Hydrogen leak delaying launch of a space rocket

The event occurred during the fuelling process of the primary fuel tank of the core stage of a moon rocket.
The initial slow-filling phase was terminated, and personnel had just started the fast-filling phase when a leak was detected on a quick-disconnect fitting of the liquid hydrogen line into the tank. Hydrogen was leaking into the purge can, which is a housing covering the quick disconnect fitting, part of the tail service mast umbilicals. The leak was exceeding the concentration limit of 4%, set by flammability risks, by a factor of two or three.

Engineers made several attempts to reseat the fitting on-site and on-launch, by allowing it to warm up and replacing the seal, or by closing the fuelling valve and drain it, then increasing pressure on the ground transfer line using helium to force the fitting back into place. Eventually, the launch on 3rd of September had to be postponed.

In the following days, engineers troubleshooted the leak. During the cryogenic tests, the 21st September, already during the slow-filling phase a leak at the same location was detected. The connection was again warmed up and refixed, and the filling operation continued at lower pressure than before. This time the hydrogen concentration in the housing remained under the allowed limit, and the refuelling could continue up to full fill and the pressurisation tests.

During the next launch attempt, the 15th of November, the fast-fuelling phase was successfully completed, but a valve at the base of the mobile launcher, used to replenish the core stage with hydrogen, showed a leak above safety limits. The hydrogen flow was stopped, and a team of engineers had to go to access the pad and tighten several bolts to troubleshoot a valve. The rocket lifted off one day later.

The INITATING CAUSE was a hydrogen leak from a liquid hydrogen connection, above the allowed maximal concentration in air.

One possible reason of the loss of confinement capacity of the connection could have been an error of over-pressurisation of the liquid hydrogen line of the quick-disconnect interface, which may have damaged a seal. During the early phase of hydrogen transfer operations called chill-down, aiming at cooling down the lines and propulsion system prior to flowing liquid hydrogen, an inadvertent command was sent that temporarily raised the pressure in the system.

A second explanation of the leak could have been a small indentation on the eight-inch-diameter liquid hydrogen seal, found during the post-mortem examination of the seal.

These sources do not identify with certainty a ROOT CAUSE. From the suggested leak explanations, it derive that one root cause factor could have been the cryogenic procedure.

ground tank, connection

DESCRIPTION OF THE SYSTEM
The Space Launch System core stage was the central, tallest component of the rocket that launches the Orion spacecraft. It functioned as the rocket's primary fuel tank, housing a liquid hydrogen tank , a liquid oxygen tank, and four engines providing the initial thrust for ascent.
The core stage is 212 feet (64.6 meters) long and has a dimater of 27.6 feet (8.4 meters).
The LH2 tank has a volume of 537,000 gallons (2 million liters), able to host approximately 140 tons of hydrogen. Its nominal pressure is 8.5 bar, the operative pressure is around 1.4 bar.
For more infromation see https://www.nasa.gov/reference/space-launch-system-core-stage/

DESCRIPTION OF THE FUELLING SYSTEM
Two tail service mast umbilicals provide liquid oxygen and liquid hydrogen fluid lines and electrical cable connections to the Space Lauch SysteM core stage to support propellant handling during pre-launch operations.
For more details, see https://www.nasa.gov/wp-content/uploads/2018/06/fs-2018-02-250-ksc-ml_umbilical_fact_sheet.pdf?emrc=3f5e9f (accessed October 2025)
https://www.youtube.com/watch?v=oeuU_ffBlGI&t=26s

The delays in the launch have not been monetirised.

This event, prolonged throughput several weeks, is similar to an earlier case of elusive leaks related to the launch of a space shuttle, a period called by NASA the ‘the summer of hydrogen’ (see HIAD_1223).
It shows how difficult is to identify the exact location of hydrogen leak in complex technical systems. Due to its volatility, the leaking hydrogen quickly diffuses into the space around the leak, so that several fixed detectors placed in different positions can detect hydrogen simultaneously. Despite the progress in fundamental knowledge of liquid hydrogen behaviour and in engineering system dealing with a liquid at the most extreme temperatures, loss of confinement remains a concern. In this recent case, however, engineers required much less time than back 20 years ago to troubleshooting the leak.

Engineers replaced two seals, one surrounding the 8-inch line used to fill and drain liquid hydrogen from the core stage, and another surrounding the 4-inch bleed line used to redirect some of the propellant during tanking operations.
They also updated the cryogenic loading procedures and developed new ground automation allowed for slower transition of temperatures and pressures during tanking, to reduce the likelihood of leaks caused by rapid changes in temperature or pressure.