STORY LINK: "Discovery's Slow Crawl to the Pad" on Space.com
Slow Crawl to the Pad
by Michelle Evans, June 2006
During preparations for flight into space, the Space Shuttle goes through many transitions. Once the major components are individually made ready for flight, they then all have to be brought together, or stacked, in the Vehicle Assembly Building (VAB) at the Kennedy Space Center. First the Solid Rocket Booster (SRB) segments are mated on top of each other on the Mobile Launch Platform in one of the high bays of the VAB. Next, the External Tank (ET) is positioned between the twin SRBs. Finally, the Orbiter is secured to the back of the ET and everything is upright and ready to roll to the launch pad.
A rollout can be an amazing thing to watch. It is the first time the full stack of the shuttle system sees the light of day, approximately four to six weeks prior to a scheduled launch.
The Crawler Transporter (CT) is driven underneath the launch platform and shuttle stack, then lifts it off the VAB hard stand. At this point you have literally a full launch pad and space vehicle on the CT’s back. This load weighs 12 million pounds.
The VAB high bay doors telescope upward, the crawler’s diesel engines are brought to life, the driver in his glass enclosed cab puts the monstrosity in gear and away it goes!
Okay, it might not be that dramatic since once the CT is up to full speed that means they have achieved the lighting fast speed of almost one mile-per-hour. For those of us watching the progress on the lawn area just outside the VAB, we have to watch closely to see when there is real movement of the stack.
On the most recent rollout (Space Shuttle Discovery on STS-121, now scheduled to lift off on July 1 at 15:48 EDT), the start of rollout was delayed about 45 minutes, and then we had to watch closely for another 15 minutes or so before any of us could truly be sure that mission was on its way. One mile an hour may be the top speed, but most of the time it is much less.
The grey launch platform slowly moved out of the shadow of the VAB interior. Atop this we saw the back side of the shuttle stack: the ET and SRBs only. Hidden behind these is the underside of the orbiter. A black triangle of thermal protection tiles on each side of the SRBs is the first indication that the orbiter is even there as we see the outer tips of Discovery’s delta wings.
Eventually, the CT and its load are outside the VAB structure and on their way to the launch pad. River rock from Alabama provides the cushion on which this behemoth moves. The rock is crushed beneath the crawler as it rides on eight tracked tread belts, each with 57 one ton treads. Slowly moving past us, more and more becomes visible. The beautiful white and black of the orbiter fully reveals itself, bolted to the spine of the External Tank.
The whole operation is fondly reminiscent of childhood memories of Gerry Anderson television shows like the Thunderbirds. The operation is quite different than the way it is handled in Russia, where their boosters are transported on their side on a railroad car cradle, only to be swung vertically after arriving at the launch pad.
Many people ride along on the crawler’s journey. When those of us on the ground spy someone on the launch platform next to a booster, or standing below the tail of the orbiter, we truly get the scale of what we are witnessing.
Getting out of the VAB is only the start. For those of us covering the event, our work day has just begun. Once the stack is heading down the 130-foot wide crawlerway, we look for other vantage points to record the progress.
Eventually, we return twice to specific points along the route. First at the wide turn the transport must make as it moves to the specific launch pad chosen for this mission, and finally, at the pad itself.
At the midway point, we drive to the viewing spot by going under the edge of the crawler. The rumble and power of the gigantic structure passes over our heads, the shuttle reaching into the cloud-speckled blue sky. The weight of the universe seems to bear down on us. Instinctively we duck, even inside the safety of our bus.
In position at the turn point, we watch in awe as the stack slowly makes its way toward us, and once again, over our heads. How can such a huge mechanism even move, let alone transport an entire spacecraft and booster system across the miles of Florida swampland? It seems incomprehensible.
The day draws to a close. The Sun is touching the horizon as we make our way to the final stop at the launch pad. Here the perimeter gates are pushed open by a large forklift, the CT moves inside, and stops long enough to level the towering vehicle as it prepares to traverse the 5-degree incline to the top of the launch complex. Hydraulic jacks over three feet in diameter raise or lower each end of the CT up to six feet, to keep everything within a fraction of a single degree of level.
Inexorably, the CT inches closer to the summit above the flame trench, where it will finally drop its burden.
Lights at the pad come up and provide pinpoints of illumination against the slowly darkening sky. The last rays of the Sun disappear and the purple sky serves as a backdrop for the final few feet as the Space Shuttle approaches the point where the crawler comes to rest, gently dropping the platform and stack onto the hard stand from where it will eventually take flight. Then the CT itself backs away to head home to the parking area beside the VAB. This round trip mission has been performed over 150 times during the Apollo and Space Shuttle missions, and will probably continue for years to come as the replacement for the shuttle eventually comes on line.
Total time on this 4.2-mile crawl to the launch pad was seven hours and forty-five minutes. A slow and arduous beginning to a journey that will culminate in a 17,500 mile-per-hour blast into Earth orbit to move humanity just a little closer to the stars.