A floodlit Endeavour is heading into the complex ballet of the final part of the STS-118 launch countdown procedure, after the Rotating Service Structure (RSS) was retracted Tuesday night.
No issues are currently being worked, as the milestone of filling the External Tank (ET) with LH2 and LOX draws nearer. NASASpacefight.com will be continuing extensive live multi-level coverage – and will once again be the best place to follow a shuttle mission on the internet.
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Article:
When Endeavour launches, her crew, consisting of Commander Scott Kelly, Pilot Charles Hobaugh, and mission specialists Richard Mastracchio, Barbara Morgan, Tracy Caldwell, Alvin Drew Jr., and Canadian Space Agency’s Dafydd Williams, will be heading for a space station that, after decades of anticipation, looks suspiciously like the final product.
Although the origins of the International Space Station (ISS) date back to a 1984 State of the Union address given by President Ronald Reagan, the first components did not reach orbit until 1998, and only after the June flight of space shuttle Atlantis (STS-117) did the orbital outpost begin to resemble the early design drawings.
Dominate in those early sketches was a symmetrical space station flanked on either side by enormous eye catching solar arrays, and this is how the ISS looks today – a beautiful structure. Still, more work is to be done; construction continues with STS-118, the first post Columbia accident flight of the space shuttle Endeavour.
Just a few short hours after Endeavour and the ISS dock, the Canadarm will pull the integrated truss segment S5 from shuttle’s cargo bay and subsequently hand it over to Canadarm2, the station’s robotic manipulator.
Astronauts will skillfully operate the station’s arm and position the S5 next to the already attached S3/S4 truss segment. The first of two starboard solar array wings is anchored to S3/S4 segment, and the S5 will serve primarily as an extension piece providing clearance between this solar array wing and a future one to be attached further out on the truss.
In addition, the approximately 2000kg aluminum constructed S5 segment – measuring 3.4m by 4.5m by 4.2m – will continue the pathway for power and cooling fluid transfer along the truss. The first of four planned extravehicular activities (EVAs), or space walks, by Mastracchio and Williams, will be devoted to securing S5 to the truss, and removing restrains that held the station component together during the shuttle’s forceful climb to orbit.
These two astronauts will venture outside again two days later to begin the process of replacing a failed control moment gyroscope (CMG) with a working unit hauled into orbit by Endeavour. The ISS has four such gyroscopes, and these rapidly spinning wheels – a phenomenal 6,600 revolutions per minute – play a major role in controlling the station’s attitude.
A first year physics course explains why CMGs are attached to the ISS; should the ISS begin an unwanted rotation about a particular axis the resulting net angular momentum can be nullified by changing in a specific way the orientation of one or two CMGs. The re-orienting process, however, can not go on indefinitely. Over time a CMG will reach the physical limit of an axis gimbal, at which point its usefulness will have come to an end.
To correct this situation rocket thrust or some other mechanism must be used to control ISS attitude while the CMG swings back into a workable position. This process is called CMG de-saturation.
Originally this flight was to include a crew rotation: Clay Anderson to replace Sunita (Suni) Williams, who arrived at the Station last December. However, external tank repairs required after the February hail storm forced the flight of Atlantis to slip from March to June, and caused a subsequent delay in Endeavour’s flight by over a month. After some consideration it was deemed prudent to launch Anderson aboard Atlantis, STS-117, and return Suni Williams to earth on the same flight.
Anderson is on the ISS now (along with Fyodor Yurchikhin and Oleg Kotov) and, given that he previously trained with the STS-118 crew, will participate in two of the four spacewalks planned while Endeavour is docked. His space walking duties involve helping Mastracchio relocate the Crew and Equipment Translation Aid (CETA) carts. The CETA carts support EVA duties and share the truss rail system with the station robotic arm’s mobile base system.
In addition, Anderson will replace communication components and install equipment that, in the future, will allow the Orbiter Boom Sensor System (OBSS) to be securely attached to the outside of the station.
Also in Endeavour’s cargo bay will be the third station external stowage platform (ESP-3) and a single SpaceHab module. Placement of the ESP-3 (on the truss, near the port solar array wing) will be handled completely by robotics: shuttle arm operators will be Caldwell and Morgan, Hobaugh and Anderson the Canadarm2 drivers.
As the name implies, the stowage platforms function as a convenient external location to secure tools, spare parts, and other equipment that can be accessed easily by a spacewalker or Canadarm2. And for those wondering, the ESP-1 is on the Destiny module and ESP-2 is adjacent to the Quest airlock.
SpaceHab is a pressurized module that remains secure to the shuttle, and serves as additional storage space for food, water, and supplies which eventually will be transferred to the station. For the return trip home, SpaceHab will be loaded with, well, a fair amount of garbage and other clutter from the ISS.
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