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After the Space Shuttle has been rolled out to the launch pad on the
Mobile Launcher Platform (MLP), all pre-launch activities are controlled from the Launch Control Center (LCC).
After the Shuttle is in place on the launch pad support columns, and
the Rotating Service Structure (RSS) is placed around it, power for the vehicle is activated. The MLP and the Shuttle are then electronically and mechanically mated with support launch pad facilities and ground support equipment. An extensive series of validation checks verify that the numerous interfaces are functioning properly.
Meanwhile, in parallel with pre-launch pad activities, cargo
operations get underway in the RSS's Payload Changeout Room.
Vertically integrated payloads are delivered to the launch pad
before the Shuttle is rolled out. They are stored in the Payload Changeout Room until the Shuttle is ready for cargo loading. Once the RSS is in place around the orbiter, the payload bay doors are opened and the cargo is installed. Final cargo and payload bay closeouts are completed in the Payload Changeout Room and the payload bay doors are closed for flight.
Pre-launch Propellant Loading. Initial Shuttle propellant loading
involves pumping hypergolic propellants into the orbiter's aft and forward Orbital Maneuvering System and Reaction Control System storage tanks, the orbiter's hydraulic Auxiliary Power Units, and SRB hydraulic power units. These are hazardous operations, and while they are underway work on the launch pad is suspended.
Since these propellants are hypergolic -- that is they ignite on
contact with one another–oxidizer and fuel loading operations are carried out serially, never in parallel.
Finally, dewar tanks on the Fixed Service Structure (FSS), are
filled with liquid oxygen and liquid hydrogen, which will be loaded into the orbiter's Power Reactant and Storage Distribution (PRSD) tanks during the launch countdown.
Final Pre-launch Activities. Before the formal Space Shuttle launch
countdown starts, the vehicle is powered down while pyrotechnic devices – various ordinance components – are installed or hooked up. The extravehicular Mobility Units (EMUs) – space suits – are stored On Board along with other items of flight crew equipment.
When closeouts of the Space Shuttle and the launch pad are
completed, all is in readiness for the countdown to get underway.
Launch Control Center. While the VAB can be considered the heart of
LC-39, the Launch Control Center (LCC) can easily be called its brain.
The LCC is a 4-story building connected to the east side of the VAB
by an elevated, enclosed bridge. It houses four firing rooms that are used to conduct NASA and classified military launches of the Space Shuttle. Each firing room is equipped with the Launch Processing System (LPS) which monitors and controls most Shuttle assembly, checkout and launch operations. Physically, the LCC is 77 ft. high, 378 ft. long and 181 ft. wide.
Thanks to the LPS, the countdown for the Space Shuttle takes only
about 40 hours, compared with the 80 plus hours usually needed for a Saturn/Apollo countdown. Moreover, the LPS calls for only about 90 people to work in the firing room during launch operations – compared with about 450 needed for earlier manned missions.
From the outside, the LCC is virtually unchanged from its original
Apollo-era configuration, except that a fourth floor office has been added to the southwest and northwest corners corner of the building.
The interior of the LCC has undergone extensive modifications to
meet the needs of the Space Shuttle era.
Physically, the LCC is constructed as follows: the first floor is
used for administrative activities and houses the building's utilities systems control room; the second floor is occupied by the Control Data Subsystem; the four firing rooms occupy practically all of the third floor, and the fourth floor, as mentioned, earlier is used for offices.
During the Shuttle Orbital Flight Test program and the early
operational missions, Firing Room l was the only fully-equipped control facility available for vehicle checkout and launch. However, as the Shuttle launch rate increased during the first half of the 1980s, the other three firing rooms were activated. Although NASA operates the firing rooms, the Department of Defense uses Firing Rooms 3 and 4 to support its classified, Shuttle-dedicated missions. Additionally, Firing Room 4 serves as an engineering analysis and support facility for launch and checkout operations.
Launch Countdown. As experience was gained by launch crews during
the early years of the Space Shuttle program, the launch countdown was refined and streamlined to the point where the average countdown now takes a little more than 40 hours. This was not the case early in the program, when countdowns of 80 hours or more were not uncommon.
The following is a narrative description of the major events of a
typical countdown for the Space Shuttle. The time of liftoff is predicated on what is called the launch window – that point in time when the Shuttle must be launched in order to meet specific mission objectives such as the deployment of spacecraft at a predetermined time and location in space.
Launch Minus 3 Days. The countdown gets underway with the
traditional call to stations by the NASA Test Director. This verifies that the launch team is in place and ready to proceed.
