The orbiter has several noteworthy systems. The most important are the 5 GPCs, the 3 APUs, the 3 Fuel Cells, the 3 SSMEs, the OMS. the RCS, the payload bay, cooling systems, the RMS, the TPS, the Aerosurfaces, and later on, the APDS, and OBSS.
The GPCs (longest explanation)
The 5 GPCs, or General Purpose Computers, are 5 computers that have control of all the systems in the orbiter. For all of you old-age science fiction aficionados, imagine 5 HAL 9000s controlling the shuttle... only without AI, following a set pattern of instructions. In order to embrace the horror of it should it have AI (artificial intelligence), I give you a quick photoshopped image:
"I'm afraid I can't let you do that, Dave..." ...5 times.
Ok, so it's not very reassuring imagining our favorite 1960's science fiction insanely homicidal computer, or rather, 5 of them, controlling the air we breathe. This ain't the case, lol. The GPCs ensure automatic perfect operation of the orbiter's systems, and accept input by means of 3 keypads, 2 of them beside the commander's seat and pilot's seat, respectively, and one on the aft-flight deck. In other words, it executes commands, it doesn't 'think' them.
...yet.
Actually, 4 of those GPCs are the primary control computers, and GPC #5 is the BFS, or backup flight system. The first 4 are constantly checking each other and #5. If one of the main computers fails, the others vote it out of system control. To date, there have been no GPC failures, and therefore, no need to use the backup flight system. But, NASA loves to double (or quintuple in this case) up on everything.
The GPCs have several programs, but the most used are 3 programs which control the 3 phases of any mission, called Operational Sequences, or OPS. Each OPS is divided into major modes, for example, OPS 201, and OPS 202, which means OPS 2, Major Mode 01 and 02. And each major mode is divided into Specialist Functions, or SPEC, which control individual aspects of the major mode. Try viewing it like this: You have a computer with an installed operating system, like Windows or Mac OS X (the OPS). You have many programs, but at the moment, you are using Word (the major mode). In Word, you can write documents, or letters, or envelopes, you can spell-check, print, and many more things (the SPEC functions).
The first program is OPS 1, which controls the launch, 8 1/2 minute ascent, and orbit insertion of the shuttle. OPS 101 through 106 control specific portions of final countdown, liftoff and ascent with the solid rocket boosters and the shuttle main engines, the remaining ascent with the shuttle main engines, and orbit insertion. Once orbit insertion is complete, the orbiter's GPCs are transitioned to OPS 2.
OPS 2 controls the in-orbit operations of the orbiter, and is less dynamic than the other 2 main OPSes. Therefore, GPC 5, the BFS, and if I'm not mistaken, GPC 3, are shut down. OPS 201 and 202 have several functions, ranging from orbit maneuvers and course/altitude corrections, operation of the KU antenna and star trackers for tracking satellites or the ISS to payload bay doors and radiators operation, RMS arm operations, orbiter cooling systems, and the like. It is mostly controlled through the aft keypad, unless it's an OMS burn. Once the mission is complete, the GPCs, plus GPC 5, are transitioned to OPS 3.
OPS 3 controls the deorbit, entry, approach and landing phases of the mission. OPS 301 thru 305 control specific aspects of the pre-deorbit configuration, the deorbit burn, the entry into the atmosphere, course corrections, approach, final approach and landing.
The APUs
The APUs, or auxiliary power units, are only used in the launch and entry parts of the mission, and are otherwise shut down. They are the ones responsible for the hydraulics controlling the aerosurfaces and the main engine nozzle gimbal (AKA aiming the engine nozzle). Anyone familiar with planes knows most or all planes have an APU, for the aero surfaces' hydraulics. The orbiter has three.
The Fuel Cells
The fuel cells are compact power plants, using liquid hydrogen and liquid oxygen on the orbiter's internal tank to produce electricity for the entire orbiter, and water as a byproduct of the process. How more eco-friendly can you get? Oh. They're worth AN ARM AND A LEG (best effect reached when said in a low, deep voice). The orbiter has three of them.
The SSMEs
Picture taken from STS-128-Discovery's live coverage, 2 seconds prior to liftoff.The 3 Space Shuttle Main Engines provide the thrust for the liftoff, and complete ascent of the orbiter. They are fueled by the External Tank, with a mix of roughly 2 parts liquid hydrogen and roughly 1 part liquid oxygen. The exhaust plume the engines generate is no more than super-heated ol' water vapor. Once the engines shut down at the end of ascent and the External Tank is cut off and jettisoned, the engines become useless for the rest of the mission. In other words, the movie Armageddon was a big fat hoax. But a pretty darn good one.
The engines begin to pressurize using their turbopumps a few seconds away from main engine start, which is 6.6 seconds before liftoff, and each powers on 0.12 seconds after the other. Maybe turning them on at the same time is bad, but turning them on one at a time at 6.6, 6.5 something, and 6.4 something seconds is safer? Meh.
The OMS
The Orbital Maneuvering System, the 2 small engines above the left and right SSME, respectively, is what provides for the orbital course correction burns to be executed, known as OMS burns. They have their own propellant, and are used for orbit insertion, changes in the orbit to catch up to a satellite or the ISS, changes in the orbit altitude, and of course, the final deorbit burn. The burns are executed by the GPCs, in which the burn targets are loaded and executed.
