After nearly 12 years of development, NASA has completed the last major test of its Space Launch System rocket and Orion spacecraft, setting the stage for a series of missions to expand and sustain a U.S. human presence beyond low Earth orbit. The final hurdle, like much of the Space Launch System (SLS) development, was not without problems. It took NASA four tries between April 3 and June 20 to fully fuel the 322-ft.-tall rocket's core and upper stages with 750,000 gal. of cryogenic liquid oxygen and liquid hydrogen at Kennedy Space Center's Launch Complex 39B. "The beast was alive," says John Blevins, SLS chief engineer at NASA's Marshall Space Flight Center. The practice launch countdown, known as a wet dress rehearsal (WDR), entailed one trip back to the Vehicle Assembly Building (VAB) for repairs. The fourth WDR on June 20 ended with 29 sec. left in the simulated countdown due to a hydrogen leak in a 4-in. quick-disconnect fitting that attaches from the tail service mast on the mobile launcher to the rocket's core stage. The leak, which was detected earlier in the day, prevented a bleed of liquid hydrogen through the core-stage engines to thermally condition them for flight. That situation would have scrubbed an actual launch attempt. After efforts to stem the leak failed, the launch team opted-for the purposes of the WDR-to mask it with software, enabling the ground launch sequencer to continue into the terminal phase of the countdown.
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