It took NASA four tries to fully fuel the new Space Launch System moon rocket, and even though fresh problems cropped up during the most recent practice countdown Monday, senior managers said they were pleased with the giant booster's performance.
"We think that we had a really successful rehearsal," Tom Whitmeyer, deputy chief of NASA Exploration Systems Development, said Tuesday. "We know that we'll have a handful of items that we need to address ... and I think we'll take a couple of days and get through that and then we'll make a decision on what's the best path forward."
Assuming repairs of a leaking hydrogen fitting are successful, managers could decide to carry out yet another fueling test of some sort, or they could conclude they have enough data now to head into a late-summer launch campaign without losing time on another dress rehearsal that might only provide incremental improvement.
During a teleconference Tuesday with reporters, Whitmeyer and other senior managers would not speculate on what comes next. But Mike Sarafin, the Artemis moon program mission manager at NASA headquarters, said the SLS rocket has now accomplished most of the agency's pre-flight objectives despite not getting all the way to the end of Monday's practice countdown.
As it was, the team made it down to T-minus 29 seconds — just 20 seconds short of the goal — and engineers understand what caused the early cutoff.
"I would say we're in the 90th percentile in terms of where we need to be overall," Sarafin said. But "there are still some open items that we need to go off and look at ... to say that we're ready from a flight rationale standpoint."
Running years behind schedule and billions over budget, NASA is in the final stages of testing the giant SLS rocket and its complex systems before launching it on the program's maiden flight: sending an unpiloted Orion crew capsule on a flight 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 solid-fuel boosters, added an upper stage from United Launch Alliance, and then attached an Orion crew capsule, built by Lockheed Martin.
The 330-foot-tall rocket, the most powerful ever built for NASA, was hauled from the iconic Vehicle Assembly Building out to launch pad 39B in March for a practice countdown and fueling test, one of the last major milestones on the road to launch.
The goal was to load the rocket with 750,000 gallons of supercold liquid oxygen and hydrogen fuel and then count down to T-minus 9 seconds, the point at which the engines would begin igniting during an actual launch, to test complex computer systems, software and hardware under flight-day conditions.
Along with attempting to run through a normal countdown, the team also planned to test its ability to halt the clock and recycle: to make sure the system can handle problems that might force a last-second delay during an actual countdown.
But a frustrating series of mostly ground-system problems, along with a hydrogen leak in a fuel line fitting, trouble with a jammed second stage helium valve, and a shortfall of gaseous nitrogen used in fire prevention systems, derailed three fueling attempts in a row.
The rocket was hauled back to the VAB for repairs and then back out to the pad earlier this month. During its fourth fueling test Monday, engineers were finally able to fully load the SLS, pumping 750,000 gallons of oxygen and hydrogen into the four tanks making up the first and second stages.
But before the tanks were full, engineers discovered a new problem: a leaking 4-inch hydrogen quick-disconnect fitting. The system is used to route hydrogen through the four RS-25 first stage engines to properly cool, or condition, them prior to ignition.
The ground launch sequencer computer running the countdown monitors thousands of parameters, including the status of the 4-inch "bleed line," and it is programmed to stop the clock if specifications outlined in launch commit criteria are violated.
When the bleed line issue cropped up Monday, engineers had to quickly come up with a way to work around the problem so they could continue propellant loading and get into what is known as "stable replenish," constantly topping off the tanks as hydrogen and oxygen warm and boil off.
They managed to do that by instructing computers to ignore sensors readings that would have indicated a leak, and the team finally got all four tanks fully loaded. That set the stage for the terminal phase of the countdown, the action-packed final 10 minutes leading up to launch. Or, in this case, leading up to a computer-commanded cutoff.
The original goal was to count down to T-minus 33 seconds, the point where the ground computer would hand off to the SLS's on-board flight software, and then to recycle back to T-minus 10 minutes. The idea was to test the system's ability to recover from a problem. The plan then called for resuming the count and continuing all the way down to T-minus 9.3 seconds.
But on Monday, because of the leaking quick-disconnect fitting in the hydrogen bleed line, and time lost troubleshooting, managers opted to forego the recycle at 33 seconds and to continue the countdown past the handoff to the rocket's flight computer.
While the ground computer could be told to ignore the sensors indicating a leak, the flight computer's software could not be easily modified, and launch director Charlie Blackwell-Thompson said engineers expected it to stop the countdown once it took over.
"We certainly have the capability within the ground launch sequencer to inhibit monitoring on those types of parameters, but we have less capability on the flight side for that," she said. "And so we knew that once it sensed that condition, that we would have a cutoff."
And that's exactly what happened. The countdown stopped at T-minus 29 seconds, four seconds after the handoff occurred.
What happens next is not yet clear. The next realistic lunar launch period, based on the motions of Earth and moon and the Orion capsule's planned trajectory, opens on Aug. 23. Another fueling test could push the flight beyond that, but NASA has not yet addressed launch dates.
In the meantime, Blackwell-Thompson said, "You follow the data. And so we'll go collect the data, and we'll see where the data takes us."
Editor's note: An earlier version of this story misstated which company manufactured the solid-fuel boosters.