NASA counts down to within 29 seconds after launching the large SLS rocket

Enlarge / NASA’s Space Launch System rocket, reflected in the swing pool at the Kennedy Space Center in Florida, is rolling out for a fourth attempt at a wet test on June 6, 2022.

Trevor Mahlmann

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

And so NASA made the difficult decision to roll the big rocket back into the vehicle assembly building for repairs, adding a few months of delays to a program that was already many years behind schedule. After this work was completed in early June, the NASA SLS rocket and Orion spacecraft rolled back to the launch pad for a fourth attempt.

The painful decision turned out to be the right one. In more than 14 hours on Monday, NASA largely succeeded in completing this fuel 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,” he said Charlie Blackwell-Thompson, Launch Director for Artemis.

She and other NASA officials joined a conference call with journalists on Tuesday to discuss the results of the fourth “wet dress test” test, which is meant to find out the tricks of counting down the rocket to launch before launch day. So far, the test largely seemed to work. NASA arrived within T-29 seconds after launch during the test, close to the intended target of T-9.3 seconds, before the test ended just before the rocket’s four main engines were ignited.

During the teleconference, NASA officials declined to answer specific questions about whether a fifth test would be necessary – to get the countdown to T-9.3 seconds – or when the rocket could be ready for its debut launch. Referring to a desire to review more data, officials said they expected to provide this information in a few days. From their comments, however, it sounded like the officials could lean towards a fifth test.

A handful of technical issues arose 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 while refueling. This 4-inch hydrogen line is one of several that are released from the rocket just before the ascent and is connected to the tower’s tail service mast.

NASA failed to solve the problem with a leaky seal during the last part of Monday’s test, so it chose instead to mask the leak from the ground launch sequence, the ground – side computer that controls most of the countdown. This posed no risk to the rocket during the test, but had to be fixed before an actual launch.

With this bit of masking, the NASA launch team managed to recover 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 the fuel tanks on top. When the ground launch sequence was handed over to the rocket’s built-in computer for the last part of the countdown, the aircraft computer automatically ended the count.

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

This fuel test is the last major obstacle between the SLS rocket and a launch attempt later this year. There is still work to be done, and the agency must decide whether a new wet cladding test is necessary. But Mike Sarafin, the Artemis I mission leader, said he believed NASA to date has completed about 90 percent of the test targets.

In addition to fixing the leaking hydrogen seal, NASA must continue to roll the rocket back to the vehicle assembly building to install and reinforce the flight termination system. This work probably precludes a launch attempt no earlier than the end of September.