The first item of business is to checkout the backup flight system
and the software stored in the mass memory units and display systems. Backup flight system software is then loaded into the Shuttle's fifth general purpose computer (GPC's).
Flight crew equipment stowage begins. Final inspection of the
orbiter's middeck and flight decks are made, and removal of work crew module platforms begin. Loading preparations for the external tank get underway, and the Shuttle main engines are readied for tanking. Servicing of fuel cell storage tanks also starts. Final vehicle and facility closeouts are made.
Launch Minus 2 Days. The launch pad is cleared of all personnel
while liquid oxygen and hydrogen are loaded into the Shuttle fuel cell storage tanks. Upon completion, the launch pad area is reopened and the closeout crew continues its prelaunch preparations.
The orbiter's flight control, navigation and communications systems
are activated. Switches located on the flight and mid- decks are checked and, if required, mission specialist seats are installed. Preparations also are made for rollback of the Rotating Service Structure (RSS).
At launch minus ll hours a planned countdown hold -- called a
built-in hold – begins and can last for up to 26 hours, 16 minute depending on the type of payload, tests required and other factors. This time is used, if needed, to perform tasks in the countdown that may not have been completed earlier.
Launch Minus 1 Day. Countdown is resumed after the built-in hold
period has elapsed. The RSS is rolled back and remaining items of crew equipment are installed. Cockpit switch positions are verified, and oxygen samples are taken in the crew area. The fuel cells are activated following a fuel cell flow through purge. Communications with the Johnson Space Center's Mission Control Center (MCC) are established.
Finally, the launch pad is again cleared of all personnel while
conditioned air that has been blowing through the payload bay and other orbiter cavities is switched to inert gaseous nitrogen in preparation for filling the external tank with its super-cold propellants.
Launch Day. Filling the external tank with liquid oxygen and
hydrogen gets underway. Communications checks are made with elements of the Air Force's Eastern Space and Missile Center. Gimbal profile checks of the Orbital Maneuvering System (OMS) engines are made. Preflight calibration of the Inertial Measurement Units (IMU) is made, and tracking antennas at the nearby Merritt Island Tracking Station are aligned for liftoff.
At launch minus 5 hours, 20 minutes -- T minus 5 hours, 20 minutes
– a 2-hour built-in hold occurs. During this hold, an ice inspection team goes to the launch pad to inspect the external tank's insulation to insure that there is no dangerous accumulation of ice on the tank caused by the super-cold liquids. Meanwhile, the closeout crew is preparing for the arrival of the flight crew.
Meanwhile, the flight crew, in their quarters at the Operations and
Checkout (O&C) Building, eat a meal and receive a weather briefing. After suiting up, they leave the O&C Building at about T minus 2 hours, 30 minutes for the launch pad – the countdown having resumed at T minus 3 hours.
Upon arriving at the white room at the end of the orbiter access
arm, the crew, assisted by white room personnel, enter the orbiter. Once on board they conduct air-to-ground communications checks with the LCC and MCC. Meanwhile, the orbiter hatch is closed and hatch seal and cabin leak checks are made. The IMU preflight alignment is made and closed-loop tests with Range Safety are completed. The white room is then evacuated and the closeout crew proceeds from the launch pad to a fallback area. At this time, primary ascent guidance data is transferred to the backup flight system.
At T minus 20 minutes a planned 10-minute hold begins. When the
countdown is resumed on-board computers are commanded to their launch configuration and fuel cell thermal conditioning begins. Orbiter cabin vent valves are closed and the backup flight system transitions into its launch configuration.
At T minus 9 minutes another planned 10-minute hold occurs. Just
prior to resuming the countdown, the NASA Test Director gets the "go for launch" verification from the launch team. At this point, the Ground Launch Sequencer (GLS) is turned on and the terminal countdown starts. All countdown functions are now automatically controlled by the GLS computer located in the Firing Room Integration Console.
At T minus 7 minutes, 30 seconds, the orbiter access arm is
retracted. Should an emergency occur requiring crew evacuation from the orbiter, the arm can be extended either manually or automatically in about 15 seconds.
At T minus 5 minutes, 15 seconds the MCC transmits a command that
activates the orbiter's operational instrumentation recorders. These recorders store information relating to ascent, on-orbit and descent performance during the mission. These data are analyzed after landing.
