The launch team, made up of people from NASA’s Exploration Ground Systems (EGS) program and prime launch processing contractor Jacobs, is preparing to conduct the final test prior to the launch of the Artemis 1 mission to the Moon. Following the rollout of the first integrated Orion spacecraft and Space Launch System (SLS) rocket to Launch Pad 39B at the Kennedy Space Center in Florida overnight on March 17 into March 18, the Artemis 1 vehicle and the Mobile Launcher are being connected to pad services and powered up for testing.
The launch control team is conducting the tests out of firing rooms in the Launch Control Center of Launch Complex 39, while ground support crews are overseeing hands-on work at the pad. The vehicle was rolled to the pad for a last big test; a full countdown demonstration called the Wet Dress Rehearsal (WDR). Currently scheduled for April 1 through April 3, the test will culminate with propellant loading and a near-complete countdown.
Getting set up at the pad for testing
The Artemis 1 Orion and SLS vehicle were rolled out of High Bay 3 of the Vehicle Assembly Building (VAB) at KSC on the evening of March 17 after almost a year of integration on Mobile Launcher-1 (ML-1). Stacking of the SLS Solid Rocket Boosters (SRBs) began in November 2020, when it was expected that the Core Stage Green Run would be completed early the following year.
Stacking of the remaining vehicle elements and first-time testing and checkouts stretched through the rest of 2021 and into March 2022 before final closeouts were completed ahead of the first rollout. After Crawler Transporter-2 raised the Mobile Launcher (ML) off the pedestals in the VAB, the stack began rolling at 5:47 pm Eastern time on March 17.
The rollout was completed when the crawler lowered the ML down onto Pad 39B’s pedestals, with “hard-down” occurring at 4:17 am Eastern the next morning, March 18. Once at the pad, the launch team began getting the vehicle set up for a few engineering tests – which will later be followed by the major test, the Wet Dress Rehearsal (WDR).
The series of firsts will continue throughout the Artemis 1 campaign; following the first-ever rollout, the vehicle is now being taken through its first-ever pad flow.
NASA is using this first pad flow with a flight vehicle as not only a dress rehearsal for the launch countdown timeline but also to perform all pre-launch tasks once at the launch pad. “The whole reason we’re going to the pad is to go through the timelines necessary to get to launch,” Jeremy Graeber, Assistant Launch Director for Artemis 1, said in a March 14 interview with NASASpaceflight.
(Photo Caption: The Artemis 1 flight vehicle and Mobile Launcher-1 are carried by Crawler Transporter-2 from the VAB to Launch Pad 39B at dusk on March 17. The operation took about 10.5 hours to complete and set the stage for the final pre-launch testing at the pad.)
“We have done a massive amount of testing on each of these pieces of flight hardware from the design side all the way up to everything we’ve done in the Vehicle Assembly Building. But there are parts of testing that we have not been able to do yet, and that’s why we’re going to the launch pad and to go through this entire flow, Wet Dress pad flow, to perform those tests that we have not yet had a chance to do yet and to be in this configuration.”
Because this is the first time the launch team is running through the on-pad preparations for a launch countdown, NASA decided to add extra time to the schedule for final countdown preparations. Additional Program-Specific Engineering Tests (PSETs) for this first integrated Orion-SLS vehicle were also planned in the first few days after arrival at the pad.
“Our timeline at the pad does allow for the opportunity to get through all of that testing, really to establish what our timelines are,” Graeber explained. “[We want] to ensure that we understand — since it’s the first time that we’re going through it — that we capture any lessons learned [and] any things that we could optimize or make more efficient because when we do come back for a launch, we want to have all of our procedures well-oiled and efficient so that we can be at the pad for as short a period of time as possible prior to launch.”
“So there is some additional time built in to allow for us to really understand that and ultimately so that we hit our mark for the Wet Dress dates that we targeted,” he added. “We want to make sure there’s a little bit of ability to maneuver in that timeframe so that we can get started with Wet Dress on time as planned.”
About two weeks of time were planned at the pad to complete the PSETs, perform countdown preps, and start the WDR countdown on April 1 with T-0 planned for April 3. “At this point, we are scheduled on the range for Wet Dress proper for April 1 through the 3rd, with T-0 on the 3rd,” Graeber said. “So we’re on the range and I fully anticipate that we’re going to stick to that date.”
“As with everything [for] a first-time flow we’re going to be looking at things on a shift-by-shift, hour-by-hour basis for any adjustments that we might need to make [to the schedule]. I wouldn’t say that there’s an intention to move any of those dates whatsoever; that’s our plan, but we’re always going to be evaluating our progress, and if there’s a path that makes sense, then we’ll be exploring that.”
