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PRE-LAUNCH OPERATIONS<\/p>\n
After the Space Shuttle has been rolled out to the launch pad on the
\nMobile Launcher Platform (MLP), all pre-launch activities are
\ncontrolled from the Launch Control Center (LCC).<\/p>\n
After the Shuttle is in place on the launch pad support columns, and
\nthe Rotating Service Structure (RSS) is placed around it, power for
\nthe vehicle is activated. The MLP and the Shuttle are then
\nelectronically and mechanically mated with support launch pad
\nfacilities and ground support equipment. An extensive series of
\nvalidation checks verify that the numerous interfaces are functioning
\nproperly.<\/p>\n
Meanwhile, in parallel with pre-launch pad activities, cargo
\noperations get underway in the RSS’s Payload Changeout Room.<\/p>\n
Vertically integrated payloads are delivered to the launch pad
\nbefore the Shuttle is rolled out. They are stored in the Payload
\nChangeout Room until the Shuttle is ready for cargo loading. Once
\nthe RSS is in place around the orbiter, the payload bay doors are
\nopened and the cargo is installed. Final cargo and payload bay
\ncloseouts are completed in the Payload Changeout Room and the payload
\nbay doors are closed for flight.<\/p>\n
Pre-launch Propellant Loading. Initial Shuttle propellant loading
\ninvolves pumping hypergolic propellants into the orbiter’s aft and
\nforward Orbital Maneuvering System and Reaction Control System
\nstorage tanks, the orbiter’s hydraulic Auxiliary Power Units, and SRB
\nhydraulic power units. These are hazardous operations, and while
\nthey are underway work on the launch pad is suspended.<\/p>\n
Since these propellants are hypergolic — that is they ignite on
\ncontact with one another–oxidizer and fuel loading operations are
\ncarried out serially, never in parallel.<\/p>\n
Finally, dewar tanks on the Fixed Service Structure (FSS), are
\nfilled with liquid oxygen and liquid hydrogen, which will be loaded
\ninto the orbiter’s Power Reactant and Storage Distribution (PRSD)
\ntanks during the launch countdown.<\/p>\n
Final Pre-launch Activities. Before the formal Space Shuttle launch
\ncountdown starts, the vehicle is powered down while pyrotechnic
\ndevices — various ordinance components — are installed or hooked
\nup. The extravehicular Mobility Units (EMUs) — space suits — are
\nstored On Board along with other items of flight crew equipment.<\/p>\n
When closeouts of the Space Shuttle and the launch pad are
\ncompleted, all is in readiness for the countdown to get underway.<\/p>\n
Launch Control Center. While the VAB can be considered the heart of
\nLC-39, the Launch Control Center (LCC) can easily be called its brain.<\/p>\n
The LCC is a 4-story building connected to the east side of the VAB
\nby an elevated, enclosed bridge. It houses four firing rooms that
\nare used to conduct NASA and classified military launches of the
\nSpace Shuttle. Each firing room is equipped with the Launch
\nProcessing System (LPS) which monitors and controls most Shuttle
\nassembly, checkout and launch operations. Physically, the LCC is 77
\nft. high, 378 ft. long and 181 ft. wide.<\/p>\n
Thanks to the LPS, the countdown for the Space Shuttle takes only
\nabout 40 hours, compared with the 80 plus hours usually needed for a
\nSaturn\/Apollo countdown. Moreover, the LPS calls for only about 90
\npeople to work in the firing room during launch operations —
\ncompared with about 450 needed for earlier manned missions.<\/p>\n
From the outside, the LCC is virtually unchanged from its original
\nApollo-era configuration, except that a fourth floor office has been
\nadded to the southwest and northwest corners corner of the building.<\/p>\n
The interior of the LCC has undergone extensive modifications to
\nmeet the needs of the Space Shuttle era.<\/p>\n
Physically, the LCC is constructed as follows: the first floor is
\nused for administrative activities and houses the building’s
\nutilities systems control room; the second floor is occupied by the
\nControl Data Subsystem; the four firing rooms occupy practically all
\nof the third floor, and the fourth floor, as mentioned, earlier is
\nused for offices.<\/p>\n
During the Shuttle Orbital Flight Test program and the early
\noperational missions, Firing Room l was the only fully-equipped
\ncontrol facility available for vehicle checkout and launch. However,
\nas the Shuttle launch rate increased during the first half of the
\n1980s, the other three firing rooms were activated. Although NASA
\noperates the firing rooms, the Department of Defense uses Firing
\nRooms 3 and 4 to support its classified, Shuttle-dedicated missions.
\nAdditionally, Firing Room 4 serves as an engineering analysis and
\nsupport facility for launch and checkout operations.<\/p>\n
Launch Countdown. As experience was gained by launch crews during
\nthe early years of the Space Shuttle program, the launch countdown
\nwas refined and streamlined to the point where the average countdown
\nnow takes a little more than 40 hours. This was not the case early
\nin the program, when countdowns of 80 hours or more were not uncommon.<\/p>\n
The following is a narrative description of the major events of a
\ntypical countdown for the Space Shuttle. The time of liftoff is
\npredicated on what is called the launch window — that point in time
\nwhen the Shuttle must be launched in order to meet specific mission
\nobjectives such as the deployment of spacecraft at a predetermined
\ntime and location in space.<\/p>\n
Launch Minus 3 Days. The countdown gets underway with the
\ntraditional call to stations by the NASA Test Director. This
\nverifies that the launch team is in place and ready to proceed.<\/p>\n
The first item of business is to checkout the backup flight system
\nand the software stored in the mass memory units and display systems.
