SLS rolled back to VAB for final launch preparations

by Adrian Beil

After a successful Wet Dress Rehearsal (WDR), NASA Exploration Ground Systems (EGS) rolled back the Space Launch System (SLS) rocket to the Vehicle Assembly Building (VAB), which should be the last time this is performed before launch. The Artemis 1 vehicle will now undergo final readiness checks and launch preparations ahead of its debut flight planned for later this year.

Currently, NASA is targeting the launch window between August 23 and September 6 for the launch of Artemis 1, but there is still work to be done before that first flight, which could impact the schedule going forward.

The recent test revealed a leak in the hydrogen bleed line connection that needs to be fixed in the VAB, as well as the final checkouts of the rocket and the testing and arming of the Flight Termination System. To fix the hydrogen leak, NASA will replace a seal on the quick disconnect of the tail service mast umbilical (TSMU).

Recent Testing

The June 20 WDR countdown demonstration test performed a complete fill of the cryogenic tanks and conducted the 10-minute-long terminal countdown sequence until a cutoff at T-29 seconds.  Due to the hydrogen leak at one of the quick disconnects on the Core Stage liquid hydrogen TSMU, the pre-start thermal conditioning of the stage’s four RS-25 engines could not be completed.

The readiness of the engines to start is critical to whether or not the countdown and launch sequence can continue to the point of ignition, and since the engines would not be in a ready-to-start condition without the thermal conditioning, a mask was applied so that the ground computers would not abort the terminal countdown.  With the engine start readiness masked, the ground and flight computers managed to get all the way down to the T-29 second point in the sequence.

At T-29 seconds, the SLS flight computers had just taken control of the launch sequence; without the mask for the ground computers, they immediately saw that RS-25 engines were not ready to start, issued a hold, and the countdown was aborted at that time.  After reviewing data from the test, NASA declared that the WDR was complete and gave the green light for the final rollback in the campaign.

Along with declaring the WDR test complete, NASA also decided to conduct a separate thrust vector control (TVC) system test of the two SLS solid rocket boosters (SRB). In this integrated test, the ground and flight computers started up the two hydraulic power units (HPU) in each SRB, and the hydrazine-powered system moved the two giant nozzles at the bottom of the boosters successfully. After this test was concluded, the system was successfully shut down.

The SRB hydraulic TVC test could not be completed in the WDR, as the countdown stopped just short of the point where the booster hydraulics are started for launch at T-28 seconds.  After the special SRB HPU test was conducted overnight from June 24 into June 25, the hydrazine was drained to ensure a safe transport back to the VAB.

This was the fourth WDR test run overall for the rocket. The initial attempt on April 3 was scrubbed after a group of supply fans stopped working in the pad area. This resulted in a second attempt on April 4. This attempt was first delayed again after the gaseous nitrogen purge flow had some issues, and was later scrubbed as one part of the Mobile Launcher was not in the desired configuration to perform hydrogen loading on the rocket.

SLS at LC-39B during preparations for the first WDR attempts in April. (Credit: Nathan Barker for NSF)

The third attempt was performed on April 14, without any load on the ICPS. During this attempt, the GN2 supply line showed some issues again, which delayed the attempt further. This test, however, was able to load LOX and LH2 on the Core Stage, showing some progress compared to previous WDR attempts. Unfortunately, the concentration of hydrogen gas around the umbilical lines spiked over the hard limit of 4%, which triggered an abort in the loading progress, and with that scrubbing the WDR attempt.

The Rollback

SLS had its first motion at 4:12 AM EDT (08:12 UTC) today and was rolled back together with Mobile Launcher 1. The move was performed by Crawler Transporter-2 (CT-2) and was completed at approximately 2:30 PM EDT (18:30 UTC) where the rocket was placed back in the VAB after it had traveled seven km from Pad 39B. This was slightly faster than the expected travel time of 11 hours from first motion to hard down.

SLS stopped for Crew Access Arm extension before finishing the journey into the VAB. (Credit: Thomas Burghardt for NSF)

The rollback was originally planned for the night of June 30, but an issue with the condition of the crawlerway postponed plans.  “The inclined pathway must be precisely level with an even distribution of the rocks that make up the crawlerway in order to support the load of the mobile launcher and rocket that it will carry,” a NASA blog post said.

After returning to the VAB, SLS has another six to eight weeks of final launch preparations ahead of the rollout for the debut mission. This still makes the planned launch window possible, although the margins are slim.

Work in the VAB includes the final testing and checkout of the Flight Termination System as well. The complicated clock for this system will determine the usable launch opportunities in every possible window going forward, as it can only be checked out and readied in the VAB and is only valid for a period of 20 days.  Together with the complicated restocking of liquid hydrogen that would be needed for multiple attempts, this is one of the biggest watch points for the launch campaign.

NASA will also perform post-WDR vehicle inspections, start the late stow on the Orion payloads for the debut flight, replace an ICPS avionics box and perform a software refresh to make sure it is up to date, and install the flight batteries ahead of the last and final rollout to the pad, which then would result in the first launch attempts for the Artemis 1 mission.

For the Artemis 1 mission, NASA partnered with the German Aerospace Center and the Israel Space Agency for the primary payload. The plan is to conduct a mission called Matroshka AstroRad Radiation Experiment (MARE), which will conduct a variety of radiation-related observations during the Artemis 1 mission. Furthermore, the mission will have several CubeSats loaded in the Orion Stage Adapter on top of the second stage.

NASA has not yet set an official launch target besides indicating the late August/early September launch window they would like to launch in, and that is understandable, given so many items remain on the checklist. Once troubleshooting and repairs of the hydrogen leak are clearer and the rest of the VAB work schedule is better defined, the agency should be ready to announce a launch date and the selected mission duration, as that is highly dependent on the selected launch period and launch day.

(Lead photo: SLS arriving at the VAB for final launch preparations. Credit: Stephen Marr for NSF)

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