The RCS
The Reaction Control System is an attitude and position maneuvering system, comprised of several small jets in the front and back of the orbiter. They fire either constantly, or in pulse mode, making attitude control a breeze. This ensures the orbiter is not spinning uncontrollably, and remains in one set attitude. Also, it's used to be able to match and control the relative movement of the orbiter to the satellite they're trying to grapple, or to the ISS.
The AeroSurfaces
This is the method of control during atmospheric flight. Powered by the 3 APUs, the surfaces are similar to those on a plane, but with slight differences. The normal ailerons and elevators used for pitch and roll on a plane are combined into one on the orbiter and are called the elevons. When deflected into the airstream, they control turning and banking, and pitch. The body flap, a flap in the aft of the orbiter, juts out just below the 3 SSMEs. It serves to protect the engines from reentry, and to provide another means of managing sinking rate. There are 2 additional flaps in the back of the vertical tail in the back of the orbiter. If one opens to the left, the orbiter yaws to the left. If the other flap opens to the right, it yaws to the right. If both flaps open, it becomes a nifty speed-brake.
This system is used to manage the energy of the orbiter, meaning the relation between the speed it's travelling at and how fast it is sinking down, and remember, without engines, the orbiter has only one shot of reaching the runway and landing safely. So one has to make sure he's going fast (or slow) enough, while at the same time making sure the sink rate isn't too fast in order to reach the runway in time, while at the same time staying on course.
The Payload Bay and Cooling Systems
The payload bay is used for ferrying cargo to space and/or back. It supports several payload options. Inside the right side of the payload bay is the RMS arm, and since 2005, on the left side is the OBSS extension. The cargo is protected during launch and entry because of the payload bay doors, which are closed during those phases, Once in orbit, the doors are opened, since the radiators for the cooling system are stored in the inside door panels, coursing with freon coolant, keeping the orbiter systems and cabin within safe operating temperatures. Also inside the payload bay, at the front left edge of the payload bay, is the KU Antenna, which supports high-speed communications like video downlink.
The RMS and OBSS
The Remote Manipulator System is the orbiter's robotic arm, designed and constructed by CSA, the Canadian Space Agency. The code name for the arm is "Canadarm". It has 3 points of fixed movement, at the shoulder (imagine your arm latched onto your shoulder. The point where your shoulder becomes your arm is the point I'm talking about), at the elbow (ditto), and the wrist (double-ditto). The shoulder controls pitch (up/down) and yaw (sideways left-right movement). The elbow only has pitch control. Finally, the wrist, where the "hand" of the RMS, called the End Effector, has controls for all 3 attitude position controls: pitch, yaw and roll (roll is spinning in place to the left or right). These 3 points are used so as to move the end effector to a grapple point. The end effector has a camera to aid with the grappling target. Once contact is made, the end effector is commanded to capture, where the grapple pin is firmly grabbed by the end effector, and thus, executing a successful grapple of whatever it is that needs to be grappled. The arm can then move the grappled object around, and release it whenever it need be. Applications for this include taking a satellite out of the payload bay and hoisting it above the orbiter before releasing it and leaving it behind, or the reverse, possibly for servicing by the astronauts before being released again, or for taking it back to Earth.
The Orbiter Boom Sensor System is an extension boom to the Canadarm, also made by the CSA. It has a grapple point on one of the ends and in the middle of the extension boom. At the other end are laser sensors and cameras. Its use is simple, to be grappled by the RMS, and maneuvered so that the sensors are aiming at the underside of the orbiter. The TPS is scanned for damage, and if no damage is found, the TPS is cleared for reentry. The inspections are done on the second day of flight and 2 days prior to deorbit.
The TPS
The Thermal Protection System, AKA the thermal tiles, these line the nose, the leading edges of both wings, and the whole underside of the orbiter. Their job is to dissipate the heat of the ionized airstream and prevent it from harming the orbiter. Such a very important system is always checked, before, during, and before the ending of a mission, to ensure the safety of the vehicle and crew during reentry (checks were implemented into the mission after the Columbia disaster). They aren't your regular floor and wall tiles. Rather, these are tiles made of high-temperature-resistant material, varying in composition, according to the area it protects. The Reinforced Carbon Carbon Tiles (not a typo... they use the word carbon 2 times), or RCC Tiles, are the strongest tiles, and are used in the nose and wing leading edges, since these are the areas where maximum heating occurs. The other areas use lighter materials, since the heat dissipation isn't as much.
The APDS
The Androgynous Peripheral Docking System is a major upgrade done to the orbiters when the ISS became operational. This allows docking with ISS-Harmony's PMA, or Pressurized Mating Adapter. It's latched onto the front of the payload bay, where the old airlock door of the orbiter was. It allows for more storage room inside the orbiter's crew cabin, as it essentially becomes an extension of the middeck, and when docked to the ISS, is like an interconnecting tunnel between both vehicles. It has a capture ring that is extended, and as soon as it comes in contact with the ISS's mating adapter, it captures it in a process known as soft-dock. Once relative movement between both vehicles has stabilized to 0, the ring retracts, bringing the two vehicles together until the seal is airtight. Several botls and latches are driven to ensure that the seal isn't broken, and once this process is complete, it is known as the completion of hard-dock. The APDS them pressurizes the space between both hatches, and after leak checks, both hatches are opened and the two vehicles are officially one. Once the hatches are closed when joint operations complete, the air is let out of the space between both hatches, and when leak checks confirm that both hatches are securely closed, the APDS releases the latches and bolts, and, thanks to a spring-loaded mechanism, separates both craft.
On my next blog, I will *hopefully* explain the launch milestones, unless I see that further explaining is needed before that.
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