\n Backup flight system software is then loaded into the Shuttle’s
\nfifth general purpose computer (GPC’s).<\/p>\n
Flight crew equipment stowage begins. Final inspection of the
\norbiter’s middeck and flight decks are made, and removal of work crew
\nmodule platforms begin. Loading preparations for the external tank
\nget underway, and the Shuttle main engines are readied for tanking.
\nServicing of fuel cell storage tanks also starts. Final vehicle and
\nfacility closeouts are made.<\/p>\n
Launch Minus 2 Days. The launch pad is cleared of all personnel
\nwhile liquid oxygen and hydrogen are loaded into the Shuttle fuel
\ncell storage tanks. Upon completion, the launch pad area is reopened
\nand the closeout crew continues its prelaunch preparations.<\/p>\n
The orbiter’s flight control, navigation and communications systems
\nare activated. Switches located on the flight and mid- decks are
\nchecked and, if required, mission specialist seats are installed.
\nPreparations also are made for rollback of the Rotating Service
\nStructure (RSS).<\/p>\n
At launch minus ll hours a planned countdown hold — called a
\nbuilt-in hold — begins and can last for up to 26 hours, 16 minute
\ndepending on the type of payload, tests required and other factors.
\nThis time is used, if needed, to perform tasks in the countdown that
\nmay not have been completed earlier.<\/p>\n
Launch Minus 1 Day. Countdown is resumed after the built-in hold
\nperiod has elapsed. The RSS is rolled back and remaining items of
\ncrew equipment are installed. Cockpit switch positions are verified,
\nand oxygen samples are taken in the crew area. The fuel cells are
\nactivated following a fuel cell flow through purge. Communications
\nwith the Johnson Space Center’s Mission Control Center (MCC) are
\nestablished.<\/p>\n
Finally, the launch pad is again cleared of all personnel while
\nconditioned air that has been blowing through the payload bay and
\nother orbiter cavities is switched to inert gaseous nitrogen in
\npreparation for filling the external tank with its super-cold
\npropellants.<\/p>\n
Launch Day. Filling the external tank with liquid oxygen and
\nhydrogen gets underway. Communications checks are made with elements
\nof the Air Force’s Eastern Space and Missile Center. Gimbal profile
\nchecks of the Orbital Maneuvering System (OMS) engines are made.
\nPreflight calibration of the Inertial Measurement Units (IMU) is
\nmade, and tracking antennas at the nearby Merritt Island Tracking
\nStation are aligned for liftoff.<\/p>\n
At launch minus 5 hours, 20 minutes — T minus 5 hours, 20 minutes
\n— a 2-hour built-in hold occurs. During this hold, an ice
\ninspection team goes to the launch pad to inspect the external tank’s
\ninsulation to insure that there is no dangerous accumulation of ice
\non the tank caused by the super-cold liquids. Meanwhile, the
\ncloseout crew is preparing for the arrival of the flight crew.<\/p>\n
Meanwhile, the flight crew, in their quarters at the Operations and
\nCheckout (O&C) Building, eat a meal and receive a weather briefing.
\nAfter suiting up, they leave the O&C Building at about T minus 2
\nhours, 30 minutes for the launch pad — the countdown having resumed
\nat T minus 3 hours.<\/p>\n
Upon arriving at the white room at the end of the orbiter access
\narm, the crew, assisted by white room personnel, enter the orbiter.
\nOnce on board they conduct air-to-ground communications checks with
\nthe LCC and MCC. Meanwhile, the orbiter hatch is closed and hatch
\nseal and cabin leak checks are made. The IMU preflight alignment is
\nmade and closed-loop tests with Range Safety are completed. The
\nwhite room is then evacuated and the closeout crew proceeds from the
\nlaunch pad to a fallback area. At this time, primary ascent guidance
\ndata is transferred to the backup flight system.<\/p>\n
At T minus 20 minutes a planned 10-minute hold begins. When the
\ncountdown is resumed on-board computers are commanded to their launch
\nconfiguration and fuel cell thermal conditioning begins. Orbiter
\ncabin vent valves are closed and the backup flight system transitions
\ninto its launch configuration.<\/p>\n
At T minus 9 minutes another planned 10-minute hold occurs. Just
\nprior to resuming the countdown, the NASA Test Director gets the “go
\nfor launch” verification from the launch team. At this point, the
\nGround Launch Sequencer (GLS) is turned on and the terminal countdown
\nstarts. All countdown functions are now automatically controlled by
\nthe GLS computer located in the Firing Room Integration Console.<\/p>\n
At T minus 7 minutes, 30 seconds, the orbiter access arm is
\nretracted. Should an emergency occur requiring crew evacuation from
\nthe orbiter, the arm can be extended either manually or automatically
\nin about 15 seconds.<\/p>\n
At T minus 5 minutes, 15 seconds the MCC transmits a command that
\nactivates the orbiter’s operational instrumentation recorders. These
\nrecorders store information relating to ascent, on-orbit and descent
\nperformance during the mission. These data are analyzed after
\nlanding.<\/p>\n
At T minus 5 minutes, the crew activates the Auxiliary Power Units
\n(APU) to provide pressure to the Shuttle’s three hydraulic systems
\nwhich move the main engine nozzles and the aero-aerosurfaces. Also
\nat this point, the firing circuit for SRB ignition and the range
\nsafety destruct system devices are mechanically enabled by a
\nmotor-driven switch called the safe and arm device.<\/p>\n
At about T minus 4 minutes, 55 seconds, the liquid oxygen vent on
\nthe external tank is closed. It had been open to allow the
\nsuper-cold liquid oxygen to boil off, thus preventing over
\npressurization while the tank remained near its full level. Now,
\nwith the vent closed, preparations are made to bring the tank to its
\nflight pressure. This occurs at T minus 2 minutes, 55 seconds.<\/p>\n
At T minus 4 minutes the final helium purge of the Shuttle’s three
\nmain engines is initiated in preparation for engine start. Five
\nseconds later, the orbiter’s elevons, speed brakes and rudder are
