When a mission's planned in-orbit operations have been accomplished, the emphasis on board the orbiter turns to the task of preparing the vehicle for its return to Earth. Usually, the last full day in orbit is devoted primarily to stowing equipment, cleaning up the living areas and making final systems configurations which will facilitate post-landing processing.
The crew schedule, or timeline, is designed such that crew members are awake and into their "work day" 6 to 8 hours before landing. At about 4 hours before deorbit maneuvers are scheduled, the crew and flight controllers have finished with the Crew Activity Plan for that mission. They now work from the mission's Deorbit Prep handbook, which covers the major deorbit events leading up to touchdown. Major events include the "go" from MCC to close the payload bay doors, and the final OK to perform the deorbit burn which will bring the orbiter back to Earth.
However, before the deorbit burn is performed, the orbiter is turned to a tail-first attitude. (That is, the aft end of the orbiter faces the direction of travel.) At a predesignated time, the OMS engines are fired to slow the orbiter down and to permit deorbit. The RCS thrusters are then used to turn the orbiter back into a nose-first attitude. These thrusters are used during much of the reentry pitch, roll and yaw maneuvering until the orbiter's aerodynamic, aircraft-like control surfaces encounter enough atmospheric drag to control the landing. This is called Entry Interface (EI) and usually occurs 30 minutes before touchdown at about 400,000 ft. At this time, a communications blackout occurs as the orbiter is enveloped in a sheath of plasma caused by electromagnetic forces generated from the high heat experienced during entry into the atmosphere.
As the orbiter glides toward a landing, initially at a velocity of 25,000 feet per second at the EI point, its velocity is gradually slowed by a series of banks and roll reversals. As the atmospheric density increases, the forward RCS thrusters are turned off, while the aft RCS jets continue to maneuver the orbiter until a dynamic pressure of 10 lb. per square foot is sensed by instruments on board. At this point, the ailerons on orbiter's delta-shaped wings begin to operate and the aft RCS roll thrusters are stopped.
When the dynamic pressure reaches 20 lb. per square foot, the orbiter's wing elevators become operational and the RCS pitch thrusters are stopped. A speed brake on the vertical tail opens when the orbiter's velocity falls below Mach 10. Then, at Mach 3.5, the rudder is activated and the final RCS burns -- the yaw jets -- are stopped. The orbiter is now at an altitude of 45,000 ft., and is beginning what are called "area energy management maneuvers" which enable it to intercept the landing approach corridor at the desired altitude and velocity.
As it nears the landing site, the orbiter is steered into the nearest of two heading alignment circles called HACs. Each has a radius of 18,000 ft. The orbiter is now in subsonic flight, at 49,000 ft., and about 22 mile from its touchdown point.
In the future, final approach and landing will be controlled at this point the commander takes over control of the orbiter for final approach and landing maneuvers by the Microwave Scanning Beam Landing System (MSBLS) -- called autoland -- which will take over control 2 minutes before touchdown while the orbiter is at an altitude of 15,489 ft., 9.8 mile from the runway touchdown point, traveling at a speed of 410 mph. This phase of the flight will be completely automatic and the crew's main task will be to monitor the MSBLS.
The initial orbiter landing approach is at a glide slope of 19 degrees. This is six times steeper than the 3-degree glide slope of a typical commercial jet airliner as it approaches landing.
Just before the orbiter touches down, flare or pull-up maneuvers are required to bring it into its final landing glide slope of l.5 degrees. At touchdown -- nominally about 2,500 ft. beyond the runway threshold -- the orbiter is traveling at a speed ranging from 213 to 226 mph.
Once the orbiter has rolled to a stop on the runway, post-landing activities get underway involving the Orbiter Recovery Convoy. Mission responsibility has shifted from the Johnson Space Center back to the Kennedy Space Center.
The Orbiter Recovery Convoy consists of a number of specially-designed vehicles and a team of specialists who safe and service the orbiter and assist in crew egress. Included in the convoy are ll special vehicles and units. A brief description of these follows.
Scape Trailer . Self-Contained Atmospheric Protection Ensemble (SCAPE), vehicle, parked at a midfield location during landing, contains the equipment necessary to support recovery including recovery crew SCAPE suits, liquid air packs, and a crew who assist recovery personnel in suiting-up in protective clothing.
Vapor Dispersal Unit. The Vapor Dispersal Unit is a mobile wind-making machine able to produce a directed wind stream of up to 45 mph. It is an adaptation of a standard 14-ft. agricultural wind machine designed to protect fragile agricultural crops from frost damage or freezing. It is used by the recovery team to blow away toxic or explosive gases that may occur in or around the orbiter after landing. The fan can move 200,000 square feet of air a minute.
