The crawler-transporters, formally known as the Missile Crawler Transporter Facilities, are a pair of tracked vehicles used to transport spacecraft from NASA's Vehicle Assembly Building (VAB) along the Crawlerway to Launch Complex 39. They were originally used to transport the Saturn IB and Saturn V rockets during the Apollo, Skylab and Apollo–Soyuz programs. They were then used to transport Space Shuttles from 1981 to 2011. The crawler-transporters carry vehicles on the mobile launcher platforms used by NASA, and after each launch return to the pad to take the platform back to the VAB. The two crawler-transporters were designed and built by Marion Power Shovel Company using some components designed and built by Rockwell International at a cost of ( in 2022) each. Upon its construction, the crawler-transporter became the largest self-powered land vehicle in the world. While other vehicles such as bucket-wheel excavators like Bagger 288, dragline excavators like Big Muskie and power shovels like The Captain are significantly larger, they are powered by external sources. The two crawler-transporters were added to the National Register of Historic Places on January 21, 2000. Specifications The crawler-transporter has a mass of and has eight tracks, two on each corner. Each track has 57 shoes, and each shoe weighs . The vehicle measures . The height from ground level to the platform is adjustable from , and each side can be raised and lowered independently of the other. The crawler uses a laser guidance system and a leveling system to keep the Mobile Launcher Platform level within 10 minutes of arc (0.16 degrees; about at the top of the Saturn V), while moving up the 5 percent grade to the launch site. A separate laser docking system provides pinpoint accuracy when the crawler-transporter and Mobile Launch Platform are positioned in the VAB or at the launch pad. A team of nearly 30 engineers, technicians and drivers operate the vehicle, centered on an internal control room, and the crawler is driven from two control cabs located at either end. Before the launch the crawler-transporter is removed. The crawlers were overhauled in 2003 with upgrades to the Motor Control Center, which houses the switchgear and electrical controls of all of major systems on board; a new engine and pump ventilation system; new diesel engine radiators; and replacement of the two driver cabs on each vehicle (one on each end). As of 2003, each crawler had