\nmoved through a pre-programmed series of maneuvers to position them
\nfor launch. This is called the aerosurface profile.<\/p>\n
At T minus 3 minutes, 30 seconds, the ground power transition takes
\nplace and the Shuttle’s fuel cells transition to internal power. Up
\nto this point, ground power had augmented the fuel cells. Then, 5
\nseconds later, the main engine nozzles are gimballed through a
\npre-programmed series of maneuvers to confirm their readiness.<\/p>\n
At T minus 2 minutes, 50 seconds, the external tank oxygen vent hood
\n— known as the beanie cap — is raised and retracted. It had been
\nin place during tanking operations to prevent ice buildup on the
\noxygen vents. Fifteen seconds later, at T minus 2 minutes, 35
\nseconds, the piping of gaseous oxygen and hydrogen to the fuel cells
\nfrom ground tanks is terminated and the fuel cells begin to use the
\non board reactants.<\/p>\n
At T minus 1 minute, 57 seconds, the external tank’s liquid hydrogen
\nis brought to flight pressure by closing the boil off vent, as was
\ndone earlier with the liquid oxygen vent. However, during the
\nhydrogen boil off of, the gas is piped out to an area adjacent to the
\nlaunch pad where it is burned off.<\/p>\n
At T minus 31 seconds, the Shuttle’s on-board computers start their
\nterminal launch sequence. Any problem after this point will require
\ncalling a “hold” and the countdown recycled to T minus 20 minutes.
\nHowever, if all goes well, only one further ground command is needed
\nfor launch. This is the “go for main engine start,” which comes at
\nthe T-minus-10-second point. Meanwhile, the Ground Launch Sequencer
\n(GLS) continues to monitor more than several hundred launch commit
\nfunctions and is able automatically to call a “hold” or “cutoff” if a
\nproblem occurs.<\/p>\n
At T minus 28 seconds the SRB booster hydraulic power units are
\nactivated by a command from the GLS. The units provide hydraulic
\npower for SRB nozzle gimballing. At T minus 16 seconds, the nozzles
\nare commanded to carry out a pre-programmed series of maneuvers to
\nconfirm they are ready for liftoff. At the same time — T minus 16
\nseconds — the sound suppression system is turned on and water begins
\nto pour onto the deck of the MLP and pad areas to protect the
\nShuttle from acoustical damage at liftoff.<\/p>\n
At T minus ll seconds, the SRB range safety destruct system is
\nactivated.<\/p>\n
At T minus 10 seconds, the “go for main engine start” command is
\nissued by the GLS. (The GLS retains the capability to command main
\nengine stop until just before the SRBs are ignited.) At this time
\nflares are ignited under the main engines to burn away any residual
\ngaseous hydrogen that may have collected in the vicinity of the main
\nengine nozzles. A half second later, the flight computers order the
\nopening of valves which allow the liquid hydrogen and oxygen to flow
\ninto the engine’s turbopumps.<\/p>\n
At T minus 6.6 seconds, the three main engines are ignited at
\nintervals of 120 milliseconds. The engines throttle up to 90 percent
\nthrust in 3 seconds. At T minus 3 seconds, if the engines are at the
\nrequired 90 percent, SRB ignition sequence starts. All of these
\nsplit-second events are monitored by the Shuttle’s four primary
\nflight computers.<\/p>\n
At T minus zero, the holddown explosive bolts and the T-O umbilical
\nexplosive bolts are blown by command from the on-board computers and
\nthe SRBs ignite. The Shuttle is now committed to launch. The
\nmission elapsed time is reset to zero and the mission event timer
\nstarts. The Shuttle lifts off the pad and clears the tower at about
\nT plus 7 seconds. Mission control is handed over to JSC after the
\ntower is cleared.<\/p>\n
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MARSHALL PAYLOAD OPERATIONS CONTROL CENTER<\/p>\n
The Payload Operations Control Center (POCC) operated by the NASA’s
\nMarshall Space Flight Center (MSFC), Huntsville, Ala., is the largest
\nand most diverse of the various POCCs associated with the Space
\nShuttle program. Since its functions in many respects parallel those
\nof other POCCs operated by private industry, the academic community
\nand government agencies, a description of what it does, how it
\noperates and who operates it will serve as an overview of this type
\nof control center.<\/p>\n
The Marshall POCC — like all POCCs — is a facility designed to
\nmonitor, coordinate, and control on-orbit operation of a Shuttle
\npayload, particularly Spacelab. During non-mission periods it also
\nis used for crew training and simulated space operations. It is, in
\neffect, a command post for payload activities, just as the JSC
\nMission Control Center (MCC) is a command post for the flight and
\noperation of the Space Shuttle.<\/p>\n
Both control centers work closely in coordinating mission
\nactivities. In fact, the Marshall POCC originally was housed in
\nBuilding 30 at JSC, adjacent to the MCC. It has since been moved to
\nBuilding 4663 at Marshall and is an important element of the
\nHunstville Operations Support Center (HOSC), which augments the MCC
\nby monitoring Shuttle propulsion systems.<\/p>\n
The Marshall POCC Capabilities Document states that the “POCC
\nprovides physical space, communications, and data system capabilities
\nto enable user access to payload data (digital, video, and analog),
\ncommand uplink, and coordination of activities internal and external
\nto the POCC.”<\/p>\n
Members of the Marshall mission management team and principal
\ninvestigators and research teams work in the POCC or in adjacent
\nfacilities around-the-clock controlling and directing payload
\nexperiment operations. Using the extensive POCC facilities they are
\nable to communicate directly with mission crews and direct experiment
\nactivities from the ground. They also can operate experiments and
\nsupport equipment on board the Shuttle and manage payload resources.<\/p>\n
The POCC operations concept requires a team consisting of the
\nPayload Mission Manager (PMM) directing the POCC cadre which has
\noverall responsibility for managing and controlling POCC operations.
