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  #1  
Old March 11th 04, 02:58 AM
ypauls
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Default Space Shuttle

Dear Astronomy Experts
I have heard the shuttle descends rapidly. I was sitting in a plane at
30,000 feet and thought, how long would it take the shuttle to touch down
from that height? Anyone know the rate of decent?
Cordially
ypauls


  #2  
Old March 11th 04, 05:05 AM
Opus Penguin
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Posts: n/a
Default

ypauls wrote:
Dear Astronomy Experts
I have heard the shuttle descends rapidly. I was sitting in a plane
at 30,000 feet and thought, how long would it take the shuttle to
touch down from that height? Anyone know the rate of decent?
Cordially
ypauls




ENTRY
The entry phase of flight begins approximately five minutes before entry
interface, which occurs at an altitude of 400,000 feet. At EI minus five
minutes, the orbiter is at an altitude of about 557,000 feet, traveling at
25,400 feet per second, and is approximately 4,400 nautical miles (5,063
statute miles) from the landing site. The goal of guidance, navigation and
flight control software is to guide and control the orbiter from this state
(in which aerodynamic forces are not yet felt) through the atmosphere to a
precise landing on the designated runway. All of this must be accomplished
without exceeding the thermal or structural limits of the orbiter.
The entry phase is divided into three separate phases because of the
unique software requirements. Entry extends from EI minus five minutes to
terminal area energy management interface at an altitude of approximately
83,000 feet, at a velocity of 2,500 feet per second, 52 nautical miles (59
statute miles) from the runway and within a few degrees of tangency with the
nearest heading alignment cylinder in major mode 304.

TAEM extends to the approach and landing capture zone, defined as the
point when the orbiter is on glide slope, on airspeed, and on runway
centerline, which occurs below 10,000 feet and is the first part of major
mode 305. The orbiter attains subsonic velocity at an altitude of
approximately 49,000 feet about 22 nautical miles (25 statute miles) from
the runway.

Approach and landing begins at the approach and landing capture zone, an
altitude of 10,000 feet and Mach 0.9 and extends through the receipt of the
weight-on-nose-gear signal after touchdown, which completes major mode 305.

The forward RCS jets are inhibited at entry interface.

At 400,000 feet, a pre-entry phase begins in which the orbiter is
maneuvered to zero degrees roll and yaw (wings level) and a predetermined
angle of attack for entry. The flight control system issues the commands to
the roll, yaw and pitch RCS jets for rate damping in attitude hold for entry
into the Earth's atmosphere until 0.176 g is sensed, which corresponds to a
dynamic pressure of 10 pounds per square foot, approximately the point at
which the aerosurfaces become active.

When the orbiter is in atmospheric flight, it is flown by varying the
forces it generates while moving through the atmosphere, like any other
aerodynamic vehicle. The forces are determined primarily by the speed and
direction of the relative wind (the airstream as seen from the vehicle). The
direction of the airstream is described by the difference between the
direction that the vehicle is pointing (attitude) and the direction that it
is moving (velocity). It may be broken into two components: angle of attack
(vertical component) and sideslip angle (horizontal component).

To rotate the orbiter in the atmosphere, aerodynamic control surfaces are
deflected into the airstream. The orbiter has seven aerodynamic control
surfaces. Four of these are on the trailing edge of the wing (two per wing).
They are called elevons because they combine the effects of elevators and
ailerons on ordinary airplanes. Deflecting the elevons up or down causes the
vehicle to pitch up or down. If the right elevons are deflected up and the
left elevons are deflected down, the orbiter will roll to the right-that is,
the right wing falls and the left wing rises. The fifth control surface is
the body flap, located on the rear lower portion of the aft fuselage. It
provides thermal protection for the three main engines during entry, and
during atmospheric flight it provides pitch trim to reduce elevon
deflections. The sixth and seventh control surfaces are the rudder/speed
brake panels, located on the aft portion of the vertical stabilizer. When
both panels are deflected right or left, the spacecraft will yaw, moving the
spacecraft's nose right or left, thus acting as a rudder. If the panels are
opened at the trailing edge, aerodynamic drag force will increase, and the
spacecraft will slow down. Thus, the open panels are called a speed brake.