At T minus 5 minutes, the crew activates the Auxiliary Power Units
(APU) to provide pressure to the Shuttle's three hydraulic systems which move the main engine nozzles and the aero-aerosurfaces. Also at this point, the firing circuit for SRB ignition and the range safety destruct system devices are mechanically enabled by a motor-driven switch called the safe and arm device.
At about T minus 4 minutes, 55 seconds, the liquid oxygen vent on
the external tank is closed. It had been open to allow the super-cold liquid oxygen to boil off, thus preventing over pressurization while the tank remained near its full level. Now, with the vent closed, preparations are made to bring the tank to its flight pressure. This occurs at T minus 2 minutes, 55 seconds.
At T minus 4 minutes the final helium purge of the Shuttle's three
main engines is initiated in preparation for engine start. Five seconds later, the orbiter's elevons, speed brakes and rudder are moved through a pre-programmed series of maneuvers to position them for launch. This is called the aerosurface profile.
At T minus 3 minutes, 30 seconds, the ground power transition takes
place and the Shuttle's fuel cells transition to internal power. Up to this point, ground power had augmented the fuel cells. Then, 5 seconds later, the main engine nozzles are gimballed through a pre-programmed series of maneuvers to confirm their readiness.
At T minus 2 minutes, 50 seconds, the external tank oxygen vent hood
– known as the beanie cap – is raised and retracted. It had been in place during tanking operations to prevent ice buildup on the oxygen vents. Fifteen seconds later, at T minus 2 minutes, 35 seconds, the piping of gaseous oxygen and hydrogen to the fuel cells from ground tanks is terminated and the fuel cells begin to use the on board reactants.
At T minus 1 minute, 57 seconds, the external tank's liquid hydrogen
is brought to flight pressure by closing the boil off vent, as was done earlier with the liquid oxygen vent. However, during the hydrogen boil off of, the gas is piped out to an area adjacent to the launch pad where it is burned off.
At T minus 31 seconds, the Shuttle's on-board computers start their
terminal launch sequence. Any problem after this point will require calling a "hold" and the countdown recycled to T minus 20 minutes. However, if all goes well, only one further ground command is needed for launch. This is the "go for main engine start," which comes at the T-minus-10-second point. Meanwhile, the Ground Launch Sequencer (GLS) continues to monitor more than several hundred launch commit functions and is able automatically to call a "hold" or "cutoff" if a problem occurs.
At T minus 28 seconds the SRB booster hydraulic power units are
activated by a command from the GLS. The units provide hydraulic power for SRB nozzle gimballing. At T minus 16 seconds, the nozzles are commanded to carry out a pre-programmed series of maneuvers to confirm they are ready for liftoff. At the same time – T minus 16 seconds – the sound suppression system is turned on and water begins to pour onto the deck of the MLP and pad areas to protect the Shuttle from acoustical damage at liftoff.
At T minus ll seconds, the SRB range safety destruct system is
At T minus 10 seconds, the "go for main engine start" command is
issued by the GLS. (The GLS retains the capability to command main engine stop until just before the SRBs are ignited.) At this time flares are ignited under the main engines to burn away any residual gaseous hydrogen that may have collected in the vicinity of the main engine nozzles. A half second later, the flight computers order the opening of valves which allow the liquid hydrogen and oxygen to flow into the engine's turbopumps.
At T minus 6.6 seconds, the three main engines are ignited at
intervals of 120 milliseconds. The engines throttle up to 90 percent thrust in 3 seconds. At T minus 3 seconds, if the engines are at the required 90 percent, SRB ignition sequence starts. All of these split-second events are monitored by the Shuttle's four primary flight computers.
At T minus zero, the holddown explosive bolts and the T-O umbilical
explosive bolts are blown by command from the on-board computers and the SRBs ignite. The Shuttle is now committed to launch. The mission elapsed time is reset to zero and the mission event timer starts. The Shuttle lifts off the pad and clears the tower at about T plus 7 seconds. Mission control is handed over to JSC after the tower is cleared.
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MARSHALL PAYLOAD OPERATIONS CONTROL CENTER
The Payload Operations Control Center (POCC) operated by the NASA's
Marshall Space Flight Center (MSFC), Huntsville, Ala., is the largest and most diverse of the various POCCs associated with the Space Shuttle program. Since its functions in many respects parallel those of other POCCs operated by private industry, the academic community and government agencies, a description of what it does, how it operates and who operates it will serve as an overview of this type of control center.
The Marshall POCC -- like all POCCs -- is a facility designed to
monitor, coordinate, and control on-orbit operation of a Shuttle payload, particularly Spacelab. During non-mission periods it also is used for crew training and simulated space operations. It is, in effect, a command post for payload activities, just as the JSC Mission Control Center (MCC) is a command post for the flight and operation of the Space Shuttle.