\nIts scientific counterpart, the investigator’s operations team, is
\nthe group that conducts, monitors and controls the experiments
\ncarried on the Shuttle, primarily those related to Spacelab.<\/p>\n
Generally, POCC operations are carried out by a
\nmanagement\/scientific team of 10 key individuals, headed by the
\nPayload Operations Director (POD), who is a senior member of the
\nPMM’s cadre. The POD is charged with managing the day-to-day mission
\noperations and directing the payload operations team and the science
\ncrew.<\/p>\n
Other POCC key personnel include:<\/p>\n
MISSION SCIENTIST (MSCI) who represents scientists who have
\nexperiments on a specific flight and serves as the interface between
\nthe PMM and the POD in matters relating to mission science operations
\nand accomplishments.<\/p>\n
CREW INTERFACE COORDINATOR (CIC), who coordinates communications
\nbetween the POCC and the payload crew.<\/p>\n
ALTERNATE PAYLOAD SPECIALIST (APS) is a trained payload specialist
\nnot assigned to flight duty who aids the payload operations team and
\nthe payload crew in solving problems, troubleshooting and modifying
\ncrew procedures, if necessary, and who advises the MSCI on the
\npossible impact of any problem areas.<\/p>\n
PAYLOAD ACTIVITY PLANNER (PAP), who directs mission replanning
\nactivities, as required, and coordinates mission timeline changes
\nwith POCC personnel.<\/p>\n
MASS MEMORY UNIT MANAGER (MUM) who sends experiment command uplinks
\nto the flight crew based on data received from the POCC operations
\nteam.<\/p>\n
OPERATIONS CONTROLLER (OC), who coordinates activities of the
\npayload operations team to insure the efficient accomplishment of
\nactivities supporting real-time execution of the mission timeline.<\/p>\n
PAYLOAD COMMAND COORDINATOR (PAYCOM), who configures the POCC for
\nground command operation and controls the flow of experiment commands
\nfrom the POCC to the flight crew.<\/p>\n
DATA MANAGEMENT COORDINATOR (DMC), who is responsible for
\nmaintaining and coordinating the flow of payload experiment data to
\nand within the POCC the DMC also assesses the impact of proposed
\nchanges to the experiment timeline and payload data requirements that
\naffect the payload downlink data.<\/p>\n
PUBLIC AFFAIRS OFFICER (PAO), who provides mission commentary on
\npayload activities and serves as the primary source of information on
\nmission progress to the news media and public.<\/p>\n
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SPACE TRACKING AND DATA ACQUISITION<\/p>\n
Responsibility for Space Shuttle tracking and data acquisition is
\ncharged to the Goddard Space Flight Center, Greenbelt, Md. This
\ninvolves integrating and coordinating all of the worldwide NASA and
\nDepartment of Defense tracking facilities needed to support Space
\nShuttle missions.<\/p>\n
These facilities include the Goddard-operated Ground Network (GN)
\nand Space Network (SN); the Deep Space Network (DSN) managed for NASA
\nby the Jet Propulsion Laboratory (JPL), Pasadena, Calif.; the
\nAmes-Dryden Flight Research Facility, (ADFRC) Edwards, Calif.; and
\nextensive Department of Defense tracking systems at the Eastern and
\nWestern Space and Missile Centers, as well as the Air Force Satellite
\nControl Network’s (AFSCN) remote tracking stations.<\/p>\n
Ground Network. The Ground Network (GN) is a worldwide network of
\ntracking stations and data-gathering facilities which support Space
\nShuttle missions and also maintain communications with low
\nEarth-orbiting spacecraft. Station management is provided from the
\nNetwork Control Center at Goddard. Basically, commands are sent to
\norbiting spacecraft from the GN stations and, in return, scientific
\ndata are transmitted to the stations.<\/p>\n
The system consists of 12 stations, including three DSN facilities.