On the flight deck display and control panel (panel F7 between the
commander and pilot) are the surface position indicators, which display the
position of each aerodynamic control surface.

The aft RCS jets maneuver the spacecraft until a dynamic pressure of 10
pounds per square foot is sensed; at this point, the orbiter's ailerons
become effective, and the aft RCS roll jets are deactivated. At a dynamic
pressure of 20 pounds per square foot, the orbiter's elevators become
effective, and the aft RCS pitch jets are deactivated. The orbiter's speed
brake is used below Mach 10 to induce a more positive downward elevator trim
deflection. At Mach 3.5, the rudder become activated, and the aft RCS yaw
jets are deactivated (approximately 45,000 feet).

Entry flight control is maintained with the aerojet DAP, which generates
effector and RCS jet commands to control and stabilize the vehicle during
its descent from orbit. The aerojet DAP is a three-axis rate command
feedback control system that uses commands from guidance in automatic or
from the flight crew's RHC in control stick steering. Depending on the type
of command and the flight phase, these result in fire commands to the RCS or
deflection commands to the aerosurfaces.

In the automatic mode, the orbiter is essentially a missile, and the
flight crew monitors the instruments to verify that the vehicle is following
the correct trajectory. The onboard computers execute the flight control
laws (equations). If the vehicle diverges from the trajectory, the crew can
take over at any time by switching to CSS. The orbiter can fly to a landing
in the automatic mode (only landing gear extension and braking action on the
runway are required by the flight crew). The autoland mode capability of the
orbiter is used by the crew usually to a predetermined point in flying
around the heading alignment cylinder. In flights to date, the crew has
switched to CSS when the orbiter is subsonic. However, autoland provides
information to the crew displays during the landing sequence.

The commander and pilot can select automatic or CSS flight control modes.
The crew can select separate modes for pitch and roll and yaw (roll and yaw
must be in the same mode). The body flap and speed brake have automatic and
manual modes.

Automatic pitch provides automatic control in the pitch axis, and the
automatic roll and yaw provides automatic control in the roll and yaw axes.
During entry, the automatic mode uses the RCS jets until dynamic pressure
permits the aerosurfaces to become effective; the aft RCS jets and
spacecraft aerosurfaces are then used together until dynamic pressure
becomes sufficient for aerosurface control only.

Control in the pitch axis is provided by the elevons, speed brake and body
flap. The elevons provide control to guidance normal acceleration commands,
control of pitch rate during slap-down (landing) for nose wheel load
protection, and static load relief after slap-down for main landing gear
wheel and tire load protection. The speed brake provides control to guidance
surface deflection (open/close, increase/decrease velocity) command. The
body flap provides control to null elevon deflection.

Control in the roll and yaw axes is provided by the elevons and rudder.
The elevons provide control to guidance bank angle command during terminal
area energy management and autoland and control to guidance wings-level
command during flat turns, 5 feet above touchdown. The rudder provides yaw
stabilization during TAEM and autoland and control to guidance yaw rate
command during flat turn and subsequent phases.

When the orbiter is in the automatic pitch and roll and yaw modes, the
crew's manual control stick steering commands are inhibited. In the CSS
mode, the crew flies the orbiter by deflecting the RHC and rudder pedals.
The flight control system interprets the RHC motions as rate commands in
pitch, roll or yaw and controls the RCS jets and aerosurfaces. The larger
the deflection, the larger the command. The flight control system compares
these commands with inputs from rate gyros and accelerometers (what the
vehicle is actually doing-motion sensors) and generates control signals to
produce the desired rates. If the crew releases the RHC, it will return to
center, and the orbiter will maintain its present attitude (zero rates). The
rudder pedals position the rudder during atmospheric flight; however, in
actual use, because flight control software performs automatic turn
coordination, the rudder pedals are not used until the wings are leveled
before touchdown.

The CSS mode is similar to the automatic mode except that the crew can
issue three-axis commands, affecting the spacecraft's motion. These are
augmented by the feedback from the same spacecraft motion sensors, except
for the normal acceleration (velocity) accelerometer assemblies, to enhance
control response and stability.

The commander's or pilot's RHC commands are processed by the GPCs in the
CSS mode together with data from the motion sensors. The flight control
module processes the flight control laws and provides commands to the flight
control system, which positions the aerosurfaces in atmospheric flight.