Both control centers work closely in coordinating mission
activities. In fact, the Marshall POCC originally was housed in Building 30 at JSC, adjacent to the MCC. It has since been moved to Building 4663 at Marshall and is an important element of the Hunstville Operations Support Center (HOSC), which augments the MCC by monitoring Shuttle propulsion systems.
The Marshall POCC Capabilities Document states that the "POCC
provides physical space, communications, and data system capabilities to enable user access to payload data (digital, video, and analog), command uplink, and coordination of activities internal and external to the POCC."
Members of the Marshall mission management team and principal
investigators and research teams work in the POCC or in adjacent facilities around-the-clock controlling and directing payload experiment operations. Using the extensive POCC facilities they are able to communicate directly with mission crews and direct experiment activities from the ground. They also can operate experiments and support equipment on board the Shuttle and manage payload resources.
The POCC operations concept requires a team consisting of the
Payload Mission Manager (PMM) directing the POCC cadre which has overall responsibility for managing and controlling POCC operations. Its scientific counterpart, the investigator's operations team, is the group that conducts, monitors and controls the experiments carried on the Shuttle, primarily those related to Spacelab.
Generally, POCC operations are carried out by a
management/scientific team of 10 key individuals, headed by the Payload Operations Director (POD), who is a senior member of the PMM's cadre. The POD is charged with managing the day-to-day mission operations and directing the payload operations team and the science crew.
Other POCC key personnel include:
MISSION SCIENTIST (MSCI) who represents scientists who have
experiments on a specific flight and serves as the interface between the PMM and the POD in matters relating to mission science operations and accomplishments.
CREW INTERFACE COORDINATOR (CIC), who coordinates communications
between the POCC and the payload crew.
ALTERNATE PAYLOAD SPECIALIST (APS) is a trained payload specialist
not assigned to flight duty who aids the payload operations team and the payload crew in solving problems, troubleshooting and modifying crew procedures, if necessary, and who advises the MSCI on the possible impact of any problem areas.
PAYLOAD ACTIVITY PLANNER (PAP), who directs mission replanning
activities, as required, and coordinates mission timeline changes with POCC personnel.
MASS MEMORY UNIT MANAGER (MUM) who sends experiment command uplinks
to the flight crew based on data received from the POCC operations team.
OPERATIONS CONTROLLER (OC), who coordinates activities of the
payload operations team to insure the efficient accomplishment of activities supporting real-time execution of the mission timeline.
PAYLOAD COMMAND COORDINATOR (PAYCOM), who configures the POCC for
ground command operation and controls the flow of experiment commands from the POCC to the flight crew.
DATA MANAGEMENT COORDINATOR (DMC), who is responsible for
maintaining and coordinating the flow of payload experiment data to and within the POCC the DMC also assesses the impact of proposed changes to the experiment timeline and payload data requirements that affect the payload downlink data.
PUBLIC AFFAIRS OFFICER (PAO), who provides mission commentary on
payload activities and serves as the primary source of information on mission progress to the news media and public.
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SPACE TRACKING AND DATA ACQUISITION
Responsibility for Space Shuttle tracking and data acquisition is
charged to the Goddard Space Flight Center, Greenbelt, Md. This involves integrating and coordinating all of the worldwide NASA and Department of Defense tracking facilities needed to support Space Shuttle missions.
These facilities include the Goddard-operated Ground Network (GN)
and Space Network (SN); the Deep Space Network (DSN) managed for NASA by the Jet Propulsion Laboratory (JPL), Pasadena, Calif.; the Ames-Dryden Flight Research Facility, (ADFRC) Edwards, Calif.; and extensive Department of Defense tracking systems at the Eastern and Western Space and Missile Centers, as well as the Air Force Satellite Control Network's (AFSCN) remote tracking stations.
Ground Network. The Ground Network (GN) is a worldwide network of
tracking stations and data-gathering facilities which support Space Shuttle missions and also maintain communications with low Earth-orbiting spacecraft. Station management is provided from the Network Control Center at Goddard. Basically, commands are sent to orbiting spacecraft from the GN stations and, in return, scientific data are transmitted to the stations.
The system consists of 12 stations, including three DSN facilities.
