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.
Do Not Pass Go
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.
There’s also the human element. Hundreds of individuals have a part to play as the clock ticks down to zero, from the team in mission control to the driver of the astronaut transport vehicle. This is where the Wet Dress Rehearsal truly earns it name. In a sense, launching a rocket is a bit like a theater production. Every player needs to not only have their individual role memorized, but they need to work together effectively with the larger ensemble on the big night.
Although a WDR is meant to simulate an actual launch as closely as possible, the rules are slightly different. If the rocket was actually going to be released there are other variables to contend with, such as the launch window, which is the period of time in which the rocket can actually leave the pad to reach its intended orbit. On a real launch, a delay significant enough to keep the vehicle from lifting off during its pre-determined launch window would generally result in an automatic abort. There is no such constraint for a rehearsal however, which gives teams more flexibility to conduct tests and repair work.
It should be noted that the Artemis II astronauts were not aboard the vehicle for the recent WDR, although ground teams did simulate the process of loading the crew into the Orion capsule. This is partly for the safety of the astronauts should something go wrong during the rehearsal, but is also due to the fact that the Moon-bound crew is kept in quarantine until the actual launch day to reduce the likelihood they will get sick during the mission.
Light the Fires
As mentioned above, for the purposes of the Wet Dress Rehearsal, nothing actually happens when the launch clock hits zero. It’s a test of the pre-launch activities, so actually starting up the engines isn’t part of the exercise.
But of course, testing the engines is an important aspect of launch preparation as well. Such a test is generally referred to as a static fire, where the engines are briefly run while the vehicle is physically held down so as not to leave the pad. Operationally, a wet dress rehearsal could proceed directly into a static fire. On the other hand, a full WDR is not required to perform a static fire.
While static fire tests are common for modern rockets such as the Falcon 9, NASA has decided not to conduct them during the Artemis I and II missions. The SLS rocket uses lightly modified RS-25 Space Shuttle Main Engines (SSMEs), which are not only flight proven, but were individually tested before integration with the vehicle. There is also an argument to be made that a full-up static fire on the SLS, like the Space Shuttle before it, isn’t truly possible as the vehicle’s Solid Rocket Boosters (SRBs) can only be ignited once.
The Artemis I rocket did however conduct what NASA calls a Green Run back in 2021. This saw the first stage of the SLS fire its four RS-25 engines for eight minutes to simulate an orbital launch. The first attempt at the Green Run saw the engines shut down prematurely, but they did run for the full duration in a subsequent test.
Although such a test wasn’t conducted for Artemis II, and are not expected for any of the future SLS rockets, NASA is preparing for a Green Run test on the Exploration Upper Stage (EUS). This is an upgraded second stage for the SLS which is intended to support more ambitious missions after the Artemis III landing, although the timeline and status of those missions is tenuous at best.
The Road to the Moon
According to NASA’s latest update, the issues during the Artemis II Wet Dress Rehearsal has pushed the testing campaign back until at least March, at which point they will run a second WDR. But that certainly doesn’t mean it will be the last.
While admittedly no two missions are the same, Artemis I went through four WDRs before it flew. Even then, the last one was aborted before the countdown was completed. Interestingly it was a hydrogen leak that caused that final rehearsal to be cut short, indicating that it may be a more dynamic problem than NASA realized at the time.
Even if the second WDR for Artemis II goes off without a hitch next month, that doesn’t mean the actual launch won’t be hit with its own delays due to technical glitches, poor weather, or any one of a myriad of other possible issues. Getting a rocket off the ground is never easy, and it only gets harder when there are humans onboard and the destination is farther than anyone has flown since the 1970s. An almost endless number of things need to go exactly right before we’ll see Artemis II lift off the pad, but when it goes, you definitely won’t want to miss it.