Artemis II Wet Dress Rehearsal, critical milestone to launch

Humanity’s first deep space crewed mission since 1972 is scheduled for no earlier than Sunday, Feb. 8, at 11:20 PM EST (04:20 UTC Monday, Feb. 9) from Launch Complex 39B at the Kennedy Space Center.

However, NASA has not yet chosen an official launch date, and will not do so until the Artemis II stack passes its Wet Dress Rehearsal, with a simulated T0 that had been set for Saturday, Jan. 31 at 9:00 PM EST (02:00 UTC) but is now delayed until Monday, Feb. 2 at 9:00 PM EST (02:00 UTC Tuesday Feb. 3).

The Wet Dress Rehearsal (WDR), which was pushed back 48 hours due to freezing weather and winds at KSC, is a full-scale countdown and propellant load.

Although there is a set T0 time, there is a four hour window from then where the test can be finished. The only major differences between the WDR and the actual launch countdown are that the crew will not board the Orion spacecraft Integrity, nor will the SLS ignite its RS-25 main engines or solid rocket boosters.

NASA will analyze the results of the WDR before deciding whether it is good enough to allow a launch attempt as the next step, or whether the Artemis II stack will be rolled back to the Vehicle Assembly Building (VAB) for repairs. There are only three days remaining in the February launch window – Launch Period 17 – Feb. 8, Tuesday, Feb. 10, and Wednesday, Feb. 11.

The countdown starts

Like the actual countdown, the WDR started with a “call to stations” at the L-49 hour 15 minute mark from the scheduled T0 of the test. At this point, controllers take up their positions at the Launch Control Center (LCC) next to the Vehicle Assembly Building (VAB) at KSC.

The countdown clock started at L-48 hours 40 minutes, with three major activities happening concurrently: filling the sound suppression system’s water tank, preparing the Artemis II stack for propellant load, and powering up Integrity.

The Space Launch System (SLS) rocket’s core is a Space Shuttle-derived tank structure with four L3 Harris (formerly Aerojet Rocketdyne) RS-25 engines, the same engine type used by the Shuttle orbiters, and these engines use liquid oxygen and liquid hydrogen as propellant. For Artemis II, the core stage will use engines 2047, 2059, 2061, and 2062.

Closeup of Artemis II’s main engines during the core stage’s integration with the solid rocket boosters. (Credit: NASA)

Engine 2047 was used on STS-135, the last flight of the Space Shuttle, while E2059 was used on the penultimate shuttle flight, STS-134. E2061 was used during ISS assembly missions, and replaced the failed E2063 originally assigned to Artemis II. Engine 2062 was the last RS-25 built before the Shuttle program ended and is making its first flight.

Initial preparations for loading over 733,000 gallons of propellant into the core started at L-48 hours 45 minutes and run to L-39 hours 45 minutes. From L-47 hours 30 minutes to L-43 hours 30 minutes, controllers load the Ignition Overpressure and Sound Suppression System (IOP/SS) tank with over 400,000 gallons of water.

The IOP/SS is necessary to dampen sounds and vibrations associated with the ignition of four RS-25 engines and two five segment Shuttle-derived solid rocket boosters, which generates a total of 39 mN (8.8 million lbs) of thrust.

Water deluge test at LC-39B in January 2018. (Credit: NASA/Kim Shiflett)

These sounds and vibrations can damage pad equipment and rockets. A notable example is the first Space Shuttle launch in history – STS-1 – which suffered damage to its external tank struts and tiles as well as an uncommanded body flap movement as a result of insufficient sound suppression measures.

The Integrity spacecraft, if it is not already powered at the call to stations, is powered up from L-44 hours 30 minutes to L-43 hours. After Integrity is turned on, Artemis II’s upper stage, the interim cryogenic propulsion stage (ICPS), is powered up initially from L-40 hours to L-39 hours, while the core stage is concurrently turned on from L-39 hours 30 minutes to L-38 hours 45 minutes.

After the L-38 hour 45 minute mark, controllers make final preparations of the core stage’s four RS-25 engines up until the 34 hour 30 minute mark. As always, these timings are approximations and can change depending on how the countdown flows.