GN stations are located at Ascension Island, a British Crown Colony in the south Atlantic Ocean; Santiago, Chile; Bermuda; Dakar, Senegal, on the West Coast of Africa; Guam; Hawaii; Merritt Island, Fla.; Ponce de Leon, Fla.; and the Wallops Flight Facility on Virginia's Eastern Shore. The DSN tracking stations are located at Canberra, Australia; Goldstone, Calif.; and Madrid, Spain.
The GN stations are equipped with a wide variety of tracking and
data-gathering antennas, ranging in size from 14 to 85 feet in diameter. Each is designed to perform a specific task, normally in a designated frequency band, gathering radiated electronic signals (telemetry) transmitted from spacecraft.
The communications hub for the GN is the Goddard-operated NASA
Communications Center (NASCOM). It consists of more than 2 million miles of electronic circuitry linking the tracking stations and the MCC at the Johnson Space Center. NASCOM has six major switching centers to insure the prompt flow of data. In addition to Goddard and JSC, the other switching centers are located at JPL, KSC, Canberra and Madrid.
The system includes telephone, microwave, radio, submarine cable and
geosynchronous communications satellites in ll countries. It includes communications facilities operated by 15 different domestic and foreign carriers. The system also has a wide-band and video capability. In fact, Goddard's wide-band system is the largest in the world.
A voice communications system called Station Conferencing and
Monitoring Arrangement (SCAMA) can conference link up hundreds of the 220 different voice channels throughout the United States and abroad with instant talk/listen capability. With its built-in redundancy, SCAMA has realized a mission support reliability record of 99.6 percent. The majority of Space Shuttle voice traffic is routed through Goddard to the MCC.
As would be expected, computers play an important role in GN
operations. They are used to program tracking antenna pointing angles, send commands to orbiting spacecraft and process data which is sent to the JSC and Goddard control centers.
Shuttle data is sent from the tracking network to the main switching
computers at GSFC. These are UNISYS 1160 computers which reformat and transmit the information to JSC almost instantaneously at a rate of l.5 million bits per second, via domestic communications satellites.
Space Network. Augmenting the GN and eventually replacing it, is a
unique tracking network called the Space Network (SN). The uniqueness of this network is that instead of tracking the Shuttle and other Earth-orbiting spacecraft from a world-wide network of ground stations, its main element is an in-orbit series of satellites called the Tracking and Data Relay Satellite System (TDRSS), designed to gather tracking and data information from geosynchronous orbit and relay it to a single ground terminal located at White Sands, N.M.
The first spacecraft in the TDRS system, TDRS-1, was deployed from
the Space Shuttle Challenger on April 4, 1983. Although problems were encountered in establishing its geosynchronous orbit at 41 degrees west longitude (over the northeast corner of Brazil), TDRS-l proved the feasibility of the tracking station-in-space concept when it became operational later in the year.
Ultimately, the SN will consist of three TDRS spacecraft in orbit,
one of which will be a backup or spare to be available for use if one of the operational spacecraft fails. Each satellite in the TDRS system is designed to operate for 10-years.
Following its planned deployment from the Space Shuttle Discovery
scheduled for the STS-26 mission, TDRS-2 will be tested and then positioned in a geosynchronous orbit southwest of Hawaii at 171 degrees west longitude, about 130 degrees from TDRS-1. With these two spacecraft and the White Sands Ground Terminal (and eventually a backup terminal) operational, the SN will be able to provide almost full-time communications and tracking of the Space Shuttle, as well as for up to 24 other Earth-orbiting spacecraft simultaneously. The global network of ground stations can provide only about 20 percent of that coverage. Eventually some of the current ground stations will be closed when the SN becomes fully operational.
After data acquired by the TDRS spacecraft are relayed to the White
Sands Ground Terminal, they are sent directly by domestic communications satellite to NASA control centers at JSC for Space Shuttle operations, and to Goddard which schedules TDRSS operations including those of many unmanned satellites.
The TDRS are among the largest and most advanced communications
satellites ever developed. They weigh almost 5,000 lb. and measure 57 ft. across at their solar panels. They operate in the S-band and Ku-band frequencies and their complex electronics systems can handle up to 300 million bits of information each second from a single user spacecraft. Among the distinguishing features of the spacecraft are their two huge, wing-like solar panels which provide l,850 watts of electric power and their two 16-ft. diameter high-gain parabolic antennas which resemble large umbrellas. These antennas weigh about 50 lb. each.
The communications capability of the TDRSS covers a wide spectrum
that includes voice, television, analog and digital signals. No signal processing is done in orbit. Instead, the raw data flows directly to the ground terminal. During Space Shuttle missions, mission data and commands pass almost continuously back and forth between the orbiter and the MCC at JSC.
