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Student Informational Reading: Orbiter Control

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The space shuttle orbiter is truly an aerospace vehicle. It takes off using rockets, it orbits the earth as a spacecraft, and glides to the earth's surface as an airplane. During launch and its orbit of the earth, the orbiter does not make use of its airplane control surfaces at all. Because space is a vacuum, there is no air and therefore no air molecules through which the orbiter could fly. Remember that in order for there to be a difference in air pressure between the air flowing over a wing and the air flowing under a wing, there has to be air! No air, no air pressure, no lift! So the orbiter's wings and control surfaces are useless in space, but once below an altitude of about 96.9 km (60 miles), they can be used just like control surfaces on an airplane.

The orbiter has 4 basic control surfaces. They are the split rudder, elevons, speed brake and the body flap. The split rudder is located on the tail section of the orbiter which is called the vertical stabilizer. It isused to move the nose of the orbiter to the pilot's right or left. This side-to-side motion of the nose is called yaw. Because of the orbiter's delta wing configuration, it does not have elevators or ailerons. It has a combination of these two airplane control surfaces: elevons. The orbiter has two sets of these elevons. It has an inner set and an outer set located on the delta wings. The inner elevons control the motions of pitch and roll. The outer elevons also control the motions of pitch and roll. The difference between the inner and outer elevons is that the outer elevon is used primarily at hight (faster) speeds, while the inner elevon is used primarily at the lower (slower) speeds.

The split rudder is a control surface unique to the orbiter. The split rudder has two hinged surfaces which can move together to the left and right (functioning as a rudder). It can also split apart or open (functioning as a speed brake). When deflected (or opened) it changes the way the air flows around the tail section of the orbiter. In doing this, it creates drag that slows the orbiter's speed. Because of the orbiter's need to be able to glide through the atmosphere at speeds from hypersonic to subsonic, the speed brake is used along with the wide S-turns to slow the orbiter's speed as it descends. The body flap (located just underneath the engines) is also a control surface unique to mthe orbiter. It is used to help trim the orbiter's attitude. That means that the body flap is used to keep the orbiter in its proper flight position to maintain its course. These four basic control surfaces (rudder, elevons, speed brake and body flap) work together to keep everything under control as the orbiter rushes toward its runway.

Let's follow closely a typical landing of the orbiter and see how and when each control surface is used during this phase of the flight. At about an altitude of 96.9 km (60 miles), the orbiter encounters the fringes of the earth's atmosphere moving at a speed of 28,500 km/h (17,100 mph) with its nose pitched to a 30 degree angle. Less than 8 minutes later, the orbiter has already descended to an altitude of 77 km (46.2 miles) and is maintaining a speed of 28,000 km/h (16,800 mph). At approximately this altitude the air is dense enough for the control surfaces on the orbiter to operate. The body flap, elevons, split rudder and speed brake are put into use.

The orbiter begins its series of S-turns by making a broad sweeping roll to the right which is done to slow its speed as it descends dramatically from an altitude of 70 km (42 miles) to 33 km (19.8 miles). During this descent it also slows its speed from 28,000 km/h (17,100 mph) to 4,800 km/h (2,880 mph). During speeds above 8,180 mph the elevons and the body flap are in the up position. At a speed of 8,720 km/h (5,450 mph) the speed brake is deflected to a full-out position. This causes the nose to pitch up and allows the elevons to be put in the down position while the orbiter continues to slow its speed while descending. At a speed of 3,272 km/h (2,045 mph) the elevons are returned to an up position and the speed brake is moved to a smaller deflection (or closed slightly) to reduce the positive pitch angle (upward pitch) of the nose.

At an altitude of 21 km (12.6 miles) flying at a speed of 1,700 km/h (1,020 mph), the orbiter enters the Terminal Area Energy Management (TAEM) phase of its flight. During this phase of its landing, its energy (lift versus drag) is balanced for the final approach. Since the orbiter does not use engine power to land, it has only one opportunity to land. It cannot abort and fly around the runway for another try because it has no engine power to propel it upward again. It is important that during the TAEM phase the commander achieves a balance between lift and drag to ensure the best possible landing. At 15 km (9 miles) above the earth's surface, the orbiter slows as it passes through the sound barrier. At 3 km (1.8 miles) in altitude, the orbiter has entered its final approach phase with a 10 degree angle of attack as it maintains a steep glide slope of 21 to 24 degrees. To compare, a jetliner on its final approach phase approaches a runway at a glide slope of 2 to 3 degrees. While flying at subsonic speed, the speed brake is slightly deflected to continue to slow the orbiter's speed. The body flap is used to assist in trimming the orbiter's attitude. As the orbiter descends to about 550 meters (about 1,650 feet) above the ground, the orbiter's angle of attack is slowly lowered to 3 degrees and the glide slope is flattened from 24 degrees to 3 degrees. During this part of the landing the orbiter's onboard computers are assisted by a landing system that uses a microwave scanning beam. About 2 km (1.2 miles) from the touchdown point, the main landing gear is lowered and the flare maneuver is used which pitches the nose up 10 degrees. The orbiter's speed slows to about 350 km/h (210 mph) as it touches down on the runway. The parachute is deployed and the orbiter rolls to a stop about 2.5 km (1.5 miles) down the runway.

So we see how this unique aerospace vehicle travels from the outer reaches of the atmosphere to landing on a runway far below. We see how the control surfaces work together to guide the delta-winged glider from hypersonic speeds to subsonic speed, bringing it to a halt with the deployment of its parachute at its final destination. It is the orbiter's remarkable aerospace glider design that has made it a dependable part of the space transportation system linking the earth with space.

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