(Photo Caption: Artemis 1 Assistant Launch Director Jeremy Graeber (left) and Launch Director Charlie Blackwell-Thompson (right) watch the Artemis 1 vehicle roll by the large windows in Firing Room 1 of the Launch Control Center on March 17. The launch team will be conducting a final series of tests at the pad, culminating in the Wet Dress Rehearsal countdown demonstration test planned in early April.)
In contrast to the Space Shuttle that it is derived from, SLS is designed for — eventually — a relatively short stay at the pad, with launch day anticipated less than a week after arrival from the VAB. In a nominal timeline, following rollout, there are about two days of validating pad connections, followed by two days of SRB hydraulic system servicing and final Orion ordnance installations, and then a two-day launch countdown.
“When you think about previous programs, there was a lot of work that was moved and performed at the launch pad because you had access and you had other things,” Graeber, a Space Shuttle launch team veteran, said. “For the Artemis missions, when we roll out we are truly ready to fly except for getting connected at the pad, getting all of our validations of that connection verified — basically getting the vehicle ready at the pad.”
“It’s all very critical, but it’s a very minimal set of work that’s on our plate once we get out to the launch pad and it really is to ensure we’ve got a very good set up and connections, get the vehicle configured, get the boosters serviced, and then we’re into launch [countdown]. So that is the key set of work that we’re going to go demonstrate through this WDR pad flow and it’s the same exact work that we will implement on every rollout and launch campaign for the Artemis missions.”
Following the arrival of the vehicle at the pad on March 18, the teams were scheduled to connect and validate all the pad services to the Mobile Launcher over the first four days. Completion of that work would lead into two days of PSETs that would begin on March 22.
The SLS SRB nozzles are gimbaled hydraulically, a carry-over of the same hardware used during the Shuttle program. Each booster has two hydraulic systems for redundancy, each driven by a hydraulic power unit (HPU) that is run by hydrazine fuel.
The highly toxic commodity is loaded onto each of the four HPUs through servicing ports on the outside of the SRB aft skirts, adjacent to the hydraulic system equipment that lines the inside of the skirts next to the booster nozzle. Just as during Shuttle launch preparations, workers in Self-Contained Atmospheric Protective Ensemble (SCAPE) suits will load the hydrazine from portable carts pre-staged on the zero-level “deck” of the Mobile Launcher’s platform.
(Photo Caption: Floodlights point the way to Launch Pad 39B (far left) as an International Space Station (ISS) overhead pass creates a streak in the sky above the pad during the Artemis 1 rollout on March 17.)
For the WDR pad flow, five days were allocated to complete hydrazine servicing of the Boosters; this accounts for both the first-time nature of the operation and the risk of bad weather slowing down or suspending the work. Following the SRB hydrazine load, two days of countdown preps are scheduled ahead of the call to stations for the WDR countdown to begin on April 1.
WDR will follow the launch countdown timeline
The Wet Dress Rehearsal is a full launch countdown demonstration test. It will prove that the hardware, software, and personnel can execute a launch countdown and have the vehicle ready for engine start – before stopping just short of ignition.
“Wet Dress is as close to launch as it possibly can be without a launch,” Graeber said. “We are demonstrating every bit of what it takes to do launch except for some very specific aspects that we won’t do because we’re not launching.”
The WDR countdown will follow the same two-day-long timeline of the launch countdown. The countdown includes two built-in holds, totaling two hours. “We’ll start the [countdown] clock at T-minus 43 hours and 40 minutes,” Graeber said.
“We have a one hour and thirty-minute hold at six hours and forty minutes, and then we have the thirty-minute hold at T-minus 10 minutes.” Both holds will occur at key decision points in the countdown.
In the first hold, the Mission Management Team (MMT) will decide whether or not to proceed into propellant loading. In the second hold, the MMT will decide whether or not to launch the vehicle by starting the ten-minute terminal countdown.
(Photo Caption: Some of the service connection points can be seen on the left side of the base of the Mobile Launcher during the rollout of the Artemis 1 vehicle on March 17. Services such as electrical, data, and environmental control are provided to the ML and the flight vehicle through some of these connection points when they are staying in the VAB or at Launch Pad 39B.)
“Both of our holds are working holds; there is planned work during that timeframe,” Graeber explained. “But what they are intended for is to ensure that at those two major milestones, the first one being the decision from our Mission Management Team on concurrence to proceed with cryo loading, that we have the opportunity to assess and evaluate our progress through the launch countdown.”
“That is a factor going into that MMT decision and understanding where we’re at, but we have work planned and scheduled throughout that entire hold, as well as the thirty-minute hold at 10 minutes.”
