A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021

A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021
An SLS engine burns propellant (left), then abruptly shuts down (right) during a hot fire test on January 16, 2021.NASA TV
  • NASA hit a snag during a critical test of the core stage of its moon rocket, the Space Launch System, on Saturday.
  • The test automatically ended due to an issue with an engine's hydraulic system, NASA said Tuesday.
  • The rocket was scheduled to launch in November 2021 and kick off the agency's Artemis moon-landing program, but the new issue could delay that launch — and the return of humans to the lunar surface for the first time since 1972.

NASA's mega-sized moon rocket hit a snag during a critical test on Saturday, and the error could further delay the agency's effort to send astronauts back to the moon.

The rocket, called Space Launch System (SLS), is designed to eventually stand 365 feet (111 meters) and ferry astronauts to the moon sometime in the mid- to late-2020s. The system is an essential piece of a larger program called Artemis, a roughly $30 billion effort to put boots back on the lunar surface for the first time since 1972. NASA has spent about $18 billion developing the rocket.

The SLS core stage - the system's largest piece and its structural backbone - was assembled and heavily strapped down at Stennis Space Center in Bay St. Louis, Mississippi, on Saturday for a critical "hot fire" test. For the first time, the rocket was ready to simultaneously fire its four powerful RS-25 engines as it would for launch.

A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021
Crews at Stennis Space Center lift the core stage of NASA's Space Launch System into place at test stand B-2 on January 22, 2020.NASA

The core stage is the world's largest and most powerful rocket stage, according to NASA. It hosts five mains sections, including a 537,000-gallon (2 million-liter) tank for liquid hydrogen, a 196,000-gallon (742,000-liter) tank for liquid oxygen, four RS-25 engines, avionics computers, and other subsystems. Boeing is the lead contractor for the stage, and Aerojet Rocketdyne is responsible for its RS-25 engines, which used to help propel NASA's fleet of space shuttles.

The fuel tanks were filled with 733,000 gallons of cryogenically chilled propellant on Saturday, and the engines roared to life at about 5:27 p.m. EST.


"It was like an earthquake," NASA Administrator Jim Bridenstine told reporters in a press conference after the test. "It was a magnificent moment. And it just brought joy that after all this time, now we've got a rocket. The only rocket on the face of the planet capable of taking humans to the moon was firing all four RS-25 engines at the same time."

The engines were supposed to fire continuously for eight minutes. But just one minute into the test, they suddenly shut down.

The whole thing was captured on NASA's live broadcast:

Following publication of this story, which stated there was a problem with one of the engines, Ryan McKibben, the deputy chief of mechanical operations at NASA's Stennis Space Center, contacted Insider with new details about the anomaly.

"I can assure you as the test conductor that we did not end the test early due to our engines," McKibben said. "We had a redundant sensor go out on an engine. But the engine was and is still in great shape after the test. The cutoff was due to other reasons."


This particular rocket stage is the one that's set to fly Artemis 1 - an uncrewed test flight around the moon. NASA doesn't want to push the rocket so hard as to damage it during testing, so it set conservative limits on the hardware's operations for Saturday's hot fire.

In a blog post on Tuesday, NASA said that its preliminary investigation into the hot fire had revealed that these strict limits may have been the source of the shutdown.

'No, this is not a failure'

During the hot fire, the engines were "gimbaling," or pivoting, to imitate how they would move to direct the rocket's thrust during flight. The systems that control these movements are powered by Core Stage Auxiliary Power Units (CAPUs).

A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021
Four RS-25 engines fire during the SLS hot fire test on January 16, 2021.NASA TV

When the hydraulic system on the CAPU for Engine 2 exceeded NASA's conservative limits, the flight computers automatically shut down the entire test.

"If this scenario occurred during a flight, the rocket would have continued to fly using the remaining CAPUs to power the thrust vector control systems for the engines," NASA said in its post.


Two other issues arose during the hot fire, though they weren't significant enough to shut down the operation. Engine 4 lost a redundant sensor, leading it to register a "major component failure" about 1.5 seconds after firing began. Controllers also saw a flash next to the thermal-protection blanket covering that engine, though the blog post didn't reveal any findings on what may have caused it.

At the time of shutdown, "we did still have four good engines up and running at 109%," John Honeycutt, the SLS program manager at NASA's Marshall Space Flight Center, said in the press conference.

"The amount of progress that we've made here today is remarkable. And no, this is not a failure. This is a test. And we tested today in a way that is meaningful, where we're going to learn and we're going to make adjustments and we're going to fly to the moon," Bridenstine said.

NASA may need to re-do the hot fire test

Saturday's hot fire was supposed to be the eighth and final step in NASA's "Green Run," a program designed to thoroughly test each part of the core stage ahead of SLS's first launch for Artemis 1, which currently scheduled for November 2021.

A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021
An artist's rendering of NASA's Space Launch System rocketing toward low-Earth orbit.NASA via Associated Press

But that timeline may be unrealistic now. If the hot fire went well, NASA was planning to ship the rocket to Kennedy Space Center in Cape Canaveral, Florida in February. There, workers would stack all the segments of the two boosters required for sending Artemis 1 around the moon.


It's unclear how long it will take NASA to correct the engine error and get the core stage to Florida now.

"It depends what the anomaly was and how challenging it's going to be to fix it. And we've got a lot to learn to figure that out," Bridenstine said. "It very well could be that it's something that's easily fixable and we could feel confident going down to the Cape and staying on schedule. It's also true that we could find a challenge that's going to take more time."

A critical test of NASA's moon rocket ended abruptly on Saturday, possibly dashing hopes of launching in 2021
The SLS core stage fires its engines for the hot fire test on January 16, 2021.NASA/Robert Markowitz

The agency may have to redo the hot fire test. The SLS team wanted to get to at least 250 seconds of the engines firing together to have high confidence in the vehicle. Saturday's test lasted for just 67 seconds.

"My advice would be to retest and get complete data," Wayne Hale, a retired NASA Space-Shuttle flight director, said on Twitter after NASA's updates from its preliminary investigation. "May be a couple of weeks but schedule is secondary."

It would take at least four or five days to prepare the Stennis Space Center facilities for another test. If NASA needs to swap the current engines for new ones, workers can do it on-site at the Stennis Space Center. Honeycutt estimated it would take about seven to 10 days to do that.


"This is why we test," Bridenstine said. "Before we put American astronauts on American rockets, that's when we need it to be perfect."

This story has been updated with new information. It was originally published at 8:17 p.m. EST on January 16, 2021.

Dave Mosher contributed reporting.