Any damage to the tiles, or a protrusion or bump on the underbelly of the shuttle, can cause a break in the boundary layer and allow in extreme heat. Of particular concern are the gap fillers, pieces of ceramic-coated fabric the thickness of a sheet of paper that fit between the tiles to provide cushion, which have been known to protrude. (NASA, however, says the fillers do not impose a safety concern.)
The Langley researchers imaged three shuttle missions: Discovery on March 28 (STS-119); Atlantis on May 24 (STS-125); and Discovery again on September 11 (STS-128). They also conducted two small flight-research experiments. “We added a tiny bump to Discovery’s wing, approximately a quarter of an inch, to better understand what is called a boundary layer transition or trip in the flow fields,” says Tomek. The researchers also coated two of the tiles with a material that is being developed for the heat shield of the Orion crew exploration vehicle.
The researchers are just beginning to process all the collected data into 3-D surface temperature maps, which they will compare with measurements from thermal sensors on the shuttle’s underbelly and with computational fluid dynamic models. Horvath says they will present their results at a conference in January 2010.
However, the researchers have already seen some unexpected results. A small imperfection, possibly as small as a tenth of an inch, on the opposite side from the bump purposely placed on Discovery’s wing, created high temperatures in a much larger area than what you normally see, says Horvath.
Osheroff says he is interested to see if the analysis finds different results for different orbiters. For example, Columbia was the first shuttle built and is 20,000 pounds heavier than the other orbiters. “Heating patterns depend on the attitude or orientation of the orbiter during reentry, so it would be beneficial to conduct tests for at least two flights of each orbiter.”
There are only six remaining space shuttle flights before the orbiters are scheduled to retire. Horvath says the researchers hope they can continue to image the remaining missions, but final approval is still pending. “Our ability to accurately predict thermal data will have a profound impact on designs for new vehicles,” he says.