Artemis II’s Core Stage Heads to Florida

Co-Authored by Scarlet Dominik and Emily B.

The Rollout Event

The second SLS core stage, or CS-2, is now aboard NASA’s Pegasus barge following its rollout from NASA’s Michoud Assembly Facility in New Orleans. CS-2 is set to become the heart of the first SLS rocket that will carry human astronauts as part of NASA’s Artemis II mission. Rollout of CS-2 occurred at 6:00 AM Central Time on the 16th of July, 2024–the 55th anniversary of Apollo’s 11 launch. Members of the press were permitted at the rollout of the massive, 212 feet tall stage as it was rolled out of the facility. Space Scout team members who attended captured dozens of photographs of Artemis hardware and recorded talks given at the event. Rollout culminated with the loading of CS-2 onto NASA’s Pegasus barge for transport to Florida.

Speaking at the event was Artemis II Mission Commander Reid Wiseman and Canadian Mission Specialist Jermey Hansen. “The hardware is here, the Orion spacecraft is at Kennedy Space Center, our boosters are at Kennedy Space Center. We just watched the Core Stage roll by, its next stop is Kennedy Space Center,” remarked Commander Wiseman when asked about the significance of seeing the stage which will carry his crew past the Moon. When asked by a reporter what the “Earthrise moment” of the Artemis II mission would be, Wiseman remarked that part of why the Earthrise captured by Apollo 8 was special was because it was not anticipated. By nature, we can’t know what Artemis II’s version of this moment will be until the mission finally flies. “That unexpected moment on Apollo 8 is what captured all of us, and we can’t wait for our unexpected moment.”

When astronauts were asked about any personal items they were intending to carry on the Artemis II trip around the Moon, Jeremey Hansen stated that his children received a very special gift for Christmas last year. “The thing my wife and I came up with this Christmas, we purchased (each child) a little pendant with our family birthstones in it,” he explained, “when it’s time to go I’ll take them, fly them around the moon, and bring them back to them.” Commander Wiseman stated he carries a picture of his own two children on every spaceflight, but for Artemis II intends to bring a blank piece of paper and pen to record the experience and emotions of flying by the Moon in the moment.

Matthew Sterling, Boeing First-Line Tooling and Equipment Manager, also answered questions from the press. Space Scout’s David Diebold asked what the specific process for securing the Core Stage safely aboard the Pegasus Barge looked like. Sterling explained that the pedestals of the cradle system on which Core Stage 2 is secured are bolted to standoff connections in the floor of the barge, and a series of D-ring across the Pegasus’ floor are chained to various connection points across the cradle and are appropriately torsioned. Sterling also noted that while Pegasus also carried External Tanks for the Shuttle Program, the range of dynamic load factors that most be accounted for during transportation have “increased since the External Tank days, just for added factors of safety,” and also due to the increased weight of the Core Stage itself. Diebold also asked if the rollout of Core Stage 1 and lessons learned allowed this latest event to run smoother; Sterling answered that a number of “dry-run” rehearsals using a simplistic stand-in of the core stage allowed the workforce at Michoud to maintain vital experience on the operation. Sterling explained that maintaining experience like this is a necessary practice for the SLS program, as with a slow production rate factors like employee turnover and workforce attrition must be offset.

The stage is now en route to Kennedy Space Center. Prior to rollout, NASA’s Pegasus Barge was anticipated to deliver CS-2 sometime in late July, though an official date has not been made public now that the barge is underway. Upon arrival at NASA’s Kennedy Space Center, the stage will be unloaded and transported to NASA’s Vehicle Assembly Building.

The Vehicle Assembly Building (VAB) will be where Core Stage 2 spends the majority of its remaining time on Earth. In the VAB, CS-2 will see work to be readied for flight on Artemis II and, most importantly, be integrated with other mission elements already waiting at the Space Coast.

What’s Next for Core Stage-2?

Several elements of Artemis II are already present at the Cape. The segments which make up the SLS rocket’s two Solid Rocket Boosters arrived at Kennedy Space Center in September of 2023. The Orion capsule, which will carry Artemis II astronauts around the Moon and back, has been undergoing testing at the Neil A. Armstrong Operations and Checkout Facility, also at KSC, and is nearing completion. The Launch Abort System which would carry the crew away from the launch stack in the case of a launch-related emergency has been at KSC since December of 2021. The Interim Cryogenic Propulsion Stage (ICPS) for Artemis II has been at KSC since 2021 and began pre-flight testing in April of 2022, which it has since completed. ICPS-2 will provide Orion the final push into its high Earth parking orbit after which it will be used as a target for the Orion Proximity Operations Demo. The Launch Vehicle Stage Adapter (LVSA) that will join ICPS-2 with CS-2, and the Orion Stage Adapter (OSA) which will join Orion to ICPS-2 are complete, but are waiting at the Marshall Space Flight Center.

