NASA’s Space Launch System (SLS) is ready to fly with its four shuttle-era engines. The rocket, outfitted with the RS-25 engines, recently passed a critical milestone that put the integrated system to the test, using a decades-old design on a new launch vehicle.

NASA teams successfully completed the RS-25 engine checkout tests at the Kennedy Space Center in Florida, ensuring seamless communication between the SLS core stage and its engines ahead of the first crewed mission to the lunar environment in more than 50 years. Three of the four engines being used for the upcoming Artemis 2 mission have already flown a combined 22 missions as part of NASA’s iconic Shuttle program, which ran from 1981 to 2011, while one engine will be making its launch debut.

“I learned during the Space Shuttle program to listen to the engines,” Bill Muddle, RS-25 field engineer, said in a statement. “The engines talk to you, and you have to listen and understand what they are telling you to ensure they operate properly. They will tell you if they are in ill health and need to be tweaked or if a component is having an issue and needs to be replaced. Based on the [Program Specific Engineering Test], the engines all indicated they were healthy and ready for the pad.”

The RS-25 engines were built by L3 Harris Technologies’ Aerojet Rocketdyne division for the shuttle project, and NASA has a total of four contracts with the company. SLS Block 1, which launched the Artemis 1 mission in November 2022 and is meant to launch Artemis 2 and 3, is powered by four RS-25 engines in its core stage, along with two solid rocket boosters.

Three of the four engines that are currently being used to power SLS for Artemis 2 were part of significant milestones in the Space Shuttle program. Engine 2047 flew on the final shuttle mission on July 21, 2011, while engine 2059 flew on the program’s penultimate flight. Engine 2061 was part of the mission that assisted the assembly of the International Space Station. Engine 2062, on the other hand, is a newbie, ready to make its inaugural flight.

“Every day I come to work knowing that the RS-25 field engineering team has to take care of these engines, because we know we have humans riding in Orion on top of this vehicle and these engines have to perform flawlessly,” Muddle said. “The lives of our astronauts are in the RS-25 team’s hands for the eight and a half minutes those engines are firing during launch.”

The 5.75-million-pound SLS uses components from NASA’s Space Shuttle program, including solid rocket boosters built by Northrop Grumman, as a way to improve its affordability. NASA’s original thought process, however, did not pan out too well. The launch vehicle has already gone $6 billion over budget, with the projected cost of each SLS rocket being $144 million more than anticipated. That would increase the overall cost of a single Artemis launch to at least $4.2 billion, according to a report released in 2024 by the office of NASA’s inspector general.

The giant Moon rocket faces uncertainty under the current administration’s proposed budget, which laid out a plan to phase out SLS and its Orion capsule and replace them with commercial substitutes. This week, however, the Senate approved a budget reconciliation bill that would allocate an additional $6 billion to Artemis’ current mission architecture. If signed into law, the legislation may just give SLS, and its shuttle-era engines, a fighting chance.