Origins The core stage, as with the rest of the Space Launch System, has its roots in the
NASA Authorization Act of 2010, which decreed that NASA build a
heavy-lift launch vehicle capable of lifting 70 tonnes of cargo to
Low Earth orbit by 2017. 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. These studies included an open competition between three teams – one investigating Space Shuttle-derived designs, another investigating large hydrocarbon-fueled booster rockets, and another studying modular vehicles on the basis of the
Evolved Expendable Launch Vehicles. Ultimately, the favored result of these studies was a Shuttle-derived architecture resembling both the 1990s
National Launch System-1 design proposal and the modern-day Space Launch System. However, this concept included a core stage derived from and identical in length to the Space Shuttle External Tank. It also possessed only three RS-25 engines, rather than four. By August 2011, External Tank production at MAF, contracted to
Lockheed Martin, had ended, with workers laid off and tooling broken up. Simultaneously,
Boeing began unsolicited projects, based on tooling it had acquired for the by-then defunct
Ares I rocket. Boeing had been selected to manufacture the upper stage of the Ares I, and the 2010 NASA Authorization Act instructed NASA to continue contracts signed for Ares I to the extent possible. In September 2011, a definitive design concept for the SLS core stage was presented. 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. The NASA Authorization Act of 2010 directed NASA to, where practicable, reuse Space Shuttle and Constellation program parts and contractors. To fulfill this, the manufacturing contract for the first two SLS core stages was initially awarded in 2012 under a modification (number 96) of the existing Ares stages contract to Boeing. The base contract had seen Boeing responsible for the delivery of the
Ares I upper stage; the modification changed the requirement entirely to encompass the requirements for two core stages. This modification was an undefined action that did not specify the extent of the work Boeing was to complete.
Design maturation By December 2012, the SLS core stage passed its
Preliminary Design Review, a milestone review. For several years, design maturation of the Space Launch System as a whole and of the core stage continued. In 2013, the decision was made to revert the core stage's structure from the
2195 aluminum-lithium alloy used on the Super Lightweight Tank, the definitive version of the Space Shuttle external tank, and the Ares 1 upper stage, to a harder
2219 alloy. Through 2013, the Michoud Assembly Facility began to prepare for production of core stages, receiving much of the tooling for their manufacture. In June 2014, the core stage passed its
critical design review, the final design milestone before the commencement of production. By then, hardware was already in production for the first flight article. In the same month, NASA negotiated a finalized contract for SLS stages with Boeing, as Modification 127 to the existing Ares stages contract. This contract adjustment accounted for production and delivery, in full, of the first two core stages.
Production troubles Since 2014, Boeing had begun to experience a number of issues in establishing SLS production that caused significant delays to schedule. A number of fuel tubes which had been contaminated before delivery by the supplier resulted in the decision to reinspect all engine section tubing, which caused a delay of several months. The Vertical Assembly Center, a critical tool in the manufacture of the core stage's propellant tanks, was initially improperly installed, such that the tool was not able to properly lift stage components into position. When the defect was discovered in September 2014, it became necessary to completely rebuild the VAC. Further delays resulted in February 2017 from a tornado strike at Michoud Assembly Facility, which damaged buildings and delayed production of the first core stage.
First launch campaign 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. However, the stage was not to avoid further trouble. Workplace closures in response to the rapid spread of the
COVID-19 pandemic cause months more of delays, before the culminating test of the stage, the hot-fire, was prematurely ended due to several test parameters exceeding their designated limits, which were described as overly conservative. A decision was subsequently made to repeat the test. The Green Run campaign ended in May 2021, after a successful hot-fire test, and the first core stage was shipped to Kennedy Space Center and moved into the Vehicle Assembly Building, where it underwent further work ahead of integration as the core of the first SLS. Following a period of repair work on the core stage's thermal protection system to address damage taken during the Green Run campaign, CS-1 was rotated and stacked as the central element of the Artemis I Space Launch System. 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. The first core stage flew a successful mission on November 16, 2022 before breaking up over the Pacific Ocean.
Beyond Artemis I 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. The core stage was then moved to the Kennedy Space Center by barge on July 16, 2024 before beginning integration with the rest of the stack in the Vehicle Assembly Building's High Bay 2 on December 19, 2024. In December 2022, Boeing made the decision to disperse some core stage production work to several sites, adding the
Space Station Processing Facility and
Vehicle Assembly Building High Bay 2 to the CS production flow. The company stated that the change was made to increase production capacity for core stages, to reduce cost per unit, and to enable the storage of stages not immediately needed. Projections were made of cost reductions of up to $50 million per stage as a result of the new flow, and Boeing stated that the increased use could enable it to produce two core stages per year.
Contract extension and reworking The procurement of the core stage has been extended to allow for production of more than the originally ordered two stages, and a future contract award is expected to transfer operations of the Space Launch System and with it responsibility for procurement of the core stage away from NASA. In October 2019, NASA funded and authorised the beginning of production work of the third core stage, with a full contract option finalization in December 2022 allowing work to proceed at full scale towards the production of the fourth core stage and clarifying funding for both the third and fourth. In July of that year, NASA also announced its intention to shift operations of the Space Launch System to
Deep Space Transport LLC, a joint venture of core stage manufacturer Boeing and boosters supplier
Northrop Grumman under the Exploration Production and Operations Contract. Deep Space Transport would be responsible for operating the SLS for NASA, and would also be permitted to market the rocket to other customers. The contract was not open to competition, as alternative bidders would have had to establish new production lines for the rocket's stages, including the core stage, for which an alternate production line was estimated to require up to 10 years to set up. == List of stages ==