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

During the month of April, NASA attempted to complete a critical fueling test of its large Space Launch System rocket. The space agency failed three times because of technical issues.

The decision to roll the large rocket back into the Vehicle Assembly Building for repairs added a couple of months of delays to the program. After this work was completed in early June, NASA rolled the SLS rocket back out to the launch pad for a fourth attempt.

The decision was painful, but it turned out to be the right one. Hundreds of thousands of gallons of liquid oxygen and hydrogen were loaded into the first and second stages of the SLS rocket during a 14-hour test by NASA.

"It was a long day for the team, but I think it was a very successful day for the team," said Charlie Blackwell-Thompson.

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 quirks of counting down the rocket to liftoff before launch day. The test mostly worked. During the test, NASA was able to get to within 29 seconds of liftoff, near its intended target, before ending the test.

During the teleconference, NASA officials wouldn't say if a fifth test would be needed or when the rocket would be ready for its first launch. The officials said they would give more data in a couple more days. The officials seemed to be leaning against a fifth test.

A hydrogen leak at the bottom of the mobile launch tower that supports the SLS rocket during fueling was the most significant of a number of technical problems. The hydrogen line is connected to the tower's tail service mast after it is released from the rocket.

During the last part of Monday's test, NASA was not able to solve the problem with a leaking seal, so it chose to hide the leak from the ground-side computer. During the test, there was no risk to the rocket, but it needed to be fixed before the actual launch.

The NASA launch team was able to get from T10 minutes all the way down to T29 seconds because of this bit of masking. The flight computer ended the count when the rocket's onboard computer was handed off.

NASA officials like what they see. This is the first time that the upper stage and the core stage have been in a fully-cryopreserved state. The terminal count is very fast. I was expecting that we might have a few things to talk about in terminal count, but it was very smooth. There wasn't anything to say.

The fueling test is the final hurdle between the SLS rocket and a launch. The agency has to decide if another wet dress test is needed. The Artemis I mission manager believes that NASA has completed about 90 percent of the objectives.

NASA needs to roll the rocket back to the Vehicle Assembly Building in order to install and arm the flight terminated system. A launch attempt before the end of September is likely to be precluded by this work.