March 31, 2023

NASA’s leak-plagued Space Launch System moon rocket ran into an initially worrisome problem during a fuel test Wednesday, but engineers “managed” a new leak. September 3 launch attempt derailed and was able to fill the massive booster with a full load of 750,000 gallons of supercold propellant.

They conducted two more critical tests, verifying the rocket’s four hydrogen-fueled engines required for takeoff and their ability to successfully pressurize the main stage hydrogen tank at flight level.

Launch Director Charlie Blackwell-Thompson wouldn’t speculate on whether NASA might move toward the Sept. 27 launch date as previously discussed, saying he wanted his team to review data from the test before making any decisions. But he said he was “extremely encouraged by today’s test.”

NASA’s Space Launch System Mega Rocket atop Pad 39B at Kennedy Space Center on Wednesday. Engineers conducted a full-scale fueling test to verify repairs to fix a hydrogen leak that derailed the Sept. 3 launch attempt, with another leak in the same system. This time, engineers were able to use different flow rates and pressures to fully fuel the giant rocket.


“I don’t like to go over the data, so I want the team to have an opportunity to look at it to see if we need to make changes to our loading method, our timeline, or if our loading method is better as is,” he said.

Negotiations could prove challenging as seals responsible for previous launch delays were replaced and the same system, at least initially, leaked again on Wednesday.

But even if the team concludes that September 27 is a viable target for the rocket’s first flight, it may not be enough. The Space Force Eastern Range, which oversees all military and civilian launches from Florida, has yet to rule on a request by NASA to waive a requirement to inspect batteries in the rocket’s self-destruct system.

The batteries cannot be accessed on the launch pad, and without a waiver, NASA would be forced to return the 332-foot-tall SLS rocket to Kennedy Space Center’s iconic Vehicle Assembly Building, delaying the launch for a month or more.

The long-awaited Artemis 1 mission is designed to send an unmanned Orion crew capsule on a 40-day voyage around the moon and pave the way for the first piloted Artemis mission in 2024. If all goes well, NASA plans to land two. Astronauts near the Moon’s south pole in the 2025-26 timeframe, the first in a sustained series of missions.

But engineers were distracted by hydrogen leaks and other problems during the rocket’s launch. Already years behind schedule and billions over budget, the SLS rocket was first sent to Launch Pad 39B on March 17 for a fuel test to clear the way for launch. But back-to-back scrubs were ordered on April 3 and 4 due to multiple unrelated issues.

Liquid oxygen and hydrogen propellants flow through retractable 8-inch-wide lines to the massive main stage of the space launch system that extend from two so-called tail service mast umbilicals attached to the side of the booster to quick-disconnect fittings (at left). A leak in a hydrogen fitting caused initial problems during Wednesday’s fuel test, but engineers were able to reinstall a suspected seal and successfully load the rocket with propellant.


A third test was aborted on April 14 due to a hydrogen leak near the core stage fuel line quick-connect, and the rocket was returned to the VAB for servicing. It returned to the launch pad in early June only to encounter further problems during a June 20 fuel test, when engineers were unable to cool the rocket’s engines due to a valve sticking in a different system.

The rocket was returned to the VAB for repairs in early July and returned to the pad in mid-August for what NASA hoped would be its maiden flight. But a launch try was canceled on August 29 due to more hydrogen problems, and again on September 3 when an 8-inch quick-disconnect fitting leaked.

In the run-up to the second launch scrub, NASA managers chose to disassemble fittings on the launch pad, replace an internal seal, reassemble the hardware, and conduct a fuel test to verify seal integrity. Hydrogen leaks typically occur when plumbing is exposed to cryogenic temperatures — in this case minus 423 degrees Fahrenheit.

Oxygen vapor flowed from vents on the side of the Space Launch System rocket as propellants were loaded into the upper stage of the booster.


The repair work was completed last week and testing began normally enough Wednesday, with oxygen and hydrogen flowing at reduced rates to individual core stage tanks. In an effort to minimize thermal shock when transitioning to “quick fill” mode, the loading sequence was slowed and the flow rate reduced to reduce the stress on the hardware.

But when the flow rate and pressure increase, the sensors detect an immediate build-up of gaseous hydrogen in a containment housing around the newly repaired quick-disconnect fitting, indicating a leak. The sensors detected concentrations of up to 7%, above the 4% safety limit.

Engineers then chose to heat the fittings before restarting hydrogen flow, hoping to “re-seat” the internal seal. When flow resumed, a leak was still present, but it was below the 4% threshold and the engineers were able to proceed, eventually topping off the hydrogen tank with a full load of 730,000 gallons.

A closer examination of the sensor data showed that, contrary to the initially observed behavior, the leak rate decreased as the pressure increased. This is how the fitting is designed to operate, suggesting attempts to reseat the seal are at least partially successful.

With the core stage hydrogen and oxygen tanks full, engineers proceeded to load the upper stage of the SLS rocket while conducting pressurization and engine cooling tests.

Another hydrogen leak was reported near a 4-inch quick-disconnect fitting used for cooling tests. Although engineers agreed to proceed with the density already observed, this would stop an actual launch countdown. No word yet on what, if any, impact that issue might have on launch plans.

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