\nGN stations are located at Ascension Island, a British Crown Colony
\nin the south Atlantic Ocean; Santiago, Chile; Bermuda; Dakar,
\nSenegal, on the West Coast of Africa; Guam; Hawaii; Merritt Island,
\nFla.; Ponce de Leon, Fla.; and the Wallops Flight Facility on
\nVirginia’s Eastern Shore. The DSN tracking stations are located at
\nCanberra, Australia; Goldstone, Calif.; and Madrid, Spain.<\/p>\n
The GN stations are equipped with a wide variety of tracking and
\ndata-gathering antennas, ranging in size from 14 to 85 feet in
\ndiameter. Each is designed to perform a specific task, normally in a
\ndesignated frequency band, gathering radiated electronic signals
\n(telemetry) transmitted from spacecraft.<\/p>\n
The communications hub for the GN is the Goddard-operated NASA
\nCommunications Center (NASCOM). It consists of more than 2 million
\nmiles of electronic circuitry linking the tracking stations and the
\nMCC at the Johnson Space Center. NASCOM has six major switching
\ncenters to insure the prompt flow of data. In addition to Goddard
\nand JSC, the other switching centers are located at JPL, KSC,
\nCanberra and Madrid.<\/p>\n
The system includes telephone, microwave, radio, submarine cable and
\ngeosynchronous communications satellites in ll countries. It
\nincludes communications facilities operated by 15 different domestic
\nand foreign carriers. The system also has a wide-band and video
\ncapability. In fact, Goddard’s wide-band system is the largest in
\nthe world.<\/p>\n
A voice communications system called Station Conferencing and
\nMonitoring Arrangement (SCAMA) can conference link up hundreds of the
\n220 different voice channels throughout the United States and abroad
\nwith instant talk\/listen capability. With its built-in redundancy,
\nSCAMA has realized a mission support reliability record of 99.6
\npercent. The majority of Space Shuttle voice traffic is routed
\nthrough Goddard to the MCC.<\/p>\n
As would be expected, computers play an important role in GN
\noperations. They are used to program tracking antenna pointing
\nangles, send commands to orbiting spacecraft and process data which
\nis sent to the JSC and Goddard control centers.<\/p>\n
Shuttle data is sent from the tracking network to the main switching
\ncomputers at GSFC. These are UNISYS 1160 computers which reformat
\nand transmit the information to JSC almost instantaneously at a rate
\nof l.5 million bits per second, via domestic communications
\nsatellites.<\/p>\n
Space Network. Augmenting the GN and eventually replacing it, is a
\nunique tracking network called the Space Network (SN). The
\nuniqueness of this network is that instead of tracking the Shuttle
\nand other Earth-orbiting spacecraft from a world-wide network of
\nground stations, its main element is an in-orbit series of satellites
\ncalled the Tracking and Data Relay Satellite System (TDRSS), designed
\nto gather tracking and data information from geosynchronous orbit and
\nrelay it to a single ground terminal located at White Sands, N.M.<\/p>\n
The first spacecraft in the TDRS system, TDRS-1, was deployed from
\nthe Space Shuttle Challenger on April 4, 1983. Although problems
\nwere encountered in establishing its geosynchronous orbit at 41
\ndegrees west longitude (over the northeast corner of Brazil), TDRS-l
\nproved the feasibility of the tracking station-in-space concept when
\nit became operational later in the year.<\/p>\n
Ultimately, the SN will consist of three TDRS spacecraft in orbit,
\none of which will be a backup or spare to be available for use if one
\nof the operational spacecraft fails. Each satellite in the TDRS
\nsystem is designed to operate for 10-years.<\/p>\n
Following its planned deployment from the Space Shuttle Discovery
\nscheduled for the STS-26 mission, TDRS-2 will be tested and then
\npositioned in a geosynchronous orbit southwest of Hawaii at 171
\ndegrees west longitude, about 130 degrees from TDRS-1. With these
\ntwo spacecraft and the White Sands Ground Terminal (and eventually a
\nbackup terminal) operational, the SN will be able to provide almost
\nfull-time communications and tracking of the Space Shuttle, as well
\nas for up to 24 other Earth-orbiting spacecraft simultaneously. The
\nglobal network of ground stations can provide only about 20 percent
\nof that coverage. Eventually some of the current ground stations
\nwill be closed when the SN becomes fully operational.<\/p>\n
After data acquired by the TDRS spacecraft are relayed to the White
\nSands Ground Terminal, they are sent directly by domestic
\ncommunications satellite to NASA control centers at JSC for Space
\nShuttle operations, and to Goddard which schedules TDRSS operations
\nincluding those of many unmanned satellites.<\/p>\n
The TDRS are among the largest and most advanced communications
\nsatellites ever developed. They weigh almost 5,000 lb. and measure
\n57 ft. across at their solar panels. They operate in the S-band and
\nKu-band frequencies and their complex electronics systems can handle
\nup to 300 million bits of information each second from a single user
\nspacecraft. Among the distinguishing features of the spacecraft are
\ntheir two huge, wing-like solar panels which provide l,850 watts of
\nelectric power and their two 16-ft. diameter high-gain parabolic
\nantennas which resemble large umbrellas. These antennas weigh about
\n50 lb. each.<\/p>\n
The communications capability of the TDRSS covers a wide spectrum
\nthat includes voice, television, analog and digital signals. No
\nsignal processing is done in orbit. Instead, the raw data flows
\ndirectly to the ground terminal. During Space Shuttle missions,
\nmission data and commands pass almost continuously back and forth
\nbetween the orbiter and the MCC at JSC.<\/p>\n
Like the TDRS, the White Sands ground terminal is one of the most
\nadvanced in existence. Its most prominent features include three
\n60-ft.-diameter Ku-band antennas which receive and transmit data. A
\nnumber of smaller antennas are used for S-band and other Ku-band
\ncommunications.<\/p>\n
Ground was broken in September 1987, for a second back-up ground
\nterminal at White Sands to accommodate increased future mission
\nsupport required from the TDRSS.<\/p>\n
The TDRSS segment of the Space Network, including the ground
\nterminal, is owned and operated for NASA by CONTEL Federal Systems
\nSector, Atlanta, Ga. The spacecraft are built the TRW Federal
\nSystems Division, Space and Technology Group, Redondo Beach, Calif.
