The Shuttle's Legacy, Part 2

Congratulations to the Atlantis crew and all members of the Space Shuttle team on this morning's safe landing at the Kennedy Space Center.

Part 2

Although the optimistic flight rate was never achieved; the most flights in one year was 9 in 1985, the Shuttle did provide regular access to low earth orbit and it enabled  construction of the International Space Station which has been continuously occupied since 2000.  It also proved out the concept of re-usability with the External Tank being the only element that is not recovered and refurbished after a mission.  The Shuttle also brought the Hubble Space Telescope to orbit, saved the observatory with the first servicing mission in 1993, and kept it functional with 4 subsequent repair missions.  The program opened up avenues of cooperation between the United States, the European Union, Russia, Canada, the Middle East, and Japan as astronauts from many different countries flew together.  That international cooperation continued on the space station.  Beyond the technological achievements, both the Shuttle and Space Station programs became effective diplomatic tools. 

During the period from 1986-2003, 87 successful missions were flown.  The redesigned SRB field joint worked perfectly and a number of other upgrades were phased in.  The SSME fuel and oxidizer turbopumps were redesigned to achieve better structural durability and thermal margins and the ET material was changed to a new Aluminum-Lithium alloy with higher strength than the baseline aluminum.  This allowed for a lighter tank and the 7500 Lbs savings equated to an almost equal amount of additional payload performance.  Achieving additional payload performance was key because the decision to locate the Space Station at an orbital inclination angle of 51.6 degrees with the Equator (necessary in order to accommodate Russian launches from Kazakhstan), meant that the Shuttle could not fly an optimal performance trajectory to the Station. 

February 1, 2003 brought the second great tragedy to the program as the orbiter Columbia, the first to fly into space in 1981, broke apart during atmospheric reentry over Texas.  The cause of the failure, a cracked wing leading edge due to foam shed from the ET during launch, is still troubling because the failure mode was not intuitively apparent, even after it happened.  The ET foam insulation is very light, almost the consistency of Styrofoam.  During the failure investigation, testing proved that the foam, when moving at a high velocity, could fatally damage even the tough carbon composite leading edge of the wing.  The wing breach on Columbia allowed super hot gas into the interior of the vehicle which precipitated its destruction.  Inherent in the practice of Engineering is the making of assumptions.  When designing a system and deciding what tests need to be run on that system, engineers must determine what the most likely failure modes are and focus their attention on them.  Judgment and experience are critical in this process.  It is impractical to test every possible scenario so intuition as to the sensitivity of the system guides the process.  Unlike in the Challenger failure, it was not intuitive that very lightweight foam shedding from the ET during launch could fatally damage the Orbiter wing.  Testing and the deaths of 8 astronauts proved otherwise.

The decision to include the Russians in the Space Station program allowed American astronauts to continue flying to the Station during the 32 months that the Shuttle was grounded after Columbia.  Shuttles resumed flying in 2006 with better process controls in place to limit the amount of foam shed from the ET during launch.  Note that NASA and the ET prime contractor, Lockheed Martin were unable to completely prevent foam from shedding during flight.  The foam application process control improvements were the best that could be done, and NASA Administrator Michael Griffin judged the risk to be acceptable to resume flights.  22 successful flights later, most of them used to complete construction of the International Space Station, and Atlantis has now concluded her mission, the 135^th and final flight of the Shuttle program.