Control in the roll and yaw axes is provided by the elevon and the rudder.
The elevons augment the RHC control. The rudder interface between the roll
and yaw channel automatically positions the rudder for coordinated turns. A
rudder pedal transducer assembly is provided at the commander and pilot
stations. The two rudder pedal assemblies are connected to their respective
RPTAs. Because of the roll and yaw interface, rudder pedal use should not be
required until just before touchdown. There is an artificial feel in the
rudder pedal assemblies. The RPTA commands are processed by the GPCs, and
the flight control module commands the flight control system to position the
rudder.

In the CSS mode, the commander's and pilot's RHC trim switches, in
conjunction with the trim enable/inhibit switch, activate or inhibit the RHC
trim switch. When the RHC trim switch is positioned forward or aft, it adds
a trim rate to the RHC pitch command; positioning it left or right adds a
roll trim.

Manual control (CSS mode) in the pitch axis is provided by the elevons,
speed brake and body flap. The elevons provide augmented control through the
RHC pitch command. The speed brake can be switched to its manual mode at
either the commander's or pilot's station by depressing a takeover switch on
the speed brake/thrust controller handle. Manual speed brake control can be
transferred from one station to the other by activating the takeover switch.
When the SBTC is at its forward setting, the speed brake is closed. Rotating
the handle aft, positions the speed brake at the desired position (open) and
holds it. To regain automatic speed brake control, the push button must be
depressed again. In the manual mode, speed brake commands are processed by
the GPCs, and the flight control module commands the flight control system
to position the speed brake and hold it at the desired position. The body
flap can be switched to its manual mode at panel C3 by moving a toggle
switch from auto/off to up or down for the desired body flap position. These
are momentary switch positions; when released, the switch returns to off .

In the entry phase, the RCS commands roll, pitch and yaw. Lights on the
commander's panel F6 are used to indicate the presence of an RCS command
from the flight control system to the RCS jet selection logic; however, this
does not indicate an actual RCS jet thrusting command. The minimum light-on
duration is extended to allow the light to be seen even for minimum-impulse
RCS jet thrusting commands. After the roll and pitch aft RCS jets are
deactivated, the roll indicator lights are used to show that three or more
yaw RCS jets have been requested. The pitch indicator lights are used to
show elevon rate saturation.

At approximately 265,000 feet, the spacecraft enters a communications
blackout, which lasts until the orbiter reaches an altitude of approximately
162,000 feet. Between these altitudes, heat is generated as the spacecraft
enters the atmosphere, ionizing atoms of air that form a layer of ionized
gas particles around the spacecraft. Radio signals between the spacecraft
and the ground cannot penetrate this sheath of ionized particles, and radio
communications are blocked for approximately 16 minutes.

During the entry subphase, the primary objective is to dissipate the
tremendous amount of energy that the orbiter possesses when it enters the
atmosphere so that it does not burn up (entry angle too steep) or skip out
of the atmosphere (entry angle too shallow), stays within structural limits,
and arrives at the TAEM interface with the altitude and range to the runway
necessary for a landing. This is accomplished by adjusting the orbiter's
drag acceleration on its surface using bank commands relative to vehicle
velocity. During TAEM, as the name implies, the goal is to manage the
orbiter's energy while the orbiter travels along the heading alignment
cylinder, which lines up the vehicle on the runway centerline. A HAC is an
imaginary cone that, when projected on the Earth, lies tangent to the
extended runway centerline.