\nTRW also provides software support for the White Sands facility.
\nThe TDRS parabolic antennas are built by the Harris Corp’s Government
\nCommunications Systems Division, Melbourne, Fla. Harris also
\nprovides ground antennas, radio frequency equipment and other ground
\nterminal equipment.<\/p>\n
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FLIGHT OPERATIONS<\/p>\n
The Space Shuttle, as it thunders away from the launch pad with its
\nmain engines and solid rocket boosters (SRB) at full power, is an
\nunforgettable sight. It reaches the point of maximum dynamic
\npressure (max Q) — when dynamic pressures on the Shuttle are
\ngreatest — about 1 minute after liftoff, at an altitude of 33,600
\nft. At this point the main engines are “throttled down,” to about 75
\npercent, thus keeping the dynamic pressures on the vehicle’s surface
\nto about 580 lb. per square foot. After passing through the max Q
\nregion, the main engines are throttled up to full power. This early
\nascent phase is often referred to as “first stage” flight.<\/p>\n
Little more than 2 minutes into the flight, the SRBs, their fuel
\nexpended, are jettisoned from the orbiter. The Shuttle is at an
\naltitude of about 30 miles and traveling at a speed of 2,890 miles an
\nhour. The spent SRB casings continue to gain altitude briefly before
\nthey begin falling back to Earth. When the spent casings have
\ndescended to an altitude of about 17,000 ft., the parachute
\ndeployment sequence starts, slowing them for a safe splashdown in the
\nocean. This occurs about 5 minutes after launch. The boosters are
\nretrieved, returned to a processing facility for refurbishment and
\neventual reused.<\/p>\n
Meanwhile, the “second stage” phase of the flight is underway with
\nthe main engines propelling the vehicle ever higher on its ascent
\ntrajectory. At about 8 minutes into the flight, at an altitude of
\nabout 60 miles, main engine cut-off (MECO) occurs. The Shuttle is
\nnow traveling at a speed of 16,697 mph.<\/p>\n
After MECO, the orbiter and the external tank are moving along a
\ntrajectory that, if not corrected, would result in the vehicle
\nentering the atmosphere about halfway around the world from the
\nlaunch site. However, a brief firing of the orbiter’s two Orbital
\nManeuvering System (OMS) thrusters changes the trajectory and orbit
\nis achieved. This takes place just after the external tank has been
\njettisoned and while the orbiter is flying “upside down” in relation
\nto Earth.<\/p>\n
The separated external tank continues on a ballistic trajectory and
\nenters the Earth’s atmosphere to break up over a remote area of the
\nIndian Ocean. Meanwhile, an additional firing of the OMS thrusters
\nplaces the orbiter into its planned orbit, which can range from 115
\nto 600 miles above the Earth.<\/p>\n
There are two ways in which orbit can be accomplished. These are
\nthe conventional OMS insertion method called “standard” and the
\ndirect insertion method.<\/p>\n
The OMS insertion method involves a brief burn of the OMS engines
\nshortly after MECO, placing the orbiter into an elliptical orbit. A
\nsecond OMS burn is initiated when the orbiter reaches apogee in its
\nelliptical orbit. This brings the orbiter into a near circular
\norbit. If required during a mission, the orbit can be raised or
\nlowered by additional firings of the OMS thrusters.<\/p>\n
The direct insertion technique uses the main engines to achieve the
\ndesired orbital apogee, or high point, thus saving OMS propellant.
