The Space Shuttle Program - Space Shuttle Flight Profile
crew orbiter orbit engines
The ten panels of Figure 4.5 illustrate the major steps in a space shuttle flight from launch to landing.
|Length||184.2 feet||122.17 feet|
|Height||76.6 feet||56.67 feet|
|• Gross lift-off, which will vary depending on payload weight and onboard consumables||4.5 million pounds||–|
|• Nominal end of mission landing with payload, which will vary depending on payload return weight||–||230,000 pounds|
|Thrust (sea level)|
|• Solid rocket boosters||3,300,000 pounds of thrust each in vacuum||–|
|• Orbiter main engines||–||393,800 pounds of thrust each at sea level at 104 percent|
|• Length||–||60 feet|
|• Diameter||–||15 feet|
|SOURCE: "Table," in Space Transportation System, National Aeronautics and Space Administration, Kennedy Space Center, Kennedy Space Center, FL, August 31, 2000 [Online] http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_overview.html#sts_overview [accessed January 14, 2004]|
The countdown to launch begins approximately four days before lift-off. During this time numerous systems checks are conducted on the spacecraft and its components. The flight crew is taken to the orbiter approximately 2.5 hours prior to lift-off and strapped into their seats.
The shuttle is launched in a vertical position, with its nose pointing up, as shown in the second panel of Figure 4.5. At 6.6 seconds prior to launch, the three main engines at the rear of the orbiter are ignited. These engines burn fuel contained in the external fuel tank (ET). The ET includes two separate compartments. Liquid hydrogen is kept in one compartment, and liquid oxygen in the other.
When the countdown reaches zero, the SRBs are ignited. The SRBs are metal housings filled with solid fuel (aluminum powder and other dry chemicals). Ignition of the SRBs provides the powerful push needed to lift the spacecraft off the ground and overcome the effects of Earth's gravity during ascent. The ride for the crew is very rough and bumpy while the SRBs are firing. However, the acceleration load on the humans is designed to stay below three Gs. In other words, the force of gravity "pushing" against the crew members as the shuttle accelerates is only three times the force of gravity on Earth.
Approximately two minutes after lift-off, the shuttle reaches a vertical distance of twenty-eight miles and travels at about 3,000 miles per hour. At this point the SRBs are jettisoned away from the vehicle. Their fuel has been consumed. The SRBs are equipped with parachutes that open after the boosters have fallen a specified distance. The SRBs splash into the ocean and are retrieved for reuse.
The ride becomes much smoother for the shuttle crew after the SRBs are jettisoned. The shuttle's main engines continue to fire until they have used up all of the fuel in the external tank. This occurs at 8.5 minutes after lift-off. At this point the shuttle is above Earth's atmosphere and traveling at a speed of five miles per second. The main engines are shut down and, seconds later, the external fuel tank is jettisoned away from the vehicle. The tank burns up during atmospheric reentry. Only the orbiter is left to continue the journey.
The shuttle assumes a low-Earth orbit, typically 150 to 250 miles above sea level, where it travels at about 17,600 miles per hour. It takes the craft approximately forty-five minutes after lift-off to reach its orbit.
The orbiter includes a series of small engines that allow the flight crew to maneuver while in space. These engines comprise the orbital maneuvering system (OMS) and the reaction control system (RCS).
The OMS engines are mounted on both sides of the upper aft fuselage. They provide the thrust needed to make major orbital maneuvers, for example, to move the shuttle into orbit, change orbits, and rendezvous with other spacecraft in orbit. Such instances are called orbit maneuver burns, because the engines are temporarily ignited to achieve them. Panel 5 of Figure 4.5 illustrates the shuttle undergoing an orbit maneuver burn. The RCS engines are located along either side of the orbiter's tail and on its nose. They provide small amounts of thrust for delicate and exacting maneuvers.
During orbit the space shuttle crew performs a variety of tasks depending on the mission requirements. The shuttle was designed to carry payloads into space and to serve as a short-term laboratory for science experiments.
The shuttle can carry satellites or heavy equipment intended for space station construction in its large payload bay. Some satellites are intended for LEO, while others orbit at much higher distances. Shuttle crews can deploy, retrieve, and service LEO satellites from their spacecraft. Satellites that require higher orbits can also be deployed from the shuttle. These satellites have built-in propulsion systems that boost them into their orbits once they are a safe distance away from the shuttle.
