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Shuttle landing

February 4, 2003

A public radio commentary

I have no idea why the Columbia disaster occurred. But in order to understand NASA's eventual conclusions, I've looked into what normally happens when the shuttle lands.

Before its fateful decent the shuttle was orbiting the earth in, what we'd call, an upside position. To return home the astronauts first closed the cargo doors, then used thrusters to turn the shuttle right side up. These fourteen small jets aren't used to propel the craft, just to reposition it - changing it's pitch, roll, and yaw.

Next, the astronauts fire two large jets located on either side of the tail. These jets can propel the shuttle into final orbit, adjust it for rendezvousing and docking, and, most importantly, slow the shuttle down for re-entry.

The astronauts use these engines to start a free fall toward earth - it is gravity that brings the shuttle home. The large tail engines are fired to slow the shuttle down by 200 miles per hour. This is called "retrofiring" because the engine thrust is opposite the direction of travel. It takes about twenty-five minutes to fall into the upper atmosphere.

At this point, the wings on the shuttle don't lift the craft at all - the air is too thin for them to work - so, instead the on-board computer uses small jets to adjust the shuttle. It drops nose first into the atmosphere at a forty degree angle.

The shuttle is traveling about 17,000 miles per hour - compare this to a commercial jet that flies at 500 miles per hour. At this speed, a great deal of heat is generated. So, the shuttle is covered with ceramic insulating tiles to protect it. These tiles can absorb large amounts of heat without increasing their temperature much.

As the shuttle nears the ground, the atmosphere becomes thicker and the wings begin to provide lift. From here the shuttle is flown like a glider. The shuttle makes a series of banking turns to slow the descent. Up to this point the shuttle has been controlled by a computer, but now the commander takes control. He or she picks up a radio beacon from the runway when the shuttle is about 140 miles away from the landing site, gliding at about 150,000 feet. This is about where the Columbia came apart.

If all had gone well, the commander would have increased the angle of descent to minus 20 degrees - nearly seven times that of a commercial jet - and glided in for a landing. After landing, a ground crew would meet the shuttle and take about twenty minutes to blow away noxious gases that were made during the heat of re-entry.

Instead, of course, we now wait for NASA to tell us which part of this landing sequence went wrong.

Copyright 2003 William S. Hammack Enterprises