Guidance performs different tasks during the entry, TAEM and approach and
landing subphases. During the entry subphase, guidance attempts to keep the
orbiter on a trajectory that provides protection against overheating,
overdynamic pressure and excessive normal acceleration limits. To do this,
it sends commands to flight control to guide the orbiter through a tight
corridor limited on one side by altitude and velocity requirements for
ranging (in order to make the runway) and orbiter control and on the other
side by thermal constraints. Ranging is accomplished by adjusting drag
acceleration to velocity so that the orbiter stays in that corridor. Drag
acceleration can be adjusted primarily in two ways: by modifying the angle
of attack, which changes the orbiter's cross-sectional area with respect to
the airstream, or by adjusting the orbiter's bank angle, which affects lift
and thus the orbiter's sink rate into denser atmosphere, which in turn
affects drag. Using angle of attack as the primary means of controlling drag
results in faster energy dissipation with a steeper trajectory but violates
the thermal constraint on the orbiter's surfaces. For this reason, the
orbiter's bank angle (roll control) is used as the primary method of
controlling drag, and thus ranging, during this phase. Increasing the roll
angle decreases the vertical component of lift, causing a higher sink rate.
Increasing the roll rate raises the surface temperature of the orbiter, but
not nearly as drastically as does an equal angle of attack command. The
orbiter's angle of attack is kept at a high value (40 degrees) during most
of this phase to protect the upper surfaces from extreme heat. It is
modulated at certain times to ''tweak'' the system and is ramped down to a
new value at the end of this phase for orbiter controllability. Using bank
angle to adjust drag acceleration causes the orbiter to turn off course.
Therefore, at times, the orbiter must be rolled back toward the runway. This
is called a roll reversal and is commanded as a function of azimuth error
from the runway. The ground track during this phase, then, results in a
series of S-turns.

If the orbiter is low on energy (the current range-to-go is much greater
than nominal at current velocity), entry guidance will command
lower-than-nominal drag levels. If the orbiter has too much energy (the
current range-to-go is much less than nominal at current velocity), entry
guidance will command higher-than-nominal drag levels to dissipate the extra
energy.

Roll angle is used to control cross range. Azimuth error is the angle
between the plane containing the orbiter's position vector and the heading
alignment cylinder tangency point and the plane containing the orbiter's
position vector and velocity vector. When the azimuth error exceeds an
initialized-loaded number, the orbiter's roll angle is reversed.

Thus, descent rate and downranging are controlled by bank angles-the
steeper the bank angle, the greater the descent rate and the greater the
drag. Conversely, the minimum-drag altitude is wings level. Cross range is
controlled by bank reversals.

The entry thermal control phase is designed to keep the thermal protection
system's bond line within design limits. A constant heating rate is
maintained until the velocity is below 19,000 feet per second.

In the equilibrium glide phase, the orbiter effects a transition from the
rapidly increasing drag levels of the temperature control phase to the
constant drag level of the constant drag phase. Equilibrium glide is defined
as flight in which the flight path angle, the angle between the local
horizontal and the local velocity vector, remains constant. This flight
regime provides the maximum downrange capability. It lasts until drag
acceleration reaches 33 feet per second squared.

The constant drag phase begins at 33 feet per second squared. Angle of
attack is initially 40 degrees, but it begins to ramp down until it reaches
approximately 36 degrees by the end of this phase.

The transition phase is entered as the angle of attack continues to ramp
down, reaching about 14 degrees at TAEM interface, with the vehicle at an
altitude of some 83,000 feet, traveling 2,500 feet per second (Mach 2.5),
and 52 nautical miles (59 statute miles) from the runway. At this point,
control is transferred to TAEM guidance.

During these entry phases, the orbiter's roll commands keep the orbiter on
the drag profile and control cross range.

TAEM guidance steers the orbiter to the nearest of two heading alignment
cylinders, whose radii are approximately 18,000 feet and whose locations are
tangent to and on either side of the runway centerline on the approach end.
Normally, the software is set to fly the orbiter around the HAC on the
opposite side of the extended runway centerline. This is called the overhead
approach. If the orbiter is low on energy, it can be flagged to acquire the
HAC on the same side of the runway. This is called the straight-in approach.
In TAEM guidance, excess energy is dissipated by an S-turn, and the speed
brake can be used to modify drag, lift-to-drag ratio and the flight path
angle under high-energy conditions. This increases the ground track range as
the orbiter turns away from the nearest HAC until sufficient energy is
dissipated to allow a normal approach and landing guidance phase capture,
which begins at 10,000 feet at the nominal entry point. The orbiter can also
be flown near the velocity for maximum lift over drag or wings level for the
range stretch case, which moves the approach and landing guidance phase to
the minimum entry point.

At TAEM acquisition, the orbiter is turned until it is aimed at a point
tangent to the nearest HAC and continues until it reaches way point 1. At
way point 1, the TAEM heading alignment phase begins, in which the HAC is
followed until landing runway alignment, plus or minus 20 degrees, is
achieved. As the orbiter comes around the HAC, it should be lined up on the
runway and at the proper flight path angle and airspeed to begin the steep
glide slope to the runway.

