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Key Concepts

• The Artemis 2 Wet Dress Rehearsal (WDR) encountered a hydrogen leak during fast fill, managed within established limits, but ultimately led to a test cutoff.
• The SLS hardware is designed to withstand extreme weather conditions, and a data-driven approach was used to establish a 16% leak threshold considered safe for operation.
• Infrastructure improvements, such as the new 1.4 million gallon hydrogen sphere and replenish valve redesign, contributed to the test’s successes.
• Lessons learned from Artemis 1 were directly applied to troubleshooting and contingency planning during this WDR.

Artemis 2 Wet Dress Rehearsal – Overview & Initial Findings

The Artemis 2 Wet Dress Rehearsal (WDR) involved a hydrogen leak detected in the Thermal Structural Mechanical Unit (TSMU) – the cavity between the flight and ground plates – within the 8-inch fill and drain line Quick Disconnect (QD) during Liquid Hydrogen (LH2) fast fill at approximately 12:29 local time. The leak rate reached 12-14%, approaching the established limit of 16%. A “stop flow revert” procedure was implemented to manage the leak, and a contingency procedure involving warming the QD seal, mirroring a successful tactic from Artemis 1, was attempted multiple times. Despite the leak, the core and upper stages were successfully replenished and fully loaded by 17:44 local time.

The countdown progressed to T-minus 6 minutes, initiating core stage pressurization, at which point the leak re-emerged, triggering a cutoff. Core stage drain was completed the following morning around 6:00 AM, with all commodities drained from both stages. The vehicle is now undergoing boil-off and inerting activities in preparation for pad control work. This test marked the first time both core and upper stages were loaded from the new 1.4 million gallon hydrogen sphere, a significant achievement. Design modifications to the replenish valve, identified as an issue during Artemis 1, proved successful. A late modification enabling breathing air injection for the service module purge allowed for a full closeout of the crew module, and the Crew Access Arm (CAA) successfully retracted and latched into place during the terminal count – a first for this configuration with the ground launch sequencer.

Hardware Resilience & Weather Concerns

Concerns were raised regarding the SLS hardware’s vulnerability to weather conditions while on the launch pad. The response emphasized the robust design of the SLS, particularly the cryogenic tank’s ability to handle hydrogen at -423°F, significantly lower than the ambient temperature of 25-27°F. Continuous “purges” of gas are utilized to mitigate environmental effects. During the threat of a hurricane for Artemis 1, an assessment determined it was safer to remain on the pad than to roll back the vehicle, demonstrating confidence in the hardware’s resilience, and “lighting protection” measures are also in place.

The 16% Leak Threshold – Rationale & Testing

The rationale behind the 16% leak threshold was questioned, contrasting it with the Space Shuttle program’s stricter 4% limit. The SLS team deliberately re-evaluated leak limits, conducting extensive testing on small leak volumes. This testing recognized that leaks are purged and vented, resulting in diluted hydrogen concentrations. The goal was to determine the potential for a sustained flame front versus a self-extinguishing leak.

The 16% limit was established through data analysis and imagery, providing a “comfortable margin” of safety. Testing revealed that ignition could not be achieved within the “cavity” at a 16% hydrogen concentration. An external agency was contracted to introduce hydrogen into a simulated cavity to specifically assess ignition potential. The decision was data-driven, based on empirical evidence, and confirmed that at 16% concentration, ignition was not possible.

Data & Technical Specifications

• SLS Hydrogen Tank Rating: -423°F
• Ambient Temperature: 25-27°F
• Space Shuttle Leak Threshold: 4%
• SLS Leak Threshold: 16%
• New Hydrogen Sphere Capacity: 1.4 million gallons
• Replenishment Completion Time: 17:44 local time
• Key Technical Terms: TSMU, QD, LH2, Fast Fill, Boil-off, Inerting, MMT, TZ, FTS, Purge, Cryogenic, Flame Front, Cavity.

Conclusion

The Artemis 2 Wet Dress Rehearsal provided valuable data and demonstrated significant progress, despite the encountered hydrogen leak. The successful implementation of lessons learned from Artemis 1, coupled with infrastructure improvements like the new hydrogen sphere and replenish valve redesign, highlight the program’s commitment to iterative refinement. The data-driven approach to establishing the 16% leak threshold, and the demonstrated resilience of the SLS hardware, underscore a commitment to both safety and operational efficiency as the Artemis program moves closer to crewed missions.