NASA's Artemis 1 moon mission completed a crucial preflight milestone today (June 20), wrapping up a two-day set of tests known as a wet dress rehearsal.
Those tests included fueling up Artemis 1's huge Space Launch System (SLS) rocket and performing a simulated countdown that took the vehicle and NASA's Orion capsule through most of the progressions they would endure on launch day before engine ignition. The simulated countdown reached its conclusion at 7:37 p.m. EDT (2337 GMT), bringing an end to the wet dress.
Not everything went perfectly smoothly. The Artemis 1 team noticed a hydrogen leak during fueling today, and they intentionally "masked" data associated with the issue to let the countdown continue. (During an actual launch countdown, such data would have raised red flags, NASA officials said.) This change meant the countdown was halted at T-29 seconds before "liftoff," instead of T-9 seconds as originally planned.
But that hiccup didn't dampen the enthusiasm of the Artemis 1 team much.
"It's a great day for our team," Charlie Blackwell-Thompson, Artemis launch director with the Exploration Ground Systems Program at NASA's Kennedy Space Center (KSC) in Florida, said during a webcast of the wet dress just after it wrapped up. "Really proud of them working through the loading operations and working through terminal count."
Ground teams at KSC will now likely spend the next few days preparing the Artemis 1 stack and mobile launch platform (MLP) for transport from Launch Pad 39B to the Vehicle Assembly Building (VAB), where technicians will analyze results from the series of tests.
The wet dress was the final major milestone that Artemis 1 needed to complete before launch. On the actual mission, the SLS rocket will launch an uncrewed Orion on a roughly month-long journey to the moon and back. Artemis 1 will be the first in a series of expeditions to the moon that NASA hopes lay the foundation for a permanent human presence on the lunar surface — one of the chief goals of the agency's Artemis program.
Artemis 1 will be the first flight for the SLS, the realization of over a decade of research, development and manufacturing to advance NASA's human exploration plans beyond low Earth orbit (LEO). If Artemis 1 goes well, NASA plans to fly astronauts aboard Artemis 2 and Artemis 3, with the latter mission putting astronauts down on the moon in four years or so.
Agency officials have repeatedly voiced optimism for an Artemis 1 launch at the end of August, but those hopes always hinged on a smooth wet dress rehearsal. Now, that launch window seems much closer to reality.
Procedures on the rocket and MLP systems at Pad 39B began on Saturday (June 18) with a call to stations for ground teams at 5:00 p.m. EDT (2100 GMT). That kicked off a countdown clock toward a simulated launch time of 2:40 p.m. EDT (1840 GMT) today, which was later pushed back to 4:38 p.m. EDT (2038 GMT).
True to its name, one of the first major protocols for the SLS wet dress rehearsal was filling the water tanks for the pad's sound suppression infrastructure. The system dumps nearly half a million gallons (1.9 million liters) of water onto the launch pad and flame trenches during an actual liftoff and works to absorb acoustic energy expelled from engine ignition.
Next in the process came vehicle stage startups. Operators began that on Saturday night, powering on Orion's systems first. The SLS core stage was powered up next, early on Sunday morning (June 19), followed by a series of systems checks to confirm connections between the spacecraft, ground infrastructure and boards in mission control.
With the rocket's first-stage systems active, final preparations for the vehicle's four RS-25 engines completed their own set of inspections and tests, which wrapped up on Sunday morning.
NASA officials met Sunday to assess rehearsal progress and receive a weather briefing from meteorologists with the U.S. Space Force Space Launch Delta 45, which predicted favorable weather conditions for today's tanking procedures. In an official blog post, NASA stated that ground crews on Sunday afternoon were "performing a pre-launch walk down of the rocket to ensure the Space Launch System is prepared for the upcoming propellant loading operations."
Burning the midnight oil, so to speak, teams at NASA switched on power to the SLS upper stage, known as the Interim Cryogenic Propulsion Stage (ICPS), just before 2:00 a.m. EDT (0600 GMT) today. By 2:40 a.m. (0640 GMT), all non-essential personnel were cleared from the launch pad.
Artemis 1 made its full-stack debut in mid-March, when the vehicle and the MLP first rolled from the VAB to the pad for a wet dress rehearsal that began on April 1. But technicians encountered problems during that try; three separate attempts to complete the wet dress were scrubbed due to complications during SLS fueling operations.
The Artemis 1 team rolled the Artemis 1 stack back to the VAB on April 25, then spent about a month making repairs and upgrades to pave the way for another wet dress try. Loading failures during the first wet dress put a predominant focus on those systems during this morning's cryogenic fueling procedures.
A built-in countdown hold went into effect early this morning and was expected to last 1.5 hours. During that time, mission managers conducted weather and vehicle tanking briefings and had originally received a "go" from Blackwell-Thompson, but the hold was extended when operators encountered an issue with a supply valve in Pad 39B's redundant gaseous nitrogen system. According to Derrol Nail, NASA spokesperson on the agency's live broadcast for the wet dress, the valve "would not close," though the malfunction was "not with the actual valve itself, but with the controller."
Technicians were not immediately able to determine the cause of the issue, but they replaced the valve nonetheless. The affected gaseous nitrogen line was initially designated as the pad's redundant supply. However, after replacing the valve, engineers reassigned that leg for primary use and the unaffected line as the new backup. "The manual retest of that valve is working well, according to the team that's out there," Nail said during the broadcast.
Gaseous nitrogen is used at Pad 39B for purging the fuel tanks, umbilicals and other cavities inside SLS, both before and after tanking. Artemis 1's time back in the VAB following the first wet dress attempt allowed some planned upgrades at the pad to move forward ahead of schedule. One of these upgrades included the enhancement of supply capabilities for the pad's gaseous nitrogen, which nearly doubled in capacity.
