SLS Mobile Launcher Rolls to Launch Pad 39B for Testing

The Mobile Launcher (ML), atop Crawler Transporter (CT)-2, moves up the incline to Pad 39B. Photo Credit: Jeff Seibert for

As the Space Launch System (SLS) core stage for Artemis-1 approaches the end of “Green Run” testing at the Stennis Space Center in Bay St. Louis, Miss., attention shifted to Florida earlier today as NASA drove its gigantic Mobile Launcher (ML) out to historic Pad 39B at the Kennedy Space Center (KSC) for a series of tests related to late countdown operations and a top-to-bottom “washdown” to remove foreign object debris.

Video Credit: AmericaSpace

Riding atop the 6.6-million-pound (2.9 million kg) Crawler Transporter (CT)-2, the vehicle passed smoothly through the immense doors of High Bay 3 at the Vehicle Assembly Building (VAB), brilliantly illuminated in the pre-dawn darkness by powerful xenon floodlights.

“The Mobile Launcher has officially begun its 4.2-mile journey to Launch Pad 39B,” tweeted NASA’s Exploration Ground Systems (EGS). “What a sight!”

Video: Jeff Seibert for

The purpose of the multi-hour rollout, which comes less than a year before the first SLS super-heavylift booster is due to launch the uncrewed Artemis-1 mission around the Moon, is primarily to perform a “timing demonstration” of activities scheduled to occur late in the launch countdown.

Engineers will practice laser alignments with targets on the 380-foot-tall (116-meter) ML, rehearse the removal of work platforms needed to access the SLS core stage engines and position side flame deflectors and extensible columns in a timely manner to achieve an on-schedule liftoff.

Photo Credit: Jeff Seibert/AmericaSpace

These tasks have been rehearsed individually, but the current operation enables the EGS team to simulate them in parallel. During its time at Pad 39B—a facility which last saw a crewed launch to the Moon in the form of Apollo 10, more than a half-century ago—technicians will also condut a comprehensive, top-to-bottom “washdown” of the ML, removing foreign object debris as an added safety measure.

In readiness for this morning’s rollout, the ML was secured to the crawler by means of four reinforced “pickup” points, known as crawler interface blocks. All told, it totals about 10.5 million pounds (4.7 million kg).

Photo Credit: Jeff Seibert/AmericaSpace

Its construction began more than a decade ago and was completed in August 2010, initially for the Ares V rocket, but following President Barack Obama’s cancelation of the Constellation Program it was repurposed for use with SLS. Although the ML had undertaken a short move outside the VAB in October 2010, its first 14-hour rollout to Pad 39B atop the crawler occurred in November 2011.

Contracts to extensively modify it for its new role in support of SLS were awarded to J.P. Donovan Construction, Inc., of Rockledge, Fla., in May 2013. Under the terms of the 18-month contract, the ML exhaust “hole” was expanded from 22 x 22 feet (6.7 x 6.7 meters) to 34 x 64 feet (10.4 x 19.5 meters) to accommodate the requirements of the four shuttle-era RS-25 engines and the pair of five-segment Solid Rocket Boosters (SRBs). This required the demolition of 1.5 million pounds (680,000 kg) of old steel and the installation of more than 2 million pounds (900,000 kg) of new steel.

Photo Credit: Jeff Seibert/AmericaSpace

Following this early demolition work of the original exhaust hole, the first hardware arrived at KSC early the following spring. In August 2015, J.P. Donovan and its team of subcontractors set to work constructing the 355-foot-tall (108-meter) ML tower and its two-story, 25-foot-high (7.6-meter) base structure, as well as installing 800 mechanical, fluid and electrical panels and an estimated 57 miles (91 km) of cables, tubing and pipework.

The first of several SLS-specific launch umbilicals—the Orion Service Module Umbilical (OSMU)—was installed in March 2017 and in February of the following year the Crew Access Arm (CAA) was set in place at the ML’s 274-foot (83-meter) level. Most recently, in June 2019 the ML took a trip out to Pad 39B for tests of the sound suppression water system, the cryogenic propellant flow systems and electrical and umbilical utilities.



FOLLOW AmericaSpace on Facebook and Twitter!




  1. i saw the launcher on 39 and it is huge but it sat outside the vab for along time then went inside the vab where it was set on the mobile hauler and with the launcher and the launch pad,they do look great it is almost as tall as the lightening aresters around the pad 39 b it’s great to see the movement of nasa towards space travel proud to be an american

Not For Geeks? Remembering STS-73’s Mission Impossible, 25 Years On

Success! NASA’s OSIRIS-REx ‘Touches’ Asteroid Bennu, Gathers Samples for Return to Earth in 2023