NASA has taken four attempts to fully refuel the Space Launch System’s New Moon rocket, and although new problems emerged during Monday’s latest practice countdown, top managers said they were pleased with the giant booster’s performance.
“We think we had a really successful rehearsal,” said Tom Whitemaier, NASA’s vice president of Exploration Systems Development, on Tuesday. “We know we are going to have a few more items that we need to address… I think we will take a couple of days and get that over and then we will make a decision on the best way forward.”
Assuming repairs to leaky hydrogen fixtures are successful, managers could decide to conduct another fuel test of some sort, or they could conclude that they had enough data now to head into a late summer launch campaign without wasting time on another rehearsal that might only provide incremental improvement.
During a conference call on Tuesday with reporters, Whitmire and other top managers did not speculate what would happen next. But Mike Sarafin, the Artemis Moon mission manager at NASA Headquarters, said the SLS rocket has now accomplished most of the agency’s pre-flight goals despite not having reached the end of the training countdown on Monday.
As it were, the team hit the T-minus 29 seconds – just 20 seconds before the goal – and the engineers understand the reason for the early stop.
“I would say we are in the 90th percentile in terms of where we need to be overall,” Sarafin said. But “there are still some open elements that we need to get out of and look at…to say we’re ready from a logical standpoint to fly.”
After years of behind schedule and billions over budget, NASA is in the final stages of testing the giant SLS rocket and its complex systems before launching on the program’s first flight: sending an unmanned Orion crew capsule on a journey beyond the moon and back.
To clear the way for launch, NASA test fired the rocket’s first stage engines in March 2021, shipped the stage to the Kennedy Space Center in Cape Canaveral, Florida, attached it to a pair of Northrop Grumman’s solid-fuel boosters, and added an upper stage from United Launch Alliance, then a capsule was attached Orion Crew, built by Lockheed Martin.
The 330-foot rocket, the most powerful rocket ever built for NASA, was moved from the iconic Vehicle Assembly Building to Launch Pad 39B in March for a countdown and refueling training test, one of the last major milestones on the launch route. .
The goal was to load the rocket with 750,000 gallons of supercooled liquid oxygen and hydrogen fuel and then count down to the T-minus 9 seconds, the point at which the engines would begin to ignite during the actual launch, to test the complex computer systems, software and hardware under day-of-flight conditions.
Besides attempting a regular countdown, the team also planned to test its ability to stop the clock and recycle: to make sure the system could handle issues that might impose a last-second delay during the actual countdown.
But a frustrating series of mostly ground system problems, along with a hydrogen leak in the fuel line installation, a problem with a jammed second stage helium valve, and a lack of gaseous nitrogen used in fire protection systems, hampered three consecutive refueling attempts.
The missile was brought back to VAB for repairs and then returned to the platform earlier this month. During Monday’s fourth refueling test, engineers were finally able to fully load the SLS, pumping 750,000 gallons of oxygen and hydrogen into the four tanks that make up the first and second stages.
But before the tanks were full, engineers discovered a new problem: a 4-inch, quick-fitting hydrogen leak. The system is used to direct hydrogen through the four RS-25 first stage engines to cool or properly conditioned them before ignition.
The ground launch regulator computer running the countdown monitors thousands of parameters, including the status of the 4-inch “bleed line”, and is programmed to stop the clock if the specifications described in the Launch Compliance Standards are violated.
When the bleed line problem surfaced on Monday, engineers had to quickly come up with a way to solve the problem so they could keep loading fuel and get into what’s known as “stable regeneration,” constantly topping tanks as hydrogen and oxygen warm and boil.
They were able to do this by instructing the computers to ignore sensor readings that were indicating a leak, and the team finally loaded all four tanks completely. That set the stage for the final stage of the countdown, the final 10 action-packed minutes leading up to the launch. Or, in this case, lead to computer-ordered disconnection.
The original goal was to count down to the T-minus 33 seconds, the point at which the ground computer hands over to the onboard SLS flight program, and then cycle back to the T-minus 10 minutes. The idea was to test the system’s ability to recover from a problem. The plan then called for the count to resume and to continue all the way down to T-minus 9.3 seconds.
But on Monday, due to a leaky hydrogen bleed line’s quick-disconnect installation, and time lost in troubleshooting, managers chose to forgo recycling at 33 seconds and continue the countdown after delivery to the rocket’s flight computer.
While the ground computer can be told to ignore sensors that indicate a leak, the flight computer’s software cannot be easily modified, and launch director Charlie Blackwell-Thompson said the engineers expected it to stop the countdown once it took over.
“We certainly have the capability inside the ground launch sequencer to prevent monitoring on those kinds of parameters, but we have less capability on the flight side for that,” she said. “So we knew that once she sensed this condition, we would have a cut.”
And that’s exactly what happened. The countdown stopped at the T-minus 29 seconds, four seconds after the delivery occurred.
What will happen next is not yet clear. The next realistic lunar launch period, based on the motions of the Earth and Moon and the planned trajectory of the Orion capsule, begins on August 1. 23. Another fuel test could push the flight beyond, but NASA has yet to address launch dates.
Meanwhile, Blackwill Thompson said, “You’re following the data. And so we’re going to collect the data, and we’ll see where the data takes us.”
Editor’s note: An earlier version of this story misspelled the company that made the solid fuel boosters.
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