More than 700,000 gallons of cryogenic propellant that will pass through Space Launch System plumbing will flow, we will never get a single astronaut into the cabin of Orion, and that is the point. In Artemis 2, the wet-dress test of the spacecraft makes the launch pad a full-scale systems test, during which the rocket, ground equipment, and countdown crew demonstrate the ability to act as on launch day-without the safety net of early termination.
The rehearsal will be the final significant integrated test at Launch Complex 39B in Kennedy Space Center before a crewed lunar mission. This is cleared of the pad in advance of the tanking work, and in Florida, controllers liaise with personnel in Houston and elsewhere to conduct a comprehensive countdown, including the capability of halting, recycles, and scrub operations without losing track of the operation.
The most observable one is the beginning of the fueling process, when liquid hydrogen and liquid oxygen start to fill into both core stage and the intermediate cryogenic propulsion stage. That is not a one fill up process, but a ladder of achievement-chilldown, slower flow, faster flow, topping, and constant replenish as the propellant evaporates. During the process, leak checks are performed on teams as the vehicle is thermally stressed and pressurized in a manner that cannot otherwise be done in the laboratory. NASA also seals off the umbilicals which connect the mobile launcher to the rocket and spacecraft, connections which must provide power, communications, and propellant and then neatly detach at liftoff.
One of the major motivators of the intensity of the rehearsal is history. In the 2022 campaign to prepare the first SLS flight, hydrogen leakages and other ground-interface issues kept interrupting efforts to load all the propellant. The low temperature and small size of molecules of hydrogen complicate sealing of the gases and the material that seems to be sound at ambient temperatures may change as soon as the cryogenic flow occurs creating real ways to leak. Teams eventually transitioned to a more progressive loading strategy that sacrificed velocity in favor of stability; in the case of Artemis 2, the lessons and improvements are integrated into the process, and wet dress rehearsal is described by the leadership of launch as the “best risk reduction test”.
The rehearsal is yet again influenced by weather to the extent of proving. Abnormally low temperatures around the pad compelled a test later than planned in part because Artemis 2 needs to meet temperature requirements on launch day, not fueling day safe. The reason that delay is important is due to the limited nature of lunar launch opportunities: Earth, Moon, geometry of the flight path, and light requirements only coincide on certain days. The earliest attempt of Artemis 2 was now scheduled to start no earlier than Feb. 8, then further in the window in February and more in March and April.
The rehearsal of Artemis 2 even in the absence of astronauts but shows a form of human-tended countdown previously absent in SLS tests. The operation will also have an inherent break during which the crew would be boarding Orion on the launch day, which will further complicate and time-wise extend an already lengthy process. Orion has also continued operating even days in the cold and the engineers are preparing to charge flight batteries as the rocket goes through its own battery and avionics systems.
Background This pad campaign is also involved with the rest of the U.S. human spaceflight calendar. The launch infrastructure, communications infrastructure, and recovery infrastructure of the same region are not infinite, and as such Artemis 2 final prelaunch test is more of a schedule gate than an engineering one an all hands demonstration that the system is capable of being fueled, count down safely safed and drained on command.
When the rehearsal is done to the full extent of its functions loading, holding, recycling the clock, and draining, the program comes out with something more difficult to make than a clean checklist confidence that the ground and flight systems will work together as they should when all the ground and flight systems are cold, pressurized, and running at the temperature necessary to take off.
