Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/shuttle/sts.txt

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SPACE TRANSPORTATION SYSTEM

SPACE SHUTTLE PROGRAM

The space shuttle is developed by the National Aeronautics and Space
Administration.  NASA coordinates and manages the space transportation
system (NASA's name for the overall shuttle program), including
intergovernmental agency requirements and international and joint
projects.  NASA also oversees the launch and space flight requirements
for civilian and commercial use.

The space shuttle system consists of four primary elements: an orbiter
spacecraft, two solid rocket boosters, an external tank to house fuel
and oxidizer and three space shuttle main engines.

The orbiter is built by Rockwell International's Space Transportation
Systems Division, Downey, Calif., which also has responsibility for
the integration of the overall space transportation system.  Both
orbiter and integration contracts are under the direction of NASA's
Johnson Space Center in Houston, Texas.

The solid rocket booster motors are built by the Wasatch Division of
Morton Thiokol Corporation, Brigham City, Utah, and are assembled,
checked out and refurbished by United Space Booster Inc.  Booster
Production Company, Kennedy Space Center, Cape Canaveral, Fla.  The
external tank is built by Martin Marietta Corporation at its Michoud
facility, New Orleans, La.; and the space shuttle main engines are
built by Rockwell's Rocketdyne Division, Canoga Park, Calif.  These
contracts are under the direction of NASA's George C.  Marshall Space
Flight Center in Huntsville, Ala.

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SPACE SHUTTLE REQUIREMENTS

The shuttle will transport cargo into near Earth orbit 100 to 217
nautical miles (115 to 250 statute miles) above the Earth.  This cargo
(called payload) is carried in a bay 15 feet in diameter and 60 feet
long.

Major system requirements are that the orbiter and the two solid
rocket boosters be reusable.

Other features of the shuttle:

- The orbiter has carried a flight crew of up to eight persons.  A
total of 10 persons could be carried under emergency conditions.

- The basic mission is seven days in space.

- The crew compartment has a shirt-sleeve environment, and the
acceleration load is never greater than 3 g's.

- In its return to Earth, the orbiter has a cross-range maneuvering
capability of 1,100 nautical miles (1,265 statute miles).

The space shuttle is launched in an upright position, with thrust
provided by the three space shuttle engines and the two solid rocket
boosters.  After about two minutes, the two boosters are spent and are
separated from the external tank.  They fall into the ocean at
predetermined points and are recovered for reuse.

The space shuttle main engines continue firing for about eight
minutes.  They shut down just before the craft is inserted into orbit.
The external tank is then separated from the orbiter.  It follows a
ballistic trajectory into a remote area of the ocean but is not
recovered.

There are 38 primary reaction control system engines and six vernier
reaction control system engines located on the orbiter.  The first
utilization of selected primary reaction control system engines occurs
at orbiter/ external tank separation.  The selected primary reaction
control system engines are used in the separation sequence to provide
an attitude hold for separation.  Then they move the orbiter away from
the external tank to ensure orbiter clearance from the arc of the
rotating external tank.  Finally, they return to an attitude hold
prior to the initiation of the firing of the orbital maneuvering
system engines to place the orbiter into orbit.

The primary and/or vernier reaction control system engines are used
normally on orbit to provide attitude pitch, roll and yaw maneuvers as
well as translation maneuvers.

The two orbital maneuvering system engines are used to place the
orbiter on orbit, for major velocity maneuvers on orbit and to slow
the orbiter for re-entry, called the deorbit maneuver.  Normally, two
orbital maneuvering system engine thrusting sequences are used to
place the orbiter on orbit, and only one thrusting sequence is used
for deorbit.

The orbiter's velocity on orbit is approximately 25,405 feet per
second.  The deorbit maneuver decreases this velocity approximately
300 feet per second for re-entry.

In some missions, only one orbital maneuvering system thrusting
sequence is used to place the orbiter on orbit.  This is referred to
as direct insertion.  Direct insertion is a technique used in some
missions where there are high-performance requirements, such as a
heavy payload or a high orbital altitude.  This technique utilizes the
space shuttle main engines to achieve the desired apogee (high point
in an orbit) altitude, thus conserving orbital maneuvering system
propellants.  Following jettison of the external tank, only one
orbital maneuvering system thrusting sequence is required to establish
the desired orbit altitude.

For deorbit, the orbiter is rotated tailfirst in the direction of the
velocity by the primary reaction control system engines.  Then the
orbital maneuvering system engines are used to decrease the orbiter's
velocity.

During the initial entry sequence, selected primary reaction control
system engines are used to control the orbiter's attitude (pitch, roll
and yaw).  As aerodynamic pressure builds up, the orbiter flight
control surfaces become active and the primary reaction control system
engines are inhibited.

During entry, the thermal protection system covering the entire
orbiter provides the protection for the orbiter to survive the
extremely high temperatures encountered during entry.  The thermal
protection system is reusable (it does not burn off or ablate during
entry).

The unpowered orbiter glides to Earth and lands on a runway like an
airplane.  Nominal touchdown speed varies from 184 to 196 knots (213
to 226 miles per hour).

The main landing gear wheels have a braking system for stopping the
orbiter on the runway, and the nose wheel is steerable, again similar
to a conventional airplane.

There are two launch sites for the space shuttle.  The Kennedy Space
Center in Florida is used for launches to place the orbiter in
equatorial orbits (around the equator), and the Vandenberg Air Force
Base launch site in California will be used for launches that place
the orbiter in polar orbit missions.

Landing sites are located at the Kennedy Space Center and Vandenberg
Air Force Base.  Additional landing sites are provided at Edwards Air
Force Base in California and White Sands, N.M.  Contingency landing
sites are also provided in the event the orbiter must return to Earth
in an emergency.

