DEORBITING THE SHUTTLE/LANDING SEQUENCE
To return to Earth the astronauts must perform a number of checklist items including cleaning up the crew cabin, reinstalling the mission and payload specialist seats, powering down scientific experiments or the Spacelab, and closing the payload bay door s. If the doors fail to close an astronaut must perform an EVA to remove four shear pins that allow the doors to be manually closed; the orbiter cannot reenter with the payload bay doors open. Next the astronauts don the pressure suits which they wore during ascent to prevent loss of oxygen and pressure during descent and to accomplish the contingency abort if it becomes necessary.
After the astronauts are suited and seated, the shuttle commander orients the shuttle using the RCS so that the OMS engines are pointing in the direction of the orbiter motion. An OMS burn performed in this position slows down the orbiter so that its new perigee point is about 6 miles or 36,000 feet above the Earth's surface. Another RCS maneuver points the nose forward and sets up a space-atmosphere interface attitude of about 30º nose up. This insures that the thermal energy is concentrated on t he heat tiles. From a point halfway around the world the shuttle begins its entry.
The orbiter enters the Earth's atmosphere as it travels toward its new perigee point. A spacecraft normally increases velocity as it travels from apogee to perigee, but the drag induced by the Earth's atmosphere begins to slow down the shuttle. This decrease in speed causes the orbiter to increase its descent rate as the perigee point changes due to the spacecraft slow down. The pilots continue to orient the spacecraft using the RCS so that its bottom surface faces down with the nose facing forward.
About 30 minutes after the deorbit burn the shuttle begins to penetrate the Earth's atmosphere in earnest. Tremendous heat builds up on the orbiter's underside until it reaches a maximum at 20 minutes before landing. Thermal protection of the spacecraft is vital to human survival; this protection depends upon 32,000 silica glass tiles. These tiles vary from a measurement of six inches by six inches to eight by eight inches. They range in thickness from one-half an inch to 3.5 inches and are the consistency of chalk. Twenty thousand of these tiles are called high temperature reusable surface insulation (HRSI) and cover the areas most likely to encounter intense heat such as the bottom of the orbiter and its nose. These tiles are painted black and resist temperatures up to 1300ºF by radiating 90% of the heat back into the Earth's atmosphere. Low temperature reusable surface insulation tiles are painted white and resist temperatures up to 1200ºF. These tiles cover the upper side of the or biter's wing and the sides closest to the nose.
Because of the intense heat generated on the orbiter's nose and leading edges of the wings, reinforced carbon-carbon with a temperature resistance of 2300ºF is used to cover these surfaces. The rest of the orbiter including the top of the wings and the payload bay only encounter mild heat effects so they are covered with a thin layer of white insulation called flexible reusable surface insulation (FRSI) which protects up to 700ºF.
The reentry heat also causes another phenomenon called ionized communications blackout. The energy causing the heating strips away the electrons from the nitrogen and oxygen molecules causing positive ions which ensheath the sides and bottom of the orbiter thus causing loss of communications from 25 minutes before landing until 12 minutes before landing.
During the last 16 minutes before landing the orbiter performs four S-turn maneuvers to slow it down. Each of these turns removes energy from the vehicle very much like that experienced by a giant slalom skier. At this time the flight control systems such as the elevons and the rudder have sufficient air pressure to accomplish the maneuvers and the RCS is turned off. The last S-turn is performed five minutes prior to landing while the orbiter's speed is still MACH 2. At 5 minutes before landing the shuttle is at 83,000 feet. Its target is a 15,000 foot runway which looks like a skinny postage stamp at this altitude. At 86 seconds prior to landing the orbiter is at 425 miles per hour and at 13,000 feet; at this point the autoland sequence begins. Approaching the runway from this altitude, the shuttle has a 22º glide slope and a rate of descent approaching 22,000 feet per minute. The average airliner uses three degrees and a rate of descent of 700 feet per minute. At 17 seconds prior to touch down the glideslope is changed from 22º to 1.5º. At 14 seconds prior to touchdown the landing gear is lowered and then touchdown occurs at 215 miles per hour. When all three gear are firmly on the runway a small drag chute is released to slow the orbiter further and expend less energy on the wheel brakes. The orbiter rolls to a stop and then a convoy comes out to safe the craft. The major fact to remember is that this entire landing sequence is done without any power and the astronauts are flying nothing more than a large glider.
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