In the TAEM prefinal phase, the orbiter leaves the HAC, pitches down to
acquire the steep glide slope, increases airspeed and banks to acquire the
runway centerline, continuing until it is on the runway centerline, on the
outer glide slope and on airspeed.

The approach and landing guidance phase begins with the completion of the
TAEM prefinal phase and ends when the orbiter comes to a complete stop on
the runway. The approach and landing interface airspeed requirement at an
altitude of 10,000 feet is approximately 290 knots, plus or minus 12 knots,
equivalent airspeed, 6.9 nautical miles (7.9 statute miles) from touchdown.

Autoland guidance is initiated at this point to guide the orbiter to the
minus 19- to 17-degree glide slope (which is more than seven times that of a
commercial airliner's approach) aimed at a target approximately 0.86
nautical mile (1 statute mile) in front of the runway. The descent rate in
the latter portion of TAEM and approach and landing is greater than 10,000
feet per minute (approximately 20 times higher than a commercial airliner's
standard 3-degree instrument approach angle). The steep glide slope is
tracked in azimuth and elevation, and the speed brake is positioned as
required.

Approximately 1,750 feet above the ground, guidance sends commands to keep
the orbiter tracking the runway centerline, and a preflare maneuver is
started to position the orbiter on a shallow 1.5-degree glide slope in
preparation for landing, with the speed brake positioned as required. At
this point, the crew deploys the landing gear.

Final flare is begun at approximately 80 feet to reduce the sink rate of
the vehicle to less than 9 feet per second. After the spacecraft crosses the
runway threshold-way point 2 in the autoland mode-navigation uses the radar
altimeter vertical component of position in the state vector for guidance
and navigation computations from an altitude of 100 feet to touchdown.
Touchdown occurs approximately 2,500 feet past the runway threshold at a
speed of 184 to 196 knots (211 to 225 mph). As the airspeed drops below 165
knots (189 mph), the orbiter begins derotation in preparation for nose gear
slap-down.

The navigation system used from entry to landing consists of the IMUs and
navigation aids (TACAN, air data system, microwave scan beam landing system
and radar altimeter). The three IMUs maintain an inertial reference and
provide delta velocities until MSBLS is acquired.

Navigation-derived air data-obtained after deployment of the two air data
probes at approximately Mach 3-is needed from entry through landing as
inputs to the guidance, flight control and crew display. TACAN provides
range and bearing measurements and is available at approximately 145,000
feet, nominally accepting the data into the state vector before 130,000
feet. It is used until MSBLS acquisition, which provides range, azimuth and
elevation commencing at approximately 18,000 feet. Radar altimeter data are
available at approximately 9,000 feet.

TACAN acquisition and operation are completely automatic, but the crew has
the necessary controls and displays to evaluate TACAN system performance and
to take over if required. When the distance to the landing site is
approximately 120 nautical miles (138 statute miles), TACAN begins
interrogating six navigation region stations. As the spacecraft proceeds,
the distances to the remaining stations and to the next-nearest station are
computed, and the next-nearest station is selected automatically if the
spacecraft is closer to it than it is to the previous locked-on station.
Only one station is interrogated if the distance to the landing site is less
than approximately 20 nautical miles (23 statute miles). Again, TACAN
automatically switches from the last locked-on navigation region station to
begin searching for the landing site station. TACAN azimuth and range are
provided on the CRT displaying the horizontal situation. TACAN range and
bearing cannot be used to produce a good estimate of the altitude position
component, so navigation uses barometric altitude derived from the air data
system probes.

MSBLS acquisition and operation are also completely automatic, and the
flight crew can evaluate system performance and take over if necessary.
MSBLS acquisition occurs at approximately 18,000 feet and about 8 nautical
miles (9.2 statute miles) from the runway. The range and azimuth
measurements are provided by a ground antenna located at the end of the
runway and to the left of the runway centerline. Elevation measurements are
given by a ground antenna to the left of the runway centerline, about 2,624
feet from the runway threshold.

