SLS Mobile Launcher set for a test rollout to 39B in August

by Chris Bergin

Crawler Transporter -2 (CT-2) is tracking an August road trip down the Kennedy Space Center (KSC) Crawlerway carrying the modified Mobile Launcher (ML) to Pad 39B. Testing towards this milestone has passed several milestones, including a lift and weigh test of the huge structure earlier this month.

The upcoming rollout test will lay the foundations for validating both the CT and ML are ready to support the first rollout of the entire Space Launch System (SLS) stack that will launch on the rocket’s maiden flight, Exploration Mission -1 (EM-1).

The CT and ML have rolled out together before back in 2011. However, this was in their Constellation Program (CxP) configuration, before the ML was heavily modified from hosting Ares I rockets to SLS.

The 2011 rollout of the Ares I ML – via NASA

Following the cancellation of the Constellation Program, the ML – after a brief period of unemployment – was reassigned to hosting SLS Block 1 launches, albeit via a large scale modification effort that has been several years in the making.

With new umbilicals and a completely rebuilt launch mount, the SLS ML now sports an array of new arms that will link the huge structure to the rocket. However, the process of converting the ML has not been as smooth as hoped.

NASA confirmed the ML is leaning, portrayed as “some deflection”, but is classed as an understood condition. The lean will not impact on the ability to transport the ML up and down the Crawlerway, but it will provide some additional considerations during the mating process with SLS.

One such example is how a slight “deflection” will play on critical connections such as how the Ground Side Umbilical Plates on the Umbilical Arms mate up with the Flight Side Umbilical Plates on the rocket.

One of the numerous umbilical plates that will mate with SLS from the ML

An example of how sensitive such hardware is to a lack of a perfect seal was seen during the later Shuttle missions, such as STS-133, with leaks from the Ground Umbilical Carrier Plate (GUCP) causing several scrubs.

Notably, the SLS program has paid close attention to the mitigation of such concerns with the new rocket.

Checks on the connections between the ML and SLS will be carried out both inside the Vehicle Assembly Building (VAB), where the mating process will take place, followed by more checks once the stack has arrived at the pad.

Rollout to the pad will gain another test in August when the SLS ML takes its first trip to Pad 39B in August.

A milestone towards that roll took place at the start of this month when Crawler Transporter -2 (CT-2) was driven under the ML for a test lift.

The CT – which itself has undergone extensive modifications to bring it up to standard for its next roll with SLS rollouts – was driven to the East Refurb Site near the VAB, which is where the ML currently resides.

The CT team the carefully moved under the ML for a lift that also allowed for the structure to be weighed now it has “put on weight” since the installation of the umbilical arms. This process was carried out three times in total, with the CT performing the tasks without issue.

CT-2 under the SLS ML – via NASA

CT-2 was then driven back to its home in the Crawler Transporter Yard, with the testing – which covered practice lifting procedures, the validation of interface locations, confirming the weight and developing a baseline for modal analysis – deemed to be a success.

Reviews will now take place over the coming weeks to confirm the rollout date for CT-2 and ML, which is currently listed as August 16.

Once at the pad, similar test objectives to those conducted in 2011 with the Ares I ML rollout are likely to be mirrored. These tests included the structural response of the ML during rollout, structural clearance, HVAC (Air Conditioning) pressurization, and Tower Fire Suppression testing – all while ML is powered up via Pad 39B’s power supplies.

Pad 39B itself has also undergone a large number of modifications since the 2011 rollout test, with the latest including the installation of a brand new Flame Deflector.

Measuring approximately 57 feet wide, 43 feet high and 70 feet long, the deflector’s north side is slanted at about a 58-degree angle and will divert the rocket’s exhaust, pressure and intense heat to the north at liftoff.

Two side deflectors soon will be installed. They will help to contain and protect the vehicle and surrounding pad structures from the solid rocket boosters during liftoff.

This new deflector is rated to cope with SLS as it evolves into more powerful variants, which will see SLS move from the Block 1 vehicle to the Block 1B rocket in the mid-2020s.

As previously noted, the current SLS ML is unable to host the taller Block 1B without around two years of additional modifications. Those changes were set to take place in the gap between EM-1 and EM-2. However, NASA has since opted to build a second ML specific to the Block 1B SLS.

Side-by-side comparison from 2016 of the Block 1 vehicle and ML umbilical tower configuration (left) and the Block 1B configuration. Prior to funding ML-2, the single Mobile Launcher was going to be partially taken apart and rebuilt to go from the left configuration to the right. Subsequent design iterations for the Block 1B tower have changed the EUS umbilicals and added a Vertical Stabilizer Damper above. Credit: NASA

Just this month NASA confirmed that plan by publishing a solicitation for ML2.

“NASA’s purpose for ML2 at KSC is to serve as the launch platform for the Space Launch System (SLS) “Block 1B” (B1B) vehicle, both crewed Orion and future uncrewed payload configurations,” noted the document.

“NASA’s intent is to incorporate flexibility and expandability (modularity) into the ML2 design, necessary to accommodate future modifications for the “Block 2” (B2B) vehicle crewed and uncrewed payload configurations.

“NASA’s concept for ML2 is similar in nature to and concept of operations as Mobile Launcher 1 (ML1), currently in the build and test phase.  ML2 is envisioned to be a two-story base structure with similar dimensions to ML1 base with an approximate 400-foot service tower.”

The document adds that it intends to award a single cost reimbursement contract with the anticipated period of performance is not to exceed 44 total months including design and construction.

NASA is looking to add more SLS Block 1 missions to keep ML1 occupied ahead of Block 1B’s debut, which will also mark the need for another contract award to modify ML1 to cater for the larger rocket.

In the second half of the 2020s, NASA will have two Block 1B configured Mobile Launchers. However, there is no sign of the flight rate exceeding one mission per year. The requirement to enter another round of modifications on the MLs for the Block 2 SLS is not expected until the 2030s, based on the timeline for that rocket to enter service.

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