Countdown For Launch Of SLS To Begin Saturday

Credit: Ben Smegelsky/NASA

CAPE CANAVERAL—After more than 30 countdown simulations, two core-stage Green Run static engine firings and four launchpad wet dress rehearsals, countdown clocks at Kennedy Space Center were scheduled to start ticking down on Aug. 27 for the long-awaited launch of the first Space Launch System (SLS) rocket.

The mission, Artemis I, is intended to send an uncrewed Orion capsule on a six-week flight test around the Moon.

The 45-hr., 10-min. countdown was set to begin at 10:23 a.m. EDT. Barring weather concerns or technical issues, the SLS, carrying an uncrewed Orion capsule, was expected to lift off at 8:33-10:33 a.m. EDT on Aug. 29 from Kennedy’s Launch Complex 39B. The last launch from the pad was for the October 2009 mission of the SLS-precursor Ares 1-X prototype. 

After years of development and testing, only the last 29 sec. of the SLS countdown was expected to break new ground. During the fourth and final wet dress rehearsal (WDR), which took place on June 20, NASA planned to practice launch operations all the way to 9 sec. before liftoff, which is just before the rocket’s four Aerojet Rocketdyne RS-25 main engines would ignite for an actual launch.

However, a hydrogen leak stopped the countdown at the T-minus-29 sec. mark, 4 sec. after the ground launch system handed over to the flight software. The leak, which was located in a 4-in. quick-disconnect (QD) fitting that attaches from the tail service mast on the mobile launcher to the rocket’s core stage, nixed a planned bleed of liquid hydrogen through the core stage engines for thermal conditioning. The SLS was returned to the Vehicle Assembly Building for repairs.

The premature cutoff left one WDR key objective unfulfilled, prompting the launch team to insert a test—known as the hydrogen kick-start—during the actual countdown. “We’ll demonstrate it end to end for the first time on the day of launch,” Artemis Mission Manager Mike Sarafin said. “If we do not successfully demonstrate that, we are not going to launch that day.”

The test will be conducted after the core stage is filled with cryogenic liquid hydrogen and goes into replenish mode, which occurs about 2 hr. before launch. “We’ll close the vent valve on the core stage, which puts some pressure on it … then verify that 4-in. QD where we had the issue during wet dress rehearsal is working correctly,” Artemis Launch Director Charlie Blackwell-Thompson said.

If the repair was successful, a bleed-off of liquid hydrogen would run for about 45 min. That would be followed by a vented bleed and a newly added pre-pressurization test. “Normally that would happen in terminal count—we would have done it in wet dress, but we weren’t able to do that because of the issue that we had—and so we’ve added that into the launch countdown at a kind of a quiescent point so that we can understand that prior to getting into terminal count where you have a lot of other dynamics in your system,” Blackwell-Thompson said.

“We believe we have taken all of the actions to correct the problem, but of course we can only test it at ambient conditions in the Vehicle Assembly Building,” she said. “When we hit it with cryos at the pad during the bleed kick-start is when we’ll know that we’ve got that problem fully behind us.

“We’re looking forward to getting those last 29 sec. off the countdown,” she added.

Irene Klotz

Irene Klotz is Senior Space Editor for Aviation Week, based in Cape Canaveral. Before joining Aviation Week in 2017, Irene spent 25 years as a wire service reporter covering human and robotic spaceflight, commercial space, astronomy, science and technology for Reuters and United Press International.