During entry, the commander's and pilot's altitude director indicators
become two-axis balls displaying body roll and pitch attitudes with respect
to local vertical/local horizontal. These are generated in the attitude
processor from IMU data. The roll and pitch error needles each display the
body roll and pitch attitude error with respect to entry guidance commands
by using the bank guidance error and the angle of attack error generated
from the accelerometer assemblies. In atmospheric flight, the roll attitude
error and the normal acceleration error are displayed by the roll and pitch
error needles, respectively. The sideslip angle is displayed on the yaw
error needle. The roll and pitch rate needles display stability roll and
body rates by using stability roll rate, rate gyro rate and pitch rate. The
yaw rate needle displays stability yaw rate. After main landing gear
touchdown, the yaw error with respect to runway centerline and nose gear
slap-down pitch rate error are displayed on the roll and pitch error
needles. During rollout, the pitch error indicator indicates pitch error
rate.

During entry, the commander's and pilot's horizontal situation indicators
display a pictorial view of the spacecraft's location with respect to
various navigation points. The navigation attitude processor provides the
inputs to the HSI until the communications blackout is passed, at
approximately 145,000 feet. TACAN is then acquired and accepted for HSI
inputs at about 130,000 feet until MSBLS acquisition at approximately 18,000
feet some 8 nautical miles (9.2 statute miles) from the runway.

When the approach mode and MSBLS source are selected for the commander's
and pilot's HSI, data from the MSBLS replaces TACAN data. MSBLS azimuth,
elevation and range are used from acquisition until the runway threshold is
reached, and azimuth and range are used to control rollout.

At an altitude of 9,000 feet, radar altimeter 1 or 2 can be selected to
measure the nearest terrain within the beamwidth of the altimeters. This
indication is given to the altitude/vertical velocity indicator radar,
altitude and meter display from 5,000 feet to landing.

The left and right air data system probes are deployed by the flight crew
at about Mach 3. This system senses air pressures related to orbiter
movement through the atmosphere for updating the navigation state vector in
altitude, guidance in steering and speed brake command calculations, flight
control for control law computations, and for display on the alpha Mach
indicators and altitude/vertical velocity indicators.

The AMIs display essential flight parameters relative to the spacecraft's
travel in the air mass, such as angle of attack, acceleration, velocity and
knots of equivalent airspeed. The source of data for the AMIs is determined
by the position of the air data select switch. Before the deployment of the
air data system probe, the AMIs receive inputs from the navigation attitude
processor. When the air data probes are deployed, the left or right air data
system provides the inputs to all AMIs except the acceleration indicator,
which remains on the navigation attitude processor, and the radar altitude.
Neither is operational until the orbiter descends to 5,000 feet.

The three rate gyro assemblies of the flight control system measure and
supply output data proportional to the orbiter's attitude rates about its
three body axes, while the three accelerometer assemblies measure and supply
output data proportional to the orbiter's normal (vertical) and lateral
(right and left) accelerations. These assemblies are incorporated into the
flight control system for augmenting stability because of the orbiter's
marginal stability in its pitch and yaw axes at subsonic speeds.

The three IMUs constitute an all-attitude stabilized platform that also
measures and supplies output data proportional to the spacecraft's attitude
(rotation) and acceleration (velocity). They augment the rate gyro
assemblies and accelerometer assemblies.

The rate gyro assembly pitch rate (rotation) and the accelerometer
assembly normal acceleration (velocity) are used to generate elevon
(elevator) deflection commands. The rate gyro assembly yaw rate (rotation)
and the accelerometer assembly lateral acceleration generate the rudder
deflection required for directional stability. The rate gyro assembly roll
rate (rotation) generates the elevon (aileron) deflection command required
for lateral (roll) stability. The speed brake and body flap positions
generate the elevon deflection required for trim near neutral to maximize
roll effectiveness of the elevons.

