NASA has been attempting to conduct a critical fueling test of its Space Launch System rocket for nearly three months, and now the agency says it is ready to try again.

This will be NASA's fourth attempt to load the SLS rocket's first and second stages with liquid hydrogen and liquid oxygen and go deep into a countdown toward launch before ending the test at T-10 seconds. The space agency plans to call its team of engineers and technicians to their stations on Saturday evening and begin fueling operations on Monday morning, June 20.

"Our team is ready to go," said Charlie Blackwell-Thompson, NASA's launch director for the Artemis I mission, which represents a test flight for the SLS vehicle and Orion spacecraft. "We're really looking forward to getting back to this test and getting into it starting on Saturday evening and certainly looking forward to the tanking operation."

After more than a decade of development work—and tens of billions of dollars in costs—NASA and a fleet of contractors rolled a fully assembled SLS rocket out to the launch pad for the first time on March 17, 2022. During the month of April, on three separate occasions, the space agency sought to complete the fueling test. On April 16, after a third unsuccessful attempt, NASA said it would have to roll the rocket back from the launch pad at Kennedy Space Center in Florida to the Vehicle Assembly Building for repairs.

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The most significant issue was a liquid hydrogen leak at the "tail service mast umbilical," which is one of the structures on the large mobile launch tower that provides power and propellant to the rocket during the fueling and countdown process. During a call with reporters on Wednesday, Blackwell-Thompson said that this and other work had been completed. "The teams have really done a great job addressing the issues we saw in wet-dress three," she said.

Earlier this month, NASA re-rolled the rocket back out to the launch pad, 39B, in advance of the second test. Since then, NASA and its contractors have worked to prepare the vehicle for the fueling test.

Although NASA has completed a lot of its test objectives during the three previous attempts, the most dynamic parts of the wet-dress rehearsal test will come in the final hours with a fully fueled vehicle. NASA isn't there yet—during the most advanced fueling attempt in April, NASA succeeded in loading 49 percent of the core-stage liquid oxygen fuel tank and 5 percent of the liquid hydrogen tank.

A completed test will require fully loading propellants on both the core stage and the upper stage and then going into an hours-long countdown. On Monday, NASA intends to start fuel loading at 7 am ET (1100 UTC) and proceed into terminal countdown. At T-33 seconds, the plan is to recycle and enter a second countdown, this time taking the vehicle all the way down to T-10 seconds. This should occur sometime on Monday afternoon.

NASA officials have said they will not set a launch date for the Artemis I mission until the wet-dress test is completed and there is at least a preliminary review of the data. During Wednesday's call, NASA's chief of exploration systems development, Jim Free, said that August 23 to September 6 is the earliest window during which the Artemis I mission could launch.

However, such a launch date assumes a timely completion of the wet-dress test and finding few (if any) issues that require follow-up work. While this is possible, it seems unlikely given that more problems will probably crop up during propellant loading and countdown operations.