NASA made a third attempt to fuel its new Space Launch System moon rocket during a test countdown Thursday, but once again a series of technical snags triggered multiple delays, preventing engineers from pumping 730,000 gallons of liquid hydrogen and oxygen into the booster's huge first stage as planned.
Problems with a helium pressurization valve in the SLS rocket's upper stage, discovered during a fueling attempt last week, limited Thursday's propellant loading attempt to the core stage alone, but engineers were only able to fill its hydrogen tank to the 5% level and the oxygen tank to about 49%.
That's because of trouble with a gaseous nitrogen supply line, oxygen temperature violations during initial loading, an unexpected "pressure surge" when switching the liquid hydrogen flow to high speed, and indications of a hydrogen leak in a launch pad umbilical when attempting to resume "fast fill" after a halt.
Two planned terminal countdown "runs" were planned to take the count all the way to T-minus 33 seconds and then, after a recycle, down to T-minus 9.3 seconds to verify propellant management and launch day procedures. But the countdown never resumed after a final built-in hold.
Despite the significance of the testing, NASA has declined to provide realtime launch control commentary because of concern export control laws might be violated if technical information is released before it can be reviewed. Instead, the agency is using social media — tweets and blog posts — to update reporters and the public.
The helium valve problem that prevented upper stage fueling cannot be repaired at the launch pad and it's not known yet whether NASA will attempt a fourth core stage fueling test or opt to haul the rocket back to the Kennedy Space Center's iconic Vehicle Assembly Building for repairs.
At that point, the agency could roll it back out to the pad for another fueling test or press ahead for launch. Or both. NASA managers have declined to outline even tentative plans beyond the now-delayed fueling test, saying they want that data in hand before making any decisions on how to proceed.
The Space Launch System rocket is the most powerful launcher ever built for NASA, a key element of the agency's Artemis program to send astronauts back to the moon. With the "mega rocket" years behind schedule and billions over budget, NASA plans to launch an unpiloted Orion crew capsule beyond the moon and back during a maiden test flight this summer.
The dress-rehearsal countdown and fueling exercise is a critical milestone on the road to launch, allowing managers and engineers to put complex ground systems and the SLS rocket through their paces under launch-day conditions for the first time.
Problems are not unexpected given the huge amounts of super-cold cryogenic propellants involved and the intricate systems needed to safely manage them. A planned six-day countdown test for the first Apollo Saturn 5 moon rocket took 17 days to complete. Surprisingly, perhaps, the SLS dress rehearsal now appears likely to take even longer.
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