Green Run engines
NASA said the Green Run static-fire test Jan. 16 was terminated when the hydraulic system for one of four engines hit a limit intended to be conservative in order to protect the core stage. Credit: NASA

Updated 7:45 p.m. Eastern with details from briefing.

WASHINGTON — A static-fire test of the Space Launch System core stage ended early Jan. 16 when a hydraulic system for one its four engines hit an “intentionally conservative” limit during the test.

In a Jan. 19 statement, NASA said the hydraulic system for Engine 2 on the core stage “exceeded the pre-set test limits that had been established” for the Green Run test. “As they were programmed to do, the flight computers automatically ended the test.”

Later the same day, during a call with reporters, NASA officials said that the hydraulic reservoir level and hydraulic pressure in the Core Stage Auxiliary Power Unit, or CAPU, for that engine dropped below limits over “a series of milliseconds,” triggering the flight computer to end the test. That CAPU drives a thrust vector control system used to gimbal the engines, and the problem took place about one second after a gimbal sequence started 60 seconds into the test.

That also triggered a shutdown of that CAPU. “The automated software on board shut down CAPU 2 just to safe the system in case there was a problem” with the unit itself, John Shannon, vice president and SLS program manager at Boeing, said during the call.

The hydraulic system problem was not linked to a major component failure (MCF) reported by test controllers about 45 seconds after ignition. NASA said the MCF actually took place 1.5 seconds after ignition, and was caused by the loss of “one leg of redundancy” in instrumentation for Engine 4. “Test constraints for hot fire were set up to allow the test to proceed with this condition, because the engine control system still has sufficient redundancy to ensure safe engine operation during the test,” the agency stated.

NASA is still investigating what officials said shortly after the test was a “flash” seen in the vicinity of a thermal protection blanket around Engine 4. The blanket showed signs of scorching, but that was expected from standard engine operations, and temperatures in the engine section were normal.

The parameters used for the Green Run test, the agency stated, were “intentionally conservative to ensure the safety of the core stage during the test.” NASA officials previously emphasized they were taking a cautious approach to testing the core stage since it is flight hardware, intended for use on the first SLS launch, Artemis 1.

“We have to remember that the rocket we just tested is the rocket that is going to launch Orion around the Moon,” NASA Administrator Jim Bridenstine said at a briefing after the Jan. 16 test. “When we do this test, there is risk that we cannot take because this is the same vehicle that will fly Orion.”

“Our test parameters demonstrate our safety-first approach and were appropriately conservative. This core stage is a high-value flight article that will return America to deep space,” Shannon said in a company statement about the Green Run test. “Our redline limits were set to achieve data collection without unnecessarily risking the system.”

But they acknowledged in the call they may have been too conservative. The hydraulic system “had a reading, a parameter, that was maybe set a little too conservatively,” Bridenstine said. “Had this been a real launch, that parameter wouldn’t have been set so conservatively and the rocket would have continued.”

“It’s to walk the fine line between making sure, for the first time that we use any of this hardware, that we have sufficient protection in to keep the stage in a safe configuration, but also to let it operate through the test regime,” Shannon said. “There’s a judgment call in there for how you set those parameters to ensure that the stage remains in a good configuration for a further test or a launch.”

NASA has not yet decided if it will perform a second hotfire test. In comments before the first test, NASA and Boeing officials said that while the test was scheduled to last for 485 seconds, they would collect most of the data they needed after 250 seconds. However, the engine shutdown took place after just 67.2 seconds.

NASA said they want to review the data collected during the test before deciding whether to perform a second hotfire test or ship the stage to the Kennedy Space Center for final preparations for the Artemis 1 mission. “You have to understand the risk of exposing the flight core stage to another round of tests, and how does that risk trade off with the learning that we need to do,” said Kathy Lueders, NASA associate administrator for human exploration and operations.

One factor, Bridenstine said, is the rated lifetime of the core stage. He said the stage is designed to be filled with liquid hydrogen and liquid oxygen propellants nine times. That has been done twice so far: a wet dress rehearsal in December and for the static-fire test. A limited amount of propellant was loaded in the stage for the first attempt at the wet dress rehearsal in early December.

Doing another static-fire test means loading the stage with propellants at least one more time. “Every time we do something like that, it takes away one of our nine times that we can tank,” he said. “There’s reasons to do a full duration hotfire, and there’s reasons that maybe we wouldn’t do a full duration hotfire.”

One former NASA official recommended the agency conduct a second hotfire test. “My advice would be to retest and get complete data – may be a couple of weeks but schedule is secondary,” tweeted Wayne Hale, former shuttle program manager and current chairman of the NASA Advisory Council’s Human Exploration and Operations Committee.

John Honeycutt, SLS program manager at NASA, said in the call that work to recycle the engines after the truncated hotfire test is underway, either to support a second hotfire test or to get the stage ready for shipment to KSC. He previously said it would take 21 to 30 days to get the core stage ready, giving NASA time to review the data and decide if a second hotfire is needed.

“The data analysis is going to drive us and inform our decision as to whether we either proceed to the launch or we perform an additional hotfire test,” he said. “We don’t have a date just yet on when we’re going to be at that decision point.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...