Space Shuttle Re-entry
The loss of the space shuttle Columbia
on February 1, 2003, drove 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, where 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 (27,000
km/h). That really is 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 it all in a second.
To shed some speed, shuttle pilots
fire maneuvering thrusters on the shuttle's nose and tail, turning the
craft 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, with the nose facing forward and raised about 30 degrees.
These maneuvers are crucial. Too much speed, and the shuttle will slingshot
back into space. Too little, and it will burn in an uncontrolled plunge.
And only the proper, nose-up attitude puts the bottom of the shuttle in
position 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
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 the mechanical stresses
on the shuttle are intense, with shear forces exceeding 7,000 pounds per
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 degrees Fahrenheit (2,700 degrees
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 best 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.
Covered in Tile
Blunt shapes have remained constant
throughout the history of space travel. But heat-resistant materials have
not. At first, NASA relied on a process called ablation to transfer heat
during re-entry. The protective heat shield on those early NASA capsules
burned and fell away, taking large amounts of heat with it.
The shuttle requires reusable materials.
So the nose cone and leading edges of the wings, which reach the highest
temperatures during re-entry, are covered with reinforced carbon that can
withstand temperatures up to 3,000 degrees Fahrenheit (1,650 degrees Celsius).
The rest of the underside, as well as the forward fuselage, is covered
with more than 20,000 silica fiber tiles that can withstand 2,300 degrees
Fahrenheit (1,250 degrees Celsius). "Blankets" of silica fibers cover the
parts that face the lowest re-entry temperatures. 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.
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 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 assistance,
increasing the difficulty of an already challenging process.
At an altitude of roughly 83,000
feet (about 16 miles up), 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 danger the scientists,
engineers, and astronauts of the shuttle program know all too well.
August 8, 2005
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