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LAUNCH SITES

Space shuttles destined for equatorial orbits are launched from the
Kennedy Space Center, and those requiring polar orbital planes will be
launched from Vandenberg Air Force Base.

Orbital mechanics and the complexities of mission requirements, plus
safety and the possibility of infringement on foreign air and land
space, prohibit polar orbit launches from the Kennedy Space Center.

Kennedy Space Center launches have an allowable path no less than 35
degrees northeast and no greater than 120 degrees southeast.  These
are azimuth degree readings based on due east from Kennedy Space
Center as 90 degrees.

A 35-degree azimuth launch places the spacecraft in an orbital
inclination of 57 degrees.  This means the spacecraft in its orbital
trajectories around the Earth will never exceed an Earth latitude
higher or lower than 57 degrees north or south of the equator.

A launch path from the Kennedy Space Center at an azimuth of 120
degrees will place the spacecraft in an orbital inclination of 39
degrees (it will be above or below 39 degrees north or south of the
equator).

These two azimuths-35 and 120 degrees-represent the launch limits from
the Kennedy Space Center.  Any azimuth angles further north or south
would launch a spacecraft over a habitable land mass, adversely affect
safety provisions for abort or vehicle separation conditions, or
present the undesirable possibility that the solid rocket boosters or
external tank could land on foreign land or sea space.

Launches from the Vandenberg Air Force Base have an allowable launch
path suitable for polar insertions south, southwest and southeast.

The launch limits at Vandenberg Air Force Base are 201 and 158
degrees.  At a 201-degree launch azimuth, the spacecraft would be
orbiting at a 104-degree inclination.  Zero degrees would be due north
of the launch site, and the orbital trajectory would be within 14
degrees east or west of the north-south pole meridian.  At a launch
azimuth of 158 degrees, the spacecraft would be orbiting at a
70-degree inclination, and the trajectory would be within 20 degrees
east or west of the polar meridian.  Like the Kennedy Space Center,
Vandenberg Air Force Base has allowable launch azimuths that do not
pass over habitable areas or involve safety, abort, separation and
political considerations.

Mission requirements and payload weight penalties also are major
factors in selecting a launch site.

The Earth rotates from west to east at a speed of approximately 900
nautical miles per hour (1,035 miles per hour).  A launch to the east
uses the Earth's rotation somewhat as a springboard.  The Earth's
rotational rate is also the reason the orbiter has a cross-range
capability of 1,100 nautical miles (1,265 statute miles) to provide
the abort once around capability in polar orbit launches.

Attempting to launch and place a spacecraft in polar orbit from the
Kennedy Space Center to avoid habitable land mass would be
uneconomical because the shuttle's payload would be reduced
severely-down to approximately 17,000 pounds.  A northerly launch into
polar orbit of 8 to 20 degrees azimuth would necessitate a path over a
land mass; and most safety, abort, and political constraints would
have to be waived.  This prohibits polar orbit launches from the
Kennedy Space Center.

The following orbital-insertion inclinations and payload weights
exemplify the space shuttle's capabilities.

NASA's latest assessment of orbiter ascent and landing weights
incorporates currently approved modifications to all vehicle elements,
including crew escape provisions, and assumes a maximum space shuttle
main engine throttle setting of 104 percent.  It is noted that the
resumption of space shuttle flights initially requires more
conservative flight design criteria and additional instrumentation,
which reduces the following basic capabilities by approximately 1,600
pounds:

1.  Kennedy Space Center eastern test range satellite deploy missions.
The basic cargo-lift capability for a due east (28.5 degrees) launch
is 55,000 pounds to a 110-nautical- mile (126-statute-mile) orbit
using OV-103 (Discovery) or OV-104 (Atlantis) to support a four-day
satellite deploy mission.  This capability will be reduced
approximately 100 pounds for each additional nautical mile of altitude
desired by the customer.

The payload capability for the same satellite deploy mission with a
57-degree inclination is 41,000 pounds.

The performance for intermediate inclinations can be estimated by
allowing 500 pounds per degree of plane change between 28.5 and 57
degrees.

If OV-102 is used, the cargo-lift weight capability must be decreased
by approximately 8,400 pounds.  This weight difference is attributed
to an approximately 7,150-pound difference in inert weight, 850 pounds
of orbiter experiments, 300 pounds of additional thermal protection
system and 100 pounds to accommodate a fifth cryogenic liquid oxygen
and liquid hydrogen tank set for the power reactant storage and
distribution system.

2.  Vandenberg Air Force Base western test range satellite deploy
missions.  Using OV-103 or OV-104, the cargo-lift weight capability is
29,600 pounds for a 98-degree launch inclination and 110-nautical-mile
(126-statute-mile) polar orbit.  Again, an increase in altitude costs
approximately 100 pounds per nautical mile.  NASA assumes also that
the advanced solid rocket motor will replace the filament-wound solid
rocket motor case previously used for western test range assessments.

This same mission at 68 degrees inclination (minimum western test
range inclination based on range safety limitations) is 49,600 pounds.

Performance for intermediate inclinations can be estimated by allowing
660 pounds for each degree of plane change between inclinations of 68
and 98 degrees.

3.  Landing weight limits.  All the space shuttle orbiters are
currently limited to a total vehicle landing weight of 240,000 pounds
for abort landings and 230,000 pounds for nominal end-of-mission
landings.

It is noted that each additional crew person beyond the five-person
standard is chargeable to the cargo weight allocation and reduces the
payload capability by approximately 500 pounds.  (This is an increase
of 450 pounds to account for the crew escape equipment.)