\nOnly one OMS burn is required to circularize the orbit, and the
\nremaining OMS fuel can then be used for frequent changes in the
\noperational orbit, as called for in the flight plan.<\/p>\n
The first direct insertion orbit was accomplished during the STS
\n41-C mission in April 1984, when the Challenger was placed in a
\n288-mile-high circular orbit where its flight crew was able to
\nsuccessfully capture, repair and redeploy a free-flying spacecraft,
\nthe Solar Maximum satellite (Solar Max) — an important “first” for
\nthe Space Shuttle program.<\/p>\n
Launch Abort Modes. During the ascent phase of a Space Shuttle
\nflight, if a situation occurs that puts the mission in jeopardy —
\nthe loss, for example, of one or more of the main engines or the OMS
\nthrusters — the mission may have to be aborted. During the ascent
\nphase, there are two basic Shuttle abort modes: intact aborts and
\ncontingency aborts. NASA has attempted to anticipate all possible
\nemergency situations that could occur, and mission plans are prepared
\naccordingly.<\/p>\n
Intact aborts — there are four different types — permit the safe
\nreturn of the orbiter and its crew to a pre-planned landing site.<\/p>\n
When an intact abort is not possible, the contingency abort option
\nbecomes necessary. This crucial abort mode is designed to permit
\ncrew survival following a severe systems failure in which the vehicle
\nis lost. Generally, if a contingency abort becomes necessary, the
\ndamaged vehicle would fall toward the ocean and the crew would
\nexercise escape options that were developed in the aftermath of the
\nChallenger accident. The four intact abort modes are:<\/p>\n
Return to Launch Site (RTLS)<\/p>\n
Trans-Atlantic Abort Landing (TAL)<\/p>\n
Abort Once Around (AOA)<\/p>\n
Abort to Orbit (ATO)<\/p>\n
Since an intact abort could result in an emergency landing, before
\neach flight, potential contingency landing sites are designated and
\nweather conditions at these locations are monitored closely before a
\nlaunch. Space Shuttle flight rules include provisions for minimum
\nacceptable weather conditions at these potential landing sites in the
\nevent of intact abort is necessary.<\/p>\n
In an abort situation, the type and time of the failure determines
\nwhich abort mode is possible. There is a definite order of
\npreference for an abort. In cases where performance loss is the only
\nfactor, the preferred modes would be ATO, AOA, TAL or RTLS, in that
\norder. The mode selected normally would be the highest preferred one
\nthat can be completed with the remaining vehicle performance.<\/p>\n
In the case of an extreme system failure — the loss of cabin
\npressure or orbiter cooling systems — the preferred mode would be
\nthe one that would terminate the mission as quickly as possible.
\nThis means that the TAL or RTLS modes would be more preferable than
\nother modes.<\/p>\n
An ascent abort during powered flight can be initiated by turning a
\nrotary switch on a panel in the orbiter cockpit. The switch is
\naccessible to both the commander and the pilot. Normally, flight
\nrules call for the abort mode selection to be made by the commander
\nupon instructions from the Mission Control Center. Once the abort
\nmode is selected, the on board computers automatically initiate abort
\naction for that particular abort.<\/p>\n
A description of the intact abort modes follows.<\/p>\n
RETURN TO LAUNCH SITE (RTLS). The RTLS abort is a critical and
\ncomplex one that becomes necessary if a main engine failure occurs
\nafter liftoff and before the point where a TAL or AOA is possible.
\nRTLS cannot be initiated until the SRBs have completed their normal
\nburn and have been jettisoned. Meanwhile, the orbiter with the
\nexternal tank still attached continues on its downrange trajectory
\nwith the remaining operational main engines, the two OMS and four aft
\nRCS thrusters firing until the remaining main engine propellent
\nequals the amount needed to reverse the direction of flight and
\nreturn for a landing. A “pitch-around” maneuver of about 5 degrees
\nper second is then performed to place the orbiter and the external
\ntank in an attitude pointing back toward the launch site. OMS fuel
\nis dumped to adjust the orbiter’s center of gravity.<\/p>\n
When altitude, attitude, flight path angle, heading, weight, and
\nvelocity\/range conditions combine for external tank jettisoning, MECO
\nis commanded, and the external tank separates and falls into the
\nocean. After this, the orbiter should glide to a landing at the
\nlaunch site landing facility. From the foregoing, it can be
\nappreciated why RTLS is the least preferred intact abort mode.<\/p>\n
TRANS-ATLANTIC ABORT LANDING (TAL). The TAL abort mode is designed
\nto permit an intact landing after the Shuttle has flown a ballistic
\ntrajectory across the Atlantic Ocean and lands at a designated
\nlanding site in Africa or Spain. This abort mode was developed for
\nthe first Shuttle launch in April 1981, and has since evolved from a
\ncrew-initiated manual procedure to an automatic abort mode. The TAL
\ncapability provides an abort option between the last RTLS opportunity
\nup to the point in ascent known as the “single-engine press to MECO”
\ncapability –meaning that the orbiter has sufficient velocity to
\nachieve main engine cutoff and abort to orbit, even if two main
\nengines are shut down. TAL also can be selected if other system
\nfailures occur after the last RTLS opportunity. The TAL abort mode
\ndoes not require any OMS maneuvers.<\/p>\n
Landing sites for a TAL vary from flight to flight, depending on
\nthe launch azimuth. For the first three Space Shuttle missions, the
\ntrajectory inclination was about 28 degrees which made the U.S. Air
\nForce bases at Zaragoza and Moron in Spain, the most ideal landing
\nsites for TAL. Later Shuttle missions called for air fields at
\nDakar, Senegal, and Casablanca, Morocco, as TAL-option landing sites.