"We did go through and do a test to verify that [the gaseous nitrogen upgrades] supported all of our needs prior to rollout," Blackwell-Thompson told Space.com during a June 15 press call. "We went through all of the flow profiles, verified all of the pressures and also verified all the timelines with some amount of margin to ensure that our flow rates and our needs could be supported during our wet dress rehearsal."
Assistant launch director Jeremy Graeber joined the NASA broadcast for an update following the valve resolution, informing listeners that the "problem has been cleared, and we're at a good configuration to pick up with cryo-loading [and] no longer constrained." According to Nail, Blackwell-Thompson gave the "go" to move forward with the morning's cryogenic tanking procedures shortly before 9 a.m. (1300 GMT) today, and the count was resumed at 9:28 a.m. EDT (1300 GMT) toward an adjusted T-0 of 4:38 p.m. EDT (2038 GMT).
The two stages of the SLS rocket are fueled with liquid oxygen (LOX) and liquid hydrogen (LH2). The process of loading both involves super-cooling the propellants and chilling associated hardware prior to tanking. Technicians monitor loading rates carefully, starting with a slow-fill flow, and increasing to peak flow rates once the umbilical connections and fuel tanks acclimate to the extreme cold temperatures. Once nearly full, transfers decrease to a "top-off" speed before switching to an even slower flow, commensurate with fuel boil-off leading up to umbilical disconnect and launch.
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LOX slow-fill loading for the SLS core stage began this morning at 10:08 a.m. (1408 GMT) and reached fast-fill flow rates fifteen minutes later. In what's known as the "aft strut constraint," which refers to stress put on the connection points for the bottom of the SLS's solid rocket boosters, according to Nail, the core stage LOX tank cannot fill past 49% until the LH2 tank's depletion sensors are completely covered.
To accommodate for some of the morning's lost time, mission operators were able to speed up LH2 chill-down procedures for propellant and umbilical lines. This cooling also included vent valves for the ICPS, which experienced pressure spikes during the first Artemis 1 wet dress. Following that test, NASA technicians added these components to the chill-down procedures preceding LH2 fill for the core stage.
Operators were able to begin the LH2 slow-fill by 10:42 a.m. EDT (1442 GMT). At 11:30 a.m. EDT (1530 GMT), once depletion sensors in the bottom of the tank were submerged, LH2 transfer rates were switched to fast-flow, putting both first-stage fuel tanks on track to reach the targeted T-0 at 4:38 p.m. EDT (2038 GMT).
Though the core stage LH2 tank is the rocket’s largest — coming in at 537,000 gallons (2 million liters) — it was also the tank to be topped off first. The SLS core stage LH2 transfer began its boil-off replenishment flow at 12:42 p.m. (1642 GMT), allowing operators to begin LH2 cooling for the ICPS about 20 minutes later.
The first stage LOX tank holds 196,000 gallons (740,000 liters) — less than half the volume of the LH2 tank —but the vehicle’s liquid oxygen is far denser than its liquid hydrogen catalytic counterpart, and takes much longer to load.
The core stage LOX tank was topped off and began its boil-off replenishing flow at 1:30 p.m. EDT (1730 GMT). At the same time, operators began fast-fill loading for the upper stage LH2 tank.
However, Nail reported on the NASA broadcast that operators had encountered data indicating a hydrogen leak in the core stage tail service mast umbilical bleed line quick disconnect, a point where a line connecting the SLS and MLP are designed to separate during launch.
"The hydrogen bleed for the kickstart was halted when there was a leak detected at the quick disconnect for the core stage hydrogen," Nail said.
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The Artemis 1 team spent some time troubleshooting the issue. For example, technicians warmed up and then cooled down the quick disconnect, trying to reseat the seal. That didn't fix the leak, but the team ultimately decided to press ahead with the wet dress anyway.
The leak, and the attempts to fix it, ended up pushing the end of the wet dress back about three hours. And it also changed procedures for "terminal count," the final 10 minutes of the simulated countdown, which marked the end of the test. Terminal count ended at T-29 seconds, as noted above, and the team had to "mask" data associated with the leak so the launch computer couldn't see it and trigger a hold. But everything went through to its modified conclusion.
"During the terminal count, the teams performed several critical operations that must be accomplished for launch, including switching control from the ground launch sequencer to the automated launch sequencer controlled by the rocket’s flight software, an important step that the team wanted to accomplish," NASA officials wrote in an update tonight (opens in new tab).
The Artemis 1 team will now spend some time figuring out just how those operations went.
"Like any tests we run, we'll go take the data," Blackwell-Thompson said. "We'll go look at what we got. We'll go look at the couple of things that we didn't get. And we'll lay out a plan for how we go forward."
NASA has published a list of possible launch windows for Artemis 1 that extend through June 2023. Earlier versions of that schedule included a window as soon as next month, but NASA officials have repeatedly stressed the improbability of a July launch. An updated schedule was added to NASA's website on June 17, which removes July as an option.
The soonest that SLS can launch is likely Aug. 23, but that date is designated for a "short mission" on the schedule. NASA has categorized launch windows based on the alignment of various mission parameters influenced by the moon's orbital position. Windows are divided between long and short mission durations, which amount to 38 to 42 days and 26 to 28 days, respectively.
If checks in the VAB go smoothly, it's possible NASA will target the schedule's earliest long mission opportunity, which would kick off with a launch on Aug. 29. However, a long mission window lasting five consecutive days exists between Sept. 2 and Sept. 6, and may serve as a more favorable option. In any case, NASA officials are not likely to commit to a launch date prior to SLS inspections in the VAB, which won't begin until later this month.