“We are still working and configuring different systems and capabilities during that hold [at T-10 minutes] as we are proceeding towards our readiness for our final polls and the opportunity to resume from 10 minutes, but they are absolutely intended to allow for that opportunity to be in a known configuration and be able to allow Charlie [Blackwell-Thompson], our launch director, to make decisions about our path forward,” he added. “So we believe we’ve got plenty of time accounted for within the timeline to meet all of our objectives and the vehicle and our ground systems configured for launch based on the timeline.”
During a recent media briefing, Blackwell-Thompson said that propellant loading for WDR was expected to begin somewhere around 7 am Eastern time on the day of the planned T-0.
The majority of the thrust at liftoff and during the first two minutes of flight is provided by the SLS Solid Rocket Boosters, which are already loaded with their solid-fuel propellant. However, the SLS launch vehicle has two stages that will be loaded with liquid oxygen (LOX) and liquid hydrogen (LH2) propellant on launch day, the Core Stage and the Interim Cryogenic Propulsion Stage (ICPS).
The Core Stage ignites just before liftoff and burns for over eight minutes until the vehicle is nearly in orbit. The ICPS will perform a short burn to insert itself and Orion into Earth orbit and later reignite to send Orion to the Moon.
The cryogenic fuel and oxidizer will be loaded on the two stages in a particular order. “There is a specific sequence that we do load them and there are a lot of factors that go into that,” Graeber said. However, NASA won’t disclose information like the factors themselves because it is considered to be proprietary and secret under export control.
(Photo Caption: Crawler Transporter-2 continues its roll down the crawlerway from the VAB to Launch Pad 39B during rollout on March 17. A Dynamic Rollout Test was conducted through the entire roll from the VAB to the pad as a part of the testing and checkout activities for this first integrated Orion/SLS flight vehicle. The same test instrumentation will be used to capture data when the vehicle is rolled back to the VAB after the WDR.)
“What I can tell you is that we focus on the Core Stage first, we get Core Stage liquid oxygen going first because it takes longer, and then Core Stage liquid hydrogen comes next,” Graeber said. “There’s a lot of variability and we’re going to learn a ton at Wet Dress, but the next thing would be the ICPS liquid hydrogen and then finally the ICPS liquid oxygen; that’s the typical planned order. There is again, there’s variability and opportunities to adjust throughout the entire loading sequence as necessary, but basically that’s our loading sequence.”
Although Graeber isn’t allowed to go into the details, he was able to say that typically they would want to finish loading LOX or LH2 on the Core Stage before loading that commodity on ICPS. “Typically we’re going from one to the next for each commodity, so we’re going to go Core Stage liquid oxygen, get into replenish and then get into ICPS liquid oxygen,” he said.
“Same thing for Core Stage liquid hydrogen to ICPS; you want to get complete with one before you step into the next.”
During Shuttle countdowns, after the External Tank was loaded and into stable replenishment, a final inspection team would go to the pad and do a survey of all the external surfaces of the Shuttle vehicle. For SLS and Orion, that survey will be performed remotely, taking advantage of high-definition camera equipment.
“We do not have an inspection team that goes out to the launch pad,” Graeber said. “One of the things that was unique about Shuttle was that there was a lot of vantage points to be able to see the vehicle and so that final inspection team had a lot of places they could inspect the vehicle.”
“We don’t have as many of those, and so really our inspection is done by folks who actually have experience back in the Shuttle days as part of the final inspection team but they do it remotely via the multiple camera views that we have in place to do a full scan of the entire vehicle surface area outer mold-line.”
“[They] provide that same inspection and inspection criteria as they did with the final inspection team for Shuttle, but it’s done remotely and they support here from the Launch Control Center,” he added.
Two terminal countdown runs, with a recycle in between
For WDR, the launch team and launch systems will get the vehicle ready to fly twice, demonstrating the capability to recycle the countdown and vehicle in-between the two. SLS will have launch windows up to two hours long on a given day and under some circumstances it will be possible to make another launch attempt on the same day.
Two terminal countdown runs are planned, beginning at T-10 minutes and counting. The Ground Launch Sequencer automatically conducts the final ten minutes of the countdown, preparing the vehicle and ground systems for ignition, liftoff, and beyond.
During the first run, the launch team will perform a test objective to hold for up to three minutes with under 10 minutes remaining on the countdown clock. The objective of the test is for a cumulative hold time of three minutes, which could occur as late as T-90 seconds.
“At the 1 minute, 30 second hold [point] we will have held somewhere along the way for that total of three minutes to meet that objective,” Graeber said. “All of that hold time might not occur right at [T-minus] 1 minute, 30 seconds, but by then we will have held for three minutes in that timeframe.”