Coolant Umbilical Access. This apparatus is a stair and platform unit mounted on a truck bed which permits access to the aft port side of the orbiter where ground support crews attach coolant lines from the Orbiter Coolant Transporter.
Orbiter Coolant Transporters. This unit is a tractor-trailer carrying a refrigeration unit that provides Freon ll4 through the orbiter's T-O umbilical into its cooling system.
Purge Umbilical Access Vehicle. This vehicle is similar to the Coolant Umbilical Access Vehicle in that it has an access stairway and platform allowing crews to attach purge air lines to the orbiter on its aft starboard side.
Orbiter Purge Transporter. This vehicle is a tractor-trailer which carries an air conditioning unit powered by two 300 KW, 60 Hz electric generators. The unit blows cool or dehumidified air into the payload bay to remove possible residual explosive or toxic gases.
Cres Hatch Access Vehicle. The Crew Hatch Access Vehicle consists of a stairway and platform on which is located a white room equipped with special orbiter interface seals. It contains pressurized filtered air to keep toxic or explosive gases, airborne dust or other contaminants from getting into the orbiter during crew egress.
Astronaut Transporter Van. As its name implies, this van is used to transport the flight crew from the landing area. It is a modified recreational vehicle in which the crew can remove their flight suits and be examined by a physician while enroute.
Helium Tube Bank. This specialized vehicle is a trailer on which is mounted a 12-tube bank container which provides helium to purge hydrogen from the orbiter's main engines and lines. The bank contains 85,000 cubic feet of helium at 6,000 psi.
Orbiter Tow Vehicle. This unit is very much like the typical towing units used for large aircraft. However, it is equipped with a special towing bar designed specifically for the orbiter. It is used to move the orbiter from the landing facility to the OPF. It also is used for moving the orbiter from the OPF to the VAB.
Mobile Ground Power Unit. The final special vehicle for orbiter post-landing operations is the Mobile Ground Power Unit which provides power to the orbiter if the fuel cells have to be shut down. It can deliver a nominal load of 10 of direct power to the orbiter.
Augmenting these special orbiter recovery convoy vehicles are various conventional command and emergency vehicles.
Even before the Shuttle is launched, the recovery convoy begins its post-landing preparations by warming up coolant and purge equipment, readying ground service equipment and carrying out extensive communications checks.
During the Shuttle flight, the recovery convoy is on call in the event an earlier than planned landing is necessary.
Major activity begins at about 2 hours before the orbiter is scheduled to land. At this time chilldown of the purge and coolant units begins. About 1 hour, 40 minutes before landing, the recovery crew puts on their SCAPE suits and makes final communications checks. At 5 minutes before touchdown, the recovery convoy is ready to go to work.
After landing, the first staging position of the convoy is 200 ft. up wind from the orbiter. The safety assessment team in the SCAPE van moves to about 100 ft. of the port side of the orbiter. A SCAPE-dressed crew then moves to the rear of the orbiter using a high range flammability vapor detector to obtain vapor level readings and to test for possible explosive hazards and toxic gases. Two readings from three different locations are made to determine concentrations of hydrogen, monomethyl hydrazine, and hydrazine and ammonia. If they find that high levels of gases are present, and if wind conditions are calm, the Vapor Dispersal Unit -- the mobile wind machine -- moves into place and blows away the potentially dangerous gases.
Meanwhile, the Purge and Coolant Umbilical Access Vehicles are moved behind the orbiter and the safety assessment team continues to determine whether hazardous gases are present in the area. Once the umbilical access vehicles are in position, and as soon as it is possible to connect up to the liquid hydrogen T-O umbilical on the orbiter, the ground half of the on board hydrogen detection sample lines are connected to determine the hydrogen concentration. If the concentration is less than 4 percent, convoy operations continue. However, if it should be greater than 4 percent, an emergency power down of the orbiter is ordered. The flight crew is evacuated from the orbiter immediately and the convoy personnel clear the area and wait for the hydrogen to disperse.
If the hydrogen level is below 4 percent, the carrier plate for the starboard liquid oxygen T-O umbilical is attached to permit insertion of purge air ducts. After the carrier plates have been installed, the Freon line and purge duct connections are completed and the flow of coolant and purge air through the umbilical lines begins.
Purge air provides cool and humidified air conditioning to the payload bay and other cavities thereby removing any residual explosive or toxic fumes.
