The SLS returned to the VAB for final launch preparations

After a successful wet dress rehearsal (WDR), NASA Exploration Ground Systems (EGS) returned the Space Launch System (SLS) rocket to the Vehicle Assembly Building (VAB), which should be the last time it is done before launch. The Artemis 1 vehicle will now undergo final readiness checks and launch preparations before its debut flight, scheduled for later this year.

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

The recent test revealed a leak in the hydrogen vent line connection that needs to be repaired in the VAB, as well as final rocket checks and testing and activation of the flight termination system. To correct the hydrogen leak, NASA will replace the rear service mast umbilical quick disconnect (TSMU) seal.

Recent testing

June 20 WDR countdown demo test performed a full fill of the cryogenic tanks and conducted the 10-minute final countdown sequence to cutoff at T-29 seconds. Due to a hydrogen leak in one of the Core Stage’s liquid hydrogen TSMU fast shutdowns, pre-launch 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 the countdown and start sequence can continue to the ignition point, and as the engines would not be ready to start without the thermal conditioning, a mask was applied so that the ground computers do not interrupt the terminal countdown. With the engine ready masked, the ground and flight computers were able to reach the T-29’s second point in the sequence.

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

Along with declaring the WDR test complete, NASA also decided to conduct a separate test of the Thrust Vector Control (TVC) system on the two SLS solid rocket boosters (SRB). In this integrated test, the ground and flight computers started the two hydraulic power units (HPUs) in each SRB, and the hydrazine-powered system successfully moved the two giant nozzles into the bottom of the boosters. After this test was completed, the system was successfully shut down.

The SRB hydraulic TVC test could not be completed in the WDR because the countdown stopped shortly before the point at which the booster hydraulics were activated for the T-28 seconds launch. After the special SRB HPU test was conducted on the night of 24 June into 25 June, the hydrazine was vented to ensure safe transport back to the VAB.

This was the fourth WDR test overall for the missile. The initial attempt on April 3 was cleared after a group of supply fans stopped working in the pad area. This led to a second attempt on April 4. This attempt was first postponed again after the nitrogen gas purge stream had some problems and was later cleared because one part of the Mobile Launcher was not in the desired configuration to perform hydrogen loading on the rocket.

LC-39B’s SLS in preparation for the first WDR trials in April. (Credit: Nathan Barker for NSF)

The third attempt was made on April 14, no ICPS load. During this attempt, the GN2 power line again showed some problems, further delaying the attempt. However, this test was able to load LOX and LH2 onto the Core Stage, showing some progress over previous WDR attempts. Unfortunately, the concentration of hydrogen gas around the umbilical exceeded the hard limit of 4%, which caused an interruption of the charging progress and thus the purification of the WDR attempt.

Going back

SLS made its first move at 4:12 PM EDT (08:12 UTC) today and was brought 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 into the VAB after traveling seven kilometers from Pad 39B. This was slightly faster than the expected travel time of 11 hours from first move to hard down.

The SLS stopped to extend the crew access arm before completing the journey to the VAB. (Credit: Thomas Burghardt for NSF)

The return was originally planned for the night of June 30, but a problem with the condition of the crawlway postponed the plans. “The incline must be precisely level with an even distribution of the rocks that make up the crawlway to support the load of the mobile launch vehicle and the rocket it will carry,” a This is according to a post on the NASA blog.

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

Work at the VAB also includes final testing and verification of the flight completion system. The complex clock for this system will determine the usable startup capabilities in every possible window in the future, as it can only be checked and read in the VAB and valid for a period of 20 days only. Along with the complex restocking of liquid hydrogen that would be required for multiple attempts, this is one of the biggest watch points for the launch campaign.

NASA will also perform post-WDR vehicle inspections, begin late recovery of Orion’s payloads for the debut flight, replace the ICPS avionics box, and perform a software refresh to ensure it is up-to-date, and installed the flight batteries before the final and final pad deployment, which would then lead to the first launch attempts of the Artemis 1 mission.

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

NASA has yet to set an official launch target, other than stating the late August/early September launch window they’d like to launch in, and that’s understandable given that so many elements remain in the checklist. Once the hydrogen leak troubleshooting and repairs are clearer and the rest of the VAB’s operational schedule is better defined, the agency should be ready to announce a launch date and chosen mission duration, as it depends heavily on of the selected launch period and launch day.

(Main photo: SLS arrives at VAB for final launch preparations. Credit: Stephen Marr for NSF)

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