\n In March 1988, NASA announced that in addition to the TAL sites in
\nSpain, that two new African contingency landing sites had been
\nselected for future Shuttle missions: a site near Ben Guerir,
\nMorocco, about 40 miles north of Marrakesh with a 14,000-foot runway;
\nand at Banjul, the capital of the west African nation of The Gambia,
\nwhich has an international airfield with an ll,800-foot runway.<\/p>\n
ABORT ONCE AROUND (AOA). This abort mode becomes available about 2
\nminutes after SRB separation, up to the point just before an abort to
\norbit is possible. AOA normally would be called for because of a
\nmain engine failure. This abort mode allows the Shuttle to fly once
\naround the Earth and make a normal entry and landing at Edwards AFB,
\nCalif., or White Sands Space Harbor, near Las Cruces, N.M. An AOA
\nabort usually would require two OMS burns, the second burn being a
\ndeorbit maneuver.<\/p>\n
There are two different AOA entry trajectories. These are the
\nso-called normal AOA and the shallow. The entry trajectory for the
\nnormal AOA, is similar to a normal end-of-mission landing. The
\nshallow AOA, on the other hand, results in a flatter entry
\ntrajectory, which is less desirable but uses less propellant for the
\nOMS burn. The shallow trajectory also is less desirable because it
\nexposes the orbiter to a longer period of atmospheric entry heating
\nand to less predictable aerodynamic drag forces.<\/p>\n
ABORT TO ORBIT (ATO). The ATO mode is the most benign of the
\nvarious abort modes. ATO allows the orbiter to achieve a temporary
\norbit that is lower than the planned. ATO is usually necessary
\nbecause of a main engine failure. It places fewer performance
\ndemands on the orbiter. It also gives ground controllers and the
\nflight crew time to evaluate the problem. Depending on the
\nseriousness of the situation, one ATO option is to make an early
\ndeorbit and landing. If there are no major problems, other than the
\nmain engine one, an OMS maneuver is made to raise the orbit and the
\nmission is continued as planned.<\/p>\n
The first Space Shuttle program ATO occurred on July 29, 1985,
\nfollowing the STS 51-F Challenger launch, when one of the main
\nengines was shut down early by computer command because of a failed
\ntemperature sensor. Within 10 seconds of the shutdown, Mission
\nControl declared an ATO situation, and although a lower than planned
\norbit was attained, the 7-day mission carrying Spacelab-2 was
\nsuccessfully completed.<\/p>\n
On-Orbit Operations. Space Shuttle flights are controlled by
\nMission Control Center (MCC) — usually referred to as “Houston” in
\nair to ground conversations.<\/p>\n
During a flight, Shuttle crews and ground controllers work from a
\ncommon set of guidelines and planned events called the Flight Data
\nFile. The Flight Data File includes the crew activity plan, payload
\nhandbooks and other documents which are put together during the
\nelaborate flight planning process.<\/p>\n
Each mission includes the provision for at least two crew members to
\nbe trained for extravehicular activity (EVA). EVA is an operational
\nrequirement when satellite repair or equipment testing is called for
\non a mission. However, during any mission, the two crew members must
\nbe ready to perform a contingency EVA if, for example, the payload
\nbay doors fail to close properly and must be closed manually, or
\nequipment must be jettisoned from the payload bay.<\/p>\n
The first Space Shuttle program contingency EVA occurred in April
\n1985, during STS 51-D, a Discovery mission, following deployment of
\nthe SYNCOM IV-3 (Leasat 3) communications satellite Leasats’
\nsequencer lever failed and initiation of the antenna deployment and
\nspin-up and perigee kick motor start sequences did not take place.
\nThe flight was extended 2 days to give mission specialists Jeffrey
\nHoffman and David Griggs an opportunity to try to activate the lever
\nduring EVA operations which involved using the RMS. The effort was
\nnot successful, but was accomplished on a later mission.<\/p>\n
Each Shuttle mission carries two complete pressurized spacesuits
\ncalled Extra Vehicular Mobility Units (EMU) and backpacks called
\nPrimary Life Support Systems (PLSS). These units, along with
\nnecessary tools and equipment, are stored in the airlock off the
\nmiddeck area of the orbiter, ready for use if needed.<\/p>\n
As already mentioned, for each mission, two crew members are trained
\nand certified to perform EVAs, if necessary. For those missions in
\nwhich planned EVAs are called for, the two astronauts receive
\nrealistic training for their specific tasks in the Weightless
\nEnvironment Training Facility at Johnson, with its full-scale model
\nof the orbiter payload bay.<\/p>\n
Maneuvering in Orbit. Once the Shuttle orbiter goes into orbit, it
\nis operating in the element for which it was designed: the near
\ngravity-free vacuum of space. However, to maintain proper orbital
\nattitude and to perform a variety of maneuvers, an extensive array of
\nlarge and small rocket thrusters are used — 46 in all. Each of
\nthese thrusters, despite their varying sizes, burn a mixture of
\nnitrogen tetroxide and monoethylhydrazine, an efficient but toxic
\ncombination of fuels which ignite on contact with each other.<\/p>\n
The largest of the 46 control rockets are the two Orbital
\nManeuvering System (OMS) thrusters which are located in twin pods at
\nthe aft end of the orbiter, between the vertical stabilizer and just
\nabove the three main engines. Each of the two OMS engines can
\ngenerate 6,000 lb. of thrust. They can cause a more than l,000
\nfoot-per-second change in velocity of a fully loaded orbiter. This
\nvelocity change is called Delta V.<\/p>\n
A second and smaller group of thrusters make up the Reaction Control
\nSystem (RCS) of which there are two types: the primaries and the
\nverniers. Each orbiter has 38 primary trusters, 14 in the forward
\nnose area and 12 on each OMS pod. Each primary thruster can generate
\n870 lb. of thrust. The smallest of the RCS thrusters, the verniers,
\nare designed to provide what is called “fine tuning” of the orbiter’s
\nattitude. There are two vernier thrusters on the forward end of the
\norbiter and four aft, each generates 24 pounds of thrust.<\/p>\n