Like the TDRS, the White Sands ground terminal is one of the most
advanced in existence. Its most prominent features include three 60-ft.-diameter Ku-band antennas which receive and transmit data. A number of smaller antennas are used for S-band and other Ku-band communications.
Ground was broken in September 1987, for a second back-up ground
terminal at White Sands to accommodate increased future mission support required from the TDRSS.
The TDRSS segment of the Space Network, including the ground
terminal, is owned and operated for NASA by CONTEL Federal Systems Sector, Atlanta, Ga. The spacecraft are built the TRW Federal Systems Division, Space and Technology Group, Redondo Beach, Calif. TRW also provides software support for the White Sands facility. The TDRS parabolic antennas are built by the Harris Corp's Government Communications Systems Division, Melbourne, Fla. Harris also provides ground antennas, radio frequency equipment and other ground terminal equipment.
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The Space Shuttle, as it thunders away from the launch pad with its
main engines and solid rocket boosters (SRB) at full power, is an unforgettable sight. It reaches the point of maximum dynamic pressure (max Q) – when dynamic pressures on the Shuttle are greatest – about 1 minute after liftoff, at an altitude of 33,600 ft. At this point the main engines are "throttled down," to about 75 percent, thus keeping the dynamic pressures on the vehicle's surface to about 580 lb. per square foot. After passing through the max Q region, the main engines are throttled up to full power. This early ascent phase is often referred to as "first stage" flight.
Little more than 2 minutes into the flight, the SRBs, their fuel
expended, are jettisoned from the orbiter. The Shuttle is at an altitude of about 30 miles and traveling at a speed of 2,890 miles an hour. The spent SRB casings continue to gain altitude briefly before they begin falling back to Earth. When the spent casings have descended to an altitude of about 17,000 ft., the parachute deployment sequence starts, slowing them for a safe splashdown in the ocean. This occurs about 5 minutes after launch. The boosters are retrieved, returned to a processing facility for refurbishment and eventual reused.
Meanwhile, the "second stage" phase of the flight is underway with
the main engines propelling the vehicle ever higher on its ascent trajectory. At about 8 minutes into the flight, at an altitude of about 60 miles, main engine cut-off (MECO) occurs. The Shuttle is now traveling at a speed of 16,697 mph.
After MECO, the orbiter and the external tank are moving along a
trajectory that, if not corrected, would result in the vehicle entering the atmosphere about halfway around the world from the launch site. However, a brief firing of the orbiter's two Orbital Maneuvering System (OMS) thrusters changes the trajectory and orbit is achieved. This takes place just after the external tank has been jettisoned and while the orbiter is flying "upside down" in relation to Earth.
The separated external tank continues on a ballistic trajectory and
enters the Earth's atmosphere to break up over a remote area of the Indian Ocean. Meanwhile, an additional firing of the OMS thrusters places the orbiter into its planned orbit, which can range from 115 to 600 miles above the Earth.
There are two ways in which orbit can be accomplished. These are
the conventional OMS insertion method called "standard" and the direct insertion method.
The OMS insertion method involves a brief burn of the OMS engines
shortly after MECO, placing the orbiter into an elliptical orbit. A second OMS burn is initiated when the orbiter reaches apogee in its elliptical orbit. This brings the orbiter into a near circular orbit. If required during a mission, the orbit can be raised or lowered by additional firings of the OMS thrusters.
The direct insertion technique uses the main engines to achieve the
desired orbital apogee, or high point, thus saving OMS propellant. Only one OMS burn is required to circularize the orbit, and the remaining OMS fuel can then be used for frequent changes in the operational orbit, as called for in the flight plan.
The first direct insertion orbit was accomplished during the STS
41-C mission in April 1984, when the Challenger was placed in a 288-mile-high circular orbit where its flight crew was able to successfully capture, repair and redeploy a free-flying spacecraft, the Solar Maximum satellite (Solar Max) – an important "first" for the Space Shuttle program.
Launch Abort Modes. During the ascent phase of a Space Shuttle
flight, if a situation occurs that puts the mission in jeopardy – the loss, for example, of one or more of the main engines or the OMS thrusters – the mission may have to be aborted. During the ascent phase, there are two basic Shuttle abort modes: intact aborts and contingency aborts. NASA has attempted to anticipate all possible emergency situations that could occur, and mission plans are prepared accordingly.
Intact aborts -- there are four different types -- permit the safe
return of the orbiter and its crew to a pre-planned landing site.