The SLS Interim Cryogenic Propulsion Stage (ICPS). (Credit: NASA)

Critical preparations

The next phase of the practice countdown involves a number of battery charges and steps to prepare for propellant loading. The ICPS is powered down from L-33 hours 45 minutes to L-33 hours 10 minutes, while Integrity’s batteries are charged to 100% from L-32 hours 30 minutes to L-28 hours 30 minutes. Concurrently, controllers charge the core stage’s batteries from L-30 hours 30 minutes to L-23 hours 30 minutes.

Controllers power up the ICPS upper stage for flight from L-18 hours 45 minutes to L-17 hours 25 minutes. The ICPS stage is derived from the Delta IV family’s upper stage, with some modifications to attitude control, the liquid hydrogen tank, avionics, and to the liquid hydrogen vent and relief valve to support engine restarts in flight.  Like the RS-25 engines, the upper stage’s single Aerojet Rocketdyne RL-10 engine uses liquid oxygen and liquid hydrogen as propellants.

At the 14 hour and 30 minute mark before T0, all non-essential personnel leave LC-39B. The Ground Launch Sequencer (GLS) will activate during the period from L-12 hours 45 minutes to L-11 hours and 15 minutes, while controllers start the process to make the SLS vehicle cavity inert with gaseous nitrogen starting at the L-13 hour 15 minute mark up until T0.

The first RS-25 installation into the Artemis II core stage. (Credit: NASA/Eric Bordelon)

Typically, during the flow before launch or a tanking test, cavities like the engine compartment are filled with dry air to keep electronics from shorting due to moisture, and to allow workers to access areas inside the rocket. The inerting process is critical for keeping the SLS safe during the propellant load process and countdown to T0.

Gaseous nitrogen displaces oxygen out of the engine compartment and nearby compartments; this purge prevents ignition due to hydrogen leaks. Hydrogen leaks were problematic during the run up to the Artemis I launch on Nov. 16, 2022, and controllers will no doubt watch the propellant load very carefully during the wet dress rehearsal and final countdown to launch.

The countdown features a number of built-in holds, which accounts for the difference between the “T-minus” and “L-minus” timings, with L-minus the actual time to launch. At L-11 hours 35 minutes a built in hold starts and lasts for two hours and 15 minutes. During this countdown hold, the launch team will conduct a briefing regarding weather for tanking, followed by a “go” or “no go” poll for tanking at L-10 hours and 20 minutes.

Artemis II at LC-39B after its Jan. 17 rollout. (Credit: NASA/Keegan Barber)

The weather criteria for tanking must be met before propellant load starts. The temperature at LC-39B must be equal to or higher than 41 degrees F, while the average temperature over the previous 24 hours at the 132.5 and 257.5 foot levels on the launch tower also should be equal to or greater than 41.4 degrees F. This constraint possibly factored into the WDR delay.

The lightning forecast also cannot be greater than 20% within five nautical miles of LC-39B during tanking. In addition, if the air temperature falls into a band between 38 and 49 degrees Fahrenheit for 30 minutes, fueling can be halted depending on the wind and humidity measurements.  Higher wind and relative humidity result in a colder temperature constraint.

Propellant load

If the tanking poll results in a decision to proceed, the rocket and its RS-25 main engines will need to be chilled down to avoid thermal shock with the cryogenic liquid oxygen and liquid hydrogen start filling the respective propellant tanks in the rocket’s orange-colored core.

Beginning at L-10 hours 10 minutes, the core stage’s liquid oxygen transfer line and liquid hydrogen tanks will be chilled down, while Integrity will be cold soaked starting at L-10 hours 20 minutes. The core stage’s liquid oxygen main propulsion system – the RS-25 engines and associated pipes – will be chilled down from L-nine hours 50 minutes to nine hours 10 minutes.

Work on the tail service umbilical mast (TSMU) during the Artemis I launch campaign. The TSMUs connect propellant loading lines to the core stage. (Credit: NASA/Chad Siwik)

The rocket’s actual fueling starts with the liquid hydrogen slow fill process nine hours 25 minutes before launch, where the Artemis II core stage will see cryogenic temperatures for the first time. Artemis I’s core stage was filled with cryogenics a number of times during testing at the Stennis Space Center in Mississippi and during wet dress rehearsals at LC-39B before the first Artemis I launch attempt in August 2022.

If all goes well, the countdown will be released from its hold at T-eight hours 10 minutes, or nine hours 20 minutes before T0, and the liquid oxygen slow fill begins at that point. The fast fill begins nine hours before launch, first with liquid hydrogen, then liquid oxygen a few minutes later. Fast fill continues for the next two to three hours for both commodities.

