March 18, 2021

— NASA is now eight minutes closer to the moon.

The space agency on Thursday (March 18) ignited its new Space Launch System (SLS) core stage for a second time, succeeding at a full-duration firing two months after a first attempt at the "Green Run" test was cut off early.

The four RS-25 engines at the base of the stage burned for eight minutes — the same length of time they will need to fire during a launch — while being held down on a test stand at NASA's Stennis Space Center in Mississippi. The hot fire of the upgraded space shuttle main engines marked a significant step forward in NASA's plans to return astronauts to the lunar surface.

"What a great day, and a great test," acting NASA Administrator Steve Jurczyk said during a post-test news conference. "This is a major milestone advancing our goals for Artemis."

"Today's successful hot fire test of the core stage for the SLS is an important milestone in NASA's goal to return humans to the lunar surface — and beyond," said Jurczyk.

Pending a review of the data collected from the test and the time needed to refurbish the engines and surrounding insulation, the Boeing-built core stage will next be shipped to Kennedy Space Center in Florida, where it will be mated with a pair of recently-stacked solid rocket boosters that together will provide the thrust for the first Artemis mission launch.

NASA plans to send the uncrewed Artemis I mission around the moon. A crewed mission would then follow, leading to two astronauts — the next man and the first woman — landing at the lunar south pole on the Artemis III mission.

"We are still on path to have an opportunity to launch [Artemis I] this year," said Tom Whitmeyer, deputy associate administrator for exploration systems development at NASA Headquarters, "but we recognize also that there are things that can come up like weather and COVID and some first time operations. So our plan is to launch this year, but we'll continue to provide progress as we go through the year."

(Green) Run again

As was done on Jan. 16, technicians on Thursday filled the 212-foot-tall by 27.6-foot-wide (65-by-8.4-m) core stage with 196,000 gallons of liquid oxygen (LOX, cooled to -297 degrees Fahrenheit or -183 degrees Celsius) and 537,000 gallons of liquid hydrogen (LH2, at -423 F or -253 C).

At 4:37 p.m. EDT (2037 GMT), the countdown reached zero and the four RS-25 engines ignited. In addition to the first Green Run test and individual hot fires, the engines had been fired before on a combined total of 21 space shuttle launches, including two of the engines on the final shuttle mission, STS-135, in 2011.

SLS core stage second hot fire test. Click to enlarge video in a new window. (NASA)

Fired at 109 percent of rated thrust to simulate operation of the SLS core stage at liftoff (an increase over the shuttle, which typically flew at 104.5 percent), the hot fire test included two major thrust changes, mimicking those required for the climb into space.

At about a minute into the 499.6-second test, the four main engines were throttled back to 95 percent, as will be done to ease the load at "max q," when the SLS will encounter maximum dynamic pressure from Earth's atmosphere. After about 90 seconds, the engines were throttled back up to 109 percent.

The second major thrust change occurred as the engines were gradually throttled down to 85 percent, just before being shut down.

"This is not by any means a vanilla firing of the core stage," said John Shannon, vice president and SLS program manager for Boeing. "Since it's a highly instrumented vehicle, we're taking that opportunity to do some very aggressive maneuvers of the vector control system. So [there is] a lot of movement of the [engine] nozzles pretty much throughout the entire firing, even some very rapid movements that you normally would not see in flight. But we want to see that while we're on a test stand with the instrumented vehicle to really understand it."

The first attempt at the hot fire was shut down after only 67.2 seconds due to intentionally conservative test parameters that were intended to ensure the safety of the stage during the test. During gimballing, the hydraulic system associated with the stage's power unit for one of the four engines exceeded the pre-set test limits that had been established.

The test parameters were adjusted for Thursday's second run, allowing the full-duration hot fire to be achieved.

"Our primary goal is to not only get the hot fire data as much as we can, but also to protect the stage to be the first flight article. So we're threading the needle here between keeping the vehicle safe and gathering this data," said Shannon. "We got 15 terabytes of data from the first test. We'll get a tremendous amount of data from this test."

Generational opportunity

Thursday's hot fire marked the final trial in the "Green Run" series, which NASA conducted to validate the SLS core stage's design and performance.

"This is a generational opportunity to learn as much as we can about the rocket while we have it in this test configuration before we get to fly," said Honeycutt.

The SLS hot fire evoked the Apollo-era testing of the Saturn V rocket's S-IC first stage, which used the same B-2 test stand at Stennis. Before NASA launched the Apollo missions to the moon, the agency conducted numerous hot fires with a test article and then the first production (flight) stage.

The first full-duration, five F-1 engine hot fire was conducted with a test article of the S-IC stage on Aug. 5, 1965, following nine tests that were done with either one engine or with five but at a shorter duration. The first five-engine S-IC hot fire with a flight article (Apollo 4) was on Feb. 17, 1966.

The first hot fire atop the B-2 test stand at Stennis (then known as the Mississippi Test Facility) was on March 3, 1967. Earlier tests were conducted at the Marshall Space Flight Center in Alabama.