Core Stage 2’s first step after arriving at Kennedy Space Center will be the installation of the Flight Termination System (FTS). The FTS is a package of explosives designed to destroy SLS in a controlled explosion in the event of a launch emergency, wherein the vehicle is out of control and must be destroyed to mitigate harm. FTS installation is done while the Core Stage is horizontal inside the VAB’s transfer aisle, the massive hallway between the VAB’s four high bays.

Stacking of NASA’s Moon rocket cannot begin until Mobile Launcher 1 rolls back to VAB High Bay 3 from Pad 39B. ML-1 has been at 39B undergoing various rounds of testing since October of 2023 to verify all ground and pad-side systems are ready to support the crewed flight to the Moon. On July 19th, 39B’s sound suppression system was tested, leaving only three major test objectives to complete prior to ML-1 rollback. Stacking will begin with the twin SRBs which are assembled in five segments, followed by the forward avionics and nose assembly. The SRBs have a 12-month shelf-life that begins when the first two segments become joined, due to sustained stress placed on the boosters while left stacked. While this is not strictly required as Artemis I launched 22 months after its first SRB segments were joined, it is preferred for vehicle health that Artemis II launches within this timeframe.

Once the SRBs finish stacking, Core Stage 2 will be lifted from the transfer aisle and lowered in between the waiting boosters. Prior to this operation, CS-2 may be used to perform fit-checks on new hardware Boeing has installed in High Bay 2 for SLS production, but while there has been some interest in such a test it has not been confirmed that it will take place. After the core stage is joined with the SRBs, the LVSA will be stacked atop it, followed by the ICPS. Stacking will be completed with the integration of the OSA, followed by the complete Orion spacecraft, stacked while within its launch shroud with attached Launch Abort System.

Once the rocket is fully assembled, it will undergo an integrated test campaign, wherein systems on both the vehicle and ground-side will be put through their paces to ensure flight-ready performance. This includes verification of all of the interfaces between SLS and the mobile launcher and communications system checks. Integrated in-VAB tests will be less intensive than Artemis I, which included tests of the umbilical release systems and a countdown sequence check, neither of which are necessary following the success of Artemis I. SLS will be rolled to launch Pad 39B for a series of final checks including a full wet dress rehearsal, where the vehicle will be filled with propellant and undergo the full launch sequence up to the point where the boosters and engines would normally be lit. Following completion of these test objectives, SLS will rollback to the VAB for the installation of the batteries which power its Flight Termination System, and rollout a final time ahead of launch.

Following the completion of the test campaign, SLS and Orion will be prepared to launch the Artemis II mission, carrying Commander Reid Wiseman, Pilot Victor Glover, and Mission Specialists Christina Koch and Jeremy Hansen on a mission to cislunar space. The exact timeline of the Artemis II test campaign is unclear, but launch is currently expected in September of 2025.

How is a Core Stage Made?

Every SLS Core Stage goes through the same rough stages of production on the way from metal slabs to a finished rocket stage. First comes fabrication, where metal panels and forgings are turned into the basic structural components of the stage: rings, domes, and barrels. After these have been fabricated, they must be welded together to form tanks and other structures. After this, the finished pieces undergo non-destructive evaluation (NDE) of the welds before moving into integration preparations. Integration preparations include internal cleaning, adding primer paint to guard against corrosion, adding thermal protection foam sprays, and installing baffles into the fuel tanks. Integration is the process of adding functional subsystems to the mostly bare structures. These include avionics boxes, wiring harnesses, propellant feedlines, hydraulic lines and actuators, brackets, pneumatics, and tank fill level sensors. Integration is for many elements the longest single stage of production, and due to export control restrictions few specifics of the process are publicly known.

As elements finish integration they are prepared to be joined into progressively larger assemblies. The intertank, LOX tank, and forward skirt are joined vertically into the “Forward Join”. This is joined horizontally in MAF’s final assembly area with the LH2 tank to create a “4/5ths” assembly. The location of Engine Section integration depends on the build.

Each of these joins is followed by its own potentially lengthy integration and functional test phase, culminating in the Final Integrated Functional Test (FIFT), where all of the stage’s functions are exercised to demonstrate that it can operate as a complete standalone rocket stage. After a few more weeks of closeout and preparations, the stage is rolled out to NASA’s Pegasus barge to be transported to KSC.

Where Are The Other Core Stages Now?

Core Stage-3 is the farthest along of the three core stages currently in production. It is also arguably the stage with the most storied history and the stage that is currently slated to launch the first humans to land on the Moon since Apollo.

After completing integration preparations, the Forward Skirt was moved from its integration area to a foam spray booth for TPS application in mid-December, 2022. As of December 2023, it was expected to be ready for the forward join well ahead of mid-2024, and is likely in storage. The Intertank is also finished with integration preps and is ready for the forward join. There is still a significant amount of integration work to be done, but it is dependent on the LOX tank being joined. That operation is expected to happen in the fall of this year ahead of a 4/5ths shipment to KSC around the end of the year. 

