NASA and the Exploration Ground Systems program finally completed a full wet dress rehearsal of the Artemis 1 Space Launch System (SLS) rocket. More or less.

A wet dress rehearsal is a test of the entire launch countdown. The previous attempt was in mid-April 2022 and ended after a whole series of issues, mostly from the ground support equipment leaking in different ways. Liquid hydrogen, the rocket’s main propellant, is very powerful but is also very hard to manage in the rocket and the ground system.

SLS is a new rocket, and this test campaign is the first time its ground support equipment at the launch pad has been used, so it’s not surprising problems have been encountered.

On the morning of June 6, the Mobile Launcher with the SLS rocket rolled back out to pad 39B for the fourth attempt to finish this critical test before the launch of an uncrewed Orion capsule to the Moon later this year. Like the previous attempts, the countdown started dozens of hours before the actual “launch”.

Unlike the previous attempts, however, weather conditions did not delay the start of the count. At T minus 9 hours, the propellant load started. Loading the propellant takes time to avoid thermal shock from the extremely cold temperatures damaging the systems, such as increased tank pressure as the liquid propellant “boils off” into gas because of the relatively warmer tank which, left unchecked, could cause it to burst.

One of the problems encountered in the propellant load was a leak with the part of the ground system that loaded the liquid hydrogen and a valve on the rocket itself. Engineers tried to fix the issue by heating and cooling the valve and by overpressuring it to get the umbilical to seal to the rocket. Ultimately, they couldn’t fix the leak, so the ground computer was told to ignore the warning to avoid a scrub. 

As the count continued, the ground computer successfully noted and ignored its own warnings about the leaks. At T minus 33 seconds, the ground systems transferred control to the onboard systems. About four seconds later, the onboard systems triggered a hold because they detected the leaks the ground system had been told to ignore.

In a post-test press conference, NASA said that they expected an automated hold sometime after T minus 33 seconds because of the umbilical leak and modified countdown, but they didn’t know exactly when it would happen.

All other events up to this point, including pressurizing the tanks, switching the rocket to internal power, and activating the Flight Termination System were nominal. The Orion spacecraft itself was also tested during this Wet Dress Rehearsal, and the result was nominal.

The bottom line is that the NASA team didn’t accomplish everything they wanted, but they did achieve more than last time, just with different issues. This means the fixes they tried after the last test worked, but some work still remains. According to the press conference, the team is still reviewing data to determine if another test is required, so they do not have a firm launch date at this time.

Space is hard, but they’ll get it eventually.

More Information

Teams on Track for Artemis I Wet Dress Rehearsal Test (NASA)

Artemis I WDR Update: Third Test Attempt Concluded (NASA)

NASA’s Artemis I Moon Rocket Departs Vehicle Assembly Building for Next Tanking Test (NASA)