NASA today revealed that engineers were forced on December 20th to abort at about T-5 minutes their second attempt to do a fueled dress rehearsal countdown in preparation for the full core stage static fire test.
[S]ources said the terminal countdown started at T-10 minutes and counting and ran down to T-4 minutes and 40 seconds where an unplanned hold occurred. … The criteria for how long it should take for a liquid hydrogen replenish valve to close was violated at that point in the countdown when the valve was commanded to the close position as a part of the process to pressurize the liquid hydrogen tank for engine firing. After holding at the T-4:40 point for a few minutes, teams decided the terminal countdown test couldn’t continue.
Vehicle safing and recycle sequences were then executed.
Although the countdown ran for over half of its intended duration, the early cutoff left several major milestones untested. With the countdown aborted at that point, the stage’s propellant tanks weren’t fully pressurized, the hydraulic Core Stage Auxiliary Power Units (CAPUs) were never started, the final RS-25 engine purge sequence was never run, and the vehicle power transfer didn’t occur.
NASA management is debating now whether they can proceed directly to the full core stage static fire test, where the core stage engine will fire for the full duration of a normal launch. It could be that they will decide to waive testing what was not tested on this last dress rehearsal.
If they delay the full test to do another dress rehearsal, they risk causing a delay in the fall launch of SLS, as they need a lot of time to disassemble, ship, and reassembly the stage in Florida. If they don’t delay, they risk either a failure during the full static fire test, or (even worse) a failure during that first launch.
Considering the number of nagging problems that have plagued this test program, it seems foolish to me to bypass any testing. They not only do not have enough data to really understand how to fuel the core stage reliably, they don’t even have a lot of practice doing the countdown itself. All this bodes ill when they try to launch later this year, especially if they decide to not work the kinks out now.