During the Space Shuttle’s 30 years of service, hydrogen leaks would most often be found during the start of fueling, though some leaks developed later as certain components saw cryogenic temperatures for the first time. After experience with Artemis I, NASA upped the maximum allowable percentage of hydrogen in the umbilical purge area from four percent to 10 percent; some hydrogen will always leak due to the nature of the molecule but the flammability limit in the area was found to be closer to a 16 percent concentration.

Artemis II at LC-39B on Jan. 26. (Credit: Max Evans for NSF/L2)

Concurrently, the ICPS upper stage begins its own chilldown process to prepare it for fast fill starting just over eight hours before T0. Assuming that no issues develop – always a big if – the core stage’s liquid hydrogen topping off starts at seven hours and 40 minutes before T0, with replenishment continuing until a few minutes before T0.

The ICPS vent and relief test, followed by liquid hydrogen topping off and replenishment, begins seven hours and 25 minutes before T0, followed by Integrity’s communication system being activated six hours and 10 minutes before T0. The core stage’s liquid oxygen topping off starts at the same time, as well as the chilldown of the ICPS liquid oxygen main propulsion system.

The ICPS liquid oxygen fast fill begins at the L-six hour mark, with the core stage’s liquid oxygen replenish beginning 20 minutes later and continuing until a few minutes before T0. The upper stage’s liquid oxygen vent and relief test follows at five hours 15 minutes before launch, followed by ICPS liquid oxygen topping. From L-four hours and 40 minutes the ICPS liquid oxygen replenish phase begins, and all stages will be in the replenishment phase until shortly before T0, marking the end of the fueling process.

The Artemis II crew leaving for LC-39B during the Artemis II countdown demonstration test on Dec. 20, 2025. (Credit: Max Evans for NSF/L2)

Final countdown

On launch day, when all stages are in the replenishment phase, the flight crew leaves for the launch pad and the closeout crew works on boarding the astronauts and closing up Integrity and its launch abort system. During the WDR, the crew will not perform these steps; there was a countdown demonstration test last month where the astronauts and closeout crews did practice the same procedures as on launch day.

A built-in countdown hold starts at 40 minutes before the scheduled T0, and the hold will last 30 minutes. During this hold, the NASA Test Director (NTD) conducts a final briefing. At 16 minutes before T0, launch director Charlie Blackwell-Thompson will poll the team for a final “go” to proceed. The final hold is released at T-10 minutes with the ground launch sequencer (GLS) initiating the terminal count.

At T-eight minutes, the crew access arm retracts, and core stage tank pressurization starts at six minutes before T0. From this point onward, hold time to troubleshoot issues is limited to three minutes. Integrity is also set to internal power at the six minute mark as well, with the core stage also terminating its liquid hydrogen replenish process a few seconds later.

Current Artemis II launch director Charlie Blackwell-Thompson (center) with Stephanie Stilson and Mike Leinbach during the STS-133 launch. (Credit: NASA/Kim Shiflett)

The GLS will start the core stage auxiliary power unit and terminate core stage liquid oxygen replenish at the four minute mark. Shortly thereafter, the ICPS will terminate liquid oxygen replenish and purge sequence 4 will start. Two minutes before T0, the upper stage and boosters will go on internal battery power.

The core stage will switch to internal power ninety seconds before T0, followed by a three minute hold to test how systems and controllers perform during this process. The count will resume, with the ICPS entering terminal countdown mode and ceasing liquid hydrogen replenish.

On launch day, the SLS rocket would enter its automated launch sequence at T-33 seconds, severing the rocket’s last electrical link with Earth. However, this is when the count will stop for the purposes of the WDR. The countdown will recycle to T-10 minutes and hold before resuming and going to T-33 seconds again, to test how the systems perform during this process.

Artemis I SLS with the iconic KSC Press Site countdown clock in the foreground during a test in September 2022. (Credit: Stephen Marr for NSF)

The recycle test was meant to be performed during the Artemis I WDR, but due to some issues never got to that point. After this recycle test, teams will safe the vehicle and back out of the launch process, offloading the vehicle’s propellants. NASA will analyze the data from the WDR before deciding on a launch date, or whether to roll the SLS back to the VAB for additional work.

(Lead image: Artemis II rolling out to LC-39B on Jan. 17, 2026. Credit: Sawyer Rosenstein for NSF/L2)

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