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NASA completes key engine test of its massive SLS moon rocket

The agency hopes to eventually use the Space Launch System rocket to fly astronauts to the moon, but it has been plagued by problems

On Jan. 16, the core stage for the first flight of NASA's Space Launch System rocket undergoes a hot-fire test at NASA's Stennis Space Center in Mississippi. All four core-stage engines fired for barely a minute, rather than the intended eight minutes. (Robert Markowitz/NASA/AP)

Two months after being forced to cut short a major engine test of its moon rocket, NASA on Thursday successfully completed the test, firing four of the engines for more than eight minutes, sending a massive plume billowing into the air.

Before the test, NASA and its prime contractor, Boeing, said they needed to fire the engines for just four minutes to get the data needed to proceed with the first launch of the Space Launch System after years of development. But the RS-25 engines were able to simulate an entire flight, burning through some 733,000 gallons of propellant, while bolted down to a stand at NASA’s Stennis Space Center in Mississippi.

After the engines shut down, applause broke out in the mission control center. NASA officials said they were optimistic that the test achieved all of its milestones and would pave the way for the first launch of a booster more powerful than the one that launched the Apollo astronauts to the moon five decades ago. The SLS rocket would be used to fly NASA’s astronauts to the moon for the first time since the Apollo era.

“Clearly there’s a lot of data now that’s going to have to be analyzed,” said Bill Wrobel, NASA’s manager of the test campaign. “The engineers need to see what worked and what didn’t, or what needs to be tweaked and what doesn’t. But that said, I think the applause says a lot about how the team feels. They got through the test, and it looks pretty good right now.”

In a statement, acting NASA Administrator Steve Jurczyk said the SLS “is an incredible feat of engineering and the only rocket capable of powering America’s next-generation missions that will place the first woman and the next man on the Moon.” He said that the successful test "is an important milestone in NASA’s goal to return humans to the lunar surface – and beyond.”

The test came as the White House is preparing to nominate former U.S. senator Bill Nelson to be the next NASA administrator. If confirmed, Nelson would oversee the next phases of the program and the first launch of the SLS. That was supposed to come by the end of this year, but it’s not clear whether the rocket will be ready in time.

Completing the test, though, was a much-needed bright spot for a program that has suffered years of delays and setbacks. In January the test ran for just over one minute, and as a result NASA and Boeing were forced to redo the test, known as the “Green Run.”

The shortened January test came after sensors detected a problem with the hydraulic system that steers the rocket by moving the engines during flight. The issue would not have affected an actual launch because the space agency had set “intentionally conservative” limits to protect the rocket.

After that shortened test, there was another setback when NASA and Boeing had to replace a valve inside the rocket. That, too, has been fixed, NASA said.

The space agency intends to use the SLS rocket to propel the Orion crew capsule, built by Lockheed Martin, to the moon as part of its Artemis program. NASA had hoped to launch the first flight, known as Artemis I, a mission that would fly Orion around the moon without any astronauts aboard, by the end of this year. But given the recent delays, that appears unlikely to happen.

If NASA is able to complete that mission, it will follow up with Artemis II, a flight with astronauts around the moon, followed by Artemis III, which would return astronauts to the lunar surface for the first time since the last Apollo mission in 1972.

Artemis was a signature effort of the Donald Trump administration, which called for Artemis III to land on the moon by 2024, and one of the few embraced by President Biden’s White House. Still, NASA has ordered a comprehensive review of the program, and the 2024 mandate is no longer achievable, officials have said, given the amount of funding Congress has appropriated.

“Today’s successful SLS test brings us one critical step closer to returning to the moon and, someday, landing humans on Mars,” said Rep. Don Beyer (D-Va.), chairman of the House space subcommittee. “After years of development, it’s gratifying to see important and encouraging progress in this key system, which we hope will eventually open opportunities for other scientific missions in addition to NASA’s moon-Mars program.”

The SLS program had been beset by cost overruns and schedule delays. Earlier this year, the NASA inspector general said the total cost of the Artemis program through fiscal year 2025 would reach $86 billion, with $27.3 billion just for the SLS rocket.

The powerful rocket, standing 212 feet tall and weighing more than 2.3 million pounds, has become a symbol of Boeing’s prowess — and problems, including the deadly crashes of its 737 Max airliner and the failure of its Starliner spacecraft to reach the International Space Station during a critical test flight more than a year ago. A redo of that test is scheduled for this spring.

In addition to the four RS-25 engines, the SLS’s core stage will have two solid rocket boosters strapped to the side. The avionics computers have 18 miles of cabling and more than 500 sensors. And fully fueling the rocket with the supercooled liquid hydrogen and liquid oxygen requires 114 tanker trucks.

The RS-25 engines are not new. They were repurposed from the space shuttle, and each of the engines mounted to the SLS core stage has flown to space before. Combined, the engines served in 21 shuttle missions, including one from 1998. Two of the engines were a part of the last space shuttle mission in 2011. They have since been updated and reconfigured to work with the SLS.