The payload bay is equipped with a fifty-foot-long robotic arm called the remote manipulator system (RMS). The RMS is also called the Canadarm, because it was developed by Canadian companies. A crewmember operates the RMS from the orbiter flight deck. The RMS is used to move things in and out of the cargo bay and on and off the International Space Station and to grab and position satellites or even space-walking astronauts.
The shuttle is equipped with specialized laboratories in which crewmembers can conduct experiments related to astronomy, earth sciences, medicine, and other fields. Most of these experiments take place in pressurized modules specifically designed for shuttle flights. One such module, called SPACEHAB, is twenty feet long by fourteen feet wide, and eleven feet high. It can carry up to 9,000 pounds of payloads. Figure 4.6 shows how the SPACEHAB and other science-based payloads were configured in the payload bay for a shuttle flight.
A space shuttle crew normally consists of five crewmembers: a commander, a pilot, and three mission specialists. These are all NASA personnel. The commander has onboard responsibility for the mission, the crew, and the vehicle. The pilot assists the commander in operating and controlling the shuttle and may help deploy and retrieve satellites using the RMS. Mission specialists work with the commander and pilot and have specific responsibilities relating to shuttle systems, crew activities, consumables, scientific experiments, and/or payloads. Mission specialists are trained to perform EVAs (space walks) and operate the RMS.
In addition to the commander, pilot, and mission specialists, there may be one or two "guest" crewmembers called payload specialists. Payload specialists are not considered NASA astronauts. They perform specialized functions related to payloads and may be nominated by private companies, universities, foreign payload sponsors, or NASA. Payload specialists can also be foreign astronauts recommended by foreign space agencies.
The crew spends time in the crew module. This 2,325-cubic-foot module is pressurized and maintained at a comfortable temperature to provide what is called a "shirt-sleeve environment." The crew module includes the flight deck, the middeck/equipment bay, and an airlock. The airlock contains two spacesuits and space for two crewmembers to put on and take off these suits. Spacesuits are required during EVA activities.
The flight deck is the top level of the crew module. (See Figure 4.7.) This is where the commander and pilot spend most of their time during a mission. During launch and reentry they sit in the two seats facing the front of the orbiter with the commander on the left and the pilot on the right. The orbiter can be piloted from either seat. Two other crewmembers sit behind these seats further back in the flight deck. Any other crew members sit in the mid-deck section during launch and reentry.
The mid-deck of the crew cabin includes stations for meals, personal hygiene, and sleeping. This area includes the waste management system, a table, and stowage space for gear. In an emergency three additional seats can be placed in the mid-deck crew cabin for reentry. This allows the shuttle to carry ten crewmembers back to Earth. Such a contingency might be needed to rescue astronauts from the International Space Station.
Reentry and Landing
In order to reenter Earth's atmosphere the shuttle has to decrease its speed by a substantial amount. This is performed via a deorbit burn in which the shuttle is turned upside down with its tail toward the direction it wants to go. Firing of the OMS engines slows the spacecraft down. It then flips over and reenters the atmosphere with the nose pointed up at an angle as shown in the eighth panel of Figure 4.5. This ensures that the well-protected under-side of the orbiter takes the brunt of reentry heat.
Reentry is a very dangerous time for the shuttle. Any failure of the thermal protection system could allow super-hot gases to enter the orbiter. Reentry begins about seventy-six miles above the Earth's surface. Following reentry, the shuttle glides through the air to its landing site.
Emergency Flight Options
The shuttle program includes a variety of flight options in the event of an emergency. If there is a problem with the main engines up to four minutes after lift-off, the shuttle can undergo a procedure called Return to Launch Site (RTLS) abort. The SRBs and external tank are jettisoned and the orbiter is maneuvered into position to glide back to the launch site. If an RTLS abort is not possible, there is also the option to land the orbiter at an overseas location. This is called a Transatlantic Abort Landing or TAL. There are three TAL landing locations along the western coast of Europe and Africa: near Moron in Spain, at Ben Guerur in Morocco, and at Dakar in Senegal.
If the orbiter launches successfully but cannot reach its intended orbit, then an Abort-to-Orbit procedure is followed. This means that the spacecraft assumes a lower orbit than planned. In the event that the orbiter cannot maintain any orbit, it returns to Earth for reentry and landing. It may travel once around the Earth before it does so. This option is called the Abort Once Around.
The final emergency flight option is called the contingency abort. This procedure is undertaken if the orbiter cannot land on a landing strip for some reason. It calls for the orbiter to be put into a glide and the crew to use the inflight escape system. This includes a pole that is extended out the side hatch door. The crewmembers can then slide along the pole to the end and parachute to the ground.