If you’ve been even casually following NASA’s return to the Moon, you’re likely aware of the recent Wet Dress Rehearsal (WDR) for the Artemis II mission. You probably also heard that things didn’t go quite to plan: although the test was ultimately completed and the towering Space Launch System (SLS) rocket was fully loaded with propellant, a persistent liquid hydrogen leak and a few other incidental issues lead the space agency to delay further testing for at least a month while engineers make adjustments to the vehicle.
This constitutes a minor disappointment for fans of spaceflight, but when you’re strapping four astronauts onto more than five million pounds of propellants, there’s no such thing as being too cautious. In fact, there’s a school of thought that says if a WDR doesn’t shake loose some gremlins, you probably weren’t trying hard enough. Simulations and estimates only get you so far, the real thing is always more complex, and there’s bound to be something you didn’t account for ahead of time.
So what exactly is a Wet Dress Rehearsal? In the most basic of terms, its a practice launch where everyone involved does everything exactly the way they would on a real launch, except when the countdown hits zero, nothing actually happens.
It’s the final test of the vehicle and the ground support systems, the last check of fit and function before launch. But there’a also a logistical element. In other words, it’s not just a test of whether or not the vehicle can be fully fueled, it’s also a verification of how long that process takes. Many of the operations that are performed in the WDR would have already been tested in isolation, but this may be the first, and only, time to practice running them concurrently with all of the other elements of the countdown.