Next in line are both propellant tanks, which are preparing for integration. The LH2 tank is currently receiving its SOFI sprays, while the LOX tank has passed its proof test and is undergoing post-test evaluation ahead of primer application. The LOX tank is currently the critical path item for the Michoud work on CS-3 due to a yearlong welding delay caused by a tooling issue.

Finally, the Engine Section is the farthest along element, undergoing subsystem integration since early 2023, but it also has the most work left to go before the stage is completed. In December 2022, it was shipped to the Kennedy Space Center to begin subsystem integration after completing structural assembly. While few updates have been given since arrival, it is expected to be ready for joining to the 4/5ths assembly by the end of the year.

While resequencing and procedure changes are not uncommon for SLS due to the continual adjustment to new circumstances and learning curve, the unique situation of Core Stage-3 means that its history is the most convoluted of any so far and likely any to be built in the future. 

CS-3’s LH2 tank was originally destined for Core Stage-1 when it was originally manufactured back in 2016. After welding had completed, weld strength issues were discovered in other articles and so the tank could not be trusted and was set aside in favor of a new tank made with corrected welding equipment. A repair procedure was developed in the intervening years and was successfully used to make the tank acceptable for flight. This meant that the tank was suddenly ready for integration preparations while the LOX tank was still in the welding stage, and so it was moved ahead in the production order. 

The LOX tank, however, remained unfinished and three-quarters complete in the Vertical Assembly Center (VAC) welding tool for over a year, finally being completed in February 2024. The original aft dome was slightly too large when fully assembled due to a tooling problem and so was set aside in favor of a newly made one. The original dome may be reused on a later build.

Since the inception of the Artemis Program in 2019, Core Stage-3 was intended to launch the first crew to land on the moon since Apollo 17, the Artemis III mission. Simultaneous with that early stage of planning, NASA began procuring the lander that would be used on the mission, the debut of which moved from an original date of 2024 to no earlier than the still uncertain and likely to slip September 2026. NASA has a desire to minimize the gap between SLS launches to maintain institutional knowledge and prevent skill loss, so it is possible an alternate mission will be devised for Core Stage-3. While the mission goals of Artemis III are not set in stone, for the time being the landing is still baselined and work continues on.

Earlier in production but ramping up quickly is Core Stage-4, due to launch Artemis IV no earlier than September 2028. The Engine Section has completed structural assembly and is awaiting transport to KSC for integration next to Core Stage-3’s. Like on the first and second builds, it is likely to be the overall pacing item for Core Stage readiness.

The Intertank is still in its assembly tool being outfitted with other structural elements such as brackets and umbilical plates. It has not yet received its TPS foam ahead of the Core Stage-4 forward join and the associated subsystem integration.

The Core Stage Forward Skirt barrel finished welding in September 2022 and has been in storage since. It has likely not received its attachment rings yet due to other hardware using the required weld tools.

The propellant tanks are deep into the welding stage. The barrels and domes for both tanks are largely complete.

Unlike Core Stage-1 or 3 and their relatively tight deadlines, Core Stage-4 is not needed until mid-late 2027 at the earliest, so it is possible it will undergo a similar extended period of completed but undelivered to Core Stage-2. Core Stage-4 will also be the first Block 1B Core Stage. While there are no major alterations to the design as part of the change, the flight computers and navigation unit will be moved to the Exploration Upper Stage (EUS). These items are normally located in the forward skirt, so slightly less integration and testing will be needed. It remains to be seen if this will meaningfully affect the completion time, though. 

Even though Artemis V is not launching before 2030, Core Stage 5 has already entered fabrication. Rings for the LH2 tank domes have begun welding. No other components have been seen so far, but many are likely being fabricated and delivered right now. The Engine Section barrel is likely to appear soon, and tank barrels and domes not far behind. The Engine Section takes by far the longest amount of time, so it must be started before most other elements enter focused production.

With Core Stage 2 on its way to Cape Canaveral and Core Stages 3, 4, and 5 in production in New Orleans, NASA and Boeing are ramping up to enable more frequent SLS flights and more capable missions. Artemis II is now entering the final stages of preparations before returning humans to cislunar space for the first time since Apollo with a historically diverse crew. New processes for production of the next Core Stages aim to ensure NASA is ready to tackle future Artemis flights in a timely manner, with significant Artemis III hardware expected to arrive at the Space Coast before the end of the year. The Artemis Program’s ramp up has only just begun.

Acronyms: 

ICPS – Interim Cryogenic Propulsion Stage

LVSA – Launch Vehicle Stage Adapter

OSA – Orion Stage Adapter

FTS – Flight Termination System

LOX – Liquid Oxygen

LH2 – Liquid Hydrogen

NDE – Non-Destructive Evaluation

VAC – Vertical Assembly Center

VWC – Vertical Weld Center

EUS – Exploration Upper Stage