When an intact abort is not possible, the contingency abort option
becomes necessary. This crucial abort mode is designed to permit crew survival following a severe systems failure in which the vehicle is lost. Generally, if a contingency abort becomes necessary, the damaged vehicle would fall toward the ocean and the crew would exercise escape options that were developed in the aftermath of the Challenger accident. The four intact abort modes are:
Return to Launch Site (RTLS)
Trans-Atlantic Abort Landing (TAL)
Abort Once Around (AOA)
Abort to Orbit (ATO)
Since an intact abort could result in an emergency landing, before
each flight, potential contingency landing sites are designated and weather conditions at these locations are monitored closely before a launch. Space Shuttle flight rules include provisions for minimum acceptable weather conditions at these potential landing sites in the event of intact abort is necessary.
In an abort situation, the type and time of the failure determines
which abort mode is possible. There is a definite order of preference for an abort. In cases where performance loss is the only factor, the preferred modes would be ATO, AOA, TAL or RTLS, in that order. The mode selected normally would be the highest preferred one that can be completed with the remaining vehicle performance.
In the case of an extreme system failure -- the loss of cabin
pressure or orbiter cooling systems – the preferred mode would be the one that would terminate the mission as quickly as possible. This means that the TAL or RTLS modes would be more preferable than other modes.
An ascent abort during powered flight can be initiated by turning a
rotary switch on a panel in the orbiter cockpit. The switch is accessible to both the commander and the pilot. Normally, flight rules call for the abort mode selection to be made by the commander upon instructions from the Mission Control Center. Once the abort mode is selected, the on board computers automatically initiate abort action for that particular abort.
A description of the intact abort modes follows.
RETURN TO LAUNCH SITE (RTLS). The RTLS abort is a critical and
complex one that becomes necessary if a main engine failure occurs after liftoff and before the point where a TAL or AOA is possible. RTLS cannot be initiated until the SRBs have completed their normal burn and have been jettisoned. Meanwhile, the orbiter with the external tank still attached continues on its downrange trajectory with the remaining operational main engines, the two OMS and four aft RCS thrusters firing until the remaining main engine propellent equals the amount needed to reverse the direction of flight and return for a landing. A "pitch-around" maneuver of about 5 degrees per second is then performed to place the orbiter and the external tank in an attitude pointing back toward the launch site. OMS fuel is dumped to adjust the orbiter's center of gravity.
When altitude, attitude, flight path angle, heading, weight, and
velocity/range conditions combine for external tank jettisoning, MECO is commanded, and the external tank separates and falls into the ocean. After this, the orbiter should glide to a landing at the launch site landing facility. From the foregoing, it can be appreciated why RTLS is the least preferred intact abort mode.
TRANS-ATLANTIC ABORT LANDING (TAL). The TAL abort mode is designed
to permit an intact landing after the Shuttle has flown a ballistic trajectory across the Atlantic Ocean and lands at a designated landing site in Africa or Spain. This abort mode was developed for the first Shuttle launch in April 1981, and has since evolved from a crew-initiated manual procedure to an automatic abort mode. The TAL capability provides an abort option between the last RTLS opportunity up to the point in ascent known as the "single-engine press to MECO" capability –meaning that the orbiter has sufficient velocity to achieve main engine cutoff and abort to orbit, even if two main engines are shut down. TAL also can be selected if other system failures occur after the last RTLS opportunity. The TAL abort mode does not require any OMS maneuvers.
Landing sites for a TAL vary from flight to flight, depending on
the launch azimuth. For the first three Space Shuttle missions, the trajectory inclination was about 28 degrees which made the U.S. Air Force bases at Zaragoza and Moron in Spain, the most ideal landing sites for TAL. Later Shuttle missions called for air fields at Dakar, Senegal, and Casablanca, Morocco, as TAL-option landing sites. In March 1988, NASA announced that in addition to the TAL sites in Spain, that two new African contingency landing sites had been selected for future Shuttle missions: a site near Ben Guerir, Morocco, about 40 miles north of Marrakesh with a 14,000-foot runway; and at Banjul, the capital of the west African nation of The Gambia, which has an international airfield with an ll,800-foot runway.
ABORT ONCE AROUND (AOA). This abort mode becomes available about 2
minutes after SRB separation, up to the point just before an abort to orbit is possible. AOA normally would be called for because of a main engine failure. This abort mode allows the Shuttle to fly once around the Earth and make a normal entry and landing at Edwards AFB, Calif., or White Sands Space Harbor, near Las Cruces, N.M. An AOA abort usually would require two OMS burns, the second burn being a deorbit maneuver.
