NASA's (partially) Successful SLS Hot Fire Test

UPDATE: NASA has released a statement confirming that the vehicle and its engines remain in good condition. They have identified that the engine shutdown was caused by safety parameters in place to protect the vehicle during testing that were set too conservatively. Part of the statement reads- "During gimballing, the hydraulic system associated with the core stage’s power unit for Engine 2, also known as engine E2056, exceeded the pre-set test limits that had been established. As they were programmed to do, the flight computers automatically ended the test. The specific logic that stopped the test is unique to the ground test when the core stage is mounted in the B-2 test stand at Stennis. 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 has decided to conduct a second hot fire test, which is not expected any sooner than the last week of February. If successful, the SLS first stage core will begin its journey to Kennedy Space Center in preparation for the launch of Artemis 1.



NASA successfully lit the candle on the main engines of the first complete SLS core stage, but didn't stay lit for long. The "Green Run" is a series of tests performed at Stennis Space Center in Hancock County, Mississippi, designed to put new rockets through the ringer before certifying them for flight. A long duration 'hot fire' ignition test is the last check mark. This is a vital step towards the Artemis program's goal of putting the next Americans on the moon. The rocket from this test is expected to carry the uncrewed Artemis 1 mission into lunar orbit before the end of 2021.


(NASAtv Live Stream)

At 5:27 pm Eastern on Saturday, January 16th, all four RS-25 engines fired in unison on an assembled core for the first time. These engines have been tested independently of each other, and all of them have previously flown as space shuttle main engines, but this marks the first time the core assembly has been tested in a situation comparable to a real launch.

NASA administrator Jim Bridenstein spoke in a press conference about three hours after the test fire. "Today we got lots of data that we're going to go through and get to a point where we can make determinations as to whether or not launching in 2021 is a possibility." His demeanor was optimistic, and he believes that today's test has provided the Artemis team with a wealth of information to build on as the program moves towards returning humans to the moon. "I really think this is a good day, and we test for a reason, because we want to learn."

To shed some light on what happened in the final seconds of the test, Artemis Program Manager John Honeycut said, "When I left the team a little while ago they were beginning to pour through the data, and we will continue to do that over the next several days." He describes the sequence of events being called over the communications channel, "There was some conversation around a 'FID' on engine 4, which is our terminology for a failure identification, shortly followed by a MCF which is a major component failure. Any parameter that went awry on the engine could send that failure ID, but at the time that they made the call we did still have four good engines up and running."

The specific cause for this abort, as well as any damage that might have resulted during the unscheduled shutdown, was still unknown at the time of publishing. However, Honeycut also described a flash seen near the thermal cover surrounding the engines that may or may not be related. "Each one of the engines has a thermal blanket that's pretty robust, but is flexible enough to allow the engines to gimbal. The flash that we saw was in the area of that thermal protection blanket that was around engine 4."

The abort was triggered about 60 seconds after ignition, right around the time that the engines began their gimbal test sequence. The hot fire was intended to be 480 seconds of burn time with an expectation of 250 seconds minimum, so this test was significantly shorter than originally planned. However, this is not to say that the test was 'unsuccessful'.

(NASAtv Live Stream)

"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," said Bridenstein. "As we figure out what went wrong we're going to know what the future holds, and right now we just don't know that."

While it is easy to dwell on the things that didn't go right, there area lot of things that needed to work perfectly in order to get to this point in the first place. The Green Run is designed to push vehicles to their limit to reveal potential issues before attempting the real thing. Failures are far more costly in flight, and simply igniting the engines together is a new accomplishment.

Bridenstein adds, "This is a big milestone in an eventual shipment of this rocket to Kennedy for launch to the moon. We have to remember, the rocket that we just tested is the rocket that is going to launch 'Orion' around the moon. This is test hardware, this is also flight hardware." NASA plans to land the next Americans, including the first woman, on the moon by 2024. The mood seems to imply that they are still on track to meet that goal, but rocket science is never easy. "If you're expecting perfection on a first test, then you've never tested before."


(NASA/Northrop Grumman/Scott Mohrman)

Shown above is the solid fuel side-mounted booster for SLS being tested at the Northrop Grumman test facility in Promontory, Utah on September 2nd, 2020. When the complete rocket is assembled to launch Artemis, the core will be joined by two side mounted solid boosters. This design is similar to the way the boosters were attached to the external fuel tank of the space shuttle, but these boosters are larger and much improved. The solid boosters for Artemis 1 have already begun assembly at Kennedy Space Center.

On the pad SLS will stand 365 ft tall, and the Block 2 iteration of the vehicle will be capable of carrying more than 99,000 lbs to the moon. In addition to Artemis, SLS is expected to carry astronauts to mars as well as launching the upcoming 'Europa Clipper' probe to study Jupiter's elusive frozen moon, Europa, and investigate the liquid oceans beneath its surface.


Artist Concept of the SLS Block 1 before launch (NASA/MSFC)

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