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Spacecraft
Re-entry
Spacecraft
Re-entry
How do spacecraft re-enter the Earth's atmosphere?
BY JANE MCGRATH
Launching
a spacecraft into space is one thingBringing it back is another.
Spacecraft
re-entry is tricky business for several reasons.
When
an object enters the Earth’s atmosphere, it experiences a few forces,
including gravity and drag.
Gravity
will naturally pull an object back to earth. But gravity alone would cause the
object to fall dangerously fast.
Luckily,
the Earth's atmosphere contains particles of air.
As the object falls, it hits
and rubs against these particles, creating friction. This friction causes the object to experience
drag, or air
resistance, which slows the object down to a safer entry speed.
This friction is a mixed
blessing, however. Although it causes drag, it also causes intense heat.
Specifically, shuttles face
intense temperatures of about 3000 degrees Fahrenheit (about 1649 degrees
Celsius) [source: Hammond].
Blunt-body design helps alleviate the heat problem. When an object -- with
blunt-shaped surface facing down -- comes back to Earth, the blunt shape
creates a shock wave in front of the vehicle.
That shock wave keeps the heat
at a distance from the object. At the same time, the blunt shape also slows the
object's fall [source: NASA].
The Apollo program, which
moved several manned ships back and forth from space during the 1960s and
1970s, coated the command module with special ablative material that burned up upon re-entry,
absorbing heat.
Unlike the Apollo vehicles,
which were built for one-time use, space shuttles are reusable launch vehicles
(RLVs).
So instead of merely using
ablative material, they must incorporate durable insulation.
The Descent of a Space Shuttle
Re-entering Earth is all
about attitude control. And, no, this doesn't mean astronauts need to
keep a positive attitude (although that's always helpful). Rather, it refers to
the angle at which the spacecraft flies.
Here's an overview of a
shuttle descent:
1. Leaving orbit: To slow the ship down from its extreme orbit
speed, the ship flips around and actually flies backwards for a period of time.
The orbital maneuvering engines (OMS) then thrust the ship out of orbit and
toward Earth.
2. Descent through atmosphere: After it's safely out of
orbit, the shuttle turns nose-first again and enters the atmosphere belly-down
(like a belly-flop) to take advantage of drag with its blunt
bottom. Computers pull the nose up to an angle of attack (angle of descent) of about 40 degrees.
3. Landing: If you've seen the movie
"Apollo 13," you might remember that the astronauts return to
Earth in their command module and land in the ocean where rescue workers pick
them up. Today's space shuttles look and land much more like airplanes. Once
the ship gets low enough, the commander takes over the computers and glides the
shuttle to a landing strip. As it's rolling along the strip, it deploys a
parachute to slow it down.
The trip back to Earth is a
hot one. Instead of the ablative materials found on the Apollo spacecraft,
today's space shuttles have special heat-resistant materials and insulating
tiles that can sustain re-entry heat.
· Reinforced Carbon Carbon (RCC): This composite material
covers the nose and edges of the wing, where temperatures get the hottest. In
2003, Columbia's RCC was damaged during liftoff, causing its burn-up on
re-entry, killing all seven crew members.
· High-temperature reusable surface insulation
(HRSI): These
black silica tiles are on the bottom of the shuttle and various other places
that can reach up to 2,300 degrees Fahrenheit (1,260 degrees Celsius).
· Fibrous Refractory Composite Insulation (FRCI): These black tiles have
replaced HRSI tiles in many places because they are stronger, lighter and more
heat resistant.
· Low-temperature reusable surface insulation
(LRSI): These
white silica tiles are thinner than HRSI tiles and protect various areas from
temperatures up to 1,200 degrees F (649 degrees C).
· Advanced Flexible Reusable Surface Insulation
(AFRSI): Made of
silica glass fabric, these exterior blankets are installed on the forward upper
section of a shuttle and withstand temperatures of up to 1,500 degrees F (816
degrees C). Over the years, these have taken over for much of the LRSI material
on a shuttle.
· Felt reusable surface insulation (FRSI): This material sustains
temperatures of up to 700 degrees F (371 degrees C) and is made of heat-treated
white Nomex felt (a material used in firefighters' protective clothing).
Bitter Reminders
Just as the Challenger
disaster in 1986 reminded us how risky shuttle launches are, the Columbia
disaster reminded us just how dangerous atmospheric re-entry is.
In 2003, the space shuttle
Columbia and its seven crew members burned up as they were returning to Earth.
After
investigation, NASA discovered that damage to the left wing (that
actually occurred during liftoff), let hot air in upon re-entry and caused the
shuttle to lose control and burn up.
The Demise of the Satellite
Satellites don't have to
stay up in Earth's orbit forever. Old satellites sometimes fall back to Earth.
Because of the harsh
conditions of re-entry, they can severely burn up on their way down.
However, some of them can
survive the fall and hit the Earth's surface.
In controlled falls, engineers
manipulate the propulsion systems on a satellite to make it fall in a safe
place, like the ocean.
Objects that enter the Earth's atmosphere face a rough trip. |
The leading edges and
nose of the shuttle use RCC material.
|
In this image, NASA
workers show where the Columbia suffered tile damage during its maiden flight.
|
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