NASA tried three times during April to complete a critical fueling test of its large Space Launch System rocket. And three times, due to about half a dozen technical problems, the space agency failed.

And so NASA made the difficult decision to roll the large rocket back into the Vehicle Assembly Building for repairs, adding a couple of months of delays to a program already years behind schedule. After this work was complete in early June, NASA rolled the SLS rocket and Orion spacecraft back out to the launch pad for a fourth try.

The painful decision turned out to be the correct one. Over the course of more than 14 hours on Monday, NASA largely succeeded in completing this fueling test, loading hundreds of thousands of gallons of liquid oxygen and hydrogen into the first and second stages of the SLS rocket.

“It was a long day for the team, but I think it was a very successful day for the team,” said Charlie Blackwell-Thompson, Artemis launch director.

She and other NASA officials joined a conference call with reporters on Tuesday to discuss the results of the fourth “wet-dress rehearsal” test, which is intended to work out the kinks of counting down the rocket to liftoff before launch day. To that extent, the test largely seemed to work. NASA got to within T-29 seconds of liftoff during the test, near its intended target of T-9.3 seconds, before ending the test just prior to igniting the rocket’s four main engines.

During the teleconference, NASA officials declined to answer specific questions about whether a fifth test was going to be needed—to bring the count down to T-9.3 seconds—or when the rocket may be ready for its debut launch. Citing a desire to review more data, the officials said they expected to provide this information in a couple more days. From their comments, however, it sounded like the officials may be leaning against a fifth test.

A handful of technical problems occurred during Monday’s test, the most significant of which was a hydrogen leak in a quick disconnect at the bottom of the mobile launch tower that supports the SLS rocket during fueling. This 4-inch hydrogen line is one of several that are released from the rocket just before liftoff and are connected to the tower’s tail service mast.

NASA was unable to solve the problem with a leaky seal during the latter portion of Monday’s test, so it instead chose to mask the leak from the ground launch sequencer, the ground-side computer that controls the majority of the countdown. This posed no risk to the rocket during the test but would need to be fixed before an actual launch.

With this bit of masking, the NASA launch team was able to get from T-10 minutes all the way down to T-29 seconds and demonstrate the ability to not only fill the SLS rocket but also keep its fuel tanks topped off. When the ground launch sequencer handed off to the rocket’s onboard computer for the final portion of the countdown, the flight computer automatically ended the count.

NASA officials liked what they saw. “This is the first time that we have been in a fully cryogenic environment on both the core stage and the upper stage,” Blackwell-Thompson said. “Terminal count is a very dynamic time. I fully expected that we might have one or two things that we might need to talk about in terminal count, but it was extremely smooth. There was nothing to talk about.”

This fueling test is the final major obstacle between the SLS rocket and a launch attempt later this year. There is still work to do, and the agency has to decide whether another wet dress test is necessary. But Mike Sarafin, the Artemis I mission manager, said he believed that, to date, NASA has completed about 90 percent of the test objectives.

In addition to fixing the leaky hydrogen seal, NASA still needs to roll the rocket back to the Vehicle Assembly Building to install and arm the flight termination system. This work likely precludes a launch attempt before the end of September at the earliest.

https://arstechnica.com/?p=1861950