Gap fillers
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Gap fillers are an element of the Thermal Protection System on the Space Shuttle. The TPS is comprised, in part, by tiles that deflect the heat generated by the friction of re-entry away from the spacecraft.
There are doors and moving surfaces in areas that must be protected from heat, and this results in open gaps in the heat protection system. Some of these gaps are safe, but there are some places on the heat shield where surface pressure gradients would cause a cross flow of boundary layer air in those gaps. Gap fillers are placed here to minimize heating by preventing the formation of these vortices.
The filler materials are made of either white AB312 fibers or black AB312 cloth covers (which contain alumina fibers). These materials are used around the leading edge of the forward fuselage nose caps, windshields, side hatch, wing, trailing edge of elevons, vertical stabilizer, the rudder/speed brake, body flap, and heat shield of the shuttle's main engines.
On STS-114, some of this material was dislodged and determined to pose a potential safety risk. It was possible that the gap filler could cause turbulent airflow further down the fuselage, which would result in much higher heating, potentially damaging the orbiter. The cloth was removed during a spacewalk during the mission.