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Space Transportation System

0..Previous Menu
1..Main Menu

2..Space Shuttle Program
3..Space Shuttle Requirements
4..Launch Sites
5..Background and Status
6..Mission Profile
7..Aborts
8..Orbiter Ground Turnaround


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BACKGROUND AND STATUS

On July 26, 1972, NASA selected Rockwell's Space Transportation
Systems Division in Downey, Calif., as the industrial contractor for
the design, development, test and evaluation of the orbiter.  The
contract called for fabrication and testing of two orbiters, a
full-scale structural test article, and a main propulsion test
article.  The award followed years of NASA and Air Force studies to
define and assess the feasibility of a reusable space transportation
system.

NASA previously (March 31, 1972) had selected Rockwell's Rocketdyne
Division to design and develop the space shuttle main engines.
Contracts followed to Martin Marietta for the external tank (Aug.  16,
1973) and Morton Thiokol's Wasatch Division for the solid rocket
boosters (June 27, 1974).

In addition to the orbiter DDT&E contract, Rockwell's Space
Transportation Systems Division was given contractual responsibility
as system integrator for the overall shuttle system.

Rockwell International's Launch Operations, part of the Space
Transportation Systems Division, was under contract to NASA's Kennedy
Space Center for turnaround, processing, prelaunch testing, and launch
and recovery operations from STS-1 through the STS-11 mission.

On Oct.  1, 1983, the Lockheed Space Operations Company was awarded
the space shuttle processing contract at Kennedy Space Center for
turnaround processing, prelaunch testing, and launch and recovery
operations.

The first orbiter spacecraft, Enterprise (OV-101), was rolled out on
Sept.  17, 1976.  On Jan.  31, 1977, it was transported overland from
Rockwell's assembly facility at Palmdale, Calif., to the Dryden Flight
Research Facility at Edwards Air Force Base (36 miles) for the
approach and landing test  program.

The nine-month-long ALT program was conducted from February through
November 1977 at NASA's Dryden Flight Research Facility and
demonstrated that the orbiter could fly in the atmosphere and land
like an airplane, except without power-gliding flight.

The ALT program involved ground tests and flight tests.

The ground tests included taxi tests of the 747 shuttle carrier
aircraft with the Enterprise mated atop the SCA to determine
structural loads and responses and assess the mated capability in
ground handling and control characteristics up to flight takeoff
speed.  The taxi tests also validated 747 steering and braking with
the orbiter attached.  A ground test of orbiter systems followed the
unmanned captive tests.  All orbiter systems were activated as they
would be in atmospheric flight; this was the final preparation for the
manned captive-flight phase.

Five captive flights of the Enterprise mounted atop the SCA with the
Enterprise unmanned and Enterprise systems inert were conducted to
assess the structural integrity and performance-handling qualities of
the mated craft.

Three manned captive flights that followed the five unmanned captive
flights included an astronaut crew aboard the orbiter operating its
flight control systems while the orbiter remained perched atop the
SCA.  These flights were designed to exercise and evaluate all systems
in the flight environment in preparation for the orbiter release
(free) flights.  They included flutter tests of the mated craft at low
and high speed, a separation trajectory test and a dress rehearsal for
the first orbiter free flight.

In the five free flights the astronaut crew separated the spacecraft
from the SCA and maneuvered to a landing at Edwards Air Force Base.
In the first four such flights the landing was on a dry lake bed; in
the fifth, the landing was on Edwards' main concrete runway under
conditions simulating a return from space.  The last two free flights
were made without the tail cone, which is the spacecraft's
configuration during an actual landing from Earth orbit.  These
flights verified the orbiter's pilot-guided approach and landing
capability; demonstrated the orbiter's subsonic terminal area energy
management autoland approach capability; and verified the orbiter's
subsonic airworthiness, integrated system operations and selected
subsystems in preparation for the first manned orbital flight.  The
flights demonstrated the orbiter's ability to approach and land safely
with a minimum gross weight and using several center-of-gravity
configurations.

For all of the captive flights and the first three free flights, the
orbiter was outfitted with a tail cone covering its aft section to
reduce aerodynamic drag and turbulence.  The final two free flights
were without the tail cone, and the three simulated space shuttle main
engines and two orbital maneuvering system engines were exposed
aerodynamically.

The final phase of the ALT program prepared the spacecraft for four
ferry flights.  Fluid systems were drained and purged, the tail cone
was reinstalled and elevon locks were installed.  The forward
attachment strut was replaced to lower the orbiter's cant from 6 to 3
degrees.  This reduces drag to the mated vehicles during the ferry
flights.

After the ferry flight tests, OV-101 was returned to the NASA hangar
at the Dryden Flight Research Facility and modified for vertical
ground vibration tests at the Marshall Space Flight Center,
Huntsville, Ala.

On March 13, 1978, the Enterprise was ferried atop the SCA to NASA's
Marshall Space Flight Center.  At the Marshall Space Flight Center,
Enterprise was mated with the external tank and solid rocket boosters
and subjected to a series of vertical ground vibration tests.  These
tested the mated configuration's critical structural dynamic response
modes, which were assessed against analytical math models used to
design the various element interfaces.

These were completed in March 1979.  On April 10, 1979, the Enterprise
was ferried to the Kennedy Space Center, mated with the external tank
and solid rocket boosters and transported via the mobile launcher
platform to Launch Complex 39-A.  At Launch Complex 39-A, the
Enterprise served as a practice and launch complex fit-check
verification tool representing the flight vehicles.

It was ferried back to NASA's Dryden Flight Research Facility at
Edwards Air Force Base in California on Aug.  16, 1979, and then
returned overland to Rockwell's Palmdale final assembly facility on
Oct.  30, 1979.  Certain components were refurbished for use on flight
vehicles being assembled at Palmdale.  The Enterprise was then
returned overland to NASA's Dryden Flight Research Facility on Sept.
6, 1981.

