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After months of tests, troubleshooting and repairs, engineers began fueling the Space Launch System moon rocket for blastoff Monday on NASA’s long-overdue Artemis 1 test flight, but a hydrogen leak, similar to one that derailed an earlier fueling test, interrupted the complex procedure.

The leak developed in a launch pad service structure where propellants are fed into the rocket’s core stage through umbilicals designed to ensure a tight seal until the moment of liftoff when they are retracted. The buildup was detected in the housing around those umbilicals, known as a “purge can.”

Leaks are potentially dangerous, and sensors monitor concentrations to make sure safety limits are not violated. During Monday’s fueling procedure, higher-than-allowable hydrogen concentrations were observed when the flow rate switched from “slow” to “fast fill” at a 10 times higher rate, subjecting the plumbing to higher pressures.

After switching back to slow fill and evaluating the readings, engineers decided to resume fast fill to see if the hydrogen concentration in the tail service umbilical went back up. Concentrations higher than 4 percent are a violation of safety criteria, prohibiting launch.

The Space Launch System moon rocket atop pad 39B early Monday, awaiting a possible blastoff on a mission to send an uncrewed Orion capsule on a 42-day shakedown flight beyond the moon and back.


Liftoff originally was targeted for 8:33 a.m. EDT. It was not immediately clear what impact the weather-related fueling delay and the hydrogen troubleshooting might have on the eventual launch time, assuming the problem can be fixed before the end of a two-hour launch window.

The SLS rocket’s core stage must be loaded with 196,000 gallons of liquid oxygen and 537,000 gallons of hydrogen for takeoff. Another 22,000 gallons of oxygen and hydrogen are required for the upper stage, for a total of 750,000 gallons of propellant.

All that propellant will feed the core stage’s four shuttle-era engines. Combined with two strap-on solid fuel boosters, the rocket will generate a ground-shaking 8.8 million pounds of thrust at liftoff to propel the 5.7-million pound rocket away from pad 39B at the Kennedy Space Center.

The initial Artemis 1 test flight is intended to verify the rocket’s ability to propel Orion capsules into Earth orbit and then onto the moon. Engineers also will test the crew ship’s myriad systems in deep space and make sure its heat shield can protect returning astronauts from the 5,000-degree heat of re-entry.

NASA plans to follow the uncrewed Artemis 1 mission by launching four astronauts on a looping around-the-moon flight in 2024, setting the stage for the first astronaut landing in nearly 50 years when the first woman and the next man step onto the surface in the 2025-26 timeframe.

But first, NASA must prove the rocket and capsule will work as planned and that begins with Monday’s Artemis 1 launch.

NASA carried out four dress-rehearsal countdowns and fueling tests earlier this year and all four ran into problems. The most difficult to resolve were hydrogen leaks in the tail service mast umbilical system that derailed the initial test and in a 4-inch quick-disconnect fitting that cropped up during the most recent test June 20.

Hydrogen leaks are notoriously difficult to find and repair because they tend to show up only when the hardware is subjected to cryogenic temperatures. For hydrogen, that’s minus 423 degrees Fahrenheit.

The umbilical system leak during the rocket’s first fueling test was repaired at ambient temperatures in the Vehicle Assembly Building and it worked normally during a subsequent fueling test at the pad.

Whether the problem that developed Monday is a repeat of the initial issue was not immediately clear, but it was located in the same area.

The 4-inch quick-disconnect was repaired during a subsequent trip back to the VAB, but the fitting had not yet been exposed to hydrogen when the umbilical system leak was detected, interrupting the fueling procedure.

NASA engineers were confident the rocket was finally ready to fly, but the latest leak raised concerns another rollback to the VAB might be required.

Because of the constantly changing positions of the Earth and moon, NASA can only launch the SLS rocket during limited launch periods.

The current launch period, No. 25, opened August 23 and runs through September 6. Launch period 26 opens September 19 and runs through October 4. The period after that, No. 27, opens October 17 and runs through October 31.

Because of a requirement to service self-destruct system batteries, which cannot be accessed at the launch pad, the SLS rocket must take off by September 6 or it will have to be hauled back to the Vehicle Assembly Building.

Fixing another hydrogen leak also would require a trip back to the VAB, which almost certainly would delay launch until late September or October at the earliest.