NASA has installed all four RS-25 engines onto the Artemis II Space Launch System core stage, completing a major assembly milestone for the first crewed lunar mission since the Apollo era. Built at a factory in Canoga Park, California, these engines carry a perfect flight record from the Space Shuttle program and will fire for over eight minutes during launch to send astronauts toward the Moon. With the rocket’s upper stage now integrated inside the Vehicle Assembly Building, the hardware that will carry a crew around the Moon is taking its final shape at Kennedy Space Center.
Four Engines, Zero Failures
The RS-25 is not a new design. Originally developed as the Space Shuttle Main Engine, it flew on every Shuttle mission from 1981 through 2011. Pratt & Whitney Rocketdyne of Canoga Park, Calif., built the engines, which operated with 100% mission success across that entire program. That track record is the reason NASA chose to adapt the RS-25 for the Space Launch System rather than develop an entirely new engine from scratch. For a mission carrying humans beyond low Earth orbit for the first time in more than half a century, proven reliability matters more than novelty.
Each SLS core stage uses four RS-25 engines. During launch and flight, those four engines will fire nonstop for over eight minutes, consuming liquid hydrogen and liquid oxygen from the core stage’s two propellant tanks. That sustained burn provides the thrust needed to push the SLS through the atmosphere and toward orbital velocity before the core stage separates. Paired with two solid rocket boosters, the RS-25 cluster forms the heart of the Artemis II launch vehicle’s propulsion system.
Assembly Milestones at Kennedy Space Center
Engine installation on the Artemis II core stage began in September 2023, when technicians at Kennedy Space Center bolted the first RS-25 into place. The process required precise alignment and extensive checkout of each engine’s connections to the core stage’s propellant feed lines, avionics, and thrust structure. NASA confirmed that all four engines are now mounted, clearing the core stage for its final assembly phase.
One detail that often gets overlooked in coverage of Artemis II is that NASA swapped a 10-year-old engine with one nearly twice its age during the build process. That decision reflects a practical reality: the agency is drawing from a finite inventory of heritage Shuttle-era engines, and each unit’s maintenance history and test data determine which ones are best suited for flight. The swap shows that calendar age matters less than verified condition and performance margins when selecting flight hardware, especially for a mission that will carry astronauts farther from Earth than any crew has traveled in decades.
With the core stage engines secured, integration work shifted to the rocket’s upper segment. The Interim Cryogenic Propulsion Stage, known as the ICPS, has now been stacked inside the Vehicle Assembly Building. The ICPS provides the push that sends the Orion spacecraft out of Earth orbit and toward the Moon after the RS-25-powered core stage completes its job. Together, the two stages form a sequential propulsion chain: the RS-25 engines handle the heavy lift from the launch pad, and the ICPS handles the trans-lunar injection burn that places the crewed capsule on its looping path around the Moon.
Canoga Park’s Role Beyond Artemis II
The Shuttle-era RS-25 inventory is finite, which means NASA cannot fly the heritage engines indefinitely. To sustain the Artemis program through later missions, the agency awarded Aerojet Rocketdyne a contract valued at $1.16 billion to restart production and modernize the engine design. That agreement covers certification of updated components intended to reduce manufacturing complexity and cost while preserving the engine’s reliability baseline.
The new-production engines are being built at Aerojet Rocketdyne’s Canoga Park facility, the same San Fernando Valley site where the originals were manufactured decades ago. NASA has described the effort as a smart manufacturing initiative, applying updated fabrication techniques to parts that were previously hand-built using methods dating to the 1970s and 1980s. The goal is to produce engines that perform identically to the heritage units but cost less and take less time to assemble, with additive manufacturing and automated inspection replacing some of the most labor-intensive legacy processes.
Most coverage of the Artemis program frames the RS-25 as a safe, conservative choice. That framing is only half right. Restarting production of a decades-old engine design with modern tooling introduces its own risks. Workers and engineers who built the originals have largely retired, and institutional knowledge does not transfer automatically through updated CAD files or digitized blueprints. The $1.16 billion contract reflects the real cost of bridging that gap, not just buying metal and fuel. NASA and Aerojet Rocketdyne must demonstrate that new-production engines can match the Shuttle fleet’s performance while meeting stricter affordability targets.
Testing the Next Generation
To verify that new-production RS-25 engines meet flight standards, NASA runs extensive hot-fire test campaigns at Stennis Space Center in Mississippi. Teams there installed new-production engine E20001 on the Fred Haise test stand for a series of firings designed to validate the updated design. Each test cycles the engine through start-up, throttle changes, and shutdown while sensors capture data on pressures, temperatures, and vibration throughout the system.
These hot-fire runs serve several purposes at once. They confirm that redesigned parts behave as expected under full power, they help refine operating procedures for future launch campaigns, and they provide engineers with a direct comparison between heritage hardware and new-production units. Any deviations from predicted performance can trigger design tweaks or manufacturing adjustments before engines are cleared for use on later Artemis missions. In effect, Stennis functions as the proving ground where the next generation of RS-25s must earn the same trust the Shuttle fleet enjoyed.
For Artemis II, however, the core stage will still rely on refurbished Shuttle-era engines with extensive flight history. Those units have already been through multiple test campaigns, including acceptance firings and integrated checks with the SLS core stage. Their performance margins, combined with the upcoming ICPS and Orion system verifications, form the backbone of NASA’s safety case for sending astronauts on a lunar flyby. As the agency moves toward that mission, the work at Canoga Park and Stennis is laying the groundwork for the rockets that will follow, enabling a sustainable cadence of launches rather than a one-time return to the Moon.
Taken together, the completed RS-25 installation, the integrated upper stage, and the parallel push to modernize engine production mark a transition point for Artemis. The program is shifting from assembling its first crew-capable vehicles to proving that it can support a long-term presence in deep space. The engines now bolted to the Artemis II core stage represent both the culmination of Shuttle-era engineering and the bridge to a new generation of lunar missions, with each test firing and factory upgrade aimed at keeping that legacy alive on the road back to the Moon.
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*This article was researched with the help of AI, with human editors creating the final content.
