What is a wet dress rehearsal?

A week ago, NASA found a hydrogen leak during a wet dress rehearsal of its Artemis II mission. A dress rehearsal in rocketry is a full run-through of activities that will happen on the day of launch, to validate people, hardware, software, and timelines as an integrated system.

A dry dress rehearsal practices the countdown and important operations without loading cryogenic propellants into the rocket. Instead, the team will power up vehicle and ground systems, verify its communications equipment, simulate critical events, and validate decision-making and handoffs between launch control, engineering, range safety, and, if applicable, crew operations. Many of the testing steps use simulated sensor inputs. These rehearsals are useful to reveal logical problems in the flow of events without risking fuel leaks.

A wet dress rehearsal is the same concept taken to the closest safe approximation of launch day, and with the vehicle being fuelled with its actual cryogenic propellants (typically liquid oxygen and liquid hydrogen for large rockets). The team will cool the fuel feed lines, load the tanks, pressurise them, monitor leak detectors, and execute the countdown into its final stages. These rehearsals also keep the tanks full as the propellant warms and boils off, then execute a stop just before ignition, followed by draining and returning the vehicle to a stable configuration.

Wet rehearsals are important because only they can reveal events that happen in cryogenic conditions, e.g. leaks in seals or in the connections between the rocket and ground equipment.