During May and June 1983, Enterprise was ferried to the Paris, France,
Air Show as well as to Germany, Italy, England and Canada before
returning to the Dryden Flight Research Facility.

From April to October 1984, Enterprise was ferried to Vandenberg Air
Force Base and to Mobile, Ala., where it was taken by barge to New
Orleans, La., for the United States 1984 World's Fair.

In November 1984 it was ferried to Vandenberg Air Force Base and used
as a practice and fit-check verification tool.  On May 24, 1985,
Enterprise was ferried from Vandenberg Air Force Base to NASA's Dryden
Flight Research Facility.

On Sept.  20, 1985, Enterprise was ferried from NASA's Dryden Flight
Research Facility to the Kennedy Space Center.  On Nov.  18, 1985,
Enterprise was ferried from the Kennedy Space Center to Dulles
Airport, Washington, D.C., and became the property of the Smithsonian
Institution.  The Enterprise was built as a test vehicle and is not
equipped for space flight.

The second orbiter, Columbia (OV-102), was the first to fly into
space.  It was transported overland on March 8, 1979, from Palmdale to
NASA's Dryden Flight Research Facility for mating atop the shuttle
carrier aircraft and ferried to the Kennedy Space Center.  It arrived
on March 25, 1979, to begin preparations for the first flight into
space.

The structural test article, after 11 months of extensive testing at
Lockheed's facility in Palmdale, was returned to Rockwell's Palmdale
facility for modification to become the second orbiter available for
operational missions.  It was redesignated OV-099, the Challenger.

The main propulsion test article (MPTA-098) consisted of an orbiter
aft fuselage, a truss arrangement that simulated the orbiter's
midfuselage and the shuttle main propulsion system (three space
shuttle main engines and the external tank).  This test structure is
at the National Space Technology Laboratories in Mississippi.  A
series of static firings was conducted from 1978 through 1981 in
support of the first flight into space.

On Jan.  29, 1979, NASA contracted with Rockwell to manufacture two
additional orbiters, OV-103 and OV-104 (Discovery and Atlantis),
convert the structural test article to space flight configuration
(Challenger) and modify Columbia from its developmental configuration
to that required for operational flights.

Challenger (OV-099) was delivered to the Kennedy Space Center from
Rockwell's Palmdale, Calif., assembly facility on July 5, 1982.

NASA named the first four orbiter spacecraft after famous sailing
ships.  In the order they became operational, they are:

- Columbia (OV-102), after a sailing frigate launched in 1836, one of
the first Navy ships to circumnavigate the globe.  Columbia also was
the name of the Apollo 11 command module that carried Neil Armstrong,
Michael Collins and Edward (Buzz) Aldrin on the first lunar landing
mission, July 20, 1969.

- Challenger (OV-099), also a Navy ship, which from 1872 to 1876 made
a prolonged exploration of the Atlantic and Pacific oceans.  It also
was used in the Apollo program for the Apollo 17 lunar module.

- Discovery (OV-103), after two ships, the vessel in which Henry
Hudson in 1610-11 attempted to search for a northwest passage between
the Atlantic and Pacific oceans and instead discovered Hudson Bay and
the ship in which Capt.  Cook discovered the Hawaiian Islands and
explored southern Alaska and western Canada.

- Atlantis (OV-104), after a two-masted ketch operated for the Woods
Hole Oceanographic Institute from 1930 to 1966, which traveled more
than half a million miles in ocean research.

In April 1983, under contract to NASA, Rockwell's Space Transportation
Systems Division, Downey, Calif., began the construction of structural
spares for completion in 1987.  The structural spares program
consisted of an aft fuselage, crew compartment, forward reaction
control system, lower and upper forward fuselage, midfuselage, wings
(elevons), payload bay doors, vertical stabilizer (rudder/speed
brake), body flap and one set of orbital maneuvering system/reaction
control system pods.

Discovery (OV-103) was delivered to the Kennedy Space Center from
Rockwell's Palmdale assembly facility on Nov.  9, 1983.

Columbia (OV-102) was delivered to Rockwell's Palmdale assembly
facility for modifications on Jan.  30, 1984, and was returned to the
Kennedy Space Center on July 14, 1985, for return to flight.

Atlantis (OV-104) was delivered to the Kennedy Space Center from
Rockwell's Palmdale assembly facility on April 3, 1985.

On Sept.  12, 1985, Rockwell International's Shuttle Operations
Company, Houston, Texas, was awarded the space transportation systems
operation contract at NASA's Johnson Space Center to perform mission
support operations for Johnson Space Center, consolidating work
previously performed under 22 contracts by 16 different contractors.

On July 31, 1987, NASA awarded Rockwell's Space Transportation Systems
Division, Downey, Calif., a contract to build a replacement space
shuttle orbiter using the structural spares.  The replacement orbiter
will be assembled at Rockwell's Palmdale, Calif., assembly facility
and is scheduled for completion in 1991.  This orbiter is designated
OV-105.

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MISSION PROFILE

In the launch configuration, the orbiter and two solid rocket boosters
are attached to the external tank in a vertical (nose-up) position on
the launch pad.  Each solid rocket booster is attached at its aft
skirt to the mobile launcher platform by four bolts.

Emergency exit for the flight crew on the launch pad up to 30 seconds
before lift-off is by slidewire.  There are seven 1,200-foot- long
slidewires, each with one basket.  Each basket is designed to carry
three persons.  The baskets, 5 feet in diameter and 42 inches deep,
are suspended beneath the slide mechanism by four cables.  The
slidewires carry the baskets to ground level.  Upon departing the
basket at ground level, the flight crew progresses to a bunker that is
designed to protect it from an explosion on the launch pad.

