For more than 45 years, two crawler-transporters (CTs) have carried America's human spaceflight program on their backs. At NASA's Kennedy Space Center in Florida, one of the CTs has been undergoing a major overhaul to keep the workhorse going for many years to come. With the first phase of modification work complete, the crawler-transporter recently went for a successful test drive that proved all upgrades are working as designed.

The behemoth crawler-transporters are designed to lift the mobile launchers with launch vehicles mounted atop, then move the entire integrated stack from the Vehicle Assembly Building (VAB), where launch vehicles have been assembled since the Apollo era, to the ocean-side launch pads.

Weighing six-and-a-half million pounds and larger in size than a professional baseball infield, the crawler-transporters creep along at a maximum speed of one mile per hour when carrying a mobile launcher with a launch vehicle atop. Unloaded, the crawlers can manage about two miles per hour.

Last December, engineers began modifying crawler-transporter 2 to ensure its ability to carry launch vehicles currently in development to launch pad, such as NASA's Space Launch System (SLS) heavy-lift rocket -- the agency's heaviest launch vehicle yet.

The major efforts focused on replacing the original sets of AC (alternating current) electrical generators, upgrading the brakes, and preparing crawler-transporter 2 for lifting the additional weight of the SLS rockets. The heaviest SLS rollout weight, including the mobile launcher, is estimated to be about 14.2 million pounds. That compares to the Apollo Saturn V and space shuttle rollout weight of approximately 12.3 million pounds.

With the first phase of modifications complete, crawler-transporter 2 began a test drive that took place over several days. At Launch Pad 39A, the crawler picked up a shuttle-era mobile launcher platform and moved up and down the ramp to the pad to test how well the systems worked while carrying a load.

The next round of work will include replacing the jacking and leveling cylinders and the roller bearing assemblies to accommodate the additional weight of the Space Launch System rocket. Existing roller bearings will be replaced with a redesigned assembly that can carry the greater load. The redesign will also include an improved lubrication system and temperature monitoring to provide a longer operational life.

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