There are two different AOA entry trajectories. These are the
so-called normal AOA and the shallow. The entry trajectory for the normal AOA, is similar to a normal end-of-mission landing. The shallow AOA, on the other hand, results in a flatter entry trajectory, which is less desirable but uses less propellant for the OMS burn. The shallow trajectory also is less desirable because it exposes the orbiter to a longer period of atmospheric entry heating and to less predictable aerodynamic drag forces.
ABORT TO ORBIT (ATO). The ATO mode is the most benign of the
various abort modes. ATO allows the orbiter to achieve a temporary orbit that is lower than the planned. ATO is usually necessary because of a main engine failure. It places fewer performance demands on the orbiter. It also gives ground controllers and the flight crew time to evaluate the problem. Depending on the seriousness of the situation, one ATO option is to make an early deorbit and landing. If there are no major problems, other than the main engine one, an OMS maneuver is made to raise the orbit and the mission is continued as planned.
The first Space Shuttle program ATO occurred on July 29, 1985,
following the STS 51-F Challenger launch, when one of the main engines was shut down early by computer command because of a failed temperature sensor. Within 10 seconds of the shutdown, Mission Control declared an ATO situation, and although a lower than planned orbit was attained, the 7-day mission carrying Spacelab-2 was successfully completed.
On-Orbit Operations. Space Shuttle flights are controlled by
Mission Control Center (MCC) – usually referred to as "Houston" in air to ground conversations.
During a flight, Shuttle crews and ground controllers work from a
common set of guidelines and planned events called the Flight Data File. The Flight Data File includes the crew activity plan, payload handbooks and other documents which are put together during the elaborate flight planning process.
Each mission includes the provision for at least two crew members to
be trained for extravehicular activity (EVA). EVA is an operational requirement when satellite repair or equipment testing is called for on a mission. However, during any mission, the two crew members must be ready to perform a contingency EVA if, for example, the payload bay doors fail to close properly and must be closed manually, or equipment must be jettisoned from the payload bay.
The first Space Shuttle program contingency EVA occurred in April
1985, during STS 51-D, a Discovery mission, following deployment of the SYNCOM IV-3 (Leasat 3) communications satellite Leasats' sequencer lever failed and initiation of the antenna deployment and spin-up and perigee kick motor start sequences did not take place. The flight was extended 2 days to give mission specialists Jeffrey Hoffman and David Griggs an opportunity to try to activate the lever during EVA operations which involved using the RMS. The effort was not successful, but was accomplished on a later mission.
Each Shuttle mission carries two complete pressurized spacesuits
called Extra Vehicular Mobility Units (EMU) and backpacks called Primary Life Support Systems (PLSS). These units, along with necessary tools and equipment, are stored in the airlock off the middeck area of the orbiter, ready for use if needed.
As already mentioned, for each mission, two crew members are trained
and certified to perform EVAs, if necessary. For those missions in which planned EVAs are called for, the two astronauts receive realistic training for their specific tasks in the Weightless Environment Training Facility at Johnson, with its full-scale model of the orbiter payload bay.
Maneuvering in Orbit. Once the Shuttle orbiter goes into orbit, it
is operating in the element for which it was designed: the near gravity-free vacuum of space. However, to maintain proper orbital attitude and to perform a variety of maneuvers, an extensive array of large and small rocket thrusters are used – 46 in all. Each of these thrusters, despite their varying sizes, burn a mixture of nitrogen tetroxide and monoethylhydrazine, an efficient but toxic combination of fuels which ignite on contact with each other.
The largest of the 46 control rockets are the two Orbital
Maneuvering System (OMS) thrusters which are located in twin pods at the aft end of the orbiter, between the vertical stabilizer and just above the three main engines. Each of the two OMS engines can generate 6,000 lb. of thrust. They can cause a more than l,000 foot-per-second change in velocity of a fully loaded orbiter. This velocity change is called Delta V.
A second and smaller group of thrusters make up the Reaction Control
System (RCS) of which there are two types: the primaries and the verniers. Each orbiter has 38 primary trusters, 14 in the forward nose area and 12 on each OMS pod. Each primary thruster can generate 870 lb. of thrust. The smallest of the RCS thrusters, the verniers, are designed to provide what is called "fine tuning" of the orbiter's attitude. There are two vernier thrusters on the forward end of the orbiter and four aft, each generates 24 pounds of thrust.