Getting the Space Shuttle to the Launchpad

Most people have seen images of the space shuttle on the crawler heading for the launch pad and few of us give it much more though. But the crawler and its components are actually pretty interesting and worth a closer look.

The space shuttle orbiter, the orange external tank (ET) and the solid rocket boosters (SRBs) together make up what is called a stack. They are assembled in the Vehicle Assembly Building (VAB) on top of the mobile launch platform (MLP.)

The mobile launch platform is a two story structure made of steel. Although modifications have been made, the MLPs have been around since the Apollo days. The main structure is about 25 feet (7.6 m.) high, 160 feet (49 m.) long and 135 feet (41 M.) wide. The MLP rests on six pedestals 22 feet high.

When the shuttle, unfueled tank and SRBs are assembled on the launch platform, the entire package weighs about eleven million pounds (five million kilograms.) There are openings in the launch platform for the SRB and shuttle main engine exhaust.

On either side of the orbiter's main engines are structures called Tall Service Masts. These masts allow umbilical connections to be run through to the orbiter. These connections include a liquid oxygen line and a liquid hydrogen line which fuel the external tank. When the shuttle launches, these umbilicals pull away and retract into the masts which utilize protective doors to rotatte closed and shield them from exhaust flames. Other umbilicals include helium and nitrogen feed lines as well as ground electrical power and communication links.

The orbiter itself is not attached to the launch platform, rather there are eight attach posts, four on the aft skirt of each SRB. The posts attach to counterpart posts on the launch platform. The posts and counterpart posts fit together using explosive nuts which are set to activate during launch.

The Crawler-Transporter

It's the crawler-transporter that carries the assembled shuttle stack and the mobile launchh platform on the more than three mile (4.8 km) ride to the launch pad from the VAB. This huge vehicle also dates back to the Apollo program where it carried the massive Saturn V rockets to the launch pad. There are two of the crawlers which stand about 20 feet (6.1 m.) high,) 131 feet (40 m.) long and 114 feet (34.7 m.) wide. To get a better idea of it's size the crawler would just barely fit inside a baseball diamond. An unloaded crawler weights about 6 million pounds (2.7 million kg.)

The crawler doesn't have wheels, but instead uses eight tracks, each of which has 57 cleats and each one of those cleats weighs in at about one ton (907 kg.) Now while the crawler might get a lot of prizes for size, weight and importance of job, but speed is one category the crawlers will never get prizes for. When carrying the mobile launch platform and assembled shuttle, the crawler goes a blistering one mile per hour (1.6 kg/h.) Unloaded the crawlers can double their speed.

Since it is very important for the orbiter stack to be kept perfectly upright, the crawler has its own leveling system. This is especially important close to the launch pad where the crawlerway has a five percent grade. Once at the launch pad, the crawler uses a laser docking system to position the mobile launch platform at the pad.

So what kind of horsepower is needed to move that shuttle stack to the pad? Each crawler has two 2,750 horsepower diesel engines. The engines power four 1,000 kilowatt generators which provide electrical power to sixteen traction motors. There are cabs at either end of the crawler for an operator to steer the vehicle.

The Crawlerway

The path which the crawler uses between the VAB and launch pads is called the crawlerway. Each crawlerway consists of two 40 foot (12 m.) wide lanes separated by a 50 foot (15 m.) wide median strip, about as wide as an eight lane expressway. There's about a 3-mile (4.8-kilometer) stretch of crawlerway from the VAB to launch pad.

It takes a very special road to support the about 17 million pounds (7.7 million kg.) which make up the combined weight of the crawler, launch platform and shuttle stack, The crawlerway has four layers which make up the roadbed. The top surface consists of river stones. These stones much be made up of very hard material and they are brought in from Louisiana where stones with high quartz content can be found. The stones are 4 inches (10.2 centimeters) thick on the straight parts, and about eight inches (20.3 cm.) thick on the curves. Underneath the river stones in descending order are: 4 feet (1.2 meters) of graded, crushed stone; 2.5 feet (0.76 meter) of select fill; and 1 foot (0.30 meter) of compact fill.

Putting it all together

Once the shuttle stack is assembled on the mobile launch platform inside the VAB, the crawler comes in, lowers itself horizontally, and moves in under the launch platform. The crawler then raises itself back up, supporting the weight of the platform and shuttle stack. The journey to the pad usually takes 3-4 hours. Once at the launch pad, the crawler again lowers itself until the launch platform is resting on its base. Then the crawler moves away, back to the VAB.