NASA beefs up its ground transport vehicle

The colossal Crawler Transporter 2 (CT-2) was built by the US space agency, NASA, in the 1960s to transport spacecraft to the launch pad that included the Saturn V rockets, used for the Apollo missions to the moon, as well as the more recent Space Shuttles. The behemoth CT-2 creeps along at just 1mph and is designed to lift the mobile launcher platform that has a spacecraft mounted on top. It moves this entire integrated stack from the Vehicle Assembly Building where launch vehicles have been assembled since the Apollo era to the ocean-side launchpads. NASA is currently developing its next generation space launch vehicle called the Space Launch System (SLS). This will carry, among others, the Orion spacecraft in to orbit and beyond. The new rocket system will have the heaviest lift capability and be the most powerful rocket system ever produced. The heaviest SLS – including its mobile launcher platform – is estimated to weigh around 6,441 tonnes. The Apollo Saturn V, by comparison, had a rollout weight of approximately 5,579 tonnes. With the SLS due to launch in 2017, it means the CT-2 needs its massive roller bearing assemblies replaced if it going to take the additional weight of future rockets and carry on operating for another 50 years. The overhaul will take place around the crawler's four 'feet', the part of the carrier that holds and drives the tracks. These will also be modified and further strengthened with structural reinforcements. Mary Hanna, CT project manager in the vehicle integration and launch branch of ground systems development and operations programs at NASA, says: "We've already begun the process of removing the treads and jacking two of the crawler corners four feet off the ground. The next step is to remove and replace the roller bearing assemblies. "The rollers, shaft assemblies, sleeves and other hardware needed will amount to about a 500,000lbs (226 tonnes) of steel." Once the old roller bearing assemblies are removed, engineers will inspect the structure and integrity of the mounting holes and openings to see if any repairs are needed. After this, installation of the newly designed and fabricated bearing assemblies can take place. "We expect installation to begin in August," says Hanna. "Testing should then take place near the end of this year." The crucial nature of the roller bearings surfaced on one of the first Crawler Transporter 'test drives' back in 1965. The machine was being tested using a launch tower and was being taken along two short stretches of specially prepared and maintained surfaces known as 'crawlerways'. During the test, metal fragments were discovered on the ground. "The original rollers were simply ball bearings," says Hanna. "The roller bearing and associated assembly, including roller shafts and new sleeve bearings were redesigned and installed, and that hardware was then successfully used throughout the Apollo and Space Shuttle programs." Need to modify However, after extensive tests in November 2012, engineers identified flaws in the roller bearing design and concept. However, for one of the biggest tracked vehicle in the world, using stock bearings is unlikely to be an option. As a result, the engineers have had to redesign the assemblies and develop a number of modifications to the original to get the improvements in performance needed. "The new assemblies will help the CT-2 carry the heavier loads," says Hanna, "and will also be better lubricated. And that should provide a longer operational life." The CT-2 on its own weighs around 2,948 tonnes, but will be able to carry the 4,672 tonne mobile launcher with the attached 1,996 tonne SLS rocket (without liquid propellants). The result will be a vehicle lumbering toward the launchpad weighing just under 10,000 tonnes. Needless to say, this creates incredible pressure and strain on the bearing assemblies. "When you have that much metal on metal, carrying such huge loads, there is a tremendous amount of heat and friction," says Hanna. "That creates much of the wear and tear that we see on the Crawlers." Currently, engineers are redesigning and upgrading the units with modern roller and plain bearings, and lubrication systems, which are much more advanced than the previous assemblies. The main purpose of the work is to increase the durability and life of the bearings, which need to be able to cope with payloads of around 6,700 tonnes. The redesign will also allow the easy replacement of broken bearings during the operation. Work on the modernisation of bearing units are planned to be completed by August 2013, with testing due to take place at the end of the year. Other modifications include overhauling and updating many of the onboard systems such as upgrades to the jacking, equalisation and levelling cylinders. In addition the AC power generators are also being replaced to increase the available power from 750kW to 1,500kW.