At launch, the three space shuttle main engines-fed liquid hydrogen
fuel and liquid oxygen oxidizer from the external tank-are ignited
first.  When it has been verified that the engines are operating at
the proper thrust level, a signal is sent to ignite the solid rocket
boosters.  At the proper thrust-to-weight ratio, initiators (small
explosives) at eight hold-down bolts on the solid rocket boosters are
fired to release the space shuttle for lift-off.  All this takes only
a few seconds.

Maximum dynamic pressure is reached early in the ascent, nominally
approximately 60 seconds after lift-off.

Approximately a minute later (two minutes into the ascent phase), the
two solid rocket boosters have consumed their propellant and are
jettisoned from the external tank.  This is triggered by a separation
signal from the orbiter.

The boosters briefly continue to ascend, while small motors fire to
carry them away from the space shuttle.  The boosters then turn and
descend, and at a predetermined altitude, parachutes are deployed to
decelerate them for a safe splashdown in the ocean.  Splashdown occurs
approximately 141 nautical miles (162 statute miles) from the launch
site.  The boosters are recovered and reused.

Meanwhile, the orbiter and external tank continue to ascend, using the
thrust of the three space shuttle main engines.  Approximately eight
minutes after launch and just short of orbital velocity, the three
space shuttle engines are shut down (main engine cutoff), and the
external tank is jettisoned on command from the orbiter.

The forward and aft reaction control system engines provide attitude
(pitch, yaw and roll) and the translation of the orbiter away from the
external tank at separation and return to attitude hold prior to the
orbital maneuvering system thrusting maneuver.

The external tank continues on a ballistic trajectory and enters the
atmosphere, where it disintegrates.  Its projected impact is in the
Indian Ocean (except for 57-degree inclinations) in the case of
equatorial orbits (Kennedy Space Center launch) and in the extreme
southern Pacific Ocean in the case of a Vandenberg Air Force Base
launch.

Normally, two thrusting maneuvers using the two orbital maneuvering
system engines at the aft end of the orbiter are used in a two-step
thrusting sequence: to complete insertion into Earth orbit and to
circularize the spacecraft's orbit.  The orbital maneuvering system
engines are also used on orbit for any major velocity changes.

In the event of a direct-insertion mission, only one orbital
maneuvering system thrusting sequence is used.

The orbital altitude of a mission is dependent upon that mission.  The
nominal altitude can vary between 100 to 217 nautical miles (115 to
250 statute miles).

The forward and aft reaction control system thrusters (engines)
provide attitude control of the orbiter as well as any minor
translation maneuvers along a given axis on orbit.

At the completion of orbital operations, the orbiter is oriented in a
tailfirst attitude by the reaction control system.  The two orbital
maneuvering system engines are commanded to slow the orbiter for
deorbit.

The reaction control system turns the orbiter's nose forward for
entry.  The reaction control system controls the orbiter until
atmospheric density is sufficient for the pitch and roll aerodynamic
control surfaces to become effective.

Entry interface is considered to occur at 400,000 feet altitude
approximately 4,400 nautical miles (5,063 statute miles) from the
landing site and at approximately 25,000 feet per second velocity.

At 400,000 feet altitude, the orbiter is maneuvered to zero degrees
roll and yaw (wings level) and at a predetermined angle of attack for
entry.  The angle of attack is 40 degrees.  The flight control system
issues the commands to roll, pitch and yaw reaction control system
jets for rate damping.

The forward reaction control system engines are inhibited prior to
entry interface, and the aft reaction control system engines maneuver
the spacecraft until a dynamic pressure of 10 pounds per square foot
is sensed, which is when the orbiter's ailerons become effective.  The
aft reaction control system roll engines are then deactivated.  At a
dynamic pressure of 20 pounds per square foot, the orbiter's elevators
become active, and the aft reaction control system pitch engines are
deactivated.  The orbiter's speed brake is used below Mach 10 to
induce a more positive downward elevator trim deflection.  At
approximately Mach 3.5, the rudder becomes activated, and the aft
reaction control system yaw engines are deactivated at 45,000 feet.

Entry guidance must dissipate the tremendous amount of energy the
orbiter possesses when it enters the Earth's atmosphere to assure that
the orbiter does not either burn up (entry angle too steep) or skip
out of the atmosphere (entry angle too shallow) and that the orbiter
is properly positioned to reach the desired touchdown point.

During entry, energy is dissipated by the atmospheric drag on the
orbiter's surface.  Higher atmospheric drag levels enable faster
energy dissipation with a steeper trajectory.  Normally, the angle of
attack and roll angle enable the atmospheric drag of any flight
vehicle to be controlled.  However, for the orbiter, angle of attack
was rejected because it creates surface temperatures above the design
specification.  The angle of attack scheduled during entry is loaded
into the orbiter computers as a function of relative velocity, leaving
roll angle for energy control.  Increasing the roll angle decreases
the vertical component of lift, causing a higher sink rate and energy
dissipation rate.  Increasing the roll rate does raise the surface
temperature of the orbiter, but not nearly as drastically as an equal
angle of attack command.

If the orbiter is low on energy (current range-to-go much greater than
nominal at current velocity), entry guidance will command lower than
nominal drag levels.  If the orbiter has too much energy (current
range-to-go much less than nominal at the 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 a computer-loaded number, the orbiter's roll angle is
reversed.

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

The entry thermal control phase is designed to keep the backface
temperatures within the design limits.  A constant heating rate is
established until below 19,000 feet per second.

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

The constant drag phase begins at that point.  The angle of attack is
initially 40 degrees, but it begins to ramp down in this phase to
approximately 36 degrees by the end of this phase.

In the transition phase, the angle of attack continues to ramp down,
reaching the approximately 14-degree angle of attack at the entry
terminal area energy management interface, at approximately 83,000
feet altitude, 2,500 feet per second, Mach 2.5 and 52 nautical miles
(59 statute miles) from the landing runway.  Control is then
transferred to TAEM guidance.

