Space
Shuttle Basics
Landing
When it is
time to return to Earth, the shuttle is rotated tailfirst into the
direction of travel to prepare for another firing of the orbital
maneuvering system engines, a firing called the deorbit burn. This
engine firing, usually about three minutes long, slows the shuttle
by only a couple of hundred miles per hour, but it is enough that
it begins to descend toward the atmosphere. The engine firing takes
place usually half a world away from the intended landing site:
for example, the firing may take place above the Indian Ocean to
put the shuttle on course toward a landing at the Kennedy Space
Center. The three-minute firing is the only active brake the shuttle
will use as it heads toward a landing. The rest of its descent toward
Florida and trip halfway around the world is devoted to slowing
down using only the drag produced by the atmosphere.
 | | The
Space Shuttle Endeavour lands at the Kennedy Space Center. |
After the firing
takes place, it is about another 25 minutes before the shuttle will
descend to a point that it first encounters the effects of the atmosphere,
usually at an altitude of about 129 kilometers (80 miles) and a
range of more than 8,047 kilometers (5,000 miles) from the landing
site. Before the shuttle encounters the atmosphere, leftover fuel
is burned from the forward reaction control system steering jets
as a safety precaution. Before it reaches the upper atmosphere,
the shuttle is oriented with the nose angled up about 40 degrees
from horizontal and its wings level, an orientation that keeps the
black thermal tiles on the underside facing the majority of the
heat generated by its encounter, heat that can range as high as
1,648.9 degrees Celsius (3,000 degrees Fahrenheit) on the leading
edges of the wings and nose.
The aft steering
jets are used to control the shuttle's orientation as it descends
into the atmosphere. As it descends, however, it begins a transition
from spacecraft to aircraft, and its aerosurfaces -- the wing flaps
and rudder -- gradually become active as air pressure builds. As
those surfaces become usable, the steering jets turn off automatically.
 | | Typical
shuttle groundtrack for a landing at Kennedy Space Center. |
During its
descent, the shuttle performs a series of four steep banks, rolling
over as much as 80 degrees to one side or the other, to slow down.
The series of banks gives the shuttle's groundtrack toward landing
an appearance similar to a highly elongated letter "S".
As the shuttle
approaches the landing site, it incorporates several aids to navigation.
The first, a radio beacon from the Tactical Air Navigation System,
or TACAN, is received when the shuttle is about 225 kilometers (140
miles) from the landing site and at an altitude of about 45,720
meters (150,000 feet). The TACAN provides updated bearing information
to the shuttle to aid in steering toward the runway.
As the shuttle
continues toward landing and its speed drops to less than three
times the speed of sound, or Mach 3, two air data probes are deployed
from either side of the nose of the spacecraft. These probes provide
supplemental information on the airspeed and altitude derived from
the outside barometric pressure and wind speed.
For a normal
entry and landing, the shuttle's flight control computers are in
control of the spacecraft until it is about 40 kilometers (25 miles)
from touchdown. At that time, as the shuttle's speed drops below
the speed of sound and it is at an altitude of about 15,240 meters
(50,000 feet), the commander takes over manual control of the approach
and landing. Using an approach aid called the Microwave Scanning
Beam Landing system that is installed at shuttle landing sites,
the commander normally flies the shuttle around an imaginary cylinder
to align with the runway, sometimes completing almost a full 6.4-kilometer-
(4-mile-) diameter circle of one end of the runway.
 | | The
space shuttle being serviced immediately after landing. |
During the
circle, the shuttle's altitude drops from 15,240 meters (50,000
feet) to about 3,048 meters (10,000 feet) as it begins to align
with the runway. As it aligns with the runway, the shuttle then
begins a steep descent with the nose angled as much as 19 degrees
down from horizontal, a glide slope that is seven times as steep
as the average commercial airliner landing. During the final approach,
the shuttle drops toward the runway 20 times faster than a commercial
airliner as its rate of descent and airspeed increase. When it is
less than 610 meters (2,000 feet) above the ground, the commander
pulls up the nose and slows the rate of descent in preparation for
touchdown.
At this point,
the pilot deploys the landing gear. As the shuttle's main landing
gear touches down, it is dropping at less than 8 kilometers per
hour (5 miles per hour) and has a forward speed of about 354 kilometers
per hour (220 miles per hour). After touchdown, the pilot deploys
a drag chute from a compartment located just below the tail and
the commander begins to drop the shuttle's nose gear slowly toward
the runway. The drag chute is then jettisoned before the wheels
come to a stop to ensure that it falls clear of the shuttle. |