The Crew Hatch Access Vehicle moves to the hatch side of the orbiter. When the access white room is secured, the orbiter hatch is opened and a physician boards the orbiter to make a brief preliminary medical examination of the crew. The crew then leaves the orbiter and departs in the Astronaut Transporter Van.
The flight crew is replaced on board the orbiter by an exchange crew who make preparations for ground towing operations, installing switch guards and removing data packages from onboard experiments, if required.
Meanwhile, after allowing for a 30-minute orbiter tire cool down, the Tow Vehicle crew installs the landing gear lock pins, and disconnects the nose landing gear drag link. The Tow Vehicle is positioned in front of the orbiter and the tow bar connection is made. Finally, about two hours after landing the orbiter is towed off the runway.
After the Space Shuttle is launched, the Solid Rocket Boosters (SRB) are jettisoned at 2 minutes, 7 seconds into the flight. They are retrieved from the Atlantic Ocean by special recovery vessels and returned for refurbishment and eventual reuse on future Shuttle flights.
SRB separation occurs at an altitude of about 30 miles The separated boosters then coast up to an altitude of 47 miles and free-fall into an impact zone in the ocean about 158 miles downrange. The so-called splash "footprint" is in an area about 7 miles wide and about 10 miles long.
When a free-falling booster reaches an altitude of about 3 miles its nose cap is jettisoned and the SRB pilot parachute pops open. The pilot parachute then pulls out the 54-ft. diameter, l,100-lb. drogue parachute. The drogue parachute stabilizes and slows down the descent to the ocean.
At an altitude of 6,240 ft., the frustum, a truncated cone at the top of the SRB where it joins the nose cap, is separated from the forward skirt, causing the three main parachutes to pop out. These parachutes are 115 ft. in diameter and have a dry weight of about l,500 lb. each. When wet with sea water they weight about 3,000 lb.
At 6 minute and 44 seconds after liftoff, the spent SRBs, weighing about 165,000 lb., have slowed their descent speed to about 62 mph and splashdown takes place in the predetermined area.
The parachutes remain attached to the boosters until they are detached by recovery personnel.
Waiting near the impact area are two 176-ft.-long, specially-designed SRB recovery vessels. Their first job is to recover the main SRB parachutes. Each vessel is equipped with four 5 ft. 6 in. -diameter reels which wind the parachute winch lines onto the reel similar to the way line is wound onto a fishing reel.
The frustum-drogue parachute also is reeled in until the 5,000-lb. frustum is about 100 ft. from the recovery ship. The drogue parachute lines are then reeled in until the frustum can be lifted out of the ocean by a 10-ton-capacity crane.
Next, the empty SRB casings are recovered using a special device called the Diver Operated Plug (DOP). This procedure calls for a team of underwater divers to descend to a depth of about 110 ft. and place the DOP into the nozzle of the casing. A 2,000-ft.-long air line attached to the DOP is plugged into an air compressor on the recovery vessel. Air is pumped into the booster at 120 psi to empty water from the casing -- a procedure called "dewatering."
Under ideal weather and sea conditions, the retrieval operation takes about 5 and 1/2 hours. The recovery ships with the retrieved SRBs in tow, sail to Port Canaveral, travel north up the Banana River and dock near Hangar AF at the Cape Canaveral Air Force Station, their mission completed.
The retrieval ships take the SRBs to a dock at the Solid Rocket Booster Disassembly Facility (SRBDF) located at Hangar AF -- a building originally used for Project Mercury, the first U.S. manned space program.
The SRBs are unloaded onto a hoisting slip and mobile gantry cranes lift them onto tracked dollies where they are safed and undergo their first washing.
The casings are then taken to the SRBDF for disassembly into their four main segments: two aft skirt and two forward skirt assemblies. The main casing segments undergo further cleaning, after which they are placed on railroad cars and shipped to the manufacturing plant in Utah where they undergo final refurbishment and are again loaded with propellant.
Meanwhile, the nose cone frustums and parachutes are processed at the Parachute Refurbishment Facility in the KSC Industrial Area.
Specific procedures for refurbishment of the SRB parachutes include untangling the lines, and hanging them on an overhead monorail and automatically washing and drying them. When this is completed, and final inspections are conducted, the parachutes are folded on 64-ft.-long tables and stored in canisters for eventual reuse. Click Here for SPACE SHUTTLE PROGRAM MANAGEMENT
Information content from the NSTS Shuttle Reference Manual (1988)
Last Hypertexed Wednesday October 11 17:53:00 EDT 1995
Jim Dumoulin (email@example.com)
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