https://arstechnica.com/science/202...ueling-test-before-todays-sls-launch-attempt/Despite all of the hard work by its engineers and technicians, NASA did not really get close to firing up the massive Space Launch System (SLS) rocket on Monday.
The rocket experienced several issues during the countdown early on Monday before running into a technical problem the launch team could not solve: an RS-25 rocket engine that did not properly chill down prior to ignition.
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Beginning in April of this year, NASA conducted four separate "wet dress rehearsal" tests during which the agency aimed to fully fuel the SLS rocket and countdown to T-10 seconds, ending the test before ignition of the main engines. Each of these four tests ultimately ended prematurely, although the fourth attempt in June saw engineers bring the rocket down to T-29 seconds.
However, to reach that late stage in the countdown, NASA had to "fool" the flight computer. During the test, a 4-inch hydrogen line—smaller than the problematic 8-inch line on Monday—had a leaky seal. To complete the wet dress test, NASA chose to mask the leak from the ground launch sequencer, the ground-side computer that controls the majority of the countdown.
Because of this masking, NASA could not complete the engine chill portion of the test. Had it done so, the agency may well have uncovered the problem that caused a scrub on Monday. In hindsight, therefore, NASA probably should have completed a full wet dress rehearsal before rolling the rocket out for a launch. Instead, the agency effectively attempted a fifth wet dress test on Monday, when the world was expecting a launch
The problem with the Americans trying to go back to the Moon is that NASA is nowhere near what it used to be.
100,000 had been predicted to show up to watch the launch...they were all disappointed.