a blend of polyols, separate before application, and mixed in a spray head that allows them to release in the form a foam.

[22] The forward skirt receives its TPS application by being put on a turntable and rotated around a fixed robotic spray head that applies the foam.

[27] These elements are not immediately joined to the liquid hydrogen tank, but instead undergo substantial outfitting work before being rotated back to a horizontal configuration.

Tasks include installation of the systems tunnel, a cable routing visible on the exterior of the stage, the LOx feedlines, LH2 manifold, and completion of work inside the intertank, where avionics equipment is housed.

By contrast, subsequent stages will see the four-fifths transported to the Kennedy Space Center in Florida, where it will meet the engine section, both elements having been moved to High Bay 2 of the Vehicle Assembly Building.

Boeing plans to have each part of the vehicle as complete as possible before integrating them, allowing the core stages to be finished with minimal work in the High Bay.

Once completed, the stages are to be stored in the former Space Shuttle External Tank storage and checkout cells, awaiting stacking with the rest of an SLS vehicle.

In response to this requirement, the Marshall Spaceflight Center (MSFC) began a series of studies intended to provide a basis on which NASA HQ would decide upon a suitable launch-vehicle architecture.

[31] This proposal, however, was turned down by NASA administrator Charles Bolden, who requested a design that could be evolved to one capable of lifting 130 metric tonnes to orbit.

[33] By August 2011, External Tank production at MAF, contracted to Lockheed Martin, had ended, with workers laid off and tooling broken up.

It was to be 8.4 meters in diameter, longer than the Space Shuttle external tank, and powered by four RS-25 engines, a configuration broadly similar to the core stage as built.

During this period, the core stage began to be depicted unpainted, in the natural orange color of its thermal protection system.

Before the critical design review, the stage had been shown as painted white and black in a scheme reminiscent of the Saturn V.[41] Since 2014, Boeing had begun to experience a number of issues in establishing SLS production that caused significant delays to schedule.

However, when items were welded using the new, redesigned pins, they tended to randomly exhibit sections of low strength, which appeared in approximately one out of fifteen (6.7%) of test panels investigated after the deficiency was discovered.

[36][44] Through these issues, by the beginning of 2020, the first core stage was complete and ready to move to Stennis Space Center for the Green Run test campaign.

[50][51] Thereafter, it took part in the testing campaign of that vehicle, encountering issues related to the core stage's tail service mast umbilical connection, but successfully completing the sequence.

[53] In the 2023-24 time period, the second core stage, intended for its first crewed launch on Artemis II, was integrated in Building 103 at the Michoud Assembly Facility, with its engines fully installed.

[54] The core stage was then moved to the Kennedy Space Center by barge on July 16, 2024[55] before beginning integration with the rest of the stack in the Vehicle Assembly Building's High Bay 2 on December 19, 2024.