PeterJ, and others,
The "sane participants" of the sci.space.shuttle newsgroup have put together the following FAQ:
http://www.io.com/~o_m/columbia_loss_faq.html
Shuttle re-entries are normally under computer control until just before landing. There's an extensive sss thread on the subject here:
http://groups.google.com/groups?hl=e...109%26rnum%3D5
The following info on flight profile is taken from the FAQ. I hope this isn't considered an excessively long post:
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* Where and when did Columbia break up? (Altitude, speed, time, etc.)
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Ground controllers lost communications with Columbia at 7:59:22am
CST, at a mission elapsed time of 15 days, 22:20:22. At the time,
the shuttle was at an altitude of 207,000 feet (63,000 m), traveling
at Mach 18.3, approximately 37 miles (60 km) above the Dallas-Fort
Worth Metroplex region of Texas.
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* What was Columbia's status prior to breakup?
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First, let's look at Columbia's reentry profile. The Shuttle has 3
distinct phases to the standard reentry profile:
1) Thermal Control Phase. This lasts from Entry Interface, when the
first aerodynamic effects occur, until a speed of approximately
19,000 ft/sec (12,900 MPH, 11,200 kts, 20,900 km/hr) has been reached.
2) Equilibrium Glide Phase. This is flight at a constant attitude as
the deceleration due to drag builds up to approximately 1G.
3) Constant Drag Phase. The 1G deceleration is held until the orbiter
enters the Terminal Area Energy Management interface, after which it
is flying as a conventional, but very heavy and fast, glider. This
is usually 52 NM (59 SM, 94 km) from the landing site, at an altitude
of 83,000', and a speed of Mach 2.5 (2500 ft/sec, 760 m/sec) The
orbiter slows to below Mach 1 at about 49,000', 22 NM (25 SM, 40 km)
from the runway.
Columbia was either at the end of the first phase, or the beginning
of the second phase when she broke up.
The first phase begins when the orbiter is oriented tail-first,
and the OMS engines fire to reduce its speed by about 300 ft/second
(90 m/sec). The reaction control system then orients the orbiter nose
first to prepare for reentry. At roughly 400,000 ft altitude (122
km), Entry Interface is considered to occur. This normally takes
place 4,400 NM (5063 statute miles/3160 km) from the landing site.
The speed at this point is about 25,000 ft/second (7600 m/sec).
At this point the orbiter is maneuvered to 0 degrees roll and yaw,
and a 40 degree angle of attack. The flight control system at this
point uses the Reaction Control System to keep things aligned. The
forward RCS engines are turned off at the entry interface, and the
aft RCS system is used to maneuver the spacecraft.
The spacecraft must dissipate the tremendous amount of kinetic
energy it has. It does this by varying the amount of aerodynamic
drag that it presents on the way down. This generates a lot of
heat because of the speed of the shuttle. This heating is controlled
by changing the speed of the shuttle in small amounts. This is done
by varying the aerodynamic drag of the shuttle. Most aircraft do
this by changing the Angle of Attack. When you pull up the nose,
an airplane tends to slow down, unless an engine is used to counteract
the drag. For a re-entering shuttle, the angle of attack must be
held constant to prevent the structure from overheating.
The shuttle controls drag by rolling into a series of 'S' turns along the
flight path. Increasing the roll angle of the orbiter moves the
direction of its lift (perpendicular to its wings) away from the
vertical, causing it to descend faster. These S-turns are used to
fine tune the energy level (A fancy way of saying altitude and
airspeed) of the orbiter, something like skiers turning while going
downhill to control their speed. When a dynamic pressure of 10
psf is reached (EAS of 62 MPH (100 km/hr)), when the orbiter's
ailerons become effective for roll control. At that point, the roll
RCS engines are deactivated. At a dynamic pressure of 20 psf (EAS of
85 MPH(138 km/hr), roughly), the elevators on the orbiter become
active, and the RCS pitch engines are deactivated.
In the Equilibrium Glide Phase of the reentry, the spacecraft is
flown to maintain a constant drag level, where the flight path angle
remains constant. This is maintained until the deceleration of the
orbiter due to drag is about 1G.
In the last phase of the reentry, the 1G deceleration level is held,
reducing the angle of attack as necessary, until the Terminal Area
Energy Management interface. The RCS system continues to control
Yaw until the rudder become effective at around Mach 3.5.
Columbia was lost either at the tail end of the Thermal Control
Phase, or the early stages of the Equilibrium Glide Phase. The
ailerons and elevators were providing control, (the Q at that point
was around 75-80 psf, or an EAS of about 170 mph (275 km/hr)), and
yaw was being controlled by the RCS thrusters in the tail.
Late reports before this writing this indicate that the Flight Control
System reported that it was correcting a left yaw/roll just before
breakup.