During the entry phases described, 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
which are located tangent to and on either side of the runway
centerline on the approach end.  In TAEM guidance, excess energy is
dissipated with an S-turn; and the speed brake can be utilized to
modify drag, lift-to-drag ratio and flight path angle in 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 altitude.  The orbiter also can be flown near
the velocity for maximum lift over drag or wings level for the range
stretch case.  The spacecraft slows to subsonic velocity at
approximately 49,000 feet altitude, about 22 nautical miles (25.3
statute miles) from the landing site.

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 WP-1, the TAEM heading alignment phase begins.  The HAC
is followed until landing runway alignment, plus or minus 20 degrees,
has been achieved.  In the TAEM prefinal phase, the orbiter leaves the
HAC; pitches down to acquire the steep glide slope; increases
airspeed; banks to acquire the runway centerline; and continues until
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 spacecraft comes to a
complete stop on the runway.

The approach and landing trajectory capture phase begins at the TAEM
interface and continues to guidance lock-on to the steep outer glide
slope.  The approach and landing phase begins at about 10,000 feet
altitude at an equivalent airspeed of 290, plus or minus 12, knots 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 over seven times that of a commercial
airliner's approach) aimed at a target 0.86 nautical mile (1 statute
mile) in front of the runway.  The spacecraft's speed brake is
positioned to hold the proper velocity.  The descent rate in the later
portion of TAEM and approach and landing is greater than 10,000 feet
per minute (a rate of descent approximately 20 times higher than a
commercial airliner's standard 3-degree instrument approach angle).

At 1,750 feet above ground level, a preflare maneuver is started to
position the spacecraft for a 1.5-degree glide slope in preparation
for landing with the speed brake positioned as required.  The flight
crew deploys the landing gear at this point.

The final phase reduces the sink rate of the spacecraft to less than 9
feet per second.  Touchdown occurs approximately 2,500 feet past the
runway threshold at a speed of 184 to 196 knots (213 to 226 mph).

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Space Transportation System

0..Previous Menu
1..Main Menu

2..Space Shuttle Program
3..Space Shuttle Requirements
4..Launch Sites
5..Background and Status
6..Mission Profile
7..Aborts
8..Orbiter Ground Turnaround


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ABORTS

Selection of an ascent abort mode may become necessary if there is a
failure that affects vehicle performance, such as the failure of a
space shuttle main engine or an orbital maneuvering system.  Other
failures requiring early termination of a flight, such as a cabin
leak, might require the selection of an abort mode.

There are two basic types of ascent abort modes for space shuttle
missions: intact aborts and contingency aborts.  Intact aborts are
designed to provide a safe return of the orbiter to a planned landing
site.  Contingency aborts are designed to permit flight crew survival
following more severe failures when an intact abort is not possible.
A contingency abort would generally result in a ditch operation.

There are four types of intact aborts: abort to orbit, abort once
around, transatlantic landing and return to launch site.

The ATO mode is designed to allow the vehicle to achieve a temporary
orbit that is lower than the nominal orbit.  This mode requires less
performance and allows time to evaluate problems and then choose
either an early deorbit maneuver or an orbital maneuvering system
thrusting maneuver to raise the orbit and continue the mission.

The AOA is designed to allow the vehicle to fly once around the Earth
and make a normal entry and landing.  This mode generally involves two
orbital maneuvering system thrusting sequences, with the second
sequence being a deorbit maneuver.  The entry sequence would be
similar to a normal entry.

The TAL mode is designed to permit an intact landing on the other side
of the Atlantic Ocean.  This mode results in a ballistic trajectory,
which does not require an orbital maneuvering system maneuver.

The RTLS mode involves flying downrange to dissipate propellant and
then turning around under power to return directly to a landing at or
near the launch site.

There is a definite order of preference for the various abort modes.
The type of failure and the time of the failure determine which type
of abort is selected.  In cases where performance loss is the only
factor, the preferred modes would be ATO, AOA, TAL and RTLS, in that
order.  The mode chosen is the highest one that can be completed with
the remaining vehicle performance.  In the case of some support system
failures, such as cabin leaks or vehicle cooling problems, the
preferred mode might be the one that will end the mission most
quickly.  In these cases, TAL or RTLS might be preferable to AOA or
ATO.  A contingency abort is never chosen if another abort option
exists.

The Mission Control Center-Houston is prime for calling these aborts
because it has a more precise knowledge of the orbiter's position than
the crew can obtain from onboard systems.  Before main engine cutoff,
Mission Control makes periodic calls to the crew to tell them which
abort mode is (or is not) available.  If ground communications are
lost, the flight crew has onboard methods, such as cue cards,
dedicated displays and display information, to determine the current
abort region.

Which abort mode is selected depends on the cause and timing of the
failure causing the abort and which mode is safest or improves mission
success.  If the problem is a space shuttle main engine failure, the
flight crew and Mission Control Center select the best option
available at the time a space shuttle main engine fails.

If the problem is a system failure that jeopardizes the vehicle, the
fastest abort mode that results in the earliest vehicle landing is
chosen.  RTLS and TAL are the quickest options (35 minutes), whereas
an AOA requires approximately 90 minutes.  Which of these is selected
depends on the time of the failure with three good space shuttle main
engines.

The flight crew selects the abort mode by positioning an abort mode
switch and depressing an abort push button.

RETURN TO LAUNCH SITE.

The RTLS abort mode is designed to allow the return of the orbiter,
crew, and payload to the launch site, Kennedy Space Center,
approximately 25 minutes after lift-off.  The RTLS profile is designed
to accommodate the loss of thrust from one space shuttle main engine
between lift-off and approximately four minutes 20 seconds, at which
time not enough main propulsion system propellant remains to return to
the launch site.

