01/02/03

The loss of the space shuttle Columbia on February 1 drives home a simple truth: space travel is never routine, no matter how many shuttles launch and land.

Success depends on near-perfection, especially during re-entry. Re-entry takes less than a half-hour to complete, but it's marked by tremendous changes in gravity, pressure, and speed. Re-entry vehicles must endure kinetic and thermodynamic extremes that rival even the hazards of liftoff. There is almost no room for error. Here's how it's supposed to work.

A Speeding Bullet

First, the shuttle has to slow down. While in orbit, it's traveling at more than 17,000 miles per hour. That's literally faster than a speeding bullet. Imagine driving down a highway for five minutes, passing landmark after landmark as you go. The orbiting shuttle would speed by all of it in a single second.

To shed some of this speed, shuttle astronauts fire maneuvering thrusters located along the shuttle's nose and tail, turning the shuttle so that its tail faces forward. Then they fire the main thrusters to slow down. Another thruster burn reorients the shuttle to its proper re-entry attitude (its orientation relative to its direction of flight), with the nose facing forward and raised about 30 degrees. These maneuvers are crucial. Too much speed, and the shuttle will slingshot back out into space; too little, and it will burn in an uncontrolled plunge. And only the proper, nose-up attitude presents the bottom surface of the shuttle forward, to bear re-entry's brunt.

A Flying Brick

At an altitude of 400,000 feet (about 75 miles up), the shuttle begins to catch a little air. Aerodynamic forces take effect. All thruster maneuvering ceases, and the shuttle becomes a 120-foot glider. Onboard computers maintain proper descent attitude and speed by adjusting body flaps, elevons on the wings, and a rudder. Yet without power, and traveling at hypersonic speeds through the thin atmosphere, pilots say flying the shuttle is like flying "a brick with wings."

Plowing through air, however thin, causes drag, which slows the shuttle down--and creates gravitational forces that stress the ship and its crew. Apollo astronauts regularly endured more than 6 Gs during re-entry, while Mercury astronauts suffered up to 11 Gs, near the maximum sustained force humans can stand. Shuttle crews experience only about 3 Gs, significantly less than during liftoff, because of the shuttle's longer descent time. But mechanical stresses on the shuttle are intense. Sensor data from early shuttle missions indicate that the skin of the shuttle suffers peak shear forces in excess of 7,000 pounds per square inch, while the interior aluminum trusses and spars of the shuttle's wings face peak tensions up to 8,000 pounds per square inch.

A Blunt Bottom

Still, the most difficult challenge of re-entry comes from another source: heat. Any object moving through the atmosphere encounters friction caused by air molecules hitting its surface. At relatively low speeds, like those of conventional aircraft, the heat caused by this friction is negligible. But as speeds increase, so does friction. For some objects, the extreme speed of re-entry can produce surface temperatures greater than 5,000° F.

Two factors are crucial in foiling this heat: the shape of the re-entry vehicle and the materials used to make its surface. Strangely enough, the optimal shape for an object during re-entry is not slender and streamlined, but blunt-faced. A blunt shape creates more drag, which allows the re-entry vehicle to decelerate more in the thin upper atmosphere, so that speeds in the thicker lower atmosphere remain as low as possible. The blunt shape also produces shockwaves away from the object's surface, which deflect more heat away. The capsules used in NASA's Mercury, Gemini, and Apollo programs had wide, bluntly rounded bottoms that were oriented downward during re-entry. The flat bottom of the space shuttle provides a similarly blunt surface.

Covered in Tile

Although shape has remained constant throughout the history of space travel, heat-resistance materials have not. At first, NASA programs relied on a process called ablation to transfer heat during re-entry. The protective heat shield on early capsules was made of synthetic resins that were designed to burn and fall away, taking large amounts of heat with them.

The space shuttle, however, requires reusable, lighter weight materials. So it has more than 20,000 heat-resistant tiles covering its surface. The nose cone and leading edges of the shuttle's wings, which reach the highest temperatures during re-entry, are covered with reinforced carbon capable of withstanding temperatures up to 3,000° F. The rest of the underside of the shuttle, as well as the forward fuselage, is covered with silica fiber tiles capable of withstanding 2,300° F. Lighter "blankets" of silica fibers cover those parts of the shuttle that face the lowest re-entry temperatures, below 1,200° F. Underneath the brittle tiles is a layer of material that cushions them from the vibration, expansion, and contraction of the shuttle's aluminum frame and that insulates the frame from the radiating heat.

Briefly Alone

The tiles protect the shuttle from the heat, but they can't protect the radio. The intense heat of re-entry strips electrons right off oxygen and nitrogen molecules in the air around the shuttle, forming a highly conductive layer of ionized particles (called plasma) around the vehicle. Because radio waves cannot penetrate this layer, the shuttle can temporarily lose communication with ground control. This "ionization blackout" can occur from 25 minutes to about 12 minutes before landing. During this time, shuttle pilots must often operate without benefit of assistance from ground control, increasing the difficulty of an already challenging process.

At an altitude of roughly 83,000 feet, re-entry is over. On a good day, all that remains are five minutes of relatively cool, gentle, controlled gliding to the landing site. Yet the tremendous aerodynamic and thermal forces of re-entry always contain the potential for catastrophe. It is a tribute to the scientists, engineers, builders, and astronauts of the space shuttle program that until February 1, 2003, no re-entry accident had ever occurred.