In the entry phase, navigation software functions as it did during the
deorbit phase (three state vectors corresponding to each IMU) except that
additional external sensor data are sequentially incorporated. These data
provide the accuracy necessary to bring the orbiter to a pinpoint landing
and, to some extent, to maintain vehicle control. The TACAN system, which
becomes available at about 156,000 feet, provides slant range and magnetic
bearing to various fixed stations around the landing site. It is used until
the orbiter is approximately 1,500 feet above the ground, at which point it
is rendered ineffective by ground reflection. The air data system, which
includes two transducer assemblies attached to a probe on the left side of
the vehicle and two on the right side, provides pressures from which angle
of attack, Mach number, equivalent airspeed, true airspeed, dynamic
pressure, barometric altitude and altitude rate are computed. Only
barometric altitude is used by navigation. The other parameters are used by
guidance and flight control as well as for display to the flight crew. The
probes are normally deployed around Mach 3. The MSBLS precisely determines
slant range, azimuth and elevation relative to the landing runway. For
landing at runways with MSBLS ground stations, MSBLS data become available
at 20,000 feet for processing by navigation.

One other tool used by navigation is a drag altitude software sensor,
which uses a model of the atmosphere to correlate the drag acceleration
measured by the IMUs to altitude. This measurement, then, is only as good as
the atmospheric model on which it is based. The model is not perfect.
However, it has been determined through testing and analysis that drag
altitude data are important in keeping downrange and altitude errors bounded
during the blackout portion of entry (from approximately 265,000 to 162,000
feet). During this time, the ground is unable to uplink state vector
corrections to the orbiter, and TACAN data are not available because of the
heat-generated ionization of the atmosphere around the vehicle.

Navigation also maintains a statistical estimate of the expected error in
the state vector. This is called a covariance matrix and is propagated along
with the state vector. When an external sensor, such as TACAN, becomes
available to the navigation software, a check is made to see if the data lie
within the current expected range of error. Flight crew controls are
provided on an onboard CRT horizontal situation display to force the
software to accept or inhibit the external sensor data whether or not the
data lie within the expected range. Another control on the display may be
selected to allow the software to use the external sensor data to update its
state vector so long as the data lie within the expected range.

About five minutes before entry interface, the crew adjusts the software
to major mode 304. During this mode, which lasts until TAEM interface, five
CRTs become available sequentially and are used to monitor auto guidance and
the orbiter trajectory compared to the planned entry profile. The five
displays are identical except for the central plot, which shows the
orbiter's velocity versus range or energy/weight versus range with a
changing scale as the orbiter approaches the landing site. This plot also
includes static background lines that allow the crew to monitor the
orbiter's progression compared to planned entry profiles.

Once TAEM interface is reached, the software automatically makes a
transition to major mode 305. The CRT vertical situation 1 display then
becomes available. It includes a central plot of orbiter altitude with
respect to range. This plot has three background lines that represent the
nominal altitude versus range profile, a dynamic pressure limit in guidance
profile and a maximum lift-over-drag profile. At 30,000 feet, the scale and
title on the display change to vertical situation 2, and the display is used
through landing. When the approach and landing interface conditions are met,
a flashing A/L appears on the display.

Another prime CRT display used during entry is the horizontal situation.
In addition to providing insight into and control over navigation
parameters, this display gives the crew orbiter position and heading
information once the orbiter is below 200,000 feet.

The entry trajectory, vertical situation and horizontal situation CRT
displays, then, are used by the flight crew to monitor the GN&C software.
They can also be used by the crew to determine whether a manual takeover is
required.




  #3  
Old March 11th 04, 06:19 AM
ypauls
external usenet poster
 
Posts: n/a
Default

So the answer is a little more than 3 minutes...

The descent rate in
the latter portion of TAEM and approach and landing is greater than

10,000
feet per minute (approximately 20 times higher than a commercial

airliner's
standard 3-degree instrument approach angle).




  #4  
Old March 15th 04, 02:12 AM
chuck_sterling
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On Thu, 11 Mar 2004 04:05:56 +0000, Opus Penguin wrote:

ypauls wrote:
Dear Astronomy Experts
I have heard the shuttle descends rapidly. I was sitting in a plane at
30,000 feet and thought, how long would it take the shuttle to touch
down from that height? Anyone know the rate of decent? Cordially
ypauls




ENTRY
The entry phase of flight begins approximately five minutes before entry
interface, which occurs at an altitude of 400,000 feet. At EI minus five
minutes, the orbiter is at an altitude of about 557,000 feet, traveling at
25,400 feet per second, and is approximately 4,400 nautical miles (5,063


Where did all this come from? Surely you did not type it in as you sat
replying to that question. Just curious as to your source.

Chuck Sterling

 




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