The test objective will demonstrate the capability of SLS to hold for up to three minutes after resuming the clock at T-10 minutes and still be able to resume the countdown and launch without the need to perform a recycle. T-90 seconds is the last hold point in the countdown where that is possible; the GLS milestone at T-90 seconds is the power transfer of the Core Stage from the ground to internal power provided by four onboard batteries.
For WDR, the first terminal countdown run will continue down and then hold at T-33 seconds, the point at which control of the vehicle for the launch is handed over from GLS to the SLS flight computers in the Core Stage. That won’t happen until the second run; for the first run, the launch team will perform a recycle.
“We’ll stop at 33 just for a very short period of time and then we will give cutoff and get into our recycle,” Graeber said. “So we’ll give cutoff, we’ll do our critical safing, our non-critical safing, and then all of our recycle work to get back to the T-minus ten minute configuration to be able to demonstrate that whole turnaround, which we’ve demonstrated multiple times in emulation environments and in other opportunities.”
“This will be the first time to do it at the pad with the fully-loaded vehicle. And then once we’re back at ten, we’ll get ourselves configured and prepped and ready to be able to countdown again for the second opportunity.”
The amount of time it takes to be ready to pick up the clock again at T-10 minutes in a recycle on launch day will usually depend on how long it takes to resolve the issue that forced the previous terminal countdown to be aborted. If the problem is a technical one, the Integration Console will lead that discussion and the resolution of the issue will depend on the problem.
(Photo Caption: A close-up of the aft end of the Artemis 1 SLS on the pad on March 18. The aft end of the right-hand SLS Booster and the Core Stage engine section/boattail can be seen in this image. To the left, the two Tail Service Masts and their umbilical connections to plates on the engine section are where cryogenic liquid hydrogen and liquid oxygen will enter the vehicle during tanking operations for both the upcoming Wet Dress Rehearsal test and launch.)
If the problem was a transient weather or range safety violation at the wrong time that required a recycle, then resetting and reconfiguring systems may be what consumes the time necessary to be ready to resume again. For the WDR, the launch team will intentionally cutoff and recycle during the first terminal countdown run. Graeber said that the ballpark estimate just for recycling systems for a second attempt is about an hour.
“All of this is situational, and every instance of this could play a different role or be more primary than others, but in general a recycle takes about an hour and it could be a little less or a little more depending on when it happens [and] the reason why it happened,” he said. “Ultimately for us getting back to ten will take less than that hour but there are some things that we have to configure or reconfigure while we’re holding at ten before we’re ready to pick up from ten again for the second attempt.”
One example is the Core Stage batteries. The batteries have to be able to power all the stage systems for both nominal duration launches and contingency cases where the stage has to operate for a few minutes longer than normal. To support off-nominal cases, the batteries need to be at full charge when the Core Stage goes to internal power at T-90 seconds; in some cutoff and recycle scenarios, some of that battery life has been discharged.
During a recycle, the batteries would have to be recharged to full capacity before they would be ready for a second launch attempt. While the countdown clock can be reset fairly quickly back to T-10 minutes and holding, there are other vehicle and ground systems that will also need some time before they are ready again.
“It ranges across aspects of Orion and ICPS and the Core Stage and the Boosters and the ground systems,” Graeber said. “We anticipate that we can [complete] recycle in a two-hour launch window, just based on the likelihood of it happening right at the beginning [of the window] and us being able to do a full recycle. “[As far as more than one recycle, ] I’ll never say never, but I would expect that we can do one recycle.”
On the second terminal countdown run during the WDR, the plan is to continue counting through the T-33 seconds milestone. The GLS will hand over vehicle oversight and control to SLS, and at T-30 seconds, the vehicle flight computers will begin the Autonomous Launch Sequence (ALS).
(Photo Caption: Artemis 1 at the pad on the afternoon of March 18. The Crew Access Arm was retracted and locked during rollout to reduce structural loading on the swing arm; following hard-down at the pad, the arm was extended back in place against the Orion Launch Abort System and Crew Module hatches.)
At that point, ALS has command and control of the vehicle and GLS has command and control of ground systems. ALS will complete the final configurations of the vehicle for launch, such as starting the SRB HPUs and conducting a steering check. On launch day, GLS would begin the ignition and liftoff water release sequence, fire the hydrogen burn-off igniters (HBOI), and give ALS a go for main engine start; for WDR, the water and hydrogen burn-off systems will not be fired, since they have already been tested and are not needed for the test.
The countdown cutoff for the second attempt is planned at T-9.34 seconds. In a real launch attempt, a cutoff that late in the countdown would result in an automatic scrub. At that point in the WDR, the launch team will move into scrub turnaround procedures.
Following the WDR, the team will prepare the vehicle for roll back to the VAB to get ready for the Artemis 1 launch.
Lead image credit: Stephen Marr for NSF.