An RTLS can be considered to consist of three stages-a powered stage,
during which the space shuttle main engines are still thrusting; an ET
separation phase; and the glide phase, during which the orbiter glides
to a landing at the Kennedy Space Center.  The powered RTLS phase
begins with the crew selection of the RTLS abort, which is done after
solid rocket booster separation.  The crew selects the abort mode by
positioning the abort  rotary switch to RTLS and depressing the abort
push button.  The time at which the RTLS is selected depends on the
reason for the abort.  For example, a three-engine RTLS is selected at
the last moment, approximately three minutes 34 seconds  into the
mission; whereas an RTLS chosen due to an engine out at lift-off is
selected at the earliest time, approximately  two minutes   20 seconds
into the mission (after solid rocket booster separation).

After RTLS is selected, the vehicle continues downrange to dissipate
excess main propulsion system propellant.  The goal is to leave only
enough main propulsion system propellant to be able to turn the
vehicle around, fly back towards the Kennedy Space Center and achieve
the proper main engine cutoff conditions so the vehicle can glide to
the Kennedy Space Center after external tank separation.  During the
downrange phase, a pitch-around maneuver is initiated (the time
depends in part on the time of a space shuttle main engine failure) to
orient the orbiter/external tank configuration to a heads up attitude,
pointing toward the launch site.  At this time, the vehicle is still
moving away from the launch site, but the space shuttle main engines
are now thrusting to null the downrange velocity.  In addition, excess
orbital maneuvering system and reaction control system propellants are
dumped by continuous orbital maneuvering system and reaction control
system engine thrustings to improve the orbiter weight and center of
gravity for the glide phase and landing.

The vehicle will reach the desired main engine cutoff point with less
than  2 percent excess propellant remaining in the external tank.  At
main engine cutoff minus 20 seconds, a pitch-down maneuver (called
powered pitch-down) takes the mated vehicle to the required external
tank separation attitude and pitch rate.  After main engine cutoff has
been commanded, the external tank separation sequence begins,
including a reaction control system translation that ensures that the
orbiter does not recontact the external tank and that the orbiter has
achieved the necessary pitch attitude to begin the glide phase of the
RTLS.

After the reaction control system translation maneuver has been
completed, the glide phase of the RTLS begins.  From then on, the RTLS
is handled similarly to a normal entry.

TRANSATLANTIC LANDING ABORT.

The TAL abort mode was developed to improve the options available when
a space shuttle main engine fails after the last RTLS opportunity but
before the first time that an AOA can be accomplished with only two
space shuttle main engines or when a major orbiter system failure, for
example, a large cabin pressure leak or cooling system failure, occurs
after the last RTLS opportunity, making it imperative to land as
quickly as   possible.

In a TAL abort, the vehicle continues on a ballistic trajectory across
the Atlantic Ocean to land at a predetermined runway.  Landing occurs
approximately  45 minutes after launch.  The landing site is selected
near the nominal ascent ground track of the orbiter in order to make
the most efficient use of space shuttle main engine propellant.  The
landing site also must have the necessary runway length, weather
conditions and U.S.  State Department approval.  Currently, the three
landing sites that have been identified for a due east launch are
Moron,, Spain; Dakar, Senegal; and Ben Guerur, Morocco (on the west
coast of  Africa).

To select the TAL abort mode, the crew must place the abort rotary
switch in the TAL/AOA position and depress the abort push button
before main engine cutoff.  (Depressing it after main engine cutoff
selects the AOA abort mode.) The TAL abort mode begins sending
commands to steer the vehicle toward the plane of the landing site.
It also rolls the vehicle heads up before main engine cutoff and sends
commands to begin an orbital maneuvering system propellant dump (by
burning the propellants through the orbital maneuvering system engines
and the reaction control system  engines).  This dump is necessary to
increase vehicle performance (by decreasing weight), to place the
center of gravity in the proper place for vehicle control, and to
decrease the vehicle's landing weight.

TAL is handled like a nominal entry.

ABORT TO ORBIT.

An ATO is an abort mode used to boost the orbiter to a safe orbital
altitude when performance has been lost and it is impossible to reach
the planned orbital altitude.  If a space shuttle main engine fails in
a region that results in a main engine cutoff under speed, the Mission
Control Center will determine that an abort mode is necessary and will
inform the crew.  The orbital maneuvering system engines would be used
to place the orbiter in a circular  orbit.

ABORT ONCE AROUND.

The AOA abort mode is used in cases in which vehicle performance has
been lost to such an extent that either it is impossible to achieve a
viable orbit or not enough orbital maneuvering system propellant is
available to accomplish the orbital maneuvering system thrusting
maneuver to place the orbiter on orbit and the deorbit thrusting
maneuver.  In addition, an AOA is used in cases in which a major
systems problem (cabin leak, loss of cooling) makes it necessary to
land quickly.  In the AOA abort mode, one orbital maneuvering system
thrusting sequence is made to adjust the post-main engine cutoff orbit
so a second orbital maneuvering system thrusting sequence will result
in the vehicle deorbiting and landing at the AOA landing site (White
Sands, N.M.; Edwards Air Force Base; or the Kennedy Space Center).
Thus, an AOA results in the orbiter circling the Earth once and
landing approximately  90 minutes after lift-off.

After the deorbit thrusting sequence has been executed, the flight
crew flies to a landing at the planned site much as it would for a
nominal entry.

CONTINGENCY ABORT.

Contingency aborts are caused by loss of more than one main engine or
failures in other systems.  Loss of one main engine while another is
stuck at a low thrust setting may also necessitate a contingency
abort.  Such an abort would maintain orbiter integrity for in-flight
crew escape if a landing cannot be achieved at a suitable landing
field.

Contingency aborts due to system failures other than those involving
the main engines would normally result in an intact recovery of
vehicle and crew.  Loss of more than one main engine may, depending on
engine failure times, result in a safe runway landing.  However, in
most three-engine-out cases during ascent, the orbiter would have to
be ditched.  The in-flight crew escape system would be used before
ditching the orbiter.

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ORBITER GROUND TURNAROUND

Spacecraft recovery operations at the nominal end-of-mission landing
site are supported by approximately 160 space shuttle Launch
Operations team members.  Ground team members wearing self-contained
atmospheric protective ensemble suits that protect them from toxic
chemicals approach the spacecraft as soon as it stops rolling.  The
ground team members take sensor measurements to ensure the atmosphere
in the vicinity of the spacecraft is not explosive.  In the event of
propellant leaks, a wind machine truck carrying a large fan will be
moved into the area to create a turbulent airflow that will break up
gas concentrations and reduce the potential for an explosion.

A ground support equipment air-conditioning purge unit is attached to
the right-hand orbiter T-0 umbilical so cool air can be directed
through the orbiter's aft fuselage, payload bay, forward fuselage,
wings, vertical stabilizer, and orbital maneuvering system/reaction
control system pods to dissipate the heat of entry.

A second ground support equipment ground cooling unit is connected to
the left-hand orbiter T-0 umbilical spacecraft Freon coolant loops to
provide cooling for the flight crew and avionics during the
postlanding and system checks.  The spacecraft fuel cells remain
powered up at this time.  The flight crew will then exit the
spacecraft, and a ground crew will power down the spacecraft.

At the Kennedy Space Center, the orbiter and ground support equipment
convoy move from the runway to the Orbiter Processing Facility.

If the spacecraft lands at Edwards Air Force Base, the same procedures
and ground support equipment are used as at the Kennedy Space Center
after the orbiter has stopped on the runway.  The orbiter and ground
support equipment convoy move from the runway to the orbiter mate and
demate facility at Edwards Air Force Base.  After detailed inspection,
the spacecraft is prepared to be ferried atop the shuttle carrier
aircraft from Edwards Air Force Base to the Kennedy Space Center.  For
ferrying, a tail cone is installed over the aft section of the
orbiter.

In the event of a landing at an alternate site, a crew of about eight
team members will move to the landing site to assist the astronaut
crew in preparing the orbiter for loading aboard the shuttle carrier
aircraft for transport back to the Kennedy Space Center.  For landings
outside the U.S., personnel at the contingency landing sites will be
provided minimum training on safe handling of the orbiter with
emphasis on crash rescue training, how to tow the orbiter to a safe
area, and prevention of propellant conflagration.

Upon its return to the Orbiter Processing Facility at the Kennedy
Space Center, the orbiter is safed (ordnance devices safed), the
payload (if any) is removed, and the orbiter payload bay is
reconfigured from the previous mission for the next mission.  Any
required maintenance and inspections are also performed while the
orbiter is in the OPF.  A payload for the orbiter's next mission may
be installed in the orbiter's payload bay in the OPF or may be
installed in the payload bay when the orbiter is at the launch pad.

The spacecraft is then towed to the Vehicle Assembly Building and
mated to the external tank.  The external tank and solid rocket
boosters are stacked and mated on the mobile launcher platform while
the orbiter is being refurbished.  Space shuttle orbiter connections
are made and the integrated vehicle is checked and ordnance is
installed.

The mobile launcher platform moves the entire space shuttle system on
four crawlers to the launch pad, where connections are made and
servicing and checkout activities begin.  If the payload was not
installed in the OPF, it will be installed at the launch pad followed
by prelaunch activities.

Space shuttle launches from Vandenberg Air Force Base will utilize the
Vandenberg launch facility (SL6), which was built but never used for
the manned orbital laboratory program.  This facility was modified for
space transportation system use.

The runway at Vandenberg was strengthened and lengthened from 8,000
feet to 12,000 feet to accommodate the orbiter returning from space.

When the orbiter lands at Vandenberg Air Force Base, the same
procedures and ground support equipment and convoy are used as at
Kennedy Space Center after the orbiter stops on the runway.  The
orbiter and ground support equipment are moved from the runway to the
Orbiter Maintenance and Checkout Facility at Vandenberg Air Force
Base.  The orbiter processing procedures used at this facility are
similar to those used at the OPF at the Kennedy Space Center.

Space shuttle buildup at Vandenberg differs from that of the Kennedy
Space Center in that the vehicle is integrated on the launch pad.  The
orbiter is towed overland from the Orbiter Maintenance and Checkout
Facility at Vandenberg to launch facility SL6.

SL6 includes the launch mount, access tower, mobile service tower,
launch control tower, payload preparation room, payload changeout
room, solid rocket booster refurbishment facility, solid rocket
booster disassembly facility, and liquid hydrogen and liquid oxygen
storage tank facilities.

The solid rocket boosters start the on-the-launch-pad buildup followed
by the external tank.  The orbiter is then mated to the external tank
on the launch pad.

The launch processing system at the launch pad is similar to the one
used at the Kennedy Space Center.

Kennedy Space Center Launch Operations has responsibility for all
mating, prelaunch testing and launch control ground activities until
the space shuttle vehicle clears the launch pad tower.  Responsibility
is then turned over to NASA's Johnson Space Center Mission Control
Center-Houston.  The Mission Control Center's responsibility includes
ascent, on-orbit operations, entry, approach and landing until landing
runout completion, at which time the orbiter is handed over to the
postlanding operations at the landing site for turnaround and
relaunch.  At the launch site the solid rocket boosters and external
tank are processed